CHAPTER 7 RECOGNITION OF MINERAL ENVIRONMENTS

• Jurisdiction: United States

Mining Exploration Technology for Lawyers and Landmen
(Apr 1980)
CHAPTER 7
RECOGNITION OF MINERAL ENVIRONMENTS
Thomas E. Walthier
St. Joe Minerals Corporation
New York, New York

Ladies & Gentlemen,

It is a great pleasure and honor to be here today and address this learned audience. When I accepted, the assigned topic appeared simple enough, but as I began assembling my thoughts and putting them to paper, more and more ramifications came to mind. In short, I have found this a stimulating exercise.

Some Generalities about Mineral Deposits

First, let me stress that to be successful in exploring, one has to look where a mineral deposit lies hidden. This is not such an obvious statement as you may think. A mineral ore deposit is a very small, natural occurrence of some metal or element that has been concentrated several-hundred to several-thousand fold over its average content in the earth by natural geologic processes acting over large periods of time. For example, the average igneous rock contains 70ppm copper (70 grams per metric ton) with a range between 5 and 150ppm for common igneous and sedimentary rocks. Good grade copper ore contains 8000ppm, or a concentration of a hundred fold. Common igneous and sedimentary rocks have between 5 and 50ppm lead while good lead ore runs 30,000 to 50,000 ppm Pb (3-5%), or a 1000-fold concentration. Many ore deposits require several types, and more than one period, of natural concentration to enrich the mineral to the point that man can mine and extract the mineral efficiently. Ore bodies are geological rarities, especially so for the metals — the chance result of innumerable processes that acted, reacted and modified one another over eons of time. A certain tract of land, a county, a state either has or has not undergone those geologic phenomena of concentration. Man has no control over where nature formed her treasures, nor can we move them. We're very fortunate when we can find them! A little later I'll review criteria by which we hope to decide a hidden goodie is down there. So much, then, for those of our citizens who say, "I don't want your mine in my backyard or in my favorite playground. Go mine it somewhere else." You either mine that ore body where God put it, or it is a lost and wasted resource.

I said mineral deposits are rarities. To give some measure to this, the U. S. Bureau of Mines¹ calculated that the total land area of the United States that has been involved in mining (that's for all the metals, all the non-metals, coal mining, cement quarries, etc.) from 1930 to 1971, is less than 2/10 of 1% (actually 0.16%). If we subtract from this involvement the acreage due to surface facilities, tailings and dumps, the percentage of the land that actually contains ore under it is much less. So it is evident that the discovered mineral deposits occupy only a tiny fraction of 1% of the earth's crust. Their unit value, however, is very, very high.

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One definition of "science" is "classified knowledge" and out of the classification nature's rules, order and processes can be deduced. Over the years, earth scientists have classified geologic processes and types of mineral deposits. And so ore bodies are variously classified by geologic setting, by commodity, by theory of how they formed, where they occur, etc. It is apparent that certain parts of the world are particularly well endowed in large and rich deposits of a certain metal while other regions appear impoverished. Chile is noted for copper, tin typifies the Malay Peninsula, South Africa has gold and diamonds, Missouri is known for lead, and you are assembled here in Arizona because it is famous for copper. Similarly, it has been noted that certain rock types, in a certain configuration and evidencing a particular geologic history or alteration and metamorphosis, typify where ore bodies of a certain type, or mineral, are localized.

The most attractive place to explore, it would seem at first glance, is to go to that part of the world where that particular metal seems to be enriched and then seek within that region a duplication of the favored rock types in the right configuration, state of alteration, etc. However, as no two ore bodies are exactly alike, we must judge whether the similarity of geologic setting is sufficiently alike to be promising as an exploration bet. By analogy with classical music, we find that in describing ore bodies we have a dominant theme with many variations. Thus, similar, yet different geologic settings in a mineral rich province, constitute secondary targets where to dig, probe and hunt for the elusive treasure. Then, there are those parts of the world where the geologic setting and conditions look right but only meagre, or no, evidence of the metal exists. These are high-risk, wildcat areas, but are legitimate places to look, especially if the amount of prior exploration has been small. I'm sure I don't have to point out the least attractive combination.

I hope by now it is clear that you don't just go out and "explore." A proper approach requires the identification of "what commodity or mineral" is to be sought and then select the most attractive situations wherein to look. Serendipitous discoveries can be made — and the good explorationist must always be alert for these — but this should not constitute the dominate modus operandi.

The Basic Exploration Approach

1. Select the mineral or minerals of major interest.

2. Outline those areas and regions that are geologically favorable:

(a) with known deposits;

(b) with no known (significant) occurrences but have been little explored;

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(c) like (b) but having undergone varying amounts of prior exploration.

Notice that a new concept how a type of ore deposit may form can open up new regions not previously considered worthy of exploration.

3. Assess and eliminate localities selected in (2) above on other criteria:

(a) political and governmental attitudes and limitations;

(b) market area, remoteness and/or degree of infrastructure, and other commercial factors;

(c) financial and investment conditions.

4. Determine the extent of prior exploration:

(a) has the intensity and sophistication of prior exploration groups been sufficient that the residual discovery potential has passed the point of diminishing returns? Exploration is a confidential activity and it is a very difficult practical problem to find out what exploration techniques have been used, and by whom in a particular area.

(b) has a new technique been developed that gives substantial promise of detecting ore bodies that were unrecognizable by earlier methods?

5. Employ a field staff with broad general experience but also familiar with the particular geologic environment involved, including as much personal knowledge as possible about the subtleties of the previously discovered deposits. The staff should have the seasoned imagination to envision new variations on the basic mineralization theme, plus the courage to recommend spending large sums of money testing exploration targets, knowing that the odds of success, at any one site, are 100 to 1 or more in favor of failure. Ideally, the staff should understand the local social environment, and the customs, traditions and sensitivities within which they must operate. Those people who have previously been involved in a successful discovery are apt to be involved in other discoveries — probably because such people have the right combination of experience and personality traits.

All five of the above strategic considerations are highly subjective. What is an attractive commodity to seek? How onerous are the laws or political attitudes? Will they last? How thorough and competent was that earlier exploration? Does the new theory of ore genesis make enough sense to justify a major effort in men, time, and money to test it? Do we know enough about the geologic

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environment of a proposed area to say it is permissive? Is the field man realistic in his imagination and projections; does he possess the needed courage; does he have the physical endurance and motivation to climb the steep mountain, covered with thick underbrush to observe the critical rock exposures?

Because of all these variables, the exploration programs of different companies vary significantly with quite dissimilar results.

Earlier, I mentioned that some areas can be so intensively explored that the attraction for further search is low. While I admit the basic rationale, there is another point-of-view worth mentioning. Previous exploration may enhance the chance of success for those that follow. This is particularly true as we search ever deeper in the earth as deposits at the surface are recognized and put into production. With all of our most advanced geophysical and geochemical surveys and geological studies, we still know very little about the details of what's buried down there, especially as we search progressively deeper. We get clues and ideas from drill holes and geophysics and from these postulate what most likely resembles a good loci for ore. Study of the drill cores as they are pulled up to the surface probably will continually change the interpretation of the details of rock types and distribution. More holes are drilled. Sooner or later, barring a discovery, discouragement sets in. A new ore possibility somewhere else captures the imagination and the first site is abandoned. Meanwhile, this first project area is far from completely tested. Each drill hole measures only a few centimeters across and a couple of hundred meters long — really far from an exhaustive probe. A new group arrives on this scene and sees possibilities not given such credence by the earlier searchers. Enthusiasm of the second group mounts as it reviews all the information available. New concepts, biases, and philosophy are introduced. If these fail, the new group, too, will leave. And so it goes until, perhaps, the...