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NATIONAL MINERAL-RESOURCE ASSESSMENT:

USGS Estimate of Undiscovered Gold, Silver, Copper, Lead, and Zinc Remaining in the United States

Introduction

Why a Mineral-Resources Assessment Is Needed

     The U.S. Geological Survey (USGS) conducted a scientific study to estimate, for the first time, the amount of undiscovered gold, silver, copper, lead, and zinc that could be present in yet-to-be-discovered mineral deposits 1 kilometer or less below the surface of the ground of the conterminous United States. This estimate is based on the best information and theory available to the USGS and is the most thorough, well-documented assessment of undiscovered deposits of gold, silver, copper, lead, and zinc ever assembled at this scale. Table 1 summarizes data on U.S. and world resources of gold, silver, copper, lead, and zinc. 

     In Table 1, the estimated undiscovered U.S. resources can be described as the estimated amount of metal expected or most likely to occur in the undiscovered deposits. Statisticians define the values as "the mean of a probability distribution." To estimate undiscovered mineral resources, earth-scientists actually begin by analyzing their data and then estimating the number of undiscovered mineral deposits of a specific geologic type that they believe occur in an area. They make this estimate at several levels of certainty. For example, they may estimate from the geologic data that there is a 90-percent chance that the area has at least 1 or more undiscovered deposits of a specific type; a 50-percent chance of 3 or more deposits; and a 10-percent chance of 5 or more. They also know the amounts of metal contained in both large and small deposits of this type previously discovered in the area, nearby, and often around the world. With statistical methods and with the aid of a computer, scientists can combine their estimates with the information on the metal contents of discovered deposits to predict the probable amounts of metal expected to occur in undiscovered deposits in the study area.

 

Table 1. U.S. and world resources of gold, silver, copper, lead, and zinc
	Gold (tons)	Silver (tons)	Copper (kilotons)	Lead (kilotons)	Zinc (kilotons)
Estimated undiscovered U.S. resources	14,000	430,000	280,000	55,000	150,000
Discovered U.S. resources1	22,000	400,000	320,000	75,000	140,000
Total U.S. resources	36,000	830,000	600,000	130,000	290,000
Mineable world resources, excluding the United States2	61,000	390,000	570,000	110,000	310,000

1Estimates of discovered resources based on Singer, D.A., 1995, World class base and precious metal deposits - A quantitative analysis: Economic Geology, v. 90, p. 88-104.

2Same as the estimates of the Total, Reserve base, World total (excluding the United States) based on U.S. Geological Survey, 1996, Mineral Commodity Summaries 1996, 195 p.

How the Assessment Was Done

     The assessment was based on mineral-deposit models. These models are sets of data that describe a group of deposits that have similar geologic characteristics, form in similar geologic settings, and have distinctive sizes and distinctive amounts of metal. Earth scientists begin an assessment by outlining large regions having geologic characteristics and settings broadly similar to those of one or more mineral-deposit models. 

     Subsequently, they examine the geology of the region in detail in order to outline much smaller "tracts" of land having geologic features that more closely match those of the models (see example map below). These tracts are considered to be permissive for the occurrence of a specific type of mineral deposit. A total of 46 mineral-deposit models were used to delineate 12 regions (fig. 1, table 2) and 281 permissive tracts (fig.2) in the conterminous United States. These models fall into 5 general types of mineral deposits called vein, porphyry, replacement, stratiform-massive, and stratiform-disseminated deposits. Some of the deposit types contain all five metals, some contain only one or two, and some of these metals are present in only small amounts (table 3).

Figure 1. Map of the conterminous United States showing the 12 National mineral-resource assessment regions.

 

Figure 2. Example permissive tract map - Kuroko-type massive sulfide deposit.

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     At today's prices, the gross value of gold, silver, copper, lead, and zinc in undiscovered deposits is estimated to be $1.2 trillion. Included in this gross value are such things as the value of the metals; cost of salaries paid for labor to find the metals and to build and operate the facilities to extract and refine them; and the returns (profits) to investors in metals exploration and development. This can be compared with the gross value at today's prices of the discovered (past production plus remaining reserves) gold, silver, copper, lead, and zinc, which is calculated to be $1.4 trillion. Thus, for conventional-type deposits in the conterminous 48 States, about as much is left to be discovered as has already been discovered, excluding discovered materials that have not been produced and are not currently classified as reserves. The gross value of the five metals in undiscovered deposits by general mineral-deposit type is shown in figure 3. This assessment marks the beginning of an ongoing assessment by the USGS of all mineral resources in the United States.

Figure 3. Gross value, in U.S. dollars, of the five general deposit types.

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