CHAPTER 4 CHARACTER AND OCCURRENCE OF COPPER, MOLYBDENUM, LEAD, ZINC, GOLD AND SILVER MINERAL DEPOSITS IN THE UNITED STATES

• Jurisdiction: United States

Mining Exploration Technology for Lawyers and Landmen
(Apr 1980)
CHAPTER 4
CHARACTER AND OCCURRENCE OF COPPER, MOLYBDENUM, LEAD, ZINC, GOLD AND SILVER MINERAL DEPOSITS IN THE UNITED STATES
William B. Wray, Jr.
Van Cott, Bagley, Cornwall & McCarthy 141 East First South
Salt Lake City, Utah 84111

I. Introduction

This paper is concerned with describing the geologic character and occurrence of deposits of certain important non-ferrous metals—copper, molybdenum, lead, zinc, gold and silver— in the United States. This is a vast topic, worthy of a lifetime (or more) of study and corresponding space to report the results of such study,¹ and accordingly this paper can do little more than attempt to survey the subject and provide an overview of some general facts and common characteristics which may be of value to the landman, attorney, or other non-geologist seeking to understand for professional purposes somewhat of the nature of domestic mineral deposits.

Covered in this paper are brief discussions of (i) the geochemistry, definition, and distribution of mineral deposits of the six subject metals, (ii) the classification of the mineral deposits, and (iii) the occurrence and general physical characteristics (size, shape, vertical extent, character and minerology of ores, attitudes of the mineralized zones, and other characteristics) of the mineral deposits. Not covered, as outside the scope of this paper, are such subjects as the general geologic framework of the mineral deposits, the methods of recognition of mineralization and associated rock alteration in the field and laboratory, the techniques of exploration and evaluation of mineral deposits, and the economics of development and production of mineral deposits.² I have endeavored here to emphasize those aspects of the mineral deposits which may be of greatest professional interest to the attorney or landman. The landman and the attorney who must deal with a mineral deposit often are primarily concerned with establishing or maintaining a land position sufficient to cover a part or all of the mineral deposit. In order to successfully carry out this function it is often helpful, if not necessary, to understand the geometry and extent—known, as well as possible—of the deposit on the surface as well as in the subsurface. The

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landman and the attorney also often are concerned with the other physical characteristics of the mineral deposit: its tonnage, grade, categories of ore reserves (proved or positive, probable, and possible or prospective³ ), mineralogy, deposit variability, mode of occurrence (disseminated, manto, vein, and so forth), and other characteristics. A knowledge of the physical characteristics of the mineral deposit can be relevant and useful when, for instance, purchase agreements and mining contracts are negotiated and drafted, permit applications for land use or other operations are prepared, environmental statements and reclamation plans are drafted, exploration news releases and securities offering circulars are prepared, and when questions arise concerning approaches to take in litigation. And an understanding involving the ownership or use of mineral deposits is essential at that most basic stage (where appropriate) of deciding whether to locate a lode or placer claim.

II. Mineral Deposits

The thin, outermost shell of the earth is referred to by geologists as the earth's crust. The term is usually defined as that part of the earth lying above a zone of discontinuity in seismic velocities known as the Mohorovicic discontinuity, said zone being generally held to mark the transition between rocks more or less representative of those widely found at the surface of the earth, and rocks of higher specific gravity, which are rarely found at the surface of the earth. The thickness of the crust varies, being thickest beneath mountainous areas. The average thickness of the continental crust is about 36.5 kilometers.⁴

The rocks making up the earth's crust, especially in the continental areas, are a complex assemblage of sedimentary, metamorphic and igneous types, intermingled by faulting and folding into a heterogeneous aggregation of widely varying local composition; however, there is probably a gradual change in average composition of the continental crust from grantic at and near the earth's surface to basaltic deep within the crust. The average composition of the continental crust is probably quite close to the average of igneous rocks determined by Clarke and Washington⁵ (Table 1 below):

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Table 1

Composition of the Average Igneous Rock, Principal

Rock-Forming Elements, as Oxides, in Weight Per Cent

SiO2	59.14%	Ne2O	3.84%
Al2O3	15.34	K2O	3.13
Fe2O3	3.08	H2O	1.15
FeO	3.80	TiO2	1.05
MgO	3.49	P2O5	0.30
CaO	5.08	MnO	0.12

The average composition given in Table 1 corresponds to no particular igneous rock type, but is intermediate in composition between the compositions of the common rock types granite and basalt. The composition in Table 1 corresponds to the following abundances on an element-by-element basis:⁶

Table 2

Approximate Average Amounts (by Weight) of the

Principal Rock-Forming Elements in Continental Crustal

Rocks, in Grams per Metric Ton⁷ (parts per million)

O	466,000 g/ton	K	25,900 g/ton
Si	277,200	Mg	20,900
Al	81,300	Ti	4,400
Fe	50,000	H	1,400
Ca	36,300	P	1,180
Na	28,300	Mn	1,000

Relative to the abundances of the principal rock-forming elements, the average crustal abundances of the elements concerned with here—copper, molybdenum, lead, zinc, gold and silver—are quite low. Table 3 gives the estimated average crustal abundances of these six metals.⁸

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Table 3

Estimated Average Amounts (by Weight) of Copper, Molybdenum,

Lead, Zinc, Gold and Silver in Continental Crustal Rocks, in

Grams per Metric Ton (parts per million)

Zn	81. g/ton	Mo	1.1 g/ton
Cu	50.	Ag	0.065
Pb	13.	Au	0.0035

Thus, for example, iron is about 617 times more abundant than zinc in the continental crust, and zinc is about 23,143 times more abundant than gold. Some average abundance ratios of geologic significance for the six elements here considered are given in Table 4.

Table 4

Average Abundance Ratios for Copper, Molybdenum, Lead,

Zinc, Gold and Silver in Continental Crustal Rocks

Zinc ratios	Copper ratios	Lead ratios	Silver ratios
Zn/Cu=1.6	Cu/Pb=3.8	Pb/Ag=200	Ag/Au=19
Zn/Pb=6.2	Cu/Mo=45
Zn/Ag=1,246	Cu/Ag=769
Zn/Au=23,143	Cu/Au=14,286

A mineral deposit of any or all of the six subject metals is a local accumulation or concentration by geologic processes of these metals within a body of rock to such a degree as to be of some scientific or economic interest. If the degree of accumulation or concentration is such as to permit the metal or metals to be extracted at a profit, then the mineral deposit is said to constitute an ore deposit of the metal or metals. The degree of concentration necessary to produce an ore deposit of a given metal varies from metal to metal and from deposit to deposit, the key factors being the extent to which society desires the metal and the geological character (size of deposit, depth of burial, ease of mining, expense of extraction of the metal from the ore, and other aspects) of the mineral deposit. Other factors include for instance environmental constraints or restrictions on mining and/or processing, and restrictions caused by adverse ownership

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situations. While every deposit is unique, it is possible to indicate approximate concentration factors, or "concentration clarkes,"⁹ applicable to certain deposits containing the six subject metals, as shown in Table 5.¹⁰

Table 5

Approximate Concentration Clarkes and Minimum Average

Grades Necessary to Support Profitable Mining Operations

for Deposits Consisting of Only One Recoverable Metal

Metal	Approximate Concentration Clarkes Necessary to Support Profitable Mining Operations	Corresponding Approximate Minimum Average Metal Contents (Grades)	Deposit; Type (infra)
Zinc	494	4%	#24, #27
Copper	80	0.4%	#1
200	1%	#2, #3
Lead	3077	4%	#24
Molybdenum	727	0.08% Mo*	#11
Silver	2870	6 oz/ton**	#17
Gold	89	0.01 oz/ton***	#22
444	0.05 oz/ton	#21
1777	0.2 oz/ton	#20

For deposits containing more than one recoverable metal, and there are many in that category, the concentration clarkes and minimum average grades necessary to support profitable mining operations as indicated in the above table would be less. For instance, molybdenum can be an important byproduct of copper production in porphyry copper deposits at grades as low as 0.01% Mo, corresponding to a concentration clarke of 91.

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The concentration clarke values given in Table 5 are in a very general way indicators of the relative abundances of economic deposits—the higher the number values, the more "extraordinary" the deposits must be in terms of their grade relative to the average grade of the continental crust, and hence the rarer (and/or smaller) the deposits must be. Although many other factors enter into the abundance and distribution of mineral deposits, it can be observed that there seems to be, for instance, a far greater tonnage, if not number, of "economic-grade" copper deposits in the United States than "economic-grade" silver deposits. Gold is by any measure a very rare metal in the earth's crust, and yet a relatively modest degree of enrichment in stream or beach gravels may produce a mineable deposit.

The relationship between tonnage (size) of mineral deposits and the grade (metal content) of the deposits has been determined to closely approximate an exponential relationship: as the grade declines, the tonnage or volume of the mineral deposits averaging such a grade increases exponentially.¹¹ The relationship is illustrated below in two possible variants:

Figure 1

Illustration of Exponential Grade-Tonnage

Relationship in Two...