The Treasure of the Sierra Cobre — How Do We Find Copper Deposits?

Introduction

Copper is one of society’s most important metals, being used in wiring and plumbing in homes and factories, phone lines, automobiles, airplanes, computers, coins, and as alloys, such as bronze and brass. Although recycling helps reduce our consumption of "new" copper, we need to find additional deposits of copper to sustain our technological advances, our high standard of living, and anticipated worldwide increase in population. Where will this new copper come from? How do copper deposits form, and where should we look for new copper deposits? Do all areas of the Earth have a similar potential for copper deposits? Alternatively, does the formation of a major copper deposit require a certain set of unusual geologic circumstances?

In the classic movie Treasure of the Sierra Madre, prospectors explore a rugged part of Mexico looking for gold and silver. A modern-day boom of mineral exploration by both large and small international companies is again going on today in Mexico and the rest of Latin America, thanks largely to changes in the political and economic climate of these regions. Much of this exploration is directed toward finding large deposits of copper, an essential commodity in today’s post-industrial world. So how does one go about finding a copper deposit? If you were the manager of a mutual fund or the loan officer for a bank, would you invest in or loan money for mineral exploration? To get a feel for the process of mineral exploration — and the associated financial risks and rewards — you will work in small teams to explore for copper deposits in Sierra Cobre, a virtual exploration area. Your team begins with some limited initial information and has a limited amount of virtual dinero (Spanish for money) to buy other types of information, such as a geologic map or exploration drill holes.

Objectives

To use your limited financial resources to find a copper deposit. To do this your team will need to:

If it all works out, you’ll be rich and a superstar within your company.

Procedure

Each team starts only with a computer-generated 3D-perspective of the virtual landscape, some background information about how copper deposits form, and $200,000 in virtual dinero. Your team will decide what additional information to purchase. Available additional information (and prices in parentheses) includes the following:

For each exploration hole your team drills, you will receive a log of rock types encountered and the extent of copper mineralization. You will be provided with the overall type of ore (high, medium, or low grade) or the actual copper content of the rocks encountered in the drill hole. In geology classes linked with a chemistry class, you will receive the copper content from a chemistry student who will do the actual analysis. You should use the drill holes to search for copper mineralization and to document whether copper mineralization in any area is big enough and rich enough to be able to mine at a profit (rock that can be mined at a profit is called ore). Modern large copper deposits typically contain 0.2 to 1.0 % copper. In this exercise, ore is categorized as low-grade (0.1 to 0.3% copper), medium-grade (0.3 to 0.5% copper), and high-grade (0.5 to >1.0 % copper). Your team receives points for each drill hole with ore, according to the following: high grade – 4 points, medium grade – 2 points, and low grade – 1 point. Ten points in contiguous squares (must be touching on a side, not just corner) are needed to define an economic mine. Some drill holes also contain deep medium-grade ore, worth one bonus point if you already have ten points from your shallow ore.

Anything smaller than 10 points will not be economic to mine, considering the remote location of the area and the high start-up costs (there are no paved roads, electrical lines, or other types of infrastructure). The team that identifies the largest and highest grade copper deposit will be most likely to survive in these harsh economic times. You should try to identify the copper deposit with your existing supply of money. In an emergency, the loan officer for the World Bank (your instructor) may loan additional money if you can convince them that the next drill hole will hit ore. Good luck and may the force be with you!

Background Information

Geology of Copper Deposits

Most of the world’s large copper deposits are associated with a distinctive type of granitic rock composed of larger crystals in a fine-grained matrix. This type of rock is called a porphyry, and the texture is called porphyritic. This type of rock forms from magma that begins to crystallize at depth (to form the visible crystals) but then moves up closer to the surface where the remaining magma around the crystals cools quickly, forming the fine-grained matrix. The associated copper deposits are termed porphyry copper deposits and generally are mined in large open pits that can be several kilometers in diameter and hundreds of meters deep.

The best way to understand how such deposits form is to look at ones that formed very recently, say within the last several million years. Deposits this young are present in the subduction-related volcanic arcs of the Indonesia – New Guinea region of the southwest Pacific. Here, the copper deposits are present beneath big volcanoes that are composed of interlayered lava and ash (commonly of granitic to intermediate composition). This type of volcano is termed a stratovolcano, and includes Mt St. Helens in Washington, Mt. Fuji in Japan, and Mt. Pinatubo in the Philippines. The porphyry deposit forms from hot fluids that circulate around the magma chamber, deep within the volcano (Figure 1a). The best ores are commonly formed right above and next to the porphyry intrusion, so the top of such systems are more favorable places to look than the deeper levels. Richer ore is also commonly found in rocks, such as limestone, that reacted strongly with the hot fluids, causing high-grade copper to be deposited in a narrow zone (Figure 1a).

With time, the volcanic rocks commonly are eroded away and the copper deposit is exposed (Figure 1b). When this happens, the copper is weathered and leached from rocks near the surface and moved by rainwater to depth, where it is redeposited at the water table. This process may increase, or enrich, the copper content of the rock at depth, and is called secondary enrichment. Commonly, a deposit may only be economically viable if it has experienced such enrichment. If the deposit continues to be weathered and eroded, however, it may be totally eroded away. The deposit and enriched zone may be preserved from such erosion if they are covered by a younger rock layer (Figure 1c).

Any exposed copper deposits, such as those marked by abundant blue-green copper minerals, have been found long ago, so modern mineral exploration focuses on deposits that are just beneath the surface. Companies have developed many techniques to look for shallowly buried deposits. Even if no copper minerals are visible, rocks above a copper deposit and plants whose roots are in copper-rich rocks may contain unusually high concentrations of copper. Geologists commonly collect rock or plant samples and send them off for geochemical analyses. They also look for the kinds of volcanic rocks that form over the top of porphyry deposits or for evidence that hot fluids have passed through the volcanic rocks. They look for small knobs or outcrops of porphyry that may connect at depth into a bigger mass. Some deposits have been found by magnetic surveys because rocks around the deposits are unusually magnetic or unusually nonmagnetic. Another exploration technique involves running electrical current through the ground and finding areas where the rocks contain highly conductive copper minerals.

 

Philosophies of Mineral Exploration

Mineral exploration is a beautiful microcosm of how geological exploration and field-based research take place. The exploration geologist begins with some background information about how the type of mineral deposit being sought is formed. The geologist decides what the necessary geologic setting is for this type of deposit (e.g., near a volcano formed by a subduction zone) and where in the world such a setting has existed in the past. The exploration geologist gathers any existing information and materials about the area, such as topographic maps, aerial photographs, and geologic reports. From these materials, the geologist may narrow the search to one or more areas. Then it’s time to "go to the field" and do a reconnaissance of the area, noting the kinds of rocks present and their inferred subsurface extent, observing any evidence of mineralization, and beginning to reconstruct the geologic history of the area, and consider any implications the geologic history has for the formation and preservation of mineral deposits.

If favorable rocks or mineralization are encountered, the exploration geologist will collect samples and send them off to a commercial geochemical lab to be analyzed. If the signs continue to be favorable, the exploration company may contract for other types of surveys, such as electrical and magnetic surveys. By this time, the company will probably have staked mining claims or otherwise acquired the exploration rights for the area of interest.

If a potential target is identified, the company will carefully decide where to begin drilling exploration holes, which are very expensive. As each hole is being drilled, a geologist will "sit the rig", observing the drill cuttings or drill core recovered from the hole and making detailed notes on (1) the kinds of rocks encountered and their depths, (2) any geologic structures, such as fractures that might have permitted mineralizing fluids to pass through the rocks, and (3) the presence of mineralization or alteration (minerals, such as quartz and mica, that might record the passage of hot fluids through the rocks). Samples of the cuttings or core are sent off for chemical analyses and possibly microscopic analysis. Geologists interpret the results of the chemical analyses to determine, commonly using computers, whether the mineralized area is big and rich enough to mine. Mineral economists and mining engineers enter the picture at this point to try to factor in all the variables, such as the projected costs of mining. Metallurgists study the ores to determine how best to recover the target commodity (copper or whatever) via the most cost effective manner. The final decision is based on economics and hopefully a good understanding of the subsurface geology (mining engineers do not like big geologic surprises). Most exploration and drilling programs do not find a minable deposit; this is one reason why mineral exploration is a very expensive and rather risky venture.

Economics of Copper Deposits

A large porphyry copper deposit may contain more than a billion tons of ore (a ton of rock is approximately a cubic meter — a cube that is one meter across on each side). The largest mines may move more than one million tons of rock each day! Based on a quick "back-of-the-envelope" calculation, such a mine may produce copper worth 13 million dollars per day! The figures used to derive this estimate are as follows: one million tons ore mined per day, 2,000 pounds per ton, average ore grade of 0.5% copper, and price of copper $1.30 per pound. The simple equation looks like this:

That’s a lot of money! Of course, most of this earned income goes to pay for the mine’s energy bill, water costs, equipment purchase and maintenance (big shovels and trucks each cost millions to ten’s of millions of dollars), and salaries of mine personnel (miners, truck drivers, engineers, electricians, accountants, security guards, safety officers, and of course geologists). It also costs lots to build the mill, transportation facilities (roads or trains to get the copper out), and other infrastructure (office buildings, water wells, or even a power plant). If the mineral company borrowed bank or stockholder money, then it will have to pay this back along with interest or dividends — because of this, the company wants to get the mine going right away to pay these debts off as soon as possible (the so-called "time value of money"). Companies also put money away to handle the environmental clean up when the project is finished (in many countries, the company would be required to post a huge bond to ensure that this clean-up money will be there).

Whether a copper deposit is economic depends on many factors, including the following:

Stephen J. Reynolds, Dept. of Geology, Arizona State University