A moment of fierce joy

In a near 60-year career this experience has only happened to me three times. Two of these discovery outcrops eventually led to operating mines, the third, after years of exploration and drill testing, failed to make the cut (I still think it will be a mine someday, so I will say no more on that one)).  I take only modest credit for these discoveries. I was first on the ground and got the naming rights for the prospect and subsequent mines, but was part of a team, and a large element of luck was involved. All this happened 30-50 years ago when not all the low-hanging fruit had been plucked. Prospecting discovery of significant outcropping mineralisation  happens increasingly less these days. Maybe in a remote and under-prospected third world country? If there are any such left in this globalised world.

The Greenfields (1) gold deposit near Coolgardie in Western Australia, which I identified in the course of 1:5000 scale geological mapping, was named for the field below which it lay – full of Spring grass and wildflowers and big eucalypt trees when I first came upon it in 1984. I hoped that the name would preserve the memory of this pleasant little valley long after the bulldozers had moved in. Greenfields open cut mine is long exhausted, but the Greenfields Mill still operates today as a Toll facility for other gold mines in the district.

The Magellan Pb deposit near Wiluna in Western Australia was an entirely serendipitous discovery of high-grade (2) outcropping mineralisation which I made in in 1991 in the course of checking old gold claims in nearby rocks. That day was the first time that I had used a GPS unit in the field. This was a hand-held instrument made by a company called Magellan and the size and weight of a house brick, but a revelation and a boon. I thought it a cool name so gave it to my discovery. Magellan was the first discovery in a new and unique lead province. Later prospects in the area followed the same theme and named for 15th Century Spanish and Portuguese explorers. The Magellan lead mine is still in operation today although environmental restrictions have severely restricted its operations.

Discovery of outcropping mineralisation is invariably followed by collecting rock samples for assay.  The different types of rock chip sampling and how to collect them are detailed in an earlier blog post here.

 (1) The name I gave was actually Greenfield, but the company later added the final “s”, probably because they wanted investors to think they had more than one.

 (2) The first suite of rock chip samples from Magellan, collected from over 5km of strike, came back 5%-35% Pb. Near pure cerussite.

The post The Discovery Outcrop appeared first on Roger Marjoribanks.

Here’s the thing

You know what Australia looks like. You would recognise it on a map: its general shape, the peninsulas, the great gulfs. You could draw it from memory, probably, and if you did, it might look something like this:

Figure 1

That’s a pretty good effort and would certainly serve to identify the island continent, but it does not look a bit like a real map. No one, for example, would ever think that you had painstakingly traced this outline from an atlas. What you have drawn is a cartoon. Actual coastlines are seldom, if ever, composed of straight lines. Actual continental shapes are not irregular polygons. With some notable exceptions (crystals for example, some biological structures) few things in nature are defined by the straight lines, planes and regular shapes of classical Euclidean geometry.

Now try drawing the same map again, but this time employing an irregular wriggly line to outline the coast and to create a multitude of completely imaginary small-scale bays and headlands. Here it is:

Figure 2

This looks much more realistic. Why is that? Because that’s what coastlines look like (especially rocky ones).

Geographers Wobble

What you have done in figure 2 is often referred to as “geographers’ wobble”, and not usually as a compliment. However, Geographers’ wobble is not artistic license, it is not decoration to make a map look pretty or interesting in the style of old-time cartographers (here be dragons). In the absence of a finely-detailed data base, the geographer has chosen the correct type of irregular line to use in outlining a coast – and that is not a straight line. It is a fractal line. The geographer chose to use a line with the correct  fractal dimension for his purpose.

More on fractals and fractal dimensions later.

Geologists Wobble too

When making a detailed outcrop map, geologists often use the same technique – one could call it “geologists’ wobble” (1)

Figure 3

Consider the map below, originally compiled at 1:1000 scale (the survey pegs are 40m apart). It is a portion (one of 12 field sheets) of a geological map that aims to show all outcrop of more than 1-2 m across within a mineralised prospect area.

Figure 4 An outcrop geology map a 1:1000 scale

It would be impractical to do an exact point by point survey of each of the outcrops in the area. Rather the geologist compiling the map surveyed a few points on each outcrop by triangulating on survey markers, then took compass bearings and counted paces as he walked around it, sketching in the details. Note that the style of line used to outline each outcrop is dependent upon the rock type. In this area, shale units (sh) are exposed as short, interconnected runs poking through the surface rubble: the map outline of shale outcrop thus has a very complex shape with many large-scale and small-scale re-entrants. The quartzite (qtzite) unit occurs in long, parallel-sided, strike ridges broken by weathering along an orthogonal jointing: by comparison with the shale, its map outline is simpler and more geometric in shape. The simplest outcrop shapes of all are those of the granite (gr) which has rounded outlines resulting from its massive nature and the effects of spheroidal (onion skin) weathering.

Examples – extracted from the map – of these different line styles are shown below. The outline of each unit has a different  fractal dimension which the geologist attempted to capture with an appropriate type of line.

 Figure 5 Fractal shapes of rock outcrops in figure 4

Fractals and Fractal Dimensions

The idea of a fractal dimension was developed by French-American mathematician Benoit B Mandelbrot in the 1960s and 70s as a way of quantifying the degree of complexity of folded lines and sheets. The idea became widely disseminated beyond his specialist field with the publication of his beautifully illustrated best-selling book The fractal geometry of Nature (1983: W H Freeman, San Francisco, 468pp).

In classical Euclidean geometry, a line is a one-dimensional entity (1-D) with a length but neither width nor thickness. It retains its 1-D identity no matter how complexly it is folded. Mandelbrot reasoned that if a line were sufficiently folded within the plane of a 2-D surface then it would begin to fill that surface and so approach the dimensions of the sheet within which it is embedded. In other words, depending on the degree of complexity of the folding, it will have a dimension somewhere between 1 and 2. He called this the fractal dimension and the line itself a fractal object.

Similarly, a 1-D or a 2-D object (think of a length of thread or sheet of paper, ignoring their thickness), folded within 3-D space, will have a fractal dimension somewhere between 2 and 3[2].

Fractal lines and shapes are the characteristic of natural objects. Trees, topography, turbulence, coastlines and cauliflowers, snowflakes and stock markets, protein molecules, distribution of galaxies, folded sediments, the outlines of rock outcrop.

Mandelbrot realised that fractal shapes can be quantified by comparing the detail of the object that is apparent when viewed at different scales.

Take the outline of a coast – a typical fractal line. How long is this line? Obviously, it will be much longer if we measure it with a 1-meter-long ruler than if we measure it with a 100-meter-long ruler. How much longer? That will depend upon how wriggly and convoluted the coastline at whatever level of detail we view it, and that depends on its fractal dimension. If coastlines had regular Euclidean shapes – segments of polygons, circles or ellipses, for example – we could step off its length with dividers of shorter and shorter openings and our succession of results would progressively converge on a finite answer. But for a fractal line there is no convergence: the coastline has an infinite length! The reason for this paradoxical result is that the fractal dimension you can measure will be approximately the same no matter how far you zoom down into the detail of the line.

Mandelbrot defined Fractal Dimension (FD) as the statistical ratio of the change in detail with the change in scale.  It matters little whether the FD ratio of a rocky coast is measured on a 1,000,000 scale map, on the bays and headlands of a 100,000 scale map or on a map of a rock pool on the shore at 1:1 scale. At levels of detail that range through several orders of magnitude, the Fractal Dimension of a rocky coast will be approximately the same.  That is not to say that, if you overlaid a tracing of 100 km of coastline over a tracing of 1 km of coastline, there would be a neat match, but the lines would look the same: they would have the same style.  Without further information, it would be impossible, from the lines alone, to say at what level of detail it was being viewed.  That is what is meant by saying that the Fractal Dimension is scale invariant. Fractal shapes have scale-invariant self-similarity.

The map of the rock pool is a fractal for the map of the bay in which it occurs, and the map of the bay is itself a fractal for the map of the whole rocky coastline. Similarly, small folds on the limbs of large folds can be fractals for the large fold, which may of course be a fractal for an even larger fold. This useful relationship has been known to geologists for almost 130 years, the first example of the practical use of what we now know as fractal geometry. When I was a geology student in the 1960s we were taught to call this “Pumpelly’s Rule” after USGS geologist Raphael Pumpelly (he of the eponymous mineral) who first described these relationships in 1894(4).

Figure 6: Raphael Pumpelly, 1837-1923. Geology & Mining Professor Harvard University (1866-75). Founding Member of the United States Geological Survey (1879), Director of the USGS (1884). President of the Geological Society of America (1905)

In Nature, fractal shapes are only ever self-similar through a limited range of scales. There are practical limits. Patterns are never repeated endlessly to infinity. One cannot strictly say (parodying Jonathon Swift) that:

So, Geologists observe, a fold,

Hath smaller folds that on limbs prey,

And these have smaller yet to parasite ‘em,

And so proceed ad infinitum.

However, fractal shapes and patterns created by computer through the iteration of simple non-linear algorithms can have perfect self-similarity through an infinity of scales.  The best-known example of a computer- generated set of fractals is the famous Mandelbrot Set.

The idea of the scale invariance of fractal shapes has great relevance in structural geology, where the structure of large regions (which cannot be directly observed) has to be deduced from observation of small outcrops in the field. This topic is explored in an earlier post (LINK).

The box counting method for calculating Fractal Dimension

I understand that some GIS software packages, such as ArcGIS, provide sub-programs for calculating fractal dimensions. However, it can be done manually. Mandelbrot’s insight that the Fractal dimension is the statistical ratio of change in detail with the change in scale enabled him to provide the following formula for calculating the FD of a folded line embedded in 2-D space:

C^D = N/M

Where the scale is a unit grid in 2-D space, and:

 C  is the scale reduction multiple              D  is the fractal dimension

N  is the number of squares occupied by the fractal in the high-resolution 2-D grid.

M  is the number of squares occupied by the fractal in the low-resolution 2-D grid

Figure 7 below illustrates the application of the formula to calculate the fractal dimension of a line. To demonstrate the technique and for simplicity of illustration, in the examples below I have chosen a low reduction multiple (C) of 2.  However, it should be noted that the larger the value of C, the more accurate and precise will be the calculated value of the Fractal Dimension (D).

Figure 7 Calculating the fractal dimension of a line (click for a larger, sharper image)

Some numbers

A typical rocky coastline has a fractal dimension (FD) of 1.2 – 1.3. Low-lying coasts of sediment accumulation (the 80-mile beach, for example) have lower fractal dimensions. An FD of around 1.2 was instinctively used in drawing the map of Australia shown in figure 2. That is an exaggeration as the actual coastline, averaged over the whole continent, has been recently measured at 1.114 (LINK)

The FD of the west coast of Britain has been calculated as 1.25 (LINK) (3)

The FD of the coast of Norway -  a particularly rugged and fiord-indented coastline -  has been calculated (by Slartybartfast, among others (LINK from p 139)) at 1.52.

The outline of the granite outcrop in figure 5 has an approximate FD of 1.1

The outline of the quartzite outcrop in figure 5 has an approximate FD of 1.2

The outline of the shale outcrop in figure 5 has an approximate FD of 1.3

Figure 8 The coastline of southwest Norway. Fractal Dimension 1.52

Conclusion

The terms Geographers’ Wobble and Geologists’ Wobble may sound unscientific and uncomplimentary, but they describe procedures based on an instinctive understanding that natural objects are fractal and cannot be described graphically using the lines and shapes of classical Euclidean geometry. Fractal lines provide a more accurate representation of reality.

Following Mandelbrot, that reality has come to be known as Chaos – the expression of non-linear natural processes. The descriptors of Chaos are fractals, fractal dimensions and scale-invariant self-similarity.

The post Geologists Wobble and the fractal nature of rocks appeared first on Roger Marjoribanks.

Speak to exploration geologists and you will find two opposing views about what a geologist should do when observing outcrop or drill core in the field.  Some seek merely to be unbiased objective recorders of what they see.  Others observe the rock in the wider context of their theories about region or prospect geology and ask questions of the exposure to help choose between them. I characterize these two approaches with the metaphors of  geologist-as-camera, and geologist-as-interrogator.

Geologist as camera – 1

I arrive at the exploration site to find a team of three young geologists engaged in making a geological map of their large property. Using GPS, they walk predetermined traverses spaced 400m apart. They each aim to average 8km in a day and, between them, by taking alternate lines, they cover a large swathe of country before their next field break. Observations on each geological feature along the line of march are logged into a field-hardened tablet computers by going through a series of pull-down menus and checking boxes on pre-determined questions. The geologists have only the vaguest ideas about the geology of the property they are mapping. Two of them have never thought much about this: the brightest of the three specifically rejects the notion of understanding her observations: she sees herself as an unprejudiced, objective observer – confirmation bias is not for her. Eventually all this data is computer-plotted to 2D images as a linear sequences of point observation.  Another, more senior geologist, might eventually produce a geological map by joining the dots. Probably there is a software program that can do much of this task for him (or her) and thus obviate the need for too much thought or hard work.

Geologist as Camera – 2

I travel to an exploration site on the other side of the country. Here, a much larger company is at the final stage of drill testing a substantial metal deposit. Four diamond drill rigs are going 24/7 and have reached hole number 300 and something. Stacked on pallets, kilometers of core are waiting to be logged. Four geologists are hard at work logging core laid out on long racks in a big shed. As geologists come and go on field break, or come and go through resignations and new hires, it seldom happens that any one hole is logged in its entirety by the same person. They log on to analytical spread sheets in laptop computers using pre-determined menu options – there are more than 50 columns on their spreadsheet. The same log form has been used since DDH 001. The geologists have never seen a drill section of the prospect (there are none). There are no detailed maps or level plans either. The geological model, such as it is, was produced a year before by an outside consultant who spent ten days on site. Back in the head office a specialist Ore Reserve Geologist ignores most of the vast data base of geological observations and calculates a resource based on assay numbers, virtual reality 3D string models and geostatistical techniques. For the geologists at point, slaving in the core yard, their job as 100-meter-a-day, core-logging automata is mind-numbingly boring. They count the days till their next field break, or until they have earned enough money to find some other more intellectually stimulating employment.

After a day data logging, Susan contemplates a career change. Picture by author.

These are anonymized projects, extreme examples perhaps, but based on my observations of many actual projects, with many companies in different countries over many years. Not all exploration projects are conducted like this, but I am sure that readers will recognize the method of doing geology that I describe. The geologists are not to blame. In many cases it is their first job, they are on short-term contracts, and they are doing what their employer asks and expects of them. They probably wonder what the point was of much of the time they spent learning at university and have come to believe that the essence of their job as exploration geologists is to convert light signals from their eyes into digital computer feed according to pre-set formulae. A kind of biological digital camera.  Without a pre-existing context in their brain, each observation they make has equal importance, each pixel equal weight.

There is a better way.

The geologist as interrogator

All observation is made in a particular context. The context which should guide the exploration geologist is a matrix of different competing theories about the true nature of the geology being observed. This context provides the questions that must be asked of each outcrop or each piece of drill core. It defines the strategy to be followed in the search for those critical observations that allow selection between multiple working hypotheses (1), (2). The idea of multiple working hypotheses was first propounded by 19th Century US Geological Survey geologist Thomas Crowther Chamberlin. It is a methodology now used in all fields of science research, but geologists can be proud that the first clear statement of the idea came from their profession.

Critical observations are those that fit into a pre-existing context or, just as importantly, those that do not. These are prioritized and not lost in a sea of trivial observation. The method does not guarantee the you will arrive at the correct solution. Your evidence may be less than ideal, the expression in the rocks of geological events may be atypical.  Nature can be chaotic and unpredictable: the true hypothesis you should be testing  may be a Black Swan – the one you have never thought of. In Geology – indeed in all science – neat, clean results where all the data slots in exactly are rare and should be always regarded with some skepticism. But in spite of all that, the method of multiple working hypotheses is the best known procedure for approaching the truth.

After a morning of fieldwork, Robert interrogates his lunchtime sandwich using the method of multiple working hypotheses. Picture by author.

Collecting data to test hypotheses in this way is biased observation. All geologists have cognitive biases which were imprinted when they were taught to be geologists at University, and reinforced by early-career mentoring and  reading geological literature relevant to the job.  But this bias is a good thing where it is up-front, acknowledged and constantly revised and re-focused in the face of evidence. Without bias, there is no way of separating signal from noise – not even by the most sophisticated of statistical procedures. This is a quite different kind of bias from the one that uncritically accepts evidence that confirms a single pre-existing theory, or a theory you have fallen in love with,  and ignores, or explains away, evidence which does not. That is Confirmation Bias and is rightly condemned.

All scientists have a point of view and all views must have come from somewhere. Anyone who claims to make unbiased observations merely lacks self-awareness. Their biases are unconscious or lie in the biases of whoever drew up the detailed procedure manual which they follow.

The metaphor I use for this approach is the geologist as an interrogator of each rock exposure or core piece, asking a series of relevant questions that come from his or her views as to what might be happening, with each question determined by the answer to the previous question.

RM ’14

In a previous post I explore how using the technique of multiple working hypotheses can be used when constructing a geological map (LINK)

The wider context

The true nature of scientific investigation is focused observation guided by emergent theory. This is a long established and respectable idea and contrasts with seeking to find your theory in your data after you have completed your observations.

Collecting data should not be a fishing expedition that hopes to fortuitously entangle a new understanding or ideas on its line.  Its purpose should be to provide evidence to choose between a range of pre-existing hypotheses.  It therefore follows that the hypotheses must come first.

Seeking your hypothesis in your results is such a widespread and acknowledged error in many scientific disciplines that it has been given its own acronym - HARKing[3] (Hypothesizing After Results are Known). In statistics-based research the same error is known as p-hacking [4].  Both techniques have been, and undoubtedly still are, widely used in so called science [6].

The parable of the Texas Sharpshooter provides a good example of the HARKing technique at work [5] (6):

“The Texan directs sixty shots at a barn door. He then selects the best grouping of the random impacts and draws a target around them. A carefully cropped photograph is then used to establish his sharpshooting credentials.”

The Texas Sharpshooter – Picture by author. 

This idea that theories should precede observation is the exact opposite of what is popularly believed. Sherlock Holmes, for example, is often quoted approvingly:

“It is a capital mistake to theorize before one has data”.

But Sherlock’s patronizing throw-away line to Watson could not be more wrong. Contrast it with these quotes from real scientists – as opposed to a fictional one.

In Biology:

“How odd it is that anyone should not see that all observations must be for or against some view if it is to be of any service.”  Charles Darwin.

In Fundamental Physics:

“We never draw inferences from observations alone, but observations can become significant when they reveal deficiencies in some of the contending explanations.” David Deutsch, The Fabric of Reality, 1998.

In the Philosophy of Science:

“The facts that we measure or perceive never just speak for themselves but must be interpreted through the coloured lens of ideas…. We can no more separate our theories and concepts from our data than we can find a true Archimedean viewpoint – a God’s eye view – of ourselves and the world”. Michael Schermer, Scientific American, 2007

In Medical Research:

“…you cannot find your hypothesis in your results. Before you go to your data…you have to have a specific hypothesis to check. If your hypothesis comes from analysing your data, then there is no sense in analysing the same data again to confirm it.”  Ben Goldacre, Bad Medicine, 2008.

In Psychology

“I am more and more convinced that the only way to obtain clear answers from Nature is to ask her clear questions.” Eric-Jan Wagenmakers, Professor of Neuro-Cognitive Modelling, University of Amsterdam, 2014. 

 Final words

But in spite of this, it is my observation that the geologist-as-camera view is becoming increasingly common in our profession.  This slows down the acquisition of geological knowledge and can be disastrous for understanding.

Data is not information

Information is not knowledge

Knowledge is not understanding

Understanding is not wisdom (8)

Without geological understanding, drill holes are put in the wrong place, or ten are drilled where one would have sufficed. Without good geological models that reflect reality, the certainty required to convert an Ore Resource into a bankable Proven Reserve cannot be easily or cheaply achieved.

This is a modified and updated version of an essay first posted in 2014

The post The Camera and the Interrogator appeared first on Roger Marjoribanks.

The Copper Wars, called by some the Battle of Butte, took place from 1898 to 1906 between the Anaconda Copper Company and companies owned by Fredrick Augustus Heinze. One of the minor but significant players these wars was young Anaconda geologist Reno Sales (1876-1969).  In his eighties, Sales wrote a book about his experiences int the war called Underground Warfare at Butte (1964). Much of this post is based on that book. Background details to the conflict are taken from “The Battle for Butte” (1981) by Montana historian Michael P Malone.

Butte Montana

In the late summer of 1864, a group of prospectors discovered placer gold in the alluvial gravels of Missoula Gulch, a tributary of Silver Bow Creek in NW Montana. A rush ensued, but the gravels were thin and soon exhausted and the gold miners moved on. But swarms of quartz veins, up to 30m wide and stained with secondary oxides of iron, manganese and copper, had been noted at surface over an area of 15 km2 throughout Butte Hill which overlooked Silver Bow creek to the north. Within 10 years, shafts sunk on these veins had discovered rich silver and lead ore and a new rush began.  Butte Hill gave its name to an emerging and rapidly-growing boom town based on silver and lead mining.

One of the early pioneers of the silver boom at Butte was Marcus Daly. Daly was born in 1841 near Ballyjamesduff in County Cavan, Ireland.  As a 15 year old he joined the Irish diaspora escaping the potato famine, arriving penniless and alone in New York in 1856. Travelling west, he learnt his trade as prospector and underground miner on the California goldfields and the silver mines of Nevada and Utah.  In 1880, moving to Montana, Daly paid for $40,000 for the most productive silver vein on the field – the Anaconda. Two years later (1882) a crosscut from the 300-foot level of the Anaconda opened a 10 m wide vein of high-grade chalcocite (copper sulphide: Cu₂S). Grades averaged 12% Cu, but locally reached 55% (Malone, 1981). Soon a new rush based on copper mining began at Butte as more and more bonanza copper veins were discovered and developed through numerous shafts and dozens of individual claims. All this coincided with a booming demand for copper as America began to electrify at the end of the nineteenth century: there were vast fortunes to be made at Butte.

The Butte mineral field was, and possibly still is, one of the biggest metal concentrations on the planet. Up to 2013, it had produced 9.8 million tons of copper along with very substantial tonnages of Zinc, Manganese, Lead, Molybdenum, Silver and Gold. As at that date, a resource of 4.9 billion tons of 0.49% Cu and 0.033% Mo remained to be mined (Houston and Dilles, 2013). Mining of copper and molybdenum in the Continental open cut continues to this day.

Figure 1: Geology of the Butte District. Orange lines are the mineral veins, blue lines are faults. L is the Leonard Shaft, Tw the Tramway Shaft, Ra the Rarus Shaft, Pe the Pennsylvania Shaft and A the Anaconda Shaft. Figure 3 from Houston & Dilles, Economic Geology, v108, 2013.

Figure 2: An example of detailed underground mapping at Butte by Anaconda geologists: the 1200′ underground level of the Pennsylvania mine. From Reno Sales 1914 Memoir: “Ore deposits at Butte, Montana.” 

The Apex Law

The United States Law of the Apex (or Apex Law, or Apex Rule) was established and defined by the US General Mining Law of 1872 and applies to Federal public lands, located mostly in the Western States. It was based on the idea of winner take all. If you own the highest point – called the apex – of a mineralised vein, the Apex Law allows you to mine it down dip, even beyond the point where it passes at depth through the projected vertical boundary of your claim into an adjacent property. The apex holder can continue mining down the dip of the vein past his claim boundaries, but not along its strike (i.e. the horizontal extensions). Where the claim is rectangular with long sides parallel or sub-parallel to the vein strike (as would normally be the case) then the law states that mining at depth must be constrained within the projected vertical planes that define the two short sides of your claim: provided these sides are parallel and not divergent, and provided your claim was registered first. This is called your Extralateral Right. A Sub-Fault Apex is created where upward continuity of a vein ends against an intersecting fault plane. “Continuity” is the key word here.  If the essential character and continuity of a vein can be demonstrated across a displacing fault, then the apex of the vein is the apex of the vein on the hangingwall of that fault and there is no sub-fault apex on the footwall.

What could possibly go wrong?

This absurd law, still in force today although seldom used, was written to legalize and codify what had become accepted mining practice in the Comstock silver district of Nevada – traditions probably brought there by immigrant Cornish miners and reflecting mining practice going back to pre-Roman times. But in the Comstock District (or in Cornwall for that matter) veins are usually discreet with relatively constant strike and dip. In Butte, by contrast, the mineralised veins form anastomosing swarms over an area of 15 km², with variable strike and dip, and are cut and displaced by several shallow-dip normal faults (figure 1).

By 1887, virtually every surface vein had been located and pegged in the Butte area with hundreds of claims, dozens of head frames and numerous smelters. With bonanza grades and millions of dollars at stake, it is not surprising that the Apex Law provided the source for endless litigation, illegality and corruption. This led directly to the Copper Wars. But an incidental good that came from this sordid time in mining history was the genesis of detailed geological mine mapping.

The Anaconda Copper Company Consolidates the Field

By 1898, a round of acquisitions, financed by East Coast money men, led to extensive consolidation of ownership with the Amalgamated Copper Company becoming the dominant player on the field. Later, the Amalgamated Copper Company changed its name to the Anaconda Company. To avoid confusing the reader with a multiplicity of company names, I use Anaconda Company or just Anaconda for the remainder of the essay. You will find more detail on the corporate structure at Butte in footnote  [1].

One of Anaconda’s first steps was to set up a well-resourced geology department to provide accurate surface and underground geological mapping of the vein systems and enclosing rocks. This was not just to provide information for mine development, but to arm themselves with exact scientific data to use in the litigation consequent on the Apex Law. The geology department was set up and headed by Horace V. Winchell. Among his first hires were rookie geologist Reno H Sales (in 1900, from Columbia School of Mines, New York City) and David M Brunton (an experienced Canadian geologist, inventor of the eponymous geology compass). Between them these three worked out the essential methods and procedures for accurate underground mapping of Butte’s geology. This involved the meticulous observation and measurement of all lithology, structure and mineralisation seen on underground rock faces, and the plotting and interpretation of the data on stacked Level Plans and Cross Sections throughout the mine. This scientific approach had never been attempted before at this scale or in such detail.

Reno H Sales in 1939

A New Player Comes to Town

In 1898, at around the same time as the Anaconda company was being formed, a new and colourful character entered the scene at Butte. He was Frederick Augustus Heinze (or Fritz, to his friends).

Frederick Augustus Heinze in 1910

The son of immigrant German/Irish parents, Heinz was educated in both the US and Germany and graduated a mining engineer/geologist from Colombia School of Mines – the same school as Sales.  After a brief stint working as a Mining Engineer with the Butte & Montana Company, he resigned and, with borrowed money and an inheritance of $50,000, created several companies to buy up a portfolio of many of the scattered small mines, smelters and claims in the district that had yet to be absorbed by Anaconda. This group were then amalgamated in 1901 into the United Copper Co. Where the Anaconda Company had set up a geology department  to help defend their rights under the Apex Law, United Copper set up a legal department of 37 lawyers to achieve the same end. They were led by Heinze’s able lawyer brother Frank, and he set to work aggressively pursuing Anaconda through the courts for all real or imagined rights.

Heinze promoted himself and United Copper as the champions of the little miner against the creeping hegemony of the “evil” Anaconda and their Wall Street capital backing. He bought and edited a local daily newspaper – called the Reveille – which constantly attacked Anaconda and its senior employees, accusing them of corruption, perjury and other illegal activity. Butte locals referred to this paper as the Reviler, Sales called it a smear sheet.

Full page cartoon in the Butte Revielle.  It depicts a sinister Reno Sales as a burglar lurking in a dark alley. In his hand is a gladstone filled with “Tools, False Affidavits and Perjuries”. Figure reproduced from Sales, 1964.

Heinze financed the electoral campaigns, at local, state and Federal Level, of politicians and judges  whom he thought would be favourable to his cause (Malone, 1981). Heinze also used his considerable oratorical and demagogic skills to address crowds of miners from the Butte Hotel balcony, or from the Courthouse steps. He became a hero to his employees, reducing their working hours from 10 to eight hours per day whilst maintaining their daily rate of $3.75 – Anaconda were grudgingly forced to follow suit. District Judges William Clancy and Edward Harney, who were indebted to Heinze for their election (Malone, 1981) were particularly important to Heinze as they presided at the Silver Bow County Court where rulings on the application of Mining Law in much of the Butte District were given. Judge Clancy’s rulings in particular were so biased in favour of Heinze that it is hard not to believe there was a direct financial connection between them (although Sales is careful to make no claim of outright corruption).

District Court Judge William Clancy. According to Sales, his trident beard was usually decorated with samples from his morning’s breakfast. Image from Montana State Library. 

Through the period  1898 to 1906 there were literally hundreds of individual battles between the two companies, and they were conducted not just in the courts and but as physical confrontations between miners at the rock faces underground. Many of these disputes ran simultaneously: some lasted, unresolved, for the full eight-year period of the conflict. Writing in his old age in 1964, Reno Sales was intimately involved in most of these battles and describes them in great detail in his book.

I retell the story of just two of these battles, to indicate the nature and scale of the conflict.

The Battle for the Pennsylvania

The most productive claim owned by United Copper was the Rarus. Ore coming up the Rarus haulage shaft provided the main feed for their concentrator and smelter located on the claim. The Rarus claim contained a system of interconnecting veins which dipped mostly south at a steep angle and so, with some wishful thinking, might pass at depth into the adjacent Pennsylvania and Michael Devitt claims of Anaconda (figure 4). The future of the small Rarus orebodies, and of its ever-hungry smelter, therefore critically depended on proving before a Judge that Rarus had extra-lateral mining rights under the Apex Law into the Anaconda ground.

Figure 4: A portion of the Butte mineral field showing claims held around 1901. The claims shaded with dots were owned by Anaconda. The Rarus claim (with no shading) and its extension to the SE (diagonal lines) were owned by Frederick Heinze and the United Copper Company. Other parties owned the  unshaded claims to the west. Figure reproduced from Sales, 1964. 

The situation was complicated by the large, northeast trending, shallow northwest-dipping, Rarus Normal Fault that ran through all these claims and displaced veins in the area by up to 120 m. The Rarus Fault was by then well-established through geological mapping – both at surface by the United States Geological Survey (USGS) and underground by the Anaconda Company. The Rarus veins lay on the hangingwall of the fault: the rich Pennsylvania and Michael Devitt veins that Heinze coveted were on the footwall. However, the direction of throw on this fault made it virtually impossible that the Rarus veins ever were continuous with vein systems of Michel Devitt and Pennsylvania. The The only way for Heinze to establish extra-lateral rights was to go to court and deny the existence of the Rarus Fault. To establish this, Heinze was able to obtain (“buy”, might be a better word) the testimony of a series of mining “experts” – one of them a former USGS geologist who, in that role, had put his name to the USGS geological map portfolio of the district!

Now, as it happened, the Michael Devitt claim lay outside the jurisdiction of Silver Bow County, so Heinze had to make his case before a Judge sitting in the Federal Court at Helena (the Capital of Montana). But, for his claim on Pennsylvania, he was able to make his case before his good friend, County Court Judge William Clancy. With a long line of expert witnesses for both sides, these court cases took weeks to resolve and were expensive for both parties. Unsurprisingly, Federal Judge Hiram Knowles rejected Heinze’s petition. Judge Clancy, on the same evidence, and by a stroke of the judicial pen, removed the Rarus Fault from the equation and granted Heinze extralateral rights to Pennsylvania. Heinze supporters claimed that Judge Knowles was a former employee of Anaconda, but there was no evidence to support that.

With appeals, supplementary legal claims and counterclaims and political maneuvering at both Local and State level, the Michael Devitt dispute went on for several years. Throughout that time United Copper were mining and hauling to surface through the Rarus shaft huge tonnages of ore illegally mined from both the Michael Devitt and Pennsylvania properties. Injunctions were issued, mine inspectors appointed. There were visits by US Federal Marshalls. Derisory fines for contempt of $20,000[2] were imposed on Frederick Heinze [3] and $2000 and $1000 respectively on his Chief Engineer Alfred Frank and the Rarus Mine Foreman Josiah Trerise. Heinze paid no heed to these legal impediments and continued to mine Anaconda ore.

So it came about that throughout the year 1903 both Anaconda and the United Copper were mining ore within the Pennsylvania Lease between the 400’ and 1000’ levels. The mining levels of the Rarus and Pennsylvania mines differed by 50’ in elevation. Both Anaconda (Pennsylvania) and United Copper (Rarus) sought advantage by advancing drifts[4] above the level of the opposition in order to  extract the ore first. Almost daily, Pennsylvania workings would break into the workings of Rarus, and vice versa. When this happened, the openings were vigorously defended by opposing groups of miners, using high pressure water hoses, throwing rocks and smoke grenades  or blowing lime. Strangely, according to Sales, the miners regarded all this as something of a game, and confrontation seldom continued after they returned to surface. But it would not stay a game for long…

Figure 5: Fighting in the Pennsylvania Mine in 1903. Reproduced from Sales, 1964.

But it would not stay a game for long…

By October 1903, all attempts by Anaconda to gain access to Rarus workings so that they could document their illegal ore extraction for use in Court, had failed. With the approval of the Federal Court, Anaconda then commenced a raise[5] from the Pennsylvania 600 level to intersect the United Copper stopes on the Rarus 800 level (6).  The raise would provide Anaconda with their own access to the illegal United workings (see figure 6).  When Pennsylvania raise broke through into Rarus 800, Rarus miners tried to stop access by tipping mine waste into the new shaft and lighting large fires of rubber and leather where a downdraft would drive the fumes into the shaft and the Pennsylvania workings below.

Figure 6: The scene of the crime (not to scale).

Early on New Year’s day 1904, thinking the Pennsylvania mine would be deserted due to the holiday, Rarus miners lowered a large box of dynamite down the shaft and detonated it remotely. But although the Pennsylvania was quiet, two miners – Frederick Divel and Samuel Olsen – were working that morning on the 600 level. They had been given the special task of erecting an airtight wooden bulkhead near the foot of the shaft to block the noxious fumes of burning rubber. The blast from the explosion killed Fred Olsen instantly. Sam Divel was horribly wounded and died a few hours later.

It is hard to escape the conclusion that the dynamiting of the Rarus shaft was done with the full knowledge and probable instigation of Rarus management. However, at the subsequent Coronial Inquest, all employees of the Rarus Mine, from the foreman to lowest miner, swore on oath they knew nothing of the tragic event. Thomas Thyack, the Rarus Shift Boss who was on the level at the time of the explosion could not (or would not) give the names of the other miners present on the level because, he said, he only knew them by their first names. Attorney Peter Breen (United Copper) and Attorney L O Evans (Anaconda) were only prevented from coming to blows in the court by the intervention of the Coroner. The Jury’s verdict was manslaughter by person or persons unknown. Despite Anaconda’s offer of a $5000 reward for information, no one came forward, no one was ever charged with the crime.

Figure 7 Death of Fred Olsen

The Battle for the Leonard Deeps

Among the many scattered small claims bought by Heinze in his initial acquisition phase was the tiny 1.7 ha (4.2 ac) Minnie Healy.

To the south of Minnie Healy was the Tramway Claim in which Anaconda had majority ownership and United Copper minority ownership, but there was an existing court injunction against either company mining the Lease.  The Tramway vein dipped north and was projected to enter Minnie Healy at depth. But Anaconda had the apex, and clear extra-lateral rights.

Immediately north of Minnie Healey were a group of claims wholly owned and actively mined by Anaconda. The ore was from the rich Coluso, Gambetta-3 and Leonard veins – which were amongst the most productive of the Butte field. These veins dipped steep south, converging below the 700’ level into a horsetail zone with outstanding tonnage potential (see Figures 2 and 7).

Figure 8: A vertical cross-section (view to west) of vein systems in the vicinity of the Minnie Healy claim of United Copper. Redrafted, with some additional annotation. From Sales, 1964

The Minnie Healy claim contained several small anastomosing veins with modest remaining copper reserves. These veins had variable but steep dips to the south, but there was no evidence that they might trend at depth beyond the vertical boundaries of the Claim. In spite of this, it is almost certain that the main reason why Heinze had bought Minnie Healey was to establish extralateral mining rights into the rich, adjacent Leonard-Colusa-Gambetta-3 vein system of Anaconda.  Heinze could be confident of a favourable outcome since was a case that would be heard before Judge Clancy

And so began yet another of the endless series of apex hearings in the Silver Bow County District Court.

As an expert witness for Anaconda, Reno Sales showed the court a geological section (figure 8) based on detailed geological mapping using the methods that had been developed and carried out by the Anaconda geological team over the preceding few years. Appearing for United Copper, their Chief Engineer Al Frank produced a “geological” section which showed a broad, fuzzy mineralised zone dipping north from the Minnie Healey to enclose the coveted Leonard-Coluso-Gambetta-3 deeps below the 700 level in the Anaconda ground. Franks’ “geological” section was based on little more than wish fulfilment. It omitted the Rarus Fault [7] whose existence, of course, would have been detrimental to his position. Frank’s interpretation of the geology consisted of a section with a single broad dash line showing the course of the vein systems across the two claims.  I have superimposed Frank’s “geology” as the broad purple dash line on Sale’s section (figure 8) .

The new geological science of accurate underground mapping won the day. Judge Clancy denied United Copper’s claim for extra-lateral rights and placed an injunction on their mining in the Anaconda ground. But in an absurd twist, he at the same time placed a similar injunction on Anaconda from extracting ore in their own mine from below the 700 level! Presumably, he thought this the Judgement of Solomon.

Although not quite the judgement he had sought, Heinze was undaunted. He reflected that what goes on underground is easy to conceal. An injunction to cease and desist only affects those companies who feel compelled to obey the ruling. With Anaconda excluded from the deep levels of their own mine, Heinze felt free to crosscut from Minnie Healey and commence illegal mining of the Coluso-Leonard deeps. As Anaconda subsequently found out when they eventually re-gained access to their mine, many thousands of tons of rich copper ore were removed. At the same time, in early 1904, Heinze constructed a secret crosscut to the south from the 1000’ Minnie Healy level and commenced to mine the Tramway vein, despite the previous Federal Court injunction against both Anaconda and United Copper from mining this vein.

Anaconda Wins the War

In February of 1906, Anaconda bought all the assets of Frederick Augustus Heinze with which they were currently in litigation (amounting to well over 100 active cases) for $10.5 million. United Copper retained some producing assets at Butte but became a minor player. Heinze took his money and left for New York with the intention of turning his great fortune (by then estimated at $25 million) into an even greater one. With his brothers Frank, Otto and Arthur, he set up a brokerage firm and acquired a bank. Through these they attempted to corner the all the stock in the United Copper Company so as to drive up the price and make a killing through the forced buying by short sellers (by that stage the Heinze brothers hoped to hold all the stock). But they miscalculated, the scheme failed spectacularly, and the brothers went bankrupt (8).  The scandal caused a run on a number of banks and instigated the great Wall Street crash of 1907 and the subsequent creation, in 1913, of the Federal Reserve System (Malone, 1981; King 2012).

Heinze was down, but not yet quite out. He continued as the main shareholder and principal of United Copper, but its share price, which had fetched $80 in its glory days, fell to 80 cents and went into receivership in 1911. F. Augustus Heinze returned to the west and resumed his specialty of apex litigation in the Coeur D’Alene district of Idaho. He died in 1914 at the age of 44 through cirrhosis of the liver.

In 1906, Sales succeeded Horace Winchell as the Chief Geologist of the Anaconda Copper Company (see Reno Sales biography, Perry & Meyer, 1968). Under Sales, the Anaconda Company became renowned for the excellence of its geological work. The quality of mine mapping for which North American geologists are known was a direct result of the foundations laid by Reno Sales and his colleagues in Butte around the turn of the 19th century. The geology department of the Anaconda Copper Company was one of the important factors contributing to its growth through the 20^th Century to become one of the world’s major mining houses.

Reno Sales was director of the geology department of Anaconda until his retirement in 1947. In his long career, he was awarded an honorary Doctor of Science degree from Montana State College (1935), the Penrose Medal of the Society of Economic Geologists (1939) and the Egleston Medal for distinguished engineering achievement (1942). He was President of the Society of Economic Geologists in 1937.

Reno Sales died in 1968 at the age of 82.

Some Final Thoughts

In 1906 the Anaconda Copper Company was left in almost complete possession of the field at Butte and so, in that sense, were entitled to be called the winner of the Copper Wars.  Frederick A Heinze had surrendered the field:  but he left richer by more than ten million dollars and would not have considered himself a loser.  But winners get to write the history.  That history is a satisfying one of Good Guys v. Bad Guys, where the guys in the white hats win through in the end after years of struggle and that is the story I have retold here. But was Frederick Heinze entirely the one-dimensional, opportunistic, greedy rogue portrayed by Sales?  At the time, Heinze portrayed himself as a battler for the rights of miners, claim holders and small mining companies, threatened by a wealthy corporation with bottomless pockets. Even although Heinze’s main motivation was to build a wealthy corporation for himself, there is no reason to suppose that he was not sincere in his desire to improve the lot of miners. Undoubtedly, many in Butte during the nineteen noughties saw Heinze as their champion, and it is clear that his employees were fiercely loyal to him.  I suspect that Anaconda’s motives during the war were not always pure and their own methods not always ethical or legal. Heinze’s efforts to counter the technical presentations of Anaconda geologists before the courts may appear ludicrous, but we are seeing this through the eyes of Reno Sales – hardly an unbiased reporter. Heinze did not have the luxury of a well-resourced geology department to support him.

********

Personal note:

I was an employee of the Anaconda company from 1976 to 1985. In the Spring of ’77 I spent a week in Butte where, in a small down-town secondhand bookshop, I bought a copy Reno Sales 1964 Memoir, “Underground warfare at Butte”.

 Sources

King, Gilbert. September 2012: The copper king’s precipitous fall. Smithsonian Magazine. www.smithsonianmag.com

Houston R. A. and Dilles J.H.: 2013: Structural geologic evolution of the Butte district, Montana. Economic Geology, V108, pp 1397-1424.

Malone, Michael P 1981: The Battle for Butte – Mining and Politics on the Northern Frontier, 1864-1906. University of Washington Press, 281p

Meyer, C., Shea E. P., Goddard C. C., Staff: 1970. Ore deposits at Butte, Montana. In: Ore Deposits of the United States, Gratton-Sales Volume 2, AIME, pp 1373-1416.

Perry V. D. and Meyer C., 1970: Biography of Reno H Sales. In: Ore Deposits of the United States, Gratton-Sales Volume 1, AIME, pp xvii-xxiii.

Reno H Sales, 1914: Ore deposits at Butte, Montana. AIME Transactions v46, pp 4-106.

Reno H Sales, 1964. Underground warfare at Butte. Caxton Printers, 77 p.

Footnotes:

The post The Copper Wars of Butte and the Invention of Underground Geological Mapping appeared first on Roger Marjoribanks.

One of the most important structures for any mineral explorer to understand are faults.

What, exactly, is a fault? To geologists the answer seems so obvious that few of them (even the writers of many geology textbooks or dictionaries) ever bother with a definition. And when they do, they very often get it wrong. When thinking of a fault, geologists usually have in mind a planar dislocation or fracture where the rocks on either side have slid past each other with the displacement across the fault lying in the plane of the fault itself.  Geological dictionaries and structural geology textbooks (or at least the random selection I have read) often reflect this same misunderstanding and offer a definition specifying a direction of movement along the plane of the fault.  They also frequently describe faults are “brittle fractures”, and add that the observed fault movement must always be “observable”.

The authoritative Glossary of Geology[1] with 36,000 defined terms defines a fault as:

A fracture, or zone of fractures, along which there has been displacement of the sides relative to one another parallel to the fracture. (my italics)

The Oxford Dictionary of Geology and Earth Sciences[2] offers this:

Approximately plane surface of fracture caused by brittle failure and along which observable relative displacement has occurred between adjacent rocks. (my italics)

Hobbs, Means & Williams in their well-known (and otherwise impeccable) structural geology textbook[3] provide this definition:

A (fault is a) planar discontinuity between blocks of rock that have been displaced past one another in a direction parallel to the discontinuity. (my italics)

Neville J Price, in his 1964 textbook on rock mechanics (4), describes a fault as:

A fault is a fracture which exhibits obvious signs of differential movement on either side of the plane.” (my italics)

and adds…

 Joints are cracks and fractures in rock along which there has been extremely little or no movement. (my italics)

From that universal source Wikipedia [5]:

A fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement (my italics)

Or this one, perhaps the worst of all, which I found on the internet here:

 if rocks on both sides of the (fault) plane have moved relative to each other, parallel to the plane of the fault (faults are shear fractures)…Joints, if there is no component of displacement parallel to the (fault) plane (joints are extension fractures). (my italics)

Finally, my last example, and probably the best (although still flawed) from www.brittanica.com :

Fault in geology, a planar or gently curved fracture in rocks of the Earth’s crust, where compressional or tensional stresses cause relative displacement of the rocks on either side of the fracture.

Why these definitions are wrong

In the quotes above I have placed the words I find problematic in italics. They are the words that define fault movement as taking place parallel to (or along) the fault plane. They are the words that require the fault mechanism to be brittle (or a “fracture”, which implies the same thing). They are the words that require fault movement to be greater than some vaguely-defined (“obvious”, “significant”, “observable”) and arbitrary minimum value.

If all the criteria of these definitions were strictly applied, they would exclude almost all structures that geologists normally understand by the term fault. They would make it impossible to understand and interpret the multitude of second order structures that occur within a fault, and provide a means of interpreting its history and movement direction. But more importantly, from the point of view of the exploration geologist, these definitions make it impossible to fully understand and predict the emplacement of epigenetic mineral veins.

A fault is a planar zone of rock failure across which relative movement has taken place. The mechanism of that failure may be brittle or ductile. Most faults formed through a combination of both mechanisms.

Any section across a fault, such as an outcrop face, a geological map, or section, is only capable of showing the resolved component of movement on that section. This means that if a pre-fault structure such as a bedding plane is displaced across the fault trace, then, in the general case, the displacement you see is apparent and relates only to marker beds of that orientation. Other beds, with a different orientation, may show different amounts, or even different senses, of apparent displacement. Some displaced beds may show no apparent movement at all on the section on which they are viewed.

The above discussion on apparent displacement applies where the relative movement of the rock masses on either side of the fault have moved laterally past each other along the fault plane. As I have shown, many definitions of geological faults either explicitly or implicitly assume this. But rock masses may also move towards each other or away from each other across the fault plane. This creates a whole new set of geometries.

An accurate definition of a fault must avoid any assumptions about direction of fault movement or the mechanism of fault formation.

Discussion on Fault Movement

Fault Movement Vectors (FMVs) define the relative movement that has taken place between the rock masses on either side of a fault at the end of any given fault movement.  FMVs are the direction of movement of any point on one side of the  fault with respect to any point on the other side. FMVs can be shown as two parallel arrows pointing in the direction of relative movement – one arrow for the rocks on either side of the fault. These arrows may point towards each other. They may point away from each other. They may lie at any angle to the plane of the fault.  Any plane that includes the these arrows is the plane of the FMVs.

If the plane of the FMVs is parallel to the fault plane, then the rock masses on either side of the fault must have moved laterally past each other and the deformation mechanism is that of simple shear If the arrows are normal (i.e. at right angles) to the fault, then the type of displacement across the fault is known as pure shear.  Pure shear may be compressional or extensional. In compression (where the FMVs point towards each other) the rocks on either side of the fault have moved towards each other and there has been a necessary reduction in the volume of the affected rocks. In extension (where the FMVs point away from each other) the rock masses on either side of the fault have moved away from each other, and there has been an increase in the volume of the affected rocks.

Extensional faults are of vital interest to the exploration geologist because the extension provides the space, and the creation of the space the  driving force, for emplacement of epigenetic mineral veins – an important source of ore.

A more detailed discussion on the dynamics of fault formation can be found in my earlier blog post here, entitled: The movement of faults.

The diagram shows a series of two-dimensional slices through rocks affected by different dynamic styles of faulting. The red opposed arrows are the Fault Movement Vectors and indicate the direction of net movement of any point on either side of the fault trace. The sections are all in the plane of the FMVs. Click for a larger, sharper image.

Simple shear and pure shear faults are end members of a continuum of styles of displacement across a fault. Even where simple shear is the predominant deformation mechanism, some parts of the fault will locally exhibit the effects of pure shear. Conversely, in dominantly pure shear structures, there will be zones where the structures observed formed through the mechanism of simple shear. If the FMV arrows lie some angle between 0⁰ and 90⁰ to the fault plane, then fault deformation took place by some combination of simple shear and pure shear mechanisms.

Marker Bed Movement Vectors (hereafter, MMVs) are the paired arrows with which geologists use to decorate their maps and sections in order to indicate relative displacement of pre-fault planar structure across a fault.  These structures are typically sedimentary marker beds, but may be veins or even pre-existing faults. In the general case where the fault is viewed on a random section, and without further data, MMV arrows indicate apparent displacement only. The MMV plane is any plane which contains the arrows and, by definition, is always parallel to the fault plane.  However, the displacement of a single plane across a fault is incapable of fully defining the relative movement of the rock bodies on either side of the fault – only the relative displacement of points on either side of the fault can do that. When a fault is viewed on a random section – such as an outcrop, a mine opening, a geological section or a piece of diamond drill core – marker beds with different orientation may give opposed MMVs across the same fault. The angle which the line of intersection of marker bed and fault makes with the Fault Movement Vectors will determine the amount of apparent displacement on that section. An angle of 90 degrees produces maximum displacement; an angle of 0 degrees will produce no apparent displacement. MMV arrows are only equivalent to FMV arrows in the special case where the section on which the displacement is observed is parallel to the FMV plane (as in the left hand map view of the diagram below).

When viewing a fault on the FMV plane, all marker beds will show the same amount and sense of displacement irrespective of their orientation (see figure below). Conversely, if differently-oriented beds show the same amount and sense of displacement, then the plane on which that displacement is being viewed must be the plane of the FMVs.

Many geologists, including the writers of some geological textbooks and dictionaries, confuse FMVs and MMVs, but FMVs are the more fundamental measure of fault movement.

A plan view and vertical sections across a strike-slip, simple shear fault affecting differently-oriented marker beds. Red arrows are the Fault Movement Vectors. White arrows are the Marker Bed Movement Vectors. The map view (left) is in the plane of the Fault Movement Vectors: marker beds therefore show the true displacement. Sections AB and BC are planes at right angles to the FMVs: marker beds displacements are apparent only. Vectors. Click for a larger, sharper image. 

Discussion on Joints

The amount of movement that has taken place across a fault can vary through several orders of magnitude – from a fraction of a millimeter to hundreds of kilometers. However, locally developed, brittle fracture surfaces of limited extent across which insignificant displacement has taken place are usually called joints. The suggestion by Neville Price (see his definition above) that there may have been no movement across a joint is of course nonsense: if there had been no movement, there would have been no fracture. By “insignificant” I mean difficult or impossible to see with the naked eye.  However, in practice, a displacement can only be quantified where the fracture affects a marker surface – and sometimes not even then.

Joints are presumably the category of fracture which the lexicographers sought to exclude from their definition of faults by their requirement that displacement be “significant”, “observable” or “obvious”.

Joints form in the same way as faults and should be regarded as a – somewhat vaguely-defined -sub-category of brittle faulting. There is no logic for imposing an artificial division between faults and joints based on some arbitrarily defined amount of movement – what has the resolution of the human eye got to do with rock mechanics?

All faults are caused by stress. In faults with a significant strike extent, the causative force is usually the deviatoric stresses associated with tectonism. Joints can also be caused by tectonic stress but may also be the result of changes of non-deviatoric stresses. Examples are the change in lithostatic stress caused by weathering or rock excavation, and thermal gradients associated with igneous activity. Stress is an abstract force which can only be deduced (if you are lucky). It cannot therefore be used as the basis for a definition of a physical structure such as a fault. That would be putting the cart before the horse.

Joints, as you will have gathered, are notoriously difficult to define. However, even with the  fuzzy definitions that are out there (including mine), the term “joint” remains a useful field term for describing arrays of small-scale brittle fractures.

But joints are micro-faults, nonetheless.

Discussion on Brittle and Ductile Deformation

The method of deformation that enables faults to form, and movement to take place across them, can be either brittle or ductile, or, more typically, some combination of brittle and ductile. Ductile deformation is promoted by high temperature and confining pressure, but if rocks are sufficiently incompetent they can deform in a ductile manner at almost any temperature or pressure. Most faults show evidence for both styles of deformation either at different places within the same fault zone, and/or at different times during its formation. This is particularly true for large-displacement faults which typically affect a range of rocks with different physical and chemical properties, and are a composite of episodic movements that took place over a long period of time.

Where the dominant deformation mechanism is brittle, faults typically consist of tabular arrays of close-spaced, sub-parallel anastomosing fractures separating slices of lesser deformed, or unreformed, rock. This pattern is fractal in that it can occur at all scales from that of a regional map to that of a microscope slide.

Where the dominant fault deformation mechanism is ductile, the strain is more uniformly and smoothly distributed across across the width of the fault.  Ductile faults are often described as ductile shear zones, but they are faults, nonetheless.

So here is my definition of a fault:

A fault is a restricted tabular zone of high strain with relative displacement of the rocks on either side.

**********

[1] J A Jackson & R L Bates (eds), 1980: Glossary of Geology. Published by the American Geophysical Institute, 2^nd Edition, 1980.

[2] Michael Allaby 4^th Ed. 2013 online version. DOI: 10.1093/acref/9780198839033.001.001

[3] Hobbs B E, Means W D & Williams P F, 1976: An outline of structural geology. John F Wiley and Sons, 571p.

(4) Price, Neville J: 1964. Fault and joint development in brittle and semi-brittle rocks. Pergamon Press, 176p.

[5] Accessed July 2020

The post The definition of a geological fault and why most dictionaries get it wrong. appeared first on Roger Marjoribanks.

Faults are not mathematical planes (2D surfaces with length and depth but no thickness) but 3D tabular zones of deformed rock. The length and depth of a fault is always much greater than its thickness, but fault width can vary through many orders of magnitude from a fraction of a millimeter to to tens of kilometers.

There is widespread confusion in the geological literature as to the exact definition of a fault. I discuss this in another blog post here.

Small, locally developed, fracture surfaces across which insignificant displacement has taken place are called joints. By “insignificant” I mean difficult or impossible to see with the naked eye. Joints form in the same way as faults and should be regarded as a sub-category of brittle faulting.

No fault is ever strictly planar. Normal and Thrust faults (more on these later) are typically curved: steep dipping near the surface and progressively flattening with depth – a shape known as listric. In addition, at all scales, faults show irregularities – bends and bumps and jogs. During fault movement the variations from strict planarity lead to complex patterns of stress along the fault surfaces. These stress variations are the key to understanding the location and shape of ore that might form within the fault zones.

The method of deformation in a fault can be either brittle or ductile, or, more typically, some combination of brittle and ductile.

Fault Movement Vectors (hereafter FMVs) indicate the direction that  any point on one side of the fault trace has been displaced relative to any point on the other side during fault movement. On any plane passing through the FMVs, the FMVs can be shown as a pair of opposed arrows, one for the rocks on either side of the fault.

FMVs can lie at any angle to the fault surface

FMVs can lie at any angle to the fault surface. Where the FMVs are parallel to the fault plane, the fault has formed by a deformation mechanism known as Simple Shear. Where the FMVs are at 90° to the fault surface, the fault was formed through the process of Pure Shear. Where FMVs lie between 0˚ and 90˚ to the fault plane, deformation was accomplished by a mixture of both simple shear and pure shear mechanisms.

In Simple Shear Faults, the rocks on either side of the fault zone have moved laterally with respect to each other. FMVs are parallel to the fault plane.

In Pure Shear Faults, the rocks on either side of the zone have either moved towards each other in compression or moved apart in extension[1]. For this to happen, there must be a reduction or increase in the volume of the rocks affected by the external stress field.

Most fault zones are the result of both simple shear and pure shear deformation styles. The relative proportion of these two processes can vary both across and along the fault zone.

Because changing rock volume is difficult, they deform much more easily by the mechanism of simple shear than by pure shear. Thus, displacements of more than a few meters indicates that the dominant mechanism was probably that of simple shear. Note the deliberate use here of the vague terms: “dominantly”, “more than”, “few” and “probably”.  In dealing with real rocks in the field, as opposed to simplified textbook examples, that is the best that can be done.

Because of the above, the most commonly occurring map-scale faults are those where the amount of displacement attributable to simple shear is greatest. For most purposes, and to a first order approximation, map scale faults can be regarded as dominantly simple shear faults.

As rock is incompressible, faulting can only reduce its volume by the physical removal of material from the faces of the fault zone. This happens predominantly by means of selective solution (or in extreme cases, melting) of rock material into fluids within the fault zone. The process is promoted by high temperature and high confining pressure and controlled by the chemical/mineralogical nature of the affected rocks. The dissolved material in solution moves along (laterally and upwards) the fault zone. Movement is driven by pressure and temperature gradients as well as by the “pumping” effects of periodic seismicity[6].  It will ultimately be deposited as vein material (typically quartz or calcite) elsewhere in the fault in regions that are under relative tension. Left behind in the fault zone are the relatively insoluble rock components such as clay or graphite.  Any puggy, clay-rich material in a fault is the insoluble residuum of material lost through pressure solution during pure shear compression. This type of fault fill is usually described as fault gouge.

In the upper few kilometers of the earth’s crust, where confining pressure and temperature are relatively low, rocks have little strength under tension. Pure shear extension stress creates fractures – planar zones of extension – that (unless at or near the surface) will suck in fluids from along the fault zone or from adjacent rocks. The presence of such pressurized fluids can aid the propagation of a tensional fracture.  The fluids deposit vein material in the fracture.  Igneous fluids (magma) may crystallize as dykes or sills.   Vein filled zones in former tensional sites of a fault are the so-called “dilational jogs” that host many epithermal ore deposits.

The important point to remember is that pure shear deformation, as opposed to simple shear deformation, always results in a change in volume – either an increase or a decrease – of the affected rocks. 

Figure 1: The diagram shows a series of two-dimensional slices through rocks affected by different dynamic styles of faulting. The red opposed paired arrows are the Fault Movement Vectors and indicate the relative net movement of any point on either side of the fault trace. The sections are all in the plane of the FMVs. Where the vectors are parallel to the fault plane (as in a), the opposed blocks have moved laterally past each other: sinistral if to the left (as shown) or dextral if to the right. However, FMVs can lie at any angle to the fault plane. Where FMVs point towards each other, and are not parallel to the fault plane, the stress state is known as trans-pression; if the arrows point away from each other, and not parallel to the fault plane (b), the stress state is known as trans-tension. Two end member states, either of pure compression or pure extension, occur where the vectors are normal to the fault (b & c). There is a continuum between these different styles of fault movement, not just between different faults, but within any one fault at different places and at different times during its formation. Click for a larger, sharper image.

Simple shear and pure shear faults are end members of a continuum of styles of displacement across a fault. Even where simple shear might be the predominant mechanism, different parts of the fault will locally exhibit the effects of pure shear. Conversely, in dominantly pure shear structures, there will be zones where the structures observed formed through the mechanism of simple shear.

Faults develop incrementally over geological time through the accumulation of large numbers of relatively small movements. During this process, each part of a final fault structure may have been sequentially subjected to, and show the effects of, both displacement mechanisms. Therefore, in addition to evidence for different structural structural styles that operated in different parts of a fault at any one time, at any one point in a fault different styles of faulting may have operated over time. Typically, early formed structures are destroyed by later movement, but this is not always the case.

 The displacement of marker beds

Any marker bed intersected by a fault will be displaced by an amount which depends on the angle which the bed makes with the plane which contains the FMVs. If the angle is 0˚ there will be no apparent lateral displacement of the bed across the fault on any section.  The amount of lateral displacement will increase with increasing angle. Maximum displacement is reached when the angle is 90˚. On figure 2 below, the FMVs are parallel to the fault plane. The blue marker beds intersect the fault plane at an angle of around 45⁰ to the FMVs, both to the east and to the west. The vertical anticlinal axial plane (green dash line) makes an angle of 0⁰ with the FMVs.  Fault movement thus displaces the marker beds to the east and to the west, but has no apparent effect on the axial plane.

This is a simple geometrical consequence and applies whether the fault mechanism is simple shear or pure shear.

Figure 2: Block diagram of a vertical simple shear fault (red) with S block down displacement. The fault displaces a marker bed (blue) that dips to E and W on opposite limbs of an upright anticline. On all sections parallel to the Fault Movement Vectors (i.e. any vertical section) the displacement of all marker beds or structural planes is the same, irrespective of orientation, and is parallel to the FMVs. When the structures are viewed on any section that is not in the vertical plane (in this example, the plan or map view) differently-oriented structures will show differing amounts, or even sense, of displacement. Click for a larger, sharper image.

The displacement of a single plane across a fault is incapable of providing the FMVs for the fault.

However, the displacement of a unique linear structure across a fault plane will provide a measure of the FMVs. A frequently occurring linear structure is the line of intersection of two differently oriented marker beds. Even although not directly observed, the line of intersection of the marker beds can be easily calculated and its intersection with the fault plane plotted. In the map and section shown in figure 3 below, the intersection of a NE-dipping bed (blue) and a vertical dyke (green) on either side of a fault defines two displaced points across the fault. The line joining the points is the Fault Movement Vector. In this example it has a sinistral strike slip movement (70%) plus a small component (30%) of dip-slip S-block down movement. The analysis only works, of course, if the FMVs lie within the plane of the fault (i.e., a simple shear fault).

 Figure 3: The displacement of a point across a simple shear fault defines the Fault Movement Vector.

Simple rules for interpreting simple shear faults

For a first pass interpretation of any geological section across a simple shear fault:

1. On any section or plan, if marker units of differing orientation are displaced in the same sense and by the same amount, then that section or plan must lie in the plane of the FMVs for that fault.
2. On any section or plan, if marker units of differing orientation are displaced by different amounts and/or in different senses, then that section or plan must lie at an angle other than zero to the FMV plane for that fault. From which it follows that:
3. In a horizontal section (usually called a map) the fault movement is STRIKE-SLIP (i.e. a transcurrent fault) if marker beds of different orientation show the same displacement across it.
4. In a vertical section, the fault movement is DIP-SLIP (a thrust or normal fault) if marker beds of different orientation show the same displacement across it.

 The descriptive nomenclature of Simple Shear Faults

In faults where the dominant displacement style is that of simple shear, the fault can be described in purely geometrical terms as either strike-slip (where the movement parallel to the strike of the fault), dip-slip (movement parallel to the dip of the fault), or oblique-slip (a direction between dip-slip and strike-slip). 

A more fundamental classification of simple shear faults is based on the orientation of the stress axes which cause them.  There are three classes: Transcurrent Faults, Normal Faults and Thrust Faults.

Scottish geologist Ernest Anderson who first proposed it in 1905

Transcurrent faults are strike-slip.  Normal and Thrust faults are dip-slip. Reverse faults are usually included in this scheme as steep-dipping Thrust faults. This 3-fold  classification of faults (four, if you count Reverse faults separately) is sometimes called “Andersonian” after the Scottish geologist Ernest Anderson who first proposed it in 1905[2] The classification reflects the orientation the three orthogonally-resolved principal external stress axes (greatest, least and intermediate) in the upper part of the earth’s crust. Here, these axes are dominantly either parallel to the earth’s surface, or at right angles to it[4]. If the direction of greatest stress is vertical, Normal faults may form; if the intermediate stress direction is vertical, Transcurrent faults may form; if the least stress direction is vertical, Thrust or Reverse faults may form. This is certainly a simplification, but Anderson’s classification of simple shear faults stands up remarkably well.

 Consider an orthogonal section across a simple shear fault with a dip-slip displacement that affects a layered sequence whose dip is less than the fault (Figure 3).  The fault (if dipping less than 90˚) will separate a hanging wall block from a footwall block. Where the hanging wall has moved up relative to the footwall, such faults are called reverse or, if the dip is less than around 45˚, they are called thrust faults. In all thrust or reverse faults, movement has shortened the affected rock sequence in the horizontal direction but increased it in the vertical direction. There is no volume loss. If reverse or thrust faults affect strata whose dip is shallower than the fault, there will be vertical repetitions of marker beds on any section across it.

In steep drill holes, where there is repetition of stratigraphy, and older units overlying younger, the presence of thrust faulting is indicated.

Where the hanging wall block has moved down relative to the footwall, dip-slip faults are called Normal. With Normal faults, the affected rocks have been extended horizontally and compressed vertically. Where Normal faults affect shallow-dip strata, elements of the sequence might be missing on vertical sections. 

In steep drill holes, missing stratigraphy is a good indicator of the presence of normal faulting.

Figure 4:  Vertical sections through dip-slip Simple Shear Faults drawn in the plane of the Fault Movement Vectors (red arrows). On the left, Thrust Faults leading to horizontal compression and vertical thickening of a sequence. On the right, Normal Faults leading to horizontal extension and vertical thinning. Click for a larger, sharper image.

 Recognizing Dominantly Pure Shear Faults

left-stepping bends in sinistral faults and right-stepping bends in dextral faults are regions of transtension

Transpression and transtension stresses occur where FMVs are at an angle to the fault plane so as to cause either compressive or tensile stress across it. This situation arises at bends in simple shear faults such as the steep portions, or ramps, of thrust faults (transpression) or the steeper dipping portions of normal faults (transtension). In transcurrent or strike slip faults, bends that tend to oppose fault movement (i.e. left-stepping bends in dextral faults or right stepping bends in sinistral faults) are areas of transpression: bends that are congruent with the sense of fault movement (i.e. left-stepping bends in sinistral faults or right-stepping bends in dextral faults) are regions of transtension. Structures typical of regions of transpression are fault gouge, folds, cleavages, thrusts, back-thrusts and flower structures. Structures typical of regions of transtension are vein-filled dilational jogs, normal faults and graben.

left-stepping bends in dextral faults or right stepping bends in sinistral faults are areas of transpression

During their formation, extensional faults move rocks apart and create a new volume which (except at the surface) sucks in fluids from further along the fault zone or from wall rocks. The fluids may be meteoric water or derived from metamorphic processes or an igneous source.  The fluids deposit vein material (typically quartz or calcite) or will crystallize as an igneous dyke. The presence of this epigenetic material in the fault plane is the main way of identifying such faults. 

 Figure 5: Before and after block diagrams along with a plan view of an Extensional Pure Shear Fault (FMVs  – red arrows – point away from each other and are at right angles to the fault plane).  On the plan view, the displacement of the marker bed (MMVs – black arrows) is not he result of simple shear. Click for a larger image.

A pure shear compressional fault has lost material from the fault face. The presence of clay gouge in the fault zone indicates that material has been lost through pressure solution. Other structures that may be present in the fault zone such as fine penetrative cleavage parallel to the zone margins, are also indicative of compression.

 Figure 6: Before and after block diagrams of a pure shear compressional fault (i.e. the FMVs point towards each other and are at right angles to the fault plane). Note that the dextral strike-slip displacement of the marker bed across the fault (MMVs) is not caused by simple shear. Click for a larger image.

 First posted November 2013.  Modified July 2020, April 2021

[1] I have always found the terms “simple shear” and “pure shear” unfortunate and non-intuitive. For a start, the word “shear” or “shearing” in all non-technical dictionaries refers only to the lateral movement of two bodies past each other. And what is the logic in calling one type of deformation “simple” and one type “pure”?  However, the terms are long established and well defined in rock mechanics. We must live with them.

[2] Sibson R H, Moore J McM & Rankin A H 1975: Seismic pumping and hydrothermal fluid flow mechanisms. J  Geol Soc London Vol 131, pp 653-659

(3) E M Anderson, 1905: The dynamics of faulting. Trans Geol Soc Edinburgh, vol 8, pt 3

(4) There are good theoretical reasons why this is so, but that explanation lies beyond the scope of this essay.

The post The movement of faults appeared first on Roger Marjoribanks.

Within or adjacent to a fault zone, various minor structures can be present that enable the sense of movement across the fault to be determined.  These structures are often called kinematic indicators.

In Part 1 of this series of posts, I classified kinematic indicators as T (tension), S (compressive), R (simple shear) and C (laminar flow). The part described how and why these structures formed and explained their use as kinematic indicators.  Part 2 consisted largely of photographs of actual structures in outcrop and drill core. In this final post, simple tabulated rules are provided that enable the various classes of structure to be identified in outcrop.

Most faults, especially large ones, have poor outcrop so when you are lucky enough to find an exposure of a fault it is worthwhile spending time examining it in detail to find out what it can tell you.  The type of exposure that is most likely to provide useful kinematic indicator structures is a section that cuts at right angles (orthogonally) across the full width of the zone. A stream section offers the best chance of finding such an exposure.   Although relatively rare in nature, orthogonal or near orthogonal sections across fault zones are frequently exposed in man-made outcrop such as road cuttings, trenches, the walls of open cuts, underground openings or in drill core. Next to an orthogonal section, the next most useful fault exposure for the structural geologist to work with is an exposure of the face of a fault – the plane of movement.  This is probably the most common type of natural fault exposure.

Don’t expect every exposure of a fault to contain sense of movement indicators that can be reliably identified and interpreted.  In fact, most will not. The minor structures within fault zones can be chaotic, apparently contradictory and difficult to assess in three dimensions.  If you are not certain how to identify the minor fault structures that you see it is better to walk away and try for a better exposure elsewhere. 

Being able to distinguish between different sense of movement structures is vital. Confusing S surfaces with T surfaces, for example, leads to radically-different interpretations of the direction of fault movement. The Table below sets out the criteria that can be used to identify structures.   Note that no single criteria is definitive and many of the fields overlap. The more criteria that a particular structure meets, the more certain is its identification.  

It is worth bearing in mind that identifying one single structure, or even a number of structures from a single outcrop, does not give 100% certainty as to the overall fault movement.  You have good evidence, but the important point is that a credible working hypothesis about the nature of the fault can now be constructed.  From this hypothesis predictions can be made which can tested against the evidence from future exposures of the structure.  This is the Scientific Method and from this process knowledge is gained.

The post Sense of movement structures in fault zones Part 3: Identification Criteria appeared first on Roger Marjoribanks.

Within or adjacent to fault zones, various minor associated structures can be present that enable the sense of movement across the fault to be determined. These structures are often called kinematic indicators.

This is the second of three posts (see part 1 and part 3 ) about sense of movement  indicator structures in fault zones. In the first part, the theory behind sense of movement indicators is explained and it is recommended that this part be read in conjunction with part 2.  This post provides illustrations of sense of movement structures as seen in outcrop and drill core. In Part 3, the criteria for identifying the different classes of structure are tabulated.

Preamble

 Knowing the movement that has taken place across a fault makes it possible to predict the location and orientation of tensional zones – and hence potential veins of ore – within or adjacent to the fault.

 If the fault is later than the ore body, knowing its movement allows prediction of the location of any mineralisation that may be displaced by it.

Determining the strike of a fault is usually simple. Determining its dip requires some vertical exposure or drill hole information.  However, to completely define a fault, it is necessary to know the movement (or displacement) vector and the amount of movement that has taken place  between the two sides of the fault.

For all these reasons, knowing the sense of movement on a fault is of the greatest importance.

Some cautionary words

1. Faults develop over time through the accumulation of hundreds of relatively small movements.   Although these will add up to a final overall displacement vector, at any given point in a fault zone it is possible to find the preserved evidence for a short term or localised movement that might contradict the overall displacement.  You would be unlucky if the one indicator you found happened to be one of these outliers, but nevertheless, many separate observations of kinematic indicators are needed before you can be absolutely certain of the overall displacement.

2. Remember also that many faults are re-activated by successive tectonic events, and later movements may be completely different to earlier ones.   As a general rule, movement indicators seen in a rock will reflect only the latest phase of movement that has taken place, but like all rules, there are always exceptions.

3. Rock structure is 3-dimentional, but most structures seen in field exposure or on maps and sections are observed on two-dimensional surfaces that are randomly-oriented with respect to the structure.   When interpreting sense of movement structures it must be borne in mind that a 2-D surface is only capable of showing  the resolved component of movement on that surface.  The true movement direction (the movement vector) may lie at any angle to that surface. To determine the true movement direction it is necessary to have some idea of the 3-D shape of the structure. This is best done by observing the structure on several differently oriented planes.

4. All the examples shown in this post (with the exception of “M“) show structures observed in the plane of maximum movement i.e. the plane containing the greatest and least stress axes for that deformation (see Part 1) or else in a view looking directly at the fault face (a very common type of natural exposure).

 Brittle Structures

A:  Slickenlines (parallel to pencil) developed on a fault surface. The surface containing these lines is called a slickenside – old Cornish miners term. An alternative name for slickenlines is striations, or, if they are deep enough, grooves.  These are mechanical scratch marks on the fault surface. Slickensides are common where the fault surface is coated with clay (as in the example shown) or graphite. They indicate the line of movement but not its direction or vector. In this example the  fault is dip-slip, but it could be either normal or reverse. Slickenlines are easily formed and easily destroyed (you can usually make a few yourself with the point of your hammer): as such they are generally unreliable indicators of overall fault movement.

B.  The movement across a small fault at a high angle to layering is easily determined by the displacement of marker beds. However, in this case, even without the marker beds, fault displacement could have been determined by the presence of small calcite-filled dilational jogs along the fault. Such jogs are a type of mineral filled T (tension) surface characteristic of brittle deformation. Although this is an insignificant fault, through a fractal relationship, it gives insight into the movement of an adjacent, unexposed, larger fault of the same generation. This unusual rock from Jervoise, Northern Territory, Australia is a bedded tourmalinite – which I interpret as a metamorphosed, boron-rich, exhalative sediment.

C.  View of the exposed face of a fault which cuts a large (pre-fault) quartz vein.  On the face, short, lineated, crescentic fractures trend away from the fault surface at an angle of around 20 degrees. The low angle and the presence of lineation on the surfaces indicates that these are not tensional (T) fractures but Reidel Shears (R). The attitude of the shears and the horns of the crescents point to the direction of movement of the missing block (it has moved to the right indicating a sinistral strike-slip movement).  Kerimenge Fault, Wau District, Papua Nugini.

D.  A zone of sigmoidal quartz-filled tension (T) fractures indicates sinistral strike-slip movement on a fault cutting meta-basalt. Poseidon pit, Higginsville, Western Australia.

Sigmoidal quartz tension veins in Proterozoic sandstone, Ashburton Province, Western Australia

(above) Gold-bearing Tension veins as splays from a small thrust fault exposed on the wall of the Sigma underground Mine, Val D’or, Canada.  Note the S surfaces (foliation) at 90 degrees to the tension veins. Illustration taken from Robert F & Poulsen KH 2001: Vein formation and deformation in greenstone gold deposits. Reviews in Economic Geology v14 pp 111-152.

Sheeted quartz fibres with down-dip facing terminations exposed on a vertical fault surface. The missing plate has moved down relative to the exposed face – 70% normal fault dip-slip + 20% dextral strike-slip, movement. Underground gold mine near Kadoma, Zimbabwe. Photo by Mathew Swift. 

E.  Calcite fibres showing strong lineation and a pronounced “shingling” effect coat an exposed vertical fault surface. These structures should not be confused with Slickenlines (see A). The facing of the steps (white crescentic lines) indicates sinistral strike-slip movement on the fault (i.e. the missing plate has moved to the right).  Fibres are an example of an overlapping  stack of mineral filled dilational jogs – a category of T fracture. Although fibres are a more useful and significant movement indicator than Slickenlines, they are generally typical of joints and faults with small movement along them.   Any really significant fault movement will usually grind them off.   This example from the Snow Mountains, Sichuan, China.

Acicular tremolite crystals (fibres) form sheeted overlapping veins on a fault face in peridotite. Missing block has moved down and to the right. RAV8 open cut nickel sulphide mine, Ravensthorpe, Western Australia

F.  A fine penetrative compressive foliation (S surfaces) in a small clay-filled fault zone indicates dominant normal fault movement. Host rocks are weathered granite (saprolite).

G.  A fine penetrative foliation at 30degrees to the fault zone (S surfaces) shows rotation into the top and bottom shears of a thrust zone. This is an example of a small thrust duplex. Host rocks are sandstone.

H.  A small thrust duplex cutting dolerite. S-surfaces in the fault zone are sigmoidal shaped and clay coated (brown staining). This example from the Nyankanga gold mine, Tanzania.  At this mine, the thrusts are post ore and have a significant effect on ore distribution.

Ductile Structures

I.  A sawn surfaces in HQ drill core (left) shows the effects of ductile reverse faulting on a distinctive big feldspar gabbro.  The appearance of the undeformed gabbro can be seen in the core sample at right. A sense of movement can be determined  from the asymmetric chlorite “tails” on the margins of relict anorthosite feldspar crystals. In the Mt Ida gold camp, reverse faults such as this are the controls on high grade ore shoots. Note: sense-of-movement indications can only be obtained from oriented drill core.

J.  More sense-of-movement indicators from oriented drill core.  Note asymmetric “tails” on the sedimentary clasts at right. Reverse faulting at this gold deposit controls distribution and orientation of high grade ore shoots.

K.  Large orthoclase phenocrysts (that grew during deformation by potassic metasomatism) have grown within, and then been flattened, in a quartz-feldspar-biotite mylonite zone. A dominant pure flattening strain (pure shear) across the rock is indicated, although one phenocryst has slightly asymmetric “tails” suggesting a component of dextral strike-slip movement (can you spot it?). The Redbank Zone is a major E-W trending ductile shear zone with a long and complex history. It is at least 20 km wide and can be traced for over 600 km across Central Australia..

L.  A view of the backs (roof) of the Hemlo underground gold mine (the rock-bolt gives scale). The strong foliation of a major shear zone contains an amphibolite band which has broken and been pulled apart in a brittle fashion (boudinage). White quartz and albite has grown in the strain shadows between the amphibolite fragments. As with the previous example, a  dominant pure flattening strain is indicated.

M.  A Section and Level Plan from an underground gold mine. The mine lies within a regional ductile shear zone.  The zone contains lenses of undeformed felsic porphyry – probably part of a once continuous sill intruded during deformation.  Quartz vein stockworks in the porphyry contain high-grade gold values. Individual veins occupy T (tension) fractures and their orientation indicates that shear movement on the zone was oblique-slip, with a resolved sinistral strike-slip component on plan view, and reverse fault component on cross section. Individual veins are too small to map, but their distribution and shape is reflected in the contoured gold values. Meekatharra is in the Achaean age Yilgarn Craton of Western Australia.

N. A ductile shear zone affecting an Ordovician age basaltic agglomerate at Gympie, Queensland Australia. Asymmetric “tails” on the clasts and a small asymmetric fold pair indicate a dextral strike-slip movement on the fault.

Quartzite cobbles in sheared Ordovician conglomerate. Note asymmetric “tails” indicating a sinistral sense of shear. Gympie region. Queensland.

O.  An amphibolite in a migmatite zone has resisted ductile deformation and partial melting. The amphibolite has broken into separate fragments.  In the low-strain zones between the fragments thick zones of quartz-feldspar have grown. Another example of boudinage. Pure flattening strain only.

All comments, questions even crticisms are welcome. Please email me using the details under the “contact” tab.

The post Sense Of Movement Structures in Fault Zones: Part 2: Examples appeared first on Roger Marjoribanks.

Within or adjacent to fault zones, various minor associated structures can be present that enable the sense of movement across the fault to be determined. These structures are called movement or kinematic indicators.

Faults are the host to epigenetic, vein-type metal deposits. Identifying the nature of a fault and the sense of movement across it enables prediction of the likely shape and attitude of any high grade ore shoots that might be associated with it (see my previous post on drill hole targeting). One of the best ways to determine sense of movement (also called the movement vector or the kinematic indicator) is through associated indicator structures that can be observed in outcrop and drill core. Some large movement indicator structures or patterns of structures can also be identified at map scale.  This post describes these structures: why they form, what they look like and how they can be interpreted.

This post is in three parts. Part 1 gives the theory of  how sense of movement structures form and their kinematic significance. Part 2 provides illustrations of actual structures from exposures in the field, in mines and in drill core, while Part 3 provides a tabulated summary of the objective criteria by which the different classes of sense-of-movement structure can be identified and interpreted.

Stress and Strain

Structures in rock are an example of deformation.  Another word for deformation is strain. The strain that you observe is the cumulative effect of forces that acted on the rock over time.

A force acting on any given point in a rock is known as stress. Strain is the tangible effect that you observe: stress is the intangible cause that can only be deduced. However, the relationships between strain and stress are well known and provide a high level of predictability to structural geology. Higher, at any rate, than many other areas of geological science.

Every point in a rock is acted upon from all directions by stresses. However, because the infinity of stresses will, to various degrees, tend to cancel out or reinforce each other, they can all be resolved mathematically into just three stresses that are mutually at right angles to each other. The three orthogonally arranged stress directions are known by the Greek letter sigma (written thus: σ). Ignoring the gravity vector, in most rocks, in most places and at most times the three stress axes will be of approximate equal magnitude – a state is known as isostatic stress.  However, tectonic movements, isostatic adjustment or heat flow can lead to the stresses being locally unequal.  When this happens it is called deviatoric stress. Deviatoric stress causes rock structures to form.  With deviatoric stress, the direction of greatest stress is called σ1 (sigma one); the direction of least stress is called σ3 (sigma three) and the stress with an intermediate value is called σ2 (sigma two).

Figure 1: Graphic showing the three principal directions of resolved stress that act upon every point of a rock in the ground that is subject to deviatoric stress. Sigma 1 is always positive. Sigma 3 and sigma 2 are usually positive but may be locally negative.

For reasons that should be sufficiently obvious, the bulk of rock deformation takes place as a result of the stress difference between σ1 and σ3. This means that, for most practical purposes, we need only show deformation in the plane containing these two axes. This is called a “plane strain” analysis. It is a simplification, but a useful one. It means that we can illustrate stress-strain relationships with two-dimensional diagrams in the plane of sigma 1 and sigma 3, as in the graphic below.  On the diagram the stress axes sigma one and sigma three are shown as arrows, the lines represent planes – potential rock structure – that extend into the page parallel to σ2.

Figure 2: Structures that may form in the plane containing the principal (σ1) and least (σ3) stress axes. The intermediate stress axis (σ2) is normal to the page. Red green and purple lines show structures that might form and their angular relationships to the stress axes.

This diagram should be familiar to anyone who has gone through a university course in structural geology: yet it is widely misunderstood. I have been asked: “how can you simultaneously get compressive and extensional structures in the same rock?” and: ” how can two faults cross each other in the form of an X, and yet neither displace the other?”.  The answer is, you can’t, at least not in the same rock, at the same place and at the same time. That is not what this diagram means.  The graphic is a  representation of the mathematical formula which predicts the attitude of planes where the maximum amount of shear force will be resolved, given two primary stress axes of unequal value. Whether any structures  actually form in a stressed rock, as well as the type and mix of structures,  is dependent on other factors such as the difference in magnitude between σ1 and σ3, the strength of the affected rocks, the temperature and  the rate at which stress is applied. But given that a particular structure will form, the diagram predicts what its attitude will be relative to the stress axes.

Theory, mathematical computation, experimental laboratory rock deformation and field observations have established that:

• Planes that lie at right angles (normal) to σ3 will be planes of relative tension: in rocks such planes might be marked by open brittle fractures or joints, or (more likely) by fissures filled with vein material or igneous dykes and sills.
• Planes which lie at right angles to σ1 will be planes of relative compression: in rocks, such planes will be cleavage or stylolites or perhaps fracture zones coated with clay or graphite indicating that material has been lost by pressure solution from the sides of the fracture (the clay, or fault gouge, is an insoluble residuum of this process).
•  Planes which lie at angle of between 25°- 40° to σ1 are planes that have the maximum  resolved component of simple shear across them[1].  Simple shear is where the rock on one side of a fracture moves laterally with respect to the opposite side. The exact angle depends on the nature of the rocks being deformed with the smaller angle being a feature of shears in stronger (more competent) rock such as quartzite. The higher angle characterizes shears forming in less competent rocks such as siltstone. These planes in actual deformed rocks may become simple shear faults.

Structures in Zones of Simple Shear

Real faults are not mathematical planes: as well as length and breadth they have a width which may range through many orders of magnitude. All faults are tabular fault zones. Consider one of the red lines – a potential fault zone – on the diagram above.  The amount of relative  movement between the two sides of the zone could be a few millimeters or hundreds of kilometers. Structures which can show the sense of movement are second order structures which occur within, or immediately adjacent to, the fault zone. To understand these structures we therefore need to consider the stress and strain that exists within an actively moving fault zone.

The fault zone itself is the primary structure caused by a Primary or First Order Stress Field.  However, within the zone itself, the shear movement re-orients stress axes to a new orientation called Second Order Stress. Second Order stresses produce second order structures that reflect the movement direction.

At this point it is useful to introduce the idea of the Strain Ellipsoid. This is a mathematical concept, but one that is easily visualized in geometrical terms. Think of a sphere of perfectly elastic (squashable) material such as rubber ball. This sphere represents a rock in its undeformed (unstrained) state. Now subject the ball to a tri-axial stress field : it will compress most in a direction that is parallel to σ1: the material displaced will extend the ball in the other two dimensions most noticeably in the direction where it is least constrained, i.e. parallel to σ3. As a result, the sphere becomes an ellipsoid, called the Strain Ellipsoid. The strain ellipsoid does not represent an actual rock structure (perfectly elastic spheres do not exist in nature): it is a graphic which illustrates the mathematical relationship between stress and strain. 

Figure 3. The effect of stress on an imaginary unit sphere.

In simple shear zones, we only need consider the section through the ellipsoid that contains σ1 and σ3 axes. This section is the strain ellipse.

Now consider what happens inside a simple shear zone[2]. A good model for such a zone is a deck of cards on the end of which two lines and a circle have been drawn, as shown in left diagram below.

Figure 4: A deck of cards provides a useful model to illustrate the strain effects of simple shear.

Now we push the deck sideways by sliding each card to the right over the one below by the same small amount . The end face of the deck now has a trapezoid shape as shown at right, above. We have applied a dextral simple shear rotational force across the pack.  The resulting strain is entirely two dimensional (or monoclinic) as no deformation has taken place at right angles to the page. The blue line inscribed on the undeformed pack that formed an original acute angle with the direction of shear is now shortened by the shear movement.  It has thus undergone a compressive strain. The red line, whose initial orientation formed an obtuse angle with the shear direction, is extended by the shearing (extension strain). The initial circle is deformed to an ellipse whose long axis is tilted in the direction of shear – this is the strain ellipse. It easy to show mathematically that those lines that were initially at 45° to the boundaries of the shear zone will show the maximum amount of strain – either a shortening or an extension – depending on their original orientation relative to the shear direction. From this we can deduce that within a zone of simple shear (i.e. within a fault zone) the principal and least stress axes (σ1 and σ3) must, at every given moment during deformation, be oriented at 45° to the boundaries of the zone.

At last we get to the killer diagram, one that has graced a dozen textbooks and a hundred learned papers. Here it is in simplified form:

Figure 5. Graph showing orientation of greatest and least stresses (σ1 and σ3) within a zone of simple shear (i.e. a fault zone). The blue ellipse is the strain ellipse that exists at every instant during shear (known as the instantaneous strain ellipse). The orientations of the various rock structures which can form are shown. Compare these with figure 2.

S – (green lines) these are planes of compression – they could be ductile foliations of various kinds, the axial planes of folds, even stylolites. The angle which they make with the zone boundaries forms an obtuse angle with the direction of shear (i.e.  the surface is congruent with the movement direction). This gives the sense of movement.

T – (yellow) these are planes of tension. They are sometimes characterised as E (for extension) surfaces. Joints, open fractures, veins, dykes. Their orientation forms an acute angle with the direction of shear (i.e. is the surface is opposed to the movement direction) and this gives the sense of movement on the fault zone.

R and R’ – (red) these are complementary planes of simple shear (faults) known as Reidel Shears (R) and Antithetic Reidel Shears (R’). They form at an angle of 25°- 40° on either side of σ1. Reidel shears have a sense of movement that is congruent with the shear direction of the primary structure. Antithetic Reidel Shears have a sense of movement that is opposed to the movement of the Primary Shear. R shears are almost always better developed and have greater movement along them than R’ shears. 

R and R’ shears are often lineated in a direction parallel to sense of movement. Although both R and T surfaces make an acute angle with the shear direction and thus have a similar orientation,  that angle is always much lower for R surfaces than for T surfaces. Bottom line is this: if a secondary planar surface in a fault zone makes an angle of 40 degrees or greater with the fault zone boundary, then it is probably a T surface. If it makes an angle of 25 degrees or less then it is probably an R surface.  If it makes an angle of 40-25 degrees, then look for lineations.

C – (from the French  cisaillement, meaning shear). These are planes of laminar flow related to movement on the Primary Fault Zone and are parallel to the boundaries of the Zone. They are sometimes referred to as Y surfaces, but the French usage has priority. They characterize zones in which a large amount of ductile simple shear has taken place.  Strictly, C surfaces should not be in this diagram (figure 5) at all because, unlike the other structures illustrated, they are not directly related to the instantaneous strain ellipse.  Rather, C surfaces originate as compressive S surfaces which have been physically rotated into parallelism with the shear direction (as illustrated in the next two figures). They are typically lineated in the direction of shear movement.

More about Tensional (T) Structures

As tension veins develop over time they can become very wide, forming mineral filled offsets in a shear zone. Such distinctive offset veins are known as dilational jogs. Dilational jogs are a characteristic development of brittle deformation, i.e. structures that form in competent rocks in the upper few kilometers of the crust.  A right stepping mineral filled jog in a shear zone (illustrated below) indicates a dextral sense of shear (movement to the right). A left-stepping jog indicates a sinistral sense of shear (movement to the left).

Figure 6:  Development of a dilational jog.

Dilational jogs can become very elongate. In these cases, it is likely that only the vein would be exposed and the nature of the structure as an offset in a shear zone would be hard, if not impossible, to spot in field exposure.

Figure 7:  An elongate dilational jog. In typical field exposure, only the vein would be exposed.

Veins occupying adjacent dilational jogs on the same shear surface can grow so as to overlap each other forming a stacked composite sheeted vein. The veins coat the surface like tiles or shingles on a roof – hence the common description of this structure as “shingling”.  A section through such stacked veins is shown below (figure 8). However, the most common field exposure is the face on view of the shear surface (figure 9).  Minerals comprising the veins (usually quartz or calcite) grow in the direction of extension, at right angles to the T surfaces.  This typically produces a strong mineral lineation in the direction of shear.

Figure 8:  Section through stacked dilational jogs. Note the overlapping veins look like tiles or shingles.  The steps are T surfaces.  They face the same way and indicate movement direction. The typical exposure of this structure is looking face on at the shear surface (figure 9).

Figure 9:  Sketch of an exposed fault surface coated with thin sheets of overlapping tension veins. The leading edges of the veins  (T surfaces) form a series of asymmetric steps which face towards the direction of movement of the missing fault block. Within the veins, minerals have grown in the direction of extension (i.e. at right angles to the T surfaces) typically defining a strong lineation in the direction of shear. These are known as “fibres”.  For a photograph of this structure see Part 2 (E)

  Rotational or Non Co-Axial Strain

Within a zone of simple shear, stress axes are always oriented at 45° to the boundaries of the zone. Under  appropriate conditions the stress may produce any of the various structures shown in figure 5, above.  Structures start small then evolve and grow within the active shear zone over time. Any physical structure, once formed, will tend to be rotated by flow of material in the shear zone – even as it continues to grow. As a result, a long-lived planar structures will develop a distinctive sigmoidal shape, as  illustrated on the left of the diagram below.    

Figure 10:  Progressive development of structures in a simple shear zone. The strain ellipsoid that exists at any instant is shown in blue . At left, are time stages (numbered 1-4) in the development of a quartz filled vein during continued dextral shear. At right, the progressive growth of an S surface (a ductile foliation) is exactly analogous to that of the vein although its orientation differs from the vein by 90°.

A tensional fracture initiates parallel to the stress axis σ1. That means it forms at 45° to the zone margin in a direction that is opposed to movement (stage 1, above left). The fracture begins where strain is greatest and that usually is in the centre of the zone. Fault fluids may deposit minerals (typically quartz or calcite) in the dilational fracture to form a vein. As shear movement continues, the early vein is physically rotated in a clockwise direction (stage 2).  Obviously, a vein in solid rock can only rotate if the material surrounding it is able to deform in a ductile manner.  Rotated tension veins are thus characteristic of deep level structures where a combination of brittle and ductile styles of deformation can operate, or where the fault fill material is relatively incompetent. This style of vein development contrasts with that of dilational jogs (see above), where brittle deformation predominates throughout vein development.

As it rotates, the vein continues to thicken and extend by growth at its tips. The new growth at the tips is controlled by the stress so is at 45° to the zone margins, making an angle to the older, now rotated, central part of the vein. The process continues (stages 3 & 4) with new growth at the extending tips always  forming at 45° to  the zone boundaries. The end result is the sigmoidal vein shape seen in stage 4.  Vein formation ceases when the bulk of the vein has rotated so much that there is no longer any resolved component of extension across it and is no longer able to accept new mineral deposition.

A compressive or S surface (the black lines of figure 10) such as a ductile foliation would seldom if ever form in a shear zone in the same place or at the same time as a tension fracture. However under appropriate conditions it will be initiated where strain is greatest – usually the central part. The surface forms parallel to the stress axis σ3, that is: at 45° to the zone boundaries in a direction congruent with shear movement (stage 1, above right). Its development over time now follows the same path as that of the tension vein with the early formed foliation being physically rotated towards parallelism with the boundaries of the shear zone whilst, at the same time, becoming extended by new growth at 45° to the zone boundaries. Once parallel with the principal shear direction it is known as a C surface. Shear movement takes place along C and low-angle S surfaces creating mineral lineation oriented in the direction of shear, as shown in the block diagram below.

Figure 11: In ductile shear zones,  compression results in a foliation that initially forms at 45° to shear margins (S surfaces)  but in the centre of the zone becomes rotated into planes of laminar shear parallel to zone margins (C surfaces).  C surfaces typically are strongly lineated by oriented growth of elongate minerals or mineral aggregates.

In zones of strong ductile simple shear C and S surfaces are often partitioned into alternate laminae at millimeter to centimeter scale. This distinctive structure is known as S-C structure and is illustrated below. Such structures can provide an unequivocal sense of movement for the zone. However, in many examples, this type of structure is open to the alternative explanation that it a spaced cleavage, reflecting a second generation of folding, affecting an earlier S surface. To distinguish between the two explanations you need to know whether the C surfaces are parallel to the zone margins – not always easy when confronted with a small outcrop in the middle of a wide ductile shear zone.

Figure 12.  S-C structure in a ductile shear zone. This is interpreted as caused by dextral shear although the alternative explanation of a spaced cleavage is often also available. 

The above analysis is based on the assumption that structures in shear zones initiate in the central part of the zone where strain is greatest. However, in narrow shear zones with discrete and well defined margins the greatest strain may occur along the boundaries of the zone. A compressive S fabric in the centre of such zones will tend to rotate into parallelism with the shear direction (C surfaces) along the margins of the zone. This will yield shapes as shown below:

Figure 13: In shear zones with well defined margins simple shear strain is often greatest along the zone boundaries. In the centre of the zone compressive S foliation is at 45 degrees to the margins, but are rotated to parallelism with the shear direction (C surfaces) as they approach the contacts. Blue ellipses are the final strain ellipse showing the cumulative effects of both rotational and non-rotational strain. Where the shear zone is shallow-dipping and the movement produces a horizontal compression of the rocks, this structure is known as a thrust duplex (see illustrations G & H in Part 2).

Structures in high temperature ductile shear zones

In a ductile shear zone which has developed under conditions of high stress and temperature and across which a large amount of movement has taken place, almost all structure, no matter how initially formed, is rotated into parallelism with the shear surface C. However, minor inhomogeneity can lead to variations in strain that produce distinctive sense of movement structures. Some of these are illustrated below.

Figure 14: Structures in high temperature ductile shear zones. In A, a phenocryst or clast (blue dots) that has resisted deformation creates two low-pressure zones on its margins in which new mineral growth (coloured green) has occurred during deformation. Typically, the clasts range from a few millimeters to a few centimeters across. The asymmetry of these “tails” indicates the shear direction (compare this diagram with E). In B, the clast and attached tails has been physically rotated in the direction of shear, dragging the “tails” with it to create a spiral structure. In C, a marker surface in the shear zone (often an early formed quartz vein) is folded into an asymmetric fold pair. The axial planes of the folds mark S surfaces and their orientation relative to zone boundaries provides the sense of shear. In D, the clast has been split apart in a brittle fashion and new mineral growth (usually quartz or calcite) has grown in the low-pressure site between the separated pieces. This process is called boudinage (another French word).  In this case, the overall strain is one of  pure flattening and no sense of shear can be determined. Lastly, E shows symmetric “tails” of new mineral growth attached to the clast (compare to A). There is no sense of simple shear. Only a pure flattening strain in a direction normal to the boundaries of the ductile shear zone can be deduced.

Scaling it Up

It is important to remember that the widths of fault zones may range from a few millimeters to tens of kilometers. The structures shown on figure 6 therefore range in size from things that you might hope to see in a thin section or hand specimen to structures that might be shown on a regional geology map. Many structural geologists have used a diagram such as figure 6 as a template to lay on a regional map and so, from their strike orientation, read off the identity of the various structures. Such exercises are seldom convincing since knowledge of the strike of a structure provides insufficient evidence to enable its true nature to be determined.

Reidel shears (second order structures) can themselves be significant faults zones. Within these zones, third order stresses operate to provide third order structures. And, of course, within third order structures we can expect to find fourth order structures. And so on: it is an example of scale-dependent self-similarity, a fractal relationship. However, if we proceed much beyond third order structures, we have boxed the compass with the orientations of potential structures that can result from the original, first order, far field, causative stress.  At this point, in the real world, the whole analysis process usually breaks down into unconvincing complexity. There are just too many assumptions and selectable options.  Interpretations, although plausible, usually reflect wishful thinking. Analyses of this kind are however still attempted. They are generally known as Moody-Hill analyses after a pioneering and well known paper by these authors (on the San Andreas Fault Zone) published in 1956 (ref below).

Figure 15:  The relationships of First and Second Order Stress and Strain. This type of analysis, when applied at regional scale, is called a Moody-Hill analysis. Such analyses often invoke third, fourth and even higher orders of stress.

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TARGETING DRILL HOLES

It is a truism that ore bodies are rare and hard to locate. If this were not so, they would hardly be worth finding.  Explorers search for them by drilling holes into the ground.  A single drill hole produces a very small sample of rock: ore bodies, even the largest of them, are of insignificant size relative to the barren rocks that surround them. Even after an initial discovery hole has been made into a potential ore body, if the subsequent holes are poorly positioned, the ore body may remain undiscovered, or, at best, an excessive number of holes will be needed before its true shape, attitude and grade are defined. Western Mining Corporation, after their first serendipitous intersection of the blind Olympic Dam Au-Cu-U deposit in South Australia, required another nine holes (and 1 million  1975 exploration dollars) before they could find it again. And this was one of the largest metallic ore bodies on the planet.

For the most efficient path to discovery, the explorationist has to make use of all available knowledge.

When geologists drill targeted holes they are testing a mental model of the size, shape and attitude of a hoped for ore body. The more accurate that model, the greater the chance that the hole will be successful. The model is the result of extensive detailed preparatory studies on the prospect, involving literature search, examination of known outcropping mineralisation, geological mapping at regional and detailed scales as well as geochemical and geophysical studies. Compared to drilling, such preliminary studies are relatively cheap. Each drill hole into a prospect, whether it makes an intersection of mineralisation or not, (and perhaps especially if it does not), will increase geological knowledge and lead to modification or confirmation of the model and so affect the positioning of subsequent holes. The first few targeted holes into a prospect are always hard work, this is the steepest part of the learning curve, and it is in how this stage of exploration is approached that exploration geologists most clearly reveal their true worth.

In order to most efficiently define the size and shape of a potential ore body, drill holes will normally be aimed at intersecting the boundaries of the mineralisation at an angle as close to 90° as possible. If the expected mineralization has a tabular, steep-dipping shape, the ideal drill holes to test it will be angle holes with an inclination opposed to the direction of dip of the body. If the direction and amount of dip is not known (as is often the case when drilling in an area of poor outcrop, or testing a surface geophysical or geochemical anomaly), then at least two holes with opposed dips, intersecting below the anomalous body, will need to be planned in order to increase the chances of intersection the target at depth. Flat-lying mineralization (such as a recent placerdeposit, a supergene enriched zone above primary mineralization, or perhaps a manto deposit) is normally best tested by vertical holes. These are not the only considerations.  Holes are normally positioned to intersect mineralization at depths where good core or cuttings return can be expected. If the target is primary mineralization, the hole will be aimed to intersect below the anticipated level of the oxidized zone.

Once an intersection in a potential ore body has been achieved (a situation often described as having a “foot-in-ore”, although “toe-in-ore” would often be more accurate), step out holes from the first intersection are then drilled to determine the extent of the mineralization. The most efficient drill sampling of a tabular, steep-dipping ore body is to position deep holes and shallow holes in a staggered pattern on alternate drill sections. However, the positions selected for the first few post-discovery holes depend on confidence levels about the expected size and shape of the deposit and, of course, on the minimum target size sought. Since the potential horizontal extent of mineralization that has an expression at surface is usually better known than its depth extent, the first step out hole will in most cases be positioned along strike (at a regular grid spacing in multiples of 40 or 50 m) from the discovery hole, and aimed to intersect the mineralization at a similar depth. Once a significant strike extent to the mineralization has been proven, deeper holes on the drill sections can be planned.

Epigenetic[1] vein or lode type deposits occur as the result of mineral deposition from fault fluids in localised dilation zones that result from fault movement. These dilation zones may lie within the main fault or in splays from or adjacent to it. High grade ore shoots therefore will tend to have the same shape and orientation as the dilation zone[2].  Dilation zones in faults are typically highly elongate, with a flat, blade-like cross section. An elongate ore body of this sort is known as an ore shoot. If the long axis of the shoot has a shallow pitch[3] on the fault plane, any hole targeted to drill below a surface indication or initial discovery hole is likely to pass below the shoot and so miss it. If the ore shoot pitches steeply, a hole collared along strike from an initial discovery hole may lie well beyond the shoot. Obviously, being able to predict the pitch of an ore shoot is important. How can we do this? The answer lies in understanding the nature of the fault structure which controls it.

The shape and orientation of dilation zones reflect the stresses that produced the fault and local variations in the physical properties of the rocks being faulted.  A detailed theoretical treatment of the stress/strain relationships of faults is beyond the scope of this blog but can be found many standard texts (for example Ramsey and Huber, 1983,) and published papers (for example Nelson, 2006). However, the exploration geologist is a generalist with little time or opportunity for reading academic papers.  She needs practical rule-of-thumb guidelines.  For that purpose, the following brief description of fault geometries will be found useful in predicting the attitude of high grade epigenetic ore shoots that might have formed within them.

Most faults that form in the upper few kilometres of the earth’s crust result from principal stress directions that are, to a first approximation, oriented either parallel to, or normal to, the earth’s surface.  This has produced three common classes of fault :  normal faults, thrust (or reverse[4]) faults and strike-slip faults, depending on which one of the three orthogonal principal stress directions is vertical.  Normal faults are the commonest type of fault to form in the upper 1-3 km kilometres of the crust: they are steep dipping, but tend to flatten with depth.  In normal faults, the direction of movement across the fault lies in the direction of the dip of the fault such that fault movement produces a horizontal extension of the crust. Thrust and reverse faults are generally shallower-dipping than normal faults. Once again the direction of movement across the fault lies in the direction of the dip of the fault but in this case the movement direction is such that that it causes a horizontal compression or shortening of the crust.  

Strike-slip faults are always steep dipping with a movement or displacement direction in the direction of the strike of the fault. This movement is either left-lateral (where, on looking across the fault, the rocks have been displaced to the left) or right-lateral (a displacement to the right). The technical terms for left-lateral and right-lateral is are sinistral and dextral.  

The three fault categories are illustrated in the figures below.

Sections illustrating the geometrical relationship of tensional openings (and hence potential sites for epigenetic ore deposits) to the three principal classes of fault.  In each illustration, the principal dimension of the dilational vein is at right angles to the page.

How can we know what category of fault we are dealing with? To classify faults as normal, thrust or strike-slip, it is necessary to know (1) the attitude of the fault and (2) the movement direction (or movement vector) across it. The movement vector can be determined from the displacement of marker beds across the fault (based on field mapping or drill hole interpretations) and from observing sense-of movement indicators that can be seen in outcrop or drill core. 

Once we know, or suspect, the category of fault that mineralisation is associated with, the following rules of thumb can now be used to predict the likely attitude of dilational zones and hence of high grade ore shoots within or adjacent to it. This is a simplified summary. A more detailed presentation of theory can be found in Sibson 1996 and Cox et al 2001.

NORMAL FAULTS

As already stated, Normal Faults are the commonest type of fault to form in regions of crustal extension in the upper 1-3 kms of the crust. They are the typical structures that host epithermal veins in young magmatic arcs.

The long axes of dilation zones will tend to be sub-horizontal and to lie within portions of the fault that are steeper dipping than the rest of the fault, or within steep dipping spays that trend off, or are adjacent to,  the main fault (see figure above). By contrast, the shallower-dipping portions of such faults will be unmineralised or poorly mineralised. These local bends in fault attitude are known as dilational jogs (a term first defined by McKinstry in 1948). For this type of fault an initial ore discovery should be followed up by drilling a hole to along strike from the discovery hole to intersect the target at approximately the same depth. Two examples (chosen from dozens of potential examples) of epithermal vein gold mineralisation hosted within Normal Faults are shown below.

Example of vein hosted gold mineralisation: The Berenai Vein in the Rawas Gold Camp of South Sumatra, Indonesia. The red-brown colour of the mineralised zone is caused by oxidised pyrite. Figure taken from Marjoribanks 1999,  

Example of an outcropping, bonanza-grade, epithermal vein hosted by a normal fault: The Newcrest Gosowong gold deposit, Halmahera Island, Indonesia. Note gold widths and gold grades show a positive linear relationship to dip on the Gosowong Fault. Figure taken from Oldberg et al, 1999.

THRUST AND REVERSE FAULTS

Shallow-dip thrusts, often located along incompetent bedding horizons, are common in the shallow levels of the crust in zones of crustal compression (orogenic zones).  They seldom contain any significant mineralisation.

Reverse faults on the other hand are the characteristic type of fault to form between 3-10 km below surface and are the typical focussing mechanism for the location of mesothermal vein gold deposits in Archaean, Proterozoic and Palaeozoic slate belts.

The principal dimension of zones of relative dilation on reverse faults will tend to be sub-horizontal and lie within those portions of the fault that are shallower dipping than the main fault plane, or within shallow dipping splays that trend away from, or occur adjacent to, the main fault plane. On the other hand, the steeper-dipping portions of such faults will be unmineralised or poorly mineralised (Sibson et al 1988). For this type of fault, as with mineralisation encountered in normal faults, any initial intercept should be followed up by drilling a hole to the same depth as the discovery hole and along strike from it.  I could give dozens of examples of deposits of this kind.  Two are shown below:

Gold lodes hosted by tensional jogs in a fold-related reverse fault. Section through underground mine at Bendigo, Victoria, Australia (Lower Palaeozoic). The figure is taken from Cox et al, 1991.

Block diagram showing the relation of gold bearing veins to reverse faulting and a favourable dolerite host rock in the Groundrush open cut gold mine, Tanami Complex (mid-Proterozoic), Central Australia. From Marjoribanks 2011.

STRIKE SLIP FAULTS

Crustal-scale strike-slip faults (think the San Andreas in California, The Tintina of the Yukon and Alaska, The Highland Boundary in Scotland, the Barisan of Sumatra, the Cadillac of the Abitibi) are known as transform faults. Transforms  are tight, straight structures and poor places to look for mineralisation. However, minor strike slip faults can show the irregularities needed to focus ore veins in dilational sites.

The long axes of dilation zones will tend to be steep-plunging.  For sinistral strike-slip movement, a zone of relative dilation occurs in any left-stepping bend in the surface trace of the fault.  For a dextral strike-slip movement, the dilation occurs in any right-stepping bend in the strike trace of the fault.  By contrast, right-stepping bends in sinistral faults and left-stepping bends in dextral faults will be zones of relative compression during fault movement and will tend to be unmineralised or poorly mineralised.  Any initial ore discovery should be followed up by a deeper hole on the same cross-section. Two examples of quartz-vein hosted gold mineralisation associated with dilational sites in strike-slip fault zones are shown below:

Surface geology plan of the famous Comstock Silver – Gold Camp, Nevada, USA. (click on image for full size plan). From Berger et al, 2003. For a fault analysis of this map, see cartoon below:

Analysis of fault patterns around the Comstock Lodes, Nevada. The mineralised Lodes are a series of normal faults that define a right-stepping dilational jog in a regional dextral strike-slip fault which carries the name Bain Spring in the north and Silver City in the south.  Compare this analysis to the detailed map, above. Based on Hudson, 2003.

Geology map of the open cut on the Lorena high-grade, gold-arsenic Lode, Cloncurry district, Queensland, Australia. From Marjoribanks, 2011. Click on image for full size version.

Block diagram of Lorena lode showing its interpretation as a steep-plunging dilational jog on a sinistral strike-slip fault.

DRILLING ON SECTION

Once a zone of mineralisation (potential ore) has been discovered, and its shape and attitude approximately outlined, it needs to be defined in detail by a follow-up program of in fill drill holes. Each drill hole provides a one-dimensional (linear) sample through a prospect. The problem facing the explorationist is how to use this restricted data to create a three-dimensional model of the mineralisation and its enclosing rocks. Our brains are not really very good at conceptualising complex 3-dimentional shapes and relationships (although good mining and exploration geologists can do this better at this than most).  The best way to solve the problem is to concentrate drill holes in a series of vertical cross sections[5]. Each section is thus a plane of relatively high data density and will facilitate interpretation.  A series of parallel interpreted drill sections are two-dimensional slices through the prospect: they can be assembled (stacked) to produce a three dimensional model. Formerly, section interpretations were often plotted onto clear perspex sheets which were then physically assembled into a frame so that they could be viewed as a whole.  Today, mining software allows digitised interpreted sections to be used as a basis for creating three-dimensional virtual reality shapes of ore bodies and rock masses which can then be rotated and viewed from all angles on a monitor.  Although the software allows stunning presentation of results, the key interpretation stage is still the manual interpretation of two-dimensional drill sections.

This magnificent solid model of the high grade Zn-Pb-Ag sulphide ore bodies at Broken Hill, New South Wales covers a volume that extends along strike for around 10km and to a depth of 4km. The flat top of the ore body shows where it is truncated by the surface. It was constructed from historical Level plans showing stoped out areas. Today such a model would probably be constructed using virtual reality software, but would never have the same impact.

Where drill holes deviate off section, assay and lithology data can be projected orthogonally (i.e. in a direction at right angles to the section) onto the drill section plane. Such projections are usually done by mining/exploration software programs. On these programs it is possible to specify the width of the “window” on either side of the section from which data will be projected.  Obviously, if holes are not drilled at right angles to the strike of the feature, orthogonal projection will tend to distort true sectional relationships – a problem which will be exacerbated the further the data has to be projected (the wider the “window”) onto the section.   

REFERENCES

Berger B.R., Tingley J.V. & Drew L.J (2003): Structural localisation and origin of compartmentalised fluid flow, Comstock Lode, Virginia City, Nevada. Economic Geology, v98, pp 387- 408.

Cox SF Knackstedt MA and Braun J (2001) Principals of structural control on permeability and fluid flow in hydrothermal systems. Econ Geol Reviews14, 1-24.

Hudson D.H. (2003): Epithermal alteration and mineralisation in the Comstock district, Nevada. Economic Geology, v98, pp 367-385.

McKinstry HE (1948) Mining Geology. Prentice-Hall, New York, 680p.

Nelson EP (2006) Drill hole design for dilational ore shoot targets in fault fill veins. Econ Geol 101, 1079-1085.

Marjoribanks RW (1999: unpublished consulting report.

Marjoribanks RW (2010): Geological Methods in mineral exploration and mining. Springer 238p

Marjoribanks RW (2011): Unpublished Consulting Report for Tanami Gold NL

Oldburg DJ, Raynor J, Langmead RP & Coote JAR, (1999): Geology of the Gosowong epithermal gold deposit. In Pacrim ’99 Conference papers pp 179-185

Ramsey J & Huber M (1983) The techniques of modern structural geology. Volume 1: Strain Analysis. Academic Press, 307p.

Sibson RH Robert H and Poulsen KH (1988) High-angle reverse faults, fluid pressure cycling and mesothermal gold-quartz deposits. Geol 16, 551-555.

Sibson RH (1996) Structural permeability of fluid driven fault fracture meshes. J Structural Geol 18, 1031-1042.

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