A process of mineral discovery

I have made discoveries, and the feeling is amazing. I’m talking of finding gold in my previous life as a geologist, but this sense of discovery applies whenever something new is found or deduced. But how does discovery occur?

Setting

So here’s the scene: I’m a relatively young geologist with some miles and trials behind me, and have become employed as a mine geologist in a century-old mining district. The odds of discovering something new are, on one hand, minimal because of the age of the district and the amount of mining that has already occurred. On the other hand, the geologic engine that drove the processes that created several gold-and-silver deposits was pretty large, and one old adage of exploration is: if you want to find elephants, go to elephant country.

I was definitely in elephant country.

Other key players included an extremely bright exploration geologist and a mine manager from Hungary. The exploration geologist saw the world on a different scale than did I, so our conversations were often energetic as we tried to convince each other of the reality of our respective visions. The mine manager – Josef – was an incredible man, bringing his old-world work ethic and management techniques to a gold mine that mixed old technology with new.

Mine models

Mine models are imaginary creations built on a mixture of what is known and what could be. That may sound simple, because what is known should form a sound foundation for extrapolating outward into the unknown. But we’re talking geology here, and much of what was considered known was really formed of not-so-solid interpretations between widely spaced occurrences.

For example, several drill holes that seem to be associated with the same style of mineralization can be connected (think connect the dots) to create what appears to be a continuous mineralized zone. Sometimes these holes are tens of feet apart, and sometimes they are hundreds or even thousands of feet apart.

People look at these data points differently. My exploration geologist associate would see evidence of continuity where I would see a lot of blank space between the points. It wasn’t about who was right and who was wrong. It was simply two different perspectives on the same factual data.

This means the extrapolation from the known data is really very imaginary. I’ll try to give an example. If you saw a damaged photograph of a neighborhood, and only one out of every ten homes was discernible in the damaged photo, and each of the homes you could see was yellow, then a reasonable conclusion would be that most or all of the homes you could not see are also yellow.

In reality, this kind of homogenous neighborhood is rather unlikely to occur. Some folks would fill in the blanks based only on the limited data they see, and others would fill the empty spots by applying experience gathered elsewhere. There are many more possible scenarios when you’re willing to fill in with a variety of possibilities, and that was the kind of geologist I was.

Our mine model was not a physical creation like you might have made in school out of multiple layers of cardboard. No. It was a couple of idealized views that cut across (called a cross section) the mineralized zone, and some description of the geologic characteristics of such mineralization. Those characteristics are like a fingerprint. When a geologist sees some of the fingerprint, it’s time to look deeper to see if interesting mineralization is present.

New kid

So back to the setting. I was definitely the new kid. I came into the situation at a time when opinions about the orebody and surrounding rock were becoming solidified. As the new kid, I had some freedom to question the status quo as a way of learning the geology in the mining district.

As I learned about the local geology and the generally accepted mineralization model, I became aware of the differences in how various people viewed the information. (Throughout this post I’ll keep coming back to the ideas of scale and perspective.)

As mentioned previously, some folks saw continuity between widely spaced data points. I am aptly named Thomas, because instead of seeing continuity, I saw doubtful areas between the points.

When you challenge the status quo, a variety of things can happen. People resist change when it might change how they function. For those outside the sphere of geologists, some of our conversations probably looked like a clash of egos. But the reality was we were engaged in deep, intense conversations driven by the classic scientific method of gathering data, creating theories, and testing those theories. A mine model is simply a theory to be tested and refined.

Gathering data

Part of a mine geologist’s job is to map the working parts of the mine, creating an ever-growing, ever-changing body of information. That information describes past and current mine workings, and the nature of mineralization observed.

Mapping in this particular mining district was very low tech. A mine opening is a three-dimensional void in solid rock. That void has a width and height at any given point. The simplest way to describe the basic mapping procedure is this: stretch a long level measuring tape from a known point, draw a line to scale that represents that tape, and at regular intervals, measure the distance left and right of the tape. One can do this in a list form and draw it later, but it is usually better to do both: capture the raw data and draw the mine opening while you are there. Sometimes measuring the distance above and below the tape is helpful when the “back” is not level.

(Special mining terms: face, ribs, and back. Imagine you are in a tunnel. The tunnel ends in solid rock. When you are facing the solid end of the tunnel, that surface is called the face. To your right and left are the ribs. Over your head is the back.)

Assuming the opening is following mineralization, then the geology of the working face is mapped, and usually samples are collected. Back in the office, the new map is combined with the series of earlier maps to update the record for that part of the mine. Analyses of samples are plotted on the appropriate map sheets so one can see the changes in mineralized and unmineralized rock as mining progresses.

Over time, not only is the geology recorded by this mapping as to length and width of mineralization, but also vertical changes are described as mining advances up toward the earth’s surface or downward.

Making theories

Geology is a science. It is the field application of mathematics, biology, botany, chemistry, and physics.

But it is also an art. The art of geology involves the way an individual geologist perceives the world. It involves deductive reasoning based on factual data. At some point, it also involves inductive reasoning, where the logic of what must follow from facts is extrapolated. In other words, facts lead to conclusions, and those are then extended beyond the factual to ranges of probability.

As I examined older maps made by previous geologists, and incorporated my own observations, I began to notice relationships between gold concentration and geologic characteristics. For example, when the rock adjacent to veins showed evidence of being broken (brecciated) and healed with silica, gold concentration in the vein increased. When a steeply inclined vein showed a change that was less steeply inclined, gold concentration increased.

I deduced about a dozen of these key relationships between observable characteristics and gold concentration. Remember that my scale of observation was very fine grained. I was seeing the mine up close, while my exploration counterpart was seeing the mine from afar. My conclusions were based on what I observed from deep inside the mine; his were based on what he observed from outside the mine. Both of us had access to a common body of information derived from drilling in and around the mine.

Different purposes = different theories

The goal of a mine geologist is, at minimum, to replace every ton of ore mined with a fresh ton of ore. Ore is rock that will cover the cost of mining, milling to extract the valuable components, marketing and disposal. When ore exceeds this value, profit is made.

The goal of an exploration geologist is somewhat different: find a new vein or a new orebody. The nature of exploration work is conditioned on working at a larger, coarser scale.

Where a mine geologist sees individual trees, an exploration geologist sees not trees but a whole forest.

I was lucky to have worked on both sides of this fence before. I had been a mine geologist and an exploration geologist, and that gave me an appreciation of the differences of scale involved. It also made it much easier for me to grasp how variable data can be , and how tricky it is to extrapolate that data.

The model that was emerging from my examination of available data was different than the generally accepted model that had been in play for many years in this particular mining district.

To test it, I made some predictions about future gold concentrations in various parts of the mine, and as we accessed those areas, I was gratified to see the predictions held up pretty well.

Other veins

We had a pretty good handle on what controlled mineralization in the working mine. But what about other quartz veins in the vicinity of the mine? Would they also follow the new model?

A few drill holes pierced an unnamed vein several hundred feet away from the working mine. I looked at the logs of those holes, and postulated it was actually a known vein that was offset by bending and faulting. I examined that vein where it daylighted on the surface and saw it had the characteristics of a gold-bearing vein according to the new model. The drill cores for this vein also showed these characteristics.

To test the evolving model, we drilled a series of holes through this vein. Before we did so, I predicted where gold concentrations would be highest, based on the new model. It was an amazing feeling to see the analyses of the vein samples from the drill core as predicted. In one sample, visible gold was evident.

I also applied the new model to a much older mine thousands of feet away, and saw the same basic relationships involving wallrock alteration and changes in the steepness of the vein. The highest gold concentrations corresponded to the areas where wallrocks were the most broken and healed, and where the veins bent. The model fit.

Chaos and vein formation

About this time I was concentrating on trying to shift my thinking from matching physical characteristics to actually understanding the physical and chemical processes that ultimately created gold ore in this mining district.

At the time, I was also reading a book that is now one of my all-time favorites called Chaos: Making a New Science. The concepts in this book were critical to what happened next.

I had been observing interesting textures in the quartz veins in both the old and new mines. Bland, barely banded vein was generally not profitable to mine. When banding occurred with certain fine-grained minerals, the vein was much more likely to be ore grade. Black-and-white banding might be ore, or might not, depending on why the bands were black. And the highest grades of ore were almost always associated with cauliflower-like textures, often broken repeatedly and healed by black quartz again and again.

The concepts in the book helped me see beyond the microscopic scale, and I began to see repetitions of the geometric textures across larger parts of the mine. And I was recognizing larger features repeated at the microscopic level in vein samples. I even estimated the fractal dimension of vein textures in various parts of the mine, and correlated those values with gold concentrations.

This recognition of patterns repeated at different scales got me wondering why. What could cause such an interesting thing? I came to no firm conclusions other than it was unlikely we could predict linear relationships in an orebody created by chaotic processes. The process of inquiring and examining and thinking was stimulated by this book, and it lead to the next evolution in the mine model.

Breaking and boiling

The consensus of most geologists I knew was this system of quartz veins had once been a hot spring system. What we were seeing now was the chilled, fossilized remains of a once dynamic hot spring environment.

What would cause the breaking of wallrock and subsequent healing with quartz? Possibilities were movement, dissolving of minerals between rock fragments, and rapid depressurization.

What would cause the lack of banding, and the very evident banding, and the incredibly complex banding of veins? The more breaking and healing of wallrocks we saw, the more complex were the characteristics of the veins.

And the most complex vein segements were also the thickest. Thin veins were usually poorly banded and had little wallrock brecciation. Thick veins were almost always more complex, had signficantly more wallrock brecciation, and the highest gold concentrations. Oddly, thick veins also occurred where veins bent vertically or horizontally.

When we added the observed characteristics together, one possible conclusion was a hot spring system that repeatedly sealed up, then ruptured. The main conduit of water flow would correspond with the thickest vein segments.

Amending the model

So we had a fossilized hot spring system that had repeatedly sealed and ruptured. One observation did not match this simple model, and it confounded me for some time.

What we were seeing was high gold concentrations extending more deeply than the thick portions of veins. Gold grades extended below thick zones, and the deeper we went, the narrower and less concentrated became the gold bearing roots of the mine. The shape was almost like the roots of a tooth.

Now imagine a bottle of carbonated soda. If you shake it up and pop the cap, you know what happens: when it depressurizes suddenly, it erupts. Gas in solution comes out of solution when the pressure is released suddenly; this is a phase transition.

Now imagine your bottle of soda is actually very, very tall. When you shake up this bottle and suddenly remove the cap, the system still erupts. One of the most interesting observations is the point where the liquid changes to liquid dominated by bubbles actually shoots down the neck, until it weakens and finally peters out.

This model matches what we observed in the mine: a fluid-dominated hot spring that sealed and ruptured repeatedly, with a phase transition interface that shot down, down, down. Gold mineralization was most strongly associated with areas where this boiling (bubbles forming in the liquid) occurred.

It is likely the hot wall rocks contributed to the sudden boiling upon rupture, in a process known as superisoenthalpic boiling.

Adding to the model again

The boiling hot spring model fit observed characteristics well. It was reasonably predictive: based on geometry and texture, we could usually estimate the rough gold concentration of the rock.

So we now had a tool that worked for deeper parts of the mine. But as we turned our gaze upward, we had little data. The ancient surface where the hot spring had vented was now buried under a younger formation of rock. We could not see the vent, but the model predicted one should exist.

The vent could have been eroded away before the young rock was deposited. But what we observed was more and more brecciation and healing of rock as we neared the ancient surface. Some widely spaced drill holes showed low concentrations of gold, but not at the concentration we had come to expect. The rock in those drill cores was much less cohesive, that is, it broke apart much easier than deeper rock.

Based on these and other factors, I postulated the eruption zone was intact, and could have gold concentrations sufficiently high to mine at a profit.

Clash of the models

The old model did not allow for ore-grade mineralization where the hot spring system had vented to the ancient surface. Why? Because although the handful of drill holes through the eruption zone did contain gold, the concentration found was less than ore grade.

Where some geologists would connect the dots and conclude the intervening rock was, or was not, mineralized, I saw large gaps in the data. The other problem with the old model is it assumed the same mining method we had been using would be used, and thus the same cost for determining profitability formed the basis for labeling the rock as ore or waste.

We drilled additional holes from inside the mine, up through the eruption zone, and found good gold concentrations in many holes. Some were sufficiently high to mine with the mining method we were using at the time. Other values were just below the amount necessary to cover costs. However, it looked like there was a significant amount of this subgrade material.

Success

Ultimately, we mined that rock. We were able to do so by delineating a large resource and investing in a different mining method. What makes this such a bittersweet discovery is some folks were absolutely convinced there was no potential for profitable extraction of this resource. The problems we faced were with naysayers, not with the technology or capital needed to get this gold.

This discovery added several years of life to the mine, benefiting the many employees and their families, shareholders in the corporation, and the local community. It would not have been possible without some outside-the-box thinking, without the constant revisiting of the geologic model, and the support and encouragement of Josef, our mine manager.