Introduction to Geology and Resources of 

Gold, and Geochemistry of Gold

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Chapter A

Introduction to Geology and Resources of 

Gold, and Geochemistry of Gold

The Case for Gold An Introduction to Geology 

and Resources of Gold in the United States

By DANIEL R. SHAWE

Geochemistry of Gold in Hydrothermal Deposits 

By SAMUEL B. ROMBERGER

U.S. GEOLOGICAL SURVEY BULLETIN 1857

GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES

DANIEL R. SHAWE and ROGER P. ASHLEY, Scientific Editors 

LM.H. CARTER, Technical Editor
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988

First printing 1988 

Second printing 1991

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Geology and resources of gold in the United States.

(U.S. Geological Survey bulletin : 1857)

Supt. of Docs. no. : I 19.3:1857-A

Bibliography: p.

Contents: ch. A. Introduction to geology and resources of gold, and 

geochemistry of gold.

1. Gold ores United States. 2. Geochemistry United States. 

I. Shawe, Daniel R., 1925- . II. Ashley, Roger P. (Roger Parkmand), 

1940- . III. Carter, LM.H. IV. Series.

QE75.B9 no. 1857, etc. 557.3s [553.4'1'0973] 88-600238 

[QE390.2.G65]
CONTENTS

The case for gold An introduction to geology and resources 

of gold in the United States

Abstract Al

Introduction A2

Acknowledgments A2

Gold as a store of wealth the uses of gold A2

How much gold? A2

Factors influencing the price and production of gold A3

The role of gold in world monetary systems A6

Where gold comes from A6

Scope of this series, and conventions used A7

References cited A8

Geochemistry of gold in hydrothermal deposits

Abstract A9

Introduction A9

Nature and source of solutions A9

Sources for gold A12

Mechanisms of gold transport and deposition A15

References cited A22

FIGURES

Al. Chart showing gold production, United States consumer price index, and

price of gold, 1845-1987 A4

A2. Schematic diagram of hydrothermal mineralizing environment A10 

A3. Schematic diagram of generation of a mineralizing solution A12 

A4. Schematic diagram of processes responsible for releasing gold to leachable

sites in a source rock A12 

A5. Periodic table of the elements illustrating relationship between gold, silver,

and copper A13 

A6. Graph calculating volume of source rock required to form gold deposits

containing various amounts of gold A15 

A7. Eh-pH diagram for Au-Cl-H2O system at 25 °C and aqueous chloride

concentration of 20 ppm A16 

A8. Oxygen activity-pH diagram for the system Au-NaCl-S-H2O: relative

solubilities of gold chloride and bisulfide complexes A17

Contents III
A9. Oxygen activity-pH diagram for the system Au-Fe-NaCl-S-H2O: relative

stabilities of iron minerals and gold A18 

A10. Oxygen activity-sulfur activity diagram for the system Au-Fe-NaCl-S-H2O:

relative stabilities of iron minerals and gold A19 

All. Oxygen activity-pH diagram for the system Au-Fe-CaO-K2O-Al2O3-SiO2-

NaCl-C-S-H2 O: relative stabilities of aluminosilicate alteration minerals and

carbon, and relative stabilities of iron minerals and gold A19 

A12. Oxygen activity-sulfur activity diagram for the system Au-Fe-As-NaCl-C-S-H2O:

relative stabilities of arsenic and carbon phases and relationships of

figure A10 A20 

A13. Oxygen activity-sulfur activity diagram for the system Au-Fe-As-NaCl-S-H2O:

relative stability field of arsenopyrite, and relationships of

figure A10 A21 

A14. Gold solubility versus oxygen activity diagram showing solution path during

oxidation A22

TABLES

Al. Temperatures of formation and salinities for selected epithermal precious

metal deposits All

A2. Summary of gold contents for selected rock types A14 

A3. Partial list of aqueous gold species occurring in natural systems A16

IV Contents
GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES

Introduction to Geology and Resources of Gold, and 

Geochemistry of Gold

The Case for Gold An Introduction to Geology and Resources of 

Gold in the United States

By Daniel R. Shawe

Abstract

Gold has long been a store of wealth, whether held by 

governments or in private hoards. Of the total of about 3 

billion troy ounces of gold that has been mined worldwide 

through man's history, about one-third is held by central 

banks, another third is held privately, and the last third is 

fabricated in jewelry (also a store of wealth) and in forms 

useful in industries such as electronics and dentistry.

Since the British Gold Standard was established in 

1717, the price of gold generally has been fixed officially, and 

thus it has tended to remain stable. The U.S. Congress in 

1792 adopted a gold standard of $19.39 per troy ounce, 

which lasted to 1834 when the official price was set at $20.67. 

In 1934 the official U.S. price (effectively, the world price) was 

raised to $35 per ounce. At times during this long period of 

stable gold price, aberrations such as the American Civil War 

and financial panics caused either temporary de facto 

abandonment of the gold standard or suspension of specie 

payments. The U.S. government declared a de facto 

abandonment of the standard in 1968 and then an official 

abandonment in 1971, at the time of the collapse of the 

Bretten Woods Agreement, which had established the U.S. 

dollar as the world's monetary standard at gold parity of $35 

per ounce. Thereafter, the world gold price rose rapidly. A 

peak of $850 per ounce was reached in January 1980; since 

then the price has settled in the range of $300-$500 per 

ounce.

Gold production in the United States prior to 1848 was 

less than 2 metric tons annually. Discovery of gold in California in that year led to annual production of 60-100 metric

Publication approved by the Director, U.S. Geological Survey, 

April 26, 1988.

tons during the 1850's. Gold production gradually declined 

thereafter, until discoveries in South Dakota, Alaska, Colorado, and Nevada provided new sources to help finance the 

country's expanding economy, culminating in peak annual 

production of more than 150 metric tons just before World 

War I. Following the war, production declined sharply, until 

the depression of the early 1930's and the gold price increase 

in 1934 again stimulated gold mining. An all time high in 

annual gold production of 187 metric tons was reached just 

prior to World War II. The government order restricting gold 

mining in the United States during the war resulted in a 

precipitous drop in annual production to less than 50 metric 

tons. After the war, a brief increase in annual production 

tailed off into a gradually declining trend near 50 metric tons 

annually. The rapid price increases in the 1970's were not 

matched by increased gold production until 1981, reflecting 

the long lead times required to develop gold mines. Annual 

gold production in the United States presently again exceeds 

100 metric tons.

Because of the importance of gold as a store of wealth, 

and because of the historical bond between gold and money, 

it is hoped that the descriptions of the geology of gold 

provided in this series will help increase the supply of gold, 

and thus they may also benefit the world's monetary 

systems.

Gold has been mined throughout the world from a great 

variety of types of deposits and of all geologic ages. 

Excepting placers, most of the world's gold production has 

come from Precambrian terranes, and commonly from deep 

geologic environments. In the United States most production 

has come from deposits of younger geologic age and 

shallower geologic environments. Aside from the

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A1
Precambrian-age Homestake deposit at Lead, South Dakota, 

the United States' largest producer, most of the major gold 

deposits are of Mesozoic and younger age. They occur 

predominantly in intermediate to shallow geologic environments such as the mid-level quartz-vein system of the 

California Mother Lode and numerous porphyry-copper 

systems of the Western United States, and shallow-level 

bonanza-type deposits such as the Comstock Lode in 

Nevada and Cripple Creek in Colorado, and recently, a 

variety of disseminated deposits mostly in Nevada.

This series describing the geology of gold deposits in 

the United States consists of chapters on the geochemistry of 

gold, the distribution of gold in the United States, descriptions of major gold terranes, and descriptions of 

individual deposits or districts. Deposits are grouped as 

those in metamorphic rocks; mesozonal deposits consisting 

of polymetallic replacements and veins, porphyry copper 

systems, and mid-level gold-quartz veins; a variety of epizonal deposits, including bonanza and disseminated types; 

and placer deposits.

INTRODUCTION

The value of gold in international political and 

commercial relations makes a secure and substantial 

domestic source of gold a significant national asset. This 

series is being prepared in the belief that a broader 

understanding of the geologic occurrence of gold will 

assure a more extensive and a more efficient exploitation 

of this valuable resource, and thereby contribute to the 

national well-being.

ACKNOWLEDGMENTS

A.R. Wallace, G.A. Desborough, T.A. Steven, and 

R.P. Ashley of the U.S. Geological Survey, and I.E. 

Tilton of the Mineral Economics Department, Colorado 

School of Mines, Golden, Colo., have offered me 

suggestions that have materially improved this introductory report.

In addition to the authors of reports in this series, 

many people have contributed to its preparation. Initial 

planning was by R.P. Ashley, W.F. Cannon, A.V. Heyl, 

T.P. Miller, W.C. Prinz, D.R. Shawe, J.F. Slack, R.B. 

Taylor, and E.W. Tooker, all of the U.S. Geological 

Survey. D.R. Shawe and R.P. Ashley coordinated work 

on the series. R.B. Taylor and E.W. Tooker provided 

much useful advice and suggestions during preparation 

of the series. V.E. Swanson edited several manuscripts. 

D.P. Buscher computer-drafted many of the illustrations 

using the U.S. Geological Survey's GSDRAW system.

GOLD AS A STORE OF WEALTHTHE USES OF GOLD

Gold has been prized by man from distant 

prehistory because of its beauty, rarity, and durability.

From its use as an article of adornment, it grew to be a 

stable item of trade and eventually became what is now 

called money. Other metals and printed (paper) money, 

however, now have replaced gold coinage in circulation 

worldwide. Though in theory these substitutes represent 

a reserve of gold that is held as backing by the issuer of 

the substitute money, in fact, this support of money by 

means of a gold reserve is seldom strictly true. Nevertheless, with the organization of industrialized nations 

and the development of international commerce, gold 

became the primary instrument for settlement of 

international debt, whether used in direct payment, or as 

backing, officially or de facto, for national currencies 

(that is, as monetary reserves). Thus gold is accumulated 

and held as reserves by national banking institutions, the 

so-called central banks. But individuals worldwide 

commonly do not trust the vagaries of national and 

international politics and economics, and of wars, and 

they prefer to hold their own reserves as private hoards of 

gold. Whether held privately, or as a public hoard, gold 

has long been a store of wealth.

With modern advances in technology, gold now is 

finding more and more industrial uses, particularly in 

electronics. However, with respect to the total store of 

gold the amount involved in industrial uses is minor, and 

likely to remain so. Because of the high price of gold, 

industrial applications tend to use very small amounts of 

gold.

Of the total gold available in the world today 

which represents virtually all the gold that has ever been 

mined about one-third is held by national central banks 

and other official agencies such as the World Bank and 

the International Monetary Fund as a support, albeit 

imperfect, for monetary systems. At the end of 1983 

central bank gold holdings were about 35,000 metric tons 

(Mining Journal, March 8,1985). Another third is held in 

private hoards, as a hedge against inflation and sudden 

domestic or international instabilities, or for speculation 

purposes. The last third is in jewelry (in some respects a 

store of wealth) and in other fabricated uses such as in 

the electronics industry and dentistry (Stoehr, 1984; 

Davenport, 1981).

HOW MUCH GOLD?

Probably at least 5,000 years have passed since the 

first gold was mined on this planet, during which time 

slightly more than 3 billion troy ounces (about 94,000 

metric tons; fig. Al) have been produced. Most of this 

has been produced since A.D. 1500 (Kavanagh, 1976), 

and about half of the total has been mined within the last 

35 years (Stoehr, 1984). The increasing quantity of 

available gold was highlighted by Stoehr (1984) in the 

following figures: assuming an even distribution of all 

available gold among all living persons on the planet, in

A2 Geology and Resources of Gold in the United States
the year 1800 there was 0.14 oz per person, in 1900 0.31 

oz, in 1980 0.71 oz, and projected at current trends to 

2000 there will be about 1 oz per person. This 

remarkable acceleration in gold production has resulted 

from improved extraction technology and a better understanding of the geologic environments of gold, coupled 

with an ever-increasing demand that in recent years has 

substantially increased the price of gold.

Despite the long-term increase in gold production 

worldwide, gold mining has been sporadic through time 

and from locale to locale, owing in part to nature's 

uneven allotment of gold to different geologic environments and to man's unstable political and economic 

activities. Figure Al shows the production of gold in the 

United States and in the world during the period 

1845-1987.

FACTORS INFLUENCING THE PRICE 

AND PRODUCTION OF GOLD

The British Gold Standard was established in 1717 

by Sir Isaac Newton, then Master of the Mint, at £4.248 

per troy ounce. The standard lasted for more than 2 

centuries. In 1792 the U.S. Congress adopted a bimetallic 

(gold-silver) standard for the new Republic, fixing the 

price of gold at $19.39 per troy ounce (U.S. gold price 

history from Cycles, 1981) at a fixed ratio of 1 to 15 gold 

to silver, at parity with the British pound sterling. The 

United States suspended specie (coinage, particularly of 

gold and silver) payments in August 1814 following the 

War of 1812 with Great Britain, and resumed them in 

February 1817. The gold price then remained constant at 

$19.39 until 1834, when because of inflation a new price 

was set at $20.67, at a fixed ratio of 1 to 16 gold to silver. 

The financial panic of 1837 brought on a one-year 

suspension of specie payments. Specie payments also 

were suspended briefly during the panic of 1857. The 

Civil War brought on a further specie suspension in 

December 1861 which lasted until 1879. Unofficial gold 

prices soared during the war (fig. Al) but dropped 

substantially on "Black Friday," September 24, 1869, at 

the time of the "silver panic." With the panic of 1873, the 

U.S. adopted a de facto gold standard that brought the 

gold price down close to its earlier fixed price.

In January 1874, Congress provided for redeemability of paper money in gold effective January 1879. 

This action did not generate a rush for gold, but instead 

it induced an increase in gold deposits in banks and an 

increased demand for government securities (Wormser, 

1967), because of the strengthening effect of the gold 

backing upon the nation's monetary system. Thus, specie 

payments were resumed in 1879 at $20.67 per troy ounce, 

and in 1900 the U.S. again officially (de jure) adopted the 

gold standard. During World War I, between September 

1917 and June 1918, free export of gold from the United

States was prohibited. Following World War I in 1919 

Britain was forced off the gold standard because of 

inflation that diminished the value of gold. Gold prices in 

Britain, as well as in South Africa where the world's most 

significant gold production was already underway, 

thereafter improved gradually until 1925, when Britain 

and South Africa, with inflation controlled, returned to 

the old standard of £4.248 per troy ounce. Then in March 

1933, following the 1929 stock market crash and collapse 

of the Federal banking system in the United States, the 

Congress prohibited U.S. citizens from owning gold, and 

the administration in January 1934 fixed the official price 

of gold at $35.00 per troy ounce. But by the end of 1932 

the Great Depression had already destroyed the gold 

standard in Britain. As a consequence, the gold price 

there rose to £8.40 per troy ounce by January 1940, 

where it was fixed through World War II. In 1944 as a 

result of the Bretten Woods Agreement, the U.S. dollar 

became the world's monetary standard at gold parity of 

$35 per ounce (data in part from Mining Journal, Dec. 

1986 suppl.) .

In 1968 the U.S. government declared a de facto 

abandonment of the gold standard, and within 3 years the 

price increased slightly to about $40 per troy ounce. The 

Bretten Woods system collapsed in 1971 (Lehrman, 

1986). At that time also the United States officially (de 

jure) came off the gold standard, and citizens were again 

allowed to own gold, and thereafter the price rose 

rapidly. A significant increase in the price of gold to an 

average of about $160 per ounce followed the oilproducing and exporting countries' (OPEC's) fourfold 

increase in oil prices in 1973. After a temporary decline 

to $115 in 1976, the gold price soared to a peak of $850 

per troy ounce in January 1980, following further oil 

price increases, the U.S.S.R.'s invasion of Afghanistan, 

the Iranian revolution and occupation of the U.S. 

Embassy in Teheran, and general worldwide economic 

uncertainty. The average price for the entire year of 1980 

was well above $600 per ounce. Since 1980, and in 

response to several factors including a strong U.S. dollar, 

a drop in oil prices, and a drop in interest rates, the price 

of gold declined, somewhat erratically, to an average 

price of about $315 per ounce in 1985. With a weakening 

dollar and new increases in oil prices and interest rates, 

the price of gold again climbed, and in May 1987 was 

above $450 per ounce, where it now (December 1987) 

rests.

Gold production in the United States prior to 1848 

was less than 2 metric tons annually (fig. Al). Discovery 

of gold in California in that year led to the great gold rush 

of 1849, following which, up to the beginning of the Civil 

War, 60 to nearly 100 metric tons (in 1853) of gold were 

produced annually. This period was that of peak production from the California goldfields, where placer production exceeded lode production by a margin of 2 to 1.

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A3
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METRIC TONS
Depletion of the placers and new laws constraining 

placer mining were major factors in the subsequent 

decline in production. During the Civil War, and 

following the prewar decline in California gold production, U.S. production temporarily increased as 

unofficial gold prices soared. Although the Comstock 

Lode (Nevada), which was discovered in 1859, produced 

silver that financed a large part of the Union's war costs 

(for example, Smith, 1943), as well as substantial gold, it 

had little effect on the immediately prewar decline in gold 

production. Likewise the discoveries of the Homestake 

deposit in 1876, the Juneau goldfields in 1880, the Yukon 

placers in 1882, and the Cripple Creek (Colorado) 

district in 1891 had no immediate effect on a gradually 

but erratically falling U.S. gold production. However, by 

1893 the cumulative effect of the newly discovered 

sources of wealth, spurred by an ever-growing need for 

capital to finance a rapidly expanding U.S. economy, 

sharply increased gold production.

The financial panics in the United States in 1857, 

1869, and 1873 had little effect on production of gold. In 

each instance the panics occurred during or just prior to 

periods of declining gold production. The 1857 panic was 

in a period in which gold specie payments continued, 

whereas the 1869 and 1873 panics were in a period in 

which specie payments were suspended, and unofficial 

gold prices were substantially higher than the fixed price 

of gold (fig. Al).

Discovery of the Nome (Alaska) gold placers in 

1897, the Tonopah (Nevada) gold-silver district in 1900, 

and the Goldfield (Nevada) district and the Fairbanks 

(Alaska) placers in 1902 added impetus to U.S. gold 

production, which soared into the range of 100 to more 

than 150 metric tons annually during the years 

1899-1918. This period saw peaks of production of gold 

from Cripple Creek, Fairbanks, Nome, Tonopah, 

Goldfield, and the Comstock Lode (Koschmann and 

Bergendahl, 1968).

During World War I, gold production dropped 

appreciably, under a fixed price. A precipitous drop

Figure A1 (facing page). Annual world and United States 

gold production, United States consumer price index, and 

yearly average price (per troy ounce of gold), 1845-1987. 

World gold production from Ridgway (1929), Ryan (1965), 

Anonymous (1982), Anonymous (1986), and various issues 

of Mining Journal (1980-1987); U.S. gold production from 

U.S. Geological Survey (1883-1923), U.S. Bureau of Mines 

(1924-1931), U.S. Bureau of Mines (1932-1984), and various 

issues of Mining Journal (1980-1987); U.S. consumer price 

index from U.S. Bureau of the Census (1970) and Facts on 

File Yearbook (1971-1986); price of gold (average annual 

price for 1968-1987) from Cycles (1981) and various issues 

of Mining Journal (1980-1987); dotted interval 1861-1879 

indicates "unofficial" or black market price during and 

following the Civil War.

occurred in 1918, following the United States entry into 

the war and the prohibition of the export of gold. Gold 

production generally declined until the stock market 

crash of 1929. Gold production then rapidly increased, 

culminating at the start of World War II in the largest 

annual production of gold ever in the United States. This 

burgeoning gold production was accelerated in 1934 by 

the official increase of almost 70 percent in the fixed 

price of gold. Resurgence of gold mining, following the 

financial crash of 1929 and with the impetus of the 1934 

price increase, was made possible by the large reserves of 

gold still available in the Yukon placers, the AlaskaJuneau lode mine, the Mother Lode mines, the 

Homestake mine, and lesser districts in Idaho (for 

example, the Boise Basin), Montana (Virginia City-Alder 

Gulch), Nevada (Eureka; the Comstock), New Mexico, 

Oregon, and Washington (Republic). Porphyry copper 

deposits in Utah, Arizona, Montana, Nevada, and New 

Mexico also contributed substantial byproduct gold 

during this period.

An all-time high in annual U.S. gold production of 

about 187 metric tons was reached in 1940, just prior to 

United States entry into World War II. The most 

catastrophic drop in gold production in U.S. history came 

during World War II, a period of constant gold price, 

following the government order that closed down 

domestic gold mines. When the ban on gold mining was 

lifted at the end of the war, gold production spurted 

momentarily and then declined irregularly as inflation 

eroded the value of the fixed gold price of $35 per oz. 

Production never again reached its prewar level, for in 

addition to the damping effect of inflation, mining could 

not resume in many of the abandoned mines whose 

workings were caved or flooded. Discovery of the Carlin 

gold deposit in 1962 and its early production starting in 

1965 had little influence on the overall U.S. production of 

gold. In spite of sharply rising gold prices and increasing 

discoveries in the years 1973-1980, production of gold 

continued to fall.

Figure Al shows that production of gold in the 

United States between 1845 and the end of World War II 

closely paralleled that in the rest of the world. However, 

after a momentary spurt that followed the war, gold 

production in the United States gradually decreased up 

to 1980; during the same period, world production 

soared. The discrepancy probably can be attributed to 

several factors, including high U.S. wages relative to the 

rest of the world, and the fact that gold mines worldwide 

except in the United States were generally maintained 

during the war. By 1981, even though prices began to 

drop substantially, gold production rose sharply and it 

continues to rise. Much of the lag in production can be 

attributed to the long lead times required to put new gold 

mines into production. Exploration and development 

spurred by the gold price increases between 1973 and

Introduction to Geology and Resources of Gold, and Geocnemistry of Gold A5
1980 did not begin to reach fruition until 1981. 

Moreover, a significant advance in the technology of gold 

extraction during the past decade, notably the development of cyanide heap leaching (Nettesheim, 1987), has 

greatly accelerated the production of gold in the United 

States.

THE ROLE OF GOLD IN WORLD 

MONETARY SYSTEMS

The importance of gold as a store of wealth, 

including its role in world monetary systems, is the 

principal driving force that leads to efforts to find and 

produce gold. This series, intended to provide a more 

thorough understanding of the geology of gold and 

consequently an increase in the supply of gold, thus may 

have a bearing on gold's influence upon world monetary 

systems. This brief discussion is not intended to show 

what the interrelationships of gold and monetary systems 

ought to be, but instead it is meant only to emphasize the 

historical bond between gold and money.

The interrelationships between gold and monetary 

systems are complex and have long been debated; 

opinions are far from unanimous. Nevertheless, there is a 

general recognition that a tie exists between the two. The 

connection has been expressed in many ways. For 

example: "Whatever mineral you have in your mine, you 

are mining for gold" (Mining Magazine, July, 1985, p. 5). 

"The price of gold***is determined not only by the 

supply and demand for gold, but by the supply and 

demand for money" (Wall Street Journal, Sept. 4, 1986, 

p. 26). "If there is success in restoring price stability and 

confidence in our currency, tighter linkage of our 

monetary system to gold may well become supererogatory" (U.S. Gold Commission, as reported by 

Mining Journal, April 30, 1982, p. 5). "Under fixed 

exchange rates based on a true international gold 

standard, even during periods of very rapid growth as 

during the first century and a half of the Industrial 

Revolution global financial balance is maintained by 

virtue of the fact that total international reserves increase 

through increased gold production, not increased dollar 

(or other reserve-currency) debt" (Lehrman, 1986).

Inasmuch as monetary systems are extensions of 

national policy, and therefore susceptible to the changing 

world scene, gold transactions also are responsive to 

world events. Despite the difficulty of predicting even in 

the short term how gold price and production are going 

to respond in a free world market, gold nevertheless is a 

sensitive indicator of certain domestic and international 

conditions such as inflation and wars. For example, the 

general parallelism between the price of oil and gold in 

recent years has been apparent, although this relationship might not persist over the long term. According

to the Mining Journal (Feb. 12,1982, p. 115), the relative 

appeal of gold is based on (1) inflation and other anxiety 

factors, (2) cost of money and the yield on alternative 

assets, (3) promotional and marketing efforts, (4) price 

of gold relative to other commodities, and (5) supplyindustrial demand outlook for gold. (See also Kaufmann, 

1987.) In the last week of February 1983, gold tumbled 

from about $500 per oz to about $400 per oz, in concert 

with a drop in the U.S. prime interest rate from about 12 

percent to 10.5 percent. Under today's uncontrolled gold 

price, if the value of the dollar falls, the price of gold 

increases. The great complexity of interplay of various 

influencing factors however can cause some otherwise 

reliable indicators to become fallible. What can be stated 

is that a stable gold price reflects generally stable 

economic conditions worldwide, discounting political and 

military influences. Furthermore, the gold price is not 

appreciably influenced by supply factors such as newly 

mined gold, inasmuch as new gold makes only a trivial 

impression on the total supply of gold. Effects of shortterm fluctuations in the world supply may be apparent, 

however, as for example when the U.S.S.R. in December 

1981 sold a large amount of gold contributing to a 

moderate decline in the world price (Mackay-Smith and 

Leger, 1981).

With the discontinuance of a world gold standard 

instigated by U.S. action in 1971, the price has been 

controlled by the free market, and it has sought its own 

level. But discontinuing the use of gold as a world reserve 

currency has not eliminated the need for such a standard, 

and the U.S. dollar has assumed this role. A silver-based 

world monetary system was established as a result of the 

1922 Genoa Conference, and continued in 1944 as a 

result of the Bretten Woods Conference (Mueller, 1986). 

The system was abandoned by the U.S. in 1971 with 

suspension of dollar-gold convertibility, as the United 

States and Britain could no longer redeem currency in 

gold while other countries used dollars and pounds as 

official reserves (Lehrman, 1986). The dollar, however, 

simply by its overwhelming dominance in world economic activity nevertheless retained a de facto role as the 

world's reserve currency. But because U.S. dollars 

basically are financing world growth and trade, there 

cannot be a net increase in world monetary reserves. The 

dollar, therefore, becomes an asset for foreign central 

banks but a liability for the United States (Mueller, 1986; 

Lehrman, 1986).

WHERE GOLD COMES FROM

Gold has been mined throughout the world from a 

great variety of deposit types and of all geologic ages (for 

example, Boyle, 1979). Gold has been produced from the 

Precambrian shield areas of Africa, North and South 

America, Australia, India, and Scandinavia, principally

A6 Geology and Resources of Gold in the United States
from quartz veins, and in southern Africa and Brazil 

from widespread "reefs" or "bankets" which generally 

are thought to be relatively thin fossil placer deposits in 

conglomerates. The gold-bearing reefs of the Witwatersrand in South Africa have been the world's 

greatest source of gold, having produced about one-third 

of the total supply. Paleozoic-age deposits occur widely, 

but they are concentrated in the Ural Mountains, Kazakhstan, and southern Siberia in the U.S.S.R., in southeastern Asia, in eastern Australia, in the Appalachian 

region of the United States, and in Europe. These 

deposits are mostly quartz veins and stockworks. The 

Muruntau gold deposit in Kazakhstan, a complex of 

quartz veins and associated stockworks of quartz veinlets 

in Paleozoic calcareous siltstones, shales, and schists near 

Paleozoic granodioritic plutons, is the world's largest 

producing mine. Mesozoic deposits of gold occur mostly 

in the cordilleran belts at the western margins of North 

and South America, and in eastern Siberia and China. 

Quartz veins again are the dominant type, in some 

districts forming great regional systems such as the 

productive Mother Lode of California. Deposits of Cenozoic age have a distribution similar to, though wider 

than, those of Mesozoic age, occurring also throughout 

southern Europe and eastward through southern Asia to 

the western Pacific area of Indonesia, New Zealand, and 

Japan. Gold deposits include bonanza-type quartz veins, 

porphyry-copper systems, and disseminated types. 

Finally, gold placers are found throughout the regions of 

bedrock occurrences of gold. Tertiary and Quaternary 

placer deposits such as those in Alaska, the western 

foothills of the Sierra Nevada of California, and Siberia 

have yielded large amounts of gold.

The types of deposits, excepting placers, that have 

dominated gold production in the United States have not 

been the same as those that have been most productive 

elsewhere in the world. Throughout most of the world, 

major gold production has come from Precambrian 

terranes, and commonly from deep geologic environments. In the United States most production has come 

from deposits of younger geologic age and of shallower 

geologic environments. Aside from the Precambrian-age 

Homestake deposit at Lead, S. Dak., the United States' 

largest producer, most of the major gold deposits have 

been of Mesozoic and younger age. They occur 

predominantly in intermediate to shallow geologic 

environments, as for example the mid-level quartz-vein 

system of the California Mother Lode and numerous 

porphyry-copper systems of the Western United States, 

and shallow-level bonanza-type deposits such as the 

Comstock Lode in Nevada and Cripple Creek in Colorado, and recently, a variety of disseminated deposits 

mostly in Nevada.

Historically, the types of gold deposits exploited at 

any given time have depended on an ever-expanding

understanding of the occurrence of gold, modified by a 

technology of extraction that has evolved in response to 

discovery. Primitive societies won their gold from placers, 

which have remained major but highly varied sources to 

this day. Lode deposits (in the strict sense of fissure 

veins) have been major sources for millennia. The highly 

productive gold-bearing reefs of the Witwatersrand have 

been mined for just a century. Byproduct gold is a fairly 

recent phenomenon, and disseminated gold in "Carlintype" and similar deposits was first recognized quite 

recently (modified from Shawe and others, 1982).

SCOPE OF THIS SERIES, AND 

CONVENTIONS USED

The resurgence of gold mining in the United States 

in the 1980's, coupled with the present depressed 

condition of most of the remainder of the mining 

industry, focuses new interest on the geologic occurrence 

of gold. Increased understanding of the distribution of 

gold in the Earth's crust allows a more effective 

exploitation of this valuable commodity. Localized 

concentrations of gold (gold deposits) occur in a great 

variety of geologic environments, and descriptions of 

important examples of these deposits provide valuable 

models to guide assessment of gold resources and the 

search for new gold deposits. The objectives of this series 

thus are to give descriptions of the major geologic 

terranes in which gold occurs, as well as descriptions of 

significant gold mining districts and deposits throughout 

the United States. Some deposits that have been mined 

principally for base metals or silver but that have 

produced significant amounts of gold also will be 

described. In addition to the recently recognized types of 

gold deposits that show great promise for substantial 

future gold production, many "conventional" types of 

gold deposits offer significant potential. Among these 

latter are deposits whose production was interrupted by 

World War II and that never recovered thereafter 

because of adverse economic conditions, but which with 

a favorable gold price may be viable again.

An accompanying report in this chapter discusses 

the geochemistry of gold in hydrothermal fluids. Reports 

in subsequent chapters will describe the geologic and 

geographic distribution of productive (> 10,000 oz of 

gold) districts in the United States, and broad geographic 

clusters of gold deposits that in general also are 

characterized geologically. Each geographic region (gold 

terrane) exhibits major lithologic (or lithogenetic) units 

and structural elements with which gold deposits are 

associated. Ages of the deposits will be discussed to 

clarify the relation of gold mineralization to particular 

geologic events.

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A7
Later chapters, the bulk of the series, will describe 

individual gold deposits and districts. The deposits and 

districts described are considered significant because 

they have had production or they contain announced 

reserves of > 1,000,000 troy oz of gold, or because they 

are considered to represent an important geologic type of 

deposit. Our coverage has not been exhaustive, and a few 

deposits that have had significant production will not be 

described because of similarity to certain described 

deposits. Also, some important recently discovered 

deposits are not included among our descriptions. The 

deposits and districts described have been grouped for 

convenience as gold deposits in metamorphic rocks; 

mesozonal gold deposits including polymetallic replacements and veins, porphyry copper systems, and mid-level 

gold-quartz veins; epizonal gold deposits including 

bonanza-type and disseminated gold deposits; and placer 

gold deposits.

This collection of reports was planned initially for 

publication as a single volume, but now is being published in appropriate groupings as separate chapters in a 

series, with the hope that late reports can be incorporated 

in final chapters. Because of the delay entailed in 

obtaining some reports, a few of those submitted earlier 

had become outdated. As much as possible, those reports 

have been updated.

Metric units are used throughout the series 

generally, except that gold amounts are given as either 

troy ounces (oz) or as metric tons (t) (1,000,000 troy 

ounces = 31.1 metric tons and 1 metric ton = 32,151 troy 

ounces), and gold grades are given as ounces of gold per 

short ton (oz Au/ton). Descriptions of placer gold 

deposits give gold grades as ounces of gold per cubic yard 

(oz Au/yd"). No standard conversion exists for oz Au/yd 

to oz Au/ton, because of variation in the density of placer 

gravel. The term "fineness" is used to indicate the 

proportion of pure gold in native gold that is almost 

universally alloyed with lesser amounts of mostly silver. 

For example, a fineness of 860 indicates 860 parts per 

thousand pure gold. The term "tenor" is commonly used 

interchangeably with grade. Map and cross section 

elevations, and mine levels, are given in feet.

Manuscript received by scientific editors August 1987

REFERENCES CITED

Anonymous, 1982, Roskill's Metal Databook, 3rd edition 1982:

London, Roskill Information Services Ltd., p. 57. 

Anonymous, 1986, World Metal Statistics Year Book 1986:

London, World Bureau of Metal Statistics, p. 36. 

Boyle, R.W., 1979, The geochemistry of gold and its deposits:

Geological Survey of Canada Bulletin 280, 584 p. 

Cycles, 1981, 200 years of American gold prices: v. 32, no. 7,

p. 5.

Davenport, C.H., 1981, Gold standard or not: Barren's

Magazine, Nov. 2, 1981, p. 9, 21-23. 

Facts on File Yearbook, 1971-1986, Annual consumer price

index data: New York, Facts on File, Inc., pages vary. 

Kaufmann, T.D., 1987, The witchcraft and logic of gold pricing Politics, inflation, speculation, and the value of the

dollar are all contributing factors: Mining Engineering, v.

39, p. 857-858. 

Kavanagh, P.M., 1976, Gold reserves of the world, Presidential

address to the Geological Association of Canada: Geoscience Canada, v. 3, no. 3, p. 151-155. 

Koschmann, A.H., and Bergendahl, M.H., 1968, Principal

gold-producing districts of the United States: U.S.

Geological Survey Professional Paper 610, 283 p. 

Lehrman, L.E., 1986, An exorbitant privilege: National Review,

Nov. 21, 1986, p. 40-46. 

Mackay-Smith, Anne, and Leger, J.M., 1981, Gold price skids

$11.90, battered by Soviet selling: The Wall Street

Journal, Dec. 22, 1981. 

Mining Journal, various issues 1980-1987: London, v. 294-309,

pages vary.

Mining Magazine, 1985, Going for gold: July 1985, p. 5. 

Mueller, John, 1986, The reserve-currency curse: The Wall

Street Journal, Sept. 4, 1986, p. 26. 

Nettesheim, M.C., 1987, The impact of heap leaching on the

U.S. comparative advantage in gold mining: Mining and

Metallurgical Society of America, presentation, June 30,

1987; preprint, 23 p. 

Ridgway, R.H., 1929, Summarized data of gold production:

U.S. Bureau of Mines Economic Paper 6, 13 p. 

Ryan, J.P., 1965, Gold, in Mineral facts and problems: U.S.

Bureau of Mines Bulletin 630, p. 387-393. 

Shawe, D.R., Ashley, R.P., Steven, T.A., Taylor, R.B., and

Tooker, E.W., 1982, United States gold resource profile A discussion: Economic Geology, v. 77, no. 5,

p. 1242-1244. 

Smith, G.H., 1943, The history of the Comstock Lode,

1850-1920: University of Nevada Bulletin, v. 37, no. 3,

305 p. 

Stoehr, R.J., 1984, Some thoughts on the current situation and

a look ahead: Mining Engineering, Nov. 1984, v. 36,

no. 11, p. 1519-1521. 

U.S. Bureau of the Census, 1970, Historical statistics of the

United States, Colonial times to 1970: U.S. Bureau of the

Census, Part 1, p. 210-212. 

U.S. Bureau of Mines, 1924-1931, Mineral resources of the

United States: U.S. Bureau of Mines annual publications,

pages vary. 

U.S. Bureau of Mines, 1932-1984, Minerals yearbook: U.S.

Bureau of Mines annual publications, pages vary. 

U.S. Geological Survey, 1883-1923, Mineral resources of the

United States: U.S. Geological Survey annual

publications, pages vary. 

Wall Street Journal, 1986, Sell some gold: New York, Sept. 4,

1986, p. 26. 

Wormser, F.E., 1967, Monetary realism for the gold miner:

1967 Metal Mining and Industrial Minerals Convention,

American Mining Congress, Denver, Colorado, September 10-13, 1967; preprint, 16 p.

AS Geology and Resources of Gold in the United States
Geochemistry of Gold in Hydrothermal Deposits

By Samuel B. Romberger1

Abstract

The geochemistry of gold in hydrothermal deposits is 

influenced by the source of transporting solutions, metal 

source, mechanisms of transport, and the mechanisms of 

ore deposition. The solutions may have a magmatic source, 

or may be derived from any one of the following, depending 

on deposit type: sea water, connate water, meteoric water, or 

water evolved during metamorphism. Where the solutions 

have a nonmagmatic source, the gold must be derived from 

aquifer or other rocks through which the solutions migrate. 

No one rock type appears to be a favorable source for gold 

in terms of its abundance. Instead, the suitability of a rock as 

a source for gold will be controlled by its permeability, 

accessibility of the gold to the transporting solutions, and the 

chemical composition of the solutions.

Gold may be transported in aqueous solutions as a 

variety of complexes; however, aurous chloride or bisulfide 

species are most likely in hydrothermal systems, depending 

on temperature and solution composition. In most deposits 

formed between 200 and 300 °C, gold is best transported as 

a bisulfide complex; however, at higher temperatures typical 

of magmatic systems, gold is transported as a chloride 

complex. That a gold thioarsenide complex may exist is 

evaluated based on the ubiquitous association of gold and 

arsenic in primary hydrothermal deposits and the occurrence 

of thioarsenide species.

If the gold is transported as a chloride complex, 

reduction, increase in pH, and (or) decrease in temperature 

may result in deposition. If it is transported as a bisulfide or 

thioarsenide complex, oxidation, increase or decrease in pH, 

and (or) decrease in sulfur activity will cause deposition; the 

effect of temperature on the solubility of these complexes is 

uncertain. Because oxygen activity strongly affects solubility 

of gold bisulfide complexes, oxidation is the preferred 

mechanism of gold deposition. The oxidation of hydrothermal solutions may be produced by boiling, mixing with 

oxygenated ground waters and (or) contact with^a host rock 

containing an oxidized mineral assemblage. Because of the 

high solubility of gold bisulfide complexes compared to the 

amount of gold in the ore deposits and source rocks, the 

mineralizing solutions seldom may be saturated with gold. 

Gold may be coprecipitated with sulfides as these minerals 

use up the sulfur forming the complexes. Recrystallization 

during metamorphism or other superimposed events would

Colorado School of Mines, Golden, CO 80401.

then produce fine-grained gold along grain boundaries and 

as patches within the sulfides.

INTRODUCTION

In the development of models for the origin of 

various types of gold deposits, it is essential to 

understand how the metal behaves in the geochemical 

environment. This is true particularly for deposits of 

hydrothermal origin because of the complex nature of 

the solution-mineral equilibria for gold. In hydrothermal 

deposits four factors must be considered in developing an 

understanding of their origin: (1) nature and source of 

the mineralizing solutions, (2) source of the gold, (3) 

mechanisms of gold transport, and (4) mechanisms of 

gold deposition. Each of these factors will be considered, 

although emphasis will fall on the sources of gold and the 

mechanisms of transport and deposition because 

extensive literature already exists on the nature and 

source of mineralizing solutions.

Figure A2 is a schematic representation of the 

general gold mineralizing system which illustrates three 

of the four factors (parts of the system) given above. It 

presents the environment of mobilization, or source rock, 

the environment of transport, and the environment of 

deposition; note that in the environment of deposition, 

the gold may be dispersed or concentrated, depending on 

the physical and chemical nature of the environment. The 

nature of each part of the system will depend on the 

composition of the other parts. For example, the 

mechanisms of gold precipitation in the environment of 

deposition will depend on how this metal is transported. 

This, in turn, will depend on the availability of suitable 

complexing agents in the source environment. In more 

fundamental terms, the occurrence of gold in the 

environment of deposition will depend on its availability 

in the source area. The diagram implies that the 

transport of gold will depend on the activities of electrons 

and protons (oxidation state and pH of the solution 

respectively).

NATURE AND SOURCE OF SOLUTIONS

The source of solutions responsible for hydrothermal mineralization has been much debated. Five sources 

have been proposed for various deposit types: (1) 

meteoric (ground water); (2) sea water; (3) connate, or

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A9
Environment 

of

Mobilization

X

 - / / H + 1

/ /

X S

Environment

X of _

_ . Deposition

^ /, -

1 "I

e~

J

H+1l

\^

i^l/J_ "j

e~ Concentration or 

DispersionFigure A2. Schematic diagram of the hydro-thermal mineralizing environment illustrating 

the environments of source rock, transport, and deposition (concentration, solid square) or 

dispersion (dashed square).

formation, water; (4) magmatic water; and (5) metamorphic water. Taylor (1979) summarized a large number of 

oxygen and hydrogen isotope data available for precious 

metal deposits and concluded that the hydrothermal 

solutions responsible for their formation were dominated 

by meteoric water. Based on detailed geochemical 

studies, Radtke and others (1980) concluded that the 

gold and other materials deposited at Carlin were transported in solutions dominated by meteoric water. In their 

development of genetic models for epithermal precious 

metal deposits Buchanan (1981) and Berger (1982, p. 

119-126) assumed these systems to be fossil geothermal 

areas where the hydrothermal solutions were chemically 

evolved heated ground waters. Their models are based 

on sound field relationships as well as geochemical 

studies of known precious metal deposits and active hot 

spring areas. Finally, Henley and Ellis (1983) discussed 

similarities between present-day geothermal systems and 

epithermal gold deposits, implying the importance of 

ground water in the formation of the epithermal deposits. 

The importance of sea water and connate water in 

the formation of gold deposits is less well documented. 

Initial concentration of gold by hot spring activity on 

the sea floor may have been important in the genetic 

history of Archean vein deposits (Ridler, 1970; Karvinen, 

1982). Fripp (1976) concluded that the gold-bearing 

Archean banded iron-formations of Rhodesia represent 

chemical precipitates deposited from waters issuing from 

submarine hot springs. These stratiform ore bodies occur

with sulfide and sulfide-carbonate facies banded ironformation containing alternating layers of chert, arsenopyrite, pyrrhotite and ankerite. The gold occurs within 

arsenopyrite grains. Textural and structural features suggest that the gold and sulfides were deposited before 

deformation and metamorphism. Bavington and Keays 

(1978) found a significant enrichment of gold in interflow 

sedimentary rocks in a komatiite sequence in Western 

Australia. These rocks are interpreted as chemical 

precipitates formed around hot springs on the Archean 

sea floor. Even though the associated volcanic rocks 

contain average (crustal abundance) gold contents, the 

gold concentration of the interflow sedimentary rocks is 

as much as several hundred parts per billion (ppb).

Hallager (1984) concluded that the goldcontaining iron-rich chemical sedimentary rocks at Jardine, Mont., were deposited by hot spring activity on the 

Archean sea floor. The gold is closely associated with 

sulfide facies ironstone which lies within a thick sequence 

of quartz-mica schists and quartzites. The close association of the gold with a carbonate iron-formation at the 

Homestake deposit suggests that the gold may have been 

deposited initially by a process similar to that for ironformation. These deposits appear to have many 

similarities to syngenetic massive sulfide deposits that 

have been interpreted to be formed from deep circulating 

heated and chemically evolved sea water (Hutchinson, 

1973; Ohmoto and Skinner, 1983; Hutchinson and Burlington, 1984).

A10 Geology and Resources of Gold in the United States
Fehlberg and Giles (1984) interpreted the Spargoville deposit in the Archean Eastern Goldfields Province 

of Western Australia to have a volcanic-exhalative origin. 

Fine-grained gold occurs in conformable bands of 

arsenopyrite-bearing actinolite-quartz-carbonate rock 

within a metagreywacke sequence. These sedimentary 

rocks are associated with a felsic volcanic center within a 

greenstone belt. Fehlberg and Giles (1984) called upon 

connate water within the sedimentary-volcanic sequence 

as a possible source for the mineralizing solutions.

Gold occurs in anomalous amounts in many 

porphyry copper deposits around the world (Kesler, 

1972; Titley, 1978; Sillitoe, 1979, 1980, p. 52-68, 1981). 

Most geochemical studies suggest that the solutions 

responsible for primary hydrothermal mineralization in 

these deposits were derived from magmatic water (Roedder, 1971; Taylor, 1979). However, an overprint of 

meteoric-water-derived mineralization is often indicated 

by stable isotope studies. It seems apparent that 

magmatic water may be important in the transport of 

gold in the porphyry copper systems.

The importance of waters released during metamorphism in the formation of gold veins has received 

considerable attention in recent years. Boyle (1959) 

originally proposed a metamorphogenic origin for the 

Archean greenstone-hosted veins in the Yellowknife 

district of the Northwest Territories. Groves and others 

(1984) interpreted the Archean vein deposits of Western 

Australia as being formed from hydrothermal solutions 

liberated during metamorphism. Fyfe and Kerrich (1984) 

developed a general model for the formation of Archean 

greenstone-hosted lode gold deposits which relies heavily 

on the transport of materials by solutions released during 

metamorphism. This model is based on extensive geochemical data on the distribution of major, minor, and 

trace elements in the veins and wall rocks (Kerrich and 

others, 1977; Kerrich and Fryer, 1979; Kerrich and Fyfe, 

1981; Kerrich and Hodder, 1982; Kerrich, 1980, 1983).

Buchanan (1981) and Roedder (1984) summarized 

the composition of gold mineralizing solutions based on 

fluid-inclusion evidence and inferences from field relationships. In most hydrothermal deposits the solutions 

which transported and deposited the metals are no longer 

present in a quantity large enough to analyze and 

evaluate in detail. Solutions may be present in fluid 

inclusions in ore and gangue minerals; however, these 

are commonly too small to analyze directly, or they may 

represent an event superimposed on the original veinforming episode. In addition, these fluid inclusions 

represent mineral-depositing and not necessarily 

mineral-transporting solutions. Nevertheless, fluid inclusions can yield valuable information on the physicochemical conditions of hydrothermal mineralization 

(Roedder, 1979; Roedder and Bodnar, 1980; Bodnar, 

1983).

The range of temperatures determined for hydrothermal gold deposits is 100-400 °C, with most occurring 

between 200 and 300 °C. Table Al summarizes the 

homogenization temperatures of various epithermal 

precious metal deposits discussed by Buchanan (1981). 

The temperature of ore deposition was considered to be 

similar to that of fluid-inclusion homogenization because 

these systems were at, or very close to, the boiling curve 

for the solutions (Buchanan, 1981). In addition, Nash 

and Theodore (1971) reported temperatures of formation and salinities for gold-quartz veins at Copper 

Canyon, Nev., to be 250-330 °C and 0.3-12.2 wt. percent 

NaCl equivalent. Nash (1975a, 1975b) determined 

formation temperatures for gold veins in the Park City 

district, Utah, and Sneffels-Telluride district, Colorado, 

to be 220-300 °C and 250-315 °C respectively. 

Batchelder and others (1978) gave a temperature for the 

formation of deposits in the Tintic district of Utah as 

150-300 °C and salinities of less than 0.1 to 3.1 wt. 

percent NaCl equivalent.

Higher temperatures and salinities have been 

determined for gold-bearing deposits closely associated 

with magmatic activity. Moore and Nash (1974) reported 

temperatures and salinities for the Bingham Canyon 

porphyry deposits of 400-600 °C and up to 36 wt. percent 

NaCl equivalent, respectively. Huang and others (1978) 

determined temperatures of 450-600 °C and salinities of 

up to 42 wt. percent NaCl equivalent for the tactile 

deposits at Ely, Nev. T.G. Theodore and others (written 

commun., 1985) gave temperatures of 350-550 °C and 

salinities of up to 25 wt. percent CaCl2 for the TomboyTable A1. Temperatures of formation and salinities for 

selected epithermal precious metal deposits (Buchanan, 

1981)

[Leaders (--), no data]

District Salinity2

Oatman, Arizona............................. 220

Pachuca, Mexico.............................. 200-250

Comstock, Nevada........................... 250-300

Guanajuato, Mexico........................ 230 <1

Tonopah, Nevada ............................ 240-265 < 1

Goldfield, Nevada............................ 200-300

Creede, Colorado ............................ 250

Eureka, Colorado............................ 285-293 0.8

Rochester, Nevada .......................... 270-310 6

Bodie, California.............................. 215-245

Tayoltita, Mexico............................. 265 3.4-8.4

Seven Troughs, Nevada.................. 240-318

Ramsey-Talapoosa, Nevada........... 221

Colqui, Peru...................................... 270 2-10

Lake City, Colorado........................ 250 ______ 3

Th - temperature of homogenization, °C. 

Salinity in weight percent NaCl equivalent.

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A11
Minnie skarn deposits at Battle Mountain, Nev. Finally, 

E.W. Tooker (written commun., 1985) indicated a 

temperature of formation for veins in the Butte, Mont., 

district of 300 °C.

Geochemical data available suggest that sodium 

chloride is the most important dissolved compound in the 

solutions; lesser amounts of calcium and potassium are 

present also. Carbon dioxide may be present in 

significant quantities in inclusions from certain types of 

deposits, particularly those hosted by Paleozoic carbonate sedimentary rocks. Groves and others (1984) 

reported that fluid-inclusion data from gold veins in 

Western Australia indicate that the mineralizing 

solutions were CO2-rich and relatively dilute, with 

salinities generally between 1 and 3 wt. percent NaCl 

equivalent. Temperatures of formation were between 

300 and 450 °C. R.M. Smith (written commun., 1985) 

suggested that the fluid salinities were very low in the 

Grass Valley vein deposits in California, and formation 

temperature was about 500 °C.

SOURCES FOR GOLD

If hydrothermal systems responsible for the 

formation of gold deposits are dominated by meteoric 

water, sea water, or metamorphic water, source rocks 

must be somewhere in the flow path of the mineralizing 

solutions. Figure A3 represents the generation of a 

mineralizing solution by showing a gold-barren solution 

of variable pH and oxidation state interacting with a 

source rock containing dispersed gold. A prerequisite for 

these solutions to be able to react with the source is that 

the source rock must be permeable. As the solutions 

migrate through the rock the gold is released in some 

soluble form such as complexes. The input solutions may 

be the source of the complexing agents, or the complexing agents may be derived from the source rock as well. 

Gold trapped in the crystal structures of rock-forming 

minerals will not be in a form accessible to the solutions 

unless these minerals go through complete recrystallization during which the gold can be released. Figure A4 

schematically illustrates that impurity components can be 

removed to accessible sites by various processes of 

recrystallization where the gold is no longer accommodated by the "new" minerals. Such changes can occur 

during weathering, diagenesis, metamorphism, or hydrothermal alteration. Gold dissolved in volcanic glass may 

be released during devitrification. The efficiency with 

which the gold is subsequently mobilized will depend on 

the nature of the solutions (see section on mechanism of 

gold transport and deposition).

Geochemical studies have shown that wall rocks 

can serve as sources for gold, at least on a local scale. 

Stephenson and Ehman (1971) reported a depletion of 

gold in altered metamorphosed intrusive mafic rocks

& 

Metal 

Dmplexes 

(sol'n)

Output 

(Leachate)

Process 

(Source)

Figure A3. Schematic representation of the generation of a 

mineralizing solution wherein a barren solution reacts with a 

source rock containing dispersed gold.

Figure A4. Schematic diagram of processes (alteration, 

recrystallization, devitrification, or metamorphism) responsible for releasing gold (small circles; left side of diagram) to 

leachable sites (grain boundaries and open spaces; right 

side of diagram) in a source rock.

adjacent to gold-quartz veins in the Rice Lake-Beresford 

Lake area of southeastern Manitoba. Jenkins (1980) 

detected a decrease in gold content in altered Precambrian metamorphic rocks adjacent to veins in the 

Jamestown district of Colorado. Klein (1980) found a 

statistical decrease in gold and other related components 

in altered metamorphosed ultramafic and mafic rocks 

surrounding veins in the Seminoe Mountains of Wyoming. In addition, other studies have been done to show 

that other components occurring in veins can be derived 

from the wall rocks (Kerrich, 1983).

A consideration of sources for gold must begin 

with its behavior during various geochemical processes. 

Gold belongs to group IB of the periodic table along with

A12 Geology and Resources of Gold in the United States
copper and silver (fig. A5) and very commonly follows 

these elements during processes such as magmatic 

differentiation. Analyses of meteorites indicate that gold 

has a distinct siderophile tendency with a secondary 

chalcophile tendency, whereas silver and copper have 

strong chalcophile tendencies. Gold is concentrated in 

the metallic phase, or in the sulfide phase in the absence 

of metal. Siderite meteorites contain up to 10 ppm (parts 

per million) Au with an average of 1.2 ppm. The gold 

content of chondrites averages between 0.17 and 0.34 

ppm, depending on the type. The highest amounts occur 

in the metal-rich enstatite chondrites, whereas the carbonaceous chondrites contain the least. There is a strong 

partitioning of gold into the metallic phase compared to 

the sulfides and silicates in heterogeneous meteorites 

(Allman and Crocket, 1978).

n

IB

no. 79, m* 196.967

Figure A5. Periodic table of the elements illustrating the 

relationship between goid, silver, and copper.

In some of the earlier work on the behavior of gold 

during magmatic processes, Vincent and Crocket (1960) 

demonstrated a positive correlation between gold and 

copper, and showed that gold preferred copper sulfides 

to iron sulfide, oxides, or silicates in the Skaergaard 

intrusion. Rowe (1969) also found a correlation between 

these two elements in the Great Lake sheet of Tasmania, 

a differentiated tholeiitic dolerite. The maximum gold 

concentration occurred when the magma began to 

crystallize copper-bearing sulfides. Keays and Kirkland 

(1972) suggested that gold was derived from mafic to 

ultramafic dikes containing abundant copper sulfides in 

the Woods Point dike swarm deposits at the Thomson 

River copper mine, Victoria, Australia. Keays and Scott 

(1976) found a correlation between gold and chromium 

contents in mid-Atlantic ridge basalts and suggested that 

at least some of the gold is tied up in chromian spinels. 

They found no strong relationship between the gold and 

sulfur contents, although their data suggest that the 

samples high in gold are also high in copper. Finally, 

Dupuy and others (1981) found a similarity in the 

behavior of gold and copper in the noncumulate ultramafic rocks of New Caledonian ophiolites, where these 

metals appear to have been enriched in the liquid relative 

to the residue during partial melting. However, in the

mafic and cumulate phases gold showed no relationship 

to copper and appeared to be concentrated during the 

crystallization of chromian spinel.

Allman and Crocket (1978) and Boyle (1979) 

tabulated the gold contents of a wide variety of rock types 

from many different locations. Table A2 lists representative abundances for selected rock types from these 

tabulations as well as from other recent publications. 

Significant variations in gold content occur between and 

within rock types; however, some generalizations can be 

made about gold abundances and lithology. Ultramafic 

and mafic rocks appear to be enriched in gold relative to 

the average crustal abundance (5 ppb), although there 

are exceptions. Rozhkov and others (1973) determined 

the gold content of kimberlite and ultramafic inclusions 

from various Russian localities to average 3.9 and 4.8 

ppb, respectively. They concluded that gold contents 

between 3 and 7 ppb are typical of upper mantle 

material, and that variations were a result of differences 

in the degree of partial melting during magma formation. 

Mitchell and Keays (1981) suggested that the bulk of the 

gold in the upper mantle was present in an intergranular 

phase which may represent an immiscible sulfide melt. 

They determined the average gold content of whole rock 

spinel Iherzolite to be about 0.6 ppb Au. Oshin and 

Crocket (1982) studied the gold content of ultramafic 

rocks in the ophiolite complex at Thetford Mines, Quebec. They reported a gold content of 1.5 ppb for tectonite 

peridotite and 0.27 ppb for dunite lenses within the 

harzburgite. Cumulate phases within the complex 

contained similar concentrations. The gold content 

increased during progressive magma fractionation; gabbros within the complex had the highest concentrations 

of about 1.7 ppb. These authors suggested that sulfur 

saturation of the magma may play a role in the 

enrichment of gold in later differentiation products.

The occurrence of gold-quartz veins in Archean 

ultramafic and mafic rocks has been attributed to the fact 

that these rocks were anomalously high in gold and 

served as good source rocks (Viljoen and others, 1970; 

Anhaeusser, 1976; Keays and Scott, 1976; Pyke, 1976; 

Groves and others, 1984). However, there appears to be 

no detectable difference between the gold content of 

mineralized and unmineralized rocks (Anhaeusser and 

others, 1975). Tilling and others (1973) detected no 

appreciable differences in gold content between granitic 

and metavolcanic rocks in greenstone belts.

Saager and others (1982) compared the gold 

content of rocks from South African Archean greenstone 

belts with that of European Paleozoic ultramafic rocks. 

For the rocks from greenstone belts they obtained an 

average of 10.8 ppb (range 0.1-372 ppb), and for the 

ultramafic rocks they determined an average of 2.0 ppb 

(range 0.1-25 ppb). They found no relationship between 

gold content and major and trace element contents and

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A13
Table A2. Summary of gold contents for selected rock 

types

[Leaders (--), no data; data selected from Tilling and others, 1973; 

Allman and Crocket, 1978; Boyle, 1979]

Rock type

Granite ...............................

Granodiorite. .....................

Diorite ................ ................

Monzonite. ......... ................

Syenite ................................

Gabbro ...............................

Peridotite, dunite, etc.......

Rhyolite ..............................

Andesite .............................

Basalt..................................

Conglomerate....................

Sandstone........... ................

Siltstone.............. ................

Shale...................................

Limestone ..........................

Slate and argillite..............

Schist...................................

Gneiss.................................

Quartzite ............................

Amphibolite....... ................

Hornfels..............................

Range Average

ppb Au

.... 0.2-3.3

.... 1.3-4.3

.... 1.4-4.4

.... 0.75-0.9

.... 1.4-4.4

.... 0.5-9.9

.... 1.4-11

.... 0.5-3.5

.... 0.6-5.4

.... 0.3-9.5

.... 11-100

.... 2.1-109

.... 5.9-18

.... 2.3-57

.... 1.1-38

.... 0.3-10

.... 1.1-13.5

.... 0.6-4.3

.... 2.4-7.3

.... 2.7-12

.... 6.6-22.5

1.7

3.0

3.2

-

1.8

4.8

6.6

1.5

52] D.ZI

3.6J

--

3.01

3.0 j

2.5

2.0

1.0

2.2

3.9

4.9

--

8.5

No. of

samples

310

380

-

261

--

580

149

188

696

--

105

28

20

135

114

37

2

-

43

concluded that the gold was carried by accessory sulfides 

in the volcanic rocks, with perhaps some atomic gold 

occurring along grain boundaries. A sulfide-free 

phlogopite-peridotite from the Paleozoic ultramafic suite 

contained 0.1-0.5 ppb Au.

Keays and Scott (1976) determined the gold 

content of pillow basalts on the mid-Atlantic Ridge. The 

fresh interiors of the pillows contained from 0.2 to 0.7 

ppb Au, whereas the glassy rims contained 0.3-2.7 ppb. 

These authors concluded that the glassy material better 

preserves the original magma chemistry and the interiors 

are much more susceptible to post-emplacement alteration by sea water. Crocket and Teruta (1977) obtained 

similar gold contents for mafic rocks drilled from the 

mid-Atlantic Ridge, leg 37, Deep Sea Drilling Project. 

These authors detected a slight increase in gold values 

with depth. Anoshin and Kepezhinskas (1972) studied 

recent volcanic rocks of the Kurile-Kamchatka province 

and found no relationship between gold content and 

differentiation index in a basalt-andesite-liparite rock 

sequence. The average gold content for all rocks was 

about 1.8 ppb. They found the gold to be fairly uniformly 

distributed in the rock-forming silicates and concluded 

that it occurred as uncharged atoms. However, mafic

rocks containing sulfides, especially chalcopyrite, were 

enriched in gold.

The intrusive equivalents of mafic to intermediate 

lithologies show a slightly higher gold content than do 

extrusive types. During magma differentiation in a closed 

system, the gold is excluded from rock-forming silicates 

and will be concentrated in residual liquids along with the 

volatile components. Eventually these will be incorporated in the rock if the melt crystallizes totally within 

the magma chamber. However, if the magma vents to the 

surface, the gold, volatiles, and other incompatible 

elements will be lost. This trend appears to be reversed in 

ultramafic systems, where the gold may be incorporated 

in early crystallized refractory minerals such as spinels 

(Keays and Scott, 1976; Dupuy and others, 1981).

Korobeynikov (1981a) concluded that feldspars in 

igneous and metamorphic rocks are the carriers for 

background levels of gold, and noted that gold content 

decreased in these minerals from mafic to felsic igneous 

rocks. The highest gold values of about 7 ppb were found 

in plagioclase from intrusive basalts. In porphyritic 

varieties the plagioclase phenocrysts contained less gold 

(approximately 1 ppb) than the coexisting groundmass 

(approximately 6 ppb). Korobeynikov (1981b) also 

showed that biotite in igneous rocks has the highest gold 

content of all rock-forming silicates. The highest values 

(15-90 ppb) were found in biotites from gabbroic rocks.

There are fewer data on the gold content of various 

types of metamorphic and sedimentary rocks compared 

to igneous rocks (table A2). There appears to be a 

decrease in gold with increase in grade of metamorphism 

in quartzofeldspathic terranes. This is probably due to 

the removal of gold in solid solution and along grain 

boundaries during recrystallization. The gold is 

mobilized in the fluids released during prograde metamorphism. However, Pchelintseva and Feldman (1973) 

concluded that the gold content of metamorphic rocks 

was controlled by the original composition of the protolith and not by the grade of metamorphism. Sighinolfi 

and Santos (1976) obtained an average gold content of 

1.5 ppb for Archean granulites from Brazil. They 

concluded that high-grade metamorphism may not 

significantly affect the distribution of gold and that the 

composition of the protolith is important in determining 

its content. They conceded that gold may be mobilized 

during dehydration but emphasized that most of it will be 

reprecipitated not far from the source.

A review of the preceding geochemistry reveals 

three factors which are important in any discussion of 

possible source rocks for gold: (1) no one rock type 

appears to be enriched in gold; (2) gold content in mafic 

rocks may increase slightly with increasing age; and (3) 

gold content in mafic varieties varies significantly 

depending on geographic location. Exceptions to the lack 

of enrichment of gold are the chemical sediments

A14 Geology and Resources of Gold in the United States
occurring as interflow units in volcanic terranes and 

iron-formation (Fripp, 1976; Bavington and Keays, 1978; 

Saager and others, 1982). These rocks may represent 

protore concentrations in the eventual formation of 

metamorphogenic veins in greenstone terranes. In the 

absence of such enrichments, certain rock types may 

serve as sources because the gold occurs in an easily 

mobilized state. Saager and others (1982) suggested that 

gold occurring in association with sulfides is readily 

available to later hydrothermal solutions, and a metamorphic overprint is very important in the genesis of 

gold-quartz veins in greenstone terranes. Most such 

rocks have been metamorphosed to at least greenschist 

facies, during which the gold could have been released 

into a more accessible site.

The increase in gold content with age in mafic 

rocks is less well documented and not well understood. 

This relationship is particularly noticeable in comparing 

Archean with Paleozoic ultramafic and mafic suites 

(Saager and others, 1982). The increase in gold content 

with age may be related to the progressive differentiation 

and depletion of metals and volatiles in the upper mantle 

(Keays, 1984). The geographic variation in gold content 

in mafic rocks has been interpreted to be due to inhomogeneities in the mantle (Anoshin and Kepezhinskas, 

1972; Tilling and others, 1973; Saager and others, 1982). 

However, these inhomogeneities may have been produced by different degrees of partial melting at different 

times and places during an earlier differentiation of crust 

or upper mantle.

Consideration of source rocks with near normal 

abundances of gold requires that the volume of rock 

accessible to the leaching solutions is large enough to 

supply all the gold in a given deposit. Figure A6 gives the 

calculated volume of source rock required to form a 

deposit containing a given amount of gold. If a source 

rock contained 10 ppb Au, approximately 10 km3 of the 

rock would be required to produce a deposit containing 

10 ounces. Approximately 1 ppb would be removed 

from the source, a depletion which would be very difficult 

to detect using present analytical methods. If the source 

rock contained 225 ppb Au, only 0.05 km' would be 

required. Of course, if the efficiency of extraction was 

increased, the volume of source rock required would be 

decreased proportionately.

MECHANISMS OF GOLD TRANSPORT 

AND DEPOSITION

The solution chemistry of gold is not well 

understood because of its apparent low solubility in most 

solutions of geologic interest. Previous studies on the 

transport of gold in hydrothermal systems have 

contrasted its solubility as chloride species in acid 

oxidizing solutions with that as various sulfide species

lOOOr , 0 o/o extraction

10° !<T I03 10° 10' 

Ounces Gold

IOC

Figure A6. Calculated volume of source rock required to 

form gold deposits containing various amounts of gold. The 

diagram assumes a 10-percent efficiency in extraction and 

deposition, meaning that 10 percent of the gold in the original 

source ends up in the deposit. Three lines shown are for 

sources with 10,25, and 50 ppb Au. A fourth line for 225 ppb 

was used to determine the volume of rock required to 

produce the gold veins in parts of the Front Range mineral 

belt where bostonite dikes containing this amount of gold 

occur (Budge, 1982).

under reducing conditions. As early as 1887 Becker 

(1887) suggested that gold was soluble in sulfidecontaining solutions, based on his studies of mercury 

deposits in California. Stokes (1905) and Lenher (1918) 

demonstrated experimentally that gold is soluble in 

various chloride solutions at elevated temperatures. In 

the first relatively comprehensive experimental study on 

gold solubilities, Ogryzlo (1935) was able to dissolve 

significant quantities of this metal in weak hydrochloric 

acid solutions and in sodium bisulfide solutions at 

elevated temperatures and pressures. He concluded that 

in acid systems gold is transported as a chloride complex 

and deposition occurs as a result of reduction and (or) 

neutralization. In reduced sulfide solutions gold is 

transported as a bisulfide complex and deposition occurs 

through oxidation. Krauskopf (1951) arrived at the same 

conclusions based on a theoretical treatment of gold 

solubility. Smith (1943) concluded that gold is transported as sulfide complexes because in order for it to be 

soluble as chloride complexes, the solutions must be free 

of reducing agents. He stated that natural systems will 

have significant amounts of ferrous iron and H2 S which 

will tend to reduce the gold out of solution.

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A15
In more recent studies, Helgeson and Garrels 

(1968) evaluated gold solubilities as chloride complexes 

and concluded these species to be important in the 

formation of hydrothermal deposits. Henley (1973) 

experimentally evaluated the solubility of gold in chloride 

solutions at temperatures between 300 and 500 °C. He 

concluded that the gold was transported as an uncharged 

gold chloride complex at high temperatures, but 

conceded that ionic or sulfide complexes may predominate at lower temperatures. In contrast, the 

transport of gold as various aqueous sulfide species has 

been supported by Weissberg (1970), Seward (1973, 

1984), and Grigoryeva and Sukneva (1981). The most 

quantitative of these studies is that of Seward (1973), 

who evaluated the solubility of gold as bisulfide complexes as a function of temperature, pH, and concentration of various sulfide species. Solubilities were 

observed that were far higher than required to form gold 

deposits (see p. A17) under reasonable geologic 

conditions. In addition, he was able to postulate the 

complex stoichiometries Au(HS)2 and Au2S(HS)2 2~ 

from the solubility measurements. Grigoryeva and 

Sukneva (1981) suggested that arsenic and antimony also 

may be involved with gold complexing as well as reduced 

sulfur species in some natural hydrothermal systems.

Several aspects of the behavior of gold in aqueous 

systems can be summarized using the Eh-pH diagram in 

figure A7. This is a standard diagram calculated at 25 °C 

showing the upper and lower stability limits for water at 

1 atm. pressure and the upper limit of typical surface and 

near-surface water compositions labelled Eh = 0.859-pH 

(Sato, 1960). This diagram and others to follow have 

been calculated using thermodynamic data from Wagman and others (1969), Naumov and others (1974), 

Robie and others (1978), and Earner and Scheuerman 

(1978). In solution gold can be in the aurous (Au + ) or

auric (Au + ) state depending on the oxidation potential. 

However, auric species occur only at high Eh values at or 

above the stability limit of water, particularly with 20 ppm 

chloride present. This value was chosen because it 

represents the average chloride content of near-surface 

ground water in the vicinity of weathering sulfide 

deposits. Therefore, it may be concluded that in hydrothermal systems gold will most likely occur either as the 

aurous ion or as a complex containing this species.

Table A3 gives the possible aurous complexes 

taken from various sources listed in the preceding 

paragraph. Many of these need not be considered 

because they require solution compositions outside the 

range that are typical of hydrothermal systems (AuOH°, 

AuBr2~, AuI2~, Au(CN) 2", AuS"). The thiosulfate 

complex appears to be quite stable, as thiosulfate 

solutions are used in some heap leaching procedures 

(Berezowsky and Sefton, 1979). However, the existence 

of thiosulfate is unknown in most hydrothermal systems.

1.4

1.2

Eh

2 4 6 8 

PH

10

-0

Figure A7. Calculated Eh-pH diagram for the system 

Au-CI-H2O at 25 °C assuming an aqueous chloride concentration of 20 ppm.

Thioarsenide complexes have been postulated (Grigoryeva and Sukneva, 1981; Romberger, 1984) because of 

the common occurrence of arsenic with gold in a wide 

range of deposit types from Archean quartz veins to 

Tertiary hot springs. However, the stability relationships 

for such complexes, if they do indeed exist, are unknown. 

The same rationale was used to suggest the existence of 

a telluride complex (Romberger, 1984).

Table A3. Partial list of aqueous gold species occurring in 

natural systems

Au+, Au3+ ,

AuOH°, AuOH2+, Au(OH)2 +, Au(OH)3°,

Au(OH) 4", Au(OH) 52", Au033", 

AuCI2", AuCI3°, AuCU", 

AuOHCIs", Au(OH) 2CI2", Au(OH) 3CI~ 

AuBr2~, AuBr3°, AuBr4", 

Aul 2",

Au(HS) 2~, Au2S(HS) 22", AuS", 

Au(S2 03) 2 3", 

Au(CN) 2", 

(AuAsS2°, Au(HTe) 2 ")

A16 Geology and Resources of Gold in the United States
The solubilities and relative stabilities of the 

chloride and bisulfide complexes are shown in figure A8, 

which is an oxygen activity-pH diagram calculated at 

250 °C and assuming a solution containing 1 molal NaCl 

and 0.01 molal total sulfur (5.8 wt. percent NaCl and 320 

ppm S, respectively). This 250 °C temperature is based 

on temperatures of formation for a wide variety of 

epithermal gold deposits (table Al). The solution 

composition is considered to be in the right order of 

magnitude for hydrothermal gold-depositing systems; 

significant changes in these solution parameters would be 

required to change the solubility relationships appreciably. The gold solubility as chloride complexes is 

calculated using thermodynamic data from Wagman and 

others (1969); the solubility of the bisulfide complexes is 

calculated from stability constants derived by Seward 

(1973). Only the Au(HS)2 species is considered here; 

the effect of including Au2S(HS)22~ would be to increase 

gold solubility in the alkaline region. This diagram 

confirms the conclusions of earlier investigators that gold 

will be soluble as the chloride complex in acid oxidizing 

solutions and as the bisulfide complexes under reducing 

conditions in solutions where sulfur would occur as 

either HiS or HS". It is apparent from the shape of the 

contoured solubility surface that in the case of the 

chloride complex, increase in pH and (or) reduction can 

result in gold deposition from a saturated solution. The 

maximum solubility of gold as bisulfide complexes occurs 

at slightly alkaline pH's just below the boundary between 

sulfide and sulfate. Both increase and decrease in pH can 

result in gold deposition, depending on the starting 

composition of the solution. However, because of the 

steep slope on the solubility surface towards increasing 

oxygen activity, oxidation is a much more efficient 

mechanism for gold deposition.

Henley (1973) and Seward (1973) showed that the 

solubility of gold as chloride and bisulfide complexes 

respectively increased with increasing temperature. 

Therefore, cooling would be an adequate mechanism for 

gold deposition from a saturated solution. However, 

temperature affects other parameters, such as oxygen 

activity, which may have a greater influence on solubility, 

so that temperature effects in natural systems are 

difficult to evaluate. This discussion will assume temperature to be relatively constant, which is not unreasonable considering that geothermometric studies 

indicate little temperature variation within any one 

mineralizing episode.

Pressure is even more difficult to assess. Generally, 

increase in pressure increases mineral solubility; however, Seward (1973) noted that its effect on gold 

solubility in sulfide solutions depended on both 

temperature and pH. In epithermal systems where the 

pressure-temperature conditions are very close to the 

liquid-vapor curve for the solution, the only significant

-I0r

-50

Figure A8. Calculated oxygen activity-pH diagram for the 

system Au-NaCI-S-H2O at 250 °C showing the relative 

solubilities of gold chloride and bisulfide complexes: 1 m. 

NaCl and 0.01 m. S in aqueous solution.

effect of pressure will be to influence the solubility of 

volatile components such as carbon dioxide and sulfur 

gases. These are very important in controlling the pH and 

oxygen fugacity of the solutions, as will be discussed later 

(Drummond, 1981).

The solubilities shown for bisulfide complexes far 

exceed values expected in typical hydrothermal mineralizing systems even if in error by one or two orders of 

magnitude. For this reason Romberger (1984) suggested 

that mineralizing solutions may be undersaturated with 

respect to gold, and the gold is coprecipitated with 

sulfides during their deposition. The gold is subsequently 

released to the native state through some process of 

recrystallization. If this is the operating process, the 

usefulness of these phase relations is limited to show how 

the gold is transported and the stability relationships of 

the associated minerals; little can be concluded about the 

mechanisms of deposition of the gold itself.

Figure A9 superimposes the phase relations for 

iron sulfide and oxides on the gold solubility relationships 

plotted in figure A8 assuming the concentration of 

dissolved iron to be 10 ppm. The quality and usefulness 

of this and subsequent diagrams will depend on the 

reliability of the thermodynamic data used for their

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A17
calculation. The patterns will remain the same; however, 

the absolute limits on the coordinates will change 

somewhat with small (a few kilocalories) differences in 

thermodynamic parameters. Therefore, even though the 

data may not be of the highest quality, these diagrams are 

conceptually very useful in establishing relative solutionmineral equilibria and trends in parameters which 

influence the systems. The stable common minerals 

shown here, and others discussed later, serve as 

indicators of the conditions of ore deposition, so that the 

presence of pyrite in a hydrothermal mineral assemblage 

suggests that the oxygen activity-pH conditions of the 

solutions responsible for gold deposition fall within the 

pyrite field. Pyrite is very common in many hydrothermal 

gold deposits, and in most of these a close paragenetic 

relationship obtains between pyrite and gold. Figure A9 

clearly shows that for these deposits the gold is most 

likely transported as a bisulfide complex and the most 

likely mechanisms of deposition are oxidation, 

diminished activity of dissolved sulfur, and (or) coprecipitation with pyrite. In contrast, in natural systems 

dominated by oxidized species such as hematite, gold 

would be transported as a chloride complex and 

deposition might occur as a result of neutralization and 

(or) reduction.

Figures A8 and A9 were prepared assuming the 

concentration of dissolved sulfur remains constant. 

However, this may not be the case in natural systems, 

because the precipitation of sulfides will decrease the 

activity of dissolved sulfide species, as will the loss of 

volatile sulfur species if the hydrothermal system is 

boiling. Figure A10 shows the gold solubility relationships superimposed on the relative stability of various 

iron minerals in terms of the activities of oxygen and 

sulfur gas at 250 °C and a constant pH of 5. The positions 

of the pyrite, pyrrhotite, and hematite fields are shown as 

in the earlier diagram. However, chlorite is shown here to 

replace the magnetite field because of the common 

occurrence of chlorite as an alteration mineral in gold 

deposits and the nearly total absence of magnetite. (See 

Barton and others, 1977, for the rationale used for 

replacing magnetite with chlorite in these diagrams.) The 

diagram shows that within the stability field of pyrite, the 

solubility surface slopes steeply in the direction of lower 

82 activities so that gold and pyrite can be coprecipitated 

as a result of declining sulfur activity. The diagram also 

does not preclude the coprecipitation of gold with pyrite 

in a hydrothermal system undersaturated with respect to 

gold.

Many hydrothermally altered rocks contain 

abundant chlorite and pyrite, which would suggest that 

these rocks would buffer the system in terms of oxygen 

and sulfur activities along the boundary between these 

two minerals. However, the solubility contours are subparallel to this boundary, suggesting that gold would not

-lOr

-20

o

o 

o> 

o

-40-

Figure A9. Calculated oxygen activity-pH diagram for the 

system Au-Fe-NaCI-S-H2O at 250 °C showing relative 

stabilities of various iron minerals (heavy solid lines) 

superimposed on the gold solubility relationships (dashed 

lines) from figure A8; 1 m. NaCI, 0.01 m. S and 10 ppm Fe in 

aqueous solution.

be deposited in systems buffered by this assemblage; yet, 

the contrary is observed. An alternative explanation is 

that the gold is coprecipitated with pyrite. In addition, 

Romberger (1984) showed that if gold is transported as a 

thioarsenide complex, the solubility contours are oblique 

to this phase boundary, and deposition would occur as a 

result of reductions in both oxygen and sulfur activities.

Alteration and gangue minerals that occur with the 

ore assemblage can be very useful in indicating the 

chemical conditions of deposition and also the chemical 

changes associated with ore precipitation. In addition, 

when the temperature and residence time for solutions in 

source rocks or aquifers are sufficient, the solutions 

begin to take on a chemical character which is governed 

by those source rocks. For example, solutions derived 

from carbonaceous sedimentary rocks will be reducing 

and can be assumed to be high in reduced sulfur species. 

It is the contrast between the geochemical environments 

of the host and source rocks which commonly is 

responsible for the chemical changes causing ore 

deposition.

A18 Geology and Resources of Gold in the United States
-25-

-20

Figure A10. Calculated oxygen activity-sulfur activity 

diagram for the system Au-Fe-NaCI-S-H2O at 250 °C and pH 

5 showing the relative stabilities of various iron minerals 

(heavy solid lines) and gold solubility relationships (light solid 

lines); 1 m. NaCI in aqueous solution.

Figure All superimposes the stability relationships for the common potassium aluminosilicate 

alteration minerals and the phase relationships for 

various forms of carbon onto the earlier derived solutionmineral equilibria at 250 °C and assuming an aqueous 

potassium concentration of 100 ppm, a calcium 

concentration of 100 ppm, and COi partial pressure of 1 

atm. Alunite and kaolinite would characterize alteration 

assemblages produced by acid to slightly acid solutions; 

sericite would be formed by neutral to slightly alkaline 

solutions; and potassium feldspar would be a product of 

alkaline solution alteration (neutral pH at 250 °C is 

approximately 5.5). Carbonate minerals would be 

deposited by solutions with oxygen fugacities above the 

graphite field and at pH's above the carbonate solubility 

line shown. At lower oxygen fugacities below the carbon 

field, methane would be stable. In this diagram the 

stability field of graphite has been calculated. However, 

the stability field of more complex forms of carbon would 

have the same shape and be in a similar position to that

shown. Hydrocarbon fields would fall between methane 

and graphite, depending on the carbon-to-hydrogen 

ratio.

Figure All shows clearly that solutions which 

attain chemical equilibrium with pyritiferous carbonaceous sedimentary rocks would be reduced in nature and 

be of the composition appropriate for the solution of gold 

as bisulfide complexes. The pH of these solutions would 

be determined by CChiCHi ratios or the silicate 

assemblage in the source rocks.

Sericite is a common mineral occurring in 

alteration haloes around gold veins in a wide variety of 

deposit types and geologic terranes from veins in Tertiary 

volcanic rocks (Casadevall and Ohmoto, 1977) to veins in 

Archean greenstone terranes (Boyle, 1979). Spatial 

relationships suggest that the sericite is genetically 

related to gold deposition in many places. Figure All 

shows that the stability field of sericite overlaps the area 

of maximum gold solubility as a bisulfide complex, and 

therefore sericite would be expected as the most 

common potassium aluminosilicate in alteration

-I0r

-20

-40-

-50

Figure A11 . Calculated oxygen activity-pH diagram for the 

system Au-Fe-CaO-K2O-AI2O3 -SiO2 -NaCI-C-S-H2O at 250 

°C and CO2 partial pressure of 1 atm. showing relative 

stabilities of aluminosilicate alteration minerals (light vertical 

solid lines) and carbon (lightest solid lines) superimposed on 

the stability relationships for iron minerals (heavy solid lines) 

and gold solubilities (dashed lines); 1 m. NaCI, 0.01 m. S, 0.1 

m. Ca, 100 ppm K and 10 ppm Fe in aqueous solution.

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A19
assemblages produced by solutions originating in this 

field if simple oxidation was the prevailing mechanism of 

deposition.

Adularia (orthoclase) is commonly found spatially 

associated with precious metals in many epithermal gold 

deposits (Buchanan, 1981). In a vertically zoned hydrothermal system, the adularia zone usually narrows and 

disappears less than 100 meters above the ore zone. In 

figure All adularia indicates alkaline pH's. The presence 

of this mineral in hydrothermal veins can be due to 

increases in pH as a result of the loss of carbon during 

boiling (see p. A22), or the original solutions had a 

higher initial pH.

Argillic alteration assemblages are widespread in 

many gold districts and may occur as haloes around 

relatively deep precious and base metal veins, or in quite 

shallow precious metal and hot springs systems. These 

assemblages may consist of varied proportions of kaolinite, montmorillonite, illite, sericite and (or) alunite. All of 

these indicate solution pH's at or below 5. Kaolinitealunite-silica assemblages also are found as layered caps 

around hot spring centers, many of which contain 

anomalous amounts of gold. Figure All shows that 

alunite and kaolinite would be expected in assemblages 

formed by acid oxidizing solutions, and these minerals 

are referred to commonly as the acid assemblage. 

Alunite would not be expected to form directly from 

solutions transporting gold as bisulfide complexes even 

though there appears to be a genetic relationship 

between alunite and gold deposits. The acid assemblage 

has been interpreted to be a result of downwardpercolating sulfuric acid solutions which, in turn, had 

been a product of mixing of oxidized ground waters with 

H2S vapors rising from below. This assemblage is 

diagnostic of epithermal gold deposits showing evidence 

of boiling, and it may be a reflection of events occurring 

at depth in a vertically zoned hydrothermal system 

(Berger, 1982; Henley and Ellis, 1983).

Arsenic occurs ubiquitously in hydrothermal gold 

deposits of all types, so a consideration of the phase 

relations for this element is appropriate here. In primary 

ores arsenic occurs most commonly as realgar or arsenopyiite, depending on deposit type. Orpiment occurs 

associated with realgar in certain sedimentary rockhosted deposits. Figure A12 shows the stability fields for 

various arsenic minerals and compounds superimposed 

on the oxygen-sulfur activity diagram previously given as 

figure A10. The presence of arsenic sulfides limits the 

oxygen fugacity in these hydrothermal systems. In 

chemically complex natural systems, arsenic oxide would 

be replaced by other phases such as scorodite, 

FeAsO4.2H2O. This mineral is found in some 

disseminated gold deposits; however, it is a weathering 

product of the original sulfide ores. Also shown are the

position of the carbon field at a CO2 pressure of 1 atm. 

and the boundary between reduced and oxidized sulfur 

species.

In view of these phase relationships, it is noteworthy to recognize that a very common, if not 

ubiquitous, assemblage in many disseminated gold 

deposits is pyrite, realgar, and gold in a carbonaceous 

host. Figure A12 shows that solutions in equilibrium with 

this assemblage would transport gold as the bisulfide 

complex, and deposition from saturated solutions would 

occur as a result of oxidation or reduction of sulfur 

activity. Solution compositions conducive to the transport 

of gold as the chloride complex lie at oxygen fugacities 

well above the stability fields of sulfides, and chloride 

complexing is considered unimportant at temperatures 

typical for most gold deposition.

Figure A13 shows the stability field for arsenopyrite in terms of the same parameters used above. Certain 

Precambrian gold deposits are dominated by the 

assemblage chlorite, arsenopyrite, and iron sulfide, either 

pyrrhotite or pyrite. Even though the temperature of 

formation of these deposits is higher than 250 °C (Groves

O.lppbAu 

0.01 ppb Au 

-40-

-20 -15 -10

log as .

Figure A12. Calculated oxygen activity-sulfur activity 

diagram for the system Au-Fe-As-NaCI-C-S-H2O at 250 °C, 

pH 5, 1.0 m. NaCI, and CO2 partial pressure of 1 atm., 

superimposing the relative stabilities of arsenic (light solid 

lines) and carbon phases (diagonal line pattern) on the 

relationships shown in figure A10.

A20 Geology and Resources of Gold in the United States
and others, 1984), the phase relations shown here suggest 

that the mineralizing solutions were dominated by 

reduced sulfur species and gold was most likely transported as bisulfide complexes.

To summarize the solution chemistry described 

herein, in hydrothermal systems where gold may be 

accompanied by pyrite and various arsenic sulfides, the 

composition of the solutions had to be such that the gold 

was transported as bisulfide complexes. Gold chloride 

complexes would predominate only in a few systems 

dominated by hematite or other oxides, or at higher 

temperatures than those determined for typical epithermal deposits. Alteration assemblages containing sericite and (or) potassium feldspar are consistent with the 

solutions being near neutral or slightly alkaline. Even 

though the acid assemblage, including alunite and kaolinite, commonly occurs in gold deposits, it is probably 

formed above the zone of gold deposition by processes 

which are indirectly related to those responsible for the 

precipitation of gold itself. Gold may occur with alunite 

as a result of the expansion of the hydrothermal system 

from below and the superposition of a depositional event

-25-,

-30

CM 

O

o 

o»

O

-35

-40

-20

O.lppbAu 

-15 -10

log aS2

Figure A13. Calculated oxygen activity-sulfur activity 

diagram for the system Au-Fe-As-NaCI-S-H2O at 250 °C, pH 

5, and 1.0 m. NaCI, showing the stability field of arsenopyrite 

(heavy solid lines) superimposed on the solution-mineral 

equilibrium relationships of figure A10. Aspy, arsenopyrite; 

Orp, orpiment; Py, pyrite; Chl, chlorite; Po, pyrrhotite.

onto an earlier formed argillic assemblage. The evidence 

for this is the occurrence of fresh euhedral pyrite with 

alunite and (or) kaolinite, an assemblage not predicted to 

form contemporaneously based on the phase 

relationships.

If the gold is transported as bisulfide complexes, 

the most efficient mechanisms of deposition from 

saturated solutions at relatively constant temperatures 

are oxidation and (or) decrease in sulfur activity. 

Changes in pH probably are not important in the 

deposition of gold itself, although they do occur as 

evidenced by the zoned nature of alteration assemblages 

(Buchanan, 1981). Hydrothermal solutions most 

certainly are not saturated with respect to gold through 

most of their existence. Figure A14 shows the path that a 

solution may take starting out with a gold concentration 

of 0.004 ppb and buffered by the mineral pair pyrrhotite 

plus pyrite (Weissberg and others, 1979). The solution 

would not become saturated with respect to gold upon 

oxidation until the composition intersected the solubility 

surface (crosslined on fig. A14) as shown. However, 

upon piercing the saturation surface, the solubility of 

gold decreases rapidly and deposition would occur. 

Alternatively, the solution may never become saturated 

and the gold could be coprecipitated with sulfides. Once 

these mechanisms of deposition have been established, 

the possible natural processes responsible for these 

changes must be addressed. Processes occurring in 

hydrothermal systems which may result in solution 

oxidation are boiling (or effervescence), mixing with 

more oxygenated ground waters, or contact with a more 

oxygenated host rock. Processes which may result in 

reduction in sulfur activity are boiling and (or) 

effervescence and precipitation of sulfides.

Drummond (1981) addressed the chemical 

changes occurring during both boiling and fluid mixing. 

During boiling there will be a partitioning of reduced 

volatile components into the vapor phase, resulting in 

oxidation of the residual liquid. Drummond (1981) 

calculated the solution paths for several compositions in 

terms of oxygen fugacity and pH at 250 °C and found 

that, depending on the starting composition, the solution 

has an initial increase in pH followed by a sharp increase 

in oxygen fugacity. His solution composition paths are 

consistent with transport of gold as a bisulfide complex 

and precipitation as a result of oxidation due to boiling.

Many epithermal gold deposits have shown 

evidence that boiling was occurring in the mineralizing 

system; the results of several studies have been 

summarized by Buchanan (1981). Numerous more 

recent studies also result in the conclusion that boiling is 

an important part of the mineralizing process, as there 

appears to be both a spatial and temporal relationship 

between a boiling event and precious metal deposition. 

The models proposed by Buchanan (1981) and Berger

Introduction to Geology and Resources of Gold, and Geochemistry of Gold A21
I ppm -

I ppb0.001 -

o 

en

'6

IO

io-9

(ppb)

Po Oxide/Chl.

-50 -40 -30

log

Figure A14. Calculated gold solubility versus oxygen 

activity at 250 °C, pH 5, 1.0 m. NaCI, 0.01 m. S, and CO2 

pressure of 1 atm. showing the solution path during oxidation 

of a solution starting with 0.004 ppb Au buffered by the 

mineral pair pyrite-pyrrhotite. NZ, New Zealand.

(1982) incorporate boiling into their mineralizing 

systems. The mineralization commonly is restricted to a 

relatively narrow vertical range and its elevation 

commonly shows a spatial relationship to a paleosurface. 

The geology of many of the deposits suggests that they 

formed at relatively shallow depths, commonly with 

evidence of hot spring activity, so boiling of solutions at 

temperatures in the vicinity of 250 °C is not an 

unexpected event.

In addition to oxidation and gold deposition, 

boiling will have other relevant effects on the mineralizing system. Alteration assemblages within the 

narrowly telescoped vertical zonation may be explained 

by changes in the pH of the solutions, which itself is a 

result of loss of acid volatiles such as CO2. The acid 

assemblage already described is a product of sulfuric acid 

solutions formed by the oxidation of H2S vapors, which 

had to originate in a boiling system. This same loss of 

H2S would result in the decrease of sulfur activity in the 

residual liquid and may cause gold deposition from a

saturated solution if the gold is transported as bisulfide 

complexes. Finally, gold itself may be transported in the 

vapor phase as the highly speculative thioarsenide species 

AuAsS2 , as this is a neutral molecule. This species 

would redissolve in the condensate forming in the cooler, 

shallow parts of the system and precipitation would result 

from oxidation. This mechanism would result in a very 

shallow near-surface zone of gold deposition, a feature 

recognized in many hot spring and hydrothermally 

mineralized systems.

Mixing of rising hydrothermal solutions with cooler 

oxygenated ground water may be an important 

mechanism for gold deposition in some systems. Theoretically, deposition should occur along the mixing 

interface. However, these are very dynamic systems, and 

convection, turbulence in open structures, and refluxing 

would destroy any regular mixing surface, so that kinetics 

would be more important in controlling mineralization 

than simple solution-mineral equilibria. The same would 

be true for transporting solutions entering oxygenated 

host rocks, particularly where the water:rock ratio is high. 

The rocks would take on the chemical characteristics of 

the hydrothermal solutions with time, and the driving 

forces for deposition would disappear until a change in 

the hydrologic regime occurred. Earlier formed ore 

minerals may be redissolved by later solutions dominated 

by either ascending hydrothermal solutions or descending ground water in areas of recharge in a convecting 

system. The result will be periods of acidification and 

(or) oxidation alternating with those of reduction and 

(or) neutralization, producing complex and unpredictable zoning and paragenetic relationships.

Thus, geochemical models can be postulated which 

incorporate plausible natural processes and at the same 

time are consistent with fundamental solution-mineral 

equilibrium principles. However, care must be taken 

when applying these models so that equilibrium principles fit paragenetic relationships, and also it must be 

understood that the deposits are commonly products of 

multiple events. Each event will have its own compositional history in terms of oxidation state, pH, and 

sulfur activity, among other parameters; and the result of 

superimposing these events one upon the other must be 

considered. Finally, the hydrologic environment is very 

important in controlling ultimate source rocks, the 

affected hosts for ores, and the kinetics of the reactions 

responsible for ore deposition.

Manuscript received by scientific editors September 1984

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introduction to Geology and Resources of Gold, and Geochemistry of Gold A25

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