Besides what we are pleased to call the riches of a mine are riches relative to a distinction which nature does not recognize. The spars and veinstones which are thrown out in the m b h h of our mines may be as precious in the eyes of nature, as conducive to the great object of her e c m y , and are certainly as characteristic of mineral veins as th+ ores of silver or gold to which we attach so great a value.

John Playfair, 1802

Ores are rocks or minerals that can be mined, processed, and delivered to the marketplace or to technology at a profit. Ores are generally subdi- vided into the categories of metallic, nonmetallic, energy, and water. In many quarters, me refers only to metals or metal-bearing minerals, but many nonmetallic minerals, such as sulfur and fluorite, are included in mod- ern common usage of the term. Building stone and industrial materials, such as abrasives, clays, refractory materials, lightweight aggregates, and salts, are also ores, but they are classified separately as nonmetallics, in- dustrial minerals and rocks, or ZM and R's (Figure 1-1). Ore minerals are considered to be naturally occurring compounds valued for their metal con- tent, so further processing after mining-generally including concentration (extractive metallurgy), smelting, and rehing-is implied. Industrial min-

-1

INTRODUCTION

Figure 1-1. A rock mass that is both an ore and an orebody, since it can be mined from this quarry for use as a building stone at a profit. Sheeting joints parallel to the surface coupled with two nearly vertical joint sets visible in the photo permit easy separation of useful-sized blocks from the Crotch Island granite quarry, Hancock County, Maine. (Photo by E. S. Bastin.)

erals, such as salt, garnet, or asbestos, may need some upgrading with respect to associated worthless materials, but they are all used for their own specific physical or chemical properties rather than for anything they contain. They are not "broken down" to be useful. The term economic mineral is applied to both ore minerals and industrial minerals.

Not all minerals containing a given element need be classified as ore minerals. For example, most iron silicates, such as biotite and fayalite, are not mined for their contained iron and therefore are not ore minerals, while hematite and magnetite are. However, any mineral of a precious metal is probably an ore mineral. Depending on geologic circumstances, an ore may be a rock containing veinlets, disseminations, or small amounts of useful minerals, or it may be massive, essentially solid metal sulfide or oxide. Although both metallic and nonmetallic minerals are widely distributed in the rocks of the Earth's crust, only under exceptional circumstances are they concentrated in orebodies in amounts sufficient to permit economic recovery and in a form that permits that recovery. Most ore minerals are associated with valueless material called gangue, and many ores grade lat- erally or downward into protore-mineralized rock that is too lean in ore minerals to yield a profit. As Playfair stated so well in 1802, the economic value of an ore mineral does not set it apart genetically from the worthless pyrite, sericite, calcite, or other gangue mineral or rock with which it is associated and which is unavoidably mined with it (Figure 1-2). The study

2

INTRODUCTION

Figure 1-2. Vast Bingham Canyon open-pit mine near Salt Lake City, Utah. Each lower bench is 16 meters high; the upper ones are 25 meters. The dust cloud is from an explosive charge set to shatter the ore, which is taken out in railway gondola cars. The deeper rock is ore because it contains about 2 wt % disseminated sulfides of copper, molybdenum, and iron, with some gold and silver. But that means that about 99% of the rock, 99% of the volume of the hole, is gangue and is discarded into waste piles elsewhere. (Courtesy of Kennecott Minerals Company.) .

of ore deposits thus becomes a specialized part of the broader field of pe- trology-petrography. Continued recognition of the fact that ore minerals and gangue minerals are normally "part and parcel" of the rocks that contain them has given rise to a new perception of the subdiscipline called economic petrology, which brings the tools of the petrologist-thin-section petrog- raphy, polished surfaces for electron microprobe and optical mineragraphic study, physical geochemistry, and mineralogy-more and more to bear on economic geology's problems. I t is thus a premise of this book that mineral concentrations--ore deposits where profitability of extraction is real-are generally normal, interpretable extensions of the host rocks that contain them.

3 -

INTRODUCTION

MINERAL RESOURCE PROBLEMS

Many demographic and economic influences combine to dictate the prices of metals, and hence the level of activity of exploration, mine development, and mining itself. As this chapter was written, in July 1983, industrial activity was near historically low levels, and the doldrums in metal-consum- ing sectors such as automobile manufacture and commercial-residential con- struction had dragged metal prices to all-time inflation-adjusted lows. Three years earlier, however, base- and precious-metal pricesawere soaring a t the highest relative levels in decades. Clearly, metal prices are in general cyclic. Most metals are produced by so many diverse countries and cultures that ca r t e l sand thus price management by the producershave succeeded only rarely and briefly. Although prices are currently low on world markets, and although use patterns are changing, prices will doubtlessly be pushed up in the 1980s by increasing needs and consumption in underdeveloped nations, by overall global population growth, and by the historic human tendency to translate success and economic well-being into material con- sumption. The United States finds itself having to compete more assidu- ously each year with other countries for raw materialebauxite and chro- mite, for example-that used to be routed to American industrial centers, but which now go to Japan and eastern and western Europe.

Ultimately and progressively, the world will have to adapt to short- ages brought about by the consumption of its nonrenewable natural re- sources. Economics, politics, human needs and priorities, and geologic char- acteristics will all determine the geography and volume of mineral production, as they do now. However, many American readers will be surprised to learn that although the United States markets prodigious amounts of copper from a scattering of huge open-pit mines in its western states (Figure I-2), it is a net importer of copper. Copper can be produced more cheaply and shipped to eastern ports more economically from Chile, for example, than from southern Arizona. Although the authors intend a global emphasis in this text, some figures pertaining to the United States demand in global perspective are given in Tables-1-1 to 1-4. Americans should note that the United States is economically, that is, realistically, self-sufficient in only three of the thirty-five commodities listed-a scant 10 percent-lithium, phosphate, and molybdenum. In the 'principal world producers' column of Table 1-1, Canada is listed 20 times, the United States 17, the U.S.S.R. 10, South Africa 9, Mexico 7, and Brazil 5 times. The United States is so productive because of its endowment, because it has been the principal seat of twentieth century technologic and economic growth, and because of the international security that a strong domestic reserves position provides. But the United States dependence on foreign sources of supply of many crucial commodities is obvious and problematic, and the commercial-tech- nologic necessity of maintaining friendly trade relations with many nations is clear. More universally, the ultimate need for global-scale harmony, co-

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- - -

MINERAL RESOURCE PROBLEMS

operation, and international homogeneity of purpose must be the goal of all nations, governments, and peoples.

A concept frequently referred to in the pages that follow is the dis- tinction between orebody and ore deposit, mineral deposit, ore-mineral occurrence, or just ore occurrence. The first denotes the distribution in a specific volume of "the naturally occurring material from which a mineral or minerals of economic value can be extracted a t a reasonable profit" (AGI Glossary of Geology, 1980). It thus depends on geography, energy costs, type and degree of dilution, tenor (the grade or amount of a commodity actually present), depth in the crust, and many other variables. Mineral deposit carries no necessary profitability implications and usually denotes subeconomic or incompletely evaluated occurrences of ore minerals. Min- eral occurrences or ore-mineral occurrences are uneconomic but still anom- alous concentrations of minerals that are common ore minerals elsewhere. A distinction is also made between reserves and resources. Reserves include orebodies "in production," ores known by drilling or other specific measure- ment to exist, or ores reliably inferred to exist in a specific place (Figure 1-3). Resources include reserves and all other potentially viable mineral deposits that are either unknown or uneconomic at present but that can still reasonably be expected to exist. Note in Figure 1-3 that actual mining activity involves only the stippled area. The cross-ruled portion of "undis-

Total Resources I I Identified Undiscovered 1

Decreasing economic feasibility

- Increasing degree of T geologic assurance

Figure 1-3. Geologic-economic classification of mineral reserves and resources. (Adapted fromU.S.G.S.sources.)

TABLE 1-1. U.S. Consumption and Pmductlon and World Production, Resetves, and Sources of 35 Mineral Commodltles In Order of Increasing U.S. Oomestlc Self-Sufliclency.

Commodity

Newly Mined Material, 1982

U.S. U.S. Consumption, U.S. Consumption, Production, in Metric Tons Million $ %

Ratio of Reserves to Cumulative

Approximate Demand; World 1976-2000

Consum~tion.- . 1982, in U.S. Entire Metric Tons Alone World

Chromium

Manganese

Cobalt

Tantalum

Niobium Thorium

Tin

Platinum group

Antimony

Aluminum Asbestos

Fluorite

Nickel -

Gold

Princiud World producers, 1982 (Most Important First)

U.S.S.R., South Africa,

Finland, Zimbabwe

U.S.S.R., South Africa,

Gabon, Brazil

Zaire, Zambia, Japan, Canada

Malaysia, Thailand, Canada

Canada, Nigeria, Brazil Australia, Brazil, India,

Malaysia Malaysia, Bolivia, South

Africa South Africa, U.S.S.R.,

Zimbabwe P.R. China, Bolivia,

South Africa Jamaica, Australia U.S.S.R., Canada, South

Africa, Zimbabwe,P.R. China

Mexico, U.S.S.R., Mongolia, P.R. China

Canada, New Caledonia, U.S.S.R., Norway

South Africa, U.S.S.R., Canada

Bismuth Silver Tungsten Potassium

Cadmium Beryllium Zinc

Selenium Mercury

Titanium Lead Barite

Copper

Iron

Sulfur

Lanthanides

Uranium

Vanadium

phosphate rock

Lithium

Molybdenum

e = estimated; NA = not available.

Sources: U.S. Bureau of Mines via National Research Council, 1981, and Engineering and Mining J o u m l , March 1983.

Mexico, Peru, Canada Mexico, Peru, Canada Canada, Bolivia, U.S. U.S.S.R., Canada,

Israel, U.S. Canada, Mexico, U.S. P.R. China, Brazil, U.S. Canada, Mexico, western

Europe Canada, Japan, U.S. U.S.S. R., Spain, Italy,

U.S. Australia, Canada, U.S. U.S., Canada, Mexico U.S., P.R. China, Peru,

India Chile, U.S., Canada,

Zambia, Zaire U.S., Canada, Australia,

Brazil U.S., Canada, France,

Mexico U.S., Australia,

Malaysia, P.R. China Canada, South Africa,

Australia South Africa, U.S.,

Finland, P.R. China US., Morocco, U.S.S. R.,

P.R. Chiia U.S., Brazil, Namibia,

Zimbabwe U.S., Chile, Canada

Table 1-2. Gmloglc octurrencs and sonnes of 33 mlnenl commodltles by broad genetlc class.

Chromium ManganeseCobalt Tantalum Niobium

Thorium Tin Platinum p u p Antimony Aluminum

Asbeatoa Fluorine Niel Gold B i u t h

Silver TungstenPotPsaium Cadmium Beryllium

Ziae Selenium M.m a t o n i ~ m

Lead

Barite copperIron SuWlr Lanthanides 000

URnium V a n a d i i 0.

Phosphate rock Lithium Molybdenum

Each circle represents 26% of mual world production. Open circles = U.S. sourcea; solid circles = non-U.S. production (see right-hand column of Table 1-1). Elements are annnged from top to bottom in order of inamaing U.S.self-suffleiency. "Secretionn include8 Missiseippi Valley type; "Intermediate Felsic" includes porphyries.

Source: In part h m National Re& Council, 1981.

Table 1-2. (continued)

Commodity

Chromium Manganese Cobdt Tantalum Niobium

Thorium Tin Platinum group Antimony Aluminum

Asbestos Fluorine Nickel Gold Bismuth

Silver Tungsten Potassium Cadmium Beryllium

Zinc Selenium Mercury Titanium

Lead

Barite copper Iron Sulfur Lanthanides

Uranium Vanadium

Phosphate rock Lithium Molybdenum

Chemical Mechanical Secretion Sedimentation Sedimentation Weathering (Chapters 19 (Chapter 15) (Chapter 16) (Chapter 17) and 20)

M Jor Additional Potential Sources

Laterites Seafloor nodules Seafloor nodules

Aluminum-rich sedimen- tary and igneous rocks

Phosphate by-product S e h r.nodules

Nonmarine brines

Greisem, skarns

Zinc-silicate laterite Coal by-product

By-product of copper-moly- porphyries

Alkalic igneous complexes Seafloor nodules

Petroleum, cod, gypsum Phosphate by-product

Granites Vanadium-rich shales, oil by-

product, layered mafic in- trusions

Lithium clays, brines

INTRODUCTION

TABLE 1-3. u.$. Rependence on Forelgn Sources for Some of Its Mlnerals in 1980.

Less than half imported from foreign sources, so more than 50% self-sufficient Copper Tellurium Iron Stone Titanium (ilrnenjte) Cement Lead Salt Silicon Gypsum Magnesium Barite Molybdenum Rare earths (lanthanides) Vanadium Pumice Antimopy

One-half to three fourths imparkd j h m foreign s m e s Zinc Nickel Gold Cadmium Silver Selenium Tungsten Potassium

Threefourths to 90% imported from fweign sources Aluminum

Platinum

Tin

Tantalum

More than 90% imported from foreign sources Manganese Cobalt Chromium Titanium (rutile)

S o m e : Adapted from U.S.G.S. sources.

Bismuth Fluorine Asbestos Mercury

Niobium Strontium Sheet mica

covered resources" should be proportionally much larger, and is in large part what this book is all about.

'THE ROLE OF ECONOMIC GEClLOGY

Although the understanding of ore-deposit genesis and occurrence has led to tremendous exploration success in the 1970s in porphyry copper, Climax-type molybdenum, volcanogenic copper-zinc-lead, and unconformity-type uranium deposits, to name but a few, projected consumption rates of metals indicate a secure world reserves position extending into the twenty-first century only for iron. In spite of cyclic supply-and-demand-price vicissi- tudes, it is clear that corporate and government exploration activity must and will continue and that geologists must be prepared to be increasingly capable, professional, and diligent in their search.

The accelerating growth of the world's population, combined with an improving standard of living throughout the world, is greatly increasing demands for mineral products of all types. These demands will certainly

4!l

THE ROLE OF ECONOMIC GEOLOGY

TABLE 1-4. General Outlook for Wood Reserves end Resources throu()h fOOO A.O. Wlthin Each Group, Commodltles Are Llsted Id Order of Apflroximate Importance as DeterPlned by Dollar Value of Production.

Group 1-Reserves adequate to fill needs well beyond the year 2000. Coal Phosphorus Construction stone 8ilicon Sand and gravel Molybdenum Nitrogen Uranium Chlorine Gypsum Hydrogen Bromine Titanium (except Boron

rutile) Argon

Soda Diatomite

Calcium Barite

Clays Lightweight aggregates

Potash Helium

Magnesium Peat

Oxygen Lithiunl

Group %Identified, but currently subeconomic resources adequate to liU needs beyond the year 2000.

Aluminum Vanadium Asbestos Zirconium (as zircon) Nickel Thorium Chromium Titanium (as rutile) Manganese Rare earths (lanthanides)

Group %Estimated undiscovered resources adequate to fill projected needs beyond the year 2000 and in quantities significantly greater than those of group 2. Research efforts for these commodities should concentrate on geologic theory and exploration methods aimed at discovering new resources.

Iron Platinum

Copper Tungsten

Zinc Beellium

Gold Cobalt

Lead Cadtnium

Sullur Bismuth

Silver Selenium

Fluorine Niobium

Group PIden t f ied subeconomic and undiscovered resources, together probably not enough to fill needs until the end of the century. Research on possible new exploration targets, new types of deposits, and substitutes is necessary.

Tin . Mercury ,

Antimony Tantalum

Sacme: Adapted from U.S.G.S. sources.

continue to grow. At the same time, the search for ore is becoming more complex; more and more, ore is being sought under cover and at greater and greater depths. In order to obtain sufficient supplies in the future, new geologic, geochemical, and geophysical exploration ideas and techniques must be devised to supplement the old. Recovery, recycling, and mining techniques need to be improved so that large bodies of near-surface min- erals that are not now economic can be developed with due regard for

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ecological and environmental constraints. For these reasons, the successful economic geologist must develop "exploration thinking" requiring imagi- nation, ingenuity, and a degree of optimism, as well as a thorough knowl- edge of structural geology, stratigraphy, petrology, and mineralogy, and of how fluids migrate underground. Economic geologists should also be familiar with fundamental techniques in geophysics and geochemistry, as these fields are becoming increasingly useful in the search for buried de- posits. More importantly, they need to be increasingly able to interpret the field and laboratory significance of observable geologic relationships, and to bring geologic, geochemical, geophysical, mathematical, and computer skills to bear on them.

Under what conditions and as a result of what processes are ores formed? What factors lead to the concentration of certain elements in one environment and not in another? What causes the localization of ore? Prob- ably the best way to answer these questions, and hence the best way to search for new ore bodies, is to study the structuke and genesis of known mineral concentrations and then explore geologically favorable analogous areas. One of the precepts of this text is that although no two deposits are exactly alike, they share enough unifying characteristics that they can be grouped into exploration-genetic sets which have lithotectonic-geologic characteristics, and can therefore be successfully hunted and found.

Owing to the complex nature of the Earth, and because many of the processes involved in ore deposition cannot be observed, the study of ore deposits and ore genesis is not an exact science. We work primarily with Earth-scale reaction products, the final result of the interaction of complex geochemical systems of ages, even eons, past. Interpretation of the processes that produced the reaction products that resulted in ore-mineral concentra- tions is commensurately difficult. Although the study of ore deposits is basically a field science, this does not mean that precise laboratory and experimental data are not desirable or useful; it does mean that laboratory data should be viewed critically and used with wisdom and care. Laboratory data unsupported by field evidence frequently lead to erroneous conclu- sions, and the final test of all theories and hypotheses in geology is their applicability in the field. But certainly our ability to loop between field observation, geochemical-geophysical laboratory analysis, and process eval- uation-all supported by computation and data base management-is a growing and necessary part of the role of the economic geologist searching for, or studying the genesis of, ore deposits.