* DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, DIRECTOR

BULLETIN 529

THE

ENRICHMENT OF SULPHIDE ORES

BY

WILLIAM HARVEY EMMONS

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1913

CONTENTS.

Page.

Introduction.............................................................. 9

Statement of theory.................................................... 9

Sources of information and acknowledgments........................... 11

Distinctions between primary and secondary deposits.................... 12

Physical conditions and enrichment........................................ 16

Climate.............................................................. 16

Temperature..................................................... 16

Rainfall........................................................... 18

Altitude.............................................................. 19

Relief................................................................ 19

Permeability................................. .v............. w......... 20

Influence of rate of erosion on rate of sulphide enrichment................ 2.1

Age of the primary deposits and their periods of weathering............... 22

Paleophysiography................................................'.... 23

Structural features and blind lodes..................................... 24

Glaciation............................................................. 24

The underground circulation............................................... 26

Openings in the earth's crust.......................................... 26

The level of ground water.............................................. 27

The vadose circulation................................................ 28

The deeper circulation................................................ 29

The region of nearly stagnant waters.................................... 30

The several successive zones................................................ 31

Downward change in sulphide deposits................................. 31

The oxidized zone.............................................. '...... 32

Conditions in the oxidized zone.................................... 32

Segregation of the metals in the oxidized zone....................... 33

Depth of the oxidized zone............................... .>....... 36

Submerged oxidized ores........................................... 37

Transitions between oxidized and sulphide ores..................... 38

The secondary sulphide zone................................:.......... 39

Position and extent................................................ 39

Relation to the water level......................................... 4.1

Precipitation of sulphides above the water level...................... 42

Textures of secondary sulphide ore.................................. 44

Estimates of vertical extent of portions of lodes eroded.................. 47

Chemistry of sulphide enrichment.......................................... 48

Experimental data on the solution and precipitation of the metalo........ 48

 Composition of mine waters............................................ 60

Analyses......................................................... 60

Discussion of analyses............................................. 63

Mines represented............................................. 63

3
4 CONTENTS.

Chemistry of sulphide enrichment Continued. 

Composition of mine waters Continued.

Discussion of analyses Continued. Page. 

Mineral changes indicated..................................... 65

Sulphates and sulphuric acid ............................... 65

Chlorides................................................. 66

Carbonates................................................ 67

Silica.................................................... 68

Arsenic and antimony..................................... 68

Nitrates.................................................. 68

Phosphates............................................... 68

Potassium................................................ 68

Sodium................................................... 68

Calcium.................................................. 68

Magnesium............................................... 69

Aluminum................................................ 69

Manganese.................................................. 69

Iron....................................................... 69

Copper..................................................... 70

Zinc....................................................... 70

Gold and silver............................................ 70

Changes in composition-depending on depth..................... 70

Precipitates from mine waters under superficial conditions....... 73

Oxidation and solution of certain metallic sulphides...................... 75

Factors concerned................................................. 75

Relative rate of solution of several sulphides in an oxidizing environment. ........................ :\ ................................ 76

Net results of chemical changes during oxidation of certain ores..... 78

Relative rate of solution of several sulphides in a reducing environment. 83 

Metasomatic replacement of primary sulphides by secondary sulphides 

and relations indicated by Schuermann's series..................... 84

Source of the sulphur of secondary sulphides and generation of hydrogen 

sulphide............................................................. 87

Processes of sulphide enrichment compared with the process of separating 

the" metals in qualitative chemical analysis............................ 88

Decrease in acidity of descending sulphate waters....................... 89

Changes in the state of oxidation of descending acid sulphate solutions... 90 

Rate at which hydrogen sulphide is generated from several primary sulphides by cold dilute acid sulphate waters.............................. 91

Composition of the primary ore as a factor determining the vertical extent 

of the secondary sulphide zone......................................... 92

Some chemical relations and minerals of copper......................... 95

Principal copper minerals.......................................... 95

Solubility of some copper compounds............................... 95

Nature and relations of the copper minerals.......................... 95

Occurrence of the copper minerals................................... 93

Some chemical relations and minerals of silver........................... 114

Principal silver minerals............................................ 114

Solubilities of salts of silver.......................................... . 114

Nature and relations of the silver minerals.......................... 114

Occurrence of the silver minerals.................................... 118

Some chemical relations and minerals of gold............................ 125

Solubility of gold................................................... 125

Precipitation of gold with manganese oxide......................... 128
CONTENTS. 5

Chemistry of sulphide enrichment Continued.

Some chemical relations and minerals of gold Continued. Page.

Sources of manganese in gold deposits................................ 130

Placers and outcrops...................... *......................... 131

Concentration in the oxidized zone.................................. 132

Vertical relation of deep-seated enrichment in gold to clialcocitization.. 133

Gold and silver tellurides........................................... 135

Some chemical relations and minerals of lead............................ 135

Principal lead minerals............................................. 135

Solubilities of lead compounds...................................... 135

Occurrence of the lead minerals.................................... 136

Some chemical relations and minerals of zinc............................ 140

Principal zinc minerals............................................. 140

Solubilities of zinc compounds...................................... 140

Occurrence of the zinc minerals.................................... 141

Some chemical relations and minerals of iron.............................. 143

Principal iron minerals............................................. 143

Solubilities of iron salts............................................ 143

Nature and relations of the iron minerals............................. 143

Occurrence of the iron minerals.................................... 145

Some gangue minerals................................................. 150

Nature and genesis................................................ 150

Some aluminum minerals........................................... 151

Occurrence.................................................... 151

Feldspars..................................................... 152

Sericite....................................................... 152

Kaolin........................................................ 153

Alunite..............."....,................................... 154

Gibbsite, bauxite, and diasporc.................................. 154

Silica.............................................................. 155

Sulphur and sulphates.............................................. 157

Solubility..................................................... 157

Occurrence.................................................... 158

Carbonates.......................................................... 161

Summary review of several classes of deposits................................. 162

Copper deposits with little or no chalcocite .............................. 162

Copper deposits with shallow chalcocite zones............................ 164

Copper deposits with moderately deep chalcocite zones.................. 164

Copper deposits with deeply oxidized zones............................. 165

Gold deposits exhibiting secondary concentration....................... 166

Silver and silver-gold deposits.......................................... 168

Review of mining districts.................................................. 170

Butte, Mont.......................................................... 170

Bingham, Utah....................................................... 177

Bisbee, Ariz., by F. L. Ransome....................................... 179

Globe and Miami, Ariz., by F. L. Ransome.............................. 183

Ray, Ariz., by F. L. Ransome......................................... 186

Morenci, Ariz.......................................................... 187

Ely, Nev., by A. C. Spencer............................................ 189

Santa Rita and Hanover districts, N. MX ......... v..................... 191

Jerome, Ariz., by F. L. Ransome...................................... 192

Burro Mountain district, N. Mex........................................ 193

Cananea, Mexico....................................................... 194
6 CONTENTS.

Review of mining districts Continued. I»age.

Nacozari, Mexico...................................................... 196

Silverbell, Ariz....................................................... 197

San Francisco region, Utah, by B. S. Butler............................ 197

ShaetaCounty, Cal..................................................... 199

Yerington, Nev........................................................ 200

Vancouver, British Columbia........................................... 201

Snowstorm lode, Cceur d'Alene district, Idaho........................... 202

Copperopolis, Cal..................................................... 202

Boleo, Lower California.... ...............................^............. 203

Braden Copper mines, Chile............................................ 203

Queen of Bronze mine, Oreg............................................ 204

Encampment district, Wyo............................................ 204

Ducktown, Tenn...........................'........................... 205

Gossan lead, Va........................................................ 207

Stone Hill, Ala......................................................; 207

Ely, Vt............................................................... 207

La Dicha, Mexico..................................................... 208

Santiago, Cuba....................................................... 208

Pinar del Bio, Cuba.................................................... 209

Boundary district, British Columbia.................................... 209

San Jose, Mexico...................................................... 210

La Reforma mine, Campo Morado, Mexico.............................. 211

Velardefia district, Mexico.............................................. 211

Sudbury, Ontario..................................................... 212

Prince William Sound, Alaska.......................................... 214

Kasaan Peninsula, Prince of Wales Island, Alaska....................... 214

Bonanza mine, Chitina copper belt, Alaska.............................. 215

Mother Lode district, Cal. ...........*.................................. 216

Nevada City and Grass Valley,"Cal...................................... 216

Ophir, Cal............................................................. 217

Silver Peak, Nev...................................................... 218

Edgemont, Nev........................................................ 218

Douglas Island, Alaska................................................ 218

Berners Bay region, Alaska............................................... 219

Black Hills, S. Dak................................................... 219

Hedley district, British Columbia....................................... 220

Appalachian districts.................................................. 221

Annie Laurie mine, Utah............................................. 222

Manhattan, Nev...................................................... 222

Bullfrog, Nev........................................................ 223

Gold Circle, Nev..................................................... 223

Delamar mine, Nev................................................... 223

Exposed Treasxire mine, Cal............................................ 224

Bodie, Cal........................................................... 225

Little Burro Mountains, N. Mex....................................... 2*5

Marysville, Mont....................................................... 226

Summit district, Colo.................................................. 226

Bohemia mining district, Oreg......................................... 228

San Juan region, Colo ................................................. 228

Cripple Creek, Colo................................................... 229

Georgetown quadrangle, Colo.......................................... 232

Goldfield, Nev....................................................... 233
CONTENTS. 7

Review of mining districts Continued. Page.

Philipsburg, Mont. ................................................... 233

Neihart, Mont......................... .V ............................ 234

Tintic district, Utah................................................... 235

Georgetown, Colo. (silver deposits)...................................... 236

Breckenridge, Colo................................................... 237

Creede, Colo.......................................................... 239

Rico, Colo............................................................ 240

Lake City, Colo....................................................... 240

Aspen, Colo........................................................... 241

Leadville, Colo....................................................... 242

Eureka, Nev......................................................... 244

Pioche, Nev......................................................... 245

Tonopah. Nev............................................ __._...... 245

Comstock lode, Nev .................................................. 246

St. Eugene mine, Moyie, British Columbia............................ 249

Cobalt, Ontario ...................................................... 250

Rossland, British Columbia................'........................... 251

Coeur d'Alene. Idaho (lead-silver deposits) ...................... ..... 252

Index................................................................... 253

THE ENRICHMENT OF SULPHIDE ORES.

By WILLIAM HARVEY EMMONS.

INTRODUCTION.

STATEMENT OF THEORY.

Sulphide ores exposed to weathering at or near the surface of the 

earth break down and form soluble salts and minerals that are stable. 

under surface conditions. No metallic sulphide that is long exposed to 

air and water remains unaltered. Iron sulphides, which are present in 

practically all sulphide ores, are changed by weathering to iron oxides, 

and the changes are attended by the liberation of sulphuric acid. 

Many of the metals form soluble sulphates with sulphuric acid, and 

when conditions favor their migration downward they are carried in 

solution to depths where air is excluded. Unoxidized rocks are in 

general alkaline. Acid solutions that encounter such rocks in the 

regions where air is excluded will lose acidity, and as the solutions 

approach a neutral or alkaline condition some of the metals they contain are deposited. If the solutions of the metallic sulphates encounter 

metallic sulphides in depth precipitation may take place, or there may 

be an interchange between the metals in solution as sulphates and 

the metallic sulphides. Thus as a result of precipitation or chemical 

interchange the metals are redeposited and certain portions of the 

ore bodies become enriched.

The theory of sulphide enrichment, announced in 1900, has been 

found to have wide application. Many deposits, especially of ores of 

copper, are leached near the surface and are richer below the leached 

zones. The ore at still greater depths is of lower grade. According 

to the theory the copper leached from the upper zone has been carried 

downward and redeposited, forming an enriched zone, and the deeper, 

lower-grade sulphide ores are assumed to be like the original or 

primary mineralization of the whole deposit. Some deposits of gold 

ores and of silver ores show similar features, but the changes in the 

character of the ore in such deposits are generally less notable. Relatively few deposits of lead and zinc have clearly defined secondary 

sulphide zones, although some such deposits show appreciable enrichment. The theory of sulphide enrichment has now become fairly well 

understood and has proved to be of considerable economic value, for 

it has been successfully applied in the development of many deposits 

of the metallic sulphides.
10 I'HE ENRICHMENT OF SULPHIDE ORES.

Similar geologic processes operating under approximately similar 

conditions produce approximately similar results, and any theory 

formed to explain the relations that exist in some deposits might 

reasonably be cited to explain the absence of like relations in other 

deposits. It has been found, however, that the theory of sulphide 

enrichment does not apply to all deposits and that many valuable 

deposits of sulphide ores are primary. It is not applicable even to 

all deposits of copper-bearing sulphides, in which secondary changes 

that have taken place are recorded more clearly than in deposits of 

other metals. Moreover, it is not clear why some deposits show 

extensive enrichment and others show little or none.

As data are accumulated some of the geologist's most cherished 

hypotheses may be overthrown. Until recently nothing seemed to 

rest on a firmer foundation than the belief that the presence of chalcocite is almost certain evidence of enrichment by cold solutions, but 

recent studies have shown that, without much doubt, this mineral is a 

primary constituent of certain copper ores that have been deposited 

by ascending thermal waters.

The recent development at Bisbee, Ariz. (p. 182), of extensive and 

deep oxidized ores related to a pre-Comanche erosion surface which is 

not parallel to but is inclined to the present underground water level; 

the development at Miami, Ariz. (p. 185), of an extensive chalcocite 

zone not definitely related to but apparently older than the present 

topography; and the exceptional features of the secondary deposits 

in several other districts clearly indicate that the theory of sulphide 

enrichment can not be applied perfunctorily to any deposit. The 

deductions that may be drawn from it are generally of so great 

commercial importance that its use is fraught with danger unless it 

is used with an adequate understanding of the general geology of the 

region containing the deposits to which it is applied.

The chemistry of sulphide enrichment offers an especially attractive 

field for experimental study because the natural conditions' of temterature, pressure, and concentration can be closely duplicated in 

the laboratory, but at present the results of chemical experiments in 

ore enrichment leave much to be desired. It should be acknowledged 

at the outset that some of the data concerning sulphide enrichment 

appear to be conflicting and that many of the problems are now 

only in process of solution.

This paper is offered not as a contribution of many original data, 

but rather as a summary of a part of the existing knowledge of the 

subject. I hope it may suggest to those who are engaged in geologic 

work and in mining some lines of approach to the problems to be 

solved. The solution of these problems must depend very largely on 

the observations of .the engineer or geologist who can watch the
SOtJRCES OF iN-FOKMATIOif. 11

changes in the successively excavated parts of an ore body. He has a 

distinct advantage over the official geologist, who visits it only at one 

stage of its development, perhaps after the most illuminating exposures 

have been destroyed. To the chemist this paper is an appeal for more 

experimental data on the important mineral syntheses involved in the 

processes. I realize fully that there may be serious omissions and 

possibly serious errors in the discussion of the complex and scattered 

data here presented, and I shall esteem it a favor if anyone whose 

statements I may have misquoted or misinterpreted will set me 

right, and if those whcPhave had superior opportunities for study of 

certain districts will correct any wrong impressions that I may have 

given.

SOURCES OF INFORMATION AND ACKNOWLEDGMENTS.

Among the papers that treat the subject of sulphide enrichment 

are those of S. F. Emmons, 1 W. H. Weed,2 and C. K. Van Hise.3 These 

papers were issued in 1900, almost contemporaneously, and in them 

the writers, working independently, assembled the geologic and 

chemical data bearing on the problem, formulating a clearly stated 

theory from scattered and undigested observations. Five years 

later Kemp 4 wrote a summary of the processes of secondary enrichment of ore deposits of copper, and in 1910 Ransome 5 issued a comprehensive review of the criteria of downward sulphide enrichment, 

in which he treats concisely the various processes of enrichment and 

their results. The nearly related processes of oxidation of ore deposits 

were discussed by Penrose in 1894. 8

During the 12 years that have passed since the first papers on sulphide enrichment were issued a number of detailed reports have 

appeared, treating the geology and ore deposits of certain mining 

districts. In several of these papers the chemical processes involved 

are discussed in the light of the field relations of the deposits. The 

contributions of Lindgren, Ransome, Spencer, Boutwell, Irving, 

Graton, Spurr, Garrey, Ball, Butler, Lawson, and Gordon are particularly valuable. Since 1905, when the work of collecting the 

mineral statistics of the United States was placed in the hands of 

field geologists, the annual reviews of resources, production, and 

development that have appeared in the reports entitled "Mineral

1 Emmons, S. F., The secondary enrichment of ore deposits: Trans. Am. Inst. Min. Eng., vol. 30,1901, 

pp. 177-217.

2 Weed, W. H., The enrichment of gold and silver veins: Trans. Am. Inst. Mm. Eng., vol. 30,1901, pp. 

424-448.

«Van Hise, C. R., Some principles controlling the deposition of ores: Trans. Am. Inst. Min. Eng., vol. 

30,1901, pp. 27-177.

* Kemp, J. F., Secondary enrichment In ore deposits of copper: Econ. Geology, vol. 1, 1906, pp. 11-25. 

' Ransome, F. L., Criteria of downward sulphide enrichment: Econ. Geology, vol. 5,1910, p. 205.

 Penrose, R. A. F., jr., The superficial alteration of ore deposits: Jour. Geology, vol. 2, 1894, p. 288.
12 THE ENRICHMENT OF SULPHIDE ORES.

resources of the United States" have been a never-failing source of 

information. The chapters on gold, by Waldemar Lindgren and H. 

D. McCaskey; those on copper, by L. C. Graton and B. S. Butler; 

those on lead and zinc, by C. E. Siebenthal; and those on the rarer 

metals, by F. L. Hess, have furnished many valuable data.

Data on the chemistry of the processes of sulphide enrichment 

have been accumulated rapidly since 1900. The paper by Schuermann 1 published in 1888 was for ma|iy years the most important 

source of information respecting the behavior of sulphides in certain 

reactions involving double decomposition. In 1907 Weigel 2 published 

a paper showing the solubilities in water of the metallic sulphides, 

which, as was pointed out by R. C. Wells,3 correspond closely in 

order to the series found by Schuermann. In 1903 H. V. Winchell 4 

published the results of experiments in chalcocitization of sulphide 

ores. More recently experiments in the solution or precipitation of 

the metals have been made by Sullivan, Wells, Alien, Stokes, Buehler 

and Gottschalk, Brokaw, Cooke, Grout, A. N. Winchell, and many 

others.

I wish to acknowledge my indebtedness to my colleagues of the 

United States Geological Survey, especially to Messrs. Waldemar Lindgren, F. L. Ransome, A. C. Spencer, and B. S. Butler, who have read 

portions of this paper and have generously contributed unpublished 

data on certain districts. Mr. R. C. Wells, of the United States Geological Survey, and Dr. W. H. Hunter, of the University of Minnesota, 

have read critically certain parts of this paper, where the problems of 

physical chemistry are treated, and many of the reactions have been 

discussed with Dr. E. T. Alien and associates, of the Carnegie geophysical laboratory at Washington, and with Mr. F. W. Clarke. of the 

United States Geological Survey.

DISTINCTIONS BETWEEN PRIMARY AND SECONDARY DEPOSITS.

In this paper I apply the term primary to all bodies of ores whose 

chemical and mineral composition have remained essentially unchanged by superficial agencies since the ores were deposited. These 

include sulphide ores that have replaced the wall rock and are "secondary" after rock-making minerals or sedimentary beds. A secondary ore, as the term is here used, is one that has been altered by 

superficial agencies. The term is not restricted to pseudomorphous 

replacements but is used to include also material deposited by super1 Schuermann, Ernst, Ueber die Verwandtschaft der Schwermetalle zum Schwefel: Liebig's Ann. der 

Chemie, vol. 249,1888, p. 326.

2 Weigel, Oskar, Die Loslichkeit von Schwennetallsulflden in reinem Wasser: Zeitschr. phys. Chemie, 

vol. 58,1907, pp. 293-300.

8 Wells, R. C., The fractional precipitation of sulphides: Econ. Geology, .vol. 5,1910, pp. 1-14. 

< Winchell, H. V., Synthesis of chalcocite and its genesis at Butte, Mont.: Bull. Geol. Soc. America, voL 

' 14,1903, pp. 269-276.
DISTINCTIONS BETWEEN PEIMAEY AND SECONDABY DEPOSITS. 13

ficial processes in fractures in and near the primary ore bodies. It 

is unfortunate that the term "secondary" is used with different 

meanings and that the distinction between primary and secondary 

ores can not everywhere be sharply drawn, but the diverse usage of 

the terms has become firmly fixed in the literature, and ambiguity 

can generally be avoided only by using qualifying phrases.

A discussion of the genesis of primary ore deposits does not come 

within the scope of this review, but in order to distinguish clearly 

between primary and secondary ores as the terms are here used, it is 

desirable to mention briefly the several classes of primary deposits. 

Any attempt to classify ore deposits by means of the data now available is hazardous. Sharp divisions are not generally found hi nature, 

and any classification based on genesis should be regarded merely as 

a convenient means of comparison and study. Not only do some 

classes of primary ores overlap one another, but primary ores formed 

at moderate depths by cold solutions and those formed very near the 

surface by ascending thermal waters are in many respects similar to 

deposits formed by descending sulphate solutions in processes of 

secondary alteration, and it is not everywhere practicable to draw 

sharp distinctions between them. The following classification 

includes the more important groups of primary ores:

Syngenetic deposits; contemporaneous with the inclosing rocks:

Sedimentary beds; mechanical, chemical, organic, etc.

Magmatic segregations; consolidated from molten magmas. 

Epigenetic deposits;, deposited later than the inclosing rocks:

Pegmatite veins; deposited by "aqueo-igneous" magmatic solutions.

Contact-metamorphic deposits; deposited in intruded rocks by fluids passing from 

consolidating intruding rocks.

Deposits of the deep vein zone; formed at high temperature and under great 

pressure, generally in and along fissures.

Deposits formed at moderate and shallow depths by ascending hot solutions.

Deposits formed at and near the surface by ascending hot solutions.

Deposits formed at moderate and shallow depths by cold meteoric solutions.

The deposits of any of these groups may be changed by various 

geologic processes. They may be buried deeply and altered by 

dynamic metamorphism, or, if exposed to surface agencies, they may 

be leached or enriched by superficial processes.

Sedimentary beds are the sources of a great many economic 

products such as coal, clay, oil, iron ore, or manganese. Of the 

sulphide ores, however, very few are sedimentary. Some of the 

copper deposits in the "Red Beds" have been considered sedimentary. The best known example is the " Kupferschiefer" of 

Mansfield, Germany. Workable sulphide deposits of sedimentary 

origin in the United States are unknown to me.

Magmatic segregations are products of the differentiation of igneous 

magmas. Genetically considered they are in the strict sense igneous
14 . THE ENRICHMENT OF SULPHIDE ORES.

rocks. These deposits include ore bodies of considerable economic 

importance, among them some of the magnetic iron ores of the 

Adirondack Mountains, in New York. No large sulphide deposits of 

this kind are known in the United States. The nickel-copper deposits of Sudbury, Ontario, are the best-known. examples of this 

group in North America. These deposits have been planed off by 

glacial erosion in recent geologic time and do not show any considerable enrichment by superficial processes.

Pegmatite veins are very nearly related to magmatic segregations. 

They are magmatic segregations or end products of crystallization 

that have been thrust, like igneous dikes, into openings in rocks 

already consolidated. As they are probably formed from eutectic 

solutions, many pegmatite veins tend to approach a fairly uniform 

composition and are also generally characterized by large crystals of 

rock-making minerals. Pegmatites that have not moved from the 

parent magma and are not related to openings in rocks could properly 

be classed with syngenetic deposits, as magmatic segregations, but 

some authorities reserve the latter term for the more basic differentiation products. In pegmatites the sulphides are present, as a rule, 

in small quantities, and except some deposits of rare elements they 

have very little economic importance as sources of the metals.

Contact-metamorphic deposits are formed in intruded rocks by 

fluids given off by intruding igneous magmas.. Many of the ores of 

such deposits contain the sulphides of copper, zinc, and lead, intergrown with heavy silicates and calcite. In general the ore bodies of 

this class are not clearly related to determinable fissures. Many of 

these deposits have been enriched by superficial alteration. As a rule 

the changes are easily followed in the field, for many minerals that are 

formed by contact-metamorphic processes are not formed by processes 

of superficial alteration, and vice versa. In general sulphide enrichment does not extend to great depths in contact-metamorphic ores; 

the tough, fibrous, or platy minerals of the gangue do not favor 

extensive fracturing, and the presence of calcite favors precipitation 

of secondary carbonates near the surface. Noteworthy exceptions 

to this rule are known.

The deposits of the deep vein zone are mineralogically related, more 

or less closely, to contact-metamorphic deposits. They have formed 

in and along openings in rocks, however, and in the main they 

approach the tabular form more closely than do the contact-metamorphic deposits. As pointed out by Lindgren, who first defined the 

group, the deposits of the deeper zone have formed under conditions 

of high temperature and pressure, which prevail also in contact metamorphism. Because high temperature and pressure are necessary 

for their genesis these deposits do not form at moderate or shallow 

depths, at least not in fissures that extend to the surface, and there-
DISTINCTIONS BETWEEN PRIMARY AND SECONDARY DEPOSITS. 15

fore they are seldom found in the more recent rocks. The deposits 

of this group are closely affiliated with the contact-metamorphic 

deposits on the one hand and with deposits formed at moderate depth 

on the other and can not be sharply divided from the latter. If this 

class is, made to include only deposits that are related to openings in 

rocks and that carry in the gangue some of the contact-metamorphic 

minerals (such as heavy silicates, magnetite, or specularite), it may be 

said that the examples found in the United States are but little affected 

by sulphide enrichment. The gold deposits of this group are generally not manganiferous, and silver is not an important constituent of 

most of them. On the other hand, some important sulphide deposits 

of copper that carry pyrrhotite and magnetite might be included in 

this group, and sulphide enrichment has played an important part in 

the genesis of a number of these pyrrhotitic copper deposits.

The deposits formed at moderate and shallow depths by ascending 

thermal solutions constitute a very important group, which includes 

numerous deposits of ores of copper, gold, silver, lead, and zinc. Many 

of these deposits show important sulphide enrichment and illustrate 

the downward migration of the metals. Some of them are noted for 

relatively small bodies of very rich ore, locally developed in a larger 

lower-grade deposit. This group, more than any other, may be characterized as the "bonanza" group. Some of the bonanzas are due 

to sulphide enrichment, but doubtless many of them are primary.

Much may be said for the theory that some of the metals are precipitated most effectively from ascending thermal alkaline solutions 

in the zone where they first encounter descending ground water. 

Whether the solutions are alkali chlorides, alkali sulphides, or alkali 

carbonates, a decrease in temperature and partial oxidation would 

cause precipitation. An alkali sulphide solution containing iron 

would be converted to a ferrous sulphate solution, and from such 

a solution gold would be deposited almost completely. Some of the 

metals may be precipitated from alkali sulphide solutions merely by 

dilution. In the deposits of this group, more than in any other, the 

presence of bonanza ore has little genetic significance. Each deposit 

and each ore shoot is a separate problem. Even its relation to the 

present surface is not everywhere conclusive, for obviously the primary bonanzas may have a genetic relation to a former zone of 

ground water a zone which is itself superficial but which may 

not be determinable.

Deposits of sulphide ores formed at or very near the surface by 

ascending hot solutions are mainly of scientific rather than economic 

importance, although a few of them have been exploited for the 

metals. As atmospheric oxygen is present in the superficial zone 

the minerals they contain include the hydrous oxides and various 

sulphates, and as the pressure is nearly atmospheric the temperature
16 " THE ENRICHMENT OF SULPHIDE ORES.

can not be much above 100° C. Consequently the minerals form 

under physical conditions that are not greatly different from those 

found in the zones of surface alteration and sulphide enrichment 

where ores are exposed to the action of surface agencies.

The deposits formed at moderate and shallow depths by cold solutions include a large number of important deposits of lead and zinc 

in the Mississippi Valley and many copper deposits in Colorado, New 

Mexico, and Utah. Examples could be multiplied, but these define 

the type. The conditions under which these deposits form are doubtless closely similar to those which exist in zones of sulphide enrichment, 

and many of the minerals formed are found also in the secondary 

sulphide zones. Much evidence has been cited to show that these 

deposits were formed by ground water that gathered its metallic contents from great masses of rocks in which the metals were sparingly 

disseminated. The metallic salts, chiefly sulphates and chlorides, 

were gathered in water channels, and the metals were deposited as 

sulphides where conditions were favorable. In many examples some 

form of organic material supplied the precipitating agent. If deposition had taken place on an older sulphide these deposits would be 

classed as secondary sulphide ores, but in general there is no evidence 

that bodies of older sulphide ore occupied the place of the deposits. 

These ores are therefore considered primary, although they have been 

leached by ground water from an older metalliferous rock.

PHYSICAL CONDITIONS AND ENRICHMENT.

The processes of sulphide enrichment depend on the physical conditions, the environment, and the geologic history of the deposits and 

on their chemical and mineral composition. Of the purely physical 

conditions, the climate, altitude, and relief are important. Permeability of the deposits is an essential condition, for if solutions 

can not find access to the lower horizons the metals dissolved near 

the surface may be scattered. The duration of the period of weathering is important also, for under similar conditions the amount of 

solution is directly proportional to the time during which the deposits 

are exposed to weathering. Briefly, there is scarcely a feature of the 

geologic history of a deposit that may not affect the extent and 

character of its enrichment.

CLIMATE. 

TEMPERATURE.

A warm climate, in so far as it favors chemical action, is favorable 

to sulphide enrichment. Deposits in high latitudes are not so 

likely to show extensive migration .of the metals, because low temperature decreases chemical activity, and freezing prevents solution. 

Where the ground is frozen to considerable depths during the winter
PHYSICAL CONDITIONS AND ENKICHMENT. 17

and thaws out only a short distance below the surface during the 

summer, thorough weathering can not extend to great depths. 

Assuming the persistence of the present relation between latitude 

and climate, it may be said that bonanzas of secondary ore are probably less numerous and less extensive in higher than in lower latitudes.1

In late, geologic tune large areas in northern latitude have been 

'glaciated, and in many places the surface has been planed off by ice 

erosion. The altered zones of many, deposits have doubtless been 

removed. Thus the deposits of the North differ from those of lower 

latitudes in two essential respects in environment and in geologic 

history. To what extent the smaller development of secondary 

sulphide ores in the North depends on present temperature and other 

climatic conditions, and to what extent it is due to recent glacial 

action, are questions that probably will not be answered until more

data are available concerning deposits in driftless areas in high latitudes. In Alaska, Canada, and New England there are but few 

sulphide deposits of proved secondary origin that are comparable 

in extent or value to those which have formed at lower latitudes. 

The available evidence indicates that the large and important ore 

bodies of these countries are mainly of primary origin. The gold deposits of Douglas Island, Alaska, the copper deposits of Kasaan Peninsula, Latouche Island, and Prince William Sound, and the nickel 

and copper deposits of the Sudbury region are not deeply oxidized. 

Carbonate ores extend to a depth of 300 feet in the Copper Mountain 

mines, Prince of Wales Island, and chalcocite and native copper lie 

at least 200 feet deep at the Goodro mine.2 In many of the deposits in the North, however, the ore has proved to be of approximately uniform grade to considerable depths.

On the other hand, it is well known that great climatic changes 

have taken place in many regions, and that temperatures have .been 

by no means constant throughout geologic time. Therefore, because 

the processes of weathering of ore bodies are slow to-day in certain 

places, it does not follow that they have always been inactive there, 

and under some conditions secondary deposits formed in older 

geologic periods in places protected from erosion should be preserved 

to-day.

The famous Bonanza mine of the Chitina copper belt, Alaska, 

described by Moffit and Maddren, 3 presents some unusual features,

i Winchell, H. V., Prospecting in the North: Min. Mag., vol. 3, 1910, pp. 436-438. Brock, K. W., Discussion of H. V. Winchell's article "Prospecting in the North:" Min. Mag., vol. 4, 1911, pp. 204-205.

sWright, F. E. and C. W., The Ketchikan and Wrangell mining districts, Alaska: Bull. U. S. Geol. 

Survey No. 347, 1908. Wright C. W., Discussion of H. V. Winchell's article "Prospecting in the North:" 

Min. Mag., vol. 4, 1911, p. 359.

»Mofflt, F. H., and Maddren, A. G., Mineral resources of the Kotsina-Chitina region, Alaska: Bull. 

U. S. Geol. Survey No. 374, 1900, pp. 80-86.

72293° Bull. 529 13 2
18 THE ENRICHMENT OF SULPHIDE OEES.

which should be mentioned here. The principal deposit is a great 

mass of chalcocite and bornite, the ore carrying considerable silver 

and occupying a fractured zone in limestone. There is no evidence 

that the ore has replaced older sulphides. Although Moffit and Maddren do not state definite conclusions as to the origin of the ore, their 

discussion clearly implies a genesis independent of magmatic waters. 

No other conclusion seems to be warranted by the facts available. 

Geologic and physiographic studies appear to indicate that the 

Bonanza ore deposit was formed at considerable depth by meteoric 

waters, which implies that at some period processes of solution and 

precipitation have operated vigorously in this high latitude.

The rich native silver ores of Cobalt, Ontario, have been considered secondary by some geologists, but all who have studied these 

deposits are not agreed as to their genesis.1 Doubtless some enrichment has taken place in the Copper Cliff and Vermilion mines of the 

Sudbury region, Ontario. In the Vermilion mine, according to 

-Barlow, native copper, probably derived by alteration from chalcopyrite, is found 900 feet below the surface.2 In general, the 

secondary sulphide zones of deposits in western Canada in about 

the same latitude as these are not extensive, but some deposits 

for example, the St. Eugene mine, in British Columbia show 

unmistakable evidence of sulphide enrichment. The developments 

in southwestern Canada are not sufficient to permit conclusive statements. There is doubtless a relation between latitude and sulphide 

enrichment, but it is one which can not be expressed as an invariable 

rule.

On the other hand, many sulphide deposits in lower latitudes do 

not show sulphide enrichment. Examples are mines lying in the 

foothill copper belt, California (see p. 202); La Reforma mine, 

Mexico (p. 211); the Copper Queen mine, Velardena, Mexico (p. 211); 

and several deposits of the Braden Copper Co., Chile (p. 203). According to J. M. Moubray,3 several deposits in the Kafue copper 

district, in northern Rhodesia, between 13° and 14° south of the 

Equator, show copper sulphides at the very surface. In the Sable 

Antelope mine of this region superficial alteration is practically absent.

RAINFALL.

Since water is the agent of ore enrichment, abundant rainfall is 

favorable to the formation of secondary ores. The activity of ground 

water depends principally, however, on the natural acids and other 

compounds which it dissolves, and the waters in regions of oxida1 Mfller, W. O., Notes on the cobalt area: Eng. and Min. Jour., vol. 92, 1911, pp. 645-649. Emmons, 

S. F., Types of ore deposits, San Francisco, 1911, p. 140.

2 Barlow, A. E., The nickel and copper deposits of Sudbury, Ontario: Ann. Kept. Geol. Survey Canada 

vol. 14, pt. H, 1904, p. 106.

'Moubray, J.M., Discussion of H. V. Winchell's article, "Prospectingin the North:" Min. Mag., vol. 4, 

1911, pp. 117-118.
PHYSICAL CONDITIONS AND ENBICHMBNT. 19

tion and solution are generally not saturated with acids. Thus, even 

in arid regions a moderate supply of ground water under conditions 

favorable to permeability may bring about appreciable results in 

relatively brief geologic periods. Some of the silver-gold deposits of 

the Great Basin clearly show sulphide enrichment, although the 

primary ores were deposited as late as or later than the Miocene 

epoch. Ores that form some of the best examples of sulphide 

enrichment seen in the United States are found in early Tertiary 

deposits that occur in the arid Southwest, where the annual rainfall 

is not more than 10 or 12 inches. In some of these deposits, 

however, much of the secondary ore was probably formed when the 

rainfall was greater. Concerning this point it is obviously difficult 

to draw any general conclusions, because data regarding the former 

climates of many arid regions are inadequate. 

In arid countries the ground-water level is likely to He deep, and

the zone of solution is generally not clearly differentiated from the 

zone of precipitation. Above the water level in ore deposits in arid 

regions there may be considerable amounts of primary sulphide ore, 

of secondary sulphide ore, and of oxidized ore, all at approximately 

the same horizon. In such deposits the secondary sulphide ores, 

though occurring through greater vertical ranges, may not be concentrated in small volume.

ALTITUDE.

As a rule, the relief is great in areas of high altitudes, and erosion 

is consequently more rapid. Moreover, in such areas temperatures 

are lower and conditions are less favorable to solution. Deposits 

located at very high altitudes, where rocks are disintegrated by 

frost and carried away unweathered as talus and bowlders, are not 

so likely to be extensively enriched as are deposits that lie at lower 

altitudes. On the other hand, the processes of enrichment are 

effective under some conditions at considerable altitudes. Many 

deposits in Colorado that outcrop at altitudes over 10,000 feet above 

sea level and some in Montana that outcrop at about 8,000 feet 

above sea level contain extensive zones of secondary ores.

BELIEF.

In so far as strong relief supplies head, it is favorable to deep and 

rapid circulation of underground water, and it is likewise favorable 

to relatively deep enrichment. In base-leveled regions underground 

circulation is sluggish and the nearly stagnant waters can not descend 

far into the zone of primary sulphides without losing the valuable 

metals which they dissolve higher up. Calculations made for lodes 

in the base-leveled region that includes Ducktown, Tenn., show that 

all or nearly all of the secondary copper in the chalcocite zones may
20 THE ENBICHMENT OF SULPHIDE OEES.

be accounted for as a result of the leaching of the gossan below the 

old base-leveled surface. Under the conditions which prevailed in 

that district at least, it appears that a large part of the copper 

dissolved and reprecipitated in any secondary sulphide zone that was 

formed while the country was being reduced to base-level had been 

probably redissolved and scattered when the country was baseleveled. In deposits that were more permeable and in those that 

had been enriched to greater depths the conditions would have been 

more favorable to the preservation of the secondary ores formed 

before base-level had been approached. The available data regarding 

enrichment of base-leveled" deposits are scant, for nearly all ore 

deposits that clearly show sulphide enrichment are in areas of moderate 

or strong relief.

PERMEABILITY.

Permeability is essential for sulphide enrichment. If the primary 

deposits are not permeable the solutions that pass downward through 

the oxidized zones will move laterally along the contact between 

oxidized and sulphide ores and ultimately will escape into fractures 

in the wall rock or reissue as springs at some level below the points 

of entry. If they do not encounter a reducing environment the metals 

may be scattered. In rocks that have been shattered by strong 

movements since the primary ore was deposited there is generally 

more extensive and deeper enrichment than in deposits that have 

been but slightly fractured. Brittle minerals like quartz and chert 

fracture readily, and deposits composed largely of the brittle minerals 

are generally more deeply enriched than deposits of tough or elastic 

minerals. Many of the heavy silicate-sulphide ores of contactmetamorphic origin that carry a gangue of abundant fibrous amphibole, 

mica, chlorite, or like minerals do not show sulphide enrichment to 

great depths.

In a great many districts, especially in the western part of the 

United States, pronounced earth movements have occurred since 

the ores were formed. Many deposits fill fissures or were formed 

in zones of fracturing that are planes of weakness along which 

movements took place before ore deposition. To relieve subsequent 

stresses the same zones of fracture may be again fractured, like a rod 

that breaks where welded. Such later fracturing has taken place in 

many mineral deposits after the primary ore was deposited and consequently many of the planes of later movement may parallel the 

veins, following one wall or the other or cutting irregularly across 

the ore. If the deposit is composed of material that is easily fractured, such as quartz or chert, the ore may become brecciated and 

may therefore contain many small openings. A large number of 

small openings, such as may result from fracturing or.shattering of
PHYSICAL CONDITIONS AND ENRICHMENT. 21

the primary sulphides, are more favorable to concentrated enrichment than a few larger openings, for a larger surface of primary ore 

is exposed to solution along many small openings than along a few 

large openings. Moreover, inasmuch as friction is greater along small 

openings, the descent of solutions in them is retarded and therefore 

reactions that result in precipitation of secondary sulphides may be 

brought nearly to completion at relatively shallow depths. Very 

small openings, such as the pore spaces of minerals and other capillary 

or subcapillary openings, are not effective water channels because 

friction along them retards circulation. Under some conditions, 

however, mineral-bearing solutions do penetrate these minute 

openings and deposit ore in them. In certain deposits near Globe, 

Ariz., according to Ransome, 1 specks of secondary chalcocite ore 

are embedded in solid veinlets of quartz and in silicified schist, 

indicating a migration of copper, for short distances at least, through 

exceedingly minute openings. . Although water may penetrate such 

minute openings, it does not circulate freely in them but is nearly 

stagnant. It would be supposed that the metals would not be 

carried far in such openings before precipitation, at least not in 

rocks that are readily attacked by the solutions, but if the openings 

are lined with siliceous alkali-free minerals or with other minerals 

that are but slowly affected by the solutions, the reactions which 

reduce and tend to neutralize the. solutions and to precipitate the 

metals would take place more slowly and the metals might be carried 

in solution to greater depths.

INFLUENCE OF BATE OF EROSION ON BATE OF SULPHIDE

ENBICHMENT.

In so far as strong relief is favorable to rapid erosion it is unfavorable to thorough leaching. Where erosion is slow the outcrops and 

upper portions of deposits are exposed to processes of weathering for 

periods long enough to favor thorough leaching and, if the metals are 

reprecipitated at lower depths, to fa'vor ore enrichment. On the 

other hand, erosion may be delayed to a point beyond which it is 

unfavorable to solution and precipitation. The downward migration of the zone of oxidation exposes new surfaces to solution, making 

masses of fresh sulphides available for reconcentration. Consequently where metals dissolve readily, comparatively rapid erosion 

may favor rapid concentration. The metallic contents of many 

deposits of secondary ores represent not only what has been leached 

from the gossan now exposed but also what has been dissolved from 

portions of the deposits that have been carried away by erosion.

»Ransome, F. L., Criteria of downward sulphide enrichment: Econ. Geology, vol. 5,1910, pp. 217-218.
22 THE ENRICHMENT OP SULPHIDE ORES.

AGE OF THE PRIMARY DEPOSITS AND THEIR PERIODS OP

WEATHERING.

Other conditions being similar, the amount of enrichment must 

depend on the length of time the deposits have been exposed to 

weathering and erosion. In general, weathering has acted for a 

shorter time on late Tertiary deposits than on middle Tertiary, 

early Tertiary, or Cretaceous deposits. The age of the deposit is not, 

however, invariably the most important factor in determining the 

extent of its enrichment, for some of the middle or late Tertiary 

deposits, such as those in the southwestern part of the United States, 

show more extensive migration of the metals than is shown by some 

older deposits which have been exposed to weathering for a much 

longer time.

In the United States the bonanza deposits of .the precious metals 

are in the main the younger ones, for in general the Paleozoic and 

older deposits are less rich, though many of them are more nearly 

uniform in value. Many of the middle or late Tertiary deposits of 

the precious metals in the Great Basin region such as those of 

Tonopah, 1 the Comstock lode,2 and Tuscarora, Nev. show unmistakable evidence of enrichment, yet there are good reasons for supposing 

that the primary ores of many of these young .deposits were originally 

somewhat richer nearer the surface than at greater depths. .The 

deposits of Cripple Creek, Colo.,3 and of Goldfield, Nev.,4 show a 

similar relation as to values and depth, yet sulphide enrichment has 

probably not been appreciably important in either of these districts. 

Gold is generally slow of solution, especially where the environment 

is not favorable to solution. As pointed out by Graton,5 some of the 

gold deposits of the Appalachian region are workable at their outcrops, where gold has remained undissolved ever since the peneplanation of the region, which probably took place as early as Tertiary 

time.

In general, the sulphide deposits that show the most clearly defined 

secondary zones are those of copper. The periods of primary deposition of all the known important copper deposits of determined age in 

the United States, except those of Tintic, Utah, probably antedate 

the Miocene, and all have long been exposed to erosion and weathering.

1 Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, p. 95.

2 Becker, G. F., Geology of the Comstock lode and the Washoe district: Man. U. S. Geol. Survey, vol. 3, 

1882, p. 273.

3 Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, p. 204 and Chapter XI.

«Ransome, F. L.,Geology and ore deposits of Goldfleld, Nev.: Prof. Paper U. S. Geol. Survey No. 66, 

1909, pp. 191-195.

' Graton, L. C., Reconnaissance of some gold and tin deposits of the southern Appalachians, with notes 

. on the Dahlonega mines by Waldemar Lindgren: Bull. U. S. Geol. Survey No. 293,1906, p. 67.
PHYSICAL CONDITIONS AND ENRICHMENT. 23

According to Ransome, 1 the deposits at Bisbee, Ariz., may have 

been exposed to processes of alteration as far back as the Cretaceous. 

The copper deposits of Velardena, Mexico, according to Spurr and 

Garrey,2 are probably later than middle Tertiary. The deposits of 

the Braden mine of Chile, as shown by Pope Yeatman, 3 are likewise 

of comparatively late age. In neither of these districts where Tertiary ores are developed are the secondary sulphide zones conspicuously shown. Many copper deposits of the eastern United States 

are much older, the more important ones having been formed in 

Paleozoic or earlier periods. Although some of these deposits have 

long been exposed to erosion, their secondary sulphide zones, though 

rich and well defined, are not nearly so deep nor so extensive as some 

in the younger deposits of the West.

I have found no record of secondary sulphide zones that were 

unquestionably formed in pre-Mesozoic time, although zones of oxide 

enrichment as old as the Archean are known. According to Van 

Hise and Leith 4 some of the rich specular hematites of the Vermilion 

range are the metamorphosed products of surface enrichment that 

were indurated and infolded before the beginning of Algonkian time.

PALEOPHYSIOGRAPHY.

Inasmuch as sulphide enrichment depends on the action of suriace 

agencies, it is important to know as far as possible the details of the 

history of any deposit considered, the length of time it has been 

exposed to weathering, and whether faulting or folding or a second 

episode of primary ore formation has taken place since it was first 

formed. In short, any geologic or physiographic data might have 

a bearing on the problem of enrichment.

If the present topography is like that which prevailed when primary 

deposition took place and this may be the case if the deposits 

were formed in comparatively late geologic time then the richer ore 

of the primary deposits may have an obvious relation to the present 

surface. In some deposits of gold and silver ore the maximum precipitation of the metals appears to have taken place at relatively short 

distances below the surface that existed at the time of deposition. Thus 

the primary ore may show a comparatively constant change in value, 

which may decrease with increasing depth. In general, the more 

remote the period of primary deposition the less the probability that 

the important features of the present topography are similar to those 

which existed when the primary ores were deposited and the less

i Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. P^per U. S. 

Geol. Survey No. 21,1904, p. 160. Also this bulletin, p. 179.

»Spurr, J. E., and Garrey, G. H., Ore deposits of the Velardena district, Mexico: Econ. Geology, vol. 3, 

1908, pp. 724-725.

»Yeatman, Pope, The Braden copper mines: Min. and Sci. Press, vol. 103,1911, pp. 769-772.

«Van Hise, C. B., and Leith, C. K., The geology of the Lake Superior region: Mon. U. S. Geol. Survey, 

Tol. 52,1911, p. 142.
24 THE EJTBICHMENT OF SULPHIDE OBES.

the probability of error in attributing the deposition of a rich primary 

 sone to secondary processes.

STRUCTURAL FEATURES AND BLIND LODES.

Lodes that do not outcrop are termed "blind" lodes. Some of 

them do not outcrop because the primary ores did not extend to the 

present surface, others have been faulted off near the surface, and 

still others have been covered by formations deposited later than the 

ores. A fissure across a sedimentary series may end abruptly just 

below or at a bed of shale, and the ore that fills it will then be exposed 

only where the shale has been eroded away. The portion of a lode 

that is now capped by an impervious shale will not have been enriched 

by oxygenated waters unless, because of certain structural conditions, 

such waters were conducted laterally below the shale bed to the lode. 

Such structures as thin saddle reefs, thin anticlines, and thin flatlying beds are not particularly favorable to extensive sulphide 

enrichment, because they may be eroded in comparatively short 

time and because the conditions below the deposits they contain 

are not likely to be so favorable to precipitation as they are in 

deposits or in parts of deposits that are underlain by sulphide ores. 

In the absence of material below that may react with the solutions 

and readily cause precipitation, the metals are likely to be scattered. 

Nearly vertical or steeply dipping tabular deposits are in a position 

favorable for secondary concentration, but sulphide enrichment may 

be extensive also in large isodiametric deposits, or in thick deposits 

that stand in any attitude.

Deposits that outcropped at one time but are now covered by 

sedimentary rocks or by later lava flows may have been exposed to 

weathering and may have been enriched by oxygenating water before 

the beds covering them were laid down. It can not safely be assumed 

that the rich ores below the beds covering such deposits are primary 

and therefore likely to extend downward to indefinite depths. The 

secondary zones would obviously be related to a topography that 

existed long ago and not to the present one.

Systems of postmineral fractures in ore and country rock are 

structural features of obvious importance in the consideration of the 

enrichment of any ore deposit.

GLACIATION.

In comparatively late geologic time a considerable portion of North 

America was capped by a continental ice sheet, which removed by 

erosion the loose debris and the surface rock over great areas. Glaciation was most extensive in northern latitudes, but the continental 

glacier extended southward as far as Ohio and Missouri rivers, and 

smaller glaciers accumulated in the more lofty mountain ranges of
PHYSICAL CONDITIONS AND ENRICHMENT. 25

the American Cordillera. Many of the ore deposits that lay hi the 

paths of the glaciers were planed off, and the ores hi then* upper 

zones were scattered in the rocky material which was left when the 

ice had melted. Erratic fragments of such deposits have been carried far from their sources and have been the cause of much fruitless 

prospecting.

The outcrop of an ore body may be removed gradually by erosion 

by water, but weathering generally precedes erosion. The solutions 

may leach the valuable minerals from the outcrop and may precipitate them at a lower level, where they will be preserved. But weathering does not attend erosion by ice, and chemical action at low temperatures is slight; consequently the metals present in the portions 

of the deposits that are removed are likely to be scattered. The 

extent to which the ore deposits in a glaciated region were weathered 

or otherwise altered by surface agencies before the glacial period 

began can not be estimated. The amount of rock removed by the 

continental ice sheet is known to be considerable, however, for the 

drift which it deposited is in many places more than 200 feet thick. 

It is probable that glacial erosion was hi places equally great or 

greater. Whatever the amount of ice erosion, it appears to have been 

sufficient to remove the highly altered zones in most parts of northern 

North America.

As stated already, the processes of solution and enrichment are 

retarded in regions of low temperature. The areas in which ice 

erosion has been most vigorous are those in which the lower temperatures prevail to-day, and there is reason to suppose that the deposits 

in these areas were not so deeply altered before the glacial epoch as 

were similar deposits at lower latitudes. In Canada and in Alaska 

there are few large deposits of sulphide ores which are clearly of secondary origin. If the deposit at the Bonanza mine hi the Chitina 

copper region, Alaska, is primary no large rich secondary sulphide 

deposits in Alaska are known to me. (See pp. 17 and 215.) The 

sulphide ores now exploited in Canada, except possibly the deposits 

at Cobalt, in the silver-bearing region of Ontario (which some have 

considered of secondary origin), and certain well-authenticated examples hi British Columbia, are generally believed to be primary. 

I know of no important secondary deposits in New England. Small 

deposits of chalcocite ores were exploited in the Ely district, Vermont. In a copper deposit at Milan, N. H., where the sulphides outcrop at the very surface, no considerable amount of oxidation has 

taken place below 30 feet, and in general oxidation is trivial at even 

shallower depths. Only a little chalcocite enrichment has taken place, 

the secondary ore consisting of primary yellow sulphides coated with 

thin films of chalcocite, adding to its value probably not more than 

1 per cent.
26 THE ENEICHMENT OF SULPHIDE ORES.

In Norway and Sweden, according to Vogt, 1 the surface has been 

polished clean by the Quaternary ice sheet, and secondary alteration 

is insignificant.

Glaciers do not erode their beds equally at all places. In their 

higher portions, where the ice is accumulating, pressures are greater, 

the ice is more rigid, and erosion is more vigorous. Near the margins, 

where the ice is melting, deposition exceeds erosion and the deposit 

of drift protects the surface from wear. These differences are very 

conspicuous in some mountainous sections of the West where the 

glaciers covered only portions of the country and the processes are 

more clearly shown. In some of the ranges of Montana, Colorado, 

and Utah, where ore deposits are numerous and varied, the evidences 

of mountain glaciation are conspicuously preserved. At some places 

the mountain glaciers' seem to have removed very little of the altered 

ore, for the secondary sulphide zones and even the oxidized ores are 

intact, and some of these appear to be too extensive to have formed 

since the Quaternary glacial epoch. The Amethyst lode at Creede, 

Colo., has an extensive secondary zone, and one end of this lode was 

overridden by the ice in late geologic time. In general, erosion by 

mountain glaciers has been localized, the maximum wear taking 

place near the heads of the glaciers.

Erosion by the continental glaciers is also somewhat erratic, for 

great differences in the effect of the action of ice may be seen in a 

comparatively small area. In the Mesabi range of Minnesotia the 

hard, fresh country rock is polished clean in places, whereas a few rods 

away and at but slightly lower elevations thick bodies of cellular, 

almost powdery iron-oxide ore remain intact. These facts suggest 

that other important secondary zones may be encountered when the 

area overridden by the continental ice sheet is more thoroughly 

developed.

THE UNDERGROUND CIRCULATION.

OPENINGS IN THE EARTH'S CRUST.

As emphasized by Van Hise in his treatise on metamorphism 2 the 

outer part of the earth's crust may be divided into three zones, differentiated by character of deformation an upper zone of fracture, 

a lower zone of flowage, and a middle zone of combined fracture and 

flowage. The zone of fracture is near the surface. The openings in 

the rocks of this zone are comparatively stable because the weight 

of the overlying load of material is less than the crushing strength of 

the rock. At greater depths, where the differences in the stresses 

exceed the strength of the rocks, openings, if formed, would almost

»Vogt, J. H. L., Problems in the geology of ore deposits, in PoScpny, Franz, The genesis of ore deposits, 

1902, p. 675.

» Van Hise, C. R., A treatise on metamorphism: Mon. U. S. Geol. Survey, vol. 47,1904, pp. 187-191; Principles of pie-Cambrian North American geology: Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1,1896, p. 589.
THE UNDERGROUND CIRCULATION. 27

immediately be closed by pressure. It is estimated that for all but 

the strongest rocks flowage would begin at depths not greater than 5 

or 6 miles, where the weight of the overlying mass is greater than the 

crushing strength of the rocks, but there are some reasons for extending this estimate of the zone of fracture to even greater depths. This 

problem has recently been investigated experimentally by F. D. Adams * 

and mathematically by L. V. King.2 Small holes were bored in cylinders of granite that were inclosed in hollow cylinders of nickel steel. 

Thus confined, the granite sustained loads of nearly 100 tons per 

square inch, a load more than seven times as great as that which will 

crush it at the surface of the earth in the usual laboratory tests. 

Adams concludes that, under the conditions of pressure and temperature that are believed to prevail within the earth's crust, cavities and 

fissures may exist in granite to a depth of at least 11 miles and may 

exist at still greater depths if they are rilled with water, gas, or vapor, 

owing to the pressure exerted by the liquid or gas on their inner surfaces. 

Sulphide enrichment, however, is probably confined to the upper 

part of the zone of fracture, and although transfers of certain materials may take place in the deeper zones, these do not come within the 

scope of this investigation. The deepest bodies of secondary sulphide 

ores now exposed lie at comparatively shallow depths; most of them 

less than 1,000 feet below the surface, and nearly all that have been 

developed lie at depths less than 2,000 feet. The depth at which 

precipitation takes place depends not only on the rate at which the 

solutions are carried downward but also on the rate at which they 

react on the walls of their conduits. Although openings may exist at 

depths of several miles, they would not become channels of circulation 

unless they were connected. There is without doubt a tightening of 

the rocks a few hundred feet below the surface, for, as pointed out by 

Kemp 3 and by Finch,4 the lower levels of many deep mines are dry.

THE LEVEL OF GROUND WATER.

The terms "water table" and " level of ground water" are generally used to describe the upper limit of the zone in which the openings 

in rocks are filled with water. This upper limit of the zone of saturation is not a plane but a warped surface. It follows in general 

the topography of the country but is less accentuated. It is not 

so deep below a valley as below a hill but rises with the country 

toward the hilltops and in general is higher there than in the valleys. 

Although the water in the zone of saturation does not move rapidly,

1 Adams, F. D., An experimental contribution to the question of the depth of the zone of flow in the earth's 

crust: Jour. Geology, vol. 20,1912, pp. 97-118.

> King, L. V., On the limiting strength of rocks under conditions of stress existing in the earth's interior: 

Jour. Geology, vol. 20,1912, pp. 119-138.

s Kemp, J. F., The r61e of the igneous rocks in the formation of veins, in PoSepny, Franz, The genesis 

of ore deposits, 1902, p. 696.

«Finch, J. W., The circulation of underground aqueous solutions and the deposition of lode ores: Proc. 

Colorado Sd. 800., vol. 7,1904, pp. 193-252.
28 THE ENBICHMENT OF SULPHIDE OEES.

it is hot stationary. If there is a lower outlet, it will move toward 

that point. Its movements are slow, however, and it may follow 

a very circuitous route before it issues again at the surface. It 

follows the paths of least resistance, and if these are downward the 

water may sink to great depths before it rises, under pressure, to 

make its exit at some point which is lower than that at which it 

first entered the belt of saturation. Thus the water table maybe 

considered a kind of indicator that registers the differences between 

the loss or leakage of the zone of saturation and the addition from the 

surface.

As the country is eroded, the water level moves downward and, 

within certain limits also, it changes with the seasons. In dry 

years it is deeper than in wet years, and in dry seasons it is deeper 

than, in wet seasons. The difference of elevation between the top 

of this zone hi a wet year and in a dry year is normally greater 

under the hilltop than on the slopes and in the valleys. In mines 

where the ground, is open the level of ground water probably changes 

with every considerable rain. Consequently there is a zone that 

is above ground-water level in dry periods but below it in wet 

periods, and in moist hilly countries this zone may be of considerable vertical extent. Thus the water table oscillates, though 

in general it moves downward with degradation of the land surface.

THE VADOSE CIRCULATION.

Of the rain that falls on the surface a part is drained off by rills 

and streams, another part is evaporated, and still another part soaks 

deep into the ground, passes downward, and is added to the water of 

the zone of saturation. The zone mentioned under the preceding 

heading a zone that lies above the zone of saturation and may be

relatively dry during a dry period but soaked with water after a 

wet period includes openings which in a relatively dry tune are 

filled with air; consequently the water that soaks into the ground 

after a subsequent rain or snow is aerated and thus becomes a more 

active agent of solution. The downward movement of such water 

toward the zone of saturation, has been termed the "vadose" l 

circulation. The depth or thickness of this vadose zone is variable, 

for its lower limit depends on the variable level of ground water. 

Near permanent streams or lakes and other bodies of water this limit 

is not much higher than they are. In moist hilly countries its depth 

from the surface varies from a few feet to several hundred feet. In 

arid regions, where the rainfall is low and evaporation is rapid, this 

zone may extend to much greater depths. It is, in the main, a zone

1 Posepn^, Franz, The genesis of ore deposits, 1902, p. 18. PoSepnJ- Includes In the "vadose" circulation 

water below the "permanent water level."
THE UNDEEGEOUND CIECULATION". 29

of solution; consequently, its rocks are open and circulation within 

it is comparatively rapid.

THE DEEPER CIRCULATION.

The circulation of the water in the belt of saturation depends on the 

relief of the country and on the number, continuity, spacing, and size of 

the openings in the rocks. Under hydrostatic head the waters in this 

zone move to points of less pressure and issue at points lower than 

those of entry. If the deposit is tight and there are no deep outlets 

the principal movement is shallow, following down the grade of the 

undulating water table. As a rule movement in the deeper zone is 

much slower than in the vadose zone, because the openings are less 

abundant and also because they are smaller, so that friction on their 

walls is greater. Some have maintained that the deeper circulation

is fairly vigorous, but so far as may be inferred from the developments

of deep mines it appears that the underground circulation, in many 

places at least, is exceedingly sluggish. The depths to which the 

solutions descend is a question concerning which there are great 

differences of opinion. In some rocks, under favorable structural 

conditions, surface waters are conducted, in porous beds or along 

fractured zones, several thousand feet below the surface, but in other 

rocks little or no water is collected at depths of more than a few 

hundred feet.

The shallowness of the zone of the meteoric circulation has been 

emphasized by Kemp * and by Finch.2 In the copper-bearing region 

of Keweenaw Point, Mich., the lower ends of several shafts-that penetrate many layers of bedded rocks are dry and dusty. One of these 

shafts is sunk about a mile befow the surface, but no water is raised 

from the deeper levels. At Przibram, Bohemia, no water is raised 

from depths below 2,500 feet, although the workings extend below 

3,500 feet. The water raised in the Dives-Pelican mine, Georgetown, 

Colo., from the sump, 2,000 feet below the surface, was not greater 

in quantity than that which was pumped from the mine when the 

bottom of the shaft was at higher levels. The drainage tunnels at 

Cripple Creek become nearly dry a few years after they are run, 

indicating that the waters in that area are stored in the funnel of 

the volcanic complex which is surrounded by relatively impervious 

granite and crystalline schists.3 In the lower levels of deep mines of 

Butte, Mont., there is very little water except that which flows in 

from higher levels. In many comparatively deep mines of the arid

1 Kemp, J. F., The r6le of the igneous rocks in the formation of veins, in PoSepny, Franz, The genesis of 

ore deposits, 1902, pp. 681-809.

2 Finch, J. W., The circulation of underground aqueous solutions and the deposition of lode ores: Proc. 

Colorado Sci. Soc., vol. 7, 1904, pp. 193-252.

3 Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, p. 238.
30 THE ENRICHMENT OF SULPHIDE ORES.

Southwest no body of standing water has been encountered. At 

Tintic, Utah, according to Finch, 1 water for drilling is conducted into 

the mines from the surface. Many deep bore holes sunk in search of 

oil and water have proved to be dry. These and many other examples indicate that the water circulation is exceedingly sluggish in 

some regions a short distance below the surface.

On the other hand, it should be noted that large volumes of water 

were lifted for considerable periods from deep levels of the Comstock 

lode, of the Granite-Bimetallic mine in Montana, of the Commodore 

mine at Creede, Colo., and of a great many other mines that have 

been developed more than 2,000 feet below the surface. From this 

it appears that local differences in the underground circulation are 

extreme. The amount of fracturing of the deposits and especially 

the size of the openings seem to be the most important factors controlling the rate of the circulation. Friction that retards flow multiplies with decrease in size of channels.

THE REGION OP NEARLY STAGNANT WATERS.

The zone of the deeper circulation varies greatly in depth and 

vertical extent. Its water is discharged at points that are not lower 

than the lowest elevation of the country, and if numerous points of 

discharge are located along a lode that outcrops at several different 

elevations there will be a considerable lateral movement of the waters 

toward these, for the solutions move to points of less pressure. If 

lower rocks are saturated and their openings are filled, the solutions 

descending from above will find any lateral outlet that is available. 

In some deposits the solutions doubtless descend to points lower 

than the lowest outlets and rise again to issue at such outlets, but 

such circulation is probably slight compared to the circulation that 

keeps closer to the surface owing to the nearness of points of issue 

which are located at the surface. Where there are structures that 

afford passages like inverted siphons there may be a considerable 

movement of water below the lowest outlet, but under conditions 

of fairly regular spacing.of openings along the lode the circulation 

becomes less and less vigorous as depth increases below the lowest 

outlet. There is thus a division, probably everywhere somewhat 

indefinite, between the sluggish deeper circulation and a zone of 

static or nearly stagnant waters below it. This conception has been 

clearly developed by Finch in his paper on the circulation of underground waters.2 There is much evidence that in some rocks the top 

of this zone lies within a few hundred feet of the surface or even less, 

but where there are deep open fissures it may be much deeper. Finch 

cites considerable evidence to show that rocks are generally dry below

i Finch, J. W., The circulation of underground aqueous solutions and the deposition of lode ores: Proc. 

Colorado Scl. Soc., vol. 7,1904, p. 216. 

'Idem, p. 209.
DOWNWARD CHANGE IN SULPHIDE DEPOSITS. 31

depths of 1,000 or .1,500 feet, except where they are strongly fractured. Long dry crosscuts, so familiar to all who have worked in 

deep mines, point clearly to this conclusion.

THE SEVERAL SUCCESSIVE ZONES. 

DOWNWARD CHANGE IN SULPHIDE DEPOSITS.

As is clearly set forth by R. A. F. Penrose, jr., S. F. Emmons, 

W. H. Weed, and others, many sulphide deposits show characteristic 

changes from the surface down the dip. At and near the surface 

the deposits are generally oxidized and stained with limonite. The 

outcrop and the upper part of the oxidized portion of the deposit 

may be poor. Below this there may be rich oxidized ores; still 

farther down, rich sulphide ores; and below the rich sulphides, ore of 

relatively low grade. This lowest ore is commonly assumed to be the 

primary ore, from which the various kinds of ore above have been 

derived. The several kinds of ore have a rude zonal arrangement, 

the so-called "zones" being, like the water table, highly undulatory. 

They are related broadly to the present surface and generally to the 

hydrostatic level but may be much more irregular than either, for 

they depend in large measure on the local-fracturing in the lode which 

controls the circulation of underground waters. Any zone may be 

thick at one place and thin or even absent at another. If these 

zones are platted on a longitudinal vertical projection it is seen that 

the .primary sulphide ore may here and there project upward far 

into the zone of secondary sulphides, or into the zone of enriched oxides, 

or into the zone of leached oxides, or may even be exposed at the 

surface. The zone of sulphide enrichment (which is not everywhere 

present) may project upward far into the zone of rich oxidized ore, 

or into the zone of leached oxides, or may outcrop at the surface. 

The zone of sulphide enrichment nearly always contains considerable 

primary ore, and very commonly the so-called secondary ore is merely 

the primary ore containing in its fractures small seams of rich secondary minerals. The zone of enriched oxides is generally found 

above the lowest level reached by the water table. This zone in 

places extends to the outcrop. The zone of secondary sulphides in 

moist countries is in general below the water level. In arid countries 

it may be partly or entirely above the water level.

All these zones except that of the primary ore are, broadly considered, continually descending. Ore taken from the outcrop may 

represent what was once primary ore; afterward, enriched sulphide 

ore; still later, oxidized enriched sulphide ore; later still, leached 

oxidized enriched sulphide ore; and finally it became the surface ore 

or gossan. Through more rapid erosion at some particular part of 

the lode any one of these zones may be exposed, and hence an outcrop of ore of any character is possible.
32 . THE ENRICHMENT OP SULPHIDE OEES.

THE OXIDIZED ZONE. 

CONDITIONS IN THE OXIDIZED ZONE.

In the presence of air and water sulphide ores break down and 

form soluble salts and minerals that are stable under surface conditions. No metallic sulphide that is long exposed to the action of 

these agents remains unaltered. Iron sulphides, which are present 

in practically all deposits of sulphide ores, are changed to iron oxides, 

and such a change is attended by the liberation of iron sulphates and 

sulphuric acid, which under favorable conditions dissolve many of 

the minerals. In many deposits relatively stable basic iron sulphates 

are formed, but even these eventually break down.

The oxidizing zone is in the main the zone of solution. Precipitation also takes place in this zone, especially the precipitation of the 

oxides and hydrous oxides of iron, aluminum, manganese, and 

silicon. By redeposition deposits of the more valuable metals are 

formed also in this zone. Solution generally exceeds precipitation, 

however, and by solution the mass is reduced and open spaces are 

enlarged. In limestones many of these spaces, called "watercourses," are large enough for a man to pass through. The increase 

in the size and in the volume of the openings renders the downward 

circulation comparatively free in the zone of oxidation. Regarding 

the processes of solution at Bisbee, Ariz., .where oxidized ores are 

exceptionally developed, I quote the following from Ransome:*

The most marked physical effect of the oxidation of the ore bodies-has been a great 

increase in the porosity of the masses acted upon. This, by enabling solutions to 

percolate easily through the partly oxidized zone, has greatly facilitated the migration and concentration of the desulphurized ores and their segregation in workable 

masses from the bulk of the lirnonitic and clayey "ledge matter."

The oxidized material is not only more porous but much softer and more plastic

than the original mineralized limestone and hence greatly weakens by its presence

the rocky structure in which it occurs. The overlying limestones, no longer adequately supported, fissure and settle clown upon the soft plastic ore and gangiie. The 

access of solutions is thus still more facilitated, and the processes of oxidation and 

solution proceed so much the faster. That part of the surface which is underlain by 

oxidizing ore bodies is thus rendered less resistant to erosion, other things being equal, 

than the surrounding country.

Some of the metals for example, gold dissolve very slowly in 

the zone of oxidation. If the other materials in an ore deposit are 

taken away, however, the ore may be enriched by decrease in volume, 

in the manner elucidated by Rickard.2

The hydrous oxides, once formed, are comparatively stable, and 

under certain conditions the more valuable metals dissolve more

1 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U S. 

Geol. Survey No. 21, 1904, p. 159.

2 Rickard, T. A., The formation of bonanzas in the upper portions of gold' veins, in Posepny, Franz. 

The genesis of ore deposits, 1902, pp. 734-755.
THE OXIDIZED ZONE. 33

readily than the other vein materials in the oxidized zone. Consequently the oxidized zones are in general leached of the valuable 

metals. Leaching is generally incomplete, however, for owing to the 

removal of the outcrops by erosion, new surfaces are presented to 

attack.

As already stated, the oxidized zone is generally above a secondary 

sulphide zone. As the latter is the richest part of many deposits and 

as the zone of oxidation is descending the processes of oxidation 

attack materials that are comparatively rich. In many deposits the 

first effect of oxidation is to convert the richer sulphides to rich oxides; 

consequently the lower part of the oxidized zone may be as rich as or 

even richer than the secondary sulphide zone.

Since copper may replace iron sulphides almost completely in the 

altered zones of some deposits the gossans in some regions represent, 

the oxidation product of a chalcocite zone nearly free from iron. Thus 

the outcrop may be only slightly stained with iron. Some important 

bodies of copper ores at Butte, Mont., at Morenci, Ariz., at Bingham, 

Utah, and at Cananea, Mexico, have outcrops composed of lightcolored kaolinized rocks that are not highly ferruginous.

- SEGREGATION OF THE METALS IN THE OXIDIZED ZONE.

As a result of surface alteration the metals are at many places 

segregated in the oxidized zone. At Ducktown, Tenn., a pyritic 

copper ore is converted near the surface into a high-grade iron ore. 

Below the iron ore is an exceptionally rich chalcocite ore, and below 

this the low-grade copper ore, the original deposit from which the iron 

ore and the rich copper ore have been derived. At Ducktown, as in 

many other districts/the processes of alteration are clearly related to 

the water level which divides the zone of solution from the zone of 

precipitation of copper, but in certain districts where the water level 

is lower nodules of oxidizing ore show the same zonal arrangement. 

Many of these nodules or spheres are made up of successive shells, 

each inclosing a smaller one, like the layers of an onion. Although 

the spherical shape of some of these nodules may be original, many 

of them have become rounded by oxidation. Where irregular blocks 

are inclosed by fractures to which water and oxygen have access, 

processes of oxidation, extending inward from the fractures to 

approximately equal depths, tend to round off irregularities of the 

unaltered mass, because the inequalities are the more exposed. The 

processes may be compared to concentric weathering at the surface.

At the Southern Cross mine, near Cable, Mont., nodules of goldbearing pyrite are surrounded by shells of limonite which clearly have 

been derived fiom the oxidation of the iron sulphide in place. Gold 

is concentrated in the outer shells by diminution of mass, but volume 

72293° Bull. 529 13 3
34 THE ENEICHMENT OF SULPHIDE ORES.

for volume the tenor of the iron sulphide and iron oxide is approximately the same. At Leadville, Colo., according to Ricketts, 1 

nodules of galena carry six times as much silver as the cerusite 

crusts that surround them. A number of assays cited by S. F. 

Emmons 2 show a silver content of 420 ounces per ton of galena and 

only 28.6 ounces per ton of the corresponding cerusite crusts. In 

view of the fact that the galena of Carbonate Hill averages only 145 

ounces per ton, 1 these figures seem to indicate addition of silver to 

the galena, as well as leaching of silver from the carbonate.

To illustrate the separation of copper and iron in the zone of oxidation, I quote from Lindgren, Graton, and Gordon3 the following 

description of oxidizing nodular masses in the Apache No. 2 mining 

district, New Mexico:

The primary ore consists here of a gangue of extremely coarse calcite with a little 

iron and practically no magnesia. When this is dissolved in acid there remains a 

skeleton of small films and grains of quartz whose presence would hardly be expected 

in the apparently homogeneous cleavage pieces. The calcite contains grains of 

chalcopyrite and small cubes of pyrite. During oxidation this primary low-grade ore 

becomes surrounded by crusts of secondary calcite, limonite, hematite, malachite, 

and chrysocolla. The iron and copper separate, the former being deposited in the 

recrystallized calcite as hydroxide or oxide, while the copper minerals form a thin 

crust directly adjoining the primary ore and gradually traveling inward as the oxidation progresses. This is exactly what happens under the more intense conditions 

of artificial oxidation or the roasting of chalcopyrite in metallurgical work. The 

heat in the presence of oxygen will gradually concentrate the copper sulphide or .oxide 

in the center of the lump, while the ferric oxide forms a shell which can easily be 

knocked off.

Considered in more detail, the narrow ring of oxidized copper ore consists of alternate 

narrow bands of malachite and chrysocolla, the latter being due to the quartz distributed microscopically through primary calcite. The malachite always forms the 

inner zone and projects into the fresh calcite as tufts of slender needles. In places a 

thin layer of calcite will separate the copper minerals. The wide outer crust consists 

of recrystallized calcite which only in part has the same orientation as the central 

cleavage piece of primary ore. This recrystallized calcite contains limonite in flocculent masses, in places distributed concentrically, but does not carry even a trace of 

copper. At the outer edge of the specimen the limonite changes to dark-brown 

hematite.

At Bisbee, Ariz., the segregation of the metals in the zone of oxidation is shown on a grand scale. As stated by James Douglas,4 a 

large mass of ore developed between the 200 and 400 foot levels of 

the Copper Queen mine consists of a core of compact pyrite very 

lean in copper surrounded by a shell of rich copper ore. According

iKicketts, L. D., The ores of Leadville and their modes of occurrence, Princeton, N. J., 1883, p. 37.

2Emmons, S. P., Geology and mining industry of Leadville, Colo., with atlas: MOD. U. S. Geol. Survey, 

vol. 12,1886, pp. 553-554.

s Lindgren Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68, 1910, pp. 55-56.

'Douglas, James, The Copper Queen mine, Arizona: Trans. Am. Inst. Min. Eng., vol. 29,1900, p. 531.
THE OXIDIZED ZONE. 35

to Ransome,1 a mass of pyrite was noted on the 1,000-foot level of 

the Lowell mine, lying generally parallel with the bedding of the 

limestones and in contact with partially oxidized ore both above 

and below. Some native copper was seen in the oxidized ore close to 

the pyrite. Similar residual masses of worthless pyrite surrounded 

by good ore, usually containing chalcocite, were seen on the 1,050-foot 

level of the Calumet & Arizona mine.

The processes that operate to form the small nodules are probably 

similar to those that operate to form the larger masses. Many 

details of the chemistry of these processes have not been worked out, 

but it is generally recognized,that oxidation favors the solution of 

some compounds and the precipitation of others. Dilute acid in the 

presence of air dissolves the sulphides of both copper and iron. 

Iron, but not copper, is precipitated by hydrolysis of sulphates. 

Dilute acid in the absence of air dissolves iron sulphide but not 

copper sulphide, for copper sulphide is precipitated at the expense 

of iron sulphide if air is excluded. The spherical bodies of enriched 

sulphide ore in the zone of oxidation, which are crusted with lowergrade, presumably leached oxidizing ore, doubtless illustrate in a 

small way the same processes that result in the rearrangement of the 

metals in the several zones that are related to depth.

The separation of lead carbonate and zinc carbonate during the 

oxidation of ore bodies containing zinc and lead sulphides is conspicuously shown in some districts, especially in ore bodies inclosed 

in limestone. At Leadville, Colo., deposits of sphalerite, galena, and 

pyrite are overlain by large oxidized ore bodies, some of which consist of argentiferous cerusite containing but little zinc and others 

of iron-stained smithsonite containing little lead. At some places a 

zone of smithsonite is below one of lead carbonate. At the Kelly 

mine, in the Magdalena district, N. Mex., these processes have taken 

place on a smaller scale but are very clearly indicated. There, 

according to Lindgren, Graton, and Gordon,2 the primary ore consists of zinc blende, magnetite, a little pyrite, and galena. As stated 

by these writers 

The depth of oxidation is about 300 feet. * * * In the oxidized stopes nearer 

the surface the zinc and lead part company. The oxidized zinc ores form wide stopes 

in which caves large enough for a man to crawl into are coated with beautiful botryoidal, 

light-green masses of smithsonite. The crust of this material is almost 3 inches thick, 

and underneath is a dark powdery material rich in manganese but also containing 

much zinc. The lead stopes are much smaller and are composed of almost pure 

"sand carbonate" with occasional bunches of galena.

i Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21,1904, p. 146.

8 Lindgren, Waldemar, Graton, L. C.,and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68,1910, p. 55.
36 THE ENRICHMENT OF SULPHIDE ORES. 

DEPTH OF THE OXIDIZED ZONE.

The depth of the zone of oxidation and the extent of oxidation 

within that zone depend upon the permeability of the ore and its 

character and composition. Conditions differ greatly in different 

districts and even in different deposits in the same district. The 

depth of thorough oxidation is generally less than the depth of the 

vadose circulation, for oxidation follows the depression of the water 

level. Where the ground-water level has been depressed by relatively rapid climatic change rather than by the gradual downward 

migration of ground water that attends the normal degradation of a 

country, the rate of its-depression may be more rapid than that of 

the zone of oxidation, and hi consequence the sulphide ores may be 

marooned in the vadose zone. In an arid country oxidation is 

probably slow, for it depends in a measure on the supply of oxygenbearing waters. Thus at Tonopah, at Morenci, and in some other 

districts in the Southwest the lower limit of oxidation has lagged 

far behind the downward-migrating water level.

Some deposits are oxidized to great depths. At Bisbee, according 

to Ransome, 1 partial oxidation has been noted at depths more than 

1,600 feet below the surface. At Tintic recent developments have 

exposed oxidized ores more than 2,000 feet below the surface. The 

Old Abe mine, hi the White Oaks district, Lincoln County, N. Mex., 

according to Lindgren, Graton, and Gordon, has been worked to a 

depth of 1,380 feet, yielding rich oxidized ores.2 The water level in 

this mine is 1,300 feet below the surface. In the Brooklyn mine at 

Bingham, Utah,3 oxidation is said to extend 1,450 feet below the 

surface. In the Snowstorm mine, in the Cceur d'Alene district, 

Idaho, according to Ransome, carbonates of copper are found about 

1,200 feet below the outcrop of the lode. 4 At Creede, Colo., partial 

oxidation has extended locally more than 1,000 feet below the 

surface.

At Tonopah, Nev.,6 the depth of oxidation is variable. In veins 

that outcrop oxidation has taken place locally to depths greater than 

700 feet. Veins which do not outcrop but which are capped by volcanic rocks later than the ore show comparatively little oxidation. 

A single fracture line or a fault line may divide the oxidized from 

the unoxidized ore and rock. In this district no standing ground

' Eansome, F. L., written communication.

2 Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68,1919, p. 60.

»Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 215.

« Eansome, F. L., and Calkins, F. C., The geology and ore deposits of the Coeur d'Alene district, Idaho: 

Prof. Paper U. S. Geol. Survey No. 62,1908, pp. 150-152.

* Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, p. 90.
THE OXIDIZED ZONE. 37

water has been encountered in the mine workings, which extend to 

depths greater than 1,100 feet.1

At Przibram, Bohemia, hi a comparatively moist climate, oxidation extends, according to Beck, from 200 to 900 feet below the 

surface. In some other moist districts, as at Ducktown, Term., the 

oxidized ores are not more than 100 feet below the present surface.

At Butte, Mont., according to Reno Sales,2 in the vicinity of the 

St. Lawrence and Mountain View mines, the depth of oxidation is 

nearly 400 feet, although in many parts of the camp it is only 10 feet. 

The depth of oxidation in this district depends largely on the composition of the country rock to a greater degree, in fact, than on the 

topographic features. The more complete the alteration and pyritization the greater the depth of oxidation. In the central copper area, 

where the "granite" is strongly altered and pyritized, the upper limit 

of sulphides is roughly a horizontal plane, although the surface contours 

in the same area show variations of 300 feet. Over the remainder of 

the copper area the average depth of oxidation is approximately 50 

feet. The depth of oxidation in the veins is influenced to a slight 

degree by the physical character of the veins themselves. Under like 

conditions a quartz vein is oxidized to a greater depth than a fault or 

"granite vein," owing to the greater impermeability of the clay and 

crushed "granite" of veins of the latter class.

At Tintic, Utah, the limit of oxidation hi fractured limestone is at 

least 2,000 feet deep, but the ground-water level and zone of unaltered 

sulphides in igneous rocks scarcely half a mile away is only 200 or 

300 feet from the surface.3

At Cripple Creek, Colo., partial oxidation is found hi some places 

at depths of 1,200 feet,4 but in other places it is relatively shallow.

SUBMERGED OXIDIZED ORES.

As already stated, the oxidized ore is generally above the groundwater level, and in arid countries oxidation is seldom complete at 

depths considerably above that level. In deep mines little smears or 

veinlets of oxidized material, generally a mixture of kaolin and 

limonite or of kaolin and manganese oxide, are found at considerable 

depths. In moist countries such veinlets were probably deposited 

below the water level. As the solutions descend, their acidity is 

reduced, and some of the metals which may be held in acid solutions 

would be precipitated by a decrease of acidity.

' Spurr, J. E., op. cit., p. 96.

* Sales, R. H., Superficial alteration of the Butte veins: Econ. Geology, vol. 5,1910, p. 19.

»Tower, G. W., Smith, G. O., and Emmons, S. F., Tintic Special folio (No. 65), Geol. Atlas'U. S., U. S. 

Geol. Survey, 1900, p. 5. Emmons, S. F., The secondary enrichment of ore deposits, in Posepny, Franz, 

The genesis of ore deposits, 1902, p. 438. Lindgren, WaMemar, oral communication.

* Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1908, p. 196.
38 THE ENRICHMENT OF SULPHIDE ORES.

Extensive oxidation is probably limited to those parts of deposits 

that lie above the ground-water level at the time of oxidation. In 

some deposits, however, the ores are extensively oxidized below the 

water level. According to B. S. Butler,1 the ores of the Harrington-Hickory mine, near Milford, Utah, are oxidized at least 100 

feet below the present water level. At Bisbee, Ariz., the depth of 

oxidized ore ranges from 200 to 1,600 feet and the deeper oxidized ores 

are submerged. As shown by Ransome (p. 182), the altered ores are 

related to a tilted pre-Comanche erosion surface rather than to that 

existing to-day.

Where ore bodies are extensively oxidized far below the groundwater level it is reasonable to infer that the level of the ground water 

has risen. The amount of atmospheric oxygen that water may dissolve and carry far below the water level is very small (see p. 90) 

and probably would be insufficient to oxidize large masses of sulphides, even in a comparatively long period.

TRANSITIONS BETWEEN OXIDIZED AND SULPHIDE ORES.

In moist countries, where the level of ground water is near the 

surface, the transition between the almost completely oxidized ore 

and the unoxidized sulphides is generally abrupt. This is especially 

noticeable in the pyrrhotitic copper ores of Ducktown, Term., where 

the contact between the two is at some places as sharp as a knife 

blade. In pyrrhotitic ores of the Encampment district, Wyoming, 

chalcocite ores occur immediately below the gossan.2 In the pyritic 

copper ores of Butte, Mont., according to Sales, the transition is 

sharp. He says: 3

Examined from the surface downward the oxidized portion of a copper vein will 

show but little variation in physical character or mineral composition between the 

outcrop and the sulphide ore. The line of separation marking the change from oxidized to sulphide ore is very sharp and clean cut. There is no partial oxidation of 

the vein; no mixture of sulphides and oxides. The entire change in any single cross 

section of a vein takes place within 2 or 3 feet vertically. Generally the sulphide 

ore, through slight changes in the relative abundance of certain minerals, indicates 

the proximity of the oxides, while in the case of the oxides there is seldom, if ever, 

any change to indicate a nearness to sulphides. In the copper belt the minor veins, 

stringers, iron pyrite seams, country rock, etc., are all oxidized and bear the same 

relation to the surface and to depth of oxidation as do the large veins.

In some other districts, however, there is no such sharp dividing 

line between the oxide and the sulphide ore. At Bingham, Utah, 

according to Boutwell,4 the transition from the zone of oxidation to

i Oral communication.

* Spencer, A. C., The copper deposits of the Encampment district, Wyoming: Prof. Paper U.S. 

Geol. Survey No. 25,1904, p. 55:

»Sales, R. H., Superficial alteration of the Butte veins: Econ. Geology, vol. 5,1910, p. 19.

«Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 221.
THE SECONDARY SULPHIDE ZONE. 39

the zone of sulphide enrichment is gradual, as it is also at Morenci, 

Ariz., where the zone of secondary sulphides is oxidized through a 

considerable vertical distance. Says Lindgren: l

The oxidation does not extend down to any well-defined water level, which must be 

far below the deepest workings, but acts most irregularly, sometimes leaving fresh 

metamorphic limestone at the surface and again reaching down to a depth of 400 feet 

along fissures and faults. The present ore bodies probably fell an easier prey to oxidation by reason of their richness in sulphides and their favorable exposure to percolating waters. In such position are the upper ore bodies of the Detroit and Manganese 

Blue. In other cases the oxidation was facilitated by means of fault planes and porphyry dikes.

At Bisbee, Ariz., the zone of transition between the oxide and 

sulphide ore is extensive. According to Ransome: 2

Within the transition zone between completely oxidized and unaltered sulphide 

ores, which has a maximum depth or thickness of about 900 feet, the oxidizing processes 

are controlled to a large extent by recent irregular fissuring and by the relative permeability of the various sulphide masses to generally descending solutions. Fissures 

cutting through masses of lean pyrite are almost invariably accompanied by streaks 

of rich ore, often containing chalcocite, together with cuprite and native copper. 

Where there are several such fissures near one another important ore bodies result. 

The general association of profitable ore with fissured, broken, permeable ground is 

well recognized in practical operations and turned to good account in underground 

exploration.

THE SECONDARY SULPHIDE ZONE. 

POSITION AND EXTENT.

The secondary sulphide zone is generally below a zone of oxidation. It is not everywhere developed, not even in copper ores that 

are capped with gossan. ' Examples of such deposits are cited on a 

following page. In many deposits the transition between the 

oxidized and secondary sulphide zone is sharp, being essentially at 

the ground-water level. In such deposits the secondary ores extend 

downward to various distances below the water level. The vertical 

extent of the secondary zone differs widely in different districts. In 

some of the districts of the southern Appalachians the chalcocite 

zones occupy only a few feet vertically. At Ducktown, Tenn., in all 

except one mine the average thickness of the secondary chalcocite 

zone is probably between 3 and 8 feet, but some secondary chalcopyrite is developed far below this zone. In the East Tennessee mine, 

in this district, the vertical range of chalcocite is about 125 feet. In 

the Encampment district, Wyo., the vertical extent of chalcocite 

is at least 200 feet. In the disseminated deposits hi porphyry at 

Bingham, Utah, the zone of workable sulphides, mainly chalcocite

1 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43, 1905, p. 197.

2 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21,1904, p. 145.
40 THE ENRICHMENT OP SULPHIDE OEES.

and chalcopyrite, has an average vertical extent of 418 feet,1 and 

owing to the rugged topography the vertical range is much greater. 

At Morenci, Ariz., the belt of maximum deposition of secondary 

chalcocite is in general from 200 to 400 feet below the surface, although 

veins of chalcocite are said to be encountered at greater depths, 

especially in the Coronado mine. In this mine also, according to 

Lindgren,2 chalcocite in places reached the surface. At Bisbee, 

Ariz., chalcocite, partly oxidized, is found within 200 feet of the 

present surface and extends to depths at least 1,500 feet below the 

surface. At Globe, Ariz., in the Old Dominion mine, chalcocite 

has been found more than 1,200 feet below the surface and has a 

vertical range of at least 800 feet. In the Miami Inspiration ore 

zone, according to current reports, chalcocite is found 900 or 1,000 

feet below the surface. In all these districts chalcocite is regarded 

as a mineral which is in the main if not altogether secondary. At 

Butte, Mont., chalcocite has been found at depths 2,800 feet below 

the surface. In the upper levels of the Butte mines secondary 

chalcocite is abundant, but recent investigations, especially those of 

Reno Sales, indicate that the chalcocite of the deeper levels is probably primary. Except at Butte no data now available indicate 

deposition of secondary chalcocite at depths greater than 1,500 feet 

below the outcrop. It is not at all improbable, however, that 

secondary chalcocite deposited at greater depths may be revealed by 

future exploration.

Examples of enrichment of silver sulphide deposits at depths from 

1,000 to 1,200 feet below the surface are well authenticated. Doubtless secondary deposits of silver occur at greater depths, but it is 

questionable how far the several species of the silver-bearing minerals may indicate geologic processes. I know of no examples of 

the precipitation of appreciable amounts of gold by descending 

solutions at depths more than 1,000 feet below the surface. In 

general, gold that is dissolved by surface waters is precipitated at 

relatively shallow depths.

The data reviewed above show that no definite depth can be 

fixed below which processes of enrichment are not effective. The 

maximum precipitation occurs at comparatively shallow -depths, 

however, and there is little reason to suppose that these processes 

are effectively operative in the deeper part of the zone of fracture. 

The depth to which the metals are carried depends on local climatic conditions, permeability, and the chemical or mineralogic 

environment.

»Seventh Ann. Kept. Utah Copper Co., Dec. 31,1911.

a LIndgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Qeol. Surrey No. 43, 1905, p. 210.
THE SECONDAEY SULPHIDE ZONE. 41 

RELATION TO THE WATER LEVEL.

. The ground-water level, the so-called water table, has frequently 

been regarded as indicating the top of the secondary sulphide zone 

at the time that zone was formed. As has been stated, the water 

level is not stationary but oscillates, although it tends to move downward as the major drainage channels approach grade. If the water 

level is comparatively high or if the lode is much fractured, and 

particularly if it contains large spaces, there is probably but little 

precipitation of the secondary sulphides above the water level. There 

are two reasons why this should be true; one is that the solutions 

descend in open rocks at a comparatively rapid rate through the fractured and partly weathered zone of somewhat cellular ore above the 

shallow water level, and the other is that several of the sulphides of 

the more valuable metals that are dissolved readily in an oxidizing 

environment are not readily precipitated in such an environment. 

If the rocks are open atmospheric oxygen should find ready access 

to the ore above the zone of saturation. But even if precipitation 

should take place the sulphides precipitated would later be exposed 

again to oxidation and solution.

The sulphides below the water level are protected from the oxygen 

of the air, however, and solution of some metals is retarded if not 

prevented. The solution in acid sulphate of copper sulphide, silver 

compounds, and gold requires an oxidizing agent. It is well known 

that a small concentration of ferrous sulphate, which is surely present 

 in the reducing zone below the water level, will drive gold and 

silver from solution. Copper sulphide, which dissolves readily in 

sulphuric acid in the presence of air, is not dissolved in its absence. 

Hydrogen sulphide is generated by the action of acid on several 

sulphides and in the presence of the faintest trace of hydrogen sulphide 

copper sulphide is not dissolved not even in boiling concentrated 

acid. 1 Thus it would be supposed that under these conditions the solution of copper, silver, and gold would be inhibited at ground-water 

level or a short distance below it. Since a liter of water can absorb 

under surface conditions only about 6.84 cubic centimeters of atmospheric oxygen, the amount carried dissolved, even in a saturated 

solution, can not be great. The oxygen that is made available by the 

hydrolyzation of water when ferric sulphate is reduced to ferrous sulphate and that which combines to form the higher oxides of manganese might delay reduction, but the delay would be only temporary. 

Although oxygen is required for the solution of gold, silver, and copper, the sulphides of zinc and at least some of the sulphides of iron 

might be dissolved at depths considerably below water level, for they

1 Alien, E. T., oral communication.
42 THE EtfRlCHMEtf? OF SULPHIDE ORES.

are attacked by acid sulphate even in the absence of an oxidizing 

agent.

Many deposits of secondary sulphide ore in the arid Southwest are 

well above the present ground-water level. The lower limits of some 

of these deposits have been reached by mining, and below some of them 

lie considerable bodies of pyrite and chalcopyrite ore which have been 

penetrated by mine workings that encountered no standing underground water. As has been already stated, the climate in this area 

has changed from humid or subhumid to arid in comparatively recent 

time, and it is possible that some bodies of secondary sulphide ore 

have been marooned by the rapid descent of the ground-water level 

that attended the change. At Morenci, Ariz., nearly all the mines are 

dry in and below the chalcocite zones of these deposits. Of these 

Lindgren l says:

Chalcocite perhaps forms at the present time in the upper levels of the belt occupied 

by this mineral where copper-sulphate solutions from oxidizing chalcocite above are 

abundant and free oxygen absent. * * * Direct oxidation has, in fact, already 

penetrated to the deepest levels attained in the pyritic zone; at present it works here 

chiefly along fissures and seams but is probably slowly spreading.

I regard the chalcocite zone as formed about an ancient water level, much higher 

than the present. During the epoch of the Gila conglomerate the water level was 

surely at least several hundred feet higher than it is now, and it was probably still 

higher during Tertiary time, in which a moist climate most likely prevailed.

PEECIPITATION OF SULPHIDES ABOVE THE WATEE LEVEL.

In arid districts, where the water level lies very deep, the descending metal-bearing solutions may encounter a reducing environment in 

the so-called vadose zone, especially in rocks that contain only minute 

openings, through which tjie water soaks downward from the surface, 

excluding the admission of any considerable amount of air. It would 

be supposed that the oxygen present in such waters and in the imprisoned air would be used up before the descending solutions encountered 

any zone of permanent saturation, or that the solutions might become 

reduced before reaching ground-water level, so that the metals could 

readily be precipitated.2

Any former water level doubtless maintained the usual relation to 

the topography that existed when it prevailed, and it is generally 

untenable to assume that the topography which controlled a former 

water level in a region where the climate has changed recently was 

very different in its major features from the present topography. 

Stope sheets and other available published information show that 

many of the chalcocite zones hi copper deposits of arid regions have

' Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district: Prof. Paper U. S. Geol. 

Survey No. 43,1905, p. 206.

1 Finch, J. W., The circulation of underground aqueous solutions and the deposition of lode ores: Proc. 

Colorado Sci. Soc., vol. 7,1904, p. 238.
THE SECONDARY SULPHIDE ZONE. 43

a pretty definite relation to the major features of the present topography, and it therefore seems possible that when they were formed they 

bore the usual relation to the water level then prevailing. The disseminated chalcocite ores of Miami, Ariz., are, however, not definitely 

related to the present topography, which was apparently developed 

after the main period of enrichment. 1 At Ray also, according to 

Ransome, 2 the layer of disseminated chaicocite ore has many irregular 

undulations that apparently have no dependence on the present 

topography.

It is not at all unlikely that secondary sulphide ores have formed 

independently of any present or past water level. As stated by 

Finch,3 deposits exposed to processes of alteration in an arid region 

may lie in -gaseous zones that are analogous to the aqueous zones of 

more humid regions. Oxygen predominates in the surface zone, 

whereas carbon dioxide and other heavier gases may prevail at greater 

depths. Thus in an arid region there may be an oxidizing gaseous 

zone and below it a reducing gaseous zone in which precipitation may 

take place in the absence of oxygen. Circulation by evaporation 

would be possible in the lower zone.

Precipitation by evaporation is undoubtedly a factor to be considered in shallow oxidizing zones and may account for the veinlets 

of clayey limonite that are found in cracks in some sulphide ores at 

depths considerably below the zones of nearly complete or welladvanced oxidation. The material precipitated by the evaporation 

of the mineral waters should be chemically similar to that deposited 

by evaporation under surface conditions, and, so far as analyses show, 

mineral waters that accomplish sulphide enrichment generally carry 

iron, aluminum, and other metals of low value greatly in excess of the 

more valuable metals. Unless, by processes of selective precipitation, 

certain compounds only could be removed from the solutions in depth, 

nearly all of the material dissolved in the oxidizing environment and 

carried below into the reducing environment would be precipitated. 

There would be a downward circulation of water and dissolved mineral 

matter. The upward circulation of evaporated water might carry some 

of the dissolved salts, but few data are available on this point. Weed 

states that ferrous, copper, and zinc sulphates are carried by the moisture of air circulating in mine openings. He thus accounts for efflorescence of these salts on mine workings.4 Since mineral matter could 

not readily escape, however, the enrichment that could take place 

where downward-moving aqueous solutions could not reissue would

' Ransome, F. L., this bulletin, p. 185.

* Ransome, P. L., this bulletin, p. 187.

s Finch, J. W., The circulation of underground aqueous solutions and the disposition of lode ores: Proc. 

Colorado Sci. Soc., vol. 7,1904, p. 240.

* Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No 74, 1612, p. 99.
44 THE ENKICHMENT OF SULPHIDE OKES.

depend on the sum total of the openings available, for appreciable 

replacement under these conditions would probably not occur. The 

secondary materials that are deposited under such conditions as a 

whole would not be much if any richer than the residues of descending waters, although even under these conditions the valuable minerals might be segregated somewhat by selective precipitation.

TEXTURES OF SECONDAEY SULPHIDE ORE.

In the zone of oxidation solution ordinarily much exceeds precipitation. The oxidized ore is almost invariably of spongy, open texture and contains numerous solution cavities. In the secondary sulphide zone solution does not exceed precipitation to the same extent. 

In the sulphide zone numerous solution cavities may be developed, 

but these are likely to be filled, partly or completely, by secondary 

minerals. The texture of secondary sulphide ore varies widely. 

Some of the chalcocite ore of Ducktown, Term., is little more than a 

sponge of chalcocite from which iron, copper, and other sulphates 

have been dissolved. On the other hand, some secondary sulphide 

ores consist largely of massive chalcocite inclosing numerous primary 

minerals, the pore space in the ores being quite subordinate.

The solution of primary sulphides and the precipitation of secondary sulphides may go on simultaneously, the secondary minerals 

replacing the primary. The best proof of replacement is offered 

where the later minerals have assumed the crystal forms of the earlier 

minerals. Pseudomorphs of chalcocite or covellite after pyrite, chalcopyrite, or zinc blende are the most common. Such replacements of 

pyrite are clearly shown in the Miami-Inspiration ore zone at Miami, 

Ariz., 1 and at other places. Examples scarcely less suggestive are 

found in districts where masses of primary sulphide not having crystal 

form but of distinctive occurrence have been replaced by secondary 

ores. In the No. 20 mine near Ducktown, Tenn., where the primary 

ore, consisting chiefly of massive pyrrhotite, pyrite, and chalcopyrite, 

includes small rounded masses of quartz, garnet, and actinolite, the 

secondary ore consists of chalcocite and other minerals and includes 

similar rounded masses of gangue minerals partly dissolved. At Morenci, Cananea, Bingham, Ely, Santa Rita, and in other districts containing disseminated deposits of secondary chalcocite, where the mineralized material below the secondary zone consists of a granular or a 

porphyritic igneous rock containing numerous small masses and veinlets of sulphide ore, chiefly pyrite and chalcopyrite, the same rock in 

the secondary zone contains small masses and veinlets of chalcocite, 

with some pyrite and chalcopyrite, showing an arrangement of spacing 

similar to that shown by the minerals in the lower zone. Such evi1 Ransome, F. L., Criteria of downward sulphide enrichment: Econ. Geology, vol. 5, 1910, p. 214.
THE SECONDARY SULPHIDE ZONE. 45

dence of replacement is scarcely less convincing than pseudomorphs 

having the regular boundaries of older crystals. In some secondary 

deposits the small rounded- cores of an older sulphide are coated 

over with shells of the secondary sulphide. As pointed out by Irving,1 

this in itself is evidence of replacement. That solution and precipitation went on simultaneously is indicated in 'such' deposits by the 

even spacing of the cores. If solution had been completed before 

precipitation began, the small, rounded, partly dissolved masses 

would have settled and would have been massed closer. Of the chalcocite ore of the Old Dominion mine, at Globe, Ariz., Ransome 2 says:

When the chalcocite is examined closely, particularly with a lens, it shows an 

indistinct unevenness of texture suggestive of the obscurer forms of pisolitic structure 

observed in some bauxites. Critical scrutiny of the inclosed grains of pyrite discovers 

the fact that their outlines are rounded and that the chalcocite has a more or less distinct concentric, shelly structure around each grain. These facts at least strongly 

suggest that the chalcocite has been formed at the expense of'the pyrite and that the 

minute structure observable in chalcocite now free from pyrite records the former 

presence of that mineral and its subsequent replacement by the sulphide of copper.

Much of the material which is generally termed secondary sulphide 

ore consists essentially of shattered and fractured primary sulphide 

ore, the cracks in which are filled with later sulphides or with angular 

fragments of the earlier sulphides crusted over with those that were 

introduced later. Such textures do not invariably indicate processes 

of sulphide enrichment by descending solutions. Many authentic 

examples show that, in the course of primary mineralization, the ore 

first deposited has been fractured and that solutions from below have 

deposited later sulphides in the fractures. In many deposits, moreover, the later sulphides are richer than the original fractured ore.

Pseudomorphous replacement indicates a change of physical conditions or of chemical environment. Minerals that were stable under 

certain conditions have been dissolved and other minerals have simultaneously been deposited. On the other hand, fractured ore cemented 

by later minerals may be a result of normal and perhaps continued 

deposition from below. If, however, the minerals that fill the later 

cracks are those that are commonly formed by descending solutions, 

and if they do not persist in depth, the assumption that they are secondary may with considerable confidence be regarded as confirmed.

In many deposits the genesis of such later veinlets is perplexing. 

As the matter now stands, the mineralogic criteria are unsatisfactory, 

for few if any of the sulphides are formed invariably under a single 

set of conditions.

The texture of the secondary veinlets is important, however, and it 

may throw considerable light on their genesis. Concerning this point

ilrving, J. D., Replacement ore bodies, in Types of ore deposits, San Francisco, 1911, pp. 289-290. 

2 Ransome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. 8. Geol. Survey No. 12, 

1903, p. 121.
46 THE ENRICHMENT OF SULPHIDE OEES.

certain observations by Hynes 1 on ores from the Mina Mexico vein, 

Sonora, Mexico, are noteworthy. The polished surfaces of several 

specimens prepared by him showed primary ore composed of pyrite 

and quartz, not containing any antimony mineral, cut by veinlets of 

ore composed of tetrahedrite and quartz. In the later veinlets the 

quartz is everywhere inclosed in the tetrahedrite and is generally 

idiomorphic. Its crystals are distributed with great regularity 

through the tetrahedrite, suggesting definite or eutectic proportions.

So far as is now known the crystallization of minerals in eutectic 

proportions does not occur in descending sulphate waters. The 

texture of the chalcocite ores of Virgilina should be mentioned in 

this connection. In the upper levels of the mines the ore is cut by 

small veinlets of secondary chalcocite, but polished surfaces of ore 

from the lower levels, as shown by F. B. Laney,2 reveal an intimate 

intergrowth of chalcocite and bornite which suggests the pattern 

of a graphic granite. Some of Laney's sections 3 are particularly 

suggestive of precipitation under conditions that prevail at great 

depths and of processes of crystallization that are not known to 

operate under the temperatures and pressures that prevail in the 

zone of sulphide enrichment.

When in the course of the deposition of ores the minerals are 

deposited layer on layer in open spaces, the minerals forming the 

last layer or crust may differ from those formed earlier, and when 

cavities are broken open the minerals last deposited appear to be of 

late age. The position of the mineral lining the cavity does not 

certainly indicate that it was deposited by descending waters, for 

the last ores deposited by ascending waters in open spaces would be 

in a similar position. If, however, the walls of cavities and fractures 

are coated with the hydrous oxides of iron or manganese, and if sulphides have formed along with these oxides, there is small probability of error in determining whether the sulphides were formed by 

ascending or descending solutions, for the hydrous oxides are rarely 

deposited with primary sulphide ores. Likewise the intimate association of native metals with such hydrous oxides is generally evidence of deposition by oxidized descending solutions.

i Hynes, D. P., Notes on the geology of the Mina Mexico vein: Econ. Geology, vol. 7,1912, pp. 280-286. 

See also Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. 

Survey No. 74,1912, p. 76 (fig. 13, showing chalcocite inclosing idiomorphic crystals of quartz and pyrite).

* Laney, F. B., The relation of bornite and chalcocite in the copper ores of the Virgilina district of North 

Carolina and Virginia: Econ. Geology, vol. 6,1911, pp. 399-411.

8 See especially that figured on PI. VII, fig. 2, in the paper just cited.
THE ENRICHMENT OF SULPHIDE ORES. 47

ESTIMATES OP VERTICAL EXTENT OF PORTIONS OF LODES

ERODED.

Where there is reason to suppose that the primary ore was of 

approximately uniform composition before secondary alteration took 

place, it is possible to estimate the vertical extent of the portion of 

the deposit that has been eroded. As already stated, the ore in the 

secondary zone generally contains the valuable metals that were 

present in the primary ore, those that were leached from the oxidized 

zone above the secondary sulphide ore, and those that were leached 

from the portion of the deposit that has been removed the metals 

that were carried downward by solutions in advance of erosion.

Estimates made for the Granite-Bimetallic lode, Philipsburg, 

Mont., indicate that at least 1,600 feet of material like the primary 

ore in the bottom of the mine was required to supply the valuable 

minerals in the secondary sulphide zone, in addition to those of the 

primary ore and any that may have been dissolved from the oxidized zone and reprecipitated below. Estimates for Ducktown, 

Tenn., on the other hand, show that the valuable minerals in the 

secondary sulphide zone may be fully accounted for by the reconcentration of copper formerly in the leached zone now exposed, and 

that the copper that was present in the part of the lodes removed 

by erosion was scattered, probably while the region was at base-level.

If x equals the vertical extent in feet of the part of the lode which 

has been removed from above the present apex, a equals the vertical extent in feet of the leached zone, & equals the vertical extent in feet of the enriched zone, I equals the values 1 remaining in 

the leached zone, e equals the values in the enriched zone, and p 

equals the values 1 in the primary ore, the following formula may be 

applied to ascertain the number of feet removed:

P

This formula does not take into account the changes in mass 2 in 

the ore itself nor the pore space formed, and it is recognized, of 

course, that many other factors may modify the results; for the 

values may not all be reconcentrated, and the primary ore, before 

alteration and enrichment, may not have been of equal richness 

throughout the deposit. The estimates therefore give only a rude 

approximation, but one which may .be used, hi connection with 

other geologic data, as a check on conclusions regarding the 

minimum amount of erosion that has taken place since the primary 

ore was deposited.

1 Assay contents per ton.

'If value can be expressed as assay contents per unit volume, changes in mass are accounted for,
48 THE ENRICHMENT OF SULPHIDE OKES.

CHEMISTRY OF SULPHIDE ENRICHMENT.

EXPERIMENTAL DATA ON THE SOLUTION AND PRECIPITATION

OF THE METALS.

The oxidation of pyrite is considered by many to proceed as 

follows : 1

FeS2 + 70 + H2 0 = FeS04 + H2S04 .

This equation does not express intermediate steps, nor does it represent the final products. Ferrous sulphate in the presence of atmospheric oxygen will be oxidized to ferric sulphate or to ferric sulphate 

and ferric hydroxide:

6FeS04 + 30 + 3H20 = 2Fe2 (S04) 3 + 2Fe(OH)3 .

The hydrolyzation of ferric sulphate may first give a basic ferric 

sulphate,2 but this subsequently breaks down, forming ferric hydroxide 

and sulphuric acid, as indicated below :

Fe2 (S04 ) 3 + 6H20 = 2Fe(OH)3 + 3H2S04 .

Ferric hydroxide probably corresponds to the mineral limnite, which 

on dehydration would give limonite:

4Fe(OH) 3 = 2Fe2 03 + 6H2 0 = 2Fe2 03 .3H20 4- 3H20.

According to Stokes 3 the oxidation of pyrite by ferric sulphate should 

be regarded as taking place by two independent reactions:

(1) FeS2 + Fe2 (S04 ) 3 =

(2) 2S + 6Fe2 (S04 ) 3 + 8H20= 12FeS04 + 8H2S04 .

In the presence of air the ferrous sulphate formed would change again 

to ferric sulphate.

A. N. Winchell 4 treated powdered pyrite containing a trace of copper to dripping aerated water. At the end of ten months 300 grams 

of pyrite had lost 0.2 gram and the solution circulating through the 

pyrite had gained, per liter of the solution, the amounts indicated 

below:

Milligrams. 

Fe2(S04)3 . .................................................... 27.68

H2S04 . ...................................................... 5.7

Ferrous iron ................................................. Trace.

S03 . ........................................................ Trace.

1 Alien, E. T., Sulphides of iron and their genesis: Min. and Sci. Press, vol. 103, 1911, pp. 413-414. Gottschalk, V. H., and Buehler, H. A., Oxidation of sulphides: Econ. Geology, vol. 7, 1912, pp. 15-34.

' Penrose, R. A. F. , jr. , The superficial alteration of ore deposits: Jour. Geology, vol. 2, 1894, p. 293. Brauns, 

Reinhard, Chemische Mineralogie, Leipzig, 1896, p. 368.

'Stokes, H. N., On pyrite and marcasite: Bull. U. S. Geol. Survey No. 186, 1901, p. 15.

4 Winchell, A. N., The oxidation of pyrite: Econ. Geology, vol. 2, 1907, pp. 290-294.
. CHEMISTBY OF ENRICHMENT. 49

In the total solution there was 

Milligrams. 

Fes(S04),........................................................ 332

H2S04 .......................................................... 68

Grout 1 repeated these experiments, subjecting the powdered 

mineral to repeated drying. In a year the specimens had lost from 

0.015 to 0.057 per cent. These losses are of the same order as those 

shown by the figures obtained by Winchell, namely, 0.067 per cent.

Buehler and Gottschalk 2 made numerous experiments with distilled water, leaching powdered sulphides to determine the nature of 

the alteration products. The finely powdered sulphides were placed 

upon filter paper spread upon a Hirsch filter plate, which was fitted 

in a 6-centimeter funnel. The sulphides were then washed twice 

daily with distilled water, the filtrate being caught in a flat-bottomed 

flask. The sulphides were treated alone and also when mixed with 

pyrite or marcasite or some other natural sulphide.

A series of the filtrates obtained from marcasite alone and from 

marcasite mixed with galena, sphalerite, and copper sulphides were 

tested for free sulphur dioxide, sulphites, and thiosulphates, but none 

of these compounds were present.

In experiments with pyrite the residue left on the-funnel was 

treated with carbon disulphide to test for the presence of free sulphur. 

No sulphur was found in a series of eight experiments, but a 10-pound 

sample of marcasite similarly treated showed the presence of free 

sulphur.

Tests for the presence of hydrogen sulphide as a possible decomposition product of these sulphides were made in an apparatus 

through which air could be drawn. The air was passed slowly through 

lead solution to free it from any hydrogen sulphide; it was then passed 

through a tube in which finely ground sulphide was placed; finally the 

air was again washed in lead solution. The sulphides were washed 

twice daily with water. After four months' continuous treatment 

there was no sign of blackening of the lead solution, indicating the 

absence of even traces of hydrogen sulphide. The following analysis 

of the solution and precipitate in the flask indicates hydrolysis:

Solution, 1,068 cubic centimeters.

FeS04 , 0.0602 gram; Fe2 (S04 ) a , 0.1799 gram; S03 (excess), 0.2697 

gram; Fe208 (in the precipitate), 0.0218 gram.

The conclusion of Gottschalk and Buehler is that the oxidation of 

pyrite in water proceeds according to the reaction 

FeS2 + 70 + H20 = FeSO4 + H2SO4 .

i Grout, F. F., The oxidation of pyrite: Econ. Geology, vol. 3,1908, pp. 532-534. 

8 Buehler, H. A., and Gottschalk, V. H., Oxidation of sulphides: Econ. Geology, vol. 5,1910, pp. 28-35; 

vol. 7,1912, pp. 15-34.

72293° Bull. 529 13 4
50 THE ENRICHMENT OF SULPHIDE OEES.

This does not involve the generation of hydrogen sulphide, of sulphur dioxide, or of more than an exceedingly small amount of sulphur.

In this connection it is noteworthy that these experiments were 

carried on with distilled water with very free access of air. Dilute 

acid solutions acting upon some sulphides of lead and zinc and upon 

ferrous iron will generate hydrogen sulphide. (See p. 59.)

In several experiments by Gottschalk and Buehler l the powdered 

sulphides spread on a filter were washed twice daily with distilled 

water. In most of these experiments the sulphates were passed 

through 60-mesh screens and caught on 80 or 100 mesh screens, so that 

the powders were of approximately equal fineness. In some experiments galena was caught on 200-mesh screens, giving relatively greater 

surfaces for attack. Equal masses, not equal volumes, of the several 

minerals were taken.

In most experiments the accumulated washings were about 500 

cubic centimeters. Pyrite, marcasite, galena, sphalerite, and enargite 

were attacked. Each of the metals was oxidized to sulphate and 

washed through the filter. Pyrite and marcasite were oxidized to 

ferric and ferrous sulphate and to sulphuric acid. Zinc was oxidized 

to zinc sulphate and washed through. Galena was oxidized to lead 

sulphate, but most of it remained on the filter, though some washed 

through; of the part that washed through a little was deposited in the 

flask as lead sulphate. Hydrogen sulphide and sulphur dioxide were 

not identified in any of these experiments, although in some of the 

filtrates there was a loss of sulphur which is not accounted for.

Each sulphide when treated alone was dissolved rather slowly, 

but when two were treated together the action on one of them was 

greatly accelerated and that on the other retarded. Sphalerite 

dissolved more readily in the presence of pyrite. The acid generated 

by the oxidation of iron was not the only factor that favored solution, 

for when the iron sulphide was placed on a filter above zinc sulphide 

the rate of solution of zinc sulphide was not greatly increased. Gottschalk and Buehler conclude that the acceleration of the reactions is 

due to electric currents generated by contact of minerals of different 

potential. The electric current flows from the mineral having the 

higher potential, so the one of lower potential will dissolve more rapidly, the one of higher potential being protected from oxidation and 

solution.

Gottschalk and Buehler arranged several minerals in a series analogous to the electromotive series of the metals, the values having been 

determined by the use of the Ostwald potentiometer. The results 

of the experiments are shown below. The force generated while any 

two of these minerals are immersed in a solution tends to accelerate

i Gottschalk, V. H., and Buehler, H. A., Oxidation of sulphides: Econ. Geology, vol. 7,1912, pp. 15-34.
CHEMISTRY OF ENKICHMENT. 51

the oxidation and dissolution of that sulphide which stands lower 

in the series and to retard the oxidation of the one that stands 

higher. It is noteworthy that the experiments were made with distilled water. The potential varies with the solution, and for some 

salts the relations are not those indicated in the table.

Potential, in volts, of several sulphides measured in distilled water against copper wire.

Marcasite....................... +0. 37

Argentite....................... +0. 27

Cassiterite...................... +0. 27

Chalcopyrite.............. +0.18 to 0. 30

Enargite................. +0.18 to 0. 23

Molybdenite.................... +0. 20

Covellite....................... +0. 20

Pyrite........................... +0.18

Bornite......................... +0.17

Mixed copper ore................ +0.15

Galena.......................... +0.15

Chalcocite...................... +0.14

Hematite..............!".. +0.08 to 0. 26

Cuprite.......................... +0. 05

Niccolite....................... +0. 02

Domeykite...................... +0. 01

Metallic copper................. 0. 00

Stibnite.................. -0.17 to 0. 6

Sphalerite................ -0.2 to 0. 4

Corundum................ -0. 25 to 0. 6

Metallic zinc.................... -0.83

According to this table marcasite shows the highest electromotive 

force and corundum shows the lowest for the minerals, the difference 

being more than 0.6 volt. Metallic zinc shows a negative relation to 

copper of 0.83 volt, and there is a difference of 1.2 volts between the 

highest and lowest member of the series.

A piece of pyrite and a piece of galena, each 2 inches long and about 

1 inch square, each having copper wire attached near one end, were 

placed in a small dish containing ordinary distilled water. The wire 

did not touch the water, nor were the minerals in contact. A circuit 

formed by placing a galvanometer between the free ends of the wires 

showed the same difference in potential that is shown in the above 

table and recorded a current of about 5 microamperes. In the course 

of a few days, the circuit being maintained, the galena became coated 

with lead sulphate, while a similar piece of galena, placed in the same 

solution but not in circuit, remained perfectly bright. The pyrite 

showed no evidence of oxidation.

Lawson 1 placed a mixture of pulverized chalcocite and porphyry 

in a burette and passed a 10 per cent solution of ferrous sulphate 

through it in the presence of air. The chalcocite was dissolved and 

passed from the burette as copper sulphate (CuSO4) at a fairly rapid 

rate.

Winchell 2 placed crystals of pyrite in a sealed jar in a slightly acid 

solution, containing sulphur dioxide and a dilute solution of copper 

sulphate. At the end of three months films of chalcocite were deposited on the pyrite, and its copper content, which at the beginning

' Lawson, A. C., The copper deposits of the Robinson mining district, Nevada: Bull. Dept. Geology 

Univ. California, vol. 4,1906, p. 333.

> Winchell, H. V., Synthesis of chalcocite and its genesis at Butte, Mont.: Bull. Geol. Soc. America, voL 

14,1903, pp. 272-275.
52 THE ENRICHMENT OF SULPHIDE ORES.

was 1.50, had increased to 3.60 per cent. In another jar, with similar 

reagents except sulphur dioxide, no chalcocite was deposited. Winchell suggests that sulphur dioxide is necessary for the deposition of 

cuprous sulphide. He infers that the cupric sulphate in solution 

with sulphur dioxide supplies cuprous ions for the reaction with the 

pyrite that supplies the sulphur for the chalcocite, the iron going into 

solution as ferrous salt. He found that iron was dissolved and subsequently precipitated as ferric hydrate.

Recently, in support of Winchell's suggestion, Spencer 1 has discussed the possible reactions involved in chalcocite deposition.

The inference that cuprous ions form in this reaction has not 

received universal acceptance, because cuprous salts have not been 

identified in mine waters. Cuprous sulphate is a very unstable 

compound. It may be held in solution in some substances, but it is 

stated that when placed in water it quickly becomes cupric sulphate. 

Recently, however, Wells 2 has shown that cuprous ions may exist 

in exceedingly small concentration under conditions nearly similar 

to those that prevail where sulphate waters attack iron and copper 

sulphides. Examination of eight mine waters collected by the 

writer failed, however, to reveal a trace of cuprous salt.

Oxygen tends to delay or to inhibit the precipitation of copper 

sulphides. Possibly an explanation of the behavior of the two 

chemical systems above described, one with and the other without 

sulphide dioxide, lies in the power of sulphur dioxide to remove 

atmospheric air from the solution. The oxygen would be removed 

to form sulphuric acid, and copper sulphide would then be more 

readily precipitated by pyrite. In the jar without sulphur dioxide 

enough oxygen may have been present to effectively delay precipitation.

Vogt 3 pulverized 6 grams each of chalcocite, bornite, pyrrhotite, 

chalcopyrite, and pyrite and separately treated them in 100 cubic 

centimeter jars with aqueous solutions containing 30 grams of ferric 

chloride. After standing several years at about 14° C. they were* 

dissolved in the order named. The filtrates from chalcocite and 

bornite showed a trace of sulphate, chalcopyrite a stronger trace, 

and pyrite a still stronger trace. Sulphur separated from the first 

four minerals and probably also 'from pyrite. Sulphur did not 

appear to oxidize to sulphate in the reaction with chalcocite, though 

oxidation occurred in some degree with pyrite and pyrrhotite.

iSpencer, A. C., Chalcocite deposition: Jour. Washington Acad. Sci., vol. 3,1913, pp. 70-75.

a Wells, R. C., Discussion of paper by F. L. Ransomeon "Criteria of down ward sulphld» enrichment": 

Econ. Geology, vol.5,1910, p. 482.

8 Vogt, J. H. L., Problems in the geology of ore deposits, in PoSepny, Franz, The genesis of ore deposits, 

1902, pp. 676-677.
CHEMISTRY OF ENRICHMENT. 53

In the table below the solubilities of several anhydrous sulphates 

are compared.

Anhydrous sulphate held in solution in a Kter of water at 18° 0.

[After Kohlrausch.)

Grams.

Ba«04 .................................................... 0. 0023

PbS04 ..................................................... .041

CaSO4 ...................................................... 2.0

AgaS04 -... '-...................-........................... 5.5

K2S04 ..................................................... 111.1

Na^SC^.............. ....................................... 168.3

MgS04 .................................................... 354. 3

................................................... 531.2

In the table below the solubilities of several hydrated sulphates 

are compared.1

Hydrated sulphates dissolved in a liter of water at about 20° C.

Grams. 

FeS04 .7H20... ............................................. 264.2

CuS04 .5H20 ................................................ 423. 3

Al2(S04)s.18H20(atO)........-................................ 868.5

MgS04 .7EC,0.. ............................ ................... 1,160.0

ZnS04 .7H20. . .............................................. 1, 610. 0

Na2S04 .10H20 (at 31.25°) ................................... 4, 800. 0

The chemical practice in the leaching process at Rio Tinto, Spain,2 

is said -to be as follows. The ore heaps are built up with open spaces 

to give ready access of air and water. In the presence of these 

the following reactions, according to Austin, take place:

FeS2 +.70 + H20 = FeS04 + H2S04 . 

The ferrous sulphate oxidizes to ferric sulphate thus: 

2FeS04 + H2S04 + 0 = Fe2 (S04 ) 3 + H20.

The ferric sulphate reacts on chalcocite and changes it to sulphate 

as follows:

Fe2 (S04 ) 3 + Cu2S = CuS04 + 2FeS04 + CuS.

This reaction is relatively rapid, and, accordingly, about half the 

copper goes into solution in a few months. The covellite molecule 

remaining is further dissolved according to the following reaction:

Fe2 (S04 ) 9 + CuS + 30 + H2O = CuS04 + 2FeS04 + H2S04 .

This reaction is slow, but in two years, under favorable conditions, 

80 per cent of the remaining half of the copper is dissolved. These 

chemical processes illustrate in a large way the processes of solution 

of chalcocite under natural conditions.

1 Data from Comey's and from Seidell's dictionaries of solubilities. I have utilized also Lindgren's compilation, Prof. Paper U. S. Geol. Survey No. 43, 1905, p. 181. 

1 Austin, L. S., The metallurgy of the common metals, 2d ed., San Francisco, 1909, pp. 351-353.
54 THE ENRICHMENT OF SULPHIDE ORES.

In connection with the processes of solution of copper at Rio 

Tinto, the following statement by Arthur L. Collins is of interest:

Reactions relied upon for the removal of copper from the zone of weathering, 

namely, the decomposition of copper sulphides by ferric sulphate, * * * are employed at Rio Tinto on a very large,scale in the treatment of copper ore. The liquors 

from the lixiviation of heap-roasted ore were run over "raw" fine ore, originally to 

lessen the consumption of iron in the precipitation tanks and to secure a cleaner 

precipitate. But this was found to be also an' efficient method of extracting part of 

the copper content of raw pyrites, and great heaps of mixed "raw fines " and lixiviated 

roasted ore, aggregating millions of tons, gradually giving up their copper in solution, 

largely by means of this reaction, became a feature of the Rio Tinto landscape. The 

supposed reaction for the reprecipitation of copper in secondary copper ores from 

cupric sulphate solutions by pyrites can hardly take place under these conditions; it 

would upset the commercial process.1

An explanation of the apparent contradiction in the theory of 

chalcocite enrichment is offered by S. F. Emmons.2 He notes that 

near the surface, where oxidation and leaching take place at Rio 

Tinto, the air has free access to the ore, whereas air is not admitted 

to ores that lie at great depths where copper is precipitated.

Copper is precipitated from sulphate solution by alkaline silicates. 

E. C. Sullivan 3 placed finely ground materials such as orthoclase, 

albite, amphibole, and clay gouge in flasks with copper sulphate, silver sulphate, and other salts. They remained several days at room 

temperature, being occasionally shaken. The solutions were dilute 

and the precipitation in many of the flasks was nearly complete. 

Whether contact was maintained for a few hours or a few months 

made little difference. Analyses of the filtrate before and after 

the reactions showed that considerable material, especially the 

alkalies, had been dissolved from the silicate. The filtrate from 25 

grams of powdered orthoclase with 40 cubic centimeters 0.1 normal 

sulphuric acid (2.50 millimols) gave the following results: 4

Orthoclase and sulphuric acid: content of 40 cm 3 .

Constituent.

SiOs.... .........................................................................

AlsOg...........................................................................

CaO... ..........................................................................

MgO............................................................................

KjO... ..........................................................................

NasO... .........................................................................

Total......................................................................

Grams.

0.0242

.0313

Undet.

.0010

.0556

.0108

.1229

.1536

Millimols.

aO.92

03

<iQ

.17

1.71

2.14

o This is the number of divalent millimols to which 0.0313 gram alumina is equivalent.

i Collins, A. L., in Posepny, Franz, The genesis of ore deposits, 1902, p. 621.

* Emmons, S. F., idem, p. 756.

8 Sullivan, E. C., The interaction between minerals and water solutions, with special reference to geologic 

phenomena: Bull. U. S. Geol. Survey No. 312,1907, pp. 37-64.

* Idem, p. 50.
CHEMISTEY OF ENRICHMENT. 55

The acid has dissolved somewhat less than its own equivalent of 

bases from the orthoclase. No change in the concentration of the 

sulphate radicle could be detected.

Albite (NaAlSi308) in contact over night with cupric sulphate 

solution gave the following results:

Experiment with albite and cupric sulphate.

Constituent.

Copper in 50 cm3;

Finally.....................................................................

Content of 38.34 cm':

AlA..-......--. ............................................................

CaO........ ................................................................

NajO.... ...................................................................

Total.....................................................................

Grams.

0.1266

.0343

.0923

.0046

.0020

.0107

a . 0516

.0689

.0898

Millimols.

1.99

.54

1 4.<\

.06

.19

.83

1.08

1.41

a Including a little MgO and KSO.

Twenty-five grams powdered orthoclase were placed in contact for 

two days with 50 cubic centimeters of silver sulphate solution. The 

silver was almost completely precipitated.

Experiment with orthoclase and silver sulphate.

Constituent.

Silver in 50 cm^:

Finally.....................................................................

Content of 30.27 cm':

CaO.........................................................................

MgO........................................................................

K,O.... ....................................................................

NaaO... ....;..........................................-.....................

Total.....................................................................

Grams.

0. 1066

.0039

.1027

.0007

.0001

.0004

.0170

.0073

.0255

.0421

Milliequivalents.

0.98

.04

.94

.02

.36

.23

.61

1.01

Experiments by Sullivan on shale from Morenci, Ariz., are described 

by Lindgren 1 as follows:

Fifty grams of shale finely ground in agate mortar were placed in 100 cubic centimeters of solution of cupric sulphate containing 0.0025 gram of copper per cubic 

centimeter. After standing for four days, with occasional shaking, the solution 

contained but 5 per cent of that amount of copper. A solution of the same strength 

had lost all its copper after four months' contact with shale. A concentrated solution 

with original content of 0.7645 gram copper per cubic centimeter had but 0.7058 gram

i Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: 'Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 192.
56 THE ENKICHMENT OP SULPHIDE OEES.

copper per cubic centimeter after a few days' contact, 100 cubic centimeters of solution 

thus giving up nearly 0.6 gram copper (0.75 g. cupric oxide, CuO) to 50 grams shale.

A certain amount of acid radicle is also taken from the solution by the shale. This 

loss in one experiment with the dilute copper sulphate mentioned was something 

over 10 per cent of the total S04 .

The shale in turn gives off a part of ita constituents to the solution, about 0.2 gram 

having been found in 100 cubic centimeters after treatment with the dilute coppersulphate solution. This is 0.4 per cent of the 50 grams shale taken. Of this dissolved 

matter, mostly present as sulphate, one-third to one-half was potassium oxide, most 

of the remainder being magnesium oxide and calcium oxide, with sodium oxide, 

manganese oxide, ferric oxide, and silica in smaller quantities.

The acidity of the solution was not increased by removal of copper, analysis showing 

that copper precipitated in excess of that equivalent to the acid radicle removed 

was replaced in solution by other bases as sulphates.

Reactions of this character are probably at some places important processes in the enrichment of ore bodies, for the essential conditions of the experiments commonly exist in nature. The neutralization of the acid solutions by the alkaline silicates permits also such 

reactions as are favored by a neutral environment.

In some silver and copper mines clay gouge and highly altered 

material carry valuable minerals. This altered material may have 

been produced by the attrition of rock originally metalliferous, but 

the experiments described above suggest processes by which it may 

have been enriched. The results of experiments showing the extent 

to which these reactions may go on when sulphides also are present 

are not available.

According to Stephen H. Emm ens, 1 hydrogen sulp^de and sulphur 

dioxide form with sulphur as intermediate steps in the oxidation of 

pyrite. These compounds, however, will oxidize where oxygen is 

in excess, or in the absence of water pressure they can readily escape; 

thus hydrogen sulphide would have but little effect in precipitating 

the metals in the oxidizing zone.

The statement of Emmens that hydrogen sulphide is formed in the 

oxidation of sulphides was made as early as 1892, but it was not supported by a record of experiments and it has not received the attention that it probably deserves. Buehler and Gottschalk showed 

that pure water does not generate hydrogen sulphide in the presence 

of oxygen with sulphides, but it is formed on contact with some sulphides and weak sulphuric acid.

The crystallized and the freshly precipitated sulphides of a metal 

may show considerable difference in solubility in water, as is indicated 

by the table below, which gives the results of a number of determinations by Weigel, 2 who used a sensitive apparatus for measuring 

these very low solubilities. Weigel showed also that the amount

i Emmens, S. H., The chemistry of gossan: Eng. and Min. Jour., vol. 54,1892, pp. 582-583. 

1 Weigel, Oskar, Die Loslichkeit von Schwermetallsulfiden in reinem Wasser: Zeitschr. phys. Chemie, 

TOL 68, 1907, pp. 293-300.
CHEMISTRY OF EKUICHMENT. 57

of lead sulphide dissolved in water increases for a short period and 

then decreases, owing probably to the precipitation from the solution 

of crystallized galena, or some dense form which is less soluble than 

the freshly precipitated sulphide.

Solubility of the sulphides of the heavy metals in distilled water at 18° C.

[Mols X10-" per liter.]

Crystallized. Freshly precipitated (probably amorphous). 

MnS. ......................... 54.5

Pyrrhotite. .................... 53. 6

Pyrite (Freiberg) .............. 48. 89

Pyrite (artificial). .............. 40. 84

Wurtzite (artificial) ............ 28. 82

Millerite (artificial)) ........... 16. 29

Greenockite (artificial) ......... 8. 99

Zinc blende (Santander) ........ 6. 55

Zinc blende (artificial) ......... 6. 63

Cu2S....... ................... 3.1

Galena (Freiberg) ............. 1. 21

Galena (artificial) .............. 1. 21

Galena (from precipitated PbS). 1. 18 

SnSa . ........................ 1.13

AgjS......... ................. .5522

SnS (crystallized) ............. .14

Schuermann,1 after extensive experimentation, established a series 

in which the sulphides of any one of the metals thereof will be precipitated at the expense of any sulphide lower in the series. He 

used 50 cubic centimeters, of about 0.1 formula weight, of solutions 

of the salts noted below with 100 cubic centimeters of water and 

introduced small amounts of metallic sulphides.

Amounts of various salts used in Schuermann' s experiments.

Grams per liter. 

CuS04 .5H20 ...............................:................ 24. 880

Pb(N03)2 ................................................... 33. 018

ZnS04.7H20 ................................................ 28. 642

NiS04 .7H20 ................................................ 28. 014

Co(N03) 2 .6H20. ............................................. 29. 014

FeS04.7H30. . ................................................ 27. 742

HgCl2 - ...... ~~ ............................................. 27. 054

CdS04 .|H20. . ............................................... 25. 541

Bi02.H2(N03) ......... ....................................... 30. 311

MnS04.5H20 ................................................ 24. 042

AgN03 . ..................................................... 16. 955

SnCl2.2H20 .................................................. 22. 406

PdCl2 ............. .................. :........................ 17.694

P.s203 .......................'................................ 9. 884

MnS..............

ZnS..............

FeS...............

CoS..............

NiS..............

CdS..............

Sb2S3 .............

PbS ..............

CuS..............

AgaS.... ..........

"Ri Q

HcS..............

............ 71.60

............ 70.60

............. 70.1

.............. 41.62

............ 39.87

............ 9. 00

............ 5. 2

............ 3. 6

............ 3. 51

.......... '.°. 2.1

............ .552

............ .35

............ .054

33.119

The sulphides were placed in 150 cubic centimeter flasks that were 

closed with a Bunsen valve to allow steam to escape and to prevent 

access of air. On the application of heat the metal in solution as

i Schuermann, Ernst, Ueber die Verwandtschaft der Schwermetalle zum Schwefel: Liebig's Ann. der 

Chemie, vol. 249,1888, p. 326.
58 THE EETKICHMEKT OF SULPHIDE OEES.

sulphate, as chloride, or as nitrate was precipitated as sulphide, and 

the metal of the introduced sulphide simultaneously went into solution. In general, the attack was continued for several hours. His 

series is as follows: Palladium, quicksilver, silver, copper, bismuth, 

cadmium, lead, zinc, nickel, cobalt, iron, thallium, and manganese. 

Under the conditions named any metal in solution will, according to 

Schuermann, be precipitated as sulphide if in contact with a metal 

lower in the series and the metal lower in the series will simultaneously go into solution. As a rule the reaction is the more nearly 

complete the farther apart the two metals are in the series.

In a series which he regards as showing the "relative affinity" of 

the heavy metals for sulphur Schuermann includes antimony, arsenic, 

and tin. This series is palladium, mercury, silver, copper, bismuth, 

cadmium, antimony, tin, lead, zinc, nickel, cobalt, iron, arsenic, thaiHum, and manganese. Those last named have the least affinity.

Schuermann's results have been widely applied. The term " chemical affinity" is a convenient one but is used in absence of more 

definite information to express a relation, not chemical constants. 

As pointed out by Wells, the principal results of Schuermann's 

experiments may be regarded as reactions establishing chemical 

equilibria, the salts being fixed in order of their solubilities under 

the conditions of the experiments. The positions of the commoner 

metals in the Schuermann series agree fairly well with the solubilities of the sulphides determined by Weigel, and the metals which 

replace those lower in the series generally have lower solubilities 

than those which are replaced. As shown' in the table that follows there are, however, several discrepancies between the Schuermann series and Weigel's table.

If ammonium sulphide is added to a solution in which the metals 

are dissolved (in equal molar concentration) those which have the 

lowest solubilities should be precipitated first. Under similar conditions and with equal molar concentrations the metals would be 

precipitated by ammonium sulphide approximately but not exactly in 

the order indicated in the Schuermann series. This order of precipitation would hold, however, for the sulphides of the most common 

metals and for those which are assumed to be most important in 

problems of sulphide enrichment.

In the table below Schuermann's series is compared with a series 

showing the solubilities of the metallic sulphides as determined by 

Weigel, the numbers in parentheses indicating the order of (molar) 

solubility. In Schuermann's series the metals are arranged in the 

order of their decrease of "affinity for sulphur." 1 In Weigel's 

series the order of solubility of the freshly precipitated metallic suli Schuermann, Ernst, Ueber die Verwandstchaft der Schwermetalle zum Schwefel: Liebig's Ann. der 

Chemie, vol. 249, 1888, p. 326. Emmons, S. F., The secondary enrichment of ore deposits: Trans. Am. 

Inst. Min. Eng., vol. 30,1901, p. 212. Weed, W. H., The enrichment of gold and silver veins: Trans. Am. 

Inst. Min. Eng., vol. 30, 1901, p. 428; The affinity of metals for sulphur: Eng. and Min. Jour., vol. 50, 

1890, p. 484; Review of lecture by Watson Smith, on Schuermann's reactions: Jour. Soc. Chem. Ind., 

vol. 11,1892, pp. 869-87L
CHEMISTKY OF ENRICHMENT. 59

phides is expressed in mols per liter; 1 to each amount stated should 

be applied the factor XlO~8 .

Comparison ofSchuermanris and Weigel's series. 

8chuennann's series. Weigel's series.

(1)Hg. (l)HgS-.............................. 0.054

(2)Ag. (3)Ag2S............................... .552

(3)Cu. (4) CuS............................... 3.51

(4)Bi. (2)Bi2S3 .............................. .35

(5)Cd. (6)CdS............................... 9.0

(6)Pb. (5)PbS................................ 3.6

(7)Zn. (10)ZnS............................... 70.6

(8)Ni. (7)NiS............................... 39.81

(9) Co. (8) CoS............................... 41. 62

(10)Fe. . (9) FeS................................ 70.1

(11)Mn. (ll)MnS............................... 71.6

Hydrogen sulphide is made in the chemical laboratory by treating 

artificial ferrous sulphide with acid. The generation of hydrogen 

sulphide is almost instantaneous. As recently shown by Alien, 

pyrrhotite is in reality a solid solution of ferrous sulphide (FeS) and 

sulphur. Both pyrrhotite and zinc blende dissolve in acid with 

noticeable evolution of hydrogen sulphide, whereas pyrite and chalcopyrite are almost insoluble in sulphuric or in hydrochloric acid. 

They are readily decomposed, however, in nitric, an oxidizing acid.

Several experiments were made by R. C. Wells to ascertain the 

rates at which hydrogen sulphide is generated by the action of cold 

dilute acid waters on sulphides of the metals. The ores, obtained 

from the Smithsonian Institution through the courtesy of Dr. J. E. 

Pogue, were pyrite from Leadville, Colo.; chalcopyrite from the Cactus 

mine, Utah; sphalerite from Webb City, Mo.; pyrrhotite from Ely, 

Vt.; and galena from the Motley mine, Carterville, Mo. All these 

specimens appeared to be pure. They were powdered, washed with 

hydrochloric acid, alcohol, and ether, and sifted, the material passing 

a 200-mesh sieve and that held on an 80-mesh sieve being rejected. 

Thus the material used was free from fine dust and lumps.

In preliminary experiments the minerals were used in quantities 

proportional to their densities, thus insuring approximately equal 

surfaces. Portions of the minerals placed in flasks and covered with 

0.1 normal sulphuric acid showed no visible change until a week had 

elapsed, when flasks containing pyrrhotite began to show a precipitate of basic ferric sulphate. No regularity was found in the variation 

in the acidity of the solutions on account of the simultaneous consumption and production of acid with the different minerals.

The five minerals were further exposed overnight to 0.057 normal sulphuric acid. The resulting solutions were titrated with iodine solution 

to ascertain the amount of hydrogen sulphide generated, the iodine solution used being, for pyrrhotite, 28.5 cubic centimeters; for sphalerite, 

1.05 cubic centimeters; for galena, 0.40 cubic centimeter; for chalcopyrite, 0.29 cubic centimeter; for pyrite, 0.28 cubic centimeter.

1 Weigel, Oskar, Zeitschr. phys. chemie, vol. 58, p. 293.
60 THE ENBICHMENT OF SULPHIDE ORES.

A set of experiments exactly similar to the above was made, the 

resulting solutions being titrated with potassium permanganate. The 

consumption of the salt with the solutions from pyrrhotite greatly 

exceeded the consumption with those from any other mineral.

In another series of experiments the hydrogen sulphide generated 

by the action of cold dilute acid on pyrrhotite, on sphalerite, and on 

galena was determined qualitatively by lead acetate paper. There 

was no proof that hydrogen sulphide was formed with either pyrite 

or chalcopyrite, and the very small quantities indicated above (0.28 

cubic centimeter for pyrite and 0.29 cubic centimeter for chalcopyrite) 

are not regarded by Mr. Wells as proof that hydrogen sulphide is 

formed by the action of acid on pyrite and chalcopyrite. The slight 

reduction of the iodine solution may represent a correction for the 

end point of the titration.

COMPOSITION OP MINE WATERS. 

ANALYSES.

The following tables show the results of analyses of 37 samples of 

water taken from mines containing deposits of sulphide ore:

Analyses of mine waters.

Waters of copper mines.

[Parts per mfllion.]

S0«.. .......

Cl. ........

COg.......

T>/-k

B407.......

Br... ......

F..........

SiO».......

K. ........

Na. .......

Li.........

Ca... ...... UP

A 1 Mn.. ......

Ni

Co........

Cu... ......

Zn... ......

Cd... ......

Fe".......

Fe'".......

Acidity: 

HjSO4 ...

1

406.5

6.8

13.2

Tr.

23.2

7.1

16.2

Tr.

151.2

28.2

0.0

.5

Tr.

.3

1.8

Alt2

544.7

118.5

Tr.

Tr.

27.2

30.5

176.9

92.7

17.9

Tr.

.4

1.8

i.4

Alk.

3

90.6

11.8

Tr.

36.1

3.5

11.1

49.0'

10.3

2.5

1.9

.9

Alk.

4

593.4

842.8

Tr.

29.5

11.4

82.8

512.1

102.6

1.4

0.0

.4

A IV

5

71,053.3

17.7

1.5

67.4

6.8

41.7

Tr.

307.7

149.2

85.2

13.2

3.5

4.6

45,633.2

411.2

| 49.8

6

2, 672. 0

13.0

Tr.

Tr.

Tf.

Tr.

47.7

13.1

39.6

132.5

61.6

83.5

12.0 I - 5

59.1

852.0

41.1

159.8

7

492.1

95.7

94.8

27.9

33.0

105.8

25.5

I" --

/ 58.1

1 1 Q

8

6, 664. 0

.1

55.6

19.8

23.4

67.6

40.6

433.0

.2

312.1

199.8

2, 178. 0

0.0

129.6

9

2,068.0

2.2

78.9

7.8

5 0

93Q fi

no o

165.0

.3

40.8

54.3

1.3

186.3

406.5

1. Green Mountain mine, Butte, Mont., 2,200-foot level in fissure in "granite," remote from known 

veins. Crosscut opened day before sample was taken. W. F. Hillebrand. analyst. Weed, W. H., Geology 

and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey No. 74.1912, p. 101.

2. Glengarry mine, Butte. Mont., 220-foot crosscut south, Very slightly alkaline. Black deposit containing FeS, much free sulphur, and probably ZnS. W. F. Hillebranq, analyst. Weed, idem. . 3. Anaconda mine, Butte, Mont., 800 feet west. Very faintly alkaline; deposits iron and manganese. W. F. Hillebrand, analyst. Weed, idem.

4. Gagnon mine, Butte, Mont.. 1,125 feet from shaft. Very faintly alkaline; deposits iron. Weed, idem.

5. Mountain View mine. Butte, Mont., second level. W. F. Hillebrand, analyst. Clarke, F. W., The 

date of geochemistry: Bull. U. S. Geol. Survey No. 330,1908, p. 546; also Weed, loc. cit.

6. St. Lawrence mine, Butte, Mont. W. F. Hillebrand, analyst. Clarke, loc. cit.; also Weed, loc. cit.

7. Ruth mine, Robinson mining district. Nevada, bottom of inclined shaft. H. E. 1 Miller, analyst. 

Lawson. A. C., The copper deposits of the Robinson mining district, Nevada: Bull. Dept. Geology Univ. 

California, vol. 4,1906, p. 332.

8. Burra Burra mine, Ducktown, Tenn., first level below black copper workings. R. C. Wells, analyst. 

Emmons. W. H., and Laney, F. B., Preliminary report on the mineral deposits of Ducktown, Tenn.: 

Bull. U. S. Geol. Survey No. 470,1911, pp. 171-172.

9. East Tennessee mine, Ducktown, Tenn., 30-foot fathom level. R. C. Wells, analyst. Emmons 

and Laney, idem.
CHEMISTEY OF ENRICHMENT. 61

Analyses of mine waters Continued. 

Waters of copper mines Continued.

SC-4-. ......

Cl. ........

COa. ......

PC-4.......

B407 .......

Br.. .......

 p

SiOj. ......

K. ........

Na. .......

Li.........

Ca... ..:...

Mg.. ......

Al

Mn........

NL........

Co........

Pn

Zn... ......

Cd... ......

Fe". ...... Fe'". ......

Acidity: 

H2S04 ...

10

444.0

.7

20.6 

3.2 

3.1

18.1 

12.2 

40.1 

.3

12.8 

6.1

Tr. 

29.9

108.2

11

416.4 

.6

19.1 

2.8 

3.0

18.4 

11.5 

46.5 

.9

12.0 

4.2

Tr. 

31.3

115.1

12

415.8 

.7

37.0 

2.7 

5.2

19.7 

5.2 

14.5 

.2

28. i 

2.4

71.4 

20.3

210.2

13

476.8 

.4

49.9 

2.2 

5.5

30.4 

6.2 

19.1 

.1

ii.6

2.9

89.2 

65.9

97.5

14

5,064 

Undet.

76 

\ Undet.

436 

61 

Undet. 

236

 1,659 

Undet.

305

970

15

1,419 

18

28 

ol70

319 

36 

16 

37

73

76

Nil.

16

4,457 

22

56 

ol98

753 

86 

22 

153

60 

252

524

Nil.

17

3,898 

Undet.

40 

o97

239 

82 

44 

150

122 

190

799

Nil.

18

1,335

20 

o!4

277 

54 

9 

45

28 

52

149 

24

Nfl.

a Calculated as KsO+Na2 0.

10. No. 20 shaft (top), Ducktown, Tenn. Emmons and Laney, unpublished manuscript.

11. No. 20 shaft (bottom), Ducktown, Tenn. Emmons and Laney, idem.

12. Callaway shaft, Ducktown, Tenn., at water level. R. C. Wells, analyst. Emmons and Laney, 

Preliminary report on the mineral deposits of Ducktown, Tenn.: Bull. U. S. Geol. Survey No. 470,1911, 

pp. 171-172.

13. Callaway shaft, Ducktown, Tenn., 37 feet below water level. R. C. Wells, analyst. Emmons and 

Laney, idem.

14. Capote mine, Cananea, Mexico, 300-foot level. G. W. Hawley (chief chemist Cananea Consolidated 

Copper Company), analyst. Analyses 14 to 17 obtained through the courtesy of Mr. D. P. Hynes.

15. Capote mine, Cananea, Mexico, 400-foot level. G. W. Hawley, analyst.

16. Capote mine, Cananea, Mexico, 900-foot level. G. W. Hawley, analyst.

17. Capote mine, Cananea, Mexico, water pumped from mine. G. W. Hawley, analyst.

18. Capote, Puertacitas, Cananea-Duluth, and Democratic mines, Cananea, Mexico, water used at 

concentrator at Cananea. Circulates at concentrator in contact with ore and air. G. W. Hawley, analyst.
62 THE ENRICHMENT OF SULPHIDE ORES.

Analyses of mine waters Continued. 

Waters of gold, silver, and gold and silver mines.

S04 . ..............

Cl... ..............

C03 ... ............

N03 ...............

P04... ............

S. .................

SiOs.. ............

K.. ...............

Na................

Li.................

Ca. ...............

Sr.... .............

Mg... .............

AT.. .............. 

Mn... .............

Ni. ...............

Co................

Cu. ...............

Zn. ...............

Pb.. ..............

Fe".. .............

Fe'".. ..........

H (bicarbonates). .

Hfacids)..........

Fe8 03 . ..........

AljOa.. .........

COa.............

19

43.20

7.90

110. 50

Tr.

25.90

10.60

36.40

Tr.

37.40

12.25

.40 

.80

Tr.

.20

Tr.

} .70

37.20

20

161. 70

186. 40

1,513.44

1.60

Tr.

24.42

198.00

719. 45

2.85

146. 41

1.95

177. 67

1.06 

.57

.02

.34

1 35

3.50

1,418.01

21

380.38

1.27

115.03

30.50

8.39

57.13

148. 10

154. 03

j.......

22

474. 00

19.00

20.45

133. 40

53.40

132.00

100. 10

5.88

1.37

6.33

23

209,100.00

127. CO

616.00

535.00

1,286.00

6,590.00

9,670.00 

885.10

147.50

5,025.00

2,575.00

24

173. 40

.52

47.71

62.26

.27

72.37

Tr.

Tr.

25

272.30

13.77

241.39

59. 96

80.90

6.86

113. 70

Tr.

Tr.

26

ipn nn

16. 82

194 70

RS 09

254.10

7.84

84.08

Tr.

Tr.

27

7.70

3.16

141.80

1.10

32.70

1.00

13.40

33.60

5.70

""."27"

Tr.

4.60

Alk.

\ 4.20

28

7.80

3.10

146.60

41.40

1.60

13.70

44.35

3.35 ""i.'go

Tr.?

4.70

Alk.

1.80

19. Geyser mine, Ouster County, Colo., 500-foot level. W. F. Hillebrand, analyst. Emmons, S. F., 

The mines of Custer County, Colo.: Seventeenth Ann. Kept. U. S. Geol. Survey, pt. 2, 1896, p. 461.

20. Geyser mine, Custer County, Colo., 2,000-foot level. W. F. Hillebrand, analyst. Emmons, S. F., 

op. cit., p. 462.

21. Savage mine (Comstock lode), Storey County. Nev., 600-foot level.

22. C. & C. shaft (Comstock lode), Storey County, Nev., 2.250-foot level. N. E. Wilson, analyst. Reid, 

J. A., The structure and genesis of the Comstock lode: Bull. Dept. Geology Univ. California, vol. 4,1906, 

p. 189.

23. Central tunnel (Comstock lode), Storey County, Nev., vadose water. N. E. Wilson, analyst. Reid, 

J. A., loo. cit., p. 192.

24. Gould & Curry mine (Comstock lode), Storey County, Nev., 1,700-fopt level.

25. Hale & Norcross tunnel section (Comstock lode), Storey County, Nev.

26. Gould & Curry mine (Comstock lode), Storey County, Nev., 1,800-foot level. .

27. Federal Loan mine, Nevada City, Gal., 400-foot level. W. F. Hillebrand, analyst. Lindgren, Waldemar. The gold-quartz veins of Nevada City and Grass Valley, California: Seventeenth Ann. Rept. U. S. 

Geol. Survey, pt. 2,1896, p. 121. 28. Black Prince mine, Nevada City, Gal., 400-foot level. W. F. Hillebrand, analyst. Lindgren, idem.
CHEMISTRY OF ENRICHMENT. 63

Analyses of mine waters Continued. 

Waters of gold, silver, and gold and silver mines Continued.

S04 .......

Cl. ........

C03 . ......

N03 .......

PO,. ......

s.... ......

Si02 .......

K. ........

Na. .......

Li.........

Ca... ......

Sr. ........

Mg. .......

Al.........

Mn. .......

Ni.. .......

Co.........

Cu.........

Zn.........

Pb. .......

Fe". ......

Fe'".......

H (bicarH (acids)..

AUOs-...

C02 .....

29

258.40

Tr.

2.10

90.76

13.08

1.49

4.74

Tr.

Tr.

2.82

4.74

30

26.55

Tr.

8.00

72.48

id on

.37

4.12

Tr.

Tr.

47.40

6.30

31

280.10

Tr.

8.80

106.20

17.14

1 49

1.88

Tr.

Tr.

18.88

4.69

32

124 80

12.40

18.00

46.40

14.50

8.90

6.60

33

cn 19

Tr.

78.3

32.25

2.41

28.52

17.36

1 30

9";

1.47

Ait61.53

34

104.4

1.6

09 A.

23.2

3.5

7.5

46.2

7 3

3.2

.85

Alk

72.60

35

9 fV?Q 51

8 in

43.80

70

106.27

187. 15

93.50

155. 58

77.05

49.66

3.44

164.82

36

327.2

35.6

87.9

Tr.

64.8

3.4

148.8

68.8

6.3

.7

Tr.

.7

37

67.2

15

130

55.8

5.0

30.1

39.1

6.7

.8

.4

29-32. Rothscho'nberger Stolln, Freiberg, Germany. A. Frenzel, analyst. Beck, Richard, The nature 

of ore deposits (translated by W. H. Weed), vol. 2,1905, p. 377.

33. Bachelor mine, Creede, Colo. Iron includes some aluminum. Water is alkaline. Chase Palmer, 

analyst.

34. Solomon mine, Creede, Colo. Iron includes some aluminum. Water is alkaline. Chase Palmer, 

analyst.

35. Stanley mine, Idaho Springs, Colo. L. J. W. Jones, analyst. Jones, L. J. W., Ferric sulphate in 

mine waters and its action on metals: Proc. Colorado Sci. Soc., vol. 6.1897-1900, p. 48.

36. Mizpah mine, Tonopah, Nov., from bore hole 2,316 feet deep. Bicarbonates were reduced to normal 

carbonates. R. C. Wells, analyst.

37. Nettie mine, Butte, Mont., 300-foot level. W. F. Hillebrand, analyst. Weed, W. H., Geology 

and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey No. 74, 1912, p. 101.

DISCUSSION OF ANALYSES. . 

MINES REPRESENTED.

Of the foregoing analyses of 37 samples of water taken from mines 

containing sulphide deposits, samples 1 to 18 were taken from copper 

mines and samples 19 to 37 from gold, silver, and gold-silver mines. 

The tables include almost all the available nearly complete analyses 

of waters from sulphide deposits in noncalcareous rocks. Among 

those not included in the table are several analyses of waters from 

the district in southeastern Missouri containing disseminated lead 

deposits, discussed by Buckley, 1 and some analyses of waters from 

the Granby district and from Joplin, in southwestern Missouri. A 

number of partial analyses of waters from gold mines of Australia 

and New Zealand are reported by Don,2 and several analyses of

1 Buckley, E. R., Geology of the disseminated lead deposits of St. Francois and Washington counties: 

Missouri Bur. Geology and Mines, vol. 9, pt. 1,1908, pp. 99,171, 249.

2 Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27 1898 p. 654.
64 THE ENRICHMENT OF SULPHIDE ORES.

waters of the Homestake mine, Lead, S. Dak., have recently been 

published by Sharwood.1 Analyses of waters from the Lake Superior 

copper mines, reported by Lane, 2 have not been included in the 

tables, for these are chloride waters and their discussion would be 

irrelevant to the subject of sulphide enrichment.

Five mines are represented by two or more analyses. More 

extended series of analyses of waters taken at different depths from 

the same body of water would be very useful in investigations made 

to determine the acidity and the state of oxidation of waters as related 

to depth. The first work of this character was done by Lepsius,3 

who showed that the oxygen content of waters taken from bore 

holes decreases with increase of depth. A series of samples (Nos. 14 

to 17) from the different levels of the Capote mine, Cananea, Sonora, 

analyzed under the direction of G. W. Hawley, chief chemist of 

the Cananea Consolidated Copper Co., is especially valuable. Samples 12 and 13 were obtained from the Callaway shaft at Ducktown, 

Term.; sample 12 was taken from the top of the water level in this 

shaft, and sample 13 was taken 37 feet below. To obtain this sample 

a special device was arranged by Mr. Laney and myself for subaqueous filtration. This device consisted of a crate holding two 

1-gallon bottles, below which was attached a heavy weight to sink 

the unfilled bottles; below this was attached, with a short rope, a 

second weight for sounding. Each bottle was equipped with a small 

cylindrical filter tube charged with asbestos wool. A perforated 

porcelain disk was placed below and another above the wool to 

equalize pressure. The filter tube extended nearly to the bottom of 

the bottle to prevent air from coming into contact with the water dropping into the bottles: Each stopper was fitted with a glass exhaust 

tube extending0 from the top of the bottle to a point above the filter, 

and the upper end of each tube was fitted with a valve which let out 

air but prevented any intake of water. Rubber sheeting was fastened 

over the filter to prevent premature entry of waters and this was 

removed by means of a strong cord passed to the surface. The 

bottles were lowered slowly to prevent the stirring of the water in the 

shaft.

Many of the samples were taken from small bodies of water standing 

in mines. The water in such bodies is generally more concentrated 

than the average water of the mines and is probably not typical 

underground mine water. Samples 8, 9, 10, 11, 12, 13, 22, 27, 28,

1 Sharwood, W. J., Analyses of some rocks and minerals from the Homestake mine, Lead, B. Dak.: Econ. 

Geology, vol. 6,1911, p. 742.

8 Lane, A. C., Mine waters: Proc. Lake Superior Min. Inst., vol. 13, 1908, pp. 63-152; Michigan iron 

mines and their mine waters: Jour. Canadian Min. Inst, vol. 12,1910, p. 124.

' Lepsius, B., Ueber die Abnahme de gelosten Sauerstofls in Grundwasser und einen einfachen Apparat 

zur Entnahme von Tiefproben in Bohrlochern: Ber. Deutsch. chem. Oesell., vol. 18, pt. 2, 1885, pp. 2487- 

2490.
CHEMISTEY OF ENRICHMENT. 65

33, 34, 36, and probably several others were taken from underground 

streams or rills or from large bodies of standing water.

The composition of mine water, especially that in the upper part 

of a deposit, is changed by the opening of the mine, by which, doubtless, the water is more highly oxidized and probably its acidity is 

increased. The addition of artificial water channels and the depression of the ground-water level by pumping produces mixtures of 

water from several leyels and increases the rate of the underground 

circulation. This increase tends to dilute the waters; but, on the 

other hand, the downward change to a more highly oxidized and 

therefore a more highly acid condition will render the solutions more 

active solvents and tend to increase the concentration of metals they 

contain. Which set of processes predominates can not be shown, 

but the analyses represent only qualitatively the character of the 

solutions by which alterations of sulphide ores are brought about. 

Many of the analyses are not complete, and some analysts do not 

state whether certain elements were looked for. Some of the averages are therefore only rude approximations.

In the work of recalculating these analyses to the ionic form 

of statement I have had assistance from Messrs. H. K. Shearer, 

N. Sankowsky, and Clarence Russell.

CHEMICAL CHANGES INDICATED.

Sulphates and sulphuric acid. Sulphides exposed to air and water 

are changed to sulphates and to sulphuric acid. The iron minerals 

are the more important sources of sulphuric acid because some of 

them contain more sulphur than that necessary to balance iron when 

iron sulphate forms and because iron sulphate in the presence of 

oxygen forms the ferric salt, which hydrolyzes readily, giving basic 

ferric sulphate and ultimately limonite. Galena and zinc blende 

may oxidize to sulphates without liberating acid. The following 

equations, which represent certain stages in the reactions, illustrate 

this principle:

FeS2 + H2 0 + 70 = FeS04 + H2SO4 . 

Fe7S8 + H2O + 31O = 7FeS04 + H2SO,. 

CuFeS3 + 80 = FeS04 + CuSO,. 

= PbSO4 . 

= ZnS04 .

The oxidation of ferrous sulphate to ferric salt and the hydrolyzation of ferric sulphate take place very readily in the presence of 

oxygen.

2FeS04 + H2S04 + O = Fe2 (SOJ, + H20. 

6FeS04 + 3O + 3H2O = 2Fe2 (SO4 ) 3 + Fe2 (OH) t 

Fe2 (S04) 3 + 6H2 0 = 2Fe (OH) K+ 3H2SO.: 

72293° BulL 529 13 5
66 THE ENRICHMENT OF SULPHIDE OEES.

All the waters from copper mines (1-18) are sulphate solutions. 

Carbonates are determined in only one of these No. 1, which was 

taken at some distance from any known ore body. The combined 

acid radicles of several are, however, insufficient to balance the positive ions a fact that suggests the probability that some of the metals 

are balanced by carbon dioxide or by ions containing carbon dioxide. 

The average sulphate (S04) of 18 samples of water from copper 

mines is 5,695.3 parts per million. This figure is increased greatly 

by sample 5, an exceptionally strong sulphate solution. The average 

sulphate of the 17 other samples is 1,850.4 parts per million. The 

average sulphate of 19 samples from gold and silver mines is 11,266.8 

parts per million; if the sample of concentrated water (No. 23) from 

the Oomstock lode is eliminated the average of 18 samples is 276.04. 

In seven of the waters from copper mines the acidity has been 

determined, the average being 291 parts per million, calculated as 

sulphuric acid (H2S04). Samples 8 and 9 show appreciable acidity, 

notwithstanding a deficiency of sulphate ions to balance the metals. 

In at least 12 samples there <> is no free acid. At least eight samples 

from copper and precious-metal mines (1, 2, 3, 4, 27, 28, 33, 34) are 

alkaline.

Chlorides. The salt (NaCl) in sedimentary rocks may be dissolved 

by ground water, and in some places it is a source of commercial 

supply. From the available analyses it appears that in many 

regions the amount of salt in such rocks is small. The chlorine 

content of composite samples of 78 shales and 253 sandstones is 

only a trace, and an analysis of a composite of 345 limestones shows 

only 0.02 per cent. 1 A few rock-making minerals, such as chlorapatite, scapolite, hauyne, and nosean, contain combined chlorine; 

but all of these except apatite occur mainly in rocks of very rare types. 

In some rocks chlorine is present probably as sodium chloride in the 

solid particles contained in fluid inclusions.

Apatite, though widespread in igneous rocks, is a very stable 

mineral and consequently can not be regarded as an important source 

of chlorine, although it may afford small amounts when exposed to 

favorable conditions of weathering. The average chlorine content 

of igneous rocks, according to F. W. Clarke, is 0.06 per cent.

Chlorine is present in nearly all natural waters, which derive it 

from fine salt or salt water from the sea or other bodies of salt water. 

The fine salt is carried by the wind and precipitated with rain.2 The 

amount of chlorine in bodies of natural water varies with remarkable 

constancy with variations in the distance of the bodies from the shore. 

Several determinations of water from bodies very near the seashore

iClarke, F. W., The data of geochemistry, 2d ed.: Bull. U. S. Oeol. Survey No. 491,1911, p. 28. 

* Jackson, D. D., The normal distribution of chlorine in the natural waters .of New York and New England: Water-Supply Paper U. S. Geol. Survey No. 144, 1905.
CHEMISTRY OF ENRICHMENT. 67

show 10 to 30 parts of chlorine per million; in bodies a few miles 

away it is generally about 6 parts per million; in bodies 50 miles 

from shore it is generally less than 1 part per million.

In arid countries dust containing salt doubtless contributes 

chlorine to mine waters. Penrose, 1 discussing the distribution of 

the chloride ores, pointed out long ago that these minerals form most 

abundantly in undrained areas.

The average chlorine content of the 18 analyses of waters from 

copper mines is 63.9 parts per million. In three of these chlorine is 

not reported, but in two of the three it was not looked for. The 

average of the 15 samples is 76.7. The samples from gold and silver 

mines carried less chlorine than those of copper mines, the average of 

19 being 23.8. Exclusive of four samples showing traces, the average 

is 30.2. The chlorine content of waters of well-drained areas like 

those of Ducktown, Term., and Creede, Colo., is conspicuously low.

The amount of chlorine in deposits in or near arid regions or near the 

sea is greater. Two samples from Butte, Mont., show a large content 

of chlorine. In general the chlorine content in the samples analyzed 

is much lower than in samples of water obtained by Don 2 from a 

large number of Australasian mines and considerably lower than a 

former estimate (873 parts per million) given by me in a previous 

paper 3 on the concentration of gold by cold solutions.

Carbonates. Since sulphuric acid tends to drive carbon dioxide out 

of solution, it would not be supposed that highly carbonated waters 

would be common in mines where the ore carried much pyrite. 

That small amounts of acid sulphate and carbonates may exist in the 

same solution is shown, however, by several analyses. In the acid 

waters under consideration the carbonates of the bases would necessarily be present as bicarbonates, although this fact is not indicated 

in all the analyses.

Carbonates were determined hi only one sample of water from 

copper mines but are probably present in others. Carbonates are 

shown in 13 waters from gold and silver mines, the average of the 

13 samples being 224.6 parts per million. The average of the 19 

samples from precious-metal mines is 153.7 parts. Two waters 

from the Geyser mine, Custer County, Colo., and two from Creede, 

Colo., contain, in addition to the carbonate radicle (C03 ), considerable 

excess carbonate calculated as carbon dioxide (C02 ), but this is. not 

included in the average. Several analyses of waters from mines of 

Butte, Mont., show insufficient carbon dioxide in the bicarbonate

1 Penrose, R. A. F., jr., The superficial alteration of ore deposits: Jour. Geology, vol. 2,1894, pp. 314-316

2 Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27,1898, p. 654.

»Emmons, W. H., The agency of manganese in the superficial alteration and secondary enrichment of 

gold deposits in the United States: Bull: Am. Inst. Min. Eng., 1910, p. 774; Jour. Geology, vol. 19, 1911, 

p. 20. To obtain this estimate many partial analyses were included. These are not given in the table 

on pages 62-63.
68 THE ENBICHMBNT OF SULPHIDE OKES.

state to satisfy the bases, hence both normal and bicarbonates were 

assumed as present and the summation was raised to correspond.1

Silica. The analyses show that appreciable silica is dissolved even 

hi strongly acid mine waters. Very little is known concerning the 

conditions that favor the solution of silica hi such waters. Solutions 

that contain much alkali and carbonate generally carry larger 

quantities of silica, although the one that carries the most silica is a 

concentrated sulphate solution from the Comstock lode. The 

average silica content of 18 waters from copper mines is 44.7; that 

of waters from 19 precious-metal mines is 69.4.

Arsenic and antimony. Arsenic and antimony compounds are 

dissolved in some mine waters, for there is evidence that minerals 

containing these metals are deposited from cold solutions. Only 

traces of arsenic and antimony are reported in the analyses, but 

Dr. Hillebrand notes that considerable arsenic was present in a 

precipitate from a filtered sample of water collected by Waldemar 

Lindgren at Grass Valley, Cal., where the gold ores carry arsenical 

minerals. Traces of arsenic are reported in three waters from Butte, 

Mont.

Nitrates. Nitrates are not abundant in mine waters. In only one 

analysis (20) is nitrate (N03 ) reported (1.60 parts per million), and 

this hi a water that was questionably representative, for it appears 

to have been imperfectly filtered.2

Phosphates. Traces of phosphate are reported in eight mine waters. 

One sample contained 1.5 parts per million; others contained none, 

if determinations for phosphate were made.

Potassium. Potassium has been found in all the samples of waters 

from copper mines where it was looked for. The average of 13 

samples is 10.7. In the samples from gold and silver mines it varied 

from 1 part to 254 parts per million, the average of 14 samples 

being 53.9. If it is assumed to have been absent in the five samples 

hi which it is not reported, the average content of 19 samples is 39.7.

Sodium. Owing to its greater solubility in sulphate solutions, 

sodium is generally more abundant than potash in these mine waters. 

The 18 samples of waters from copper mines contained an average 

of at least 24.8 parts per million, and 13 samples in which potash 

was determined averaged 34.4. The average sodium content of 

14 samples of water from precious-metal mines is 131.6 parts per 

million.

Calcium. The average calcium content of 18 samples of waters 

from copper mines is 209.3 parts per million. The average of 19

i Hillebrand, W. F., Composition of mine waters, in Weed, W. H., Geology and ore deposits of the Butte 

district, Montana: Prof. Paper U. S. Geol. Survey No. 74,1912, p. 101.

* Emmons, S. F., The mines of Custer County, Colo.: Seventeenth Ann. Kept. U. S. Geol. Survey, 

pt. 2,1896, p. 462; analysis of water from Geyser mine, Silver Cliff, Colo. See also statement of W. F. Hillebrand, analyst, on page 460 of same report.
CHEMISTRY OF ENBiCHkElirt. 69

Samples of waters from precious-metal mines is 144.2 parts. Calcium was determined in every sample.

Magnesium. In 18 samples of waters from copper mines magnesium averages 47.4 parts per million. In 19 samples of waters 

from precious-metal mines it averages 390 parts. In three samples 

of such waters it is not reported. In 16 waters of precious-metal 

mines it averages 445.2 parts per million. The remarkable concentration of magnesium in sample 23 (an exceptional water) brings the 

average far above that of calcium, but notwithstanding the greater 

solubility of its sulphate, magnesium exceeds calcium in only three 

of the samples.

Aluminum. Like the alkalies and alkaline earths, aluminum is 

readily dissolved from the silicates of the ore or from the wall rock. 

In general it is more abundant in sulphate waters than in those that 

carry considerable carbonate.

The average aluminum in 12 samples from copper mines is 81.5 

parts per million. Singularly enough it is absent in five waters 

from mines at Butte, Mont., which are in "granite." It is reported 

from only eight samples of water from precious-metal mines, the 

average being 1,209.9. If analysis 23, which shows exceptionally 

high alumina, is excluded, the average of the 18 other samples is 

only 1.3 parts per million. , Manganese. In 16 samples of water from copper mines the average content of manganese is 40.8 parts per million. In one analysis 

it is not reported; in another a trace is shown. It is especially 

abundant in the waters from Cananea, Sonora, Mexico.

The average of 13 samples from precious-metal mines is 81.5 parts 

per million. It is not reported in six. The variation in manganese 

content is exceedingly great. Only two waters, one from the Cornstock lode and another from the Stanley mine, Georgetown, Colo., 

contain more than a few parts per million.

Iron. All the 18 samples of water of copper mines contain iron. 

In two of them it is stated as total iron; in 14 samples ferrous iron 

is present, the average of these being 303.9 parts per million; ferric 

iron is present in 7 samples. Ferrous iron was probably determined 

in 15 samples, the average of the 15 being 23.3 parts per million indicating that in such waters, even in the zone where air is admitted, ferrous iron predominates in many mines. None of the analyses of samples taken underground at Cananea (Nos. 14-18) show ferric iron, yet 

the sample taken from the concentrates, where it circulates over the 

tables in contact with ore and air, contains appreciable ferric iron.

In two samples of water from precious-metal mines iron is stated 

as total iron. Ferrous iron is reported from 5 of the 17 samples 

remaining, the average being 34.8 parts per million; or if it was 

determined in all 17 samples/ the average is 10.3 parts per million.
70 THE ENRICHMENT OP SULPHIDE OEES.

It is highest in the water from the Stanley mine. Appreciable ferric 

iron is reported in six samples, its average being 841 parts per 

million. Excluding sample 23, from the Comstock lode, the average 

of the other five samples is only about 5 parts per million. This is 

lower than the ferric iron in waters from copper mines and very 

much lower than the ferric iron determined by Don in many waters 

from Australasian mines.

Copper. Copper, dissolved as sulphate, is abundant in many mine 

waters. In some mines at Butte, Mont., Ducktown, Tenn., Jerome, 

Ariz., and Rio Tinto, Spain, and in the Ballygahan mine, Wicklow, 

Ireland, the waters during certain periods of development have 

been sufficiently cupriferous to yield marketable quantities of copper. 

The average of 13 samples from copper mines is 3,690 parts per 

million. In five samples it is absent or is not reported. The water 

from the Mountain View mine at Butte, Mont., is by far the most 

highly cupriferous. Exclusive of this the average of 12 samples is 

only 195.5 parts per million. Copper is reported also in a concentrated water from the Comstock lode (No. 23) and in water from the 

Stanley mine at Georgetown, Colo. (No. 35), and traces are present 

in waters from other precious-metal mines (19 and 20).

Zinc. The zinc content of the waters of 15 copper mines averages 

140.3 parts per million. The average of 19 samples of preciousmetal mines is 6.7 parts per million. In the seven of these from 

which it is reported the average is 18.3 parts per million. Some 

waters from mineral deposits of the Mississippi Valley contain abundant zinc.

Gold and silver. Small amounts of gold and silver have been 

detected in waters from the Comstock lode,1 and gold has been 

determined in a water from the Homestake mine, in South Dakota.2 

Traces of both gold and silver are present in waters from the Granite 

vein Montana.

CHANGES IN COMPOSITION DEPENDING ON DEPTH.

Waters descending from the surface through sulphide ore deposits 

pass through a changing chemical environment and are continually 

changing in chemical composition. In the oxidized zone, as already 

stated, they are oxidized waters and acid waters; when they pass 

below the region where oxygen is in excess their acidity and their 

state of oxidation change. Their reaction on some sulphides produces hydrogen sulphide, and their reaction on silicates and carbonates 

decreases acidity. Iron is generally abundant in waters of pyritic 

ore bodies, and the degree of the oxidation of the iron is important 

as an index to the state of oxidation of the water. Mineral waters

1 Reid, J. A., The structure and genesis of the Comstock lode: Bull. Dept. Geology Univ. California, 

vol. 4, 1906, pp. 189, 192.

* Sharwood, W. J., Analyses of some rocks and minerals from the Homestake mine, Lead, S. Dak.: Econ. 

Geology, vol. 6,1911, p. 742.
CHEMISTRY OF ENRICHMENT. 71

may thus be divided into several groups, depending on the degree 

of their oxidation and their acidity. Nos. 9, 10, and 11, from Ducktown, Tenn., are samples of waters containing acid and ferric sulphate. 

Samples of acid and ferrous sulphate water are represented by No. 7, 

from the Ruth mine, Ely, Nev.; Nos. 8, 12, and 13, from Ducktown, 

Tenn.; No. 35, from the Stanley mine, Georgetown, Colo.; and No. 32, 

from the Rothschonberger Stolln, Freiberg, Germany.

Of the 37 samples of water analyzed 13 or more carry no free acid, 

and 9 of these are alkaline. The alkaline waters include two (33, 34) 

from the deep levels of mines at Creede, Colo., where they descended 

to depths of 1,300 feet or more before they issued at the drain level. 

As would be supposed, ferric iron is not present in any of the nine 

analyses of alkaline waters.

Samples 27 and 28 are waters collected by Waldemar Lindgren 

from the 400-foot levels in the Federal Loan and Black Prince mines 

at Nevada City, Cal. As the temperature of these waters when collected was the same as that of the surrounding rock, they come, 

presumably, from the meteoric circulation. Both are alkaline and 

both form yellow-brown deposits consisting principally of hydrated 

iron oxide, alumina, and silica. These waters were clear when bottled, 

but when opened after several months both contained dark-colored 

deposits of silica, hydroxides, and sulphides. Dr. W. F. Hillebrand, 

who made the analyses, detected no hydrogen sulphide in either 

water, although both appeared to contain a little thiosulphate. 

Sample 27 contained metallic sulphides in solution, and an odor of 

hydrogen sulphide was noticeable near the place where it was obtained. Sample 28 contained no metallic sulphides and emitted no 

noticeable odor of hydrogen sulphide. At least eight additional 

samples (8, 9,10,11, 12, 13, 33, and 34) have been tested for hydrogen 

sulphide, with negative results. Sample 2 deposited sulphur and 

ferrous sulphides.

Although hydrogen sulphide is rare in mine waters, it forms when 

certain sulphides are in contact with acid. When exposed to the air 

it oxidizes rapidly. It is not supposed that it would accumulate, at 

least not in the zone where air has free access to the deposits, where 

most samples of mine waters are taken. On descending to greater 

depths it would be used up according to the reaction 

Fe2 (S04 ) 3 + H2S = 2FeS04 + S + H2S04 ,

or it might be used up also in the precipitation of metallic sulphides 

from sulphates. Whether it would be used first to form ferrous sulphate and afterward to precipitate metallic sulphides can not be 

shown, but this order of action occurs at least under some conditions. 

Samples 8 and 9, from Ducktown, Tenn., are especially instructive 

in this connection, for No. 8 contains much ferrous iron and no ferric iron, whereas No. 9 contains much ferric iron and only 1.3 parts of
72 THE ENRICHMENT OP SULPHIDE ORES.

ferrous iron. The ore through which sample 8 passed before issuing 

where it was taken is more than 90 per cent pyrrhotite, which generates hydrogen sulphide rapidly. The ore through which sample 9 

passed contains chiefly chalcopyrite and lime silicates, with but little 

pyrrhotite. This water contains considerable iron, almost all in the 

ferric state. Since No. 8 contains even more copper than No. 9, it 

would be supposed that the ferric sulphate is reduced to ferrous sulphate (FeSOJ before copper sulphide is precipitated, as in this water 

the iron is entirely reduced to the ferrous state while there is still a 

considerable amount of copper in solution. It is noteworthy that the 

alkaline waters (samples 27, 28, 33, 34) contain appreciable quantities 

of carbonates, with lime, potash, and soda. They carry also a certain concentration of sulphates. Two of the four alkaline waters 

from Nevada City, Cal., and two from Creede, Colo., have descended 

through considerable unoxidized material. Alkaline waters have not 

received sufficient attention in studies of sulphide enrichment.

Samples 14, 15, 16, and 17 are from the Capote mine, at Cananea, 

Mexico. The deposit is in an area of quartzite and other rocks 

intruded by diorite porphyry. The ore is in part of the disseminated 

type and carries pyrite, chalcopyrite, and zinc blende, with secondary chalcocite. I can not state the vertical range of chalcocite in 

this deposit, but it is not so abundant at the 900-foot level, where 

sample 16 was obtained, as it is at the 300-foot level, where sample 

14 was obtained. Sample 14 contains 970 parts of free acid per million, whereas the samples taken at greater depths contain no free acid.

The two pairs of samples Nos. 10 and 11 and Nds. 12 and 13 are 

from unused shafts at Ducktown, Tenn. As stated above, the 

samples were taken by lowering bottles equipped with asbestos 

niters to the desired depth, then removing the covers from the filters 

and allowing the bottles to fill. The waters from the open shafts 

are diluted by a large proportion of surface water, so they are less 

concentrated than mine waters that have traveled considerable 

distances through the rocks. In taking samples 10 and 11 the 

water was disturbed, so that the analyses of these samples are of 

less value; samples 12 and 13 were taken with special precautions to 

prevent agitation and show very well the difference in the mineral 

content of the waters at different depths. The water from the 

surface (No. 12) contains more than twice as much free acid as the 

deeper water. No. 13, taken below water level, is more concentrated with respect to silica, calcium, magnesium, aluminum, zinc, 

and iron, but the concentrations of potassium, sodium, and manganese are closely similar. The amount of copper is greatly decreased. 

This suggests that at a depth of 37 feet other metals are being taken 

into solution but the greater part of the copper has already been 

deposited. Although the deeper water contains more ferrous iron 

and more ferric iron than that at the water level, the proportion
CHEMISTEY OF ENRICHMENT. 73

of ferric to ferrous iron has increased with increase of depth, showing 

just the reverse of the conditions expected. It is thought that 

the increase in the proportion of iron in the solution with increase 

of depth has been brought about by reaction of acid on pyrrhotite 

and also that ferric iron has decreased near the surface by hydrolyzation. The. filter paper was colored red by a dustlike iron precipitate, probably basic ferric sulphate, formed presumably near the 

surface and held in suspension in the water. The formation of this 

precipitate probably accounts for the very small concentration of 

ferric iron at the surface of the standing water and possibly also for 

the somewhat smaller amount of total sulphate held in solution 

near the surface. This precipitate would probably not form so extensively under conditions affording free circulation.

The four waters from Freiberg, Germany (samples 29, 30, 31, 32), 

agree in containing considerable zinc and manganese. One of them 

on standing forms a deposit consisting chiefly of manganese oxide 

and some zinc oxide.

Sample 20 is from the 2,000-foot level of the Geyser mine at Silver 

Cliff, Colo. This water contained some nitrate, which is uncommon 

in analyses of mine waters.

Several of these waters do not contain a sufficient amount of the 

acid ions to hold in solution the metallic portion. If carbon dioxide 

was determined in such waters, a part of the metals must be dissolved as silicates or possibly even as hydroxides.

PRECIPITATES FROM MINE WATERS ITNDER SUPERFICIAL CONDITIONS.

Many waters, after issuing from mineralized areas, deposit copious 

precipitates. In some mining districts the streams which carry 

away the underground drainage from the adits have stained their 

beds for great distances from the points of issue. In certain of 

these areas, as at Cananea, Mexico, and Bingham, Utah, the gravels 

above the present streams are cemented by iron oxides, showing that 

the processes of precipitation at the surface were operative before 

the mines were opened. These deposits, formed under atmospheric 

pressure and in the presence of oxygen, are very different from the 

deposits of secondary ore that are formed at depths where sulphide 

enrichment is assumed to take place. Few if any of them are workable for the more valuable metals. Nearly all of them consist 

largely of the colloidal compounds, such as the hydrous iron oxides, 

hydrous aluminum compounds, and hydrous silica.

Many samples of waters quickly become cloudy after they have 

been filtered, either because of their filtration in air or because of 

slight changes in their temperature; or, if they have been taken 

from low depths, because of decrease in pressure precipitation takes 

place. Two samples of water taken from adits at Creede, Colo., 

and filtered in the presence of air deposited light-gray precipitates, 

presumably compounds of aluminum and silica. Of six samples
74 THE ENRICHMENT OF SULPHIDE ORES.

collected at Ducktown, Term., all were likewise clouded. Two of 

these samples, one taken 37 feet and the other 55 feet below the 

water level, were filtered under water, and although the air spaces 

left in the necks of the bottles were exceedingly small the samples 

clouded in a few hours. Several samples from Butte, Mont., deposited 

solids before analyses were made.

The results of analyses of four samples of material precipitated 

from mine waters are stated in the table below. Sample 1 is from a 

water in the Stanley mine, near Georgetown, Colo. (No. 35 in table 

above). Sample 2 is from a mine in Freiberg, Saxony. The sample 

was taken from workings that had been flooded 135 years. It contamed much manganese and considerable zinc. Sample 3 is a slime 

collected in the Federal Loan mine, 1 near Nevada City, Cal., where 

the water represented by sample 23 of the table on page 62 issued 

from the wall rock. Of this analysis Dr. Hillebrand makes the following remarks: 2

The bottle was filled with a black slimy matter emitting a disgusting odor of organic 

decomposition. The cork was forced out with ease by imprisoned gas, chiefly consisting of C02 and CH4 . The slime was said to have been white when collected; the 

subsequently developed black color is due to iron sulphide. Aside from organic 

matter and water, the deposit is essentially ferric oxide with a little arseniate, calcium 

carbonate with a little magnesium carbonate, and manganese as Mn02, besides gelatinous silica and fragments of minerals.

A deposit collected by Lindgren in the drain tunnel of the Providence mine, near Nevada City, Cal., when dried, formed dirty white 

masses and coarse green lumps. An analysis of some of the green 

pieces is given in column 4 of the following table:

Analyses of muds precipitated from mine waters.

BiOt..................... ......................................

AW),..........................................................

FesO»..........................................................

FeO. .................. . .... ... ....... . MnOj.......... cao......... . ....... .............:.::::

MgO. ..........................................................

Pb......... ....... .. .... ... ...............................

CuO.. ..................... . .............................

ZnO............... ............................................

Mo.............................................................

As.............................................................

NaiO..........................................................

S03 .. ..........................................................

KtO.......... ...............................................

1

10.85

2.87

53.57

11.46

21.14

99.89

2

o3.55

7.78

653.21

^ 9fl

6.08

.67

19.99

99.48

3

18.4

23.3

10.8

1.3

(J

34.9

99.8

4

8.31

37.57

.44

.12

.57

TVanft

.17

9 m

42.98

99.67

a Insoluble residue. 6 Some MnO present. The analysis as stated gives O, 8.43, and MnO, 44.78.

1. Precipitate filtered from water of Stanley mine. Jones, L. J. W., Ferric sulphate in mine waters and 

its action on metals: Proc. Colorado Sci. Soc., vol. 6,1900, p. 48.

2. Deposit from a mine at Freiberg, Saxony, taken from workings that had been flooded 135 years. Contains cooalt oxide, 1.36; cadmium oxide, 0.19; lead oxide, 3.45. Beck, Richard, Lehre von den Erzlagerstatten, 1901, p. 402.

3. A deposit from water from Federal Loan mine, near Nevada City, Cal. CaO equals CaCO3 and MgC03; 

organic material calculated with water. Insoluble material included with SiOj. Lindgren, Waldemar, 

The gold-quartz veins of Nevada City and Grass Valley districts, California: Seventeenth .Ann. Kept. 

U. S. Geol. Survey, pt. 2,1896, p. 122.

4. Part of deposit m drain tunnel of Providence mine, near Nevada City, Cal. Idem, p. 123.

 Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley districts, California: 

Seventeenth Ann. Kept. U. S. Geol. Survey, pt. 2, 1896, p. 121. 

«Idem, pp. 122-123.
CHEMISTEY OF ENRICHMENT. 75

OXIDATION AND SOLUTION OF CERTAIN METALLIC SULPHIDES. 

FACTORS CONCERNED.

In the presence of air the oxidation and the solution of the metallic 

sulphides take place simultaneously and it is difficult if not impossible to consider the two processes separately. The rate of solution 

depends on many factors, ampjg them (1) the solubility of the material in water, (2) the molecular, and physical structure of the material, 

(3) the solubility of the salts formed by oxidation, hydration, and 

related processes, (4) the~ composition, concentration, temperature, 

and pressure of the solutions, (5) the mineral and chemical environment, (6) the rapidity of circulation, and (7) the potential or electromotive force of the mineral compared with the electromotive force 

of the mineral or minerals with which it is in contact.

It is noteworthy that the natural conditions are not those which 

commonly exist in the laboratory, for in nature the solvent is in 

motion and is therefore bringing new and generally unsaturated 

solutions into contact with bodies that are dissolving. Thus, if 

sufficient time is afforded even minerals that in the laboratory are 

most nearly insoluble may by natural processes be completely dissolved. Even salts so insoluble as cerargyrite and anglesite may 

be entirely removed from the outcrops of ores.

The solubilities of the sulphates that are formed by these reactions 

are nevertheless important, for, other conditions being similar, it 

would be supposed that the metals that have the most soluble sulphates would be most readily dissolved. As shown by the table 

of solubilities, iron, zinc, and copper have highly soluble sulphates, 

whereas the solubilities of the sulphates of lead and silver are low. 

Although galena (p. 59) is readily attacked by cold dilute sulphate 

solutions it is nevertheless slow to dissolve in metalliferous deposits. 

This fact may indicate why lead lags behind iron, copper, and many 

other metals in its migration downward in ore deposits.

Pyrite and marcasite, which have the same formula, oxidize and 

dissolve at different rates, owing, probably, to a difference in their 

molecular structure. Cellular pyrite will oxidize more rapidly than 

dense pyrite.

The importance of an oxidizing environment as a condition for 

the solution of some metals should be emphasized.

All the experimental evidence shows that the oxidation and solution of the sulphides in pure water is slow. Under natural conditions it may be accelerated somewhat by carbon dioxide, which is 

generally dissolved in rain water and forms a weak acid that may 

start solution. Sulphuric acid, a more powerful solvent, is released 

in the presence of iron sulphides.
76 THE EtfRlCHMEtfT OF SULPHIDE OBE§.

The recent work of Gottschalk and Buehler 1 shows that several 

sulphides in contact in water form weak batteries. The current 

flows from the mineral having the higher, potential to the one having 

the lower potential, which is dissolved, the one with higher potential 

being to some extent protected from solution. These experiments 

should probably be interpreted as indicating particularly the relations that exist at the beginning/^j^lution, for the electromotive 

force varies with variations in the s^tions, which, as already stated, 

are continually changing. The relations considered are those which 

exist in the presence of free air. Under natural conditions all portions of oxidizing deposits are not equally accessible to atmospheric oxygen. Some sulphides that are readily dissolved in dilute 

acid sulphate in the presence of air are practically insoluble in its 

absence. Chalcocite, for example, is highly stable in portions of 

deposits where air is excluded, but in the presence of air and acid 

it dissolves very readily. The solution and oxidation of the sulphides depend on so many factors that general statements can not 

be applied to all conditions or combinations, each mineral association presenting, in a sense, a problem in itself.

EELATIVE BATE OF SOLUTION OF SEVERAL SULPHIDES IN AN 

OXIDIZING ENVIRONMENT.

The following table shows the order of oxidation of some of the 

sulphides, according to the views of several investigators. The 

sulphides ^hat are most readily attacked are placed highest in each 

series. The differences that appear between certain of the series 

might be interpreted as indicating differences which should be expected under natural conditions, the order of oxidation depending on 

the environment.

Order of oxidation of sulphides, according to several authorities.

1

Iron. 

Copper.

Zinc.

Lead.

Silver.

2

Arsenopyrite. 

Pyrite. 

Chalcopyrite.

Sphalerite.

Galena.

Chalcocite.

3

Chalcocite.

Pyrite.

4

Chalcocite.

Pyrrhotite.

Pyrite.

5

Chalcocite.

Bornite.

Pyrrtiotite.

Chalcopyrite. 

Pyrite.

6

Sphalerite. 

Chalcocite.

Galena.

Pyrite.

Chalcopyrite. 

Argentite.

7

Pyrrhotite. 

Sphalerite.

Galena.

/Chalcopyrite 

\ and pyrite.

1. Van Hise, C. R., Some principles controlling the deposition of ores: Trans. Am. Inst. Min. Eng, 

vol. 30,1901.P. 101.

2. Weed. W. H., The enrichment of gold and silver veins: Trans. Am. Inst. Min. Eng.. vol. 30,1901, 

p. 429.

3. Lindgren. Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43.1905. p. 180. At some places in Clifton district.

4. Enunons, W. EL, and Laney, F. B., Preliminary report on the mineral deposits of Ducktown, Tenn.: 

Bull. U. S. Geol. Survey No. 470,1911, pp. 151-172. Above the water level.

5. Vogt, J. H. L., Problems in the geology of ore deposits, in Posepny, Franz, The genesis of ore deposits, 

1902, pp. 676-677. Order of attack 01 sulphides by ferric chloride.

6. Gottschalk. V. H.. and Buehler, op. clt.,.p. 31. Table showing relative potential of several sulphides.

7. Wells, R. C. Table showing rate of attack of 0.057 normal solution H2SO< oh several sulphides. 

(See p. 69.)

i Gottscbalk, V. H., and Buehler, H. A., Oxidation of sulphides: Econ. Geology, vol. 7,1912, p. 16.
CHEMISTBY OF ENRICHMENT. 77

As already stated, the relative rate of solution of the sulphides 

probably can not be expressed in a hard and fast series, yet certain relations hold under many, probably under most conditions. Sphalerite 

is so readily dissolved by acid in the absence of air that at many places 

it is removed even from primary ores that are in a reducing environment. Consequently in many deposits the order of the rate of solution of 

sphalerite, chalcocite, and pyrrhotite in acid where oxygen is in great 

excess can not be stated. Possibly sphalerite should be placed at the 

head of the series shown in the table; possibly it should be placed 

between pyrrhotite and chalcopyrite.

The relative rate at which chalcocite and pyrite are removed where 

oxygen is in excess is shown by the relations of these minerals in 

several deposits. Lindgren 1 notes that in the Morenci district chalcocite is often oxidized, while much pyrite remains. At Ducktown, 

Term., the "black copper" ores, composed of pyrrhotite, pyrite, and 

chalcocite, which have replaced the iron sulphides near the Water level, 

quickly lose their copper content when the ores are exposed to highly 

oxygenated waters by the artificial depression of the level of the 

ground water. Solution is so rapid there that it is difficult to obtain 

rich chalcocite ores in mines that have been opened below the water 

level for a few years. At Ducktown, as at Morenci, chalcocite, where 

oxygen is present, is dissolved more rapidly than pyrite.

The pyrite in some of the leached chalcocite ore of Ducktown 

remains almost unaffected by the leaching, its bright crystals being 

distributed through a dark mass of sooty black pyrrhotite, which has 

been partly altered and from which practically all copper has been 

removed. The order of solution indicated appears to be chalcocite, 

pyrrhotite, pyrite.

As shown by Gottschalk and Buehler, galena oxidizes more readily 

than pyrite. The oxidation product, lead sulphate, is so insoluble, 

however, that it remains to cover the crystals of galena and serves to 

delay further action. Galena remains in the outcrops of many sulphide ore bodies after iron and zinc compounds have been completely 

altered. It is said that some ore deposits in the Wisconsin zinc district were discovered by farmers who plowed up crystals of galena.

Enargite also is an exceedingly persistent mineral in outcrops. 

According to Keno Sales 2 it is commonly found in or near oxidized 

ores of the Butte copper mines, and it is said that at Tintic enargite 

remains in the oxidized ores even after galena has been dissolved. 

Series 5 (table on p. 76), based on experiments of Vogt, shows the 

rate of attack by strong solutions of ferric chloride, an oxidizing 

agent, on several sulphides. Chalcocite was dissolved quickly, as it is

i Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 180.

8 Sales, R. H., Discussion of paper by F. L. Ransome, on "Criteria of downward sulphide enrichment''; 

Econ. Geology, vol. 5,1910, p. 681.
78 THE ENRICHMENT OF SULPHIDE OEES.

in secondary sulphide zones, where it is exposed to solution in the 

presence of air. Bornite also was attacked at a relatively rapid rate. 

Stokes * showed that dilute ferric sulphate attacks chalcopyrite more 

readily than pyrite. All these experiments and observations seem to 

indicate that in the zone of oxidation in many deposits the sulphides 

are dissolved in the following order: Sphalerite(?), chalcocite, pyrrhotite, chalcopyrite, pyrite, galena, enargite. This series does not correspond exactly with the series of WehV (column 7 of table), which 

was obtained from experiments with sulphuric acid alone.

NET RESULTS OF CHEMICAL CHANGES DURING OXIDATION OF CERTAIN ORES.

The results of the chemical changes that take place during the oxidation of sulphide deposits depend largely on the minerals that form 

the deposits. Some gangue minerals disappear completely, others are 

partly dissolved, and some elements of still other minerals remain in the 

mass in new combinations. Not many sets of analyses of sulphide 

ore and of corresponding oxidized ore from the same deposit are available. The data for the deposits of Ducktown, Term., are fairly 

satisfactory. The gossans of the copper deposits have been smelted 

for iron, and average analyses of thousands of tons are available from 

furnace records, through the courtesy of Mr. R. H. Lee and Mr. 

John B. Newton. Yearly averages from the smelters of sulphide ore 

from the same deposits are available through the courtesy of the 

officers of the Tennessee Copper Co. and the Ducktown Copper Sulphur & Iron Co. On the assumption that the average of several 

specific-gravity determinations of the gossan (2.2) applies to the entire 

mass and that the average specific gravity of the sulphide ore is 

4.05, the following table has been prepared to indicate the nature of 

the change by which primary ore becomes gossan. Column la shows 

the percentage weight of the constituents of the primary sulphide ore 

of the Mary mine (average of all ore smelted in 1906). Column Ib 

shows its percentage weight times its specific gravity (4.05) and may 

be regarded as expressing the number of grams in 100 cubic centimeters of the primary ore. . Column 2a shows the chemical composition of the gossan (average of two large shipments). Column 2b gives 

its percentage weight times its specific gravity (2.2, corresponding 

to 39 per cent porosity). Column 3 shows the gain and the loss of 

several constituents. Losses are shown for sulphur, silica, alumina, 

lime, iron, zinc, copper; gains for oxygen and water. Carbon dioxide 

and magnesia were not determined in the analyses of gossan but were 

probably almost entirely carried away.

1 Stokes, H. N., On pyrite and marcasite: Bull. U. S. Geol. Survey No. 186,1910, p. 33.
CHEMISTRY OP ENRICHMENT.

Chemical changes by oxidation processes at Ducktown, Tenn.

79

SiOs.................................. ...............

AIj03 ...............................................

Fe.. .................................................

MgO..... ...........................................

GaO................. ................................

co«..... ............................................

S. ...................................................

MnO ................................................

Cu.......... ........................................

Zn...... ...........................................

HjO .................................................

O...................................................

la

22.44

2.93

33.43

3.15

8.28

2.85

21.23

.44

2.45

2.79

99.99

lb

OH 88

11.87

135.39

12.76

33.534

11.54

85.98

1.78

9.92

11.30

404.954

2a

9 QH

1.57

49.9

(?) o.

.35

.65

.86

015.40

021.38

100.06

2b

91 80

3.45

109.78

' (!> 7,

1.43

1.89

22.88

47 fU

220.13

3

- 69

- 8.4

- 25.6

- 12.7

- 32.7

- 11.5

- 84.5

- 8

- 11.3

+ 33.88

+ 47.04

-182.8

o HaO and O are estimated, on the assumption that the Fe is In limonite.

Mineralogically the primary ore consists of pyrrhotite, pyrite, 

chalcopyrite, sphalerite, actinolite, calcite, tremolite, quartz, pyroxene, garnet, zoisite, chlorite, and micas. Small quantities of 

bornite, specularite, and magnetite are present and at some places 

graphite, titanite, and feldspars. The following is a close approximation to the mineral composition of the unoxidized ore:

Composition of primary ore at Ducktown, Tenn.

Pyrrhotite.................................................... 38. 5

Pyrite........................................................ 5.1

Chalcopyrite.................................................. 7.1

Sphalerite..................................................... 4.2

Quartz 10.3

Calcite....................................................... 6.0

Garnet........................................................ 3.0

Amphiboles, pyroxene, zoisite, etc............................. 25.8

100.0

Oxidation has changed this ore into a gossan consisting essentially 

of limonite with a little silica and kaolin, carrying a fraction of 1 per 

cent of copper and sulphur.

The table on page 80 shows the results of an analysis of rich concentrates of sulphide ore from the Montana Tonopah mine, Tonopah, Nev., 

and an analysis of rich oxidized ore from the Valley View vein of'the 

same district. The principal primary minerals in this district, according to Spurr,1 are quartz, carbonates, adularia, some sericite, and sulphides of silver, antimony, copper, iron, lead, and zinc. Limonite, 

wad, and horn silver are deposited in the oxidized zone, with some 

silver bromide, silver iodide, and free gold.

1 Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, p. 90.
80 THE ENRICHMENT OF SULPHIDE OEES.

Analyses of ores from Tonopah, Nev.

Siliceous matter. ................................................................

Gold............................................................................

Silver ...........................................................................

Lead. ...........................................................................

Copper. .........................................................................

Iron... . .........................................................................

Zinc ............................................................................

1

15.18

.82

25.92

6.21

1.32

9.87

1.36

5.84

2.56

19

.92

1.49

3.70

6.34

Not det.

81.72

2

16.53

.62

62.54

.32

.09

1.39

.07

.10

.38

m

.15

82.22

1. Concentrates of primary sulphide 9re from Montana Tonopah mine.

2. Oxidized ore from Valley View vein.

Discussing the composition of the primary and secondary ores 

shown by the analyses, Spun* 1 says:

Aside from the complex carbonate of lime, manganese, magnesia, and iron, the 

analysis of the primary sulphide ore indicates the presence of a large amount of silver 

sulphide argentite. Antimonial sulphides of silver, polybasite, very likely stephanite, and smaller amounts of galena, blende, pyrite, and chalcopyrite are also 

indicated. Of very great interest is the presence of a considerable amount of selenium, 

which occurs, in part at least, as a silver selenide, and the absence of its usually closely 

associated element tellurium. The chemical form of the gold is yet uncertain.

It is fair to assume that the oxidized ore in its primary sulphide state may have 

had a composition somewhere relatively near that of the primary sulphide analyzed. 

The two analyses may then be compared with the object of perceiving the changes 

effected by oxidation. There is no element which can be considered as having 

remained quantitatively unaffected during oxidation, so that merely the large relations can be glanced at. All the metals except silver and perhaps gold are present 

in the oxidized ore in much diminished proportions. The lead, copper, and zinc are 

present in small quantities. The manganese is now in the form of oxide, but very 

little remains; the iron is in the form of oxide, with some residual or secondary pyrite. 

There is much less gold in proportion to silver in the oxidized ore than in the sulphide 

ore; but this may be fortuitous and depend on the specimen selected. More than 

half the silver is in the form of sulphide, and from the very small quantity of arsenic 

and antimony present this portion must be nearly all in the form of argentite. The 

antimonial silver sulphide is very probably pyrargyrite (ruby silver), judging from 

microscopic observations. It is noteworthy that antimony and arsenic are present 

in the same proportions to one another in both analyses. There is less than a third 

as much selenium in the oxidized ore as in the sulphide ore, but the discrepancy is 

not so great as in the case of lead, copper, manganese, zinc, arsenic, and antimony; 

and this selenium seems to be still in the form of a silver selenide.

Therefore it is probable that during the process of oxidation the primary carbonates 

were attacked by surface waters and the lime and magnesia, together with most of 

the iron and manganese, removed in solution. Some of the iron and manganese 

remain as oxides. No important change in the amount of gold and silver is proved. 

The argentite has largely remained unaltered, but the polybasite (and stephanite if

1 Spurr, J. E., op. cit., pp. 92-93,
CHEMISTKY OF ENRICHMENT. 81

present) has probably been attacked, and much of the silver selenide. Part of this 

silver has been reprecipitated with little change of position as secondary argentite, 

not distinguishable from the primary argentite, while a large portion has been altered 

to chloride by the action of chlorine contained in the shallow underground waters. 

Most of the arsenic and antimony in the original polybasite and stephanite has been 

removed in solution; the rest goes to form the secondary sulphide pyrargyrite, as indicated by numerous field observations. The pyrite and the chalcopyrite have been 

attacked. Most of the iron in these sulphides has been removed; atsmall part remains 

as oxide, or rarely as residual or secondary pyrite. Nearly all the copper has been 

removed, a little remaining in the probable form of oxide.

At Cripple Creek, as noted by Lindgren and Kansome,1 the original 

vein structure is destroyed by thorough oxidizing decomposition. 

In some sheeted lodes having many small parallel fissures and joints 

these openings become closed or effaced and the lode appears as a 

homogeneous brown, soft mass. In other places a central seam is 

retained and appears as a streak of soft, more or less impure kaolin; 

in still other places a central seam is filled with white compact alunite, 

more rarely by jasperoid or opaline silica. Crusts of comb quartz, 

if originally present, lie included in the clayey seams, but neither the 

original fluorite nor the carbonates are ordinarily preserved. Very 

rich oxidized ore fills the central cavities of some lodes like a thick 

brown mud and easily flows out when the vein is opened.

These writers say: 2

In general oxidation tends to transform sulphides, sulphosalts, and tellurides to 

oxygen salts and native metals, both of which may, under certain circumstances, be 

further changed or carried away by surface waters. The silicates in the veins are 

changed to a few minerals most stable under atmospheric influences that is, kaolin, 

quartz, manganese dioxide, and limonite. The carbonates of the earthy metals are 

carried away in solution, while those of manganese and iron are changed to oxides. 

As waters of acid reaction, frequently containing free sulphuric acid, prevail during 

oxidation of vein deposits, original quartz will not be attacked. New silica, generally hydrated, may be deposited by solutions derived from the decomposition of 

the silicates.

The processes are more difficult to follow and to establish by means of analyses 

than those due to primary vein formation, for it seems to be a characteristic feature 

of oxidation to segregate the new minerals in larger masses and thus produce a less 

homogeneous product; this is no doubt due to the energetic action of oxygen and of 

the acids set free, as well as to the increasing mobility of substance near the surface. 

Oxidation tends to thorough change of composition and also to obliteration of structure 

of the original rock or vein.

Of the metallic minerals the telhirides form the most important division. The 

bond between the tellurium and the gold is not a strong one, and direct oxidation 

very easily produces residual pseudomorphs of free gold and tellurium dioxide (the 

mineral tellurite), which in presence of iron oxides easily changes to various tellurites 

such as the yellowish-green emmonsite. In the Bonanza King lode of the Midget 

mine occur veinlets of pyrite with tellurides. Within 30 or 40 feet of the surface

i Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district. 

Colorado: Prof. Paper U.S. Geol. Survey No. 54, 1906, p. 199. 

s Idem, pp. 199-200.

72293° Bull. 529 13 6
82 THE ENBICHMEffT OF SULPHIDE OEES.

oxidation has changed the tellurides to specks of native gold, while pyrite remains 

practically unaltered. The tellurides in fissures and joints, which form the most 

common mode of occurrence, will be most easily oxidized, while those occurring as 

metasomatic products in the rocks are not so readily reached by the surface waters.

Galena and zinc blende change to lead sulphate and zinc silicate, 

which may remain or even be somewhat concentrated in the altered 

mass. Molybdenite oxidizes very readily to yellow and blue molybdite and ilsemannite, products that do not seem to be easily carried 

away. Tetrahedrite yields various forms of oxidized copper compounds which, as usual, show considerable mobility. No oxidized 

products containing antimony have been recognized. The fluorite on 

exposure to oxidation loses its deep-purple color and becomes disintegrated. Eventually a part of it is dissolved in surface waters.

The following table shows the results of analyses of unoxidized ore 

from- the Moose mine (column 1) and the oxidized portion of the 

same specimen (column 2). The specimens were analyzed by F. C. 

Knight and have been described by Richard Pearce.1 Column 3 

shows an analysis by W. F. HUlebrand of wholly oxidized ore from 

the 100-foot level of Stratton's Independence mine.

Analyses of fresh and oxidized ores from Cripple Creek, Colo.

SiOj................................................................

TiOj................................................................

AliO................................................................

FesO3 . ..............................................................

CaO. ...............................................................

MgO...............................................................

KjO.... ............................................................

Na2O............... ................................................

ZnO.. ..............................................................

CuO.. ..............................................................

PbO.. ..............................................................

BijOs..................................................... -.....-...

MoOs...... .........................................................

TeOa ........ .......................................................

Au.................................................................

Ag.... .............................................................

S03 .. ...............................................................

P306................................................................

H2O ................................................................

COj..... ...........................................................

s...................................................................

1

56.70

19.35

04.20

.37

| <U3.97

.10

.022

.50

4.75

100

2

50.55

19.93

10.57

O7

.12

«12.13

//T>aA\ 70

049

2.55

3.05

100. 00

3

54.45

sr>

14 45

«9.94

61.71

1.14

48

{

0 Qfi

.41

d.53

.03

1O

.OQ25 fins

.52

.02

.10

.50

Trace.

'95.4205

a Both FeaOs and FeO present. 

6 Both MnOj and MnO present, 

c By difference.

d Possibly high.

«The difference is largely made up of combined water.

Of these analyses Lindgren and Ransome 2 say: 

From the analyses and from what is known about the normal composition of unoxidized ore the following conclusions may be drawn, although it is of course realized 

that much more analytical work would be necessary for an exhaustive treatment of 

this difficult subject.

> Pearce, Richard, Further notes on Cripple Creek ores: Proc. Colorado Sci. Soc., vol. 5,1896, pp. 11-16. 

»Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, pp. 202-203.
CHEMISTEY OF ENRICHMENT. 83

During oxidation the percentage of silica decreases moderately, probably by solution of silica set free during the decomposition of silicates. Alumina remains fairly 

constant, though it may locally concentrate to pure kaolin. The iron is apt to locally 

increase by concentration as limonite, though a part will be carried away as sulphate. 

Small amounts of lime and magnesia are probably leached from the rock, but the quantity is not greatly changed. Manganese id greatly concentrated, locally, on the seams 

of the rock. As to alkalies, the accumulation of potassium begun during the vein-forming process is continued or at least maintained during oxidation. Pyrite is converted 

into sulphuric acid and sulphates, and the percentage of sulphur is greatly decreased 

in the oxidized ore. Part of it remains as sulphate, but as there appears to have 

been no corresponding decrease of the bases it would seem likely that a considerable 

part of it was carried away as free acid. Little change is noted in the titanium, while 

phosphoric acid and zinc appear to have increased. The small quantities of other 

metals do not seem to differ notably from those observed in fresh vein material. An 

increase of water to 3 or 5 per cent is a natural consequence of the formation of kaolin 

and other hydrated salts; locally it may increase up to 14 per cent, which is the amount 

contained in pure kaolin.

Silver and tellurium are removed by oxidation, but in general gold 

has remained. It is not easy to demonstrate whether a slight enrichment has taken place or not. Lindgren and Ransome incline 

to the belief that the oxidized ores as a whole are somewhat richer 

than the corresponding telluride ore, but this difference may depend 

on the original unequal distribution of the gold.

RELATIVE BATE OF SOLUTION OF SEVERAL SULPHIDES IN A REDUCING

ENVIRONMENT.

When oxygen is present the process of solution is so intimately 

related to that of oxidation that their results can not be considered 

separately. The experiments of Wells on the solution of pyrrhotite, 

sphalerite, galena, chalcopyrite, and pyrite in dilute sulphuric acid 

were made in stoppered test tubes, and although no special preqaution was taken to exclude atmospheric air, yet the excess of hydrogen 

sulphide generated in at least three of the experiments indicates a 

reducing environment. The minerals were attacked in the following 

order, determined by the amount of hydrogen sulphide generated 

from the three first named: Pyrrhotite, sphalerite, galena, chalcopyrite, and pyrite. This series does not agree closely with any of 

the others named above. Possibly the electric currents that may be 

generated when two of the minerals are in contact would modify the 

rate of solution, but it should not be supposed that the series obtained 

under reducing conditions in acid would be identical with the series 

obtained by Gottschalk and Buehler in distilled water with free 

access of air.

The series obtained from Wells's experiments may not apply everywhere, but geologic evidence indicates that it applies to some 

deposits. Thus, in the Morenci district, according to Lindgren,1 the

1 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43, 1905, p. 183.
84 THE ENRICHMENT OP SULPHIDE ORES.

solutions that deposit chalcocite in the secondary sulphide zone 

appear not to attack pyrite as long as zinc blende is present. Pyrrhotite also is attacked more readily than pyrite in the chalcocite zone 

at Ducktown, Term. In some ore the pyrrhotite has been reduced to 

a powdery mass while pyrite remains fresh and untarnished.

METASOMATIC REPLACEMENT OF PRIMARY SULPHIDES BY SECONDARY 

SULPHIDES AND RELATIONS INDICATED BY SCHUERMANN'£ SERIES.

As already stated, the series of Schuermann does not agree exactly 

with the solubilities of all the sulphides involved; but if comparatively unimportant metals, such as arsenic, antimony, cobalt, and 

bismuth, are eliminated the series is almost the same as that indicated 

by the molar solubilities of the sulphides as determined by Weigel. 

The positions of zinc sulphide and iron sulphide are reversed, but 

according to Weigel the molar solubilities of these are almost the 

same. As Schuermann himself observed, the farther apart in the 

series any two sulphides are the more nearly complete is the replacement.

In the processes <j)f sulphide enrichment the primary sulphides are 

commonly replaced pseudomorphously by the secondary sulphides. 

Eliminating the relatively unimportant sulphides, such as those of 

cadmium and bismuth, and the sulphides of arsenic and antimony, 

which generally enter into composition with more than one othes 

element to make the more complex secondary minerals, Schuermann's series (and Weigel's series, except zinc sulphide) is as follows: 

Mercury, silver, copper, lead, zinc, iron. In the table below, for convenience in inspection, the metals are placed in order of increasing 

solubilities of their sulphides in water, the more soluble sulphides 

being placed to the right of and below the less soluble ones.

Metasomatic replacement of several sulphides. 

[In the order of Schuermann's series.]

Mercury.

Mercury.

Silver.

Copper.

Lead.

Zinc.

Iron.

Silver. Copper.

?

Lead.

On PbS.

Metasomatic.

Zinc.

On ZnS.

Pseudomorphic.

On ZnS.

Iron.

On FeS2.

Pseudomorphici

On FeSj.

"Drives 

out iron."
OHEMISTEY OF ENRICHMENT. 85

According to Schuermann's series it might be supposed that mercury 

sulphide would replace the sulphides of silver, copper, lead, zinc, and 

iron; that silver sulphide would replace the sulphides of copper, lead, 

zinc, and iron; that copper sulphide would replace the sulphides of 

lead, zinc, and iron: and so on.

The search for well-authenticated examples to fill in the several 

blocks in the table has been only partly successful. It would hardly 

be supposed that mercury sulphide would replace extensively the more 

soluble sulphides of other metals, for the solutions that transport the 

metals are, as already stated, sulphate solutions, and mercury sulphate 

has exceedingly low solubility. Moreover, mercury is driven out of 

combination by several metals and tends to form the native metal 

and the chloride. Secondary cinnabar is not unknown, however, in 

the oxidized zone. Silver dissolves in dilute sulphuric acid solutions in the presence of ferric sulphate, and at depth its sulphide is 

precipitated on those of lead, zinc, and iron. Hintze 1 mentions 

pseudomorphs of argentite after proustite, stephanite, and pyrargyrite, but no examples of pseudomorphs of argentite after sphalerite, 

galena, or pyrite are available. The occurrences of secondary argentite are generally described as incrustations on the primary sulphides 

or as veinlets cutting them. Secondary chalcocite frequently contains silver which is disseminated through and doubtless contemporaneous with the copper sulphide, but the nature of the silver mineral 

is uncertain. The precipitation of silver sulphide at the expense of 

sphalerite has recently been emphasized by Irving and Bancroft 2 

in their discussion of secondary processes of enrichment of silver ores 

at Lake City, Colo. In view of the fact that copper sulphide is 

highly stable in acid solutions in the absence of an oxidizing agent 

it is not remarkable that the replacement of copper sulphides by 

silver sulphides is not common in the deeper zones.

Copper is much more abundant in its deposits than silver and the 

nature of its changes are more easily recognized. Long ago it was 

pointed out by Genth 3 that copper sulphide replaces galena in deposits at Ducktown, Tenn., and B. S. Butler found examples of covellite replacing galena in the San Francisco district, Utah. As stated 

above, Lindgren has shown that copper sulphide replaces sphalerite 

at Morenci, and Butler found exceptionally clear examples of the 

replacement of zinc sulphide by covellite in the San Francisco district. 

It is said that copper sulphide replaces sphalerite at Butte, Mont., 

and without much doubt similar replacement is common in many 

mineral deposits. Examples of the pseudomorphous replacement

> Hintze, Carl, Handbuch der Mineralogie, vol. 1, p. 442.

8 Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. 

Geol. Survey No. 478, 1911, pp. 64-65. 

8 Genth, F. A., Contributions to mineralogy: Am. Jour. Sci., 2d ser., vol. 33,1862, p. 194.
86 THE ENRICHMENT OP SULPHIDE ORES.

of pyrite by copper sulphide are well established and numerous, this 

method being the most common mode of origin of secondary copper 

sulphide ores. Some examples are enumerated on page 104. Although there are well-authenticated examples of sulphide enrichment 

of lead deposits, the pseudomorphs of galena after zinc and iron 

sulphides are not common. Hintze l notes one replacement pseudomorphous after sphalerite and one after arsenopyrite. Statements 

are made that lead has driven iron or zinc out of its sulphide combinations, but lode ores afford surprisingly few examples of pseudomorphous replacements of sphalerite or pyrite. Possibly the strong 

tendency of galena to assume its own crystal form has obscured its 

pseudomorphic replacement of other minerals.

It has frequently been stated that zinc sulphide has been precipitated at the expense of iron sulphide and that zinc has driven iron 

out of its sulphide combination, but no examples of the pseudomorphous replacement of pyrite or marcasite by zinc blende are available. 

On the other hand Hintze 2 notes a pseudomorph of marcasite after 

zinc blende.

It would not be supposed that under the usual conditions of concentration in the secondary sulphide zone a sulphide that appears to 

the right of the serrate line in the table on page 84 would replace one 

to the left of it on the same line. Thus the copper sulphides would 

not be replaced by lead sulphide or zinc sulphide, and so on. Some 

examples, however, do not agree with the relations indicated in this 

series,3 for there are pseudomorphs of pyrite after chalcopyrite, arsenopyrite, argentite, stephanite, polybasite, ruby silver, and tetrahedrite, 

and pseudomorphs of marcasite after pyrrhotite, pyrite, galena, 

argentite, sternbergite, stephanite, polybasite, miargyrite, bournonite, 

chalcopyrite, zinc blende, and other minerals.

The conditions that exist where the pseudomorphs are formed at 

the expense of the less soluble sulphide can not be stated. It would not 

be supposed that in cold acid solutions, under conditions that prevail 

in secondary sulphide zones, the more soluble sulphides would be 

fixed at the expense of less soluble sulphides unless the metal entering 

into the composition of the more soluble sulphide were present in very 

high concentration. Probably some of the pseudomorphs mentioned 

above were formed in alkaline solutions or possibly at high temperatures, and the effect of increased temperature on the solubilities of 

the sulphides is unknown. It should be noted, however, that the 

secondary replacements that are clearly of great economic significance 

are such as would be expected from the relations indicated by the 

Schuermann series.

1 Hintze, Carl, Eandbuch der Mineralogie, vol. 1, p. 481. 

»Idem, p. 820. 

»Idem, pp. 722 and 821.
CHEMISTBY OF ENBICHMENr. 87

SOURCE OF THE SULPHUR OF SECONDARY SULPHIDES AND 

GENERATION OF HYDROGEN SULPHIDE.

The acid sulphate solutions that carry the metals downward from 

the oxidized zone to a reducing environment can not be regarded as 

an adequate source of sulphur for the secondary sulphides. Sulphides 

are almost unknown in mine waters, and in the presence of air hydrogen sulphide or any dissolved metallic sulphide would be oxidized to 

sulphate. In only two of the 37 analyses on pages 60-63 is sulphur 

reported about 1 part per million in the alkaline water of the Federal 

Loan mine, Nevada City, Cal. (No. 27), and hi an alkaline water from 

Butte, Mont. (No. 2). Traces of thiosulphate, probably due to oxidation after bottling, were noted in Nevada City samples (27, 28). 

Both of these were samples of alkaline solutions obtained from 

reducing environment.

The sulphate radicle, once formed, is exceedingly stable. It may be 

broken up by heat, but probably not at temperatures that exist under 

conditions of superficial alteration. Certain bacteria break up 

gypsum and other stable sulphates and liberate hydrogen sulphide, 

but these bacteria can hardly be regarded as an important agency for 

the generation of hydrogen sulphide, for it is doubtful whether they 

could survive the presence of copper and other salts, which most of 

these underground mineral waters carry. It is said that carbon may 

reduce the sulphate radicle to sulphide, but this statement has not 

been confirmed experimentally. Organic material, though it has 

been recognized in some mine waters, is not reported in many of the 

samples that were taken, in glass-stoppered bottles and could have 

little or no part in the concentration of most lode ores. The only 

apparent adequate source of the sulphur that combines with the 

metals to form the secondary sulphides lies in the older sulphides. 

Where these sulphides are replaced pseudomorphously by secondary 

sulphides the sulphur has evidently remained in place, and in general 

the pseudomorphous replacements are attended by loss rather than by 

gain of sulphur.

As already stated, some of the secondary sulphide ore is found in 

cracks that cut the older sulphides. In some of these cracks the 

secondary minerals have replaced an older veinlet of sulphide ore, 

but in others the ore has clearly been deposited in an open space, 

which indicates that both the metal and the sulphur that enter into 

combination to form the secondary sulphide have migrated to the point 

of deposition. In some districts even secondary chalcocite is known 

to form veinlets in the older sulphide ore, one of the best examples 

being afforded by deposits of the Virgilina district, North Carolina, 

reviewed on page 107. These relations, indicating a transfer of sulphur 

in some unoxidized form, together with experimental evidence showing 

that dilute acid in contact with several primary sulphides will generate
88 THE ENRICHMENT OF SULPHIDE ORES.

hydrogen sulphide, strongly suggest hydrogen sulphide or alkaline 

sulphides as agents precipitating some secondary sulphides. In view 

of the fact that sulphur compounds other than sulphates are practically unknown in mine waters, this conclusion might be questioned, 

but it should be recalled that of the samples available all but two 

were taken in the presence of air and that these two and many others 

contain ferric sulphate, which readily decomposes hydrogen sulphide. 

Hydrogen sulphide is used up in many ways, so it is not surprising 

that it does not accumulate in the zones of alteration.

PROCESSES OF SULPHIDE ENRICHMENT COMPARED WITH THE 

PROCESS EMPLOYED FOR SEPARATING THE METALS IN QUALITATIVE CHEMICAL ANALYSIS.

Although it has not been shown that hydrogen sulphide is actually 

generated by bringing acid sulphate into contact with pyrite or 

chalcopyrite, or that it is an intermediate product in processes of 

metasomatic replacement, yet Wells's experiments for producing 

hydrogen sulphide (p. 59) clearly indicate the power of various sulphides to reduce some solutions or to precipitate sulphides from 

soluble salts. Without much doubt the same laws of chemical 

equilibrium apply to both processes, for the mineral that evolves 

hydrogen sulphide most easily will precipitate a secondary metallic 

sulphide most readily and will reduce acid solutions of reducible 

salts most readily.

The method commonly employed in qualitative analysis for separating the metals is as follows: The solution of the metals is made 

acid with hydrochloric acid, which precipitates silver, lead, and univalent mercury as chlorides. Hydrogen sulphide is passed into the acid 

solution and precipitates sulphides of lead, bismuth, copper, arsenic, 

antimony, tin, and bivalent mercury. The filtrate is made alkaline, generally with ammonium hydroxide, and then ammonium sul- 

. phide is added, which precipitates the sulphides of iron, manganese, 

Nickel, cobalt, and zinc, also aluminum hydroxide. A review of this 

procedure may throw light on some of the natural processes of alteration and sulphide enrichment. Mine waters in the oxidized zone 

the zone of solution are acid and generally contain some chlorides. 

Silver chloride (cerargyrite), mercurous chloride (calomel), or lead 

chloride or chlorophosphate (pyromorphite) may be fixed in the 

oxidized zones of deposits carrying the metals indicated, but as 

chlorides are in general not abundant in mine waters, and as the 

chlorides named above are themselves somewhat soluble, some of the 

silver, lead, or mercury may be carried downward by the acid sulphate solutions. Hydrogen sulphide, which, as already stated, is 

generated by the action of dilute acid sulphate waters on some sulphides, will precipitate from acid solutions arsenic, antimony, tin, 

bismuth, copper, cadmium, mercury, lead, silver, and gold. These
CHEMISTEY OP ENRICHMENT. 89

metals may therefore be precipitated as sulphides before the solution 

has become less acid. Thus the sulphides of copper, lead, and silver, 

for example, may be formed at or near the water level, or, in the 

absence of a water level, in the upper part of the zone of reduction. 

As the solutions descend they tend to become neutral and ultimately alkaline. When the solutions have lost acidity, sulphides of 

zinc, iron, and some other metals may be precipitated.

The analogy of the process of separation employed in qualitative 

analysis is not complete, however, for sulphide enrichment takes 

place in moving solutions and in solutions probably more dilute; the 

changes from acid solutions to neutral or alkaline solutions are 

gradual, especially in deposits not containing minerals that react 

readily with the solutions, like soluble carbonates and pyrrhotite.

Another relation should be emphasized: Although zinc sulphide is 

 not precipitated by hydrogen sulphide in the moderately acid solution 

used in the laboratory, it is precipitated in solutions of very feeble 

acidity. Thus zinc sulphide may be precipitated from a solution of 

acetic acid. Iron sulphide is less readily precipitated in acid than 

zinc sulphide. There is not much geologic evidence that either 

iron or zinc sulphide is precipitated extensively from cold descending acid sulphate solutions which traverse lode ores in igneous or 

other noncalcareous rocks. In such rocks the descending waters 

probably remain acid to moderately great depths, for the acid 

waters do not react on silicates so rapidly as on lime carbonate. 

These conditions would delay the deposition of secondary iron 

and zinc sulphides. There is some evidence that secondary chalcopyrite, pyrite, and sphalerite are formed in small cracks in the 

lower parts of some secondary sulphide zones, but they are surely 

subordinate to secondary copper sulphides in most deposits in 

noncalcareous rocks. One is warranted in supposing that the 

precipitation of the iron and zinc sulphides can be brought about 

only by a nice adjustment of conditions; iron and zinc are not precipitated in solutions that are highly acid, and neutral solutions 

probably will not vigorously attack the primary sulphides. These 

relations appear to be in harmony with the geologic observations, 

which indicate that pyrite and sphalerite are not abundant secondary 

sulphides in many lode ores. Of the action of alkaline solutions 

under these conditions not much is known.

DECREASE OF ACIDITY OF DESCENDING SULPHATE WATERS.

In the discussion of the composition of mine waters of sulphide 

ore deposits it was stated that the waters of the shallow zones are 

generally acid waters. Moreover, there is considerable evidence 

that acidity decreases below the water level. Analyses of two 

samples of water taken from a column of water in the Callaway shaft 

at Ducktown, Tenn., indicate a decrease in acidity of more than 50
90 THE ENRICHMENT OF SULPHIDE OEES. ,

per cent within a vertical distance downward of 37 feet. Some 

analyses from the Capote mine, of Cananea, Mexico, show also a 

neutralization of acid at comparatively shallow depths. At least 

eight other samples, all except one taken at considerable depths, are 

alkaline. The conclusion is warranted, therefore, that acid solutions 

descending through sulphide ores decrease in acidity where oxygen is 

excluded; later they become neutral, and ultimately alkaline. The 

geologic data are completely in harmony with this conclusion, for 

there is abundant evidence that descending acid waters attack alkaline or alkaline-earth silicates and alkaline-earth carbonates; acid 

reacting with feldspars or sericite forms kaolin, and these reactions 

are attended by the solution of alkalies or alkaline earths as sulphates. Where limestone is attacked decrease in acidity may be 

attended by precipitation of gypsum.

There are reversals of these processes, it is true, for, according to 1 

Stokes, the precipitation of chalcocite is attended by the liberation of 

sulphuric acid. This acid, however, would soon be neutralized by 

reactions on silicates and carbonates and could not accumulate permanently. That it is so neutralized is indicated by the common 

association of secondary copper ore with kaolin or gypsum.

CHANGES IN THE STATE OF OXIDATION OF DESCENDING ACID 

SULPHATE SOLUTIONS.

The acidity of descending sulphate waters decreases below a certain depth, and because air is excluded the state of oxidation of the 

solutions likewise decreases. The degree of oxidation of iron, which 

is generally abundant in mine waters, affords a useful index to the 

state of oxidation of the solutions. The waters that pass downward 

from the oxidizing zone carry iron mainly in the ferric state. They 

may carry also some dissolved oxygen, but not much, for, according 

to Winkler, 1 at atmospheric pressure and at 16.87° a liter of water 

can dissolve but 6.84 cubic centimeters of oxygen. Underground 

waters are, moreover, seldom saturated with oxygen.

Dilute acid sulphate, which reacts on certain sulphides of iron, zinc, 

and lead'} will under some conditions release hydrogen sulphide, 

which is available (1) for reduction of the oxygen in the water, (2) for 

reduction of ferric to ferrous sulphate, or (3) for the precipitation of 

copper or other sulphides which may be held in sulphate solution.

The reduction of the oxygen in the water will probably take place 

before the copper sulphide is precipitated, since oxygen tends to 

inhibit the precipitation of copper sulphide. It is probable also that 

the oxygen of the solution is reduced before ferric sulphate is

i Hempel, Walther, Gasanalytische Methoden, 1900, pp. 129-130.
OHEMISTEY OP ENRICHMENT. 91

attacked, although both reactions may go on together. Assuming 

the presence of H2S, we may state the reactions as follows:

(1) H2S-f 0 = H20 + S.

(2) Fe2 (S04) 3 + H2S==2FeS04 + H2S04 + S. 

H2S + CuSO4 = CuS + H2S04 .

If oxygen is present the reaction may go on as indicated by (1) ; if 

ferric sulphate is present, equation (2) would be possible; with neither 

oxygen nor ferric sulphate in the solution, equations (3) are possible.

BATE AT WHICH HYDROGEN SULPHIDE IS GENERATED FROM 

SEVERAL PRIMARY SULPHIDES BY COLD DILUTE ACID SULPHATE WATERS.

As shown by experiments made by R. C. Wells (p. 59), cold 

dilute acid sulphate solutions attack several metallic sulphides and 

generate hydrogen sulphide. With equal surfaces exposed, such 

solutions in these experiments set hydrogen sulphide free at least 

four times as rapidly from zinc blende as from pyrite or chalcopyrite, and about twenty-five times as rapidly from pyrrhotite as from 

zinc blende. With pyrite and chalcopyrite the amount of hydrogen 

sulphide generated is small and the quantities determined may represent the end points of titration, for hydrogen sulphide was not identified as a product of the reaction. Doubtless sphalerite containing 

considerable iron sulphide will react more readily with acid than pure 

zinc sulphide. The quantity of hydrogen sulphide generated with 

galena, sphalerite, and pyrrhotite was sufficiently great, perhaps, to 

give the results a quantitative value.

These experiments were carried on with pure minerals that had 

been carefully examined. Minerals so pure are seldom found in large 

bodies of sulphide ores, where the sulphides generally occur in more 

or less intimate association. Gottschalk and Buehler have recently 

shown that in such mixtures weak batteries are formed and that the 

oxidation and solution of the mineral with the lowest potential will 

be increased while the solution of the mineral which is higher in the 

series will be retarded. There is no reason to suppose that the order 

of solution in an oxidizing environment like that in which the experiments of Gottschalk and Buehler were carried on would correspond 

closely to the order of attack in a reducing environment like that 

which existed under the conditions of Mr. Wells's experiments. Yet 

the associations of the sulphides may affect the rate of their solution 

in the reducing zone also. Consequently the rate of attack of acid 

sulphate solutions on various sulphide ores in the reducing zone can 

not be accurately stated. Possibly each association is a problem in 

itself. Where one of the metallic sulphides greatly predominates,
92 THE ENRICHMENT OF SULPHIDE OEES.

however, it should not be supposed that a high potential would greatly 

retard its attack by acid in the reducing environment which is assumed to exist where the secondary sulphides are precipitated.

The laboratory experiments and geologic observations indicate 

that pyrrhotite is attacked very readily by acidic solutions, at least 

where it constitutes a considerable part of the ore. The reaction 

with sphalerite was less rapid than with pyrrhotite but more rapid 

than with pyrite and chalcopyrite. Here, too, field observations support the conclusions based on the laboratory experiments available. 

Describing the chalcocitization of sphalerite at Morenci, Lindgren 1 

says: "Pyrite apparently does not precipitate Cu2S or CuS while zinc 

blende is present." Weed notes also that sphalerite is present in some 

of the primary ores at Butte, but that it is absent in the enriched 

chalcocite ores.2 The latter, however, contain pyrite. In a recent 

paper by Irving and Bancroft 8 on the deposits of Lake City, Colo., 

the precipitation of secondary minerals on sphalerite from downwardmoving sulphate solutions is emphasized.

The data available seem therefore to indicate that in a large 

number of deposits at least the action of dilute sulphuric acid in the 

absence of air on the following sulphides is probably in the same 

order as that indicated by the experiments of Mr. Wells, viz, (1) 

pyrrhotite, (2) zinc blende, (3) pyrite and chalcopyrite.

COMPOSITION OF THE PRIMARY ORE AS A FACTOR DETERMINING 

THE VERTICAL EXTENT OF THE SECONDARY SULPHIDE ZONE.

S. F. Emmons and others have emphasized the fact that the vertical extent of the secondary sulphide zone depends principally on the 

amount of fracturing of the primary ore body and the size, continuity, and character of the fractures. The course of such fractures 

determines the course of descending waters and the size, character, 

and continuity of open spaces control the rates at which the solutions 

descend.

In their descent the metal-bearing solutions react on the walls of 

the watercourses, and these reactions produce changes of chemical 

equilibria and deposition of certain metals. These changes depend 

not only on the rate at which the solutions descend but also on the 

chemical environment through which they pass. In limestone or in 

calcite gangue the downward migration of copper would be delayed 

at least temporarily by the formation of carbonates (see p. 98) and 

calcite would quickly drive gold from acid solutions in which it was 

held dissolved as chloride.

i Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 183.

> Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U.S. Geol. Survey 

No. 74, 1912, p. 78.

8 Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. 

Qeol. Survey No. 478,1911, pp. 64-65.
CHEMISTRY OF ENRICHMENT. 93

As already stated, dilute acid sulphate waters dissolve some pyrrhotite more rapidly than sphalerite and some sphalerite more rapidly than 

pyrite and chalcopyrite. The action on pyrrhotite and sphalerite is attended by th3 liberation of hydrogen sulphide, which precipitates several of the metals in acid solution. Under similar conditions, because 

the reaction is brought near to completion more quickly, the vertical extent of the zones of secondary ores should be less in primary ores which 

carry abundant pyrrhotite than in ores of pyrite and chalcopyrite 

which contain no pyrrhotite, and although such secondary zones 

might be richer they would not extend so deep.

The attack of acid sulphate solutions on sphalerite is probably less 

vigorous than on pyrrhotite but more vigorous than the action on 

pyrite and chalcopyrite; consequently deposits that carry sphalerite 

should have secondary zones which are not so extensive vertically as 

those of deposits that contain pyrite and chalcopyrite with neither 

sphalerite nor pyrrhotite. Briefly stated, the vertical extent of the 

secondary sulphide zones should vary inversely with the rate at which 

the descending acid sulphate solutions attack the primary sulphides.

The data at present available do not cover a wide range of conditions and combinations, and the relations above indicated may have 

but little quantitative value, because deposits and minerals are not 

equally permeable, postmineral fractures are not equally spaced, 

equal surfaces of the primary minerals are not exposed, all deposits 

Have not been equally eroded, and the temperatures prevailing are 

not the same. But notwithstanding these differences in the physical 

environment and geologic history of the deposits in different mining 

districts, I believe that the secondary sulphide zones of many 

deposits show relationships that depend on the composition of the 

primary ore and that may be rationally explained when inspected in 

the light of these chemical relations.

Principal mineral combinations of three groups of copper deposits.

1

Pyrrhotite ores with pyrite and 

chalcopyrite, with or without 

zinc blende.

Ducktown, Term. 

Gossan lead, Va. and N. C. 

Ely. Vt. 

Santiago de Cuba. 

Encampment, Wyo. 

Grants Pass, Oreg. 

Bingham, Utah (in part). 

Sudbury, Ontario.

2

Pyritic ore with sphalerite and 

chalcopyrite. and with little or 

no pyrrhotite.

Morenci, Ariz. 

Santa Kita, N. Hex. (in part). 

Shasta County, Cal. 

Velardefla, Mexico. 

Jerome, Ariz. 

Cananea, Mexico.

3

Pyritic chalcopyrite ores, with 

little or no pyrrhotite or zinc 

blende.

Butte, Mont, (in part). 

Bisbee, Ariz. 

Globe, Ariz. 

Miami. Ariz. 

Ray, Ariz.

In the foregoing table certain deposits have been so grouped 

that three classes of copper ores may be inspected to ascertain 

whether the secondary chalcocite zones have a greater vertical extent 

in pyrrhotite deposits, in sphalerite deposits, or in copper deposits
94 THE ENRICHMENT OF SULPHIDE ORES.

that contain little or no sphalerite or pyrrhotite. Those in the first 

column contain considerable pyrrhotite; all except one of them are 

known to have comparatively shallow chalcocite zones; in general 

the lower limit of chalcocite is from 50 to 250 feet below the present 

surface. These deposits are said to be comparatively tight and relatively impervious to the downward migration.of mineral waters; 

consequently they do not afford examples ideal for comparison. 

Nevertheless I can find no example of a deposit that carries abundant 

pyrrhotite in which secondary sulphides are shown to have been 

deposited at great depths.

On comparing the deposits of column 1 with those of column 3 

it is clearly apparent that the chalcocite zones in the latter are of 

much greater vertical extent; indeed, they include the deepest 

chalcocite zones that have been developed. In several of the 

deposits of column 3 chalcocite extends to depths ranging from 

1,000 to nearly 1,500 feet below the surface. It is found at still 

greater depths at Butte, Mont., but an increasing trend of opinion 

held by those most familiar with these deposits indicates that the 

chalcocite of the lower levels of the Butte mines is primary. In 

some of the deposits of Arizona the secondary zones are very extensive vertically owing to the great distance to the zone of saturation. 

Undoubtedly some of the deposits of column 3 are much more 

highly fractured than some of those of column 1, consequently these 

examples are likewise not ideal for comparison. Yet the fact that 

none of the deposits with deep chalcocite are known to carry appreciable pyrrhotite appears to be significant.

The deposits of column 2 have chalcocite zones which in general 

are of greater vertical extent than those of column 1 and less than 

those of column 3, but possibly the differences are too slight to 

give the figures much significance. It is a fact familiar to all, however, that rich silver ores frequently give way in depth, to primary 

ores containing abundant sphalerite, the contact zone being at some 

places comparatively narrow.

From the comparison of the several groups of deposits investigated, 

it is concluded that with approximately similar temperature, rainfall, 

erosion, head, permeability, and other conditions, the vertical extent 

of the secondary sulphide zone depends principally on the character 

and extent of fracturing in the primary ore bodies, but the vertical 

distribution of the secondary sulphides shows a relation also to the 

mineral composition of the primary ore. In general, ores containing 

abundant pyrrhotite are not enriched to depths so great as those 

containing pyrite and chalcopyrite but little or no pyrrhotite. . The 

influence of zinc blende and of a small proportion of pyrrhotite can 

not yet be positively stated.
THE ENRICHMENT OF SULPHIDE ORES. 95 

SOME CHEMICAL RELATIONS AND MINERALS OF COPPER. 

PEINCIPAL COPPER MINERALS.

The names and the chemical composition of the principal copper 

minerals are given below:

Copper............................ Cu.

Chalcanthite...................... CuS04 .5H20.

Pisardte...................... .....(Cu,Fe)S04 .7H20.

Brochantite..................... .Cu4S04 (OH)6 orr4CuO.S03 .3H20.

Nantokite.........................CuCl.

Atacamite.........................Cu2Cl(OH)3 or CuCl2 .3Cu(OH)a.

.Malachite........................ .Cu2(OH)2C03 or 2CuO.C02 .H20.

, Azurite..........................Cu3(OH)2 (C03)2 or 3Cu0.2C02 .H20.

Chrysocolla.......................CuSi03 .2H20 or CuO.Si02 .2HaO.

Dioptase..........................CuH2Si04 or CuO.SiOa .H20.

Cuprite..........................Cu20.

Tenorite..........................CuO.

Copper pitch ore. 1

Chalcocite........................Cu2S.

Covellite...........................CuS.

Bornite 2 ........................ .Cu5FeS4 .

Chalcopyrite..................... .CuFeS2 or Cu2S.Fe2S3 .

Enargite.........................Cu3AsS4 or 3Cu2S.As2Ss.

Tetrahedrite..................... .Cu8Sb2S7 or 4Cu2S.Sb2S3 .

Tennantite........................Cu8As2S7 or 4Cu2S.Ae2S3 .

Famatinite........................Cu3SbS4 or 3Cu2S.Sb2S8.

SOLUBILITY OF SOME COPPER COMPOUNDS.3 n

At 20° C. a liter of water dissolves 172 grams 4 CuS04 . At 20° C. a 

liter of water dissolves 435 grams CuCl2 . At 15° to 16° C. a liter 

of water containing 16.66 grams HC1 dissolves 61.59 grams CuCl.

NATURE AND RELATIONS OF THE COPPER MINERALS.

Copper, silver, and gold belong chemically to the same family; 

the three elements occupy exclusively the right side of the second 

column of the table of the periodic system and are somewhat closely 

affiliated. They stand apart as metals in the concentration of which 

the processes of sulphide enrichment are most clearly expressed. 

They are dissolved more or less readily in an oxidizing sulphate or 

chloride environment and are readily precipitated from acid waters 

by reactions in the sulphide environment where oxygen is excluded.

The mineral waters in the oxidizing zones of sulphide deposits are 

acid sulphate and ferric sulphate solutions. In the presence of oxygen such solutions dissolve copper very readily, and in contact with

1 A complex hydrated oxide.

2 Formula as established by B. J. Harrington (Am. Jour. Scl., 4th ser., vol. 16, 1903, p. 151). The 

older, commonly accepted formula is CusFeSs, also written Cu2S.CuS.FeS, or SCusS.FesSj.

8 Seidell, Atherton, Solubilities of inorganic and organic substances, D. Van Nostrand Co., New 

York, 1907.

« For the solubility of hydrated copper sulphate see p. 53.
96 THE ENRICHMENT OF SULPHIDE ORES.

copper compounds such a system will contain also copper sulphate. 

The copper sulphate in solution reacts with carbonates or with acid 

carbonate in solution, precipitating copper carbonate. If chlorides 

are abundant, copper chlorides may form. Cupric chloride is readily 

soluble in water; cuprous chloride oxidizes to form oxychlorides. 

In moist countries both chlorides are unstable. In arid countries 

copper chloride may accumulate as atacamite. The sulphates chalcanthite and brochantite also may be precipitated, and the basic 

sulphate brochantite once formed is stable. The silicates of copper 

are probably formed by copper-bearing solutions reacting on silicic 

acid, which, as shown by analyses, is commonly dissolved in mine 

waters. Native copper, cuprite, and tenorite are formed by the 

reduction or oxidation of various copper compounds. Frequently 

such changes are pseudomorphic after an older mineral. All the 

copper minerals mentioned above are formed in the main in the oxidized or oxidizing zone, and in sulphide ore deposits their occurrence 

below the oxidizing zone is exceptional. None of them are known 

to form in depth by deposition from hot ascending alkaline solutions.

Below the oxidizing zone, where air is excluded, copper is precipitated as sulphides; chalcocite, covellite, bornite, chalcopyrite, and 

possibly some of the more complex antimony and arsenic compounds 

are formed by these processes. Precipitation may be brought about 

by chemical interchange with pyrite, chalcopyrite, pyrrhotite, zinc 

blende, galena, and probably with some other sulphides, the process 

being mainly metasomatic replacement. The copper sulphides are 

precipitated also by hydrogen sulphide, which may be generated by 

attack of acid sulphate solutions on several of these sulphides. At 

ordinary temperatures only 3i51 X 10~* mols of copper sulphide 

dissolves in a liter of water. In the reducing environment the copper sulphides are stable. They are insoluble even in hot concentrated acid sulphate solutions if a slight trace of hydrogen sulphide 

is present.1

Iron sulphide dissolves in acid, however, and it should not be 

supposed that the double sulphides of iron and copper would be precipitated from acid solutions which contained much copper. But 

as the solutions descend they lose acidity, and copper sulphide is 

precipitated at the expense of iron sulphide, the iron going into solution. A decrease in acidity, a decrease in copper, and an increase 

of iron in solution bring about a state of equilibrium which is increasingly favorable to the precipitation of double sulphides, such 

as chalcopyrite and bornite.2

In the oxidizing zone copper is much more.soluble than gold, and, 

unlike gold, it may be dissolved in the absence of chlorides in suli Alien, E. T., oral communication.

swells, R. C., The fractional precipitation of sulphides: Econ. Geology, vol. 5, 1910, pp. 1.2-13.
CHEMICAL EELATIONS AND MINERALS OF COPPEE, 97

phate solutions. Thus many deposits which contain both copper 

and gold show a distinct segregation of gold near the surface, while 

copper ores with subordinate gold are found in depth. The Highland Boy mine, at Bingham, Utah; the United Verde mine, at Jerome, 

Ariz.; and the Mount Morgan mine, in Australia, were operated first as 

gold mines and subsequently developed large bodies of copper-gold 

ores. In two of these mines the gold has probably not been dissolved 

to any great extent; in one, the Mount Morgan mine, the solution of 

gold is clearly indicated. Even where the conditions for the solution of gold are most favorable, however, it is probably precipitated 

mainly in the upper part of the chalcocite zone. It would not 

remain in solutions that contain much ferrous sulphate, and chalcocitization is doubtless attended by the generation of abundant 

ferrous sulphate.

Silver, like copper, dissolves somewhat readily in an acid sulphate 

environment, especially if ferric sulphate is present. Silver sulphide 

is not so soluble as copper sulphide, however (see p. 59), and in the 

presence of chlorides its downward migration is delayed by its precipitation as cerargyrite, or in the presence of ferrous sulphate or in 

contact with several sulphides and gangue minerals native silver is 

deposited. Some of the great copper lodes of Butte, Mont., were 

worked for silver to depths from 200 to 400 feet below the surface, 

where the deposits changed to rich copper ore. 1 Like gold, silver 

would be driven from sulphate solutions in an environment where 

chalcocite forms. Its sulphide is even less soluble than copper 

sulphide and in depth would be precipitated as argentite. The rich 

narrow chalcocite zones at Ducktown, Tenn., contained considerable 

silver, although the primary pyrrhotite ore is only slightly argentiferous. In the north vein, Mass II, at Rio Tinto, Spain, 2 both gold 

and silver were concentrated in a thin layer just above the zone of 

copper and iron sulphides.

As already stated, both gold and silver are precipitated from 

dilute solutions by ferrous sulphate, but copper sulphate and ferrous 

sulphate may exist hi the same solution without precipitation of 

copper. It is reasonable to suppose that native silver as well as 

gold may be precipitated with chalcocite, but it is not so likely that 

under these conditions native copper and chalcocite would be precipitated simultaneously.

The rate at which certain sulphides react with acid to generate 

hydrogen sulphide and the tendency of certain ores to delay the 

downward migration of copper has already been discussed (p. 59).

1 Emmons, S. F., The secondary enrichment of ore deposits, in Po§epny, Franz, The genesis of ore 

deposits, 1902, pp. 442-443.

2 Vogt, J. H. L., Problenis in the geology of ore deposits, in PoSepny, Franz, op. cit.^, p. 676.

72293° Bull. 529 13-^ 7
98 THE ENRICHMENT OF SULPHIDE OEES.

The precipitation of gold and silver, if any were held in the copper 

sulphate solution, would be even more rapid.

Carbonates react with solutions of acid sulphate even more rapidly than pyrrhotite and likewise tend to delay the downward migration of copper, gold, and silver. If there is much lime carbonate in 

the gangue of the ore or in the wall rock, the downward migration of metallic sulphates may be checked or even inhibited. As 

stated by Bard, 1 chalcocitization is seldom extensive in carbonate 

rocks, because the copper is precipitated as Carbonate by reaction 

with calcite. To this there are some exceptions, and, appreciating 

these, Bard notes that the precipitation of some copper carbonate 

on the limestone may inhibit further action and insulate the passages 

from reaction with the descending solutions. Under such conditions copper sulphate could descend through carbonate rocks to considerable depths, where it could be precipitated by iron sulphide or 

by hydrogen sulphide if any were generated by the action of acid 

on sulphides or as copper carbonate by reactions with limestone.

In some districts, however, the primary mineralization of limestone is attended by extensive silicification. If carbonate has been 

removed, it would of course be no longer effective. The silicification of limestone at Bisbee, Ariz., has doubtless removed much of 

the carbonate in the region of the ore bodies.

A gangue of siderite, dolomite, or other carbonates would react 

with acid sulphate solutions and tend to neutralize them and to 

precipitate gold, silver, or copper, but the reaction would be somewhat less rapid than with calcite.

OCCURRENCE OF THE COPPER MINERALS.

Native copper occurs in the Lake Superior district and in zeolitic 

lodes elsewhere as the principal primary copper mineral. In the 

sulphide ores of the Cordillera of North America native copper is 

confined to the upper zone of the ore bodies and is clearly secondary. 

It is formed in many places by the reduction of cuprite and is frequently found, with copper oxides, directly above the zone of 

secondary chalcocite ores. The following reactions have been suggested but not proved :

Cu20 + 2FeS04 + H2S04 = 2Cu + Fe2 (S04 ) 3 + H20.

At Cananea, Mexico, at Cashin, Colo., and at many other places 

tabular masses of native copper cut the decomposed oxidized ore, 

suggesting the possibility that it has altered directly from chalcocite. 

At Morenci, Ariz., it is associated with cuprite, as a rule in the upper

iBard, D. C., Absence of secondary copper . sulphide enrichment in calcite gangues: Econ. Geology, 

vol. 5, 1910, p. 69.
CHEMICAL EELATIONS AND MINERALS OF COPPEB. 99

limit of the chalcocite zone. In the Williams vein, Arizona Central 

mine, in this district,1 200 feet below the surface, a vein of solid copper was found in sericitized porphyry. The vein formed a sheet of 

copper, in places 8 inches thick, standing nearly vertical. It had in 

places a fibrous structure, perpendicular to the plane of the vein, 

such as occasionally is exhibited by the chalcocite seams, of which 

it is believed to be a pseudomorphic development. In one specimen, 

according to Lindgren, two sheets of copper were found separated 

by sooty chalcocite. In view of these relations it appears that 

copper may form directly from chalcocite without the intermediary 

stage of oxidation to cuprite. Lindgren suggests this reaction:

Cu2S + 3Fe2 (S04 ) 3 + 4H2O = 2Cu + 6FeS04 + 4H2S04 .

Native copper was abundant above the third level of the Copper 

Queen ore body at Bisbee, Ariz., but comparatively rare in the lower 

portions of the oxidized ores.2 In the Calumet & Arizona mine, 

however, it is abundant on the 1,050-foot level. It occurs in part 

as incrustations of chalcocite. At Bingham, Utah,3 it is rare but 

has been observed. Native copper is extensively developed in the 

upper zones of copper deposits at Santa Rita, N. Mex. In general 

it is abundant from 100 to 200 feet below the surface. According 

to Lindgren, Graton, and Gordon,4 it is probably an alteration product 

of sulphides, but the latter appear to have been deposited in part in 

open spaces in the porphyry rather than as replacements of pyrite. 

In this district and also in the copper deposits of Lake Superior native 

copper has been found as perfect pseudomorphs after hornblende. 

According to Yeates,5 native copper in Grant County, N. Mex., is 

pseudomorphous after azurite. Native copper is not abundant at 

Butte, Mont.6

In view of the fact that ferrous sulphate, a reducing agent, probably accumulates at depths at which chalcocite forms, one would 

suppose that native copper, which is a product of reduction, might 

be deposited under some conditions along with chalcocite; this association is not uncommon, but may be due to subsequent oxidation 

and partial reduction of chalcocite to native copper. At Ducktown, 

Tenn., according to report, When mines that had been closed during

i Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 101.

' Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21,1904, p. 120.

aBoutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38, 1905, p. 110.

< Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68,1910, p. 316.

»Yeates, W. S., Pseudomorphs of native copper after azurite, from Grant County, N. Mex.: Am. Jour. 

Sci., 3d ser., vol. 38,1889, pp. 405-407.

6 Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 80.
100 THE ENRICHMENT OF SULPHIDE ORES.

the Civil War were reopened masses of native copper were found 

hanging to some of the timbers.1

CJialcanthite, blue vitriol, CuS04 .5H20, is frequently present in the 

oxidized zones as efflorescences or stalactites on open fissures, or as 

veinlets filling small crevices above the upper limit of the zone of 

secondary sulphides. At Ducktown, Tenn., it is abundant also in 

the chalcocite zone. It rarely forms far below the top of the chalcocite zone, except on surfaces of openings made by mining operations. Locally it is an ore of some importance. At Butte, Mont., 

according to Weed,2 it coats rock fragments in old stopes, in places 

so abundantly that it has been found profitable to collect it from 

time to time. In the Silverbow mine pure chalcanthite is so abundant as to almost fill some of the old drifts. Beautiful stalactites 

and shapes imitating organ pipes are developed. Weed states 

that copper sulphate is carried by the moisture of the atmosphere 

circulating in mine openings.3

Pisanite, (CuFe)S04 .7H20, is a rare sulphate of copper and iron. 

Large and beautiful specimens were found in copper deposits at 

Bingham, Utah.4 At Butte, Mont., stalactites occur, usually in 

deserted drifts.5

Brochantite, H6Cu4S010 , a basic sulphate of copper, has been 

identified at only a few places. It is of common occurrence in the 

ores of the Clifton-Morenci district, Arizona, where, according to Lindgren, 6 it is intergrown with malachite, which effectively masks its 

presence in hand specimens. In the Shannon mine, near the surface, 

in porphyry, it constitutes a rich ore body. Ransome noted it also 

in sections from the oxidized zone at Bisbee, Ariz. In all its occurrences it is probably a deposit from oxidizing waters.

Sullivan 7 obtained a substance of the composition of brochantite 

by treating silicates with copper sulphate solutions. Lindgren 8 

states that brochantite may form directly by the oxidation of chalcocite according to the following reaction:

2Cu2S + 100 + 4H20 = H6Cu4S010 + H2S04 .

Atacamite and nantokite. Atacamite, CuCl2 .3Cu(OH) 2 , the oxychloride of copper, and nantokite, CuCl, cuprous chloride, are com1 Edwards, W. F., Discussion of papers by H. A. Lee, on " Gases in metalliferous mines": Proc. Colorado 

Sci. Soc., vol. 7,1904, p. 183.

2 Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 81.

3 Idem, p. 99.

4 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 111.

6 Weed, W. H., op. cit, p. 82.

«Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 119.

* Sullivan, E. C., The interaction between minerals and water solutions: Bull. U. S. Geol. Survey No. 

312,1907, p. 61.

s Op. cit, p. 188.
CHEMICAL RELATIONS AND MINEBALS OP COPPEE. 101

paratively rare secondary copper minerals. They are probably 

formed in the oxidized zones, but on account of their solubility in 

natural waters they do not generally accumulate.

Since cuprous chloride is comparatively less soluble in water, it 

would be supposed that nantokite would be fixed more readily than 

the cupric salt atacamite. On exposure to the air, however, the 

cuprous chloride is oxidized to form atacamite. Such is said to be the 

genesis of some atacamite of Chile. In the United States the natural 

chlorides of copper are almost unknown.

Malachite, (CuOH) 2C03 , a basic cupric carbonate, is abundant in 

the oxidized zones of many cupriferous deposits and is most abundant 

in deposits that are inclosed in limestone. It was present in considerable quantity in the superficial portions of copper deposits at 

Bingham, Utah, 1 where it occurs as globular masses and also as 

bands equivalent to parts of certain rock strata. It also forms envelopes about sulphides and oxides. In the Old Dominion mine at 

Globe, Ariz., according to Ransome,2 malachite is conspicuously 

developed as veinlets in quartzite and has locally replaced the 

quartzite. Malachite is present almost universally in the oxidized 

zones of sulphide deposits and in some of them it persists in the outcrop. Its association with limonite in the gossan has encouraged 

deep development in many copper districts. It is unknown as a 

primary ore in deposits formed by thermal waters.

If copper sulphate waters should mingle with acid carbonate 

waters the following reaction 3 would probably take place:

2CuS04 + 2H2Ca(C03 ) 2 = CuC03 .Cu(OH) 2 + 2CaSO4 + 3C02 + H20.

Acid waters attacking limestone would form malachite according 

to the following reaction:

2CuS04 + 2CaC03 + 5H2O = CuC03 .Cu (OH) 2 + 2CaS04 .2H2O+C02 .

Solutions of iron sulphate and copper sulphate react with limestone 

and deposit limonite and malachite simultaneously. 4

Azurite, 2CuC03 . Cu(OH) 2 , like malachite, is a basic cupric carbonate. 

So far as known, it is not formed by ascending thermal waters. It 

is very much less abundant than malachite, but in some deposits in 

limestone, as at Bisbee, Ariz., it is plentiful. Kemp 3 suggests the 

following reaction with copper sulphate and acid carbonate solutions:

3CuSO4 + 3H2Ca(C03 )2 = 2CuC03 .Cu(OH) 2 + 3CaSO4 + 4C02 + 2H2 0.

» Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 107.

«Ransome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. S. Geol. Survey No. 12, 

1903, p. 122.

s Kemp, J. F., Secondary enrichment in ore deposits of copper: Econ. Geology, vol. 1,1906, p. 24.

* Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, pp. 189-190.
102 THE ENRICHMENT OF SULPHIDE OEES.

At Morenci it is frequently associated with kaolin 1 and is generally 

one of the last minerals to form.

In limestone azurite may be formed according to the following 

reaction:

3CuSO4 + 3CaC03 + 7H20 = 2CuC03 .Cu(OH)2 + 3CaS04 .2H2 0 + C02 .

CJinjsocolla, the bluish-green hydrous copper silicate CuOH4Si04 , 

forms rather abundantly in the outcrops and near the surface of 

some copper deposits; in others it is rare or absent. It is a common 

mineral also in the oxidized zone of some silver and gold mines. 

Although widely distributed in some lodes, it occurs at most places 

only in small bodies. It is frequently associated with malachite and 

azurite and is not known as a deposit of ascending hot waters. At 

Globe, Ariz.,2 chrysocolla replaces dacitic tuff. In this district it is 

older than malachite where the two minerals are together and it 

forms at greater depth than malachite. It is an ore mineral in several 

other deposits of the Southwest, where it is mined with copper oxides 

and carbonates. At Bisbee it seems never to have been abundant, 

but in the Calumet & Arizona mine it forms thin shells about 

kernels of cuprite, native copper, and brochantite and is in turn 

enveloped by malachite and calcite.3 At Butte, Mont.,4 it is found 

in the country rock near the veins. At Cripple Creek 5 chrysocolla 

is an oxidation product of tetrahedrite.

There is no record of the synthesis of chrysocolla in the laboratory. 

Kemp 6 has suggested that it results from gelatinizing silica and 

solutions of copper sulphate:

CuS04 + H2Ca (C03 ) 2 + H4Si04 = CuOH4Si04 + CaS04 + H2O + 2C02 .

It appears that the abundant carbonate is not uniformly a necessary 

condition for its genesis, since in the Morenci district, Arizona,7 it is 

more abundant in the ores in porphyry and in granite than in the 

ores in limestone. In some deposits in porphyry it is of commercial 

value. An interesting occurrence of highly auriferous chrysocolla is 

that of the Original Bullfrog mine in Nevada. If in the reaction 

suggested by Kemp ferrous acid carbonate were present in place of

1 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper 

U. S. Geol. Survey No. 43,1905, p. 118.

2 Ransome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. S; Geol. Survey No. 

12,1903, p. 123.

3 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21, 1904, p. 126.

* Weed, W. H., Emmons, S. F., and Tower, G. W., Butte special folio (No. 38), Geol. Atlas U. S., U. S 

Geol. Survey, 1897, p. 6.

«Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54, 1906, p. 129.

s Kemp, 3. F., Secondary enrichment in ore deposits of copper: Econ. Geology, vol. 1, 1906, pp. 24^25.

' Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper 

U. S. Geol. Survey No. 43, 1905, p. 112.
CHEMICAL RELATIONS AND MINERALS OF COPPER. 103

that of lime, ferrous sulphate would form instead of calcium sulphate. 

Minute quantities of ferrous sulphate drive gold from solutions in 

which it is dissolved as chloride. Possibly the free gold of chrysocolla 

was precipitated by ferrous sulphate.

Dioptase, H2CuSi04 , orH2O.CuO.Si02 , like chrysocolla, is a secondary 

copper silicate. It is characteristically formed in the zone of oxidation and is unknown as a deposit from hot ascending waters. It is 

known in only a few places in the United States, among them 

Morenci, Ariz., 1 where it is associated with chrysocolla in limestone.

Cuprite, Cu2O, is a common mineral of the oxidized zones of deposits 

of copper sulphides and is probably secondary in all its occurrences. It is an important ore mineral at Bisbee, Ariz., where, 

according to Ransome,2 it is found in an impure earthy condition 

mixed with limonite and ferruginous clays or in crystalline masses 

associated with *riative copper. The latter occurrence is particularly characteristic of the deeper oxidized zones in the vicinity of 

chalcocite and other sulphides. In the Calumet & Arizona mine 

cuprite is found in large crystalline masses. On the 950-foot level 

it occurs in bunches in earthy ore, penetrated by dendritic masses 

of metallic copper and spotted with little vugs of acicular malachite. It is abundant at Morenci, Ariz., at Cananea, Sonora, and 

in other western districts. At Morenci, according to Lindgren,3 

it occurs normally at the upper limit of the chalcocite zone as a 

product of decomposition of chalcocite. Similar relations are 

shown at Cananea. At Santa. Rita, N. Mex., abundant cuprite and 

native copper fill fissures in the oxidized zone and presumably are 

secondary in the main after chalcocite. At Butte, Mont., where it 

is sparingly developed and is not important commercially, 4 it generally contains masses of native copper, the line of contact between 

them being indistinct. 5 According to Boutwell 6 cuprite is not 

abundant at Bingham, Utah, but small grains covered with malachite were noted in brown altered limestone of the Commercial mine. 

Cuprite is present in the upper portion of the rich secondary ore at 

Ducktown, Tenn., and in other copper deposits of the southern 

Appalachians. The mode of its derivation from chalcocite is stated 

by Lindgren as follows :7

» Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper 

U. S. Geol. Survey No. 43, 1905, p. 111.

Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21, 1904, p. 128.

8 Lindgren, Waldemar, op. cit., p. 106.

« Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U.S. Geol. Survey 

No. 74, 1912, p. 80.

6 Weed, W. H., Emmons, S. F., and Tower, G. W., Butte special folio (No. 38), Geol. Atlas U. S., U. S. 

Geol. Survey, 1897, p. 6.

8 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38, 1905, p. 109.

' Lindgreu, Waldemar, op. dt, p. 186.
104 THE ENRICHMENT OF SULPHIDE OEES.

Lindgren considers it improbable that ferrous sulphate reduces 

cuprc sulphate to precipitate cuprous oxide, since cupric sulphate 

and ferrous sulphate appear to mix in all proportions without reaction.

Tenorite, CuO, the crystalline form of the black oxide of copper, 

is much less abundant than cuprite. The earthy, sooty variety, 

known as melaconite, is said to be present at Butte, Mont., where it 

is associated with cuprite and native copper and forms dark rims 

about cuprite. 1 According to Boutwell 2 scales of black copper 

oxide with metallic luster occur with chalcocite in ores of Bingham, 

Utah. At Bisbee 3 melaconite is found in soft clayey ores on the 

1,000-foot level of the Lowell mine. Apparently it is now being 

deposited along with wad near the 850-foot level of the Calumet & 

Arizona mine. At Ducktown, Tenn., much of the so-called black 

copper ore has proved to be sooty chalcocite.

Copper- pitch ore is a secondary material of complex character and 

somewhat uncertain composition. A sample from the Detroit mine 

in the Morenci district, Arizona,4 analyzed by Hillebrand, showed 

oxides of copper, zinc, and manganese, with considerable water and 

silica. Material that is probably of a similar nature is found at 

Bisbee and at Courtland, Ariz., and on the Ida, Montgomery, and 

Amazon claims 5 at Butte, Mont.

Chalcocite, Cu2S, copper glance, is economically the most important copper mineral. In 1906, according to Graton, 6 47 per cent of 

the copper produced in the United States was derived from chalcocite 

ores. In most of its occurrences chalcocite is clearly of secondary 

origin, for it replaces other minerals metasomatically or occurs as 

veinlets in small cracks in the primary ore. Well-authenticated 

examples are known of its replacement of pyrite, zinc blende, and 

galena. In many deposits now worked in which chalcocite is the 

principal mineral exploration extends into the lower-grade primary 

sulphide ore, which generally consists of chalcopyrite, pyrite, and 

other minerals without chalcocite.

Examples indicating replacement of pyrite by chalcocite are 

numerous. At Morenci, Ariz., according to Lindgren,7 most of the 

ore bodies owe their origin to this process. Lindgren writes:

By chalcocitization the massive veins of pyrite become transformed into solid 

masses of black dull chalcocite, while the sericitized porphyry becomes filled with

1 Weed, W. H., Emmons, S. F., and Tower, G. W., Butte special folio (No. 38), Geol. Atlas U. S., U. 8. 

Geol. Survey, 1897, p. 6.

2 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 109.

3 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21,1904, p. 128.

* Lindgren, Waldemar, The. copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 114.

6 Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U.S. Geol. Survey 

No. 74,1912, p. 81.

6 Graton, L. C., Copper: Mineral Resources U. S. for 1906, U. S. Geol. Survey, 1807, p. 410.

' Lindgren, Waldemar, op. cit, p. 185.
CHEMICAL RELATIONS AND MINERALS OF COPPER. 105

grains and little seams of the same mineral. The first constitutes high-grade ores, 

the second the low-grade. A residue of pyrite, not yet acted upon, is nearly always 

present. Thus a vein of massive sooty material cutting the shale in the Montezuma 

mine contained 96 per cent Cu2S and 2.4 per cent FeS2) and pyrite may be easily 

seen in nearly every specimen of low-grade porphyry ore. The pyrite first becomes 

coated with a black stain; in a more advanced stage the chalcocite penetrates the 

pyrite on cracks and fissures in all directions; finally it almost entirely replaces 

it. * * * Kaolin gouge always accompanies chalcocite when occurring as massive veins; in the chalcocitized porphyry the process is nearly always attended by 

a formation of a little brownish kaolin, together with inicrocrystalline quartz or more 

often chalcedony; these minerals encircle the pyrite or traverse the chalcocite as 

minute veinlets, distinctly later than the alteration of the porphyry to sericite and 

pyrite. Sericite does not form during chalcocitization; on the contrary, the kaolin 

is formed at the expense of that mineral. * * * The transition from chalcocite 

to unaltered pyrite at the lower limit of the zone is remarkably sudden. Usually a 

change takes place within 25 feet, sometimes within 10 feet, from rich chalcocite ore 

to pyrite with one-half to 1 per cent copper. Chalcopyrite is rarely found as a secondary mineral but has been observed as small grains inclosed in chalcocite veinlets.

At Bingham, Utah, according to Boutwell, 1 chalcocite incloses 

pyrite and chalcopyrite. At the Snowstorm and Park mines, in the 

Coeur d'Alene district, Idaho, chalcocite and bornite ore are disseminated in quartzite. At the Park mine chalcocite has formed on pyrite 

and chalcopyrite.2 In the Bisbee district, according to Ransome, chalcocite occurs as veinlets in and envelopes around grains of pyrite. 

Limestone altered to white claylike material is streaked with veinlets 

of chalcocite and speckled with native copper. Some solid, compact 

chalcocite is associated with bornite. 3 Examples of replacement of 

pyrite by chalcocite could be multiplied to include occurrences from 

nearly every important district of copper sulphide ores in the United 

States.

In the copper deposits of Butte, Mont., which are perhaps the 

largest in the world, chalcocite is an important ore mineral. The 

bulk of the ore is altered "granite" with disseminated grains and 

veinlets of chalcocite intergrown with pyrite or replacing that mineral 

completely. Distinct crystals are rare. In this district the history 

of the vein formation is exceedingly involved, and at least three 

periods of mineralization are recognized. The great chalcocite 

bonanzas in the upper levels at Butte were regarded by S. F. Emmons, W. H. Weed, H. V. Winchell, and others as secondary, and no 

other interpretation appears to be warranted by their occurrence and 

relations. More recently, however, masses of ore containing chalcocite have been found as deep as 2,800 feet below the surface, and Reno

1 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38, 1905, pp. 221-222.

2 Ransome, F. L., and Calkins, F. C., The geology and ore deposits of the Coeur d'Alene district, Idaho. 

Prof. Paper U. S. Geol. Survey No. 62, 1908, p. 91.

«JJansonie, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21,1904, pp. 123-124.
106 THE ENRICHMENT OF SULPHIDE OEES.

Sales l is of the opinion that a portion of the massive deep-seated 

chalcocite at Butte is of primary origin. The studies of polished 

surfaces examined microscopically by Simpson 2 of ore from the 

2,000-foot level showed massive pyrite surrounding, in the plane of 

the section, small and apparently isolated flakes of chalcocite. A 

veinlet of ore from the 1,000-foot level of the Leonard mine shows 

isolated idiomorphic crystals of quartz and pyrite surrounded by 

chalcocite. 3 It is not easy to account for secondary chalcocite in 

such surroundings.

L. C. Graton noted that chalcocite is found as deep as 970 feet in 

the Bully Hill mine, Shasta County, Cal., where it seems to be below 

the zone of secondary sulphide enrichment. He says that "there is 

no reason to believe that it is secondary." 4

According to Lewis,5 chalcocite is the original and essential ore of 

the Rocky Hill mine, in New Jersey. Likewise the chalcocite in the 

region of Christiania, Norway, is regarded by V. M. Goldschmidt 6 as 

in part a primary mineral.

The copper lodes of the Virgilina district, in Virginia and North 

Carolina, described by Laney,7 are of peculiar interest in this connection. The country is an area of schists and gneisses intruded 

by granite and diabase. The deposits are fissure veins. The level 

of ground water is 50 to 75 feet below the surface and the zone of 

secondary alteration does not appear to extend below 250 feet. The 

important mines of the district are 350 to 500 feet deep and the ore 

from the deepest levels contains almost as much chalcocite as bornite.8 

The ores in the deeper levels show little secondary fracturing, and 

from this Graton 9 concluded that the chalcocite is in part of primary 

origin. Laney subsequently worked out the paragenesis of these 

ores by a microscopic study of polished surfaces. Some of his sections 10 show a crystallographic intergrowth of bornite and chalcocite in relations that suggest the intergrowth of quartz and feldspar 

in a graphic granite. It is clear that these two minerals in this ore 

are contemporaneous, and since the specimens were obtained from 

considerable depths in tight lodes it can not reasonably be supposed

1 Sales, R. H., Discussion of paper by F. L. Ransome, on " Criteria of downward sulphide enrichment": 

Econ. Geology, vol. 5,1910, p. 681.

2 Simpson, J. F., The relation of copper to pyrite in the lean copper ores of Butte, Mont.: Econ. Geology, 

vol. 3,1908, p. 634.

a Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 76. For Weed's final conclusions see this bulletin, p. 176.

< Graton, L. C., The occurrence of copper in Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430, 

1910, pp. 104-105.

5 Lewis, J. V., Copper deposits of the New Jersey Triassic: Econ. Geology, vol. 2,1907, p. 247.

6 Goldschmidt, V. M., Die Kontaktmetamorphose im Kristianiagebiet, 1911, p. 250.

7 Laney, F. B., The relation of bornite and chalcocite in the copper ores of the Virgilina district of North 

Carolina and Virginia: Econ. Geology, vol. 6, 1911, pp. 399-411.

8 Idem, p. 399.

 Graton, L. C., Copper: Mineral Resources U. S. for 1907, pt. 1, U. S. Geol. Survey, 1908, p. 620. 

10 Especially that figured in his PI. VII, fig. 2, p. 406.
CHEMICAL RELATIONS AND MINERALS OF COPPER. 107

that the minerals were deposited by cold sulphate waters. Laney 1 

concludes that 

Chalcocite is clearly of two periods one confined to the upper portions of the vein 

more recent than and filling a network of minute factures in the bornite; the other 

contemporaneous and intergrown, often. crystallographically, with it. There is no 

evidence that any of the bornite is of secondary origin. It is therefore believed that 

in the Virgilina district the greater part of the chalcocite is a primary mineral contemporaneous with the bornite and in no way derived from it or from any other copperbearing minerals by processes of secondary alteration.

It appears possible from these relations that under some conditions chalcocite is deposited as a primary mineral by ascending hot 

waters. The intergrowth of the copper sulphide in graphic-like 

pattern is a feature that is difficult to explain on the hypothesis of 

an origin from descending sulphate waters. Ascending alkaline hot 

solutions are generally supposed to have deposited the primary ores 

of nearly all the larger copper deposits. Such solutions are doubtless variable in composition. It appears probable .that under conditions where the concentration of iron is low in the solutions chalcocite 

may form as a primary mineral instead of chalcopyrite, which is the 

commoner primary ore.

The chalcocite deposits in sandstone and shale which are very 

widely distributed in the Southwest should be mentioned here. 

Many of these deposits are in areas remote from igneous rocks and 

appear not to be related genetically to igneous processes. They 

have doubtless been deposited by cold waters and some of them 

have replaced coal or other organic material. In New Mexico,2 in 

Colorado,3 and elsewhere, such deposits have formed where there is 

no evidence that a sulphide existed previously. In the "Red Beds" 

of Oklahoma, 10 miles northeast of Stillwater, in Payne County, 

according to W. A. Tarr,4 veinlets of chalcocite are found cutting 

carbonaceous material. The chalcocite and bornite deposits of the 

Bonanza mine of the Chitina copper belt, Alaska,5 probably have a 

similar genesis. Barite is present in several of these ore bodies, 

suggesting the agency of sulphate solutions. Although these deposits are not secondary in the sense that they have formed at the 

expense of older sulphides, the conditions under which they were 

formed as regards temperature, pressure, and concentration of solu1 Laney, F. B., The relation of bornite and chalcocite in the copper ores of the Virgilina district of North 

Carolina and Virginia: Econ. Geology, vol. 6, 1911, p. 411.

2 Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68,1910, pp. 7&-77.

3 Emmons, S. F., Copper in the red beds of the Colorado Plateau region: Bull. U. S. Geol. Survey No. 

260,1905, pp. 221-232. Emmons., W. H., The Cashin mine, Montrose County, Colo.: Bull. U. S. Geol. Survey No. 285,1906, pp. 125-128. Lindgren, Waldemar, Notes on copper deposits in Chaffee, Fremont, and 

Jefierson counties, Colo.: Bull. U. S. Geol. Survey No. 340,1908, pp. 157-174.

*Tarr, W. A., Copper in the "Red Beds" of Oklahoma: Econ. Geology, vol. 5,1910, p. 223.

* Moflit, F. H., and Maddren, A. G., Mineral resources of the Kotsina-Chitina region, Alaska: Bull. 

U. S. Geol. Survey No. 374,1909, pp. 80-86.
108 THE ENRICHMENT OF SULPHIDE ORES.

tion are probably near those which prevail in processes of sulphide 

enrichment.

The conditions which are assumed to exist where secondary chalcocite 

is precipitated have already been stated. The solutions contain copper 

sulphate, iron sulphate, and probably sulphuric acid. Without much 

doubt the iron is mainly in the ferrous state. Stokes's equations 1 

indicate that ferrous salt is liberated in the reaction. Winchell 2 

states that hematite does not form where solutions are acid, as at 

Butte. According to Ransome 3 the formation of chalcocite at Bisbee 

was effected without the development of ferric oxide as a by-product. 

At Globe Ransome did not detect hematite or ferric hydrate in microscopic intergrowths of pyrite-chalcocite ores. Lindgren emphasizes the presence of ferrous sulphate as an attendant of processes 

of chalcocitization at Morenci,4 and this is suggested by analyses of 

waters from copper deposits of Ely, Nev. One of the deeper waters 

collected by Lawson carried 9.36 grains per gallon of ferrous sulphate 

and 0.4 grain per gallon of ferric sulphate.5

On the other hand, Kemp shows that iron oxide has formed between covellite and chalcocite in secondary ore of Copper Mountain, 

British Columbia,6 which suggests the possibility of hydrolyzation 

of iron sulphate a reaction which in acid solution takes place only 

with the ferric salts.

Experiments in the precipitation of chalcocite have already been 

reviewed. None of these experiments were carried out under exactly 

the conditions which probably exist in the secondary sulphide zone. 

Stokes's experiments (p. 133) were made at a high temperature and 

ferrous sulphate was not introduced. In Winchell's experiments 

the chalcocite was formed in the presence of sulphur dioxide, which 

appears to occur very sparingly, if at all, in waters from most copper 

mines, though lately Spencer7 has indicated the possibility that 

sulphur dioxide may be formed by the attack of cupric sulphate 

on pyrite. Ferrous sulphate may accomplish a similar reduction, 

but in experiments made by Mr. F. F. Grout and myself at the 

University of Minnesota to test this reaction, no chalcocite has yet 

formed with pyrite, at the end of one year.

In the synthesis of chalcocite with pyrite, Stokes found that 

covellite was formed but subsequently changed to cuprous sulphide.

1 Stokes, H. N., Experiments on the action of various solutions on pyrite and marcasite: Econ. 

Geology, vol. 2,1907, p. 22; See also Lindgren, Waldemar, The copper deposits of the Clifton-Morenci 

district, Arizona: Prof. Paper U. S. Geol. Survey No. 43, 1905, p. 183; Wells, R. C., The fractional precipitation of sulphides: Econ. Geology, vol. 5,1910, p. 1.

2 Winchell, H. V., The synthesis of chalcocite and its genesis at Butte, Mont.: Eng. and Min. Jour., vol. 

75,1903, pp. 783-784.

3 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21,1904, p. 157.

« Lindgren, Waldemar, op. clt, p. 186.

6 Lawson, A. C., The copper deposits of the Robinson mining district, Nevada: Bull. Dept. Geology 

Univ. California, vol. 4,1906, p. 333.

 Kemp, J. F., Secondary enrichment in ore deposits of copper: Econ. Geology, vol. 1,1906, p. 16.

'Spencer, A. C., Chalcocite deposition: Jour. Washington Acad. Sci., vol. 3,1913, p. 73.
CHEMICAL RELATIONS AND MINEKALS OF COPPEK. 109

(See p. 133.) There are reasons for supposing that this takes place 

under conditions of sulphide enrichment, the cupric sulphide changing to cuprous sulphide in the presence of cupric sulphate and ferrous 

sulphate: The cuprous ion has been looked for in vain in at least 

ten of the analyses of mine waters. On the other hand, Wells has 

shown that the cuprous ion probably does exist in exceedingly small 

concentration in solutions of cupric and ferrous sulphate and the 

direct precipitation of chalcocite is by no means unlikely. More data 

on this synthesis are needed, especially some on the synthesis in an 

alkaline environment.

Covdlite, CuS, is found in small amounts in many mining districts 

of North America but is not abundant in many of the larger deposits. 

As a rule it is associated with chalcocite, and it forms chiefly as a 

replacement of iron or zinc sulphides. Weed 1 estimates that covellite has formed 0.5 per cent of the ore mined at Butte. In this region 

it is secondary and forms in part as an alteration product of chalcocite. It replaces zinc sulphide at Morenci, Ariz.,2 in the San Francisco district, Utah,3 at the Poole mine in the foothill copper belt of 

California,4 and elsewhere. At Lake City, Colo.,5 according to Irving 

and Bancroft, small masses of blue sulphide, on being broken into 

minute cleavage blocks, were found to be sphalerite thinly coated 

along the numerous cleavage surfaces with covellite films.

In the Last Chance mine of the Coaur d'Alene district, Idaho, 

lumps of covellite contain minute specks of chalcopyrite, from which 

the covellite has been formed as an alteration product.8 It is abundant in the disseminated copper ores at Bingham, Utah, according 

to B. S. Butler,7 and was noted by Boutwell 8 coating chalcopyrite in 

the Northern Light mine. It forms from chalcopyrite and pyrite in 

the Rambler mine, Encampment, Wyo., and in the Eureka pit at 

Copper Flat, Nev.9 At Ducktown, Term., it covers pyrrhotite. 

According to Lindgren covellite is found coating pyrite in sericitized 

porphyry at Morenci, Ariz.,10 and it replaces pyrite also in. the Nevada 

Douglas mine, near Yerington, Nev.

1 Weed. W. E., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, pp. 72, 76.

2 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 183.

3 Butler, B. S., oral communication.

4 Rogers, A. F., A new synthesis and new occurrences of covellite: School of Mines Quart., Columbia 

Univ., 1911, p. 300.

«Irving, J. I)., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. 

Geol. Survey No. 478,1911, p. 64.

e Ransome, F. L., and Calkins, F. C., The geology and ore deposits of the Cceur d'Alene district, Idaho: 

Prof. Paper U. S. Geol. Survey No. 62,1908, p. 92.

7 Oral communication.

8 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 223.

9 Rogers, A. F., A new synthesis and new occurrences of covellite: School of Mines Quart., vol. 32, 

1911, p. 302. 

"Lindgren, Waldemar, op. cit., p. 186.
110 THE BNBICHMENT OF SULPHIDE OEES.

Since the precipitation of cupric sulphide from, cupric sulphate 

solutions involves no change of valence, some very simple equations 

may be written:

ZnS + CuS04 = CuS + ZnS04 .

CuFeS2 -f CuS04 = 2CuS + FeS04.

FeS + CuS04 = CuS + FeS04 .

H2S+CuS04 =CuS+H2S04 . 

With pyrite the reaction is probably more involved.

Like chalcocite, covellite is formed in a reducing environment,' 

doubtless in the presence of ferrous sulphate, but some ferric salt may 

be present also. A paragenesis noted by Kemp at Copper Mountain, 

British Columbia, and described by Catherinet, 1 strongly suggests 

the presence of ferric ion. The copper minerals, presumably secondary, were formed in the following order: (1) Bornite, (2) covellite and iimonite, (3) chalcocite, (4) chalcopyrite and chalcocite. 

The hydrous iron oxide appears to have formed with covellite between bornite and chalcocite. In view of the fact that Iimonite 

forms from ferric rather than from ferrous salt, this association indicates the presence of ferric sulphate. Possibly the iron was deposited 

from suspension. In this connection the occurrence of considerable 

finely divided iron oxide suspended in deep waters at Ducktown, 

Tenn., is suggestive. On the other hand, it is known that changes 

in acidity bring about reversals in the state of oxidation of some ions 

in a solution, and the iron oxide may have been precipitated from 

solution with covellite. The mass of evidence seems -to indicate, 

however, that the precipitation of oxides with copper sulphides is 

generally subordinate.

Covellite, like chalcocite, is found in ores of the "Red Beds" of 

the Southwest, where it was deposited as a primary mineral, presumably from cold solutions. Examples of its deposition by ascending thermal waters are exceedingly rare, but at Butte, Mont., it is 

found at depths as great as 1,600 feet and in associations which, 

according to Sales,2 suggest its deposition by ascending waters. A 

suite of specimens from the lower levels of the Butte mines, in a 

collection at the University of Minnesota, shows covellite intergrown 

with tetrahedrite.

Bornite, Cu5FeS4 , is found in associations that indicate its formation 

under many different geologic conditions. It has not been clearly 

identified as a pyrogenic constituent of an igneous rock, but it is 

a primary mineral of a pegmatite at Copper Mountain, British 

Columbia, 3 and it is primary in many contact-metamorphic deposits.

i Catherinet, Jules, Copper Mountain, British Columbia: Eng. and Min. Jour., vol. 79, 1905, p. 125. 

Kemp, J. F., Secondary enrichment in ore deposits of copper: Econ. Geology, vol. 1,1906, p. 16.

8 Sales, R. H., Discussion of paper by F. L. Ransome, on "Criteria of downward sulphide enrichment": 

Econ. Geology, vol. 5,1910, p. 682.

8 Kemp, J. F., Secondary enrichment in ore deposits of copper: Econ. Geology, vol. 1,1906, p. 17; The 

rfile of the igneous rocks in the formation of veins: Trans. Am. Inst. Min. Eng., voL 31,1902, pp. 182-183.
CHEMICAL RELATIONS AND MINERALS OF COPPER. Ill

It occurs in lodes that were formed at great depths and also in some 

that were formed at moderate depths, and less abundantly in deposits 

remote from outcrops of igneous rocks. 1 It is deposited on pyrite and 

other sulphides by cold copper sulphate waters, and in some deposits 

it is regarded as an important secondary sulphide. As such it is 

usually much less abundant than chalcocite. According to Sales 2 

it is both primary and secondary at Butte, Mont., where it results 

from alterations of pyrite, chalcocite, and enargite and is a transition 

product between pyrite and chalcocite. According to Weed 3 it is 

found mainly above the 1,000-foot level. Sales states that "bornite 

does not often result from the action of downward-moving surface 

waters but is generally a product of ascending solutions." Graton 4 

regards it as a primary mineral in the copper deposits of Shasta 

County, Cal., in some of which it is found as far as 970 feet below the 

surface. At Bingham, Utah,5 bornite is intimately associated with 

chalcopyrite in fhe large replacement ore bodies. It is a secondary 

mineral at Rio Tinto, Spain.6

Clialcopyrite, FeCuS2 , in the greater number of its occurrences is 

clearly primary, and in many sulphide deposits it is the onlyimportant 

primary copper mineral in the unaltered ore. A list of occurrences 

of primary chalcopyrite would include nearly all important deposits 

of copper ore in the United States. It is, however, a "persistent" 

mineral, and evidence is rapidly multiplying that it is secondary 

in many deposits. At Bingham, Utah, according to Boutwell, 7 

chalcopyrite is the most important primary copper mineral. It is 

intimately associated in physical mixtures with pyrite, much of the 

pyritic ore carrying 3.5 per cent of copper. In the disseminated 

secondary ores in porphyry a considerable part of the copper is in 

chalcopyrite. Since it is most abundant in crushed and altered 

areas, and since it generally occurs along fractures, Boutwell regards 

it as formed subsequent to the solidification of the porphyry but 

does not state the nature of the solutions which deposited it. 

It is a primary constituent of the quartz monzonite at Butte, Mont., 

and also of the copper ores, but according to Weed 8 it is not an

1 Lindgren, Waldemar, Graton, L. C.,and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68, 1910, pp. 77-78.

2 Sales, R. H., Discussion of paper by F. L. Ransome, on "Criteria of downward sulphide enrichment": 

Econ. Geology, vol. 5, 1910, p. 682.

3 Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 74.

4 Graton, L. C., The occurrence of copper in Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430, 

1910, pp. 104-105.

6 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 106.

8 Finlayson, A. M., The pyritic deposits of Huelva, Spain: Econ. Geology, vol. 5,1910, p. 419.

' Boutwell, J. M., op. cit.,p. 106.

«Weed, W. H., op. cit., p. 75.
112 . THE ENRICHMENT OF SULPHIDE ORES.

important ore mineral at Butte. At Globe it is associated with 

pyrite in the deeper ores and is abundant in some mines. It occurs 

with specularite in the Old Dominion, Gibson, and' other mines.1 

No secondary occurrence is mentioned by Ransome.2 At Bisbee it is 

in the main but not altogether primary. Some of it is of more recent 

origin than pyrite. Specimens taken from the periphery of the ore body 

on the seventh level of the Spray mine, near the inclosing altered and 

pyritized limestone, show massive chalcopyrite enveloping kernels of 

pyrite, and veinlets of chalcopyrite traverse the pyritic nuclei. Ransome says: 3 "It is necessary to conclude that this particular mass of 

chalcopyrite was formed after the pyrite and probably in part by 

alteration or replacement of the latter." According to Kemp, 4 

thin films of later chalcopyrite have been found at Butte, Mont., 

fining crevices between the tabular crystals of covellite, and it is 

clearly a secondary mineral at Copper Mountain, near Princeton, 

British Columbia, 5 where it is associated with chalcocite and covellite 

and is probably later than some of the chalcocite of the ore. In 

Shasta County, Cal., 6 it occurs with zinc blende in veinlets cutting 

the primary ore, and Graton regards it as in part of secondary origin. 

At Philipsburg, Mont., it occurs sparingly with zinc blende in veinlets 

which cut the earlier ore. In the veins of Morenci, Ariz., 7 chalcopyrite is mainly a primary mineral. In a stope of the Ryerson mine 

it is associated with chalcocite. Of this occurrence Lindgren says: 

"Such a connection would suggest its origin as due in this case to 

secondary sulphide formation by solutions from above, for it is 

known that chalcopyrite, as well as chalcocite, may form under 

such conditions." Chalcopyrite is secondary in the rich ore of the 

Gossan lead, Virginia.8 At Ducktown, Tenn., 9 and in the Rio Tinto 

mines, Spain, 10 chalcopyrite enrichment below the chalcocite zone has 

been a process of considerable economic .importance.

Of the chemistry of the precipitation of secondary chalcopyrite 

very little is known. It has not been formed synthetically by cold 

sulphate solutions in the laboratory. Since it is a sulphide of both

i Ransome, F. L., oral communication.

* Ransome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. S. Geol. Survey No. 12, 

1903, p. 121.

3 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21, 1904, p. 133.

< Kemp, J. F., Secondary enrichment in ore deposits of copper: Econ. Geology, vol. 1,1906, p. 16.

sCatherinet, Jules, Copper Mountain, British Columbia: Eng. and Min. Jour., vol. 79,1905, pp. 125-126.

6 Graton, L. C., The occurrence of copper in Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430,1910, 

p. 104.

7 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43, 1905, p. 106.

s Weed, W. H., Copper deposits of the Appalachian States: Bull. U. S. Geol. Survey No. 455, 1911, 

pp. 120-121.

»Emmons, W. H., and Laney, F. B., Preliminary report on the mineral deposits of Ducktown, Tenn.: 

Bull. U. S. Geol. Survey No. 470,1911, p. 151.

»° Finlayson, A. M., The pyritic deposits of Huelva, Spain: Econ. Geology, vol. 5,1910, pp. 410-420.
CHEMICAL RELATIONS AND MINERALS OP COPPER. 113

copper and iron, and since iron sulphide dissolves in acid, it would 

be supposed that chalcopyrite would not be precipitated in more than 

feebly acid solutions, although copper sulphide may be precipitated in 

solutions more highly acid. In view of these relations it would be 

supposed that the precipitation of chalcopyrite would be chiefly 

below the chalcocite zone. Such relations are suggested by the 

occurrences of these minerals in some districts, especially at Ducktown, Term.

Enargite, Cu8AsS4 , is a primary mineral of great value at Butte, 

Mont., is present in considerable amount at Tintic, Utah, and occurs 

in less abundance at Bingham, Utah, in Gilpin County, Colo., and 

elsewhere. At Bingham it was found lining druses, showing that it 

was one of the last minerals deposited, but according to Boutwell 1 

there is no conclusive evidence that it was deposited by sulphate 

waters. The enargite of Butte, according to Emmons and Tower, 

is primary.2 Weed 3 observes that some of it was deposited, however, 

by hot ascending solutions after the earlier chalcopyrite veins had 

formed. Both lieno Sales 4 and A. N. Winchell 5 regard enargite 

at Butte as mainly primary. It is found in the upper portions of 

veins above the water level at Butte and at Tintic, but in view of its 

great insolubility 6 in acid sulphate waters it is probably residual. 

Little conclusive paragenetic evidence is available that it is deposited 

by descending sulphate waters. At Butte it is intergrown with 

chalcocite, but as stated above some of the chalcocite of the Butte 

deposits is probably primary.

Famatinite, Cu3SbS4 , is not a common ore of copper, and little is 

known as to its origin. I know of no occurrence that has been 

described as secondary. In view of the primary origin of enargite, 

its corresponding arsenic salt, famatinite also is probably of primary 

origin.

Tetrahedrite, Cu 8Sb2S7, is a comparatively common copper mineral. 

In most of its occurrences it is of primary origin. Its genesis is 

discussed on page 124.

Tennantite, Cu 8As 2S 7 , is the arsenic salt corresponding to tetrahedrite, but it is not so common as tetrahedrite. Its genesis is discussed on page 125.

i Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 108.

* Weed, W. H., Emmons, S. F., and Tower, G. W., Butto special folio (No. 38), Geol. Atlas U. S., U. S. 

Geol. Survey, 1897, p. 6.

8 Weed, W. H., Ore deposition and vein enrichment by ascending hot waters: Trans. Am. Inst. Min. 

Eng., vol. 33,1903, p. 748.

* Sales, R. H., Discussion of paper by F. L, Kansome, on "Criteria of downward sulphide enrichment": 

Econ. Geology, vol. 5,1910, p. 681.

* Winchell, A. N., idem, p. 488.

«Buehler, H. A., and Gottschalk, V. H., Oxidation of sulphides: Econ. Geology, voL 5,1910, p. 31.

72293° Bull. 529 13 8
114 THE ENRICHMENT OF SULPHIDE OEES.

SOME CHEMICAL RELATIONS AND MINERALS OF SILVER. 

PRINCIPAL SILVER MINERALS.

The chemical composition of the principal silver minerals is given 

below:

Silver............................... Ag.

Cerargyrite.......................... AgCl.

Bromyrite........................... AgBr.

Embolite...........................Ag(Cl,Br).

lodyrite............................ Agl.

Argentite............................ Ag2S.

Pyrargyrite........................ - Ag3SbS3 or 3Ag2S.Sb2S3 .

Proustite........................... Ag3AsS3 or 3Ag2S.As2S3 .

Stephanite.......................... Ag5SbS4 or 5Ag2S.Sb2S3 .

Polybasite............. 1............. Ag9SbS6 or 9Ag2S.Sb2S3 .

Pearceite........................... AgBAsS6 or 9Ag2S.As2S3 .

Tetrahedrite .......................4Cu2S.Sb2S3 or 4(Cu2Ag2)S.Sb2S3 .

Tennantite (argentiferous)...........4Cu2S.As2S3 'or 4(Cu2Ag2)S.As2S3 .

Tellurides.

SOLUBILITIES OF SALTS OF SILVER.

The solubilities, of several silver salts in water, determined by 

Kohlrausch,1 are as follows:

Ag2SO<

5.5 

.02

Ag2C03

0.03 

.0001

AgN03

2134 

8.6

AgCl

0.0016 

.00001

AgBr

0.0001 

.0000006

Agl

0.000003 

.00000001

The upper number in each column shows the number of grams of 

the anhydrous salt held in solution in a liter of water, the lower 

number shows the molar solubility the number of mols contained 

in 1 liter of the saturated solution at 18° C. A more recent determination of the solubility of Ag2S04 is 7.7 grams per liter at 17° C.2

NATURE AND RELATIONS OF THE SILVER MINERALS.

Silver, like copper, is dissolved in dilute sulphuric acid, and the 

solubility of its sulphide, like that of the sulphides of copper, is 

increased by the presence of an oxidizing agent. Like copper, it is 

precipitated in a reducing environment by metallic sulphides or by 

hydrogen sulphide. Its sulphide is even less soluble than that of 

copper. Unlike copper, it does not form stable carbonates, sulphates, 

or oxides in the oxidizing zone. Its chloride is comparatively insoluble, however, whereas the chlorides of copper dissolve more readily

1 Determined by the conductivity method. See Smith, Alexander, Introduction to inorganic chemistry, 

rev. ed., New York, 1910, p. 544.

2 Euler, Hans, Uber Loslichkeitssrniedrigung: Zeitschr. phys. Chemie, vol. 49,1904, p. 314.
CHEMICAL RELATIONS AND MINERALS OF SILVER. 115

and rarely accumulate as ores. Although ferrous sulphate and copper sulphate are compatible even in solutions of high concentration, 

ferrous sulphate precipitates metallic silver from a dilute solution of 

its sulphate. As silver is 50 to 100 times as valuable as copper, the 

natural processes which even slightly enrich silver ores are of great 

commercial interest.

Silver sulphate, which is only slightly soluble, is formed by the 

action of concentrated sulphuric acid on silver. Ostwald l states 

that silver sulphate is more soluble in dilute sulphuric acid than in 

water and that in the reaction an acid salt is probably formed. 

H. C. Cooke, who recently made a series of experimental studies in 

the solution and precipitation of silver compounds, informs me that 

silver sulphide is dnly slightly soluble in very dilute sulphuric acid, 

but that if a little ferric sulphate is added to the solution the solubility 

of the sulphide is considerably increased. He found also that a little 

ferric sulphate is almost as effective as a considerable amount and 

believes that the function of ferric sulphate is to oxidize any hydrogen 

sulphide that is formed, removing it from the system, in which it 

would suppress further solution of silver sulphide. According to 

Vogt 2 ferric sulphate will itself attack metallic silver. Like gold 

chloride, silver sulphate is easily reduced by many minerals depositing the native metal.

The order of the solubility of the chloride, the bromide, and the 

iodide of silver are expressed by the order in which they are here 

named. All of them are very sparingly soluble in water; consequently the halogens will precipitate silver from sulphate solutions 

and the halides may remain in the upper portions of deposits, giving 

a considerable enrichment at or near the surface. "In consequence 

of the progressive insolubility, a cold solution of a bromide will 

slowly convert the precipitate of silver chloride into bromide, and a 

soluble iodide will similarly transform the bromide or the chloride 

into iodide." 8 If the three halogens were equally abundant in mine 

waters, the bromide and iodide of silver would probably predominate 

in the silver deposits, but chlorine only is abundant. Consequently 

cerargyrite, AgCl, is much more common than embolite, Ag(Cl,Br); 

bromyrite, AgBr; iodobromite, Ag(Cl,Br,I); and iodyrite, Agl.

If, in a solution containing the three halogens, chlorides are vastly 

in excess, silver chloride will be precipitated first, even if bromides and 

iodides are present, for in mixed solution the least soluble salts are 

not precipitated first if a more soluble salt is present in sufficiently 

greater concentration.

1 Ostwald, Wflhelm, The principles of inorganic chemistry (trans. by Alex. Findlay), London, 1904, 

p. 670.

2 Vogt, J. H. L., Problems in the geology of ore deposits, in Posepny, Franz, The genesis of ore deposits, 

1902, p. 677. 

»Smith, Alexander, Introduction to inorganic chemistry, rev. ed., New York, 1910, p. 629.
116 THE ENRICHMENT OF SULPHIDE OEES.

The relations of the balides of silver at Tonopah, Nev., should be 

mentioned in this connection. According to Burgess ] the silver 

halides at some places occupy fairly well marked horizons in the 

altered veins. The chloride, cerargyrite, occupies the upper zone, 

embolite the middle zone, and iodyrite the lower zone. The zones 

are usually not distinct, and in places two 9f the minerals are found 

together. Of these relations Burgess says: 2

The order of crystallization was chloride, bromochloride, iodide, which is contrary 

to what would be expected from their respective solubilities, since iodide is the least 

soluble and would be expected to precipitate first. The reason for this reversal may 

be involved in the complicated set of conditions under which the reactions occurred 

and the numerous other minerals present. The most obvious explanation seems to 

be that the chloride was precipitated first because of the great excess of alkaline 

chlorides in the solution, and that as the chlorine became reduced in quantity it was 

partly replaced by bromine, and that the iodide was formed only after the concentration of the other halogens was considerably reduced.

Notwithstanding the low solubilities of the silver halides, it seldom 

happens that all the silver dissolved in the upper parts of an argentiferous deposit is fixed as halides. The formation of the chloride 

near the surface does not entirely inhibit the downward migration 

of silver. The secondary silver sulphides are numerous and in some 

veins abundant. Among the reasons for the fairly extensive downward migration of silver, even where chlorides are formed, the following may be mentioned:

1. Chlorides are present in the natural waters in some deposits in 

amounts insufficient to precipitate as chloride all the silver that goes 

into solution as sulphate.

2. Silver chloride is itself slightly soluble in water.

3. Silver chloride and other halides of silver are soluble in an 

excess of alkali chlorides.

. From this it follows that if a silver solution in the upper part of 

a deposit contains a certain small amount of alkali chlorides some of 

the silver will be fixed as chloride; but if it contains sufficient alkali 

chloride horn silver will be dissolved and will migrate downward 

with other metals. That some silver is held in solutions containing 

both sulphates and chlorides is shown by an analysis of mine water 

reported by Reid.3

Silver is readily precipitated as argentite below the zone of oxidation on account of the low solubility of the sulphide 0.552 X10"6 mols 

per liter.4 It stands near the end of the Schuermann series, being 

preceded only by mercury, and accordingly it should replace most

1 Burgess, J. A., The halogen salts of silver and associated minerals at Tonopah, Nov.: Econ. Geology, 

vol. 6,1911, p. 13.

* Idem, p. 15.

8 Reid, J. A., The structure and genesis of the Comstock lode: Bull. Dept. Geology Univ. California, 

vol. 4,1906, pp. 180-190.

* Weigel, Oskar, Die Loslichkeit von Schwermetallsulflden in reinem Wasser: Zeitschr. phys. Chemie, 

vol. 58,1907, p. 294.
CHEMICAL BELATIONS AND MINERALS OF SILVEE. 117

other metals in sulphide combinations. Ferrous sulphate formed by 

the reaction of dilute acid and ferric sulphate waters on the several 

sulphides will precipitate metallic silver from sulphate solutions. 

With silver sulphate (Ag2S04) hydrogen sulphide, which, under some 

conditions at least, is generated by acid reacting upon zinc blende, 

galena, or other sulphides, gives argentite, which is one of the most 

important silver minerals in ores. The reaction is 

Ag2S04 + H2S = Ag2S + H2S04 .

If the reaction is with galena, or if lead sulphide is precipitated 

simultaneously with argentite, argentiferous galena may be formed. 

If arsenic and antimony are present in solution, the complex sulphosalts of these metals may be formed. Of these pyrargyrite is probably the most abundant. Very little is known of the manner of 

formation of the arsenic and antimony compounds. That both are 

dissolved in underground water is indicated by the small amounts 

of arsenic and antimony in gossans and oxidized zones of deposits 

which contain more of these metals in depth. Although several 

samples of mine waters show traces of arsenic and antimony, few 

waters contain appreciable quantities of these elements. The 

synthesis of the complex compounds they form is difficult under 

the conditions that are assumed to exist where sulphide enrichment 

takes place. It is generally supposed that most of them may be 

either primary or secondary. The data now available indicate that 

pyrargyrite and proustite are nearly everywhere secondary minerals; 

that stephanite, polybasite, and pearceite are secondary in most of 

their occurrences; that tetrahedrite and tennantite, which frequently 

contain silver as well as copper, are in general primary. The two 

minerals last named are found at some places in cracks that cut the 

primary ore, but it is not clear that their occurrence is related to 

the present topographic surface.

In the subjoined table the more important antimony sulphosalts 

of silver are put in the first column, the arsenic sulphosalts in the 

second. Tetrahedrite and tennantite are included, for they are frequently argentiferous.

Rather common. Rare. 

Pyrargyrite=3Ag2S.Sb2S3 . Proustite=3 Ag2S.As2S3 . 

Tetrahedrite=4Cu2S.Sb2S3 . Tennantite=4Cu2S.As2S3 . 

Stephanite=5 Ag2S. Sb2S3 . 

Polybasite=9Ag2S.Sb2S3 . Pearceite=9Ag2S.As2S3 .

The minerals in the first column are, as a rule, much more abundant and they are of commoner occurrence than the corresponding 

minerals in the second column. Proustite is probably less common and less abundant than pyrargyrite, tennantite less common 

and less abundant than tetrahedrite. Polybasite is not an uncommon mineral, but pearceite has been identified at only a few places.
118 THE ENRICHMENT OF SULPHIDE ORES.

If the antimony sulphosalts, where secondary, have been deposited 

mainly as results of reactions of silver-bearing sulphate waters on 

stibnite, it would appear that arsenic minerals are less common than 

the antimony minerals or else that they are less readily replaced 

under the conditions that exist in veins.

Although there is no invariable rule respecting the relations of 

the silver chloride, the native metal, the sulphide, and the arsenic 

and antimony sulphosalts in deposits containing all these minerals, 

yet in some such deposits some of these minerals have fairly well 

defined horizons. The chloride is most abundantly developed above 

the argentite ore; the antimony and arsenic sulphosalts are found 

with and below the argentite. The native metal is found with the 

chloride, and, overlapping the zone of chloride ores, it extends downward with argentite. In the Granite-Bimetallic mine, at Philipsburg, Mont., the deeper secondary ores contain very little argentite 

or native metals, the richer minerals being almost exclusively the 

dark and the light ruby silver. At Georgetown, Colo., according to 

Spurr, Garrey, and Ball, argentite predominates hi the upper part of 

the secondary sulphide zones, and the secondary arsenic and antimony sulphosilver minerals are more important below the argentite 

zone.

OCCURRENCE OF THE SILVER MINERALS.

Native silver is a primary mineral in some deposits, as in the zeolitic 

copper ores of Lake Superior, but in sulphide deposits it is generally 

or invariably secondary. In some districts it is among the most 

important ore minerals. It commonly occurs as thin flakes or as 

sheets plastered on the older minerals or as veinlets filling cracks in 

the ore, and presumably has been formed at many places through 

the reduction of silver sulphides or other silver-bearing minerals. 

In the Aspen mining district, Colo., native silver coats crystals of 

barite and is undoubtedly of relatively late deposition. Wire silver 

piercing barite is found 900 feet below the surface.1 According to 

Spurr,2 silver replaces organic remains in the Aspen district and is 

found probably 600 feet below the level of ground water. There, 

he says, in consequence of reduction by carbonaceous matter in the 

"Weber shales" it has formed instead of the complex salts of silver 

which generally constitute the richer ores in this province. By 

processes of reduction native silver forms as an alteration product 

of many minerals, such as argentite, pyrargyrite, proustite, polybasite, and stephanite. At many places it is pseudomorphous after 

these. Vogt 3 says that the native silver of Konigsberg, which is

1 Lindgren, Waldemar, oral communication.

2 Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Surrey, vol. 31, 

1898, p. 233; Ore deposition at Aspen, Colo.: Econ. Geology, vol. 4, 1909, p. 316.

»Vogt, J. H. L., Ueber die Bildung des gediegenen Silbers, besonders des Kongsberger Silbers, durch 

Secondarprocesse aus Silberglanz und anderen Sflberenen: Zeitschr. prakt. Geologic, vol. 7, 1899, pp. 

113-423,177-181.
CHEMICAL [RELATIONS AND MINEEALS OF SILVEE. 119

noted for its large and beautiful specimens, is formed by the reduction of argentite. Weed * states that secondary silver ore at Butte 

is probably first precipitated as argentite, which alters to the native 

metal. In the Comstock lode native silver is rare but has been 

noted. At Georgetown, Colo.,2 polybasite is covered with small specks 

of native silver, which is probably secondary to the polybasite. At 

Tonopah, Nev., native silver is not abundant but has been observed 

coating cracks in the primary ore and is entirely secondary.3 In 

the Cceur d'Alene district, Idaho, wire silver is associated with 

cerusite and limonite in the upper parts of several of the lead-silver 

veins.4 It was an important ore in silver veins and in oxidized portions of copper veins at Butte and in several other districts of Montana. It was found at Creede, Colo., 1,200 feet below the surface. 

In this district chalcedony and native silver form rich ore. The 

native silver in deposits of Cobalt, Ontario, is considered secondary. 

Ferrous sulphate precipitates silver from sulphate solutions, according to the following reaction: 6

Ag2S04 + 2FeS04 = 2 Ag + Fe2 (S04 ),.

This reaction doubtless accounts for some well-authenticated occurrences of native silver veinlets below the water level, but many other 

natural precipitants are known.6

Cerargyrite, horn silver, AgOl, is probably unknown as a primary 

constituent of ores deposited by ascending hot waters but is frequently developed by weathering, alteration, or enrichment at or 

near the outcrops of silver-bearing sulphide lodes. A list of its 

occurrences would include nearly all sulphide deposits in which 

silver is an important metal. In arid undrained areas it is an important ore mineral, so important that the term "chloriding" is 

generally used in such regions for pocket hunting near the surface. 

At many places even where the primary sulphide ores are not profitable the superficial chloride ores may be very rich. The chloride 

ores generally pass into the sulphides below, and the bottom of the 

zone of ore carrying horn silver is generally above the bottom of the 

zone of secondary silver sulphides. Cerargyrite is not everywhere 

confined to the shallow surface zone, however. In the Comstock lode, 

where it is.not particularly abundant, it was noted, according to 

Clarence King, at a depth of 900 feet below the surface. Cerargyrite

i Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 102.

»Spurr, J. E., and Garrey, G. H., Preliminary report on the ore deposits of the Georgetown mining district, Colorado: Bull. U. S. Geol. Survey No. 260,1905, p. 119.

8 Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No: 

42,1905, p. 95.

«Ransome, F. L., and Calkins, F. C., The geology and ore deposits of the Cceur d'Alene district, Idaho. 

Prof. Paper U. S. Geol. Survey No. 62,1908, p. 90.

 Penrose, R. A. F., jr., The superficial alteration of ore deposits: Jour. Geology, vol. 2,1894, p. 314.

'For the relations of native silver and copper minerals, see this paper, p. 97.
120 THE ENRICHMENT OF SULPHIDE ORES.

forms where waters carrying silver sulphates encounter waters bearing chlorides. Silver chloride is only slightly soluble in ordinary 

ground water and is therefore fairly stable under surface conditions.

Many deposits of cerargyrite ores contain large amounts of manganese oxide. In some of these native silver is present only in small 

quantity. The presence of manganese oxide delays the generation 

of ferrous sulphate, which, as already stated, precipitates native 

silver from ferric sulphate solution. With manganese dioxide in 

the presence of abundant chlorides horn silver would be precipitated 

rather than the native metal. Examples of horn-silver ores associated with manganese oxides include the famous deposits of Lake 

Valley,1 near Silver City, N. Mex., the deposits of the Exposed 

Treasure mine, 2 near Mohave, Cal., and some silver deposits at Neihart, 

Mont., 3 and at Leadville, Colo.4 Other deposits, however, containing 

appreciable though lower proportions of manganese, contain considerable native silver with subordinate chloride; a conspicuous 

example is the Amethyst vein of Creede, Colo.

Embolite and other Jialides. Embolite, Ag(Cl,Br); bromyrite, 

AgBr; iodobromite, Ag(Cl,Br,I); and iodyrite, Agl, are not common 

minerals, for they contain bromine and iodine, which are rare in 

mineral solutions. None of them are known to have been formed 

in hot ascending solutions. They are in the main even less soluble 

in water than cerargyrite and are therefore comparatively stable 

when once they are formed. The mutual relations of the silver 

halides at Tonopah, Nev., have already been reviewed (p. 116).

Argentite, Ag2S, is one of the commonest and most important 

secondary silver minerals, but it occurs also as a primary mineral. 

It fills postmineral cracks in the secondary zones at Georgetown, 

Colo., Neihart and Philipsburg, Mont., and many other places. 

Great bonanzas of argentite were found in upper levels on the Cornstock lode. In this lode it has been identified as far as 3,000 feet 

below the surface but not certainly as a secondary mineral. At 

Tonopah, Nev.,5 some occurrences of argentite are primary, but in 

places it coats crevices that cut the primary ore and is evidently 

secondary also. Some of the ores show argentite fringing cerargyrite, as if secondary to it. It is probably primary hi part at Butte,

1 Clark, Ellis, The silver mines of Lake Valley, New Mexico: Trans. Am. Inst. Min. Eng., vol. 24,1895, 

p. 148. Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. 

Paper U. S. Geol. Survey No. 68,1910, pp. 66, 279.

«De Kalb, Courtenay, Geology of the Exposed Treasure lode, Mojave, Cal.: Trans. Am Inst. Min. Eng., 

vol. 38,1908, p. 319.

»Weed, W. H., Geology of the Little Belt Mountains, Montana, with notes on the mineral deposits of 

the Neihart, Barker, Yogo, and other districts, accompanied by a report on the petrography of the igneous 

rocks of the district by L. V. Pirsson: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3,1900, p. 407.

<Emmons, S. F., Geology and mining industry of Leadville, Colo.: Mon. U. S. Geol. Survey, vol. 12, 

1886, p. 562.

' Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, pp. 92,94.
CHEMICAL RELATIONS AND MINERALS OF SILVER. 121

Mont., and at Tintic, Utah. Argentite is pseudomorphous after ruby 

silver (and vice versa) at Joachimsthal, Bohemia.

Silver sulphide is the least soluble sulphide of the commoner metals, 

except mercury and bismuth, and in reactions involving double 

decomposition it would be precipitated from argentiferous solutions 

before all except these sulphides. A search for pseudomorphs after 

copper sulphide, sphalerite, and pyrite has shown surprisingly few 

clearly defined examples, although it probably does replace these 

sulphides metasomatically. If sulphuric acid should in its descent 

encounter a soluble sulphide like zinc blende, hydrogen sulphide and 

zinc sulphate might be formed. The hydrogen sulphide would precipitate silver sulphide from a solution containing Ag2S04 . Argentite could be precipitated in any of the following reactions:

H2S + Ag2S04 = H2S04 + Ag2S. 

ZnS + Ag2S04 = ZnS04 + Ag2S. 

PbS + Ag2S04 =PbS04 + Ag2S.

The precipitation of argentite from silver sulphate solution does 

not necessitate a change in valence like that which takes place when 

chalcocite is formed.

Pyrargyrite, dark ruby silver, 3Ag2S.Sb2S3 , is probably the most 

important secondary silver mineral in a large number of silver mines 

in the United States. So far as known, it is confined to epigenetic 

deposits, and it is particularly conspicuous in many deposits of early 

and middle Tertiary age in the American Cordillera. It is not known 

as a primary mineral of contact-metamorphic and nearly related 

deposits. In the Granite-Bimetallic mine, at Philipsburg, Mont., 

pyrargyrite is perhaps the most important mineral. It occurs as 

small specks intimately intergrown with quartz and stibnite and may 

possibly be primary, but it is very much more abundant as a secondary mineral in this mine. Most of it lines vugs or occurs in small 

veinlets that cut across the banding of the primary ore, in which 

stibnite is abundant. At Tonopah, Nev., pyrargyrite coats crevices 

that cut the primary ore and is evidently secondary.1

Little is known of the synthesis of pyrargyrite under conditions of 

sulphide enrichment. The assumption that it forms under such 

conditions rests on paragenetic evidence and the fact that it frequently disappears with increase in depth. Pyrargyrite is an important ore mineral at Georgetown, Lake City, Ouray, and TeUuride, 

Colo.; at the Comstock lode, Austin, Tuscarora, and other districts of 

Nevada; and at Neihart, Butte, and other places in Montana. At all 

these places it is probably, in part, at least, of secondary origin.

1 Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Qeol. Survey No. 42, 

1905, p. 04.
122 THE ENRICHMENT OF SULPHIDE OKES.

Pyrargyrite is an important constituent of the ore of the famous 

silver mines of Zacatecas, Guanajuato, and Pachuca, Mexico. 

Mineralogically the ores of these deposits are nearly related to those 

of the Comstock lode and Tonopah, Nev.

On the horizon of pyrargyrite at Lake City, Colo., the folio whig, 

from Irving and Bancroft,1 is a particularly clear statement:

Ruby silver occurred, so far as could be learned, in all of the mines at the plane of 

demarcation between sulphides and oxides and, in generally decreasing quantity, 

to several hundred feet below this level. Along cracks and fissures it occurred in 

isolated masses to great depths for instance, at 1,200 feet in the Golden Fleece and at 

1,300 feet in the lima. These deep occurrences are, however, uncommon and merely 

indicate the presence of some easy line of access for downward-moving solutions.

Ruby silver has probably resulted from the solutions of silver and antimony obtained 

by the decomposition of the tetrahedrite and possibly to some extent also from the 

argentiferous galena. The chemistry of both the solution and reprecipitation of the 

antimonial and arsenical sulphur compounds has not yet been worked out in sufficient 

detail to permit a statement of the probable steps of the process, but the geological 

facts show that it has occurred. The proofs of the secondary character of the ruby 

silver are:

1. Its restriction in quantity to the upper levels of the mines.

2. Its invariable occurrence as the latest deposited mineral in the veins, either in 

cracks or crevices in shattered primary ore or as crystals in cavities.

3. Its occurrence only in isolated bunches in deeper workings, where its origin is 

probably due to the presence of water channels that permit the downward percolation 

of water from above.

4. Its complete absence from the great mass of deep-seated ore. 

It is clearly secondary in all of a large number of specimens of silver ores from the 

Southwest.

In some mines ruby silver is found at very considerable depths, 

however, possibly below the zone of secondary alteration. At 

Przibram, for example, it is said to be found 3,500 feet below the 

surface.

Proustite, light ruby silver, 3Ag2S.As2S3 , is similar to pyrargyrite 

in its occurrence and is commonly regarded as a secondary mineral. 

Whether it is ever primary is a moot question. Proustite is a secondary mineral at Lake City, Ouray, Silverton, Telluride, and Rico, 

Colo.,2 at Philipsburg, Mont., at Austin, Nev., in the Coeur d'Alene 

district, Idaho,3 and in several other districts. It has been noted on 

the Comstock lode but is not abundant there.

Discussing the genesis of argentite, proustite, pyrargyrite, stephanite, and polybasite, Ransome 4 says:

It is quite possible that further study of ore deposits may result in showing that 

some of these minerals, especially proustite, are fully as characteristic of downward 

enrichment as is chalcocite.

1 Irving, J. D., and Bancroft, Rowland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. 

Geol. Survey No. 478,1911, p. 63.

2 Idem, p. 35.

3 Ransome, F. L., and CalMns, F. C., The geology and ore deposits of the Coeur d'Alene district, Idaho: 

Prof. Paper U. S. Geol. Survey No. 62, p. 93. 

« Ransome, F. L., Criteria of downward sulphide enrichment: Econ. Geology, vol. 5,1910, p. 211.
CHEMICAL RELATIONS AND MINERALS OF SILVER. 123

Sommerlad 1 prepared proustite by heating silver chloride and 

arsenic trisulphide, and many other syntheses are known.2 I have 

no record of the formation of proustite in cold sulphate waters.

Stephanite, brittle silver, 5Ag2S.Sb2S3 , is an important mineral on 

the Comstock lode, at Tuscarora and Tonopah, Nev., at Georgetown, 

Aspen, and Rico, Colo., and in many other silver districts. It generally accompanies ruby silver and polybasite, and in some districts 

it occurs, like them, in cracks cutting the primary ore. On the 

Comstock lode, according to King,3 broken fragments of quartz 

themselves containing ore have been recemented by sheets of stephanite. In general its occurrences in the United States are not clearly 

described, and its genesis is more or less uncertain.

Polybasite, 9Ag2S.Sb2S3 , is commonly a secondary mineral. It 

occurs at Georgetown, Colo., in cracks cutting the primary ore, 4 and 

in the main is related to the present surface. At Neihart, Mont.,5 it 

occurs in postmineral fractures and in-vugs and incrusts primary 

sulphides. It occurs with barite in the Mollie Gibson mine at Aspen, 

Colo., where it alters to native silver.' It is probably secondary in 

the Comstock lode, Nevada, and in several districts in San Juan, Colo. 

At Tonopah, Nev.,7 polybasite is found as deep as 500 feet below the 

surface in fractures that cut older sulphides, but according to Spurr 

it is not certain that it was deposited by descending solutions. In 

many other districts it is regarded as a secondary mineral, for it is 

not abundant in the deeper levels.

Although polybasite has been formed synthetically no record of 

its synthesis from cold sulphate waters is available. H. C. Cooke 8 

treated powdered stibnite with cold dilute silver sulphate solution 

and obtained a rich antimony-silver ore but did not identify the 

material as polybasite.

Pearceite, 9Ag2S.As2S3 , is less common than the corresponding 

antimony salt, polybasite. It is generally assumed to be secondary. 

According to Weed,9 it constitutes much of the rich silver ore of

1 Sommerlad, Hermann, tiber einige Versuche zur Herstellung von Sulfantimoniten und Sulfarseniten 

des Silbers auf trockenem Wege: Zeitschr. anorg. Chemie, vol. 15, 1897, p. 177; tJber Versuche zur Darstellung von Sulfantimoniten und Sulfarseniten des Silbers, Kupfers und Bleis auf trockenem Wege: 

Idem, vol. 18,1898, pp. 420-447.

2 Clarke, F. W., The data of geochemistry; 2d ed.: Bull. U. S. Geol. Survey No. 491,1911, p. 624.

8 King, Clarence, The Comstock lode: U. S. Geol. Expl. 40th Par., vol. 3,1870, p. 81.

4 Spurr, J. E., and Garrey, G. H., Economic geology of the Georgetown quadrangle (together with the 

Empire district), Colo., with general geology by S. H. Ball: Prof. Paper U. S. Geol. Survey No. 63,1908, 

p. 261.

* Weed, W. H., Geology of the Little Belt Mountains, Mont., with notes on the mineral deposits of the 

Neihart, Barker, Yogo, and other districts, accompanied by a report on the petrography of the igneous 

rocks of the district by L. V. Pirsson: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, 1900, p. 407. 

' «Spurr, J. E., Geology of the Aspen mining district, Colorado, with atlas: Mon. U. S. Geol. Survey, vol. 

31,1898, pp. 224-225.

7 Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, p. 95.

8 Unpublished data.

9 Weed, W. H., The enrichment of gold and silver veins, in Posepn?, Franz, The genesis of ore deposits, 

1902, p. 494.
124 THE ENRICHMENT OP SULPHIDE ORES.

Neihart, Mont. Pearceite, intimately associated with galena, has 

been identified x from the Mollie Gibson mine, Aspen, Colo. In the 

Drumlummon mine, Marysville, Mont., according to Penfield,2 it 

lines a vug and is intimately associated with chalcopyrite, calcite, 

and quartz.

Tetrahedrite, gray copper, 4Cu2S.Sb2S3, is rather widely distributed 

but is of subordinate importance as a source of copper. The argentiferous variety, freibergite, is an important source of silver in many 

deposits. In the San Juan region, Colo., tetrahedrite is abundant 

in many deposits and is regarded as a primary constituent in this 

region.3 It is primary at Butte, but its corresponding arsenic compound, tennantite, is said to be secondary.4 In the Leonard mine 

some tetrahedrite is later than enargite.5 At Ouray and Lake City, 

Colo., according to Irving and Bancroft, 8 it was deposited by ascending 

hot waters, but mainly in the upper parts of the lodes, which at 

Lake City have a vertical range of over 5,000 feet. The corresponding arsenic compound, tennantite,. has not been recognized. 

Tetrahedrite appears to be primary in Clear Creek County, Colo.7 At 

Cripple Creek, Colo., it is primary and persists to depths of 2,000 feet 

below the surface.8 It is found also at Nevada City 9 and, intergrown 

with electrum, at Ophir,10 Cal. At Elkhorn, Mont., 11 it is probably 

the parent sulphide of rich altered silver ores.

At Bingham, Utah, according to Boutwell, 12 tetrahedrite is secondary. Crystals of pyrite are coated by chalcopyrite and the latter by 

tetrahedrite. In the Centennial mine,13 in the Empire district, Colorado, tetrahedrite forms in cracks of chalcopyrite in auriferous lodes 

and is said to be derived from it. Tetrahedrite is said to be second-

 Penfield, S, L., On pearceite, a sulpharsenite of silver, and on the crystallization of polybasite: Proc. 

Colorado Sci. Soc., vol. 5,1896, p. 211.

»Idem, pp. 217-218.

8 Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull. U. S. 

Geol. Survey No. 182,1901, p. 78; Criteria of downward sulphide enrichment: Econ. Geology, vol. 5,1910, 

p. 212.

«Weed,W.H., Emmons, S. F., and Tower, Q. W., Butte special folio (No. 38), Geol. Atlas U. 8., U. S. 

Geol. Survey, 1897, p. 6.

 Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 78.

 Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. Geol. 

Survey No. 478,1911, pp. 34,52.

' Kemp, J. F., Secondary enrichment in ore deposits of copper: Econ. Geology, vol. 1,1906, p. 22.

B Lindgren, Waldemar, and Eansome, F. L., Geology and gold deposits of the Cripple Cftek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, pp. 121-122.

 Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley districts, California: 

Seventeenth Ann. Kept. U. S. Geol. Survey, pt. 2,1896, p. 119.

« Lindgren, Waldemar, The gold-silver veins of Ophir, Cal.: Fourteenth Ann. Rept. U. S. Geol. Survey, 

pt. 2,1894, p. 273.

11 Weed, W. H., Geology and ore deposits of the Elkhorn mining district, Jefferson County, Mont.: 

Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2,1901, p. 450.

« Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, pp. 107,220,222-223.

i3 Spurr, J. E., and Garrey, G. H., Economic geology of the Georgetown quadrangle (together with the 

Empire district), Colorado, with general geology by S. H. Ball: Prof. Paper U. S. Geol. Survey No. 63,1908, 

p. 148.
CHEMICAL RELATIONS AND MINEEALS OP GOLD. 125

Rio Tinto, Spain.1 The interesting occurrences of primary tetrahedrite veinlets which cut older sulphides in the Mina Mexico vein, 

Sonora,2 are mentioned on page 46.

Tetrahedrite has not been formed synthetically under conditions 

that prevail in the secondary sulphide zone. The formula of the 

argentiferous variety, freibergite, has been written as a combination 

of chalcocite, argentite, and stibnite 4(Cu2S.Ag3S)Sb3S3 but since 

these minerals have dark streaks and freibergite has a reddish-brown 

streak, this interpretation of the formula is open to question. Possibly the ruby-silver molecule is present.

Tennantite, 4Cu2S.As2S3 , is regarded as having the same, range of 

occurrence as the corresponding antimony sulphide, tetrahedrite. It 

is not so common, however, and comparatively few detailed descriptions of its occurrence as a secondary mineral are available. It is 

said to be secondary at Butte, Mont.,3 where, according to Emmons 

and Tower, it seems to result from the decomposition of enargite, 

with which it is always associated.

Other silver minerals. Several other silver minerals, all comparatively rare species, have been regarded as secondary. Among these are 

stromeyerite,4 AgCuS; dyscrasite,5 Ag6Sb(?); and possibly stetefeldtite.6 Of these, stromeyerite corresponds in composition ,to argentiferous chalcocite. In the Yankee Girl mine, near Silverton,"Colo. 

it is associated with chalcocite and bornite. Since secondary sulphides of silver and copper are precipitated together, it would be supposed that stromeyerite would develop in many deposits. The silver 

compound in argentiferous chalcocite has not yet been identified, 

however.

SOME CHEMICAL RELATIONS AND MINERALS OF GOLD. 

SOLUBILITY OF GOLD.

Although gold belongs to the same chemical family as copper and 

silver it differs in many respects from both these metals. It forms no 

insoluble compound in the oxidized zone and its sulphide is not 

precipitated by mineral waters. Unlike copper and silver it is 

insoluble in sulphuric acid. Wurtz 7 stated, in 1858, that ferric sulphate dissolves gold, and his statement has frequently been quoted

1 Finlayson, A. M., The pyritic deposits of Huelva, Spam: Econ. Geology, vol. 5,1910, p. 411.

* Hynes, D. P., Notes on the geology of the Mina Mexico vein: Econ. Geology, vol. 7,1912, pp. 285-286.

3 Weed, W. H., Emmons, S. F., and Tower, G. W., Butte special folio (No. 38), Geol. Atlas U. S., U. S. 

Geol. Survey, 1897, p. 6.

< Emmons, S. F., The secondary enrichment of ore deposits: Trans. Am. Inst. Min. Eng., vol. 30,1901, 

p. 190.

& Smith, George, The ore deposits of the Australian Broken Hill Consols mine, Broken Hill, New South 

Wales: Trans. Am. Inst. Min. Eng., vol. 26,1897, pp. 69-78.

6 Spurr, J. E., Ore deposits of the Silver Peak quadrangle, Nevada: Prof. Paper U. S. Geol. Survey No. 55, 

1906, p. 120.

7 Wurtz, Henry, Contributions to analytical chemistry: Am. Jour. Sci., 2d ser., vol. 26, 1858, p. 51.
126 THE ENRICHMENT OF SULPHIDE ORES.

in discussions of the processes of enrichment of gold deposits. It is, 

indeed, a common statement that ferric sulphate is the principal 

agent in the enrichment of gold deposits. Stokes * showed, however, that ferric sulphate will not dissolve gold, even at 200° C., 

except in the presence of a chloride, and its insolubility in ferric 

chloride and in ferric sulphate at ordinary temperatures has been 

verified by A. D. Brokaw.2 W. J. McCaughey 3 found that gold is 

dissolved in a concentrated solution of hydrochloric acid and ferric 

sulphate and also in a concentrated solution of hydrochloric acid and 

cupric chloride, both experiments being carried on at temperatures 

from 38° to 43° C. Pearce,4 Don,5 and Rickard 8 performed experiments in which gold was dissolved in the presence of a chloride and 

manganese dioxide. Hydrochloric acid forms in the presence of 

sodium chloride and sulphuric acid, and in the presence of an oxidizing agent the hydrogen ion is removed to form water, leaving the 

chlorine in the so-called "nascent state." In this state it is in the 

uncombined or atomic condition and its attack is more vigorous. It 

is known that several oxides will release "nascent chlorine" at low 

temperatures if the solutions are sufficiently concentrated, but in 

moderately dilute solutions manganese oxides are probably the only 

common ones that are appreciably effective. Ferric chloride, ferric 

sulphate, and cupric salts are not more than a fraction of 1 per cent 

as effective as manganese salts and doubtless this fraction is exceedingly small.7

A number of experiments on the solubility of gold in cold dilute 

solutions were made at the request of the writer by Mr. A. D. 

Brokaw.8 The nature of these experiments is shown by the following statements, in which a and & represent duplicate tests:

(1) Fe2 (SO4 ) 3 + H2S04 +Au.

(a) No weighable loss. (34 days.) 

(&) No weighable loss. *

(2) Fe2 (S04 ) 3 + H2S04 + Mn02 + Au.

(a) No weighable loss. (34 days.) 

(&) 0.00017 gram loss. 9

1 Stokes, H. N., Experiments on the solution, transportation, and deposition of copper, silver, and gold: 

Econ. Geology,-vol. 1,1906, p. 650.

2 Brokaw, A. D., The solution of gold in the surface alterations of ore bodies: Jour. Geology, vol. 18, 

1910, p. 322.

s McCaughey, W. J., The solvent effect of ferric and cupric salt solutions upon gold: Jour. Am. Ch'em. 

Soc., vol. 31,1909, p. 1263.

« Pearce, Richard, Discussion of paper by T. A. Rickard, on "The origin of the gold-bearing quartz of 

the Bendigo Reefs, Australia": Trans. Am. Inst. Min. Eng., vol. 22,1894, p. 739.

6 Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27,1898, p. 599. 

c Rickard, T. A., The Enterprise mine, Rico, Colo.: Trans. Am. Inst. Min. Eng., vol. 26, 1897, pp. 

978-979.

7 Emmons, W. H., The agency of manganese in the superficial alteration and secondary enrichment of 

gold deposits in the United States: Bull. Am. Inst. Min. Eng., 1910, pp. 789-791. 

s Brokaw, A. D., op. cit., pp. 321-326.

8 This duplicate was found to contain a trace of 01, which probably accounts for the loss.
CHEMICAL BELATIONS AND MINEBALS OF GOLD. 127

(3) FeCl3 + HCl+Au.

(a) No«weighable loss. (34 days.) 

(6) No weighable loss.

(4) FeCl 3 + HCl+Mn02 +Au.

(a) 0.01640 gram loss. Area of plate, 383 square millimeters. (34 days.)

(&) 0.01502 gram loss. Area of plate, 348 square millimeters.

In each experiment the volume of the solution was 50 cubic centimeters. The solution was one-tenth normal with respect to ferric 

salt and to acid. In the second and fourth experiments 1 gram of 

powdered manganese dioxide was also added. The gold, assaying 

0.999 fine, was rolled to a thickness of about 0.002 inch, cut into 

pieces of about 350 square millimeters area, and one piece, weighing 

about 0.15 gram, was used in each duplicate.

To approximate natural waters more closely, a solution was made 

one-tenth normal as to ferric sulphate and sulphuric acid and onetwenty-fifth normal as to sodium chloride. Then 1 gram of powdered 

manganese dioxide was added to 50 cubic centimeters of the solution, 

and the experiment was repeated. The time was 14 days.

(5) Fe2 (S04 ) 3 + H2S04 + NaCl+Au. . No weighable loss.

(6) Fe2 (S04 ) 3 + H2S04 + NaCl + Mn02 + Au. 

Loss of gold, 0.00505 gram.

The loss is comparable to that found in experiment 4, allowing for 

the shorter time and the greater dilution of the chloride.

Although the concentration of chlorine in most of these experiments is greater than that which is found in many mineral waters, 

it is noteworthy that solution of gold will take place with even a 

trace of chlorine (see experiment 2&), and without much doubt these 

reactions will go on also in the presence of only minute quantities of 

manganese oxides.

The solution of gold is probably most important, however, in the 

upper parts of the oxidized zones where most of the pyrite has been 

removed, for on oxidation pyrite yields ferrous sulphate, which tends 

to inhibit solution. The small amount of sulphuric acid which seems 

to be necessary for the reactions could easily be supplied, even above 

the zone where pyrite persists, by the leaching of basic iron sulphates, 

the formation of which tends to delay the downward migration of a 

portion of the sulphuric acid that is released where ferric sulphate 

alters to limonite.
128 THE ENRICHMENT OF SULPHIDE ORES.

PRECIPITATION OF GOLD WITH MANGANESE OXIDE.

Where it is held in solution as chloride, gold is readily precipitated 

by ferrous sulphate, which, as already stated, is formed by the action 

of acid on pyrite or other iron sulphides. If much manganese oxide 

is present, however, the ferrous sulphate is immediately oxidized to 

ferric sulphate, which does not precipitate gold from solutions in which 

it is held as chloride. In the presence of manganese oxides, therefore, gold is not only dissolved in acid solution, but the conditions 

under which it is precipitated may be delayed. Gold may be carried 

in acid solution so long as the higher oxides of manganese are present.

In many gold deposits manganese oxides and gold are intimately 

associated .and without doubt have been precipitated together. 

This association is by no means uncommon. The deposits of the 

Camp Bird mine, 1 of the Tomboy mine,2 of the Amethyst vein at 

Creede, Colo.,3 of the Dahlonega mines, in Georgia,4 and of mines 

at Philipsburg, Mont.,5 are noteworthy examples.6

These relations indicate a process by which gold is precipitated 

along with manganese oxide in a reducing environment. On this 

point Mr. R. C. Wells offers the following statement:

The precipitation of manganese will occur more and more as the solution loses 

its acidity. It is well established that manganous salts in an acid environment 

are very stable; but in neutral or alkaline solutions they oxidize more vigorously, 

ore stage of their oxidation being the manganic salt, which hydrolyzes into 

Mn 2Os.H2O (manganite) with even greater ease than ferric salts into limonite.

In these ways the migration of an acidic solution would result in the transportation 

of both gold and manganese. But in a region of basic, alkaline, and reducing environment the manganese would be reprecipitated, the free acid neutralized, the chlorine absorbed by the bases and removed, and owing to the accumulation of the ferrous 

or other reducing salts the gold would be reprecipitated.

Some experiments in the precipitation of gold have recently been 

made by Mr. A. D. Brokaw, who offers me the following data in 

advance of publication. Into an acid solution in which gold was 

dissolved in the presence of manganese a crystal of calcite was introduced. On decreasing acidity of the solution with calcium carbonate gold was precipitated with manganese oxide on the surface and 

in the cleavage cracks of the calcite crystals. The reactions may be 

stated as follows:

2AuCl3 + 3MnCl2 + 6H20^2Au + 3Mn02 + 12HC1, 

or 2 AuCl3 + 3MnS04 + 6H20<=±2 Au + 3Mn02 + 3H2S04 + 6HC1, 

or 2 Au+++ + 3Mn++->2 Au

, ' Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull. U. S. 

Geol. Survey No. 182, 1901, p. 202.

2 Purington, C. W., Preliminary report on the mining industries of the Telluride quadrangle, Colorado: 

Eighteenth Ann. Kept. U. S. Geol. Survey, pt. 3, 1908, pp. 838-841.

8 Emmons, W. H., and Larsen, E. S., A preliminary report on the geology and ore deposits of Creede, 

Colo.: Bull. U. S. Geol. Survey No. 530, 1913, p. 58.

4 Laney, F. B., oral communication.

5 Emmons, W. H., and Calkins, F. C., Geology and ore deposits of Philipsburg, Mont.: Prof. Paper 

U. S. Geol. Survey No. 78 (in press).

6 See also Tolman, C. F., jr., Secondary sulphide enrichment of ores: Min. and Sci. Press, vol. 106, 

1913, p. 41.
CHEMICAL RELATIONS AND MINERALS OF GOLD. 129

These reactions indicate processes by which gold held in acid 

solution in the presence of manganese salts may be precipitated in 

the deeper zones, together with manganese oxides, when the solutions reacting on alkaline minerals lose acidity. In general, however, 

the manganiferous gold ores that are formed in the lower zones of 

gold deposits carry more manganese than gold and a larger proportion of manganese than would be precipitated by the reaction above 

indicated. Possibly some of the manganese is precipitated on neutralization as hydroxide, which oxidizes almost immediately on being 

exposed to air, or perhaps as some other manganese compound that 

is soluble in acid but insoluble in alkaline solutions.

In some deposits there is evidence that gold has been dissolved 

and reprecipitated in the surficial zone, yet the secondary gold ore 

carries no manganese, or at least not more than a trace of manganese compounds. Such relations, according to Ransome, 1 are indicated by the gold veins of Farncomb Hill, in the Breckenridge district, Colorado, where the secondary gold ore is almost free from manganese. It has been stated that gold may be precipitated from solutions in which it is dissolved as chloride either by the neutralization of 

the solution or by ferrous sulphate generated by the action of the 

solution on pyrite. Ferrous sulphate will precipitate gold even from 

strongly acid solutions in which manganese would still remain in 

solution. It follows that manganese might not be precipitated with 

gold from acid solutions even where manganese dioxide has supplied 

conditions for its solution in chloride waters; but from neutralized 

solutions the gold and manganese would go down perhaps simultaneously. Gold is precipitated from chloride solution also by native 

metals, sulphides, organic matter, and many other materials.

The size of the particles of gold has little value as a means of determining its origin. The gold deposited by the reduction or the neutralization of manganiferous compounds may be finely pulverulent or it 

may form nuggets of consider able size. Mr. Brokaw, in the experiments 

cited above, where gold and manganese were precipitated on calcite, 

obtained masses of gold distinctly visible without a hand lens.

In some deposits the purity or fineness of the gold may afford a 

means by which primary may be distinguished from secondary gold. 

In a deposit in northern Nevada the rich gold ore is electrum, a lightcolored natural alloy of gold and silver. On this, deposited presumably from cold solutions, are small masses of dark-yellow gold which 

is apparently of great fineness. This criterion should be applied with 

caution, however, and the relations should be established independently for each particular deposit, for under some conditions doubtless 

secondary gold and silver are precipitated together.

i Kansome. F. L., Geology and ore deposits of the Breckenridge district, Colorado: Prof. Paper U. S. 

Geol. Survey* No. 75,1911, p. 170.

72293° Bull. 529 13 9
130 THE ENRICHMENT OF SULPHIDE ORES.

SOURCES OF MANGANESE IN GOLD DEPOSITS.

It has been shown that the solution of gold depends on the presence 

of chlorine in the mineral waters, and the distribution of chlorine has 

been discussed briefly on page 66. But chlorine is an efficient solvent 

of gold only when in the "nascent" state, and under natural conditions nascent chlorine is released principally by manganese oxides. 

The sources and distribution of manganese are therefore of some 

importance in connection with the concentration of gold. The 

important manganese minerals are listed below.

Pyrolusite...................................Mn02 .

Psilomelane................................ Mn03 .2H20.

Manganite..................................Mn.,03 .H20.

Manganosite................................MnO.

Pyrochroite..................................Mn(OH)2 .

Szmikite....................................MnS04 .H20.

Mallardite...................................MnS04 .7H2 0.

Apjohnite...................................MnS04 .Al2(S04)3 .24H20.

Alabandite.................................. MnS.

Hauerite................................... MnS2 .

Manganostibnite............................. 9MnO.Sb208 .

Manganiferous calcite........................ (Ca,Mn)C03 .

Rhodochrosite............................... MnC03 .

Rhodonite...................................MnSi03 .

Amethystine quartz. 

Many rock-making silicates.

The commonest sources of manganese are probably the silicates of 

iron and magnesia. Manganese is more abundant in basic than in 

acidic rocks. The average of 1,038 determinations of igneous rocks 1 

is 0.10 per cent manganese protoxide (MnO), while some contain more 

than 1.0 per cent.

Amethystine quartz is an important source of manganese in some 

auriferous deposits. It is an abundant gangue mineral at Creede, 

Colo., where it is. the chief primary source of manganese. Some of 

the shipments of the ores from the Amethyst lode contain 3 or 4 

per cent of manganese.

Rhodonite is closely allied to the pyroxenes in crystallization, but, 

unlike them, it occurs as a gangue mineral formed by ascending 

waters at moderate depths. It is found in ore of the Camp Bird, 

Tomboy, and other mines of the San Juan, Colo., in the silver-bearing 

veins of Butte, Mont., at Philipsburg, Mont., and at many other 

places.

Rhodochrosite is more common in mines of the United States than 

rhodonite. It is present in many veins of the San Juan, Colo., at 

Butte and Philipsburg, Mont., at Austin, Nev., and in many other 

western districts.

1 Clarke, F. W., The data of geochemistry, 2d ed.: Bull U. S. Geol. Survey No. 491,1911, p, 27,
CHEMICAL RELATIONS AND MINERALS OF GOLD. 131

Manganiferous calcite, an isomorphous compound of rhodochrosite 

and calcite, is a source of abundant manganese in many later Tertiary 

deposits in Nevada, Montana, and Colorado. It is probably present 

in certain Mesozoic copper deposits of Shasta County, Cal., where, 

according to Graton, 1 wad occurs in the oxidized zone.

Rhodonite is considered primary in all its occurrences and 

rhodochrosite is deposited in the main from ascending hot solutions 

also, but in some occurrences it has been regarded as a secondary 

deposit from cold solutions.

Manganiferous precious-metal ores are in general deposited at 

moderate depths. Consequently they are common in middle or late 

Tertiary deposits of western North America but very rare indeed in 

contact-metamorphic deposits in the California gold veins and in 

deposits formed at equal or greater depths.2

The sulphates of manganese are soluble and do not accumulate in 

veins to any important extent; all are secondary. The oxides and 

hydrous oxides, which are very numerous, are the products of weathering of all manganese compounds. None of them are known to be 

deposited by ascending hot waters except near the orifices of hot 

springs.

The sulphides of manganese are exceedingly rare. Alabandite has 

been found in the gold mines of Nagyag, in Transylvania; at Gersdorf, 

near Freiberg, Saxony; on Snake River, Summit County, Colo.; at 

Tombstone, Ariz.; and at a few other places. Hauerite is still less 

common. I can find no record of its occurrence in the United States. 

The obvious explanation for. the rarity of manganese sulphides in 

nature is their comparatively great solubility even in very dilute 

acids, and their tendency to oxidize on exposure to air.

PLACERS AND OUTCROPS.

Those deposits in which enrichment in gold, is believed to have 

taken place are, almost without exception, manganiferous. Inasmuch as enrichment is produced by the downward migration of 

the gold instead of by its superficial removal and accumulation, it 

should follow that both gold placers and outcrops rich in gold would 

generally be found in connection with nonmanganiferous deposits; 

and this inference is believed to be confirmed by field observations.3 

Placer deposits are in general associated with nonmanganiferous lodes, 

and such lodes are generally richer at the outcrops and in the oxidized

1 Graton, L. C., The occurrence of copper In Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430, 

1910, p. 100.

* Emmons, W. H., The agency of manganese in the superficial alteration and secondary enrichment of 

gold deposits in the United States: Bull. Am. Inst. Min. Eng. No. 46,1910, pp. 813,814-816.

'Idem, pp. 767-837; Jour. Geology, vol. 19,1911, pp. 15-46.
132 THE ENRICHMENT OF SULPHIDE OEES.

zones than in depth, the enrichment being due, in the main, to a 

removal of the material associated with gold. Even under favorable 

conditions, however, gold is generally dissolved less readily than copper or silver and precipitated more readily than either. Consequently 

its enriched ores are likely to be found nearer the surface. In a 

manganiferous calcite gangue gold may accumulate at the very 

outcrop, for the solutions could not long remain acid if passing 

through alkaline rocks. Some placer deposits even are associated 

with gold lodes having a manganiferous calcite gangue.

CONCENTEATION IN THE OXIDIZED ZONE.

The concentration of gold in the oxidized zone near the surface, 

where the waters remove the valueless elements more rapidly than 

gold,1 is an important process in lodes which do not contain manganese 

or in manganiferous lodes in areas where the waters do not contain 

appreciable chloride. In the oxidized zone in some mines it is difficult to distinguish the ore which has been enriched by this process 

from ore which has been enriched lower down by the solution and 

precipitation of gold and which, as a result of erosion, is now nearer 

the surface. It can not be denied that fine gold migrates downward 

in suspension, but this migration probably does not occur to an 

important extent in the deeper part of the oxidized zone. If the 

enrichment in gold is due simply to the removal of other constituents 

it is important to consider the volume relations and the mass relations 

before and after enrichment and to compare them with those now 

prevailing. It can be shown that the enriched ore in some lodes 

occupies about the same space that it occupied before oxidation. 

Let it be supposed that a pyritic gold ore has been altered to a limonite 

gold ore, and that gold has been neither removed nor added. Limonite (specific gravity 3.6 to 4), if it is pseudomorphic after pyrite 

(specfic gravity 4.95 to 5.10) and if not more cellular, weighs about 

75 per cent as much as the pyrite. In those specimens which I 

have broken cellular spaces occupy in general about 10 per cent of the 

volume of the pseudomorph. With no gold added, the secondary 

ore should not be more than twice as rich as the primary ore, even if 

a large factor is introduced to allow for silica removed and for cellular 

spaces developed.

Rich bunches of ore are much more common in the oxidized zone 

than in the primary sulphides of such lodes. They are present in 

some lodes which carry little or no manganese in the gangue and 

which below the water level show no deposition of gold by descending 

solutions. Some of them are doubtless residual pockets of rich ore 

that were richer than the main ore body when deposited as sulphides,

i Rickard, T. A., The formation of bonanzas in the upper portions of gold veins: Trans. Am. Inst. Min. 

Eng., vol. 31,1902, pp. 198-220.
CHEMICAL RELATIONS AND MINERALS OP GOLD. 133

but others are very probably ores to which gold has been added in 

the process of oxidation near the water table by the solution and 

precipitation of gold in the presence of the small amount of manganese 

contributed by the country rock. In view of the relations shown by 

the chemical experiments it is probable that a very little manganese 

will accomplish the solution of gold, but it requires considerably more 

manganese to form appreciable amounts of the higher manganese 

compounds that delay the deposition of gold, suppressing its precipitation by ferrous sulphate. In the absence of larger amounts of the 

higher manganese compounds the gold would probably be precipitated 

almost as soon as the solutions encountered the zone where any considerable amount of pyrite or other sulphides was exposed in the 

partly oxidized ore; for oxygenated solutions dissolve pyrite, and 

even traces of the ferrous sulphate formed in the oxidizing reactions 

precipitate gold almost immediately. Many other sulphides precipitate gold very readily. From this it follows that deposits showing 

only traces of manganese, presumably supplied from the country rock, 

are not enriched far below the zone of oxidation.

VEETICAL RELATION OP DEEP-SEATED ENRICHMENT IN GOLD TO

CHALCOCITIZ ATION.

In several of the great copper districts of the West gold is a valuable 

by-product. In another group of deposits, mainly of Tertiary age 

and younger than the copper deposits, silver and gold are the principal 

metals, and copper, when present, is only a by-product. But in some 

of these precious-metal ores chalcocite is nevertheless the most abundant metallic mineral and constitutes several per cent of the vein 

matter. In many ores it forms a coating over pyrite or other minerals. Some of this ore, appearing in general not far below the water 

table, is fractured, spongy quartz coated with pulverulent chalcocite. 

A part of it contains a good deal of silver and more gold than the 

oxidized ore or the deeper-seated sulphide ore. Clearly the conditions that favor chalcocitization are favorable also to the precipitation 

of silver and gold.

The replacement of pyrite by chalcocite is, according to Stokes,1 as 

follows:

5FeS2 + 14CuS04 + 12H20 = 7Cu2S + 5FeS04 + 12H2S04 .

This reaction is considered by Spencer 2 as comprising several stages, 

which may be indicated as follows:

2FeS2 + 2CuS04 = Cu2S + 2FeSO4 + S.

3S + 2CuSO4 = Cu,S + 4SO2 .

6H2 0 + 5S02 + 2CuS04 = Cu2S + 6H2S04 .

' Stokes, H. N., Experiments on the action of various solutions of pyrite and marcasite: Econ. Geology, 

vol. 2, 1907, p. 22. 

8 Spencer, A. C., Chalcocite deposition: Jour. Washington Acad. Scl., vol. 3,1913, p. 73.
134 THE ENRICHMENT OP STTLPHIDE OEES.

The reactions may differ as to details, but without much doubt 

ferrous sulphate and acid are set free. Attendant reactions confirm 

this statement, for if calcite is present gypsum is formed by the reaction of sulphuric acid on lime carbonate, and if the wall rocks are 

sericitic kaolin is formed by the reaction of the acid with potassiumaluminum silicate, the potash going into solution as sulphate. The 

abundant ferrous sulphate must quickly drive the gold from solution, 

and it apparently follows that there may be no appreciable enrichment of gold below the zone where chalcocitization is the prevailing 

process. In deposits like disseminated chalcocite in porphyry, where 

the chalcocite occurs in flat-lying zones conforming with the present 

surface, where the ore from which chalcocite was derived carried 

gold, and where suitable solvents were provided, the gold, at least 

in the upper part of the chalcocite zone, should be rather evenly 

distributed and should increase and decrease in quantity with the 

chalcocite of the secondary ore. According to report there is a 

fairly constant ratio between copper and gold in the disseminated 

deposits at Ely, Nev., and Bingham, Utah. In the copper deposits 

at Rio Tinto, Spain,1 the secondary ores between the gossan and 

the primary sulphides carried high values in gold and silver. In 

view of the chemical relations it would appear that whatever gold 

and silver are present below a chalcooitized pyrite ore are, without 

much doubt, primary.

Gold and silver are commonly associated in their deposits, and an 

exact knowledge of the" solution and precipitation of the two metals 

hi experiments where both are present would have great practical 

value. Although gold is dissolved in chloride solutions, silver 

chloride is but slightly soluble, and high concentration of the two 

metals could not exist in the same solution. There is so little exact 

information regarding the solubilities of gold chloride and silver 

chloride in solutions containing both metals that a discussion of 

their relations is little more than speculation, yet certain data should 

be considered in this connection. Silver chloride is slightly soluble 

in water, and silver may be held in small concentration in solutions 

hi which gold also is dissolved. A mine water from the Comstock 

lode, according to Reid,2 carried 188 milligrams of silver and 4.15 

milligrams of gold in a ton of solution. Since ferrous sulphate and 

certain sulphides precipitate both gold and silver from acid solution, alloys of these metals might form as secondary minerals. 

Since ferrous sulphate is released hi the chalcocitization of pyrite, 

the secondary ores of gold and silver could be deposited simultaneously with chalcocite.

> Vogt, J. H. L., Problems in the geology of ore deposits, in PoSepny, Franz, The genesis of ore deposits, 

1902, p. 676.

1 Eeid, J. A., The structure and genesis of the Comstock lode: Bull. Dept. Geology Univ. California, 

vol. 4, No. 10,1905, p. 193.
CHEMICAL RELATIONS AND MINERALS OF LEAD. 135

GOLD AND SILVEE TELLURIDES.

Petzite, sylvanite, krennerite, and calaverite are tellurides of gold 

and silver, the precious metals being present in varying proportions. 

All are regarded as primary. Since hydrogen telluride (H2Te) is 

made by the action of acids on metallic tellurides, and since gold and 

silver tellurides are insoluble,1 telluride enrichment appears to be 

within the bounds of probability, but no clearly defined examples of 

secondary gold-silver tellurides are known to me.

SOME CHEMICAL RELATIONS AND MINERALS OF LEAD. 

PRINCIPAL LEAD MINERALS.

The chemical composition of the principal lead minerals is given 

below :

Minium. ....................................... . 2PbO.Pb02.

Massicot. ........................................ PbO.

Plattnerite....... . ............................ . .Pb02 .

Pyromorphite. ................................. .3Pb3P208 .PbCl2.

Cotunnite. ...................................... .PbCl2 .

Plumbbjarosite. . ................................. Pb[Fe(OH)2]8[S04]4 .

Leadhillite......... . ......................... . ..4PbO.S03 .2C02 .H20.

Anglesite... . ................................ . . .PbS04 .

Ceruaite.. ....................................... PbCOj.

Galena........................................... PbS.

SOLUBILITIES OF LEAD COMPOUNDS.

The solubilities of several lead compounds, as determined by Kohl 

rausch, are as follows:

PbS04

0.041 

.00013

PbCU

14.9 

.05

PbC03

0.001 

.00003

Pb(N03)2

516.6 

1.4

The upper number in each column shows the number of grams of 

the anhydrous salt held in solution in a liter of water at 18° C.; the 

lower number shows the molar solubility or the number of mols 

contained in a liter of the saturated solution. 2

The common salts of lead are bivalent; the rarer ones are quadrivalent. As shown by the table above, all the common salts have 

low solubilities except the nitrates, which are not important in processes of sulphide enrichment. Freshly precipitated lead sulphide is 

dissolved sparingly in cold water, its solubility being 0.0000036 mol 

in a liter. (See p. 57.)

i Smith, Alexander, Introduction to inorganic chemistry, rev. ed., 1910, p. 403. 

8 Idem, p. 544.
136 THE ENRICHMENT OF SULPHIDE ORES.

The native metal and the oxides of lead (minium, plattnerite, and 

massicot) are of rare occurrence.

Lead chloride is moderately soluble, so cotunnite, PbCl2 , does not 

accumulate in the oxidized zones of lead deposits. Lead chlorophosphate, pyromorphite, is much more common. Lead carbonate 

is very sparingly soluble, so cerusite, PbC03 , is a comparatively stable 

mineral, as is the sulphate, anglesite, PbS04 . Although the solubility of the sulphate is low, it is nevertheless appreciable, as is indicated in quantitative analysis, for in order to prevent loss a precipitate 

of lead sulphate formed in sulphuric acid is washed with alcohol 

rather than with water. The transfer of lead as sulphate in small 

yet appreciable quantities was shown also in the experiments of 

Buehler and Gottschalk (p. 49).

As would be supposed from consideration of the relatively low solubility of its sulphate, lead is not extensively transferred in cold a'cid 

sulphate waters. R. C. Wells has shown that galena reacts with weak 

solutions of sulphuric acid more readily than pyrite or chalcopyrite, 

the reaction being attended by the liberation of hydrogen sulphide 

(p. 59). Whether oxygen is necessary for the solution of lead sulphide, as it is for the solution of copper sulphide, I am unable to 

state. The salts that form under natural conditions have low solubilities and coat the sulphides, inhibiting or at least retarding further action. Consequently lead sulphide dissolves slowly. It is dissolved in acid to only a slight extent and, like copper sulphide, would 

be deposited in an acid environment. Although several primary 

minerals contain lead,1 galena is the only one of these that is at 

ah1 common. All the other minerals listed above the chlorides, 

oxides, sulphates, and carbonates are probably formed as secondary 

minerals only.

The paragenetic evidence that galena is commonly precipitated 

below the water level by the reduction of acid sulphate waters is not 

abundant, but in the greater number of its occurrences galena is 

regarded as primary. Lead, like gold, migrates very slowly in cold 

solutions.

OCCURRENCE OP THE LEAD MINERALS.

Native lead. Although native lead is found in a number of lead 

deposits, 2 it is in very few so abundant as to become an important 

ore mineral. It is probably formed by reduction of oxygen salts of 

lead.

Pyromorphite, 3Pb3P208 .PbCl2 , is the principal metallic phosphate 

and is the only common lead mineral containing chlorine. It is an 

alteration product of lead ores that are exposed to waters carrying

1 Clarke, F. W., The data of geochemistry, 2d ed.: Bull. U. S. Geol. Survey No. 491,1911, p. 646.

2 Hintze, Carl, Handbuch der Mineralogio, vol. 1, pp. 333-334.
CHEMICAL KELATIOHS AND MINERALS OP LEAD. 137

chlorine and phosphoric acid. At Philipsburg, Mont., at Creede, 

Colo., and in the Coeur d'Alene district, Idaho,1 pyromorphite is 

characteristic of upper parts of the zones of oxidation. In these 

deposits it is associated with limonite and commonly carries silver, 

possibly as finely divided cerargyrite.

Cotunnite, PbCl2 , as already stated, is a comparatively rare mineral. 

It has been identified at few places, one of them in the crater of 

Vesuvius. Lead chloride is more soluble than lead chlorophosphate. 

The accumulation of the chloride in the oxidized zones of lead deposits 

is limited probably by the amount of phosphate available in the 

natural solutions.

Minium, massicot, and plattnerite, all oxides of lead, are comparatively rare. All are secondary.

Leadhillite, 4PbO.S03 .2C02 .H20, a basic sulphatocarbonate of lead, 

is found in ores altered by surface agencies and is probably confined 

to such deposits.

Anglesite, PbS04 , is known only as an alteration product. Most 

commonly it forms from galena, but it has been noted also as an 

alteration product of guitermanite,2 an arsenic sulphide of lead, 

and also of other minerals. According to Penrose,3 the sulphate 

forms as an intermediate product when galena is altered to cerusite, 

and at Leadville, Colo., S. F. Emmons 4 found that thin crusts of 

anglesite occurred between galena and the crust of carbonate which 

enveloped it. It is associated with carbonates and sulphide in 

oxidized zones of deposits at Bingham, Utah. 5 Boutwell describes 

an interesting specimen consisting of a kernel of galena covered with 

anglesite, which in turn is coated with cerusite, upon which is deposited yellow lead oxide, probably massicot. In the sideritic lead ores 

of the Coeur d'Alene district, Idaho, according to Ransome, 6 no 

anglesite has been detected, but the carbonate appears to form 

without the intermediate sulphate.

Anglesite is much more common than cerusite in the oxidized lead 

ores of Lake City, Colo.,7 and is of considerable economic value in 

this district. In the upper 100 feet of the Moro vein it is particularly 

abundant. It is common also in the lead ores at Creede, Colo.8

' Eansome, F. L., and Calkins, F. C., The geology and ore deposits of the Coeur d'Alene district, Idaho: 

Prof. Paper U. S. Geol. Survey No. 62,1908, p. 103.

2 Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colo.: Bull. U. S. Geol. 

Survey No. 182,1901, p. 87. c

 Penrose, R. A. F., Superficial alteration of ore deposits: Jour. Geology, vol. 2,1894, p. 297.

< Emmons, S. F., Geology and mining industry of Leadville, Colo.: Mon. U. S. Geol. Survey, vol. 12,1886, 

p. 546.

6 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 113.

 Ransome, F. L., and Calkins, F. C., op. cit., p. 132.

'Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. 

Geol. Survey No. 478, 1911, p. 61.

8 Emmons, W. H., and Larsen, E. S., Geology and ore deposits of Creede, Colo.: Bull. U. S. Geol. Survey 

(in preparation).
138 THE ENRICHMENT OP SULPHIDE ORES.

Cerusite, PbC03 , is unknown as a primary mineral but is common as an alteration product. It was abundant in the upper 

levels of mines at Bingham, Utah, where it was generally associated 

with anglesite and galena.1 It is an important ore mineral of the 

deposits at Leadville,2 in some veins of the Coeur d'Alene district,3 at 

Creede, and in several other lead-ore-districts. It is most abundant 

in deposits where the country rock is limestone but is not limited to 

such deposits. Very commonly the carbonate replaces galena 

metasomatically. According to Penrose 4 lead sulphate may form 

intermediately between the sulphide and carbonate, and its formation 

is favored by the lower solubility of the carbonate. (See p. 135.) In 

the Coeur d'Alene district the change is not everywhere a metasomatic 

replacement for, according to Ransome,6 cerusite is common in little 

vugs or crevices in the sulphides, where it is formed on unaltered 

galena by solutions that have percolated downward from the oxidized 

zone. As stated by Ransome, the oxidation of siderite may supply 

carbonate for cerusite. The reaction is-^-

4FeC03 + 3H2 0 + 02 = Fe2 (OH) 6Fe2 03 + 4C02 . 

With carbonic acid in solution cerusite may form as follows: 

PbS04 + H2C03 = PbC03 + H2S04 .

Galena, PbS, is a "persistent" mineral a mineral formed under 

many natural conditions. The most important deposits of galena in 

the United States are the disseminated ores of southeastern Missouri 

and the lead-silver ores of the Coeur d'Alene district, Idaho.8 The 

galena in Missouri is nearly pure lead sulphide, has a perfect cleavage, 

and is generally in cubic crystals; the galena of the Cceur d'Alene 

district contains appreciable silver, is generally not so well crystallized, and has a much less perfect cleavage, so that a freshly broken

fracture resembles that of steel and the mineral is called "steel 

galena" by the miners. Some antimonial galena has a similar fracture. Some galena is rich in silver, which is probably present as 

finely divided argentite, Ag2S.

The stability of galena in the oxidized zone has already been 

mentioned. The relative insolubility.of galena in natural waters is 

shown by the experiments of Buehler and Gottschalk (p. 49), and is 

confirmed abundantly by observations in the oxidized zones of 

deposits. Galena is commonly found in sluice boxes of placer mines

1 Boutvvell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, p. 112.

2 Emmons, S. F., Geology and mining industry of Leadville, Colo.: Mon. U. S. Geol. Survey, vol. 12,1886, 

p. 546.

»Eansome, F. L., and Calkins, F. C., op. cit., p. 92.

< Penrose, R. A. F., Superficial alteration of ore deposits: Jour. Geology, vol. 2,1894, p. 297. 

' Eansome, F. L., and Calkins, F. C., op. cit., p. 112. 

«Idem, p. 91.
CHEMICAL RELATIONS ANt) MINERALS OF LEAD. .

and is plowed up in the fields of the southwestern Wisconsin zinc 

district. Even in districts where the climate is comparatively moist, 

such as the Breckenridge and Cceur d'Alene districts, galena is found 

at the very outcrops of some ore veins.1

In the Co3ur d'Alene district, according to Ransome and Calkins, 

there is little evidence of enrichment of either silver or lead by cold 

solutions. In the Breckenridge district 2 the characteristic ore of 

the upper levels is argentiferous galena, more or less altered to 

cerusite, whereas in the lower levels sphalerite and pyrite become 

increasingly abundant, suggesting enrichment in the upper levels by 

addition of lead or by subtraction of zinc, or by both.

At Leadville, Colo., according to Blow,3 stringers of galena rich in 

silver penetrate the limestone below the bodies of carbonate ores. 

As noted by Vogt,4 and also by Finlayson,5 the pyritic ore of the 

Rio. Tinto mine in Spain is cracked and is generally seamed with 

deposits of chalcocite, bornite, chalcopyrite, and galena, and the 

galena, according to Finlayson, is commonly of most recent formation. It is not stated, however, that the galena was deposited by 

descending waters.

Weed6 has described secondary galena from the sulphide zone at 

Neihart, Mont., where it is associated with zinc blende and incrusted 

with polybasite; and Irving and Bancroft 7 state that some of the 

galena of the Lake City district, Colorado, is without doubt connected 

genetically with oxidation processes. In that district a gray coating 

of a lead compound, thought to be galena, occurs in cracks of shattered 

sphalerite and covers its exposed surfaces. Possibly the gray 

sphalerite observed in many deposits of the West owes its color to 

such a veneer of galena.

A survey of the literature of lead deposits for examples giving 

evidence of considerable secondary precipitation of lead sulphide in 

lode ores has not been very fruitful of results. On the other hand, 

there is much evidence of migration of lead in cold carbonated 

solution and of its precipitation as sulphide in the important lead 

and zinc ores of the Mississippi Valley. These processes, as already 

stated, do not come within the scope of this investigation.

1 Ransome, F. L., Geology and ore deposits of the Breckenridge district, Colorado: Prof. Paper U. S. 

Geol. Survey No. 75, 1911, p. 166. Ransome, F. L., and Calkins, F. C., Geology and ore deposits of the 

Cceur d'Alene district, Idaho: Prof. Paper U. S. Geol. Survey No. 62, 1908, p. 133.

* Ransome, F. L., Criteria of downward sulphide enrichment: Econ. Geology, vol. 5, 1910, p. 206.

3 Blow, A. A., The ore chutes and recent developments of Iron Hill, Leadville, Colo.: Trans. Am. Inst. 

Min. Eng., vol. 18, 1890, p. 169.

4 Vogt, J. H. L., Das Huelva-Kiesfeld: Zeitschr. prakt. Geologic, July, 1899, p. 249.

' Finlayson, A. M., The pyritic deposits of Huelva, Spain: Econ. Geology, vol. 5,1910, p. 421.

«Weed, W. H., The enrichment of gold and silver veins, in PoSepny, Franz, The genesis of ore deposits, 

1902, p. 485.

7 Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. 

Geol. Survey No. 478, 1911, p. 97.
140 THE ENRICHMENT OP SULPHIDE ORES.

SOME CHEMICAL RELATIONS AND MINERALS OF ZINC. 

PRINCIPAL ZINC MINERALS.

The chemical composition of the principal zinc minerals is shown 

below:

Goslarite..........................ZnS04 .7H20.

Smithsonite........................ ZnC03 .

Calamine..........................Zn2H2Si05 or 2ZnO.Si04 .H20.

Willemite......................... Zn2Si04 .

Hydrozincite....................... ZnC03 .2Zn02H2 or 3ZnO.C02 .2H20.

Aurichalcite........................ 2(Zn,Cu)C03 .3(Zn,Cu)02H2.

Sphalerite........................ .ZnS.

Wurtzite.......................... ZnS.

SOLUBILITIES OF ZINC COMPOUNDS.

The solubilities of several zinc salts determined by Kohlrausch are 

stated below:

ZnSO<.

531.2 

3.1

ZnClj.

2039. 

9.2

ZnC03 .

0.04 ? 

.0003 ?

Zn(N03 )2.'

1178. 

4.7

The upper number in each column shows the number of grams of 

the anhydrous salt held in solution in a liter of water at 18° C.; the 

lower number shows the molar solubility or the number of mols 

contained in a liter of the saturated solution.

Zinc is bivalent in all its compounds. The sulphate and chloride 

are very soluble and the carbonate is more soluble than carbonate of 

lead. A liter of pure water dissolves at ordinary temperature and 

pressure 0.0000706 mol of freshly precipitated zinc sulphide. (See 

p. 59.) The principal primary zinc mineral is the isometric form, 

sphalerite. Wurtzite, the hexagonal form of zinc sulphide, is comparatively rare. In some occurrences the primary zinc minerals are 

oxides, such as franklinite, zincite, and gahnite, but as these are rare 

or absent in sulphide deposits, they are not considered here.

As zinc sulphide is easily dissolved in sulphuric acid sphalerite 

would not form in a highly acid solution. If, through reactions with 

the wall rock, the solution should become feebly acid or neutral, then 

zinc sulphide could be precipitated. Alien and Crenshaw state that 

sphalerite is precipitated from acid as well as from alkahne solutions, 

but that wurtzite is precipitated only from acid solutions and is 

probably always secondary.1 Since zinc sulphide is one of the most 

soluble of the common sulphides it could not replace sulphides of

1 Alien, E. T., and Crenshaw, J. L., The sulphides of zinc, cadmium, and mercury; their crystalline 

forms and genetic conditions: Am. Jour. ScL, 4th ser., vol. 34, 1912, p. 359.
CHEMICAL RELATIONS AND MINERALS OF ZINC. 141

copper, silver, or lead. Possibly it could replace pyrite, marcasite, or 

pyrrhotite under certain conditions, but no examples of its pseudomorphs after these minerals are known to me. It is itself replaced 

by chalcocite and covellite and probably by argentite and galena.

There is every reason to suppose that zinc sulphide in carbonate 

rocks in the zinc deposits of the upper Mississippi Valley hi southwestern Wisconsin has been dissolved by oxygenated- waters and has 

been reprecipitated as sulphide in large amounts where the solutions 

were in contact with reducing agents.

Secondary zinc blende has been found in western ore deposits in 

other than limestone, but so far as the records show its occurrence in 

western deposits is exceedingly rare. Large quantities of zinc are 

undoubtedly dissolved from the outcrops and oxidized zones of 

deposits in which it occurs as sulphide and are passed as sulphate 

downward, below the water level. To account for its disposition is 

one of the most perplexing problems in the study of mineral genesis. 

Doubtless much of it escapes to the surface and is scattered, yet 

many analyses of waters issuing from zinc deposits show much less 

zinc than would be expected. In two alkaline waters from sphaleritic 

deposits of Creede, Colo., no zinc is reported. In waters from several 

other deposits it is present in considerable amounts, but the quantity 

of it found in some of these is less than would be expected. In waters 

from several deposits at Ducktown^Tenn., the zinc in solution is less 

than the copper, although the sulphide deposits contain as much zinc 

as copper and copper is now being precipitated. Some sludges 

deposited by mine waters where they issue to the air contain considerable zinc; others contain practically none. Zinc salts do not 

hydrolyze so readily as ferric salts when exposed to air. With carbon 

dioxide they form smithsonite, hydrozincite, and aurichalcite, but 

these compounds are generally less abundant than hydrous iron oxide.

OCCURRENCE OF THE ZINC MINERALS.

Goslarite, hydrated zinc sulphate, occurs on the walls of some mine 

workings as a white filiform or mosslike efflorescence, which soon 

disintegrates to powder in the air. Owing to the high solubility of 

zinc sulphate in water, goslarite is comparatively rare in most districts, though it is abundant in some, as at Creede, Colo., and Butte, 

Mont. Weed states that zinc sulphate is carried by the moisture of 

the air circulating in mine openings.1

Zinc chloride is even more soluble than the sulphate and is unknown 

as a natural mineral.

Smithsonite, ZnC03 , is commonly found in the oxidized zones of 

zinc-bearing veins. It is most abundant in deposits in limestone.

* Weed, W. H., Geology and ore deposita of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 99.
142 THE ENRICHMENT OF SULPHIDE ORES.

It has not been reported as a primary mineral in deposits formed by 

hot ascending waters. Where zinc-sulphate waters attack limestone, 

smithsonite is deposited, calcium sulphate, the most soluble salt, 

going into solution. The reaction may be stated as follows:

ZnS04 + CaC03 + 2H20 = CaS04 .2H20 + ZnC03 .

The calcium sulphate precipitated as gypsum may remain with 

smithsonite or may be carried away in solution, since it is fairly 

soluble in cold water (2.0 grams in a liter at 18° C.).

Calamine, H2Zn2Si06 , is commonly associated with smithsonite in 

the oxidized zones of zinciferous ores. It is not known as a primary 

mineral of ore veins deposited from hot solutions.

Willemite, Zn2Si04 , the anhydrous silicate, is much less common 

than calamine. It is abundant in the primary ores of Franklin Furnace, N. J.,1 but is not reported as occurring in the secondary ores 

at many zinc deposits. It is noted as an alteration product, however, at Morenci, Ariz.,2 and appears in considerable quantities at 

Tres Hermanas, N. Mex.,8 where it is probably derived from zinc 

blende by oxidation.

Hydrozincite, the basic carbonate, occurs commonly in altered ores.

Aurichaldte, a basic carbonate of zinc and copper, is deposited by 

cold solutions, generally as drusy incrustations.

Sphalerite is the most abundant primary ore of zinc. A few observations on the occurrence of secondary sphalerite are noteworthy. 

According to Blow,4 it seems probable that at Leadville, Colo., a large 

proportion of the zinc removed from the carbonate zone by surface 

waters was redeposited as sulphide in the lodes just below the line of 

complete oxidation and that redeposition has kept pace with oxidation. In Shasta County, Cal., according to Graton,5 sphalerite that 

is later than pyrite is found in the sulphide zones of copper deposits 

in association with minerals that are probably in part of secondary 

origin. Veinlets of sphalerite cut rhodochrosite ore at Philipsburg, 

Mont. At Rio Tinto, according to Finlayson,6 some sphalerite is 

later than pyrite and chalcopyrite. At Breckenridge, Colo., according to Ransome,7 some of the sphalerite is younger than the earliest 

period of deposition, although it can not be shown that it is'a deposit 

of sulphate water. There is not much evidence, however, that zinc

1 Spencer, A. C., Franklin Furnace folio (No. 161), Geol. Atlas U. S., U. S. Geol. Survey, 1908.

* Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper TJ.'S. 

Geol. Survey No. 43, p. 111.

«Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68,1911, p. 294.

4 Blow, A. A., The ore chutes and recent developments of Iron Hill, Leadville, Colo.: Trans. Am. Inst. 

"Min. Eng., vol. 18,1890, p. 172.

* Graton, L. C., The occurrence of copper in Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430, 

1910, pp. 71-111.

* Finlayson, A. M., The pyritic deposits of Huelva, Spain: Econ. Geology, vol. 5, 1910, p. 417. 

7 Ransome, F. L., Geology and ore deposits of the Breckenridge district, Colo.: Prof. Paper U. S. Geol. 

Survey No. 75,1911, p. 169.
CHEMICAL RELATIONS AND MINERALS OF IRON. 143

blende is precipitated as a secondary mineral along with copper, silver, and gold.

Wurtzite is probably a secondary mineral in the San Francisco district, Utah, according to B. S. Butler. In this connection the recent 

experiments of E. T. Alien and J. L. Crenshaw at the Carnegie 

Geophysical Laboratory are suggestive. Zinc sulphide was precipitated as wurtzite in acid but not in alkaline solutions.

SOME CHEMICAL RELATIONS AND MINERALS OF IRON. 

PRINCIPAL IRON MINERALS.

The chemical composition of the principal iron minerals is shown 

below:

Limonite.....................................-2Fe203 .3H20.

Hematite...................................... Fe203 .

Magnetite.................................... Fe3 04 . Melanterite.................................. .FeS04 .7H20.

Jarosite..................................... .K20.3Fe203 .4S04 .6H20.

Siderite....................................... FeC03 .

Ankerite.................................... .CaC03 .(Mg,Fe,Mn)CO,.

Pyrite........................................ FeS2 .

Marcasite...................................... FeS2 .

Pyrrhotite.................................... Fe7S8 .

Arsenopyrite...................................FeAsS.

Chalcopyrite..................................FeCuS2 .

Bornite..................................... .CusFeS4 .

SOLUBILITIES OF IRON SALTS.

No. comprehensive table showing the solubilities of iron salts determined under similar conditions is available to me. The solubilities 

of ferric and of ferrous salts differ greatly, and the experimental 

difficulties in determining the solubilities are considerable, owing to 

the changes that take place in the state of oxidation .of iron. According to Atherton Seidell, 1 a liter of water at 20° C. dissolves 264.2 

grams FeSO4 .7H2O; a liter of water saturated with CO2 dissolves, 

at 6 to 8 atmospheres, 0.73 gram FeC03 ; a liter of water at 15° C. 

dissolves 400 grams of FeCl2 .4H2O (Etard); a liter of water'at 20° 

C. dissolves 478.8 grams FeCl3 . (Total FeCl3 .12H20 is 918.5 grams.)

NATURE AND RELATIONS OF THE IRON MINERALS.

Iron sulphides are comparatively unimportant as ores of iron. 

Some of the cinder obtained from burning pyrite for making acid is 

utilized in blast furnaces, and the gossans of some iron sulphide 

deposits also are smelted, but few of the iron ores of commerce are 

associated with sulphide deposits. The iron sulphides nevertheless

> Seidell, Atherton, Solubilities of inorganic and organic substances, New York, 1911.
144 THE ENRICHMENT OP SULPHIDE ORES.

play a most important part in the alteration and enrichment of 

deposits of the more valuable metals, and it is in that connection that 

they are considered here.

Most -mine waters contain iron, and the facility with which ferric 

sulphate is changed to ferrous sulphate affords the most readily 

available index to the state of oxidation of the solutions. The 

chemical processes involved in the alteration of iron sulphides, the 

reduction of ferric sulphate, and the removal of acid to form inert 

sulphates below the water level have already been discussed.

Most of the iron sulphides contain more than enough sulphur to 

form iron sulphate on oxidation, and the remaining sulphate may 

be utilized to form sulphuric acid. Ferric, sulphate readily hydrolyzes, forming basic ferric sulphate and subsequently ferric hydroxide, 

and each step sets free sulphuric acid. The property of hydrolysis of 

ferric salts is most important in connection with the superficial alteration and the enrichment of sulphide ores. It supplies a solvent for 

most of the metals; it generally results in the concentration of iron 

oxide at the surface, which may afford an indication that a deposit 

containing iron sulphide is below.

One function of basic ferric sulphate, which ultimately breaks 

down to form oxide, is to supply sulphate ion to solutions in the 

oxidized zone. Sulphates may thus be available even after all 

sulphides have disappeared.

As a result of the hydrolyzation of ferric sulphate the larger portion 

of the iron is generally fixed near the surface as iron oxide. Not all 

is so fixed, however, for the analyses of mine waters show that iron 

is almost universally present in the underground circulation. It has 

been stated above that iron passes below the water level, held in 

sulphuric acid solution mainly as iron sulphate; that the sulphate 

solution reacts on sulphides and gangue minerals; and that the acidity 

is reduced by forming inert sulphates, chiefly those of alkalies and 

alkaline earths. Along with such changes in acidity ferrous sulphate 

is produced by reduction of ferric sulphate.

It is doubtful whether ferric sulphate exists to any great extent 

far below the water level. In air a decrease in acidity favors 

hydrolyzation, but below the water level, where air is excluded, the 

decrease in acidity is attended by reduction of iron to ferrous state. 

Ferrous sulphate does not hydrolyze to precipitate iron oxide. Ferrous hydrate may be formed in neutral or alkaline environment. It 

is a light-colored salt which unites rapidly with free oxygen to form 

the brown ferric hydroxide. The mixture has a greenish appearance 

when distributed in a finely divided state through the precipitate. 1

In little cracks in sulphide ore, at considerable depths below the 

highly oxidized zone, small veinlets and putty-like masses of an

i Ostwald, Wilhelm, The prlndples of inorganic chemistry (trans. by Alex. Findlay), London, 1904, 

p. 577.
CHEMICAL RELATIONS AND MINERALS OF IRON. 145

iron oxide that is generally assumed to be limonite are conspicuous 

in many deposits. Such deposits of iron oxides in the GraniteBimetallic vein at Philipsburg, Mont., are at least 500 feet below the 

zone of the highly oxidized ore. Some of them doubtless have formed 

since the mine was opened, at places where iron sulphate solutions 

issued from the vein and came in contact with air. Others may be 

due to deposition of iron oxide or basic ferric sulphate from suspension. (See pp. 73; 110.) Possibly some are the oxidation products of 

ferrous sulphate.

E. T. Alien has shown that a solution of hydrogen sulphide and 

ferric sulphate gives ferrous sulphate and sulphur, and in cold solutions of very low acidity ferrous sulphate with sulphur and hydrogen 

sulphide gives FeS2 as pyrite and marcasite, the amount of each 

depending somewhat upon the acidity of the solution. It has been 

shown above that these conditions probably exist in the deeper zones 

in some deposits, but almost certainly they exist through a comparatively narrow chemical range. Consequently, the precipitation 

of secondary iron sulphides, although known in some, is probably 

subordinate in most deposits. Secondary chalcopyrite has been 

discussed with the ores of copper.

OCCURRENCE OF THE IRON MINERALS.

Liinonite, 2Fe2 03 .3H20, is generally the most abundant of the iron 

oxides of the altered zones, which are mainly but not altogether 

hydrated oxides. It is precipitated from solution and commonly 

occurs as a pseudomorphous replacement of pyrite and other iron 

minerals. As shown in the equations stated on page 65, pyrite is 

converted in the vadose zone to ferrous sulphate. Since oxygen is 

present in excess, the ferrous salt is converted to ferric sulphate, 

which in turn hydrolyzes to ferric hydrate.

6FeS04 + 3O + 3H20 = 2Fe2 (S04 ) 3 + 2Fe(OH) 3 . 

4Fe(OH) 3 = 2Fe2 03 .3H20 + 3H20.

These reactions indicate merely certain steps in the processes. 

An intermediate step may involve the precipitation of basic ferric 

sulphate.

The several hydrated iron oxides are:

Ratio of 

Fe203 to H8O,

Turgite..........................2Fe203.H20................... 2 to 1

Goethite........................ Fe203. H20................... 2 to 2

Limonite.......................2Fe203. 3H20.................. 2 to 3

Xanthosiderite................... Fe203. 2H20.................. 2 to 4

Limnite........................ Fe203 . 3H20................. 2 to (5

Ferric hydrate, Fe(OH) 3 , corresponds to limnite,. but through 

processes of drying or with scant supplies of water the oxides which

72293° Bull. 529 13 10
146 THE ENBICHMENT OF SULPHIDE OBES.

are less highly hydrated will form. Thus limonite forms from 

limnite and turgite forms when limonite is dehydrated. Turgite, 

which contains some combined water, but less than limonite, is 

probably more common in gossans than is generally supposed. The 

hydrated iron oxides are probably not deposited by ascending hot 

waters except near the orifices of hot springs.

Hematite is a constituent of igneous rocks, of pegmatites, of contact-metamorphic deposits, and of veins of the deep zone. It is 

rarely formed by hot ascending waters at shallow or even at moderate 

depths. The specular anhydrous variety (specularite) is not a 

common secondary mineral, yet it may be formed under the conditions that prevail in dry, hot countries. Ransome 1 has described 

minute crystals of secondary hematite in outcrops at Goldfield, 

Nev. At the Modoc mine, Morenci, Ariz.,2 it is formed by the surface oxidation of magnetite and garnet. In tropical countries some 

residual iron ores and gossans carry abundant hematite. Although 

hematite is the principal oxide in deposits like those of the Lake 

Superior iron ranges, which have been formed by the weathering of 

iron oxides, carbonates, and silicates,8 it is noteworthy that limonite 

greatly predominates in the gossans of most sulphide ores, at least 

in those that have formed in a temperate climate.

Magnetite, Fe304 , is a common constituent of igneous rocks and of 

magmatic segregations.4 It is present in some pegmatites, in most 

contact-metamorphic deposits, in vein deposits of the deeper zones, 

and in some ore bodies that are transitional in character between 

the deposits of the deep zone and those formed at moderate depth. 

It very rarely fills fissures in ore bodies formed at moderate depth, 

but at some places it occurs in greatly altered wall rock near such 

filled fissures. Where the alteration is less intense, usually at some 

distance from the ore, small dots of magnetite are said to form as a 

result of alteration of the ferromagnesian minerals by the spent ascending ore-depositing thermal solutions. Where it is primary, magnetite 

may be a residual mineral in the oxidized zone of any deposit. Minute 

dark magnetite particles, few if any of them showing crystal form, are 

found also in the outcrops of many deposits formed at moderate 

depth, in which the primary ore appears to be free from magnetite. 

Their minute size and possibly their lack of crystal form will usually 

serve to distinguish these particles from particles of primary magnetite.

i Ransome, F. L., The geology and ore deposits of Goldfield, Nev.: Prof. Paper U. S. Geol. Survey 

No. 66, 1909, p. 124.

* Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 103.

»Van Hise, C. R., and Leith, C. K., The geology of the Lake Superior region: Mon. U. S. Geol. Survey, 

vol. 52,1911, p. 188.

«Hastings, J. B., Association of magnetite with sulphides.in mineral deposits: Min. and Sci. Press, 

vol. 97, Sept. 5, 1908, pp. 333, 358.
CHEMICAL RELATIONS AND MINERALS OP IRON. 147

The extent to which magnetite may form in the outcrop of a sulphide ore body is a matter of considerable importance in connection 

with the prospecting of gossans, and it has been suggested that it 

forms in considerable quantities near the surface as a result of certain 

peculiar climatic conditions. 1 Many outcrops of magnetite have 

been regarded as the cappings of sulphide ore bodies and have been, 

followed downward in the hope of finding sulphide ores of- gold, 

silver, or copper. At most places it has been found that massive 

magnetite is of primary origin and is persistent in depth. If rockmaking minerals or the heavy silicates, such as garnet, actinolite, 

chlorite, and dark mica, are intergrown with the masses of magnetite, 

it may safely be regarded as a primary deposit and not a product 

of oxidation.

Melanterite, jarosite, and other sulphates and basic sulphates form 

abundantly in the oxidized zones of some sulphide ores. The sulphates are highly soluble and are therefore unstable. In some mines 

they are conspicuously developed, however, on mine timbers and on 

the faces of ore and wall rock. Weed states that ferrous sulphate is 

carried by moisture of the air circulating in mine workings.2 Jarosite 

and other basic sulphates are much less soluble than the sulphates 

and persist in some lodes even at the outcrops. The basic sulphates,. 

as already stated, are probably formed as intermediate products in 

the hydrolyzation of ferric sulphate to form ferric hydroxide. The 

abundance of limonite in the gossans indicates that the basic sulphates 

ultimately lose the sulphate radicle to form hydroxides. By these 

processes, doubtless, the downward migration of some of the sulphuric acid is delayed and sulphuric acid is contributed to the solutions that percolate through the oxidized ore even after the sulphides 

have been removed.

Siderite and arikerite. Siderite is rare as a constituent of contactmetamorphic deposits, but it is found in several veins of the deep 

zone and also in the gangue or in the altered wall rock of deposits 

formed at moderate depths by hot ascending water. High temperatures are not necessary for its genesis, however, for it is most abundant in cherty iron carbonate ores of sedimentary origin. In lode 

ores siderite is in the main primary. Noteworthy examples are the 

silver-lead veins of the Wood River 8 and Coeur d?Alene 4 districts,

1 Graton, L. C., The occurrence of copper in Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430, 

1910, p. 106.

2 Weed, W. H., Geology and ore deposits of theButte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74, 1912, p. 99.

* Lindgren, Waldemar, The gold and silver veins of Silver City, De Lamar, and other mining districts in 

Idaho: Twentieth Ann. Kept. U. S. Geol. Survey, pt. 3, 1899, p. 190.

«Ransome, F. L.. and Calkins, F. C., The geology and ore deposits of the Coeur d'Alene district, Idaho: 

Prof. Paper U. S. Geol. Survey No. 62,1908, pp. 95-97.
148 THE ENBICHMENT OF SULPHIDE OBES.

Idaho. It is found less abundantly also in altered wall rock at 

Tonopah, Nev., 1 at Breckenridge, Colo.,2 and in many other districts.

As a secondary mineral siderite is not abundant. Ferrous carbonate 

dissolves readily in acids 3 and it could not form in a highly acid 

environment. At greater depths, where the environment is alkaline 

or basic, ferrous carbonate could be precipitated. Siderite is present 

in fractures in ore of the Highland Boy mine, Bingham, Utah,4 and 

according to Ransome ferrous carbonate is deposited in veinlets crossing the sulphide ore of Breckenridge.5 In some of the occurrences 

ferrous carbonate has been precipitated as ankerite, a salt that also 

contains, in isomorphous combination, lime carbonate, manganese 

carbonate, or magnesium carbonate. These salts are somewhat soluble in acid, but none are highly soluble in water.

Pyrite is formed under varying conditions of temperature and from 

solutions of varied character. It is one of the group termed by 

Lindgren "persistent" minerals. It is much more abundant, however, in ores associated with igneous processes than in deposits 

formed by cold solutions or in secondary deposits, although it is not 

unknown in the latter. Pyrite is contained in some coal beds and is 

common in sedimentary rocks remote from igneous intrusives. Iron 

sulphide is not deposited in highly acid waters and is probably not a 

common or abundant secondary sulphide, but it may be deposited 

in slightly acid or in neutral solutions. At the surface and above 

ground water pyrite oxidizes to basic sulphates and to limonite, but 

it alters in the presence of oxygen more slowly than marcasite or 

pyrrhotite. In some deposits it persists to the very surface, and, 

unoxidized, it is found in placer deposits, especially in northern, 

recently glaciated areas,8 though its occurrence in placers is not 

limited to these areas, for it is found in auriferous gravels in California,7 

Colorado, and other Western States.

Pyrite as well as marcasite may be obtained by the action of hydrogen sulphide on ferric sulphate solution. Hydrogen sulphide reduces 

ferric sulphate to ferrous sulphate, with the separation of sulphur, 

and further action of hydrogen sulphide on ferrous sulphate slowly 

gives FeS2 . The reaction is 

FeS04 + H2S + S = FeS2 +H2S04 .

1 Spurr, J. E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, p. 248.

2 Ransome, F. L., Geology and ore deposits of the Breckenridge district, Colorado: Prof. Paper U. S. 

Geol. Survey No. 75, 1911, p. 87.

»Ostwald, Wilhelm, The principles of inorganic chemistry (trans. by Alex. Findlay), London, 1904, p. 580.

«Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey, No. 38, p. 116.

s Ransome, F. L., op. cit., p. 169.

6 Brooks, A. H.( A reconnaissance of the Cape Nome and adjacent gold fields of Seward Peninsula, Alaska, 

in 1900: Special publication U. S. Geol. Survey, 1901.

7 Becker, G. F., Some features of the Rand Banket: Econ. Geology, vol. 4,1909, p. 379.
CHEMICAL BELATIONS AND MINEBALS OF IKON. 149

This reaction goes on, even at ordinary temperatures, in solutions 

that contain less than 1 per cent of free sulphuric acid. In solutions 

of greater acidity and at lower temperatures marcasite will form. 1 

Examples of secondary pyrite are rare, but some have been reported.

According to Spurr, Garrey, and Ball,2 pyrite is secondary in the 

Georgetown region, Colorado, where it has probably been deposited by 

descending surface waters. Hintze states that pyrite is pseudomorphous after silver glance at Joachimsthal. Veinlets of pyrite cut 

J>yrrhotite copper ore at Ducktown, Term., and appear to have been 

deposited by descending waters. At De Lamar, Idaho, secondary 

pyrite is associated with kaolin.3 In view of the widespread distribution of iron, however, it is remarkable that so few occurrences 

of secondary pyrite have been reported from the sulphide deposits in 

the western United States.

Marcasite is rare as a primary mineral of igneous rocks, of pegmatites, of contact-metamorphic deposits, and of veins of the deep 

zones. It is rarely formed at moderate depths by hot ascending 

waters but has been formed near the surface or at the orifices of hot 

springs. It is abundant in the lead and zinc deposits of the Mississippi 

Valley, where it appears to have been formed by cold solutions near 

the surface. As pointed out by Alien,* it is to be regarded as 

a low-temperature mineral deposited in acid solutions. In some 

pyritic veins it has formed as a secondary sulphide. It may be 

formed artificially by treating ferric sulphate with hydrogen sulphide. 

If the solution contains as much as 1 per cent of free sulphuric acid 

and the temperature is not above 100°, the product is all marcasite. 

At.higher temperatures or in more feebly acid solutions pyrite will 

form.

At Ducktown, Tenn., marcasite is found sparingly in cracks 

associated with manganese dioxide about 200 feet below the chalcocite 

zone. At Goldfield, Nev., according to Ransome,5 marcasite is 

rather characteristic of soft, crushed ore and of gouge and appears 

in many places to be of later origin than most of the other sulphides. 

It is a constituent of most of the high-grade ore and in much of it is 

as old as gold, goldfieldite, etc. It occurs as concentric, radially 

fibrous shells in soft, dark, and probably enriched ore streaks. As a 

rule these shells disintegrate on exposure to the air, with the formation of yellow films, apparently containing a little free sulphur, a 

brown fibrous efflorescence of iron sulphate. It is also a fairly

i Alien, E. T., Sulphides of iron and their genesis: Min. and Sci. Press, vol. 103,1911, p. 414.

8 Spurr, J. E., Garrey, G. H., and Ball, S. H., Economic geology of the Georgetown quadrangle, Colorado: 

Prof. Paper TJ. S. Geol. Survey No. 63,1908, p. 144.

3 Lindgren, Waldemar, The gold and silver veins of Silver City, De Lamar, and other mining districts in 

Idaho: Twentieth Ann. Kept. U. S. Geol. Survey, pt. 3,1900, p. 124.

* Alien, E. T., op. cit., p. 413.

»Ransome, F. L., The geology and ore deposits of Goldfield, Nev.: Prof. Paper U. S. Geol. Survey No. 

66,1909, p. 115.
150 THE ENBICHMENT OF SULPHIDE OSES.

abundant constituent of the rich primary ore, associated with gold, 

pyrite, goldfieldite, alunite, quartz, etc.

Pyrrhotite, so far as is indicated by its geologic relations, is 

distinctly a high-temperature mineral. This conclusion is supported also by the recent synthetical work done by Alien, Crenshaw, 

Johnston, and Larsen,1 who found that pyrrhotite is formed by the 

decomposition of pyrite in hydrogen sulphide at high temperatures. 

Pyrrhotite is a primary constituent of igneous rocks 2 and commonly 

occurs in magmatic segregations 3 in pegmatite veins in contact-metamorphic deposits and in veins formed,in the deeper zones. A number 

of its occurrences are mentioned on page 93. Winslow4 mentions the 

occurrence of pyrrhotite in the disseminated lead ores of southeastern 

Missouri, but Buckley 5 does not list it with the ore minerals of this 

district. As a secondary sulphide in lode ores it is unknown.

Arsenopyrite, FeAsS, is a constituent of many pegmatite veins and 

has been noted as a contact-metamorphic mineral. It is a primary 

mineral in deposits of the deep zone and in some lodes formed at 

moderate depths. In such deposits it commonly carries gold. It is 

not known as a secondary constituent of ores enriched by descending 

sulphate solutions.

(Jhalcopyrite and bornite are discussed on pages 110-111.

SOME GANGTJE MINERALS. 

NATUKE AND GENESIS.

Among the minerals found in metalliferous sulphide ores are many 

that are formed by processes of surface alteration only. These include 

a large number of the native metals, carbonates, silicates, oxides, 

and some other compounds. Among the sulphides not one is 

known to be formed exclusively by secondary processes, although 

several are secondary in nearly all their known deposits. On the 

other hand, the primary ores include several sulphides that are almost 

unknown as secondary minerals. A list of such sulphides should 

include arsenopyrite, bismuthinite, enargite?, molybdenite, pyrrhotite, and probably others.

The data concerning the genesis of many minerals are inconclusive, 

and to determine the origin of an ore one must. generally rely on 

paragenesis and mineral association rather than on the occurrence 

of isolated species. So far as they may serve as an indication of the

»Alien, E. T., Sulphides of iron and their genesis: Min. and Scl. Press, vol. 103,1911, p. 414. Alien, E. 

T., Crenshaw, J. L., and Johnston, John, The mineral sulphides of iron: Am. Jour. Sci., 4th ser., vol. 33, 

1912, p. 169.

'Bastin, E. S., A pyrrhotitic peridotite from Knox County, Maine: Jour. Geology, vol. 16,1908, p. 124.

'Coleman, A. P., The Sudbory nickel deposits: Kept. Ontario Bur. Mines, 1904.

«Winslow, Arthur, The disseminated lead ores of southeastern Missouri: Bull. U. S. Geol. Survey No. 

132,1896, p. 28.

* Buckley, E. E., Geology of the disseminated lead deposits of St. Francois and Washington counties, 

Mo.: Missouri Bur. Geology and Mines, voL 9, pt 1,1909.
SOME GANGUE MINEKALS. 151

genesis of the ore, the gangue minerals are perhaps as important as 

the sulphides.

It is difficult to draw sharp distinctions between gangue and ore 

minerals and neither of these terms is uniformly used with the 

same meaning. The gangue minerals are commonly distinguished 

as the earthy or nonmetallic minerals associated with the metalliferous minerals in the ore. As commonly used the term gangue 

does not include all minerals without metallic luster nor does it 

exclude all minerals that contain metals. The iron-bearing silicates 

are almost universally regarded as gangue minerals in deposits of 

metals other than iron.

In the table below the minerals listed are regarded as primary. 

Probably none of these are formed by processes of alteration and 

superficial enrichment. Most of them are nonmetallic or gangue 

minerals.

Some minerals which are probably not formed by processes of alteration and superficial

enrichment of sulphide ores.

Acmite.

Actinolite.

Adularia.

Albite.

Allanite.

Amphibolea.

Anhydrite.

Augite.

Biotite.

Chromite.

Corundum.

Cryolite.

Diopside.

Emery.

Garnet. '

Graphite.

Hornblende.

Humites.

Ilmenite.

Ilvaite.

Lepidolite.

Microcline.

Orthoclase.

Pyroxenes.

Rhodonite.

Rutile.

Scapolite.

Sillimanite.

Spinel.

Tourmaline.

Tremolite.

Vesuvianite.

Wollastonite.

Zircon.

Zoisite.

A number of gangue minerals are formed by processes of vreathering and hydrometamorphism. Many of them are alteration products 

or replacements of older gangue minerals, but some are reprecipitated 

in open fissures. The term hydrometamorphism is used to define 

those changes which take place through the agency of surface waters 

at depths somewhat below the oxidizing zones possibly at horizons where the waters have become neutral or alkaline. Several 

silicates are developed by such processes, among them chlorite, 

epidote, muscovite, quartz, serpentine, and talc. Of these minerals 

all except serpentine and talc are formed also, under some conditions, by ascending thermal waters.

SOME ALUMINUM MINERALS.

Occurrence. Aluminum is abundant in the economy of nature and 

is a constituent of many rock-making minerals. It enters .into the 

composition of several vein-forming minerals. Among these are the
152 THE ENRICHMENT OF SULPHIDE OEES.

feldspars, sericite, alunite, kaolin, diaspore, gibbsite, and bauxite. The 

chemical composition of some of these minerals is shown below:

Sericite.. ....................... ..............K20.3Al2 03 .6Si02 .2H20.

Alunite........ .............................. .K20.3A1203 .4S03 .6H20. -

Kaolin ...................................... .Al"203 .2Si02 .2H20.

Diaspore. .................................... . A1203 .H20.

Although new mineral combinations result from hydrothermal 

processes, there is little evidence that much aluminum is carried 

into the veins by ascending hot solutions, for aluminum compounds 

are rare indeed in veins contained in nonaluminous rocks. Cold acid 

sulphate solutions dissolve aluminum from its compounds very 

readily, as is indicated by many of the analyses of mine waters 

(pp. 60-63). There is but little evidence, however, that aluminum 

compounds are extensively precipitated in open spaces from cold 

acid waters. Some alunitic veinlets are secondary and some kaolin 

is deposited in fractures by descending solutions, but in most deposits 

the larger proportion of the secondary aluminous gangue appears 

to be a residual product of altered feldspar, sericite, and other minerals. Without much doubt aluminum, like iron, is carried out of 

sulphide ore bodies in great quantities, for the slimes precipitated 

from mine waters commonly contain large proportions of hydrous 

aluminum compounds.

Feldspars. Of the feldspars, albite and adularia are primary 

gangue minerals of ore veins. Albite is generally formed at considerable depth; adularia nearer the surface. Neither feldspar is 

known to form in cold solutions by processes of secondary alteration, 

although adularia, according to Lindgren,1 may be deposited at or 

near the surface where the temperature could not have been much 

above 100° C.

Sericite. Of the vein minerals that carry aluminum, sericite is 

perhaps the most common. It is formed in the main as a pseudomorphous replacement of feldspar or other aluminous silicates. In 

some replacement veins it is very abundant and was probably formed 

by ascending thermal waters, although there is much evidence that 

it forms also by processes of hydrometamorphism 2 that are independent of processes of ore deposition. Hickling 3 states that sericite is 

an intermediate product of the kaolinization of potash feldspar exposed 

to atmospheric weathering. There is little evidence, however, that

1 Lindgren, Waldemar, Metasomatic processes in fissure veins, in PoSepny, Franz, The genesis of ore 

deposits, 1902, p. 532.

2 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43, 1905, p. 125.

'Hickling, George, China clay; Its waters and origin: Trans. Inst. Min. Eng. [London], vol. 36, 1908-9, 

pp. 10, 24.
SOME GANGUE MINERALS. 153

sericite forms extensively under the conditions that prevail where 

secondary sulphides are deposited.

Kaolin, Al203 .2Si02 .2H2 0, is a common product of rock alteration 

under surface conditions. It forms where aluminum silicates are 

leached by ground water that contains either carbonic or sulphuric acid. It is abundantly present in the oxidized zone and in 

the zone of enrichment in nearly all sulphide ore deposits. As 

a primary vein constituent* it is exceedingly rare. At Goldfield, 

Nev., according to Ransome,2 it was probably deposited simultaneously with alunite and gold by ascending thermal waters. In 

several other districts, as at the Rosita Hills, Colo., and Rabbit Hole, 

Nev., it is associated with alunite and commonly with diaspore or 

other hydrous aluminum oxides. Rosier 3 regards kaolin as a pneumatolytic product. According to Gregory,4 the deposits of kaolin or 

china clay of Cornwall and Devon extend to the bottom of some deep 

mines and probably to greater depths. These he regards as having 

been formed by solutions containing boric and fluoric acids. Hickling,5 however, regards the kaolin of Cornwall as entirely secondary. 

At Goldfield, Nev., kaolin was formed very near the surface by 

ascending hot waters and probably by partly oxidized sulphate 

waters. Nearly all these data appear to show that kaolin is 

indicative of acid waters.

It has been shown that precipitation of copper sulphide may liberate sulphuric acid, which is available for kaolinization of sericite 

or feldspar or other aluminum minerals, and consequently kaolin is 

commonly an associate of secondary chalcocite and covellite in 

aluminous rocks. At Butte, Mont., according to S. F. Emmons,0 

kaolin associated with secondary copper sulphides was found 1,100 

feet deep in the East Gray Rock mine, and Kirk 7 also has reported 

it at great depths. It is abundantly developed in the oxidized ores 

at Bisbee, Ariz.,8 and probably also with chalcocite ores. It is not uncommon as a secondary product at Morenci, Ariz.,9 where, according

1 Lindgren, Waldemar, The relation of ore deposition to physical conditions: Econ. Geology, vol. 2, 

1907, p. 120. Ransome, F. L., Criteria of downward sulphide enrichment: Econ. Geology, vol. 5,1910 

p. 212; discussion by S. F. Emmons, p. 477.

2 Ransome, F. L., The geology and ore deposits of Goldfield, Nov.: Prof. Paper U. S. Geol. Survey No. 

66,1909, p. 192.

»Rosier, H., Beitrage zur Kenntniss einiger Kaolinlagerstatten: Neues Jahrbuch, Beil.-Bd. 15, 1902, 

p. 231; Ueber Kaolinbildung: Zeitschr. prakt. Geologie, 1908, p. 251.

« Gregory, J. W., Criteria of downward sulphide enrichment (discussion): Econ. Geology, vol. 5, 1910, 

p. 680.

&Hickling, George, China clay; its waters and origin: Trans. Inst. Min. Eng. [London], vol. 36,1908-9, 

p. 24.

6 Emmons, S. F., The secondary enrichment of ore deposits, in PoSepny, Franz, The genesis of ore 

deposits, 1902, p. 443.

* Kirk, C. T., Conditions of mineralization in the copper veins at Butte, Mont.: Econ. Geology, vol. 7, 

1912, p. 35.

8 Ransome, F. L., The geology and ore deposits of Bisbee, Ariz.: Prof. Paper U. S. Geol. Survey No. 21. 

pp. 32,147.

 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Oeol. Survey No. 43,1905, p. 111.
154 THE ENRICHMENT OF SULPHIDE OEES.

to Lindgren, it is not formed during the serialization of the 

porphyry which accompanied the formation of the pyritic veins, but 

it accompanies in small amounts the chalcocite in the veins in porphyry and the copper carbonates in decomposed deposits in limestone. It occurs as veinlets in the quartz of several mines and as 

large masses in the Longfellow mine, where it is adjoined by limonite. 

Pure white kaolin commonly occurs also with azurite. According 

to Lindgren,1 its formation in limestone may be explained by the action 

of aluminum sulphate on free silica in the presence of cupric sulphate:

A12 (S04 ) 3 + 3CuS04 H- 3CaC03 + 2Si02 + 6H20 =

3CuO.2C02 .H20 + H4Al2Si2Oe + 3CaS04 + 3H2S04 + CO2 .

At Georgetown, Colo., according to Spurr, Garrey, and Ball, kaolin 

is crystallized in vugs.

Kaolinic ores commonly carry high values in gold and silver. 

A noteworthy example is seen at the Montgomery Shoshone mine 

of the Bullfrog district, Nevada, where the richer ores were found 

in altered claylike rhyolite. At many places rich kaolinic secondary 

ores of silver are extensively developed for example, in the Broken 

Hill lode, Australia.2

Alunite, K2O.3A12O3 .4S03 .6H2O, is a white or faintly pink mineral 

which in some of its occurrences closely resembles kaolin and 

sericite. It is formed from either hot or cold solutions. It is not 

known to have formed under high temperature and great pressure. 

The alunitic deposits of Goldfield, Rabbit Hole, and Alunite, Nev., 

and of the Rosita Hills and Rico, Colo., have been formed presumably 

by hot ascending waters mixed with oxygenated surface waters. 

The alunitic veinlets at Cripple Creek, Colo.,8 and at Clifton-Morenci, 

Ariz.,4 and the "Chinese talc" of Leadville, Colo., have probably been 

deposited by descending cold sulphate waters. Alunite is commonly 

associated with kaolin and the ranges of the two minerals are probably closely similar. Alunite is much less soluble than some other 

sulphates of aluminum and is therefore relatively stable in oxygenated solutions. The occurrence and genesis of alunite have 

recently been discussed by Butler and Gale.5

Gibbsite, bauxite, and diaspore. Gibbsite, A1203 .3H20, bauxite, 

A12O3 .2H20, and diaspore, A1203 .H2O, are hydrated aluminum oxides 

corresponding to the hydrous iron oxide series. By extreme weathering, especially in moist, hot climates, these minerals are formed

1 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 193.

2 Clark, Donald, Australian mining and metallurgy, 1904, p. 347.

8 Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, p. 125.

«Lindgren, Waldemar, op. cit., p. 119.

' Butler, B. S., and Gale, H. S., Alunite, a newly discovered deposit near Marysvale, Utah: Bull. U. S. 

Geol. Survey No. 611,1912.
SOME GANGUE MINERALS. 155

from aluminum silicates. 1 They have been reported from comparatively few ore veins and those in which they have been identified 

are almost without exception of late Tertiary age. It is believed 

that they have been formed in some deposits very near the surface 

by reactions upon aluminum silicates.2 The waters were oxidized 

solutions, probably hot sulphate waters. In the altered rhyolite 

of the Rosita Hills, Colo.,3 diaspore is associated with alunite, quartz, 

and kaolin, and in altered latite near Silverton, Colo.,* with alunite 

and kaolin. Microscopic crystals of diaspore have been identified 

at several places in the alunitic-kaolinic veins of Goldfield, Nev.6

SILICA.

It is generally assumed that ascending .hot waters are alkaline 

solutions. If they were acid at their sources they would become 

alkaline by reactions with wall rocks composed mainly of alkaline 

silicates.6 Although the alkalies are generally regarded as its natural 

solvents, silica is dissolved also by acid waters. The amount, 

though small, is appreciable even hi solutions that give strongly 

acid reactions, and each of the 37 samples of water whose analyses 

are stated on pages 60-63 carries silica. In dilute acid waters the 

silica is generally assumed to be mainly or altogether in the colloidal 

state. It would be supposed that silica would increase with carbon 

dioxide, but this is not clearly apparent from a survey of the 

analyses, nor is the increase of alkalies with silica pronounced. 

The water that is richest in silica is a strongly acid water of the 

Comstock lode (No. 23).

The secondary metallic silicates chrysocolla, calamine, and willemite have already been discussed. These are formed possibly in 

the presence of acid solutions, and to judge from their mineral association, the various forms of hydrated silica, such as chalcedony 

and chert, may be formed under similar conditions. It is doubtful whether clear crystalline quartz is deposited to any considerable 

extent by cold descending acid sulphate waters. In the zinc deposits of southwest Wisconsin, which were formed presumably by 

cold sulphate waters, quartz is notably absent. Some of the disseminated lead ores of southeastern Missouri carry 5 per cent silica, 

and an appreciable amount of silica is present in the sulphate-

> Worth, H. and F. J., Geol. Mag., 1903, p. 154. Worth, H., Min. Mag., vol. 13, p. 172,1902.

«Hayes, C. W., Geology of the bauxite region of Georgia and Alabama: Sixteenth Ann. Eept. U. S. 

Geol. Survey, pt. 3,1895, p. 547; Trans. Am. Inst. Min. Eng., vol. 24,1894, p. 243.

8 Cross, Whitman, Geology of Silver Cliff and the Rosita Hills, Colo.: Seventeenth Ann. Rept. U. S. 

Geol. Survey, pt. 2,1896, pp. 316-317.

*< Ransome, F. L., Economic geology of the Silverton quadrangle, Colo.: Bull. U. S. Geol. Survey No. 

182,1901 (reprinted in 1903), pp. 121-124.

5 Ransome, F. L., Geology and ore deposits of Goldfield, Nov.: Prof. Paper U. S. Geol. Survey No. 66, 

1909, p. 125.

 Alien, E. T., Sulphides of Iron and their genesis: Min. and Sci. Press, vol. 103,1911, p. 413.
156 THE ENRICHMENT OF SULPHIDE OBES.

carbonate waters of the mines of that region. The mine waters * 

carry considerable quantities of alkalies and alkali earths and give 

an alkaline reaction. Certain quartzose ore deposits have been 

regarded as the results of enrichment or concentration by downward-moving waters, and clear crystalline quartz is found in geodes 

remote from igneous intrusives and forms the interstitial filling of 

sandstones that have been converted into quartzites.

The association of sulphide ores with abundant quartz is commonly 

supposed to indicate that they were deposited by alkaline solutions. 

A study of the paragenesis of most deposits yields results that are in 

harmony with this conclusion, for quartz, as above stated, is deposited 

sparingly with secondary sulphide ores. In some deposits, however, 

it occurs as veinlets filling small cavities and is clearly more recent 

than the bulk of the ore. Some such veinlets may be due to deposition from descending solutions. At Tintic, Utah, evidence of deposition of quartz in late stages of ore formation is very clear.2 At Lake 

City, Colo., according to Irving and Bancroft 8

the white quartz occurs without question in more than one generation, but the bulk 

of it is younger than any of the metallic minerals, for it cuts through sphalerite, galena, 

tetrahedrite, pyrite, and even rhodochrosite in places. Veinlets of this white quartz 

also frequently cement shattered masses of the metallic minerals.

At Cripple Creek 4 quartz replaces celestite, but the period of its 

genesis is presumably uncertain. As emphasized by Van Hise,5 the 

quartz deposited by descending waters is laid down mainly below the 

zone of weathering in the "belt of cementation." Lindgren places it 

in the group with epidote, muscovite, pyrite, and other minerals that 

are formed distinctly below the zone of active oxidation. The valuable contribution of Wright and Larsen 6 on the use of quartz as a 

geologic thermometer should be mentioned here, since it suggests 

methods that may lead to a more definite understanding of the 

genesis of quartzose deposits.

Chalcedony, chert, jasper, and like minerals are cryptocrystalline 

forms of silica which are deposited under different conditions of temperature and pressure. They are more abundant in deposits formed 

by sedimentary processes, in cold-water deposits, in secondary zones, 

in sinters, and in veins of late geologic age than in veins formed at 

depths under higher temperature and pressure. In the latter crystalline quartz is more common.

> Buckley, E. R., Geology of the disseminated lead deposits of St. Francois and Washington counties, 

Mo.: Missouri Bur. Geology and Mines, vol. 9, pt. 1,1909, p. 249.

»Tower, G. W., and Smith, G. 0., Geology and mining industry of the Tintic district, Utah: Nineteenth 

Ann. Kept. U. S. Geol. Survey, pt. 3,1899, p. 692.

* Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. Geol. 

Survey No. 478,1911, p. 60.

«Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, p. 123.

& Van Hise, C. R., A treatise on metamorphism: Mon. U. S. Geol. Survey, vol. 47,1904, pp. 634-636.

«Wright, F. E., and Larsen, E. S., Quartz as a geologic thermometer: Am. Jour. Sci., 4th sen, vol. 28, 

1909, p. 421.
SOME GANGUE MINERALS. 157

The secondary chalcedonic quartz that is commonly associated with 

sulphide ores should be especially noted here. At Cananea, Sonora; 

Morenci, Ariz.; Goldfield, Nev.; and near Silverton, Colo. (Yankee Girl 

and similar deposits), many of the ore deposits are capped with highly 

siliceous outcrops, whereas the ore at short distances below the surface is not so siliceous. A concentration of silica at the surface in 

some of these deposits is unmistakable, yet in several of them not all 

the silica appears to be residual quartz of the ore accumulated at the 

surface by the removal of other minerals. Some shallow pits at 

Cananea pass through a siliceous capping into white material, 

streaked with iron oxide, consisting principally of sericite and kaolin 

with small crystals of pyrite and chalcopyrite and probably of some 

residual feldspar. This loose cellular material contains very little 

silica, and as its pore spaces carry crystals of iron and copper sulphates it is a natural inference that sulphate solutions have attacked 

the kaolin and by long-continued action have removed aluminum and 

set silica free. Analyses of mine waters indicate that some acid 

solutions dissolve aluminum in considerable quantities, for it is present in appreciable amount in nearly all the waters and was noted in 

four samples from Cananea.

Kaolin is generally regarded as a comparatively stable mineral in 

the oxidized zone. At Morenci, Ariz., however, according to Lindgren, 1 kaolin is decomposed by acids, which carry aluminum into solution. Such reactions may be attended by the liberation of silica, 

which presumably would be hydrous silica, but at the very surface, 

perhaps, the hydrous varieties are dehydrated to form quartz, after 

the manner that limonite is dehydrated in hot dry climates to form 

hematite.

Siliceous cappings are extensively developed in and near the copperbearing porphyry at Ely, Nev. Lawson 2 has shown, however, that 

some of the siliceous material or "blout" that outcrops at Ely is 

probably residual jasper.

SULPHUR AND SULPHATES. 

SOLUBILITY.

The genesis of the principal sulphates of copper, lead, zinc, iron, 

and aluminum has already been considered. In all their occurrences these minerals are probably secondary, except alunite, the 

sulphate of aluminum and potash, which is primary in some deposits. 

Several of the sulphates for example, those of magnesium, manganese, 

zinc, and silver are so soluble that they rarely accumulate in ore

1 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper 

U. 8. Geol. Survey No. 43,1905, p. 193.

1 Lawson, A. C., The copper deposits of the Robinson mining district, Nevada: Bull. Dept. Geology 

Univ. California, vol. 4,1906, p. 327.
158 THE ENRICHMENT OF SULPHIDE OEES.

deposits and are of little or no importance as vein minerals. The 

solubilities of some of the sulphates (determined by Kohlrausch) are 

stated below.

K2S04.

111.1 

.62

Na2S04.

168.3 

1.15

Ag2SO,

5.5 

.02

BaS04.

0.0023 

.00001

SrS04.

0.11 

.0006

CaS04.

2.0 

.015

MgS04.

354.3 

2.8

ZnS04.

531.2 

3.1

PbS04.

0.041 

. 00013

The upper number shows the number of grams in a solution of 1 

liter of water; the lower number the molar solubility.

Of the sulphates that occur as gangue minerals, gypsum, barite, 

and celestite are important. Anhydrite * is much less common, 

although it is abundant as a gangue mineral in some deposits of the 

Newhouse district, Utah. Under oxidizing conditions it is unstable 

and passes into the hydrated calcium sulphate, gypsum.

OCCURRENCE.

Native sulphur is not an abundant alteration product of sulphide 

ores. The sulphur of commerce is obtained from other sources. 

Small quantities of sulphur, however, are not rare in oxidized or partly 

oxidized ores. As shown by Vogt, it is formed from several sulphides 

by reaction with ferric chloride. According to Spurr, sulphur has 

formed on the outside of sulphide ore on the dump of the Leadville 

mine at Aspen.2 It incrusts sulphide ores in the Bunker Hill mine 

at Henderson, Mont., and occurs sparingly in the altered ores of 

Ducktown, Tenn. It is an alteration product of galena in the Mountain View lead mine of Carroll County, Md.,3 and in the Wheatley 

mine near Phoenixville, Pa.4 With anglesite it coats cavities in 

galena at Tintic, Utah.5

According to Stokes,8 sulphur is formed in the first stage of the 

oxidation of pyrite. The reaction is 

FeS2 +Fe2 (S04 ) 3 = 3FeS04 4- 2S.

The sulphur readily reacts with ferric sulphate to give more ferrous 

sulphate and acid:

2S + 6Fe2 (S04 ) 3 + 8H20 = 12FeS04 + 8H2S04 .

In the presence of ferrous sulphate and hydrogen sulphide, sulphur 

unites with ferrous sulphide 7 to form pyrite and with silver 8 to form

1 Lindgren, Waldemar, Anhydrite as a gangue mineral: Econ. Geology, vol. 5,1910, p. 525.

2 Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Survey, vol. 31,1898, 

p. 239.

» Williams, G. H., Zeitschr. Krystallographie und Mineralogie, vol. 22,1894, p. 575.

<Hintze, Carl, Handbuch der Mineralogie, vol. 1,1897, p. 90.

o Tower, G. W., jr., and Smith, G. 0., Geology and mining industry of the Tintic district, Utah: Nineteenth Ann. Kept. U. 8. Geol. Survey, pt. 3,1899, p. 704.

«Stokes, H. N., On pyrite and marcasite: Bull. U. S. Geol. Survey No. 186,1901, p. 15.

7 Alien, E. T., Crenshaw, J. L., and Johnston, John, The mineral sulphides of iron: Am. Jour. Sci., 4tb 

ser., vol. 33,1912, p. 169.

 Cooke, H. C., unpublished manuscript.
SOME GANGUE MINERALS. 159

argentite. By these and other processes it is probably removed from 

most deposits nearly as rapidly as it is formed.

Gypsum, CaS04 .2H20, ie a common mineral of ore veins but is rare 

in portions of deposits that have not been exposed to oxidizing solutions. According to A. E. H. Tutton,1 gypsum is decomposed at 

150° C., and possibly at 120°. It has probably been formed invariably at low temperatures. Gypsum is present in decomposed vein 

matter at Cripple Creek 2 and at Creede, Colo., and in the secondary 

sulphide zone at Butte, Mont.3 Veinlets of gypsum are abundant at 

Ely, Nev., in ores in porphyry. At Morenci, Ariz., according to 

Lindgren,4 it is present in oxidized deposits in limestone, where copper 

carbonates are formed by reaction between sulphates and calcium 

carbonates. It forms from calcite at Aspen, Colo.,5 and is in part 

postglacial, for it cements glacial debris. It is associated with the 

altered ores, at Tonopah, Nev.,8 and is present in most of the deposits 

of Goldfield, Nev.,7 where it occurs below the zone of oxidation.. In 

the Dixon mine, Bingham, Utah, according to Boutwell,8 a considerable body of gypsum was found between the carbonate lead-silver 

ore and the parent sulphides. Crystals of gypsum are coated with 

hematite in the Ajax mine at Tintic, Utah.9

Barite, BaS04 , has not been identified in igneous rocks, 10 in pegmatites, or in contact-metamorphic deposits but is found in both the 

oxidized and unoxidized portions of ore lodes. It is not unknown in 

some deposits presumably formed at considerable depths. In the 

Gold Hunter mine of the Cceur d'Alene district u it is associated with 

galena, sphalerite, and tetrahedrite. In this district it is present also 

in the ore of the Standard-Mammoth mine. In some of the sulphide 

copper ores of Shasta County, Cal., 12 barite is a characteristic gangue 

mineral. In many deposits of later age, which were presumably 

formed nearer the surface, barite is abundant. It is present in many

1 Tutton, A. E. H., The optical constants of gypsum at different temperatures: Proc. Royal Soc. London, vol. 81 A, 1908, p. 40.

2 Lindgren, Waldemar, and Ransome, F. L., Geology and ore deposits of the Cripple Creek district, Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, p. 125.

a Winchell, A. N., Criteria of downward sulphide enrichment (discussion): Econ. Geology, vol. 5, 1910, 

p. 489.

«Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43,1905, p. 120.

6 Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Survey, vol. 31, 

1898,p. 24.

«Spurr, J.[E., Geology of the Tonopah mining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, p. 96.

'Ransome, F. L., The geology and ore deposits of Goldfleld, Nev.: Prof. Paper U. S. Geol. Survey 

No. 66, 1909, p. 128.

* Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905, pp. 119, 204.

 Tower, G. W., jr., and Smith, G. O., Geology and mining industry of the Tintic district, Utah: Nineteenth Ann. Rept. U. S. Geol. Survey, pt. 3,1899, p. 694.

>o Clarke, F. W., The data of geochemistry, 2d ed.: Bull. U. S. Geol. Survey No. 491,1911, p. 555.

« Ransome, F. L., and Calkins, F. C., The geology and ore deposits of the Cceur d'Alene district, Idaho: 

Prof. Paper U. S. Geol. Survey No. 62,1908, p. 103.

i* Graton, L. C., The occurrence of copper in Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430,1910, 

p. 102.
160 THE ENRICHMENT OF SULPHIDE OEES.

lode deposits of southwestern Colorado, in some of which, as noted by 

Irving and Bancroft,1 it was formed by ascending hot waters. In 

some veins that are of great vertical extent barite is abundant in 

their higher portions but lacking at greater depths. It is inferred 

that the barite is primary but was deposited under lower pressures 

than those which prevailed in the deeper parts of the veins. In 

many Lake City deposits it is the most abundant mineral. It is 

clearly of earlier deposition than quartz, for quartz very commonly 

incrusts crystals that project into central cavities.

The deposits at Creede, Colo.,2 contain considerable barite. Some 

of this is thought to have formed as a primary constituent of the ore, 

but some that fills cracks and crevices in limonite is later than the 

first period of mineralization and is probably secondary. At Wagonwheel Gap, near by, it has formed along a fissure near which hot* 

springs now issue. At Aspen, Colo., barite is cut by veinlets of 

argentite and native silver and is intimately associated with polybasite.3 It is found with kaolinite in deposits at Goldfield, Nev.4 

Barite is commonly present in the gangue of copper deposits in the 

"Red Beds" of the Southwest that have formed through the agency 

of cold solutions.5

Although barite forms under conditions of surface alteration, it is 

not so commonly associated with the secondary sulphides as gypsum 

and kaolin. Owing to its insolubility it persists in the oxidized 

zones. In many deposits it outcrops at the surface and it is found 

in oxidized ores from which all the sulphides have been dissolved.

Celestite, SrS04 , is not reported as an original constituent of igneous 

rocks, of pegmatites, or of contact-metamorphic deposits. It occurs 

in certain ore bodies that were formed relatively near the surface by 

hot ascending waters. It is formed also in deposits far removed from 

igneous rocks, and in some lodes it is probably a secondary mineral 

deposited by cold sulphate waters. It is not nearly so common as 

barite.

Although celestite is relatively rare in ore veins, at Cripple Creek 

it is both a primary and secondary mineral. Slender needles of 

celestite coat openings in the vein fissures far below the zones of 

oxidation 8 and pseudomorphs of quartz after celestite are of frequent occurrence in some of the Cripple Creek deposits.

1 Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City, Colo.: Bull. U. S. Geol. 

Survey No. 478,1911, pp. 34, 35, 45.

8 Emmons, W. H., and Larsen, E. S., Geology and ore deposits of Creede, Colo.: Bull. U. S. Geol. Survey 

(in preparation).

aSpurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Survey, vol. 31,1898, 

p. 228.

<Ransome, F. L., The geology and ore deposits of Goldfleld, Nov.: Prof. Paper U. S. Geol. Survey 

No. 66, 1909, p. 128.

^Lindgren, Waldemar, Graton, L. C.. and Gordon, C. H., The ore deposits of New Mexico: Prof. 

Paper U. S. Geol. Survey No. 68,1910, p, 78.

 Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey No. 54,1906, p. 125.
SOME GANGUE MINERALS,. 161

Jarosite, K20.3Fe203 .4S04 .6H20, is a brownish-yellow sulphate 

that resembles limonite in color and occurrence and is easily confused with that mineral. Like limonite it forms in the oxidized zone 

of iron sulphide deposits. It occurs at Tintic, Utah, and probably at 

Goldfield, Nev.1 It is found at the outcrop of the Amethyst vein 

at Creede, Colo. Mr. B. S. Butler informs me that it is present in the 

oxidized ore in a number of deposits in Utah.

CARBONATES.

The alkali-earth carbonates are fairly insoluble in water, as indicated in the foilowing table:

Solubility of some carbonates in water at 18°.

KsCOa.

1080 

5.9

NasCOj.

193.9 

1.8

Ag2C08 .

0.03 

.0001

BaC08.

0.023 

.00011

SrC08.

0.011 

.00007

CaC03.

0.013 

. 00013

MgCOs.

1.0 

.01

ZnC08.

0.04? 

.0003?

PbCOe.

0.001 

.00003

The upper number gives the grams of anhydrous salt held in 

solution by 1 liter of water; the lower number the molar solubility or 

number of mols contained in a liter of the saturated solution. The 

determinations were made by Kohlrausch by the conductivity 

method.

In sulphide ores barium carbonate (witherite) and strontium 

carbonate (strontionite) are rare. Calcite and dolomite are more 

common as vein minerals. In accordance with their occurrence in 

.deposits formed under widely different conditions both are termed 

persistent minerals.2 Both are attacked by sulphuric acid, and 

calcite is dissolved in very dilute acid sulphate solutions. It could 

not be formed, therefore,' in moderately acid waters. "Where 

secondary in sulphide deposits, both calcite and dolomite are formed 

mainly below the zone of active oxidation, in solutions that have 

become feebly acid or alkaline.

The occurrence of the carbonates of copper, lead, and zinc has 

already been discussed. These are formed from acid solutions and 

are deposited extensively in oxidized zones. None of them are 

known to form in depth by deposition from thermal waters. Siderite (ferrous carbonate) is less common in lode deposits and is generally primary. It is deposited, however, under some conditions 

of sedimentation for example, in the "black band" iron ores. Its 

occurrence as a product of surface alteration is exceedingly rare,

i Ransome, F. L., The geology and ore deposits of Goldfield, Nev.: Prof. Paper U. S. Geol. Survey No. 

66,1909,p. 133.

8 Lindgren, Waldemar, The relation of ore deposition to physical conditions: Econ. Geology, vol. 2 

1907, p. 108.

72293° Bull. 529 13 11
162 THE ENRICHMENT OF SULPHIDE OEES.

although the ferrous-carbonate molecule is present in ankerite and 

in some other carbonates which are assumed to be secondary in some 

deposits. According to Ransome,1 impure siderite occurs in veinlets traversing sulphide ores at Breckenridge, Colo.

Rhodochrosite, the carbonate of manganese, is fairly abundant 

in many precious-metal ores. In the United States it is almost 

unknown in such deposits that are older than the Tertiary. There 

is little evidence that rhodochrosite is deposited by descending 

sulphate waters, although the manganese-carbonate molecule is 

probably present in some complex carbonates that are assumed to 

be secondary. In the bog-manganese deposits, which have formed 

by sedimentary processes, the manganese oxides predominate, but 

some of these deposits contain also rhodochrosite.2 Although the 

soluble carbonates precipitate silver carbonate from solutions of 

silver salts in the laboratory, and although silver sulphate is more 

soluble than silver carbonate, the latter is nevertheless unknown 

in mineral deposits.

SUMMARY REVIEW OF SEVERAL CLASSES OF DEPOSITS.

On the following pages a number of the principal sulphide deposits 

of copper, gold, and silver ores in North America are briefly described, in order that the several districts may readily be compared 

to ascertain the extent, character, and distribution of the secondary 

ores in them. As already stated, many of the problems arising in 

this connection are now only in process of solution. Yet by grouping these deposits in several ways, certain facts are more or less 

clearly indicated. The conclusions stated below appear to be warranted by the data presented herewith.

COPPER DEPOSITS IN GENERAL.

1. The deposits of many copper districts do not exhibit evidence 

of appreciable chalcocite enrichment. Examples: Kasaan Peninsula, Alaska; Prince William Sound, Alaska; Sudbury, Ontario; 

Boundary district, British Columbia; Copperopolis, Cal.; Frisco, 

Utah (Cactus mine); La Reforma mine, Mexico; probably San Jose, 

Velardena, and Nacozari, Mexico. The mines of the Braden Copper 

Co., operating in Chile, may be added to this list.

2. Deposits showing no chalcocite enrichment, located in countries 

that have recently been glaciated, include those of the following 

regions: Kasaan Peninsula and Prince William Sound, Alaska; Sudbury, Ontario; Boundary district, British Columbia. They include 

also deposits in countries that, so far as I can discover, have not

»Ransome, F. L., Geology and ore deposits of the Breckenridge district, Colorado: Prof. Paper U. S. 

GeoL Survey No. 75,1911, p. 169. 

»Penrose, R. A. F., jr., The superficial alteration of ore deposits: Jour. Geology, vol. 2,1894, p. 306.
SUMMARY EEVIEW OF DEPOSITS. 163

recently been glaciated: La Reforma, Copperopolis?, Nacozari, San 

Jose.

3. Deposits that do not contain appreciable chalcocite zones are 

found in countries of widely different latitudes: Alaska, British 

Columbia, Ontario, California, Utah, Mexico, and Chile.

4. Deposits that do not contain appreciable chalcocite zones exhibit 

primary ores containing pyrrhotite: Kasaan Peninsula and Prince 

William Sound, Alaska; Sudbury, Ontario; Boundary district, British 

Columbia. They also exhibit primary ores, in the available descriptions of which pyrrhotite is not mentioned: La Reforma; Copperopolis; Cactus mine, Utah; Nacozari (some bornite enrichment), San 

Jose, Velardena, Mexico; Braden, Chile.

5. All or nearly all the pyrrhotitic deposits which do not exhibit 

secondary chalcocite zones are in recently glaciated areas. Some 

of these deposits carry sphalerite.

6. All the deposits described that are located in the lower latitudes 

and that show no appreciable chalcocite enrichment have primary ores 

of pyrite and chalcopyrite. Zinc blende is wanting or not abundant 

in most of these deposits, but Velardena (Copper Queen mine) and 

Frisco (Cactus mine) are exceptions. Possibly there are some others. 

In none of these deposits is pyrrhotite reported.

7. In glaciated areas oxidized ores for this group are subordinate 

or lacking. In areas not glaciated a few of the deposits exhibit oxidized ores of some commercial importance.

8. In all or nearly all the deposits in low latitudes which do 

not show appreciable chalcocite enrichment complete oxidation has 

proceeded to comparatively shallow depths. The lodes in general 

are said to be relatively impervious. The copper-sulphate solutions 

probably move laterally and issue at the surface before reducing 

conditions are met, or, escaping into the wall rock, their copper content is scattered. La Reforma is a good example.

9. I know of no pyrrhotitic copper deposit in a low latitude or in a 

country not glaciated that shows no chalcocitization whatever. Some 

pyrrhotite at least reacts so readily with acid to give hydrogen sulphide that acid solutions may be reduced and copper sulphide may 

be precipitated even with access of some air. Precipitation of a 

copper sulphide is now in progress on ore surfaces in mines of Ducktown, Term., above the present water level. Such precipitation 

probably does not take place in the higher levels on pyrite and 

chalcopyrite.

10. The copper content of some of .the pyrrhotitic deposits in glaciated areas decreases with increase in depth. Possibly a former 

chalcocite zone was planed off by the ice and a secondary chalcopyrite 

zone was not completely removed. This is strongly suggested by 

relations in the Boundary district, British Columbia.
164 THE ENRICHMENT OF SULPHIDE OEES.

COPPER DEPOSITS WITH SHALLOW CHALOOCITE ZONES.

11. Many copper deposits exhibit shallow chalcocite zones. These 

include Ducktown, Tenn.; Stone Hill, Ala.; Gossan lead, Va.; Ely, 

Vt.; Queen of Bronze mine, Oreg.; Santiago, Cuba; Pinar del Rio, 

Cuba; and La Dicha, Mexico. Possibly Encampment, Wyo.; Shasta 

County, Cal.; and the Highland Boy mine of Bingham, Utah, should 

be mentioned here.

12. The examples cited above are outside of the limits of the continental ice sheets. A deposit on Prince William Sound, Alaska, however, shows a few inches of secondary sulphide ore.

13. The ores of all but one of these deposits carry pyrrhotite. In 

most of them it is abundant. The ores of Shasta County, Cal., carry 

considerable sphalerite but little or no pyrrhotite.

14. The chalcocite zones of most of the deposits of this group range 

in vertical dimensions from 1 foot to 10 feet. In some of the deposits, however, their vertical extent is 200 feet or more.

15. The secondary chalcocite ores of these deposits are generally 

rich. Some of them carry over 20 per cent copper.

16. Gold or gold and silver are very commonly concentrated in the 

secondary copper ores.

17. A zone of chalcopyrite enrichment may exist below the chalcocite zone, and it may be of considerably greater vertical extent than 

the secondary chalcocite zone. Examples: Ducktown, Tenn.; Rio 

Tinto, Spain; possibly others.

18. Pay ores are commonly found below the zone of probable 

chalcopyrite enrichment and some of them are of moderately high 

grade. Few, if any, however, could be classed as bonanza.

COPPER DEPOSITS WITH MODERATELY DEEP CHALCOCITE ZONES.

19. In many deposits of copper-sulphide ores secondary chalcocite 

extends to moderately great depths. Examples are Morenci, Bisbee, 

Globe, Miami, Ray, and Jerome, Ariz.; Santa Rita (Chino), N. Mex.; 

Ely, Nev.; Bingham, Utah (in part); Cananea, Sonora (in part); 

Butte, Mont. In many of these deposits the vertical extent of the 

chalcocite zone is from 200 to 1,000 feet or more.

20. These deposits are outside of the margins of the continental 

ice sheets.

21. Many of these deposits are in countries now arid.

22. Pyrrhotite is subordinate in the ores of all these deposits.

23. The primary ores of several of these districts carry appreciable 

sphalerite. Examples are Morenci, Metcalf, Jerome?, Cananea 

(West Cobre Grande and Kirk). At Morenci and Metcalf the bulk
STJMMAEY EEVIEW OF DEPOSITS. 165

of the chalcocite is within 400 feet of the surface. In the West Cobre 

Grande chalcocite passes into low-grade zincky ores about 200 feet 

below the surface. Exact data are not available for Jerome and for 

certain Bingham mines.

24. The deepest secondary chalcocite zones are in ore bodies that 

contain little or no pyrrhotite or sphalerite. Examples: Bisbee, 

Globe, and Miami, Ariz.; Bingham laccolith, Utah; Butte, Mont, 

(in part). A few deposits of this character have chalcocite zones 

that probably are not of much greater vertical extent than chalcocite 

zones in some sphalerite deposits. Examples are Ray, Ariz., and 

possibly Santa Rita (Chino), N. Mex.

25. Sphaleritic ores do not appear to check the downward migration 

of copper so readily as they check the downward migration of silver. 

(See below.)

26. Important zones of chalcopyrite enrichment below the chalcocite zones have not been clearly differentiated in any of these 

deposits.

COPPEK DEPOSITS WITH DEEPLY OXIDIZED ZONES.

27. Copper deposits having deeply oxidized zones include many of 

those of Tintic, Utah, and Bisbee, Ariz.; some of Bingham, Utah; 

and the Snowstorm mine of the Cceur d'Alerie district, Idaho.

28. None of these deposits are known to carry pyrrhotite or 

abundant sphalerite.

29. The data here presented indicate that in general oxidized zones 

above ores containing abundant pyrrhotite and sphalerite are more 

thoroughly oxidized than those above ores containing pyrite and 

chalcopyrite without pyrrhotite or sphalerite, but such oxidation 

is not likely to extend to depths so great in pyrrhotite or sphalerite 

ores. The pyrite ores of Rio Tinto, Spain, those of La Reforma 

mine, Mexico, and some of the deposits at Butte Mont., are capped 

by thoroughly oxidized material; possibly there are others.

30. The deepest zones of oxidized copper ores are in deposits in 

limestone. Examples: Tintic, Utah, and Bisbee, Ariz.

31. Although oxidation extends 2,000 feet below the surface at 

Tintic, and perhaps 1,600 feet or more at Bisbee, it is, in general, not 

complete at such depths.

32. Not all the deeply oxidized zones of copper ores are in arid 

countries (Snowstorm mine, for example).

33. The nature and vertical extent of chalcocite enrichment and 

the depth of oxidation depend upon the permeability of the deposits, 

the chemical and mineral composition of the ores and wall rock, the 

present and the past climatic environment, and the geologic history.
166 THE ENRICHMENT OF SULPHIDE ORES.

GOLD DEPOSITS EXHIBITING SECONDARY CONCENTRATION.

34. Many auriferous deposits in the surficial zone appear not to 

have been enriched by secondary agencies. The distribution of 

valuable deposits in the lodes of Berners Bay and Douglas Island, 

Alaska, shows no relation to the present surface. In this glaciated 

region there is little or no concentration of gold at the surface by 

removal of valueless material from the outcrops.

35. There is no evidence of solution and reprecipitation of gold in 

some other auriferous deposits in lower latitudes, although there may 

be an increase of values at the outcrop through oxidation that is 

attended by the removal of material with which the gold is associated. 

Examples are Mother Lode, Nevada City, Grass Valley, Ophir, and 

similar deposits of California; Homestake deposits, of the Black Hills; 

Cable mine, between Philipsburg and Anaconda, Mont.; and several 

deposits in the Appalachians. Practically all the important deposits 

in these regions have supplied placer gold.

36. There are still other deposits whose relations do not indicate 

a migration of gold and from which no placers have been exploited. 

Examples: Edgemont, Nev.; Goldfield, Nev.; Silver Peak, Nev.; Cripple Creek, Colo. (subordinate placers). Not all of these are in arid 

regions, Edgemont and Cripple Creek being moderately well watered.

37. Solution, migration, and reprecipitation are clearly indicated in 

a number of auriferous deposits. In some of them this is inferred 

from paragenetic evidence, in others from the great richness of the 

bonanzas compared with the unaltered ore and from the relations of 

the bonanzas to the present surface. Examples are found at Creede, 

Colo. (Amethyst vein); Ouray, Colo. (Camp Bird vein); Telluride, 

Colo. (Tomboy vein); some veins in quartzite near Leadville, Colo.; 

Little Burro Mountains, N. Mex. (Wyman vein); Bodie, Cal.; Bullfrog, Nev.; Manhattan, Nev.; Gold Circle, Nev.; Mohave, Cal. 

(Exposed Treasure vein); Georgetown, Colo. (auriferous lodes?); 

Marysville, Mont. (Drumlummon veins). All the deposits named 

above carry manganese in greater or less amounts.

Although they are not reviewed herein, several districts outside of 

North America may be mentioned as examples of manganiferous 

deposits with rich superficial zones. One of these is the Aroroy 

district, Philippine Islands. Here, according to Henry G. Ferguson, 

there is a difference in the behavior of gold in manganiferous deposits 

and in those containing little or no manganese dioxide at the outcrop. 

He says: 1

As far as a limited observation extends, all veins which on their outcrops show rich 

ore are those in which manganese oxide is not prominent. Evidence from development work, though very meager, seems to show that the values in these veins do not

1 Ferguson, H. G., The gold deposits of the Philippine Islands: Econ. Geology, vol. 6,1911, p. 126.
SUMMARY REVIEW OF DEPOSITS. 167

increase downward, as is the case with the manganese-bearing veins, and in some at 

least decrease greatly a few meters from the surface. The manganese-bearing veins, 

on the other hand, which at their outcrop may be nearly barren, carry values increasing 

progressively downward. This increasing value in moderate depth is shown in many 

cases, but most clearly by samples taken in the 30-meter raise to surface from the 

upper level of the Nancy mine of the Eastern Mining Co. In this case there was an 

increase downward from the outcrop from less than 1 gram to over 6 grams per ton.

A conspicuous foreign example is the Mount Morgan mine in 

Queensland, one of the largest gold deposits in the world. The ores 

in depth are sulphides and carry a considerable percentage of copper 

but relatively little gold. To a depth of 270 feet the surface ores 

contained masses of enormously rich gold ore highly stained with 

manganese. Much of this ore carried several ounces to the ton, but 

copper was almost entirely absent. As pointed out by Donald Clark, 1 

there is clear evidence of solution and precipitation of gold. It was 

not all dissolved, however, for some placer gold is found in beds of 

Cretaceous age near by.

The Hauraki gold fields, New Zealand, may be mentioned also. 

The vein filling is massive or comby quartz, with some manganese 

dioxide. According to A. M. Finlayson, 2 some of the small ore 

shoots are exceedingly rich. The bonanza zone extends 400 to 600 

feet below the surface.

38. Four districts are mentioned below in which gold has migrated 

in solution in ores that are not stated to carry manganese. Two 

of these (Haile mine, South Carolina, and Hedley district, British 

Columbia) are in regions of basic rocks, which possibly supply manganese to the solutions. Two others (Bohemia district, Oregon, and 

Summit district, Colorado) are in acidic and intermediate rocks. 

Manganese is not mentioned as a vein constituent, although its 

presence is not denied.

39. The outcrops of manganiferous gold ores are commonly leached, 

and placer deposits are not so frequently developed from them as 

from nonmanganiferous lodes. This is apparent from the inspection 

of the lists above. Two examples are conspicuous exceptions 

MarysvJHe, Mont., and Mount Morgan, West Australia. Both have 

enormously rich deposits, practically at the surface; both have supplied placer gold. The unaltered ores of one (Marysville) carry 

calcite, which reduces auriferous solutions very rapidly. Some rich 

manganiferous gold ores are 'found very near the surface at Bullfrog 

and Manhattan, Nev. At both places the gangue is calcitic.

40. It is concluded that the solution of gold depends in the main 

on the presence, simultaneously, of manganese and chlorides. Its 

migration depends on the permeability of the lodes and the material

> Clark, Donald, Australian mining and metallurgy, Melbourne, 1904, p. 267.

2 Finlayson, A. M., Problems in the geology of the Hauraki gold fields, New Zealand: Econ. Geology, 

vol. 4,1909, p. 641.
168 THE ENRICHMENT OP SULPHIDE ORES.

of the primary ores. Of the common minerals, calcite and pyrrhotite 

will probably reduce auriferous solutions most rapidly. Some other 

carbonates and sphalerite are doubtless somewhat less efficient. 

The action with pyrite and chalcopyrite is slower still.

SILVER AND SILVER-GOLD DEPOSITS.

41. Enrichment is clearly indicated in a number of silver and 

silver-gold deposits. Examples may be found at Georgetown, Breckenridge, Leadville, Aspen, Silverton, Creede, and Lake City, Colo.; 

at Philipsburg, Butte, and Neihart, Mont.; on the Comstock lode, 

at Tonopah, and at Pioche, Nev.; at Mohave, Cal.; and in the St. 

Eugene mine, at Moyie Lake, British Columbia. In several of these 

districts, but not in all, a migration of gold also is clearly indicated. 

All the districts of silver-gold deposits in which gold is assumed to 

have migrated include manganiferous ores.

42. In deposits carrying both metals, especially where chlorides 

form, secondary silver minerals are likely to be precipitated as 

bonanzas near the surface, while gold may be carried to greater 

depths. Examples are deposits of Mohave, Cal. (Exposed Treasure mine), Creede, Colo. (Amethyst lode), and probably the Comstock 

lode, Nevada, although in the Comstock district silver chloride is not 

abundant.

43. In manganiferous deposits of gold and silver where chlorides 

do not form extensively, silver is probably carried downward to 

greater depths than gold. Clearly defined examples are rare, perhaps because gold is not dissolved in cold mineral waters free from 

chlorine and because silver chloride is so nearly insoluble that precipitation of cerargyrite is almost certain to take place in the ores 

where chlorides are supplied by the ground waters, even in small 

amounts. Although some examples seem to indicate that silver 

migrates downward below the zone where gold is precipitated, the 

data are generally inconclusive.

44. Abundant pyrrhotite in the primary ore quickly halts the 

downward migration of both silver and gold. Some of the secondary 

ore of the "black copper floors" of Ducktown, Tenn., carried considerable silver. A sample assay b,y Genth showed over 1 per cent. 

Not many examples are available, probably because the pyrrhotitic 

ores carry copper and the rich superficial concentrations were worked 

out before the days of electrolytic refining. Since acid waters generate 

hydrogen sulphide on at least some pyrrhotite almost immediately, 

and since hydrogen sulphide precipitates gold and silver perhaps completely, the gold and silver could not descend to any considerable 

depth in pyrrhotitic ores. Although the principal ore deposits of Rio 

Tinto, Spain, carry little or no pyrrhotite and although zinc blende is 

not abundant in most of them, they should be mentioned here. Above
SUMMAEY EEVIEW OF DEPOSITS. 169

one of the principal deposits there is a ''floor" of rich silver and gold 

ore at or just above the top of the chalcocite zone. 1

45. In general, without much doubt, in copper deposits where 

silver and gold migrate downward it is. in the upper part of the zone 

of secondary copper sulphides that the precious metals are precipitated rather than in the lower part.

46. Many deposits of rich silver ore and some deposits of rich 

silver-gold ore terminate downward in low-grade sphalerite ores. 

Abundant examples are to be found in the regions of Georgetown, 

Breckenridge, Creede, and Lake City, Colo.; Pioche, Nev.; and 

Lake Moyie, British Columbia. In most of these districts the 

bonanzas of silver ores are comparatively near the surface. In the 

Comstock lode and at Tonopah, Nev., and in the Granite-Bimetallic 

vein at Philipsburg, Mont., a little zinc is present, but not much. In 

these the richer ores are found at greater depths. At Creede, Colo., 

in the Amethyst vein, secondary silver ore has been formed at greater 

depths than is usual in the region of Georgetown and Breckenridge, 

notwithstanding the fairly abundant zinc sulphide in the primary 

ore at Creede. This, I believe, is to be explained by excessive fracturing of the Amethyst vein, which at some places is little more than an 

ore breccia, and by the fact that soluble carbonates are almost lacking in the ore.

A conspicuous example outside of America of a deposit of rich silverlead ore passing into low-grade sphalerite ore in depth is the great 

Broken Hill lode of New South Wales. This deposit is in schists and 

in general conforms to the schistosity. Judging from the drawings 

of Capt. Warren2 it resembles in certain structural features the 

broken folds of some of the deposits of Ducktown, Term, Though 

classed as a saddle reef by J. B. Jaquet 3 on account of its bifurcating 

structure, it may be considered, for the purpose of this discussion, 

a tabular mass, since the bifurcation of the lode is shown between the 

415 and 515 foot levels, from which point it extends to the surface us 

a tabular body. Without much doubt a considerable amount of the 

lode has been eroded, and the valuable metals appear to have concentrated in the remaining portion relatively near the surface. Siliceous 

limonitic manganitic ore is said to have extended from the surface to 

about 300 feet below. This ore was 20 to 100 feet wide and, according to Phillips and Louis,4 carried 7 to 45 ounces silver and 18 per 

cent lead. At the bottom of this siliceous ore was a narrow zone of 

kaolinic ore, portions of which carried great bonanzas of silver, largely 

in the form of chloride. Below this level, on block 10, at a depth

»Vogt, J. H. L., Das Huelva-Kiesfeld: Zeitschr. prakt. Geologie, 1899, p. 250. Finlayson, A. M.,Thi 

pyritic deposits of Huelva, Spain: Econ. Geology, vol. 5,1910, p. 410.

» Reminiscences of Broken Hill, quoted by Clark, Donald, Australian mining and metallurgy, Mel 

bourne, 1904. The original is not accessible to me.

8 Jaquet, J. B., Geology of the Broken Hill lode and Barrier Ranges mineral field, New South Wale* 

Mem. Geol. Survey New South Wales, Geology No. 5,1894.

« Phillips, J. A., A treatise on ore deposits, 2d ed., by Henry Louis, 1896, p. 665.
170 THE EFBICHMENT OF SULPHIDE OEES.

between 400 and 500 feet below the surface, were rich friable sulphides 

with galena and zinc blende.1 Below the rich friable sulphides 

values decreased greatly, the ore being composed of "compact sulphides, or an intimately mixed mass of galena and blende, with a 

gangue of quartz, rhodonite, and garnet." I have no record that 

pyrrhotite is present in these ores, though it is mentioned as abundant 

in the Pinnacles mine, 9 miles southwest of Broken Hill.2

47. Auriferous and argentiferous acid solutions are probably more 

sensitive than cupriferous solutions to the presence of sphalerite, 

because ferrous sulphate will precipitate gold and silver from chloride 

and sulphate waters, whereas copper sulphate will remain in solution 

with ferrous sulphate. Hydrogen sulphide, which may be generated by 

acid and sphalerite, wih1 precipitate aU three of the metals, but hydrogen sulphide is used up to convert ferric to ferrous sulphate. Thus 

copper may remain in solution as long as any ferric sulphate is present, 

but gold and silver would go down in mixtures of ferric and ferrous 

sulphates. Under similar conditions in sphaleritic ore copper would 

descend to greater depths than gold and silver.

48. Although the chemical and mineralogic environment in depth 

have been emphasized in the foregoing discussion, it is recognized 

that permeability of the primary ores is a factor of the greatest importance in determining the depths to which the metals may descend, 

whatever the character of the primary sulphides.

BEVIEW OF MINING DISTRICTS.

BUTTE, MONTANA.

The Butte district,3 in western Montana, is an area of quartz 

monzonite (the Butte quartz monzonite, frequently caUed granite) 

which is intruded by a later aplite, or aplitic granite (in early reports 

sometimes alluded to as the "Bluebird granite"), and by rhyolite 

porphyry (in early reports called quartz porphyry).

Dikes of late Tertiary rhyolite cut the granite, and effusive rhyolite (rhyolite dacite) rests upon it. In the western part of the region 

are Tertiary lake beds more recent than the granitic rocks. These 

are composed of sand, gravel, and water-laid tuff. The rhyolite porphyry has in some reports been referred to as the "Modoc porphyry," 

and it is locally known by that name. In the most recent publicai Phillips, J. A., A treatise on ore deposits, 2d ed., by Henry Louis, 1896, p. 347.

»Idem, p. 657.

»Weed, W. H., Emmons, S. F., and Tower, G. W., jr., Butte folio (No. 38), Geol. Atlas U. S., U. S. Geol. 

Survey, 1897. Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. 

Geol. Survey No. 74,1912. Winchell, H. V., Synthesis of chalcocite and its genesis at Butte, Mont.: Eng. 

and Min. Jour., vol. 76,1903, pp. 782-784. Sales, Reno, Ore shoots at Butte, Mont.: Econ. Geology, vol. 3, 

1908, pp. 326-331; Superficial alteration of the Butte veins: Idem, vol. 5,1910, pp. 15-21. Simpson, J. F., The 

relation of copper to pyrito in the lean copper ores of Butte, Mont.: Econ. Geology, vol. 3,1908, pp. 628-636. 

Kirk, C. T., Conditions of mineralization in the copper veins at Butte, Mont.: Econ. Geology, vol. 7,19i2, 

pp. 36-82.
REVIEW OF MINING DISTRICTS. 171

tion on this area 1 it is called rhyolite porphyry. Its groundmass is 

microcrystalline, however, and, as Weed very truly states, it might 

be classed as granite porphyry.

The quartz monzonite, aplite, and porphyry, which contain aU the 

ores, are phases of the great Boulder batholith, which extends some 

64 miles southward from a point near Helena and is 12 to 16 miles 

wide. This batholith intrudes Paleozoic and Cretaceous sedimentary 

rocks and along its borders has induced contact metamorphism by 

which the typical garnet zones have been developed in the calcareous 

sediments.

Although the rocks of the batholith are in general of comparatively 

uniform composition, the Butte quartz monzonite is a somewhat 

more basic phase. The aplite represents a differentiation product 

that was forced into cracks in the quartz monzonite after that rock 

had cooled. Although there is an unusual amount of the aplite in 

the region about Butte, its economic significance is not clear. Indeed, the lodes, on passing from the quartz monzonite into aplite, 

are impoverished, and Weed 2 states that the aplite is uncongenial 

for ore deposition because it fractures with difficulty and is not 

readily replaced.

There seems to be a genetic relationship between the copper ores 

and the rhyolite porphyry, although such a relation is not conclusively proved. The rhyolite porphyry is found only in the eastern 

portion of the copper area, where it is younger than the Butte quartz 

monzonite and older than the veins, since even the oldest veins cut 

through it. The veins in the porphyry, like those in the aplite, are 

narrower and poorer than those in the .quartz monzonite. The porphyry is, however, the youngest igneous rock exposed that is older 

than the oldest veins.

The copper ores are included in an area about H miles long and a 

mile wide, and this area is almost surrounded by a much larger area 

containing closely spaced silver-bearing veins. Pronounced parallelism is noticeable in veins of both groups.

The region is highly fractured and extensively faulted; indeed, 

faulting movements have continued probably to the present day. 

In a general way the fractures, as stated by Weed, may be separated 

into two classes; the first, formed before the rhyolitic eruptions 

(Miocene), trend either (a) east and west or (&.) northwest and southeast; the second, accompanying or following the rhyolite intrusions, 

trend in the same directions as the first, but are most marked in a 

(c) northwest or (d) northeast direction. Still later, after ore 

deposition, northeast fracturing occurred.

The earliest fracturing recognizeji is that which is assumed to have 

taken place in the partly consolidated mass of Butte quartz morizo1 Weed, W. H., op. cit., p. 41. 2 Idem, p. 98.
172 THE ENRICHMENT OF SULPHIDE ORES.

nite, causing openings which were filled by the analogous but more 

acidic magma that cooled as aplite.

The rhyolite porphyry intrusion, which probably took place after 

both the Butte quartz monzonite and the aplite had been consolidated, formed irregular dikelike bodies which have a general northwest trend, though the one in the Anaconda mine runs east and west.

The most important fracturing^ that which formed the fissures 

that admitted the earliest mineral-bearing solutions occurred subsequent to both aplite and rhyolite porphyry intrusions.

Subsequent to the original deposition of minerals along the vein 

fissures several dynamic movements took place, each producing 

fracturing which affected the mineral deposits. These later movements have formed cross fissures faulting the earlier veins and 

strike fissures coincident with or parallel to and reopening earlier 

veins. Important mineralization has taken place also along cross 

fissures.

The volcanic outbreak which caused the rhyolite intrusion occurred 

subsequent to the earlier vein fissuring, as is shown by the fact that 

the silver veins are cut off, in places even cut in two, by the intrusive 

dikes of rhyolite. The relation of these rhyolite dikes to the different 

periods of late fracturing is, however, not positively known. More 

than one postmineral movement took place, for the rhyolite dikes 

have been markedly sheeted and somewhat fractured since their 

intrusion. The rhyolite intrusion itself, however, was probably 

accompanied by a certain amount of fracturing of the adjoining rocks. 

It is therefore not improbable that the secondary fracturing, which 

had an important bearing on the enrichment of the copper veins, was 

genetically connected with the intrusion of the rhyolite. 1

The ores are of two distinct classes, copper and siliceous silver. 

The copper ores contain a little silver; the silver ores rarely contain 

copper; both copper and silver ores contain a little gold, and the highgrade silver ores contain it in important amounts.2

Chalcocite, enargite, and bornite are the most common copper 

minerals. Covellite occurs in large amounts in one mine and in small 

amounts in others. Chalcopyrite is present in workable quantities 

in a few properties but is an insignificant part of the total copper output. Tetrahedrite is found as a rarity in the deep workings of a few 

mines. Chalcanthite, or native bluestone, is common in the old 

workings. Pyrite is the most common sulphide. It is estimated that 

since the beginning of mining about 75 per cent of the copper production has come from glance, 20 per cent from enargite, 4 per cent 

from bornite, 0.5 per cent from covellite, and 0.5 per cent from chalcopyrite.

»Weed, W. H., op. cit., pp. 58-59. s Idem, p. 65.
REVIEW OF MINING DISTRICTS. 173

Silver occurs native in the copper ores, especially in those from the 

upper levels. Ruby silver and indeterminable black sulphantimonites 

and arsenides occur in the siliceous silver ores. Free gold is rare but 

occurs in some silver ores and has been seen in specimens from the 

Leonard mine, where it is parasitic on glance.1 The gangiie minerals! 

include quartz, sericite, and several residual minerals of the altered 

country rock. Rhodonite and rhodochrosite occur in silver ores.

The mountains near by have been glaciated in recent geologic 

time, but no morainal material is found in the immediate vicinity of 

Butte. 2

The average precipitation is less than 20 .inches, but the loose disintegrated material near the surface favors extensive seepage of rain 

water. The few records of "first water" for the older mines appear 

to locate it at about the upper limit of the sulphide ores. This upper 

limit varies greatly, ranging from about 20 feet to 400 feet, being in 

part independent of the surface level but varying with the degree of 

fracture.

In the quartz monzonite the network of lesser fractures and the 

veins and faults are apparently all filled with water and serve as a 

vast reservoir. That this water has a natural level not far below 

that of the neighboring valleys is evident, and that it is moving 

slowly downward can not be doubted, though it can not be readily 

proved. The conditions of mineralization afford some evidence of 

this; and in the East Ridge region, where higher collecting ground 

exists, the springs became dry when the Altona mine was pumped out 

to a depth of some 200 feet, proving a connection between the mine 

waters and surface drainage. The drying out of the upper workings 

of some of the mines when the surface seepage is eliminated shows 

that in these mines the waters come from above. In short, all the 

evidence shows that the waters are surface waters seeping downward. 

They come, however, from a broad area, and not from the surface of 

the Butte mines alone.3

In opening new ground the amount of water, as stated by Weed, 

seems to be proportionate to the block of ground opened that is, 

the ground between the new level and the old workings above. In a 

fewplaces rising water was encountered at the deep levels f or instance, 

on the 2,200-foot level of the Mountain Con. mine and on the 1,600- 

foot crosscut of the Anaconda (where uprising waters were encountered some 2,000 feet from other workings) but the outflow diminished in a few weeks and finally all but ceased, indicating that the 

area from which the water came was probably drained.4

The oldest deposits are the east-west lodes, of which the Parrot, 

Anaconda, and Syndicate are examples. These occupy fissures along

i Weed, W. H., op. cit., p. 72. »Idem, p. 98. 

* Idem, p. 30. «Idem, p. 97.
174 THE ENRICHMENT OF SULPHIDE OEES.

which there was but slight tangential movement.1 The ore minerals 

in these veins consist chiefly of pyrite, chalcopyrite, chalcocite, 

covellite, etc. 'Pyrrhotite is not mentioned in the descriptions of 

these or any later deposits. I am informed by Mr. Reno Sales that 

sphalerite is widespread and abundant in the copper veins except in 

that part of the district extending from the Neversweat mine eastward through the Anaconda, St. Lawrence, Mountain View, Pennsylvania, Leonard, and Silver Bow mines. In these properties it is 

almost entirely absent, both in these and later vein systems. Westward, in the Steward, Original, and Gagnon 2 mines, sphalerite is 

abundant, as it is in the Colorado, Clear Grit, Buffalo, and Paulin 

mines and also in many of the silver lodes.

A later set of veins strikes northwest across the east-west veins. 

These contain, in addition to the minerals named as constituents 6f 

the earlier veins, large quantities of enargite. Along these veins 

evidence of movement parallel to the planes of the deposit is more 

pronounced. Although mineralization of the earlier veins is comparatively uniform, the later veins are characterized by barren patches 

separating rich ore shoots. These ore shoots, as shown by Sales,3 are 

of primary origin, the course of the mineralizing solutions having 

been determined by fault gouge which effectively dammed back the 

waters from the portions of the fissures that are barren.

A still later set of fissures, having a northeast course, has displaced 

both earlier sets. This system, according to Weed,4 contains little 

endogenous ore, but the material mined is mainly fault breccia from 

earlier deposits. In many of the veins later movement is pronounced.

The depth of oxidation of the copper deposits is treated briefly on 

page 37. The leached zone extends downward in places 300 or 400 

feet below the surface. It contains some silver but little copper. 

Below the oxidized zones enormous masses of chalcocite, with some

bornite and covellite, were encountered.

In the great ore bodies of the upper levels of the Anaconda veins 

chalcocite occurred, according to Weed,5 in masses of nearly pure 

leadlike mineral 20 feet or more wide. In depth the mineral shows 

a more crystalline structure, and it is found in all the mines in greater 

or less abundance and purity, but in the great bulk of the ores it 

forms small grains scattered through the ores.

Emmons, Weed, Tower, and many others who studied the copper 

lodes in the earlier stages of their development regarded the chalcocite 

ores as secondary deposits formed by descending waters. Weed 6

1 Sales, Reno, Ore shoots at Butte, Mont.: Econ. Geology, vol. 3,1908, p. 327.

,2 Pearoe, Richard, The association of minerals in the Gagnon vein, Butte City, Mont.: Trans. Am. Inst. 

Min. Eng., vol. 16,1888, pp. 62-64. 

«Sales, Reno, op cit, p. 326. 

«Weed, W. H., The copper mines of the world, New York, 1907, p. 326.

 Idem, p. 327.

 Idem. p. 328.
BEVIEW OF MINING DISTRICTS. 175

says, "As a general statement it may be said that the deposits of 

copper glance are secondary."

C. T. Kirk,1 after a comprehensive study of the ores of the Pittsmont mine of this district, concluded that the chalcocite was secondary, basing his argument in part on its association with kaolin.

As a result of microscopic studies of polished surfaces of ores, 

John F. Simpson 2 concluded that the copper sulphides were deposited 

in the following order: Chalcopyrite, enargite, bornite, chalcocite. 

Pyrite from the 2,000-foot level of the Mountain Con. mine is cut by 

veinlets of bornite and contains flakes of chalcocite. In ore from the 

800-foot level of'the Alex Scott mine chalcocite surrounds pyrite, 

enargite, and chalcopyrite and occurs as veins in the pyrite.

As the underground workings have been carried deeper, and as 

chalcocite ores have been developed in depth, doubt has been 

expressed more and more as to the validity of the conclusion that the 

chalcocite ores are wholly secondary. Gratori 3 in 1906 called 

attention to the fact that on the whole the copper veins in depth are 

dry. Sales 4 in 1910 advanced the opinion that the chalcocite in the 

lower levels of the lodes is primary. Mr. H. V. Winchell informs 

me that he has reached the same conclusion. Weed 5 mentions a fine 

body of chalcocite ore on the 2,400-foot level of the Anaconda group, 

and later developments, according to Sales, have exposed chalcocite 

2,800 feet deep. One of the photomicrographs from a deep level 

figured by Simpson shows pyrite inclosing small chalcocite masses 

in relations that are hard to reconcile with the conclusion that the 

chalcocite is of later age. In a collection of the School of Mines, University of Minnesota, ores from deep levels at Butte show covellite 

intergrown with tetrahedrite and chalcocite intergrown with pyrite, 

and there is no evidence that any of these minerals are replacements 

of older sulphides.

In Weed's most recent report on the district 6 he states that the 

bulk of the ore is altered quartz monzonite containing disseminated 

grains and veinlets of chalcocite intergrown with pyrite or replacing 

that mineral completely. He also notes that chalcopyrite is but 

sparingly developed, constituting but 0.5 per cent of the ore mined. 

The copper of the secondary ore must therefore have been derived in 

the main from enargite, cupiferous pyrite, or older chalcocite.

1 ,Kirk, C. T., Conditions of mineralization in the copper veins at Butto, Mont.: Econ. Geology, voL 7, 

1912, p. 35.

* Simpson, J. F., The relation of copper to pyrite in the lean copper ores of Butte, Mont.: Econ. Geology, 

vol. 3,1903, p. 635. 

» Graton, L. C., Mineral Resources U. S. for 1906, U. S. Geol. Survey, 1907, p. 400.

* Sales, Reno, Superficial alteration of the Butte veins: Econ. Geology, vol. 5,1910, p. 15. 

»Weed, W. H., The copper mines of the world, New York, 1907, p. 331.

8 Weed, W. H., Geology and ore deposits of the Butte district, Mont.: Prof. Paper U. S. GeoU 

Survey No. 74,1912, p. 76.
176 THE ENRICHMENT OF SULPHIDE ORES. r

A drawing by Bard * shows an inch vein of chalcocite cutting quartz. 

The chalcocite incloses idiomorphic crystals of pyrite and quartz, and 

both of these minerals seem to be spaced with some regularity in the 

chalcocite. The spacing suggests eutectic structure and the relations 

appear to be nearly identical with those in an ore from Mexico, 

described by D. P. Hynes,2 where tetrahedrite incloses quartz. 

Although Weed believes that chalcocite is mainly secondary at Butte, 

he also recognizes its primary origin. He says: 3

From the studies made of thin sections it is believed that the mineral is almost 

universally secondary but that it also occurs aa a primary mineral in some veins. 

It is undoubtedly in process of formation by descending waterjs at the present time, 

but as the great ore bodies found at deep levels are far below the present level of 

ground water they can not have been produced by the reactions commonly offered 

in explanation.

It should be noted here, however, that the history of vein formation at Butte and also the recent physiographic history of the Butte 

district are exceedingly involved, a fact that diminishes somewhat 

the value of generalizations for solving the problems of ore formation.

Some of the veins have been depressed by faulting 4 more than 500 

feet since they were formed. Evidence of descending water and of 

extensive solution and recent deposition is found at unusual depths. 

Watercourses are shown in some veins over 2,000 feet below the 

surface.

The physiographic history of the region has doubtless been favorable to a very deep descending circulation. In a previous paper 5 

Weed says:

The region has passed through several very pronounced changes in elevation since 

the formation of the veins in Tertiary time. In early Tertiary time the present 

topography was blocked out and the mountain ranges and deep intervening valleys 

were carved. This was succeeded by earth movements by which the streams became 

clogged or the valleys dammed, forming lakes; while volcanoes broke out at numerous 

places and showered ashes and scoria over the region. The valleys were silted up 

or in part filled by volcanic de"bris, before crustal movements drained the valleys and 

altered the divides. More recent movement, possibly still continuing, is marked by 

faults and a reversing of stream co\irses. The old valley at Butte is filled by hundreds of feet of d6bris, and a mountain wall 2,500 feet high marks a north and south 

fault line. These changes all caused a migration of water level facilitating the processes of weathering and enrichment, and the great bodies of rich copper ores of the 

region are believed to be in part due to this cause.

In 1907, according to Graton,6 the copper ores averaged 2.75 per 

cent copper, 2.3 ounces silver, and 0.014 ounce gold to the ton. 

Few data are available on the downward migration of gold and 

silver in the deposits. Although manganese minerals are abundant

i Weed, W. H., loc. cit. The original is not available to me.

a Hynes, D. P., Notes on the geology of the Mina Mexico vein: Econ. Geology, vol. 7,1912, pp. 280-286. 

See also this bulletin, p. 46. 

'Weed, W.H., loc. cit. 

«Weed, W. H., op. cit., p. 111.

 Weed, W. H., The enrichment of gold and silver veins: Trans. Am. Inst. Min. Eng., vol. 30,1901, p. 448.

 Graton, L. C., Mineral Resources U. S. for 1907, pt. 1, U. S. Geol. Survey, 1908, p. 610.
REVIEW OF MINING DISTRICTS. 177

in the silver lodes that surround the silver-bearing area, it is said that 

the principal copper ores do not contain manganese,1 and the solution of gold in them presumably has been delayed.. To judge from, 

their position, some, at least, of the gold placers that were worked 

in the early days at Butte were derived from the waste of the copperlodes. The valuable metal in the oxidized ore of the copper lodes 

was chiefly silver, the gold being of subordinate value.

BINGHAM, UTAH.

The Bingham district, Utah,2 is an area of Carboniferous quartzites 

and limestones intruded by monzonite and monzonitic porphyry and 

covered in part by andesites, andesitic porphyries, and breccias. 

The rocks are tilted, faulted, and in places highly shattered.. The 

ore deposits are in or near the intrusive monzonite or monzonitic 

porphyry. They include fissure veins in several formations, bedding-plane deposits in limestones, and disseminated deposits in 

shattered porphyry. The ore bodies that until recently have been 

most productive are large replacement deposits of sulphide ore in 

limestone. These ores consist chiefly of pyrite, chalcopyrite, sphalerite, and chalcocite, with a little bornite and enargite and their 

oxidation products. In many of the deposits a little pyrrhotite is 

present, and in some ore from the Highland Boy mine pyrrhotite 

is more abundant than pyrite and somewhat more abundant than 

chalcopyrite.3

In some of the mines oxidation is deep, extending to depths several 

hundred feet below the surface. In the Brooklyn mine 4 oxidation 

is reported to a depth 1,450 feet below the surface; in the Highland 

Boy it is only 300 feet deep. In some of the deposits chalcocite ores 

extend to considerable depths and in others they are subordinate. 

The chalcocite zone in the Highland Boy seems not to have been 

extensive. Boutwell notes that the secondary oxidized ores of copper 

and gold pass into a zone characterized by chalcopyrite tarnished 

and coated with bornite and seamed with limonite. A winze sunk 

through oxidized ore encountered gold in considerable quantities, 

some of the ore running over $30 to the ton. Five thousand tons of 

ore shipped early in the development of this mine are said to have 

averaged 12 per cent copper and $4 gold to the ton and to have 

contained much silver. It is reported also that the valuable metals 

in each of the three principal ore bodies ran higher in the upper 

than in the lower levels. The ores of the lower levels carried pyrite,

iPearce, Richard, The association of minerals in the Gagnon vein, Butte City, Mont.: Trans. Am. Inst. 

Mm. Eng., vol. 16,1888, p. 63. Emmons, S. F:, Notes on the geology of Butte, Mont.: Idem, p. 55.

2 Boutwell, J. M., Economic geology of the Bingham mining district, Utah: Prof. Paper U. S. Geol. 

Survey No. 38,1905*.

s Idem, p. 105.

4 Idem, p. 215.

72293° Bull. 529 13 12
178 THE ENRICHMENT OF SULPHIDE ORES.

pyrrhotite, chalcopyrite, considerable gold, and some silver. In 1906 

and 1907, according to Graton,1 practically all the copper from these 

deposits was derived from the pyritic ore.

Another type of ore deposit which in recent years has become 

highly productive contains copper ore disseminated in the monzonite 

porphyry. The principal deposit of disseminated ore is in the 

intrusive body at Upper Bingham known as the Bingham laccolith. 

According to Boutwell 2 this extensive mass of monzonite carries 

disseminated throughout its areal extent, so far as known, irregular 

grains of pyrite and chalcopyrite. The mineralized tract contains a 

multitude of thin unsystematized parting planes. The rock is 

greatly bleached, silicified, and sericitized in and near the areas of 

great shattering. The copper content is lowest in the oxidized zone 

at the surface. Farther down, in the unoxidized rock, the copper ore 

lies in flat scales and films in silicified walls of cracks, and in areas of 

great shattering it occurs abundantly on quartz-coated cracks and is 

disseminated through the silicified bleached walls. The quartz, the 

sericite, and possibly the biotite in this ore are later than the original 

rock-making minerals. I can find no record of the occurrence of 

pyrrhotite in these ores. Zinc blende is not mentioned in Boutwell's description of this deposit, although it is abundant in some of 

the lode ores in the monzonite for example, in that of the Last 

Chance mine.3 Although some of the metals may have been present 

in the rock when it solidified,4 the larger part of the primary sulphides 

was introduced by thermal waters after the porphyry had solidified.

Of the surface alteration of disseminated ores Boutwell says: 5

Superficial alteration has followed the deposition of chalcopyrite and pyrite. 

Pyrite about its periphery and along cracks which traverse these planes may be seen 

going over to limonite. This fact * * * doubtless explains the relative enrichment of gold values proved by assays to exist in the outer or surface portions of test 

tunnels in these copper and gold bearing intrusives. The brilliant tarnish of grains 

of chalcopyrite indicates a beginning of alteration, and thin rims of a dark grayishblack metal about chalcopyrite observed under the microscope suggest continuance 

of that process and replacement by black copper sulphide. Rims of a blue-black 

metallic sulphide occur around grains of chalcopyrite. The reason for the decrease 

in assay values of copper along certain open and water-bearing fracture zones is doubtless to be found in the well-known, fact that under the action of surface waters copper 

suffers rapid alteration and transportation.

It appears, then, that in the disseminated ores in igneous rock the copper minerals 

were deposited by hydrothermal action subsequent to the date of igneous intrusion, 

and that these sulphides are now undergoing normal superficial alteration.

The Utah Copper Co., which is exploiting the ores of the Bingham 

laccolith, is said to have developed about 300,000,000 tons, the

1 Graton, L. C., Mineral Resources U. S. for 1906, U. S. Geol. Survey, 1907, p. 404; idem for 1907, pt. 1, 

1908, p. ei^

2 Boutwell, J. M., op. cit., p. 167. 

8 Idem, pp. 160,178. 

«Idem, p. 171. 

»Idem, p. 172.
EEVIEW OF MINING DISTBICTS. 179

average content of which is about 1.5 per cent copper and from 20 

to 30 cents a ton in gold and silver. The surface of this ore body 

occupies an area of 214 acres and the average thickness of the workable ore is probably about 418 feet.1

The leached capping, which is stripped by steam shovels, has an 

average thickness of 80 feet on the Utah ground, 160 feet on the 

Boston Consolidated ground, and 173 feet on the Pay Roll ground, the 

average for the entire area being 106 feet. The mine figures indicate 

that the average depth of the bottom of the deposit now workable is 

more than 500 feet below the surface, although it extends downward 

in places more than 900 feet below the surface.2 In much of the ore, 

especially in that of lower levels, chalcopyrite is an important ore 

mineral, and considerable masses of the ore carry very little chalcocite. It is not known whether the chalcopyrite is a primary or 

secondary sulphide.

In 1911 the Utah Copper Co. produced from copper ore about a 

million dollars in precious metals, four-fifths of which was gold and 

about one-fifth silver. This is equivalent to about $0.225 per ton of 

ore and 1.07 cents per pound of copper. It is said that gold values 

are concentrated in the oxidized zone, but below it they tend to 

increase or decrease with the copper content of the sulphide ore.

BISBEE, ARIZONA.

The geology of the Bisbee (Warren) district, .Arizona, was studied 

in detail about eight years ago by Ransome.8 Since the publication of 

his report the known productive area has been much extended and 

large new mines have been developed, especially in a part of the district to whose economic possibilities this report specifically directed 

attention. In view of these later developments Mr. Ransome, who 

revisited Bisbee in 1912, has kindly prepared the following note, in 

which is presented more recently acquired information than could 

have been included in a mere abstract of published work.

NOTE ON THE BISBEE DISTRICT, ARIZONA. 

By F. L. RANSOME.

The fundamental rocks of the Bisbee district are the pre-Cambrian 

Final schist and probably also the granite mass of Juniper Flat, 

north of Bisbee, which is intrusive into the schist. Although the 

age of this granite is not directly evident from its geologic relations 

and although in Professional Paper 21 and in the Bisbee folio it 

was regarded provisionally as Mesozoic, the texture and character of

i Seventh Ann. Rept. Utah Copper Co., for 1911. 

* See cross section, idem.

B Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. S. 

Geol. Survey No. 21,10)4; Bisbee folio (No. 112), Geol. Atlas U. S., U. S. Geol. Survey, 1904.
180 THE ENRICHMENT OP SULPHIDE OBES.

the rock, in the light of subsequent experience in Arizona, suggest 

pre-Cambrian age. Resting on the schist in successive upward 

sequence are (1) the Bolsa quartzite (Cambrian), 430 feet thick; (2) 

the Abrigo limestone (Cambrian), 770 feet thick; (3) the Martin limestone (Devonian), 325 feet thick; (4) the Escabrosa limestone (Mississippian, "Lower Carboniferous"), 700 feet thick; and (5) the Naco 

limestone (Pennsylvanian, "Upper Carboniferous"), 3,000 feet or 

more thick. These beds were intruded in early Mesozoic time by 

stocks, dikes, and sills of granite porphyry and, after deep erosion, 

were covered by a thick series of Cretaceous (Comanche) beds. 

These have in greater part been eroded away from the productive 

area of the district.

Structurally the district is characterized by numerous faults, some 

older and some younger than the Cretaceous beds. In some of the 

tilted fault blocks the strata are gently folded, but folding is a much 

less conspicuous feature of the structure than faulting.

The primary ores were deposited during or after the intrusion of 

the granite porphyry and before the deposition of the Cretaceous 

beds. Their age is therefore early Mesozoic. Their deposition was 

connected with a decided but not conspicuous contact metamorphism 

of the limestones, shown by the development within these of tremolite, 

diopside, garnet, and other silicates, generally in crystals of microscopic size. The ore bodies that first gave prominence to Bisbee are 

those in the Carboniferous and Devonian limestones, but recent 

developments are bringing to light important lenticular masses of 

ore in the Cambrian limestone, and a little ore has been found in the 

porphyry.

Very briefly stated, the larger structural features to which the 

occurrence of the ores is related are (1) the northwest-southeast 

Dividend fault, with a normal throw that has brought Paleozoic beds 

on the southwest side of the fissure against Final schist on the northeast side; (2) a small stock of granite porphyry intruded on the line 

of the fault and invading the contiguous schist and Paleozoic beds; 

(3) an open synclinal structure in the down-faulted Paleozoic beds, 

which is such that these dip in part toward the porphyry stock and in 

conjunction with the fault plane form a trough pitching to the southeast; and (4) a gentle tilt to the southeast, as shown by the present 

slope of the pre-Comanche erosion surface. The ore bodies occur 

in the down-faulted fragment of a syncline, are disposed in, roughly 

semicircular fashion around the porphyry stock, and have radial 

prolongations along certain zones of fissuring.

While very irregular in form, the ore masses as a rule are roughly 

lenticular and tend to conform with the bedding of the limestones. 

The average thickness of all the ore bodies has been calculated by 

the engineers of the Copper Queen Co. to be about 33 feet. The shape
EEVIEW OP MINING DISTBICTS. 181

and position of many of the bodies, however, are determined by 

zones of fissuring and by the form of intruded porphyry masses, many 

of which do not extend directly upward to the surface. ' With 

possibly a few relatively unimportant exceptions, the known Bisbee 

ore bodies were originally deposited as metasomatic replacements of 

the limestones. In many of the ore bodies in the Abrigo limestone 

the positions of former bedding planes are clearly shown by a banding in the ore.

At the Copper Queen mine, near the northwest end of the structural trough, the ore bodies came to the surface. As development 

was pushed south and east, around and beyond the exposed porphyry 

mass of Sacramento Hill, through the Holbrook, Spray, Irish Mag, 

Gardner, Oliver, Lowell, Hoatson, and Junction shafts, the ore was 

found at greater depths. The great ore body of the Junction mine 

of the Calumet & Arizona Co., south of Sacramento Hill, whose 

existence was unknown when Professional Paper 21 was written, 

extends from a point 10 feet above the 1,300-foot level to the 1,500- 

foot level. It is 500 feet long and from 10 to 120 feet wide in plan, 

part of this width being due to duplication by faulting. South of 

the curved chain of mines mentioned He the Briggs and other important ore bodies connected with zones of north-northeast and southsouthwest fissuring in the limestone. In the Briggs mine the ore 

bodies grade outward into a huge partly oxidized mass of pyrite 

about 1,500 feet long and in places 800 feet wide.

In the Copper Queen, Lowell, and Hoatson mines the ore bodies 

are, as a rule, confined to the vicinity of intrusive masses of porphyry. 

In the Junction mine the connection between ore and porphyry is 

less close and in the Briggs mine no porphyry is known. Apparently 

the bodies of porphyry, like the fissures in the limestones, played an 

important part in controlling the movements of the ore-bearing 

solutions, but it is not believed that any considerable part of the ore 

constituents came from the porphyry masses now visible; the metallic 

elements and sulphur had a more distant source.

Up to the year 1904 nearly all the copper obtained at Bisbee came 

from oxidized or enriched ore. Of late years, however, primary ores 

have become very important. The Junction ore body, in which 

the present stopes average over 9 per cent of copper, is a nearly 

solid mass of sulphides that are mostly primary. The huge pyritic 

mass of the Briggs mine, with its included bodies of more cupriferous material, is largely primary, and the ore lenses in the Abrigo 

limestone have, as.a rule, undergone no enrichment.

The most abundant sulphides in the primary ore are pyrite and 

chalcopyrite. With these may be associated considerable bornite and 

in certain ore bodies magnetite. Of late years both sphalerite and" 

galena have been found in considerable quantity near the porphyry of
182 THE ENRICHMENT OF SULPHIDE ORES.

Sacramento Hill, but these minerals are not widely distributed in the 

copper ores. No pyrrhotite has been noted.

The altitude of this district ranges from 5,000 to 7,400 feet above the 

sea. The average rainfall is about 17 inches and the surface of the 

ground water as a rule lies deep. Water was reached in the Lowell 

mine at a depth of 1,100 feet. Afterward the Junction shaft, 1,800 

feet deep, drained the Lowell. For a time, when the lower Junction 

levels were kept open, the pumps raised over 6,000,000 gallons in 24 

hours.

The natural ground-water surface and the lower limit of oxidation 

are neither coincident nor parallel. In general in the northern part of 

the productive area much enriched sulphide ore lies above the original 

water level, and in the southern part there is considerable oxidized 

material below it. In the Junction mine, for example, there is oxidized and leached material on the 1,500-foot level (corresponding to a 

depth of 1,750 feet in the Lowell shaft), and oxidized ore is mined in 

places on the 1,600-foot (bottom) level of the Lowell mine. The 

lower depth of oxidation ranges from 200 feet to at least 1,600 feet.

Although the relative importance of the enriched chalcocitic ores 

at Bisbee has been diminished by the discovery of large bodies of 

profitable primary ore, they still supply a large proportion of the total 

copper produced. The lower limit of enrichment is irregular and ill 

defined but, like the lower limit of oxidation, is deeper in the southern 

part of the productive area than in the northern part. Secondary 

chalcocite occurs in the bottom level of the Lowell mine (1,600 feet) 

and on the 1,300-foot level (1,400 feet deep) of the Briggs mine. 

How much deeper it may go is not known. In some places sulphide 

enrichment has worked down to the bottom of a pyritic ore body; in 

others it has worked around and under residual masses of unenriched

pyritic material; and in parts of the Briggs mine large masses of

leached and oxidized material rest directly on unenriched, low-grade 

pyrite. The enriching mineral is generally chalcocite. In some 

loose, friable ore the chalcocite may occur as thin shells around grains 

of pyrite and as a sooty interstitial powder. Elsewhere the replacement is more nearly complete, the original chalcopyrite and much or 

all of the pyrite having been converted into massive chalcocite. 

Secondary chalcopyrite has been noted,1 but the greater part of this 

mineral is probably a primary constituent of the Bisbee ores.

The great depths to which oxidation and enrichment have penetrated at Bisbee and the inclined position of these zones of alteration 

with reference to the present underground water level and their 

approximate parallelism with the old pre-Comanche erosion surface 

indicate that much of the oxidation and enrichment were effected 

before the deposition of the Cretaceous formations.

1 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Frof. Paper U. 8. 

GeoL Survey No. 21,1904, p. 132.
REVIEW OF MISTING DISTRICTS. 183 

GLOBE AND MIAMI, ARIZONA.

For the following note I am also indebted to Mr. Kansome, who 

studied the Globe district i some years ago and who is now investigating the Miami copper deposits.

THE GLOBE AND MIAMI DISTRICTS, ARIZONA. 

By P. L. RANSOME.

In the Globe quadrangle, which includes the Miami district, a 

pre-Cambrian crystalline complex, consisting of Final schist and 

various granitic intrusives ranging from quartz diorite to true 

granite, is unconformably overlain by a thick series of Paleozoic 

beds. The stratigraphic column as revised after recent work in the 

adjoining Ray quadrangle is as follows:

Pre-Mesozoic stratigraphic column in the Globe region, Arizona.

Erosion surface. Feet.

8. Thick-bedded gray limestone 2 (Carboniferous)................ 1,000

7. Thin-bedded limestone 2 (Devonian)......................... 325

6. Quartzite 2 (Cambrian)....................................... 400

5. Cherty, dolomitic limestone 2 (Cambrian)...................... 250

4. Dripping Spring quartzite............

3. Barnes conglomerate.................... 

2. Pioneershale......................... X^mbnan or older

1. Scanlan conglomerate.

450

10-55

200

1-6

These rocks were extensively intruded, probably during the 

Mesozoic, by diabase, largely as great irregular sills, and were invaded 

also by certain masses of granite and quartz monzonite, including 

probably the Schultze granite, although this particular mass is not 

now in contact with the sedimentary series. All the rocks mentioned 

were covered wholly or in part by a thick flow of dacite, probably 

hi early Tertiary time.

These rocks are cut by numerous faults, some older and some 

younger than the dacite.

The copper deposits occur (1) as lodes in schist, quartzite, limestone, 

and diabase, which, where they pass through or alongside of limestone, as in the Old Dominion mine, may be connected with large 

replacement bodies in that rock; (2) as disseminations of chalcocitic 

ore in the Final schist near the Schultze granite (Miami and Inspiration mines); (3) as secondary deposits of chrysocolla in dacite tuff 

(Black Warrior mine) or in fissures (old Live Oak and Keystone 

workings).

»Bansome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. S. Geol. Survey No. 12, 

1903; Globe folio (No. Ill), Geol. Atlas U. S., U. S. Geol. Survey, 1904. 

3 Names will be given to these formations in a forthcoming publication. F. L. B.
184 THE ENRICHMENT: OF SULPHIDE ORES.

The primary ore of the lodes consists essentially of pyrite and 

chalcopyrite, with bornite and specularite * of less regular or less 

abundant occurrence and galena and sphalerite rather rare. In the 

disseminated deposits in schist the primary metallic minerals are 

pyrite and chalcopyrite with a little molybdenite.

The disseminated primary metallization was undoubtedly connected with the intrusion of the Schultze granite, the constituents of 

the sulphides probably emanating from the magma reservoir that 

supplied that rock. The lode ores were also deposited at high temperature and may also be genetically connected with the granitic 

magma. They are, however, more closely associated with the diabase, and possibly this rock had some share in their genesis. The 

known ore bodies in limestone have been modified by secondary 

processes, but their original deposition by replacement was apparently not associated with any important metamorphism of the adjacent limestone.2

The altitude of the Globe quadrangle ranges from 3,000 to 7,850 

feet and the average annual precipitation is between 13 and 20 inches. 

The ground-water level is in general deep, although in parts of the 

areas occupied by the Quaternary Gila conglomerate water is abundant within moderate distances from the surface. In the Old Dominion mine the original water level was probably between 700 and 800 

feet in depth and a large part of the present influx is derived from 

workings that tap the conglomerate-filled valley of Final Creek. In 

the Miami-Inspiration group of mines the underground water level 

is not very weU defined. Generally some water appears below a 

depth of 400 feet, but as a rule water is not abundant at depths less 

than 900 feet.

The ore bodies in limestone of the Old Dominion and neighboring 

mines have been nearly exhausted. They were large irregular masses

of oxide and carbonate ore associated with much limonite and hematite. In the lodes connected with these masses oxidized ore prevails generally to a depth of 700 to 800 feet but extends much deeper, 

as was predicted in 1903,3 in the much-faulted ground under the Gila 

conglomerate, in the western part of the mine, where it is found on the 

1,600-foot or bottom level about 1,200 feet below the surface. It is 

thus highly probable that much of the oxidation and enrichment at 

the Old Dominion lode was effected before the deposition of the Gila 

conglomerate.

1 This mineral had been seen at Globe only in association with oxidized ores when Professional Paper 12 

was written. F. L. R.

2 L. C. Graton (Mineral Resources U. S. for 1907, pt. 1, TJ. S. Geol. Survey, 1908, p. 596) has stated that 

the ores in limestone are believed to be of contact-metamorphic origin. I am not in accord with this statement. F. L. R.

8 Ransome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. S. Geol. Survey No. 12, 

1905, p. 148.
REVIEW OF MlttlffG DISTRICTS. 185

Under the oxidized ore in the Old Dominion and adjacent lodes, 

mainly in diabase, are large bodies of chalcocitically enriched ore 

which grade irregularly downward into pyrite or into pyritic ore containing chalcopyrite, bornite, and specularite. In places enrichment 

has not penetrated below 800 feet, but elsewhere it goes much deeper.

The disseminated deposits near Miami, about 6 miles west of 

Globe, have been proved during the last few years to be of great 

economic importance. Churn drilling and mining exploration have 

shown the existence of a chain of large ore bodies extending in a 

gentle curve from the Miami mine on the east through the Inspiration, Keystone, and Live Oak mines 'to the west. The extreme 

western limit of this ore belt is as yet undetermined. The total 

length of this chain as at present developed is 2 miles, the maximum 

width is one-fourth mile, and the greatest thickness of ore along any 

one vertical line is about 300 feet. Estimates by the engineers of the 

various mines give a total of 80,000,000 to 90,000,000 tons of ore 

averaging between 2 and 2.5 per cent of copper.

As a whole, the ore bodies form an irregularly undulating ribbon of 

very uneven thickness. The distance from the surface of the ground 

to the top of the ore varies widely from place to place and is not 

definitely related to the present topography, which apparently is of 

later development than the main period of enrichment. At the Miami 

mine the depth to ore is in general between 200 and 400 feet, on the 

Inspiration ground it ranges from 50 to 600 feet, and on the Live Oak 

ground it reaches 1,000 feet. The ore dips generally east in the 

Miami mine, is on the whole nearly horizontal in the Inspiration mine, 

but turns down to the west in the Keystone, and this westerly dip 

increases to 40° or 45° in the Live Oak ground. East of the Keystone a zone of strong faulting has stepped down the ore, and at the 

Miami mine another fault cuts off the ore by bringing the Gila conglomerate down against the schist. There are other important faults 

in the district, but these can not be described in this brief note.

The surface of demarcation between the leached, more or less 

rusty, barren schist and the top of the ore is definite, and in many 

places there has been some slipping between the capping and the ore. 

The downward change from ore (above 1.3 per cent copper) to 

primary material (less than 1 per cent copper) is less definite but 

takes place within a few feet.

The country rock is in general Final schist, but good ore occurs also 

in dikes and small offshoots that extend into metallized schist from 

the main granite mass, which grades into granite porphyry on its margins. Over much of the ore-bearing ground this granite porphyry is 

the surface rock, the ore occurring in the schist beneath it. Clearly 

this part of the porphyry was intruded as a flat-lying sheet.
186 THE ENRICHMENT OF SULPHIDE ORES.

In general the unmetallized Final schist is a bright-gray fissile rock, 

splitting with a lustrous, satiny sheen and showing considerable 

variations in color, texture, and degree of metamorphism or crystallinity. The essential constituents are quartz, muscovite (sericite), 

a little microcline and plagioclase, magnetite, zircon, tourmaline, 

hornblende, biotite, and chlorite. As a whole, the Final schist was 

derived by metamorphism from arkosic sedimentary beds, although 

it includes here and there a little material of probable igneous origin.

During the primary period of metallization pyrite, chalcopyrite, a 

little molybdenite, and quartz were deposited in the fractured schists, 

partly in fissures an inch or so wide but chiefly in much smaller 

cracks or along cleavage planes. During the period of enrichment 

the downward-moving cupriferous solutions replaced the chalcopyrite and pyrite wholly or in part by chalcocite.

Final expression of opinion as to the geologic date of the principal 

enrichment is withheld, but a preliminary review of the evidence 

strongly suggests that the enrichment antedated the deposition of 

the Gila conglomerate and may possibly be older than the dacite. 

Probably the process has continued to the present day, but where 

erosion has overtaken the chalcocite zone enrichment apparently has 

been temporarily checked by the fact that little pyrite is available 

to form strongly acid solutions and the copper, instead of migrating 

downward, remains near the surface, coloring the rocks brilliantly 

with chrysocolla and carbonates. As a rule, the largest ore bodies 

are not found under those surface rocks that are most vividly colored 

by copper compounds or by iron oxide,

BAY, ARIZONA.

By F. L. RANSOME.

The disseminated copper deposits of Ray are about 20 miles southwest of Globe, in the hilly depression drained by Mineral Creek, 

between the Dripping Spring and Tortilla ranges.

The geologic formations are in general the same as those in the 

Globe quadrangle, the Phial schist at Ray being intruded by two 

varieties of granite porphyry, one of which very closely resembles the 

porphyritic facies of the. Schultze granite.

The ore bodies are mainly in the schist, although masses of granite 

porphyry within the generally metallized area have also been converted to ore.

Within the productive area the altitude ranges from 1,950 to 2,900 

feet above sea, and most of the hills are steep and rugged. The 

climatic conditions are similar to those at Globe. Little information 

is available concerning, the level of ground water, which is below the 

principal mine workings. Most of the water that enters the mines 

appears to be seepage from Mineral Creek and from its tributary, 

Copper Creek.
REVIEW OF MINING DISTRICTS. 187

The ore bodies underlie a group of more or less rusty hills stained 

here and there with salts of copper. The principal area showing this 

alteration is of elongated oval shape and extends west-northwestward 

from Mineral Creek for about 2| miles with a maximum width of a 

little more than half a mile. Within this area churn drilling and 

mining have shown the presence of a continuous ore body about 8,000 

feet long and 2,500 feet in greatest width. As at Miami, the layer of 

ore has many irregular undulations that apparently have no dependence on the present topography and varies much in thickness. The 

average thickness of the ore body is 101 feet; the average thickness 

of overburden 250 feet. The depth to ore varies from 10 to 300 feet 

and the thickness of the ore from 0 to 400 feet. Chalcocite has been 

reported from one drill hole to a depth of 715 feet and the ore extends 

under Humboldt Hill to a depth of 700 feet. The quantity of ore in 

the Ray ore body as calculated by the mine engineers is over 

83,000,000 tons averaging 2.17 per cent of copper, with over 500,000 

tons of considerably higher grade.

The relations of the ore to the oxidized leached capping and to the 

underlying primary material are similar to those at Miami. The ore 

constituents in the two districts are the same. In both districts the 

primary metallization continues to unknown depths. As at Miami, 

the greater part of the chalcocite is disseminated through the schist 

(or porphyry) in small specks or in minute veinlets with quartz; but 

here and there are veinlets several inches wide in which the original 

chalcopyrite and pyrite have been partly or wholly altered to massive 

chalcocite.

Pyrrhotite, sphalerite, and galena, so far as known, do not occur 

in the disseminated copper deposits at Ray or Miami.

MOBENCI, ARIZONA.

The Morenci district, Arizona,1 is an area of pre-Cambrian granite 

and quartzitic schists above which rest unconformably about 1,500 

feet of Paleozoic sandstones, limestones, and shales that are locally 

overlain by Cretaceous shales and sandstone. These rocks are 

intruded by masses of granitic and dioritic porphyries, which form 

stocks, dikes, laccoliths, and sheets. All these rocks have participated in an uplift and a warping or doming that was succeeded by 

much faulting, which took place during latest Cretaceous or earliest 

Tertiary time. Surrounding the domed area of older rocks are 

great masses of Tertiary lavas consisting of basalt and rhyolite,, with 

some andesite.2

The ore bodies are veins and disseminated deposits in the granite 

and monzonite porphyry and contact-metamorphic deposits in the

1 Lindgren, Waldemar, The copper deposits of the Clifton-Morenci district, Arizona: Prof. Paper 

U. S. Geol. Survey No. 43,1905. 

8 Idem, p. 17.
188 THE EtfftlCSMEtfT OF SULPHIDE ORES.

limestone and shale near the porphyry. The fissure veins and 

nearly related deposits are most productive. In 1906, according 

to Graton, over 92 per cent came from the large disseminated deposits and stockworks of chalcocite along shattered zones in altered 

porphyry.1

Lindgren 2 summarizes the successive stages of mineral formation 

as follows :

Veins:

Primary processes: Pyrite, chalcopyrite, zinc blende, molybdenite (rarely -mag. 

netite), sericite, quartz (rarely tremolite, diopside, and epidote). Introduction 

of iron, sulphur, zinc, copper, potassium, and silica; elimination of calcium 

and sodium.

Processes of oxidation:

Action of sulphate solutions without oxygen: Chalcocite (rarely covellite, 

chalcopyrite, and bornite), quartz, chalcedony, kaolin, alunite. Introduction 

of copper; elimination of zinc, some iron and sulphur.

Action of directly oxidizing solutions: Cuprite, native copper, brochantite, 

malachite, chrysocolla (rarely azurite), chalcanthite, limonite, quartz. Introduction of carbon dioxide; elimination of sulphur, together with some iron 

and copper. 

Contact deposits:

Primary processes: Pyrite, magnetite, chalcopyrite, zinc blende, garnet, epidote, 

diopside, tremolite, quartz. Introduction of much iron and silica, together with 

copper, zinc, molybdenum, sulphur, possibly magnesia; elimination of carbon 

dioxide and probably some lime.

Processes of oxidation: Limonite, malachite, azurite, cuprite, rarely native 

copper and chalcocite, copper-pitch ore, chrysocolla, goslarite, zinc carbonate, 

willemite, calamine, pyrolusite, quartz, calcite, chlorite, serpentine. Introduction of carbon dioxide and additional copper; elimination of calcium, sulphur, 

some zinc, and iron.

The veins rarely show typical gossan or iron cap with an abundance 

of limonite and rich oxidized ores.3 Their Outcrops are either 

entirely barren or contain only a moderate amount of copper, but

much richer ore is ordinarily found just above the chalcocite zone. 

The upper half of the chalcocite ore is richer. It occurs as solid 

masses and in the porphyry as scattered grains. In both classes 

of deposits it replaces pyrite and zinc blende, and generally at a 

depth rarely more than 400 feet below the surface it gives way to 

these minerals, which form a very low grade unworkable ore. The 

annual rainfall is about 10 inches. No standing ground-water level 

has yet been encountered, and the upper parts of the chalcocite 

bodies are undergoing extensive oxidation.

The deposits in porphyry, as stated by Lindgren, generally show 

the following arrangements in depth.4

» Graton, L. C., Mineral Resources U. S. for 1906, U. S. Geol. Survey, 1907, p. 388. 

s Lindgren, Waldemar, op. cit., p. 194. 

» Idem, p. 207. 

«Idem, p. 201.
REVIEW OF MINING DISTEICTS. 189

Surface zone .....50 to 200 feet deep from the croppingB; contains oxidized copper

minerals or is barren. 

Chalcocite zone.. .100 to 400 feet in vertical extent; possibly more in places; contains

chalcocite and pyrite. 

Pyritic zone..... .Begins 200 to 600 feet below the surface; contains pyrite, chalcopyrite, zinc blende, and molybdenite.

The minerals of the upper two zones have been derived from those 

of the pyritic zone by processes of direct and indirect oxidation; the 

chalcocite is derived from replacement of pyrite and sphalerite 

probably by aid of cupric-sulphate solutions. The surface zone 

has been derived from the chalcocite zone by its gradual and direct 

oxidation. The pyritic zone has thus far been found to be poor in 

copper and rarely makes commercial copper ore; the chalcocite zone 

produces the richest ore and the richest part of it is near its upper 

limit; the leached zone is usually poor and sometimes practically 

barren. These changes with depth may be observed also in the 

contact-metamorphic ores, but these ores as a rule are not altered 

to depths so great.

The Coronado vein, which occupies a fault fissure between preCambrian granite and quartzite, differs in many respects from the 

deposits in the porphyry, and it is suggested by Lindgren 1 that it 

probably had a different origin. The ore minerals include pyrite, 

chalcopyrite, and chalcocite. Sphalerite is not mentioned byWendt 2 

or by Lindgren as a vein constituent, although it appears in appreciable quantities in other ores in this district. Lindgren notes that 

chalcocite ore is found in this vein as deep as 500 or 600 feet below 

the surface. More recently, according to Tolman,8 a chalcocite zone 

has been found in the Coronado mine considerably deeper than in 

any other mine in the district.

ELY, NEVADA. 

By A. C. SPENCER.

The Ely district, Nevada,4 is an area of folded and faulted Paleozoic 

limestones and shales ranging in age from Silurian to Carboniferous. 

These formations are intruded by monzonite porphyry along an eastwest zone about 9 miles long and from one-half to 1 mile wide. These 

older formations are locally overlain by rhyolite flows of Tertiary age. 

The sedimentary rocks adjacent to the porphyry intrusions are greatly 

altered, being locally garnetized or changed to jasperoid and commonly charged with great quantities of pyrite. In places near the 

igneous masses considerable amounts of chalcopyrite occur with the

> Lindgren, Waldemar, op. cit., p. 344.

5 Wendt, A. P., The copper ores of the Southwest: Trans. Am. Inst. Min. Eng., vol. 15,1837, pp. 28-52.

«Tolman, C. F., The southern Arizona copper fields: Min. and Sci. Press, vol. 99,1909, p. 890.

«Lawson, A. C., The copper deposits of the Robinson mining district, Nevada: Bull. Dept. Geology Univ. 

California, vol. 4, No. 14, 1906, pp. 287-357. Spencer, A. C., Preliminary geologic map of the vicinity of 

Ely, Nev., U. S. Geol. Survey, 1912.
190 THE ENBICHMEFT OF SULPHIDE OEES.

pyrite, and rarely zinc blende and pyrrhotite accompany these sulphides. Galena and its oxidation products occur in irregular lodes 

within the metamorphic area, principally at some distance from the 

porphyry masses. Gold ores, formerly exploited, occur mainly in the 

form of blanket lodes, the gold being associated with lead carbonate.

Of many superficial showings of copper carbonates none has been 

developed profitably, but oxidized ores of relatively high grade have 

been discovered in the Alpha mine of the Giroux Co., at considerable 

depth. This ore body is inclosed by metamorphosed and thoroughly 

oxidized sedimentary rocks that lie several hundred feet from the 

nearest mass of porphyry.

The present importance of the district centers in the low-grade disseminated ores in porphyry. The igneous rock was locally fractured 

after its intrusion, and great masses of it became infilled with veinlets 

of quartz carrying pyrite and chalcopyrite. Even away from the 

fractures the porphyry was charged with sulphides and the rock was 

greatly altered, with abstraction of lime, magnesia, soda, and iron and 

with noteworthy addition of potash. These losses and gains involved 

the destruction of hornblende and lime-soda feldspar and the formation of mica, including brown mica and sericite. The outcrops of the 

ore masses are yellowish or less commonly red and are said to carry 

not over 0.5 per cent of copper. There is an abrupt change from 

this capping to soft bluish-white porphyry ore, which carries disseminated sulphide minerals, including copper glance, as films coating 

grains of pyrite and chalcopyrite or, less commonly, completely 

replacing such grains.

One company, the Nevada Consolidated, has developed about 

49,000,000 tons of disseminated ore, the average being 1.7 per cent 

copper. The variations in depth are indicated in three sections as 

follows: 1

Thickness, in feet, of capping and of ore in workings of the Nevada Consolidated Co.,

Ely, Nev.

Section.

Average 

thickness of 

direct 

capping.

87.1

101.4

154.7

102.5

Average 

thickness of 

profitable 

ore.

190

280.3

m o

917 Q

The profitable ore is known to extend locally to depths about 600 

feet below the surface. In one place a hole was put down nearly 400 

feet below the ore body in material which carried less than 0.4 per 

cent of copper.

1 Fifth Ann. Rept. Nevada Consolidated Copper Co., for 15 months ended Dec. 31,1911, p. 8.
EEVIEW OF MINING DISTRICTS. 191

The gold and silver content of the disseminated ores is considerable. 

In 1911 the average per ton was 0.013 ounce gold and 0.079 ounce 

silver. Although less than 60 per cent of the precious metals was 

saved, their value recovered was 17.35 cents a ton.

A composite analysis of 1,000 samples of ore from the Euth mine 

shows, as stated by Lawson, sulphur, 6 per cent; iron, 5.3 per cent; 

and copper, 2.61 per cent, which may be calculated as equivalent to 

pyrite, 10 per cent; chalcopyrite, 1.8 per cent; and chalcocite, 2 per 

cent.

It may be stated as a general truth that any porphyry carrying 

more than 1 per cent of copper owes its grade to the presence of 

chalcocite, the enrichment having resulted from precipitation of this 

mineral out of solutions derived from overlying material.

In the porphyry mines the lower limit of complete oxidation is 

everywhere considerably above the water level, the difference in 

elevation being about 250 feet in the Ruth mine. While most of the 

material carrying chalcocite lies above standing water, the bottom of 

the ore seems to bear no definite relation to the water table. The 

greatest depth of porphyry ore is about 600 feet, but in the Alpha 

mine, which lies outside of the porphyry ore, enriched ore was 

found between the 700 and 1,200 foot levels. Here the water stands 

somewhat more than 1,000 feet below the surface and most of the 

lode material is fully oxidized and leached to this depth.

SANTA RITA AND HANOVER DISTRICTS, NEW MEXICO.

The Santa Rita and Hanover districts, New Mexico, 1 are adj acent and 

show similar geologic features. The country is an area of Carboniferous and older sedimentary rocks, consisting of limestone, quartzite, 

and shale. These are intruded by granodiorite, quartz monzonite 

porphyry, and related igneous rocks, and along the intrusive contacts garnet zones have developed. Considerable masses of magnetite, pyrite, and zinc blende are present. Pyrrhotite 2 is found in the 

Hanover district, and sphalerite has been mined to some extent 

for zinc.3

At Santa Rita most of the copper occurs as native metal, oxide, or 

sulphide in altered porphyry. Chalcocite with kernels of pyrite is 

present both in the quartzite and the porphyry.4

As stated by Graton, it is certain that much of the copper that 

now exists as native metal or oxide was not precipitated on pyrite 

but must have been deposited from solution in open spaces and in 

porphyry as replacement along fissures.

i Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S. Geol. Survey No. 68,1910, p. 305. 

»Idem, pp. 53, 58, 81. 

*Idem, p. 309. 

«Idem, p. 62.
192 THE ENKICHMENT OF SULPHIDE OEES.

Whether or not a mingling of solutions containing hydrogen sulphide or some other 

soluble sulphide with those containing the copper caused this precipitation is unknown. 1

The presence in chalcocite ores of cores and kernels of pyrite and, outside the ore 

bodies, of pyrite in masses similar in distribution to the bodies of ore, as well as in 

a more disseminated state, and all more or less coated with chalcocite, makes it certain that at least a great part of the chalcocite was precipitated by and replaced 

pyrite. 2

In the disseminated ores on the Chino ground some 55,000,000 

tons of ore averaging 2.24 per cent copper are said to be developed. 

Most of it is near the surface. About 70 per cent, it is said, may be 

mined with steam shovels. The average thickness of the capping, 

according to B. S. Butler,3 is 82 feet, and the average thickness of 

the ore below the capping is 107 feet.

The ore is somewhat richer than the disseminated ore in several 

other districts in the Southwest, but no data are available to show 

that zinc blende has favorably affected the grade of the concentration. I am informed that sphalerite is present in some of the deposits, but that little or none has been observed in the ore directly 

below some of the important bodies of disseminated ore.

JEROME, ARIZONA.

By F. L. KANSOME.*

At Jerome faulting has effected the relative elevation and exposure 

of a mass of pre-Cambrian schist, within which the principal ore 

bodies thus far developed are those of the United Verde mine. On 

the hillside just west of the mine the schist is unconformably overlain 

by Cambrian, Devonian, and Carboniferous beds.

In the vicinity of the mine the schist stands nearly vertical, with 

rather rough irregular cleavage, and strikes a little west of north. 

At least three varieties are distinguishable (1) a green rock, schistose 

on its margins but grading into massive material, which is evidently 

an altered dioritic intrusive; (2) a rough gray schist with abundant 

phenocrysts of quartz, apparently an altered rhyolite; and (3) satiny 

greenish-gray, very fissile sericitic schist that may be a metamorphosed sediment. The ore occurs in varieties 2 and 3, the main 

belt of dioritic rock (1) lying just west of the ore bodies. The ore is 

said to follow as a rule the layers of fine sericitic schist.

The ore shoot as a whole is of oval plan, its total horizontal length 

being about 1,300 feet and its greatest width 700 feet. Its general 

trend is north-northwest and it pitches in that direction at an angle

1 Lindgren, Waldemar, Grafton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. 

Paper U. S. Geol. Survey No. 68,1910, p. 316.

2 Idem, p. 315.

8 Butler, B. S., Mineral Resources U. S. for 1910, pt. 1, U. S. Geol. Survey, 1911, p. 205.

«Based on a brief visit in 1912.
BEVIEW OF MINING DISTEICTS. 193

of 45°. It has been worked to a vertical depth of 1,200 feet with 

no sign of approaching exhaustion. This great shoot is in reality a 

complex of smaller but nevertheless large irregular or lenticular ore 

bodies, apparently related in part to cross fissuring but showing a 

general tendency toward parallelism with the schistosity.

The important ore constituent is chalcopyrite, associated with 

varying proportions of pyrite and sphalerite. Bornite has been 

reported but was not noted in 1912. Neither galena nor pyrrhotite 

was observed at the time of visit. The sulphides occur partly in 

small irregular fractures and along planes of schistosity, but to a 

large extent they have metasomatically replaced the schist, particularly the fine-grained sericitic variety. The ore contains very 

little vein quartz or other gangue mineral.

The sulphides mentioned are primary and were deposited in preCambrian time.

Oxidized ore, containing malachite, azurite, and cuprite, extended 

to a depth of about 400 feet and in its upper part was comparatively 

rich in gold. Native copper was nowhere abundant. Below the 

level of complete oxidation there was, according to H. J.'Stevens,1 

some chalcocitic enrichment with a relatively high proportion of 

silver, but I was informed at the mine that the chalcocitic zone 

was not of great economic importance and soon gave place to primary ore.

BURRO MOUNTAIN DISTRICT, NEW MEXICO.

The Burro Mountain district is in Grant County, southwest New 

Mexico. The country is arid and lies 5,000 to 8,000 feet above the 

sea. The main mass of the mountains 2 consists of pre-Cambrian 

granite and pegmatite and a dioritic rock which locally is gneissic. 

These rocks are intruded by monzonite porphyry.

The most important deposits are impregnations and replacements 

along irregular fractures and zones of crushing. Silica and cupriferous pyrite have been introduced and constitute the primary lodes. 

Practically all the workable ores are the result of enrichment of this 

lean pyritic material. Rich oxidized minerals native copper, 

cuprite, melaconite (?), chrysocolla, melanochalcite (?), malachite, 

and azurite are found at or near the surface, but at greater depth 

chalcocite and chalcocite-coated pyrite are the ore minerals, and at 

a still lower level these give way to unaltered pyrite.

In the principal mine at Leopold, which belongs to the Burro

Mountains Mining Co. 

the zone of impoverishment or leaching is in most places less than 50 feet deep. 

The ore on the first or 100-foot level contains more iron and less copper than that on

' Stevens, H. J., Copper handbook, 1909, p. 1369.

1 Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Paper 

U. S Geol. Survey No. 68,1910, p. 320.

72293° Bull. 529 13 13
194 THE ENBIOHMENT OF SULPHIDE OEES.

the 130-foot and 160-foot levels; in fact, it appears less enriched. It was' reported 

in 1905 that the lowest or 210-foot level then being driven, had encountered lowgrade pyritic ore under the enriched ore of the upper levels, but since then important 

discoveries have been announced. 1

According to S. S. Lang 2 the chalcocite ore ends very abruptly 

against the pyrite-chalcopyrite ore below.

In the district southeast of Leopold, according to Sidney Paige,3 

the leached ground is 200 feet deep. From this the richer ores extend 

downward to a depth of 380 feet, where unaltered sulphides are 

encountered. The water level is variable from 300 to 500 feet 

below the surface.

CANANEA, MEXICO.

Cananea, Sonora, Mexico, is 25 miles south of the international 

boundary and some 38 miles by rail from Bisbee, Ariz. The country 

is mountainous and the principal deposits are 5,200 to 6,500 feet 

above the sea. The climate, although arid, is somewhat more humid 

than that of Arizona.

The region has had a complex geologic history.* An ancient granite is overlain by Paleozoic (?) quartzite and limestone and intruded 

by diabase, above which are rhyolitic tuffs and rhyolite. Intrusive 

masses of syenite and syenite porphyry are later than the sedimentary 

rocks and probably later than the tuffs, and large intrusive masses 

of diorite and diorite porphyry cut both tuffs and limestones. A 

later granite porphyry, a granodiorite, a quartz monzonite porphyry, 

and gabbro and diabase are found in intrusive masses.5

Contact metamorphism, with the development of heavy silicates 

and sulphides, has attended the intrusion of the diorite porphyry, the 

granodiorite, and the quartz porphyry. Contact-metamorphic deposits are numerous and important, but these are subordinate to the 

great lodes and disseminated ores in the sericitized. and silicified 

phases of the diorite porphyry. The principal deposits of the latter 

class lie in a belt 1 to 2 miles wide and 6 miles long, which strikes a 

little north of west, significantly parallel to the great Eliza fault. The 

contact-metamorphic deposits are composed of chalcopyrite, sphalerite, bornite, magnetite, hematite, and galena in a gangue of garnet, 

calcite, hornblende, pyroxene, and other heavy silicates. In general, 

these deposits show chalcocite enrichment to only very shallow 

depths.8

i Lindgren, Waldemar; Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. 

Paper U. S. Geol. Survey No. 68,1910, pp. 322-323.

8 Lang, S. S., The Burro Mountain copper district: Eng. and Min. Jour., vol. 82,1906, p. 395.

s Paige, Sidney, Metalliferous ore deposits near the Burro Mountains, Grant County, N. Mex.: Bull. 

U. S. Geol. Survey No. 470,1910, p. 131.

«Emmons, S. F., Cananea mining district of Sonora, Mexico: Econ. Geology, vol. 5,1910, p. 312.

»Weed, W. H., The Cananea copper deposits: Eng. and Min. Jour., vol. 74,1902, pp. 744-745; The Cananea 

district: Trans. Am. Inst. Min. Eng., vol. 32,1902, p. 428; The Cananea ore deposits: Eng. and Min. Jonr., 

vol. 76,1903, p. 383. Austin, W. L., The ore deposits of Cananea: Eng. and Min. Jour., vol. 76,1903, pp. 

310-311. Hill, R. T., The ore deposits of Cananea: Idem, p. 421; Cananea revisited: Idem, pp. 1000-1001.

6 Emmons, S. F., op. cit, p. 355.
REVIEW OF MINING DISTRICTS. 195

The ores in porphyry are composed chiefly of pyrite, chalcopyrite, 

sericite, kaolin, quartz, and other minerals. Zinc blende, although 

locally abundant, is erratic in distribution. In these ores chalcocite 

enrichment is very extensive.

Many of the ore bodies are marked by conspicuous gossan immediately or not far above the deposits. This, I believe, is invariably 

true of the garnetiferous ores in altered limestone, although in the 

latter oxidation extends only to shallow depths. Some richer ores of 

this type are seamed and coated with chalcocite, and possibly some 

chalcopyrite in these deposits is secondary, but these ores are nevertheless workable at several places, in what appears to be their primary 

concentration.

In the West Cobre Grande and Kirk mines the development is in 

both diorite porphyry and limestone. The ores are generally of good 

grade, but at the 200-foot level there is an apparent falling off in the 

amount of chalcocite and an increase in the proportion of zinc in the 

ore, which indicates sulphide enrichment.1

The most important deposits of the district are those of the Capote 

basin, which are almost exclusively fractured zones in sericitized 

porphyry. The primary ores consist mainl}'- of pyrite with a small 

admixture of chalcopyrite and in places considerable zinc blende. 

Their introduction was accompanied by extensive sericitization and 

silicification of the adjoining country rocks. They occur in closely 

spaced fractures or shear zones that traverse both eruptive and sedimentary rock, and they are disseminated in porphyry throughout the 

rock, adjoining fractures. The tenor in copper is so low that it is 

doubtful if the primary ore could have been mined at a profit if it had 

not received additional copper by downward migration or sulphide 

enrichment.2

The surface is conspicuously altered by oxidation, which has readily 

attacked the hydrothermally altered pyritized area. Along the 

Capote fault zone the ore body is capped by gossan, which lies above 

porphyry and quartzite. A chimney of ore, large on level 1, was followed down to level 7, where it had decreased in size. This chimney 

of ore appears to represent a zone of intense fracturing and one 

peculiarly favorable to concentration. Some of the fracturing is 

later than primary deposition, for traces of enrichment are found 

down to the seventh level.3

The Oversight ore body is capped by porphyry that is sihcified but 

only slightly stained with iron. The chalcocite ore is found 260 feet 

below the surface and extends downward at least 300 feet deeper, 

but its lower limit had not been determined in 1907.

> Emmons, S. F., op. cit., p. 340. * Idem, p. 341. * Idem, p. 345.
196 THE ENKICHMENT OF SULPHIDE OEES.

The Veta Grande ore body has produced remarkably rich oxidized 

ore, chiefly cuprite and native copper. Rich ore of decreasing tenor 

was found as deep as the 500-foot level. "In the middle levels there 

is much enrichment by chalcocite, but this enrichment has apparently 

not extended to the lowest level." * .

NACOZARI, MEXICO.

Nacozari, Sonora, Mexico, is 75 miles south of the international 

boundary line, in a region of hills and low mountain ranges much like 

those of Arizona. The country rock consists of volcanic tuffs, breccias, and other igneous rocks, with which are associated subordinate 

limestones and other sedimentary rocks.2

The most important mines belong to the Moctezuma Copper Co., 

controlled by Phelps, Dodge & Co. The principal developments are 

those of Los Pilares mine, at the end of a tunnel about a mile long, 

which intersects a shaft 600 feet below the surface. The largest 

deposit is a great semicircular body of relatively low grade ore that 

appears to conform with a curving fractured mass of monzonite (?) 

which follows a sheeted zone of faulting and a faulted diabase dike 

that intersect nearly at a right angle.

The ore in depth is composed of brecciated monzonite (?) the small 

fragments of which are cemented by iron and copper sulphides. The 

principal sulphides are pyrite and chalcopyrite, which in the richer 

ore are coated with bornite and a relatively small amount of chalcocite. In places the ore contains a little zinc, but lead is not reported 

to be present. At the surface the outcrop is iron-stained gossan that 

has locally weatheied into the pillars from which the mine derives 

its name.* As in some other arid regions, the iron ore has become 

dehydrated at the surface, forming specularite. The stopes of richer 

ore extend from a short distance below the 100-foot level down nearly 

to the 500-foot level. Below this the ore is said to be falling off in 

grade.

It is thus evidently to secondary enrichment that it owes its quality as pay ore. 

Above these limits the copper is leached out; within them is the greatest concentration of the richer sulphides and, while they can still be observed in the lower levels 

of the mine, it is evident to the eye that they are in decreasing proportion.4

According to W. L. Tovote 5 the chalcocite zone is insignificant.

i Emmons, S. F., op. cit., p. 347.

* Emmons, S. F., Los Pilares mine, Nacozari, Mexico: Econ. Geology, vol. 1, 1906, pp. 629-643.

»Idem, p. 631.

«Idem, p. 636.

'Tovote, W. L., Magmatic origin of ore-forming solutions: Min. and Sci. Press, vol. 104,1912, p. 602.
REVIEW OF MINING DISTRICTS. 197 

SILVERBELL, ARIZONA.

The deposits at Silverbell; Ariz., 1 are in an area of Paleozoic rocks, 

including limestones, complexly cut by an alaskite porphyry intrusion 

and by andesite and trachyte dikes and covered by andesite, rhyolite, 

basalt, and other rocks. Garnet contact zones are developed in limestone near the porphyry. The ore bodies, as stated by Tolman, 

include contact-metamorphic deposits in limestone, disseminations 

in porphyry, and fissure veins.

The contact-metamorphic deposits supply most of the ore (1909). 

These deposits carry chalcopyrite, pyrite, quartz, molybdenite, and 

occasionally wulfenite. A considerable proportion of this ore is 

oxidized. Some lean chalcocite deposits are disseminated in porphyry.

SAN FRANCISCO REGION, UTAH.

By B. S. BUTLER.

The San Francisco region, Utah, is a large mineralized area in the 

San Francisco and adjacent ranges in Beaver County, Utah. The 

sedimentary series consists of Paleozoic limestones, shales, and quartzites. These have been covered by a thick series of lavas of intermediate composition. These formations have been intrudedby quartz 

monzonite and allied rocks.

The ore deposits may be divided into three general types (1) replacement deposits in fissures in the quartz monzonite; (2) replacement deposits in the limestone, including contact deposits and 

replacement deposits along fissures, and (3) replacement fissure 

deposits in the lavas.

Of the replacement deposits in fissures in the quartz monzonite 

the ore zone in the Cactus mine is the most important. This consists 

of a brecciated zone of quartz monzonite that has been greatly 

altered and replaced by the ore minerals. These consist of the sulphides, pyrite and chalcopyrite, with small amounts of tetrahedrite 

and galena and abundant specularite in a gangue of quartz, sericite, 

tourmaline, impure siderite, anhydrite, and some barite. Oxidation 

has not extended below 100 feet and for the most part is confined to 

a space within 50 feet of the surface. The principal minerals resulting from the oxidation are limonite, azurite, and malachite, with 

 some native copper. Chalcocite is almost and possibly entirely 

absent from the deposit.

The 0 K mine, in the Beaver Lake Range, also works a deposit 

that lies in quartz monzonite and that is similar to the deposit worked 

by the Cactus mine except that it contains no specularite and that

1 Tolman, C. F., Copper deposits of Silverbell, Ariz.: Min. and Sci. Press, vol. 99, 1909, pp. 710-712.
198 THE ENRICHMENT OF SULPHIDE ORES.

the gangue is quartz and sericite. In this deposit oxidation has 

extended to much greater depth, fully 200 feet, and covellite and 

chalcocite replace pyrite and chalcopyrite. The only apparent reason for the difference hi the secondary ores here and in the Cactus 

mine is the presence in the latter of carbonate, which has probably 

prevented or retarded the downward movement of the copper content 

of the ores.

The typical contact deposits are composed of pyrite, chalcopyrite, 

magnetite, and sphalerite in a gangue of garnet, tremolite, and 

other silicates. In general oxidation has not extended to great depth, 

probably owing to the massive character of the deposits. The richer 

ores, however, have resulted from concentration in the oxidized zone.

The replacements along fissures in the limestone are typically leadsilver deposits containing minor amounts of copper and zinc. The 

ores are largely oxidized to the depth of present developments 500 

to 600 feet. The metal-bearing minerals are principally carbonates 

but include minor amounts of sulphates.

The Horn Silver mine is on the most important deposit in the volcanic rocks. This deposit occurs along a fault that has thrown the 

lavas down against the Paleozoic limestone. The lavas have been 

considerably shattered along this fault, especially in the vicinity of 

minor cross faulting, but the limestone is relatively massive. The 

ore deposits have formed largely as a replacement of the' brecciated 

lava. The primary ore consists of pyrite, galena, sphalerite, and 

minor amounts of other metallic minerals in a gangue of quartz, 

sericite, and partly altered lava. In the oxidized zone the ores are 

characteristically sulphates, anglesite being the principal mineral hi 

much of the lead ore. Complex sulphates,, such as beaverite, 

plumbojarosite, and jarosite, are rather abundant, and the oxidized 

copper ore carries much brochantite. Zinc is not abundant in the 

oxidized ores, though some carbonate of zinc occurs. In the secondary sulphide zone the copper ore consists of covellite and chalcocite, 

partly or wholly replacing sphalerite, wurtzite, pyrite, and, to a 

slight extent, galena. Rich copper ores were mined to a depth of 

about 750 feet, and enrichment along favorable channels has extended 

much deeper. The rich zinc ores of this mine are composed of sphalerite and wurtzite, together with the sulphides of other metals in 

variable quantities. The wurtzite is apparently secondary, commonly 

forming around cores of sphalerite, and the richer ores have resulted 

from the addition of the zinc in the wurtzite. Normally the zinc 

enrichment extends to greater depth than the copper enrichment, 

and secondary zinc sulphide has been replaced by secondary copper 

sulphides.

The Beaver Carbonate mine, another replacement deposit in the 

volcanic rocks, is similar in occurrence to the Horn Silver deposit,
EEVEEW OF MINING DISTRICTS. 199

except that both walls are of the volcanic rock. In this deposit 

enrichment has been slight, apparently owing to the presence of a 

local water table very near the surface.

SHASTA COUNTY, CALIFORNIA.

The copper-bearing region of Shasta County, Cal.,1 is an area of 

sedimentary rocks complexly intruded by several varieties of consanguineous igneous rocks. The oldest sedimentary series comprises 

the Kennett formation (Middle Devonian), which consists chiefly of 

black fissile shale, with scattered lenses of light-gray limestone and 

numerous gray or yellowish beds of tuffaceous material. Overlying 

the Kennett formation unconformably is the Bragdon formation 

(Mississippian), which consists chiefly of black and gray shales, 

with thin interbedded layers of tuff and sandstone and bands of 

conglomerate. Above the Bragdon is the Pit shale, of Middle and 

Upper Triassic age, consisting chiefly of shales with interbedded 

layers of volcanic tuff.

The oldest igneous rock is an altered andesite called by Diller the 

meta-andesite, which underlies and is older than the Kennett and 

Bragdon formations. A massive, less-altered andesite (Dekkas andesite) overlies the Bragdon and underlies and grades into the Pit. 

Younger than all these rocks and cutting the latest of them, the Pit 

shale, are intrusives of soda-rich alaskite porphyry, or soda granite. 

The alaskite porphyry is cut by quartz diorite, which in turn is cut 

by acidic and basic dikes that are genetically very closely related to the 

alaskite porphyry and to the pegmatites of the region. These pegmatites, according to Graton,2 are more acidic than the present quartz 

diorite and in places "pass over into siliceous masses that are virtually quartz veins and carry sulphides."

There are two centers of alaskite porphyry, and each is a center of 

ore deposition.

The important copper deposits consist of large masses of pyritic ore, surrounded in 

most places by alaskite porphyry but here and there extending into shale. The 

ores are of medium richness, yielding at present an average of about 3 to 3£ per cent 

of copper and $1.50 to $2 per ton in precious metals, generally about equally divided 

between gold and silver.3

The ores were probably formed about at the close of the Jurassic 

or the beginning of the Cretaceous. They are in part replacements 

in shattered zones of alaskite porphyry which is highly altered by 

sericitization (soda sericite). The deposits are mineralogically simple. 

Pyrite is the most abundant ore mineral, and chalcopyrite is the chief 

one that gives the ores their copper value. Sphalerite or zinc blende

»Diller, J. S., Redding folio (No. 138), Geol. Atlas U. 8., TJ. S. Geol. Survey, 1906. Gratoii, L. C., The 

occurrence of copper in Shasta County, Cal.: Bull. U. S. Geol. Survey No. 430,1910, pp. 71-111. 

»Graton, L. C., op. cit., p. 86. 

* Idem, p. 89.
200 THE ENRICHMENT OP SULPHIDE ORES.

is also present in.varying amount; on the average it possibly exceeds 

the chalcopyrite. Some galena is associated with the zinc sulphide, 

especially in the eastern districts of the region. The gangue minerals 

are gypsum, calcite, and barite. The minerals which have resulted 

from the alteration of the primary ore minerals include limonite, magnetite, wad; secondary chalcopyrite, bornite, and ch,alcocite; cuprite; 

native copper; small amounts of malachite and azurite; and several 

sulphates.

In some of the ore bornite, chalcocite, and chalcopyrite are intergrown. The two richer copper sulphides are very commonly intergrown with the chalcopyrite, in places inclosing it and in places being 

inclosed by it.

These two minerals were found in the deepest workings of the Afterthought mine 

at a depth of about 600 feet and in the deepest workings of the Bully Hill mine at 

970 feet, as well as at higher levels. In places they are intergrown with and even 

entirely surrounded by barite, and there is no reason whatever to consider them as 

other than primary constituents of the ore. Both bornite and chalcocite, as well as 

chalcopyrite, however, are also formed in the upper portions of some of the ore bodies 

by the process of secondary enrichment, but almost all such occurrences can be 

readily distinguished from those in which the minerals ore of primary origin. 1

The secondary zone was wonderfully productive of copper and 

silver at the Iron Mountain and Bully Hill mines, but at other mines 

it is either of little importance or practically absent. The secondary 

ores appear to have extended to no great depth and are now exhausted.2

YERINGTON, NEVADA.

The country at Yerington, Nev., is an area of schists and limestone 

with intrusive masses of granodiorite, or quartz monzonite, and 

related porphyries, which are overlain by a series of rhyolite flows 

and tuffs, bedded volcanic grits, andesite tuff, and andesite breccia, 

and an unconformably overlying flow of basalt.3

The copper deposits, according to Ransome,4 are (1) irregular 

bodies formed by metasomatic replacement of limestone and genetically associated with metamorphism of a kind usually attributed to 

the contact action of intrusive rock, (2) metasomatic vein deposits 

in altered limestone, and (3) metasomatic vein deposits in granodiorite.

To the first or contact-metamorphic class belong the deposits 

worked by the Bluestone, Mason Valley, and other mines. In the 

Bluestone mine the ore consists essentially of chalcopyrite dissemii Graton, L. C., op. cit., p. 105.

> Graton, L. C., Mineral Kcsources U. S. for 1907, pt. 1, U. S. Geol. Survey, 1903, p. 599.

 Ransome, F. L., The Yerington copper district, Nevada: Bull. U. S. Geol. Survey No. 380,1909, p. 99.

'Idem, p. 116.
EEV1EW OF MIKIKG DISTRICTS. 201

nated as grains and small irregular masses through limestone that 

has been altered to an aggregate consisting chiefly of a yellowishgray epidote with subordinate calcite and quartz. A little garnet is 

present.1 Zinc blende is not mentioned. In these deposits, as stated 

by Ransome 

Oxidation is not extensive and scarcely penetrates to the 100-foot level, while 

chalcopyrite is present in the croppings. The products of oxidation are malachite 

(with possibly some brochantite and libethenite), azurite, cuprite, and chalcanthite. 

These are generally associated with sulphides, and there are no large masses of thoroughly oxidized ore. A little chalcocite occurs above the 100-foot level, but there 

has been no important enrichment through the secondary formation of this mineral. 

It is evident that the ore body as a whole is too solid to have been penetrable to any 

considerable depth by oxidizing solutions and that erosion has kept pace with the 

process of weathering. 2

The vein deposits in limestone are represented by the Ludwig 

vein, which is composed mainly of coarse crystalline calcite carrying 

some garnet with pyrite and chalcopyrite. Oxidation in this deposit 

has extended to a depth of about 500 feet and has produced ore of 

shipping grade. At the base of the oxidized zone there has been 

some chalcocite enrichment.3

The primary ore of the Mason Valley mine contains more pyrite with the chalcopyrite than that of the Bluestone mine and appears to be generally of lower grade. 

Oxidation, however, has penetrated deeper and has effected some local concentration 

down to the No. 3 tunnel, although .some sulphides occur at the surface. On the No. 

3 level there is a considerable body of rich ore consisting of impure earthy cuprite 

with much disseminated native copper. The shape and extent of this body, which 

contains up to 20 per cent of copper, have not been fully ascertained. Chalcocite in 

a soft, sooty condition occurs sparingly, but there has been no important enrichment 

through the formation of this sulphide. The minerals noted in the oxidized zone 

are malachite, azurite, cuprite, native copper, limonite, gypsum, and chalcanthite.4

VANCOUVER, BRITISH COLUMBIA.

The copper deposits on the west side of Vancouver Island are located 

near contacts of granite and calcareous sedimentary rocks and are 

composed of pyrite, bornite, garnet, and other minerals. The ore of 

the Monitor mine is a mixture of chalcopyrite, pyrrhotite, and coarse 

magnetite, filling interstices and holding unreplaced rock fragments.5

According to William M. Brewer,6 some of the deposits carry magnetite and gossan at the outcrop, and rich chalcopyrite ore was found 

in one deposit within 6 feet of the surface.

i Ransome, F. L., The Yeringtoncopper district, Nevada, Bull. U. S. Geol. Survey No. 380,1909, p. 107. 

2 Idem, p. 108. 

8 Idem, p. 117. 

<Idem, p. 110.

6 Weed, W. H., The copper mines of the world, New York, 1907, p. 222.

'Brewer, W. M., The copper deposits of Vancouver Island: Trans. Am. Inst. Min. Eng., vol. 29,1900, 

pp. 483-488.
202 THE ENBICHMENT OF SULPHIDE OBES.

SNOWSTORM LODE, CCEUR D'ALENE DISTRICT, IDAHO.

The Snowstorm lode,1 3£ miles east-northeast of Mullan, Idaho, 

is a wide zone of impregnation in Revett quartzite (pre-Cambrian) 

and conforms with the bedding, which strikes N. 60° W. and dips 65° 

SW. There is no pronounced fissuring along the lode, but microscopic study shows a network of fractures or invisible capillary openings which gave access to the ore-bearing solutions. - The vein dips 

down the slope of the hill, and tunnel 3 is about 800 feet below the 

surface.

The ore is highly siliceous (over 90 per cent Si02 ) and is in wide 

demand for use ^converter linings. It carried (1904) about 4 per 

cent copper, 6 ounces silver, and 0.1 ounce gold.

In its unoxidized form the best ore consists of quartzite so crowded with little 

specks and small irregular bunches of bornite, chalcocite, and chalcopyrite as to be 

dark gray or nearly black. The microscope shows that the ore minerals to some extent 

fill irregular microscopic fissures but that for the most part they have replaced the 

interstitial sericite and siderite of the country quartzite.

Comparatively little of the ore, however, remains in its sulphide condition. Most 

of it has been oxidized to cuprite and malachite. There is no well-defined zone of 

oxidation, most of the sulphide ore occurring in No. 2 tunnel [about 460 feet below the 

outcrop measured on the dip of 65°] and some carbonate in No. 3 tunnel [about 1,200 

feet below the outcrop measured on the dip of 65°]. 2

COPPEBOPOLIS, CALIFORNIA.

The rocks in the vicinity of Copperopolis, Cal., are metamorphosed 

igneous and sedimentary rocks of the "Bedrock series" and include 

the Mariposa slate, meta-andesites, and other rocks, cut by gabbro, 

hornblende, granqdiorite, and serpentine.8 The ore occurs as overlapping lenses, many arranged en Echelon, each lens being composed 

of a complex series of bands or veinlets of chalcopyrite and pyrite 

deposited mainly parallel to the foliation of the schist. In age the 

ore, according to Reid, is earlier than the gold-bearing veins of the 

Mother Lode region. These veins are associated with the acidic 

intrusives of this part of the Sierra Nevada, which are distinctly 

later than the copper ore.

The surface zone of oxidized ore is very shallow, 30 feet or less, and a zone of secondary enrichment is entirely lacking. This is no doubt due to the fact that the 

intense lateral pressure in the rocks has prevented the formation of a vadose circulation. The actual surface is characterized by extensive gossan, in which occur seams 

of malachite, often very rich, coating the schist. A very small amount of azurite 

occurs, particularly toward the south. In the Empire ground, south to the concentrator, cuprite is found. Some very rich ore of this kind has been mined within a 

few feet of the concentrator, to the north. In a small incline shaft a few feet south

i Ransome, F. L., and Calkins, F. C., The geology and ore deposits of the Coeur d'Alene district, Idaho: 

Prof. Paper U. S. Qeol. Survey No. 62,1908, p. 151.

»Idem, p. 152. The matter In brackets is mine. W. H. E.

> Reid, J. A., The ore deposits of Copperopolis, Calaveras County, Cal.: Econ. Geology, vol. 2,1907, pp. 

880-417.
EEVIEW OF MINING DISTRICTS. 203

of the Empire shaft cuprite appears as small flakes in the gliding planes of serpentine 

and talc. The ground to the south, from the Empire shaft to the gabbro boss, shows 

the largest amount of oxidized ore, due to the greater shattering of the rocks by the 

latest igneous intrusives.

Beneath this oxidized zone occur the original sulphides chalcopyrite and pyrite 

in a gangue of country rock, the latter being usually chlorite schist. The richest ore 

is pure chalcopyrite; the copper content is lowered-both by admixture of pyrite and 

of country rock. The presence of too much schist or other rock makes a low-grade 

concentrating ore; too much pyrite produces eventually waste material. Both the 

sulphide minerals, which occur relatively in all proportions, are found in thin veinlets or bands in the foliation planes of the serpentine and talc. 1

BOLEO, LOWER CALIFORNIA.

The Boleo mines, on the east coast of Lower California, are among 

the more important copper deposits in Mexico. According to Fuchs,3 

the country is an arid dissected plateau composed of trachytic tuffs 

and conglomerate, probably Tertiary, which are cut by intrusive 

trachyte. The ore beds, which conform with the strata, are gently 

inclined to the sea. There are three beds, each of which is in general 

from 2 to 8 feet thick, and the three lodes are separated by a few feet 

of clayey tuffs and conglomerates. The ore beds are altered clayey 

tuffs and the ore is scattered through the clay in small fragments and 

nodules, hence the name. The minerals include cuprite, atacamite, 

azurite, malachite, chrysocolla, crednerite, and salt.

Stevens,3 who writes later than Fuchs, states that chalcocite and 

covellite predominate, although the lowest bed, which is partly 

below the water line, carries oxides and carbonates also. At the bottom of each bed is 6 to 8 inches of rich ore, which was once sorted 

to run about 20 per cent copper. De Launay 4 supposed that the ores 

were formed by processes of sedimentation, the mineral waters issuing 

at the time the volcanic sedimentary rocks were formed, but Weed 5 

suggests that they were formed by thermal waters after sedimentation.

BRADEN COPPER MINES, CHILE.

The mines of the Braden Copper Co. are in the O'Higgins province, 

Chile, in the Andes. The country rock is andesite surrounding a 

body of light-gray tuff, which is said to represent the crater of an 

ancient volcano.6 The diameter of the volcanic neck is about 4,000 

feet and the periphery measures about 2^ miles. It is said to be of 

Tertiary age, presumably the later Tertiary.

The ore lodes are in and around the periphery of the crater, in andesite at its contact with tuff. The minerals are pyrite, magnetite,

iReid, J. A., The ore deposits of Copperopolis, Calaveras County, Cal.: Econ. Geology, vol. 2,1907, 

p. 398.

«Fuchs, Edmond, Note sur les gisements de cuivre du Bol6o: Compt. rend. 14» sess. Assoc. frang. 

avancem. sci. (Grenoble, 1885), pt. 2,1886, pp. 410-426.

8 Stevens, H. J., The copper handbook, vol. 9,1909, p. 389.

4 Quoted by Weed, W. H., The copper mines of the world, New York, 1907, pp. 245-246.

 Idem, p. 246.

«Yeatman, Pope, The Braden copper mines: Min. and Sci. Press, vol. 103,1911, pp. 769-779.
204 THE ENRICHMENT OF SULPHIDE ORES.

chalcopyrite, bornite, secondary chalcopyrite, zinc blende, and tetrahedrite(?). The nonmetallic minerals include tourmaline, quartz, 

ankerite, calcite, chlorite, sericite, mica, epidote, and zircon. In the 

zone of oxidation are limonite, cuprite, native copper, and carbonates 

and silicates of copper.

In one ore body, in the Fortuna mine, chalcocite occurs apparently 

only on their coatings on pyrite and chalcopyrite, increasing the 

grade of the ore only slightly. In the Teniente ore body the effects 

of sulphide enrichment appear to be somewhat greater. It is concluded, however, that the copper content is mainly in the primary 

minerals. Over 10,000,000 tons of ore have been developed, the 

value being about 2.8 per cent copper.

QUEEN OF BRONZE MINE, OREGON.

The deposits of the Queen of Bronze mine, in southwestern Oregon,1 

are irregular masses of ore in fractured and fissured portions of gabbro, 

peridotite, and serpentine.

The unoxidized ore is chalcopyrite, with which are associated pyrite, pyrrhotite, and 

subordinate amounts of quartz and calcite. In the low-grade ores pyrite and pyrrhotite are the most abundant minerals. In addition to the copper content, the ores carry 

some gold and silver.

The oxidized ores are malachite, azurite, cuprite, tenorite (?), and chrysocolla. Of 

these the black ores containing tenorite or chalcocite are more abundant. The average 

content in copper was more than 10 per cent. The lower limit of the oxidized ores ia 

usually less than 90 feet from the surface, but some have been found at greater depths. 

In a small opening about 105 feet below the surface black oxide and small amounts of 

native copper were observed.

Although depths of about 300 feet have been reached in the workings, no important body of ore has been found below 125 feet.

ENCAMPMENT DISTRICT, WYOMING.

The copper deposits of the Encampment district, Wyoming, 2 are 

in an area of pre-Cambrian metamorphosed igneous and sedimentary 

rocks, which are cut by pre-Cambrian gabbro, granite, and quartz 

diorite. The principal mines are the Doane and the Ferris-Haggarty, 

both of them in fractured quartzite. The primary copper ores are 

chalcopyrite and pyrrhotite. The secondary ores include azurite, 

malachite, chrysocolla, bornite, chalcocite, and covellite.

The country is about 10,000 feet above the sea; the climate is moist 

and ground water is near the surface. The lodes are tight and some 

mines are dry 450 feet below the surface.3 At the Ferris-Haggarty 

mine there was a strong capping of spongy limonite containing no 

visible copper minerals. At the Doane mine green copper minerals

1 Diller, J. S., and Kay, G. F., Mineral resources of the Grants Pass quadrangle and bordering districts, 

Oregon: Bull. U. S, Geol. Survey No. 380,1909, pp. 76-78.

2 Spencer, A. C., The copper deposits o/ the Encampment district, Wyoming: Prof. Paper U. S. GeoL 

Survey No. 25,1904. 

 Idem, p. 44.
EEVIEW OF MINING DISTEICTS. 205

mixed with the limonite were encountered at the surface and continued to a depth of 100 feet or more.1

The richest ores occur immediately below the gossan and are composed chiefly of secondary chalcocite.2 From the position of the 

s topes in several mine sections some chalcocite appears to extend 

downward probably, 200 feet or more below the top of the chalcocite 

zone. The secondary copper ores, according to Graton,3 carried good 

values in precious metal.

DUCKTOWN, TENNESSEE.

The Ducktown district, in Tennessee, occupies an intermontane 

basin in the heart of the southern Appalachian region, near the 

Georgia-North Carolina boundary. The ore deposits outcrop between 1,400 and 1,800 feet above the sea. The climate is moist. The 

prevailing rocks of the district are sandy schists and graywackes, 

with which are interbedded mica schists. The dominant series is the 

metamorphosed product of an association of sedimentary beds, including conglomerate, grits, sandstones, and shales. The beds grade 

into one another along the strike and across the bedding. They contain small bodies of pegmatites and peculiar masses and stringers of 

an actinolite-feldspar rock which has a composition near that of 

quartz diorite. The schists are cut by dikes of gabbro, which are not 

so highly metamorphosed by pressure as the sedimentary beds. The 

schistosity and the bedding of the sedimentary rocks strike nearly 

everywhere northeast, and the prevailing dip is southeast. These 

rocks have been folded into sharp folds, many of them isoclines. 

Many of the folds were broken along the crests of anticlines 4 and pass 

into reverse strike faults that nearly everywhere dip southeast.

The ore bodies 5 are replacements of limestone lenses which probably, though not certainly, were deposited at a single stratigraphic 

horizon. They range in width from a few feet to over 250 feet and 

have shapes that are characteristic of closely folded sedimentary beds. 

Anticlines and faulted anticlines, which are characteristic of this 

region, are shown in the ore zone. The ores themselves are somewhat 

metamorphosed by dynamic processes, and the gangue minerals are 

bent and twisted, but at most places they do not exhibit a well-defined

Spencer, A. C., The copper deposits of the Encampment district, Wyoming: Prof. Paper U. S. Geol. 

Survey No. 25,1904, p. 45.

'Idem, p. 55.

»Graton, L. C., Mineral Resources U. S. for 1906, U. S. Geol. Survey, 1907, p. 410.

«Keith, Arthur, Nantahala folio (No. 143), Geol. Atlas U. S., U. S. Geol. Survey, 1907.

 Saflord, J. M., Geology of Tennessee, Nashville, 1869, pp. 469-482. Whitney, J. D., Remarks on * * * 

the east Tennessee copper mines: Am. Jour. Sci., 2d ser., vol. 20, 1855, pp. 53-57. Ansted, D. T., On the 

copper lodes of Ducktown, in east Tennessee: Quart. Jour. Geol. Soc., vol. 13,1857, pp. 245-254. Heinrich, 

Carl, The Ducktown ore deposits: Trans. Am. Inst. Min. Eng., vol. 25,1896, p. 173. Kemp, J. F., The 

deposits of copper ores at Ducktown, Tenn.: Trans. Am. Inst. Min. Eng., vol. 31,1902, pp. 244-265. 

Weed, W. H., Types of copper deposits hi the southern United States: Trans. Am. Inst. Mm. Eng., vol. 

30,1901, p. 480. Emmons, W. H., and Laney, F. B., Preliminary report on the mineral deposits of Ducktown, Tenn.: Bull. U. S. Geol. Survey No. 470,1911, pp. 151-172.
206 THE ENRICHMENT OF SULPHIDE ORES.

echistosity. They were probably deposited after the rock that now 

incloses them and the limestone they replace had been subjected to 

considerable dynamic metamorphism. The outcrops are composed 

of iron oxides and quartz and contrast strongly with the country rock. 

All the lodes have a general northeast strike. South of the Culchote 

mine, which is near the center of the productive area, they strike more 

nearly north than east; north of the Culchote they strike more nearly 

east than north. In general the lodes dip southeast, but some dip 

northwest. The deepest development underground is about 800 feet 

below the surface.

The primary ore consists of pyrrhotite, pyrite, chalcopyrite, zinc 

blende, bornite, specularite, magnetite, actinolite, calcite, tremolite, 

quartz, pyroxene, garnet, zoisite, chlorite, micas, graphite, titanite, 

and feldspars. The minerals are generally intergrown and are of contemporaneous age. Essentially the same minerals are found in all 

the deposits, but.they appear in varying proportions at different 

places in the lodes. Where its content of copper is above 1.5 per 

cent, or where its sulphur content is high, the material is ore, but 

where the proportion of actinolite and other lime silicates is greater 

and the sulphides are less abundant the material, though containing 

copper and other sulphides, is not workable.

The gossan extends from the surface to a maximum depth of 100 

feet. It carries 40 to 50 per cent iron, generally less than 12 per cent 

silica and alumina, and 0.3 to 0.7 per cent copper. The minerals are 

hydrous iron oxides, quartz, jasper, and kaolin, with a little cuprite, 

native copper, and sulphur. Below the gossan iron ores is a zone of 

dark, rich copper ores, consisting of chalcocite and other copper minerals in a gangue of sulphates, quartz, and decomposed silicates. 

Under the higher portions of the outcrops of the lodes the top of this 

zone is about 100 feet below the surface, but the depth decreases down 

the slopes, and where the lodes are crossed by running streams the 

secondary copper ores are exposed at the surface. The upper limit 

of the chalcocite zone follows the level of ground water, which in 

turn follows the contour of the country but is less accentuated.1

The secondary minerals include argentite, azurite, bornite, chalcanthite, chalcedony, chalcocite, chalcopyrite, chrysocolla, covellite, 

cuprite, gypsum, iron sulphate, jasper, kaolin, limonite, malachite, 

marcasite, melaconite, native copper, sulphur, and talc. The copper 

content of much of this ore ranges from 20 to 30 per cent.

The secondary zone is less extensive vertically than most chalcocite 

zones elsewhere. The lodes are comparatively impervious to downward circulation, and it is believed that the reactions were brought 

nearly to completion before the descending oxidized solutions had 

moved downward great distances.3

lEmmons, W. H., and Laney, F. B., op. cit., p. 170. 'Idem, p. 172.
EEVIEW OF MINING DISTRICTS. 207

In all the mines except the East Tennessee pyrrhotite is abundant, 

constituting more than half of the primary ore, and the vertical 

extent of the chalcocite zone is generally less than 10 feet. In the 

East Tennessee mine, in which pyrrhotite is much less abundant, the 

vertical extent of the chalcocite zone is at least 125 feet. The lodes 

are tight, and the downward circulation was probably slow before the 

mines were opened, but secondary chalcopyrite has been deposited 

below the chalcocite zone and probably extends 300 or 400 feet or 

more below the horizon of chalcocite.

GOSSAN LEAD, VIRGINIA.

The Gossan lead, in southwestern Virginia, is inclosed in highly 

metamorphosed crystalline schists. It strikes northeast, dips about 

45°, and may be traced almost continuously for 18 miles. It consists 

of pyrrhotite, chalcopyrite, quartz, hornblende, mica, and other 

minerals.

Beneath the gossan there is usually found from a foot to 6 feet of 

the secondary copper ores, containing near the top more or less 

carbonate and oxide and very rarely a little native copper; farther 

down, on top of the unaltered sulphides, there is more or less soft 

black ore, the so-called "smut ore" of the miners, with secondary 

chalcopyrite.1

As stated by Weed,2 there has been some postmineral fracturing, 

which, presumably, would favor a downward circulation. The character of the enrichment is nevertheless closely similar to that in the 

pyrrhotite ores at Ducktown, Tenn. As stated by Watson,3 the 

weathering measured in 20 operations has extended to an average 

depth of 34 feet, ranging from 10 to 60 feet.

STONE HILL, ALABAMA.

At Stone Hill, Cleburne County, Ala., a deposit of pyrrhotite, with 

some chalcopyrite, is inclosed in gneiss and hornblendic and other 

schists. The ore body is said to be 24 feet wide and is traceable on 

the surface for 1,200 feet.4

Below the shallow gossan are rich "black copper" ores of rather 

slight vertical extent, above the yellow sulphides.

ELY, VERMONT.

The copper deposits near Ely, Vt., are lenticular masses in mica 

schist.5 Some of the deposits have been worked to very great depths. 

The ore is composed of pyrrhotite, scattered grams and irregular

» Weed, W. H., Copper deposits of the Appalachian States: Bull. U. S. Geol. Survey No. 455,1911, p. 120.

'Idem, p. 119.

8 Watson, T. L., Mineral resources of Virginia, Lynchburg, 1907, p. 421.

* Weed, W. H., The copper mines of the world, New York, 1907, p. 275.

 Smyth, H. L., and Smith, P. S., The copper deposits of Orange County, Vt.: En'g. and Min. Jour., vol. 

77,1904, pp. 677-678, Weed, W, H., Notes on the copper mines of Vermont: Bull. U. 8. Geol. Survey No. 

225,1904, p. 190,
208 THE ENRICHMENT OF SULPHIDE OBES.

masses of chalcopyrite, and small amounts of pyrite and zinc blende. 

The gangue minerals consist of quartz, actinolite, garnet, and other 

metamorphic minerals. The main part of the ore body consists of 

very massive pyrrhotite, with small amounts of quartz and other 

minerals.1.

The country has been glaciated, and the tops of the deposits have 

very little gossan, the sulphide appearing at some places at the surface. 

The richer ores near the surface of these deposits were removed long 

ago, and published descriptions of the details of their occurrence are 

not available to me. Mr. J. W. Tyson, of the Tennessee Copper Co., 

at Ducktown, who was present when some of these deposits were 

opened, informs me that the richer black ores were found within 100 

feet of the surface and laid like a floor over the lower-grade yellow 

sulphides. The relations, he states, were closely similar to those of the 

"black ores" at Ducktown.

LA DICHA, MEXICO.

The Dicha mine, Guerrero, Mexico, is 150 miles south of the City 

of Mexico, in a mountainous country about 50 miles east of Acapulco. 

According to Weed,2 the ore body, which occurs in micaceous schist, 

is parallel to the schistosity. It is proved for 7,700 feet along the strike 

and is from 12 to 88 feet wide. The ore consists of pyrrhotite with 

streaks, bunches, and specks of chalcopyrite scattered through it. 

It is said that the primary ore carries 5 per cent copper. Mr. Robert 

T. Hill 3 informs me that the chalcocite zone is encountered at very 

shallow depths and is itself very thin, ranging in thickness from 6 

inches to 6 feet. Its contact with pyrrhotite is sharp.

SANTIAGO, CUBA.

Copper lodes are found at several places along the south coast of 

Cuba east and west of Santiago. The most important deposits are 

those of the El Cobre mine. The ore minerals of the veins include 

pyrite, chalcopyrite, pyrrhotite, and quartz.4 Oxidized and chalcocite ores were found 50 or 60 feet below the outcrop. At greater 

depths pyritic ores in a siliceous gangue were found to carry 6 per cent 

copper without precious metals in paying quantity. The old workings, now caved, extended 1,300 feet below the surface. Copper is 

precipitated from mine waters on scrap iron.

i Weed, W. H., op. cit., pp. 192-193.

»Weed, W. H. (quoting R. T. Hill), The copper mines of the world, New York, 1907, p. 251. 

«Correspondence.

* Lawrence, B. B., Copper mining in Cuba: Min. and Sci. Press, vol. 93,1906, p. 602. Weed, W. H., The 

copper mines of the world, New York, 1907, p. 200.
EEVIEW OF MINING DISTRICTS. 209 

PINAR DEL BIO, CUBA.

In the Province of Pinar del Rio, at the extreme west end of Cuba, 

gossans 150 to 200 feet wide are found. Below the gossans, at depths 

of 40 to 50 feet, are black copper ores. The sulphides include pyrrhotite, chalcopyrite, and chalcocite.1 The relations, according to 

Benjamin B. Lawrence, are similar in some respects to those seen at 

Ducktown, Tenn.

BOUNDARY DISTRICT, BRITISH COLUMBIA.

The Boundary district, British Columbia,2 lies between Columbia 

and Okanogan rivers, near the international boundary. The country is mountainous, forested, and about 5,000 feet above sea level. 

It is an area of sedimentary rocks, including limestones, argHlites, and 

quartzite, with volcanic tuffs and ash beds, probably of Paleozoic age. 

Associated with these rocks are gneisses and schists, possibly Paleozoic 

also. The later rocks are porphyries, serpentines, and granodiorite. 

Various intrusions and flows of andesite, basalt, and other rocks are 

probably Tertiary. The deposits are perhaps related to intrusions 

of alkali syenite, which cuts most of the other rocks and is of Tertiary age.

The principal deposits are in limestone or greenstone, not far from 

intruding masses. The gangue minerals, according to Brock, include 

garnet, epidote, calcite, quartz, and actinolite. The principal sulphides are pyrite, chalcopyrite, and pyrrhotite. Magnetite is abundant in some deposits, and these contain less of the iron sulphides. 

Sphalerite, galena, and arsenopyrite are also present, and the ores 

contain appreciable quantities of gold and silver.

Oxidation extends only a few feet down into the copper ores, and 

locally the sulphide ore is polished and striated by ice.

The mother lode, according to Brock, is over 100 feet wide and at 

least 1,180 feet long and is developed to a depth of about 500 feet. 

Chalcocite is practically unknown in this deposit, the principal copper 

mineral being chalcopyrite. It is reported that the valuable minerals 

decrease in amount with increase of depth, but it is not certain that 

the richer deposits near the surface are due to chalcopyrite enrichment. A lens of the Knobhill-Ironsides deposit, according to Le 

Roy,8 is nearly half a mile long and has a maximum thickness of 125 

feet and a known maximum width of 900 feet. Pay ore, according to 

Stevens, does not extend below 400 feet. Stevens says that the values

i Lawrence, B. B., Copper mining in Cuba: Min. and Sci. Press, vol. 93,1906, p. 602.

* Brock, R. W., Preliminary report on the Boundary Creek district, British Columbia: Summ. Kept. 

Geol. Survey Canada for 1902,1903, pp. 90-136. Le Roy, O. E., The geology and ore deposits of Phoenix, 

Boundary district, British Columbia: Mem. Geol. Survey Canada No. 21,1912. Stevens, H. J., The copper 

handbook, vol. 9,1909, pp. 206-207. Ledoux, A. R., The production of copper in the Boundary district, 

British Columbia: Jour. Canadian Min. lust., vol. 5,1902, pp. 171-177.

* Le Roy, O. E., op. cit., p. 57.

72293° Bull. 529 13 14
210 THE ENRICHMENT OF SULPHIDE ORES.

have declined from 1.7 per cent copper, 0.5 ounce silver, and $2 gold 

per ton to about 40 per cent less in copper, 50 per cent in silver, and 60 

per cent in gold. The ore reserves in 1910 were estimated by Otto 

Sussman at 5,595,000 tons.

At Copper Camp, near the head of Copper Creek, oxidized deposits 

of copper ores are found. In depth these give way to secondary 

sulphides, including chalcocite, below which are pyrite and chalcopyrite.

On the margins of the principal copper deposits are gold and silver 

veins. The minerals of these deposits include chalcocite, galena, 

zinc blende, pyrite, chalcopyrite, ruby silver, argentite, native silver, 

and gold. The chalcocite, rich silver minerals, and native silver and 

gold usually occur, it is said, in films around quartz crystals or in 

small crevices in quartz and calcite, indicating that they are products 

of enrichment.

SAN JOSE, MEXICO.

San Jose, Tamaulipas, Mexico, is 40 miles from Linares, on the 

Monterey & Mexican Gulf Railway. The principal productive area 

is about 2,250 feet above sea level. The deposits are in an area of 

limestone and nepheline syenite, and the milestone is intruded by 

diorite and by porphyry. 1

The limestone and the porphyry are cut by tinguaite and by basic 

dikes. Typical zones of heavy silicates with sulphide copper ores are 

developed near contacts of limestone and diorite porphyry.2 Magnetite is a contact mineral and is locally developed in large irregular 

masses. On the borders there are intermingled garnet and diopside, 

and throughout the magnetite are abundant veinlets of chalcopyrite 

and pyrite. One prominent outcrop gave the name "Piedra Iman," 

or lodestone, to the summit of Remedies Hill.

The most important copper mineral is chalcopyrite, which, according to Kemp, is almost invariably associated with pyrite. They 

appear not only as inclusions in the silicates but also as veinlets and 

as coatings in cavities. They cover in places the well-developed 

crystals of garnet so as to mold around them like a paste. The sulphides may themselves form large masses analogous to the magnetite 

and thus yield the best stopes of ore.

Ores including malachite, chrysocolla, and subordinate cuprite are 

found in the oxidized zone, especially along faults. I can find no 

mention of chalcocite in Kemp's descriptions.

1 FMay, G. I., The geology of the San Jose district, Tamaulipas, Mexico: Annals New York Acad. Sci., 

vol. 14,1904, pp. 247-295. Kemp, J. F., The copper deposits at San Jose, Tamaulipas, Mexico: Trans. Am. 

Inst. Min. Eng., vol. 36,1906, pp. 178-203.

^Kemp, J. F., op. cit., p. 189.
REVIEW OF MINING DISTRICTS. 211 

LA REFORMA MINE, CAMPO MORADO, MEXICO.

The Reforma mine at Campo Morado, Guerrero, Mexico, is in an 

area of somewhat metamorphosed sedimentary rocks (probably 

Lower Cretaceous) that are cut by granodiorite, which locally is 

sheared. Later porphyry, probably monzonite porphyry, intrudes 

the older rock.

The ore bodies, as stated by J. W. Finch, 1 are large and are extensively developed. The sulphide ore 3 consists of practically solid 

masses of pyrite, with chalcopyrite, sphalerite, and galena and little 

or no quartz. It-fills and replaces the beds, usually along the contacts with granodiorite, but is thought to be genetically related to 

the late intrusions. The Reforma body has an average width in six 

levels of about 100 feet and is continuous through the mountain. 

On the sixth level pyrite is developed for 2,000 feet on the strike between the oxidized zone at each end of the level.

The values are about equally distributed between gold, silver, and 

copper. The pyritic mass is compact and offers no large channels for 

water circulation. Oxidation is apparently thorough above the water 

level, but there has been very little reconcentration of copper in the 

oxidized zone. The copper-bearing waters appear to have drained 

laterally along the strike in the gossan to the surface or through the 

footwall slates. The zone of attack of oxidizing agents, as stated by 

Finch, appears to be measurable vertically in inches along practically 

all the profile of the pyrite, oxidation being complete as deep as it 

descends. A little covellite was found at the top of the pyrite in the 

Navajo workings, near by, but neither covellite nor chalcocite has been 

recognized in the Reforma. The sulphides appear to be impervious, 

and there is no evidence.whatever of sulphide enrichment.

VELARDENA DISTRICT, MEXICO.

The Velardena district, Durango, Mexico,2 near the Mexican International Railroad, between Torreon and Durango, is an area of folded 

Mesozoic (probably Cretaceous) limestone which is covered with 

andesitic tuffs and flows and cut by andesite intrusions. Dikes and 

irregular masses of alaskite and of trachytic alaskite and diorite 

intrude the older rocks. A siliceous phase of the diorite, probably 

quartz monzonite, is limited to a single intrusion near the Copper 

Queen mine. Contact metamorphism, with the development of 

heavy silicates and metallic sulphides, has attended the intrusion of 

several of these rocks, and an interesting feature is the metamorphism 

and development of heavy silicates in igneous as well as in calcareous

1 Finch, J. W., A geological journey in Guerrero: Min. and Sci. Press, vol. 101,1910, p. 498. 

*Spurr, J. E., and Garrey, G. H., Ore deposits of the Velardena district, Mexico: Econ. Geology, vol. 3, 

JC08, pp. 688-725.
212 THE ENRICHMENT OF SULPHIDE OEES.

sedimentary rocks. The ore deposits are of varied character and 

clearly indicate several epochs of ore deposition.

The deposits differ greatly in mineral composition, but most of 

them contain considerable zinc blende, zinc being about as abundant 

as copper. Some deposits contain also pyrrhotite.

The Copper Queen ore body, containing the principal copper ore of 

the region, consists of three main pipes or chimneys in metamorphic 

rock at the contact of the Copper Queen intrusion. The principal 

sulphides are slightly cupiferous pyrite, blende, and galena. The 

ores here mined are primary and there is no very great change of 

nature in depth. 1

STJDBTJRY, ONTARIO.

The Sudbury nickel region, Ontario,2 is in a hilly, glaciated country 

of moderate relief. The climate is cold and moist. The nickeliferous 

rocks are included in an elliptical area some 40 miles long and 20 

miles wide, the longer axis striking north of east. The central portion of the ellipse, occupied by Upper Huronian or post-Huronian 

rocks, has been weathered to a peneplain, which is surrounded by a 

hilly belt of eruptive rock. The oldest series in the region consists 

of Huronian graywacke, slate, quartzite, and conglomerate, which are 

intruded by acidic and basic eruptives.3 The Upper Huronian rocks 

(Animikie group) include conglomerate, tuffs, slates, and sandstones. 

Intruded between the Lower Huronian rocks or their igneous intrusives and the Upper Huronian sedimentary rocks is the great laccolithic mass, or sheet, probably of Keeweenawan age, which constitutes 

the Sudbury nickel-bearing eruptive. This great sheet dips toward 

its center, forming a canoe-shaped body which outcrops in a rudely 

elliptical belt having a nearly plane surface. As a result of magmatic 

differentiation the lower portion of the eruptive is norite and the 

upper portion is micropegmatite, the two rocks grading into each 

other.

The ore deposits are (1) magmatic segregations, which occur 

between the norite and the underlying rocks, especially in depressions in the Huronian or in rocks intruded in the Huronian; (2) 

deposits of nearly related genesis in or near dikes of norite that 

extend outward from the lower contact of the main laccolithic body; 

and (3*) deposits outside the laccolith, associated with norite intrusions, which possibly are connected with the principal body of the 

nickeliferous eruptive beneath the surface. The ore consists chiefly 

of pyrrhotite, which contains a small amount of pentlandite and

1 Spurr, J. E., and Garrey, O. H., Ore deposits of the Velardena district, Mexico: Econ. Geology, 

vol. 3,1908, p. 719.

2 Barlow, A. E., Report on the origin, geological relations, and composition of the nickel and copper 

deposits of the Sudbury mining district, Ontario: Ann. Kept. Geol. Survey Canada, vol. 14, pt. H, 1904. 

Coleman, A. P., The Sudbury nickel field: Eept. Ontario Bur. Mines, vol. 14, pt. 3,1905, p. 14.

s Coleman, A. P., loc. cit
EEVIEW OF MINING DISTRICTS. 213

chalcopyrite. At many places it grades imperceptibly into pyrrhotitic norite. Fissures in the eruptive rock are filled with quartz 

and sulphides, and along the contact with the older rocks sulphides 

have been deposited by contact-metamorphic processes. Pyrite 

is intimately .associated with pyrrhotite.1 Other minerals are magnetite, niccolite, cassiterite, gersdorffite, polydymite, danite, galena, 

native copper, sperrylite, and gold. The gangue includes the rockmaking minerals of norite, with some quartz, calcite, and other carbonates. Alteration products include limonite, chalcoeite, bornite, 

morenosite, annabergite, millerite, and probably several other 

species.

Rounded hills of gossan,2 indicating the presence of sulphide ore 

beneath, extend with almost unbroken continuity for miles along the 

contact of norite with underlying rocks. The offsets and isolated 

masses of norite with which some of the ore bodies are associated are 

generally made brownish by the decomposition of disseminated sulphides. Locally the covering of gossan is as much as 6 feet deep, 

although its ordinary depth is 2 or 3 feet, and it merges into sulphide 

ore beneath. Chalcoeite ores are not conspicuously developed, 

although the presence of chalcocite in some deposits is noted by both 

Barlow and Coleman. In the Vermilion mine, where the gossan is 

deepest, chalcocite and copper carbonate are developed and there is 

a concentration of platinum or sperrylite in the gossan. The relations at the Copper Cliff mine of the Canadian Copper Co. suggest a 

probable chalcopyrite enrichment. In the earlier years of the 

development of this mine copper was greatly in excess of nickel, 

averaging 6.44 per cent, while nickel averaged only 2.38 per cent. 

Subsequently their proportions were reversed, and in 1904 8 the 

matte from this deposit contained nearly twice as much nickel as 

copper. Of sulphide enrichment, C. W. Dickson 4 says that it is 

possible that the copper disseminated in the upper part of the ore 

bodies, now eroded, has been secondarily deposited by downwardmoving currents, but apparently not to any great extent, for the 

ground-water level is comparatively near the surface and there is 

little if any enriched sulphide.

When the thin surface covering is removed the pyrrhotite appears 

perfectly fresh and is without appreciable admixture of secondary 

minerals, such as are formed in the process of enrichment.

i Barlow, A. E., op. cit, p. 97. 

"Idem, p. 121. 

«Idem, p. 110.

* Dickson, C. W., The ore deposits of Sudbury, Ontario: Trans. Am. Inst. Min. Eng., voL 34,1904, pp. 

61-32.
214 THE ENRICHMENT OF SULPHIDE OBES. 

PRINCE WILLIAM SOUND, ALASKA.

The deposits of Prince William Sound, Alaska, are in graywacke 

and slates 1 of two unconformable series, closely folded but not highly 

metamorphosed. These are cut by granites, acidic dike rocks, and 

basic intrusive rocks. Basic flows altered to greenstone are commonly interstratified with the sedimentary rocks. The ores were 

deposited in zones of fracturing and shearing. The minerals in the 

ore bodies are chiefly pyrrhotite and chalcopyrite. With these in 

places are quartz, pyrite, and sphalerite, and at least one vein contains magnetite. As a rule the ore minerals are intimately intergrown 

and are thus apparently of contemporaneous origin.2

Possibly extensive surface alteration and sulphide enrichment took 

place before glaciation, and the remnants of the workable ores may be 

the roots of such enriched deposits. In the Bonanza mine, Latouche 

Island, chalcopyrite is less abundant in the lower level than it is 

nearer the surface. Since glaciation occurred there has been surface 

leaching and probably enrichment of the deposit, as is shown (1) by 

gossan, a few feet thick; (2) by a zone of black sulphides, a few inches 

thick, above the main ore; and (3) by waters that are carrying copper 

in solution and are in one locality depositing native copper and 

malachite.2

According to F. C. Lincoln 3 the teachings from glacial bowlders have 

been precipitated in crevices as malachite.

KASAAN PENINSULA, PRINCE OP WALES ISLAND, ALASKA.

The copper deposits of Kasaan Peninsula * are in an area of calcareous and other sedimentary rocks, which are interbedded with 

volcanic tuffs and cut by intruding masses of granodiorite and dikes of 

granite, syenite, and pegmatite. These intrusions were followed by 

the intrusion of dikes of f elsite and subsequently of small dikes of diabase and basalt, which cut the ore bodies. The sedimentary rocks are 

greatly metamorphosed near their contact with the intruding rocks, 

where typical garnet-epidote-magnetite contact zones are developed. 

The deposits are in the main iron-copper sulphides, intergrown with 

the garnet gangue, and are generally near the contact of the sedimentary rocks with intrusive syenite. Most of the deposits are 

worked by open pits.

The ore is composed largely of magnetite, chalcopyrite, and pyrite associated with 

hornblende and calcite, all of which are included in a more or less banded garnetepidote gangue.

1 Grant, U. S., and Higgins, D. F., Reconnaissance of the geology and mineral resources of Prince William 

Sound, Alaska: Bull. U. S. Geol. Survey No. 443,1910, p. 20.

2 Idem, p. 58.

s Lincoln, F. C., The Big Bonanza copper mine, Latouche Island, Alaska: Econ. Geology, vol. 4,1909, 

p. 212.

4 Wright, C. W., and Paige, Sidney, Copper deposits on Kasaan Peninsula, Prince of Wales Island: Bull. 

U.S. Geol. Survey No. 345,1908, p. 98. Wright, F. E. and C. W., The Ketehikan and Wrangell mining districts, Alaska: Bull. U. S. Geol. Survey No. 347,1908.
REVIEW OF MINING. DISTRICTS. 215

Surface oxidation has produced considerable limonite and some malachite and 

azurite; small particles of native copper also occur along slipping planes. These 

secondary minerals are relatively unimportant. 1

It appears from descriptions of the various mines by Wright and 

Paige that pyrrhotite and zinc blende are lacking or very subordinate 

in these deposits. Pyrrhotite occurs in two of the prospects and in 

one of these zinc blende is present in considerable amount. Chalcocite 

enrichment is practically absent, though some was noted on the Goodro 

claims 200 feet below the surface. In the short time since the 

glacial epoch the pyrite and chalcopyrite have precipitated practically no secondary copper, though some oxidation has taken place 

and in a mine at Copper Mountain carbonates were noted 300 feet 

below the surface.2

BONANZA MINE, CKETINA COPPER BELT, ALASKA.

The Chitina copper belt, Alaska,3 is an area of greenstone, mainly 

diabase, which is overlain by Triassic limestones. Above the limestones are later sedimentary rocks. The various rocks are cut by 

porphyries, diorites, and gabbros. Andesite and other volcanic rocks, 

probably of Tertiary age, are also present. The country is rugged and 

the region of the deposits has been deeply eroded. The principal deposit 

is a rudely tabular mass of nearly pure chalcocite, which occurs in a fractured or fissured zone in the limestone just above the contact with 

greenstone. The very rich ore can be traced on the surface for about 

250 feet. The deposit carries more than 60 per cent copper and 22 

ounces silver to the ton and is estimated to contain over 100,000,000 

pounds of copper.4 The thickness is indefinite, but the very rich ore, 

with its included limestone, as seen at the surface, has a width of 

approximately 25 feet, although the thickness of ore sufficiently rich 

to be mined may be greater. Below the deposit a little chalcocite and 

less bornite are found in some of the shearing planes in the greenstone 

but do not extend far into the greenstone. The quantity of each is 

small and the minerals are inconspicuous and might readily pass unobserved. A small amount of epidote is in places associated with 

the chalcocite.5

Though it extends to the very surface and accumulates in talus from 

the cliff, the chalcocite ore has no great vertical range. Owing to the 

rapid mechanical disintegration and the cold climate little or no 

gossan is developed. Open cavities in the fractured limestone have 

been filled with ice, and both the country rock and the talus on either

iWright, C. W., and Paige, Sidney, op. cit., p. 105.

* Wright, C. W., Prospecting In the North (discussion): Min. Mag., vol. 4,1911, p. 359. 

«Moffit, F. H., and Maddren, A. G., Mineral resources of the Kotsina-Chitina region, Alaska: Bull. U. S. 

Geol. Survey No. 374,1909, p. 80.

'Graton, L. C., Mineral Resources U. S. for 1907, pt. 1, U. S. Geol. Survey, 1908, p. 592. 

'Moffit, F. H., and Maddren, A. G., op. cit., p. 83.
216 THE ENKICHMENT OP SULPHIDE OBES.

side of this ridge are frozen all summer except for a few feet at the 

surface. No chalcopyrite and pyrite are recognized in the chalcocite ore. There is no evidence that either of these minerals was replaced, but immediately below the contact in the same shear zone 

bornite, chalcocite, and native copper are present in small amounts.1

It seems most unlikely that pyrite and chalcopyrite would be so 

completely replaced as to leave no vestige of Othe original mineral. 

Organic matter, especially if supplying hydrogen sulphide, could 

precipitate the copper as chalcocite, and the character and purity of 

the glance suggests a genesis somewhat similar to that of the copper 

glance deposits of Cashin, Colo., or the replacements of coaly material 

in many prospects in the West.2

In this connection the following statement of Moffit and Maddren 3 

is suggestive: The limestone ' 'is found to have a blue color, which is 

indicative of considerable carbonaceous material in its composition." 

Although Moffit and Maddren only state alternate hypotheses as to 

the mode of deposition of the ore, in their discussion of the subject 

they seem clearly to favor the one which suggests the agency of cold 

waters. A possible source of the copper is the greenstone upon which 

the limestone rests. This formation is mainly diabase and at many 

places it carries a little copper.

MOTHER LODE DISTRICT, CALIFORNIA.

The Mother Lode district, California, as described by Ransome, 4 is 

an area of crystalline schists and altered igneous rocks with intruded 

granodiorite and related rocks. The deposits are fissure veins, which 

generally trend northwestward and at many places parallel the 

schistosity of the country rock. I have no record that the ores contain manganese minerals. Placers are abundantly developed, and 

at many places rich ore is found at the very surface. According to 

Ransome, there is no evidence that the mines grow suddenly richer 

at any particular depth, nor is there any recognizable regular change 

in the value of pay shoots with depth below the zone of superficial 

weathering. Some of these deposits are very regular and uniform 

in value and have been developed to very great depths.

NEVADA CITY AND GRASS VALLEY, CALIFORNIA.

The area of Nevada City and Grass Valley, Cal.,5 includes metamorphosed Carboniferous sedimentary rocks, compressed into isoclines, 

and associated igneous rocks less intensely metamorphosed. Above

i Moffit, F. H., and Maddren, A. G., op. cit., p. 53.

» Lindgren, Waldemar, Copper, silver, lead, vanadium, and uranium ores in sandstone and shale: Econ. 

Geology, vol. 6,1911, p. 568.

s Moffit, F. H., and Maddren, A. G., op. cit., p. 25.

< Ransome, F. L., Mother Lode district folio (No. 63), Geol. Atlas U. S., U 8. Geol. Survey, 1900, p. 3.

6 Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley districts, California: 

Seventeenth Ann. Kept. U. S. Geol. Survey, pt. 2,1896, pp. 1-262.
EEVIEW OF MINING DISTRICTS. 217

these are slates with associated diabase and serpentine. These rocks 

are folded and metamorphosed but are not so intensely compressed 

as the Carboniferous. Intruded into these rocks are great bodies 

of granodiorite, probably of early Cretaceous age. The ore deposits 

are strong fissure veins, formed after the granodiorite intrusions. 

The minerals are quartz, chalcedony, magnetite, sericite, mariposite, 

pyrite, pyrrhotite, chalcopyrite, galena, blende, scheelite, arsenopyrite, tetrahedrite, stephanite, and cinnabar. Some earthy manganese ore occurs in small fissures in the granodiorite but not in 

connection with the quartz veins.

Near the surface 1 the upper part of a vein is generally decomposed, forming a mass of limonite and quartz. The decomposition 

seldom extends more than 200 feet on the incline of a vein dipping 

45°, or more than 150 feet below the surface. Fresh ore is found in 

places almost at the surface. The surface ore is generally richer 

than the fresh ore below, owing to the liberation of gold from the 

sulphides and the removal of substances other than gold. In this 

process silver also is partly removed. In some of the mines the 

lodes have been followed down the dip for 2,000 or even 3,000 feet. 

The unoxidized ore shows no gradual diminution of tenor in the pay 

shoots below the zone of surface decomposition. Within the same 

shoot there may be great variations of the tenor, but there is certainly no gradual decrease of it from the surface down. Important 

placer deposits were formed from these veins.

OPHIR, CALIFORNIA.

The rocks of the Ophir district, California,2 comprise amphibolite 

schists and massive amphibolites, with intrusions of granodiorite. 

These rocks are cut by quartz veins which fill coordinate fissures. The 

minerals are gold, electrum, some iron, copper and arsenical pyrites, 

with galena, zinc blende, tetrahedrite, and molybdenite. The gangue 

is mainly quartz, with a little calcite. The proportion of gold to silver 

varies by weight from 1:1 to 1:10, the gold predominating in value. 

Certain small ore shoots in veins in the amphibolite carry more than 

the usual tenor of gold, and the richest shoots are usually found 

where veins cross the belts rich in pyrite. According to Lindgren, 

such ore bodies may have been enriched by thermal waters concentrating gold from the pyrite in the iron belt. The common statement that the gold vein becomes barren as the depth from the surface increases is not justified, in his opinion,3 by the evidence afforded 

in the mines. The extensive development of placers, the value of

1 Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley districts, California: 

Seventeenth Ann. Rept. U. S. Geol. Survey, pt. 2,1896, p. 182.

1 * Lindgren, Waldemar, The gold-silver veins of Ophir, Cal.: Fourteenth Ann. Rept.U. 8. Geol. Survey, 

pt. 2,1894, p. 252.

8 Idem, p. 279. .
218 THE ENBICHMENT' OF SULPHIDE OBES.

the ore near the surface, and the occurrence of valuable ore shoots 

just below the surface are opposed to the notion of extensive migration of gold in these deposits.

SILVER PEAK, NEVADA.

According to Spurr,1 the deposits of Silver Peak, Nov., are lenticular masses and fissure veins in Paleozoic sedimentary rocks. 

Genetically they are related very closely to granitic rocks which, 

as shown by Spurr, have alaskitic or pegmatitic phases. They are 

probably post-Jurassic and should be grouped with the California 

gold veins, with which geologically they have much in common. 

Concerning the°Drinkwater and Crowning Glory deposits, which are 

the most important examples, Spurr says that no decided enrichment of the ores by oxidation can be established. The ores in the 

upper tunnel seem to have been locally richer than any found in the 

lower tunnel, but this difference has no evident relation to the 

surface and is probably original. The values occur as finely disseminated gold and auriferous sulphides. Placers are not mentioned.

EDGEMONT, NEVADA.

The gold deposits at Edgemont, Elko County, Nev.,2 are in an 

area of quartzite, with intrusions of granodiorite. The deposits are 

fissure veins and their gold values are comparatively uniform. The 

ore consists of pyrite, galena, and arsenopyrite hi a gangue of quartz. 

Copper carbonates and manganese minerals are rare or absent. The 

ore is stoped practically to the surface. There has probably been a 

slight amount of enrichment by removal of certain substances in 

the oxidized zone more rapidly than gold; there is no evidence that 

gold has been transferred below the water level by descending surface 

waters.

DOUGLAS ISLAND, ALASKA.

At the Treadwell mines, Douglas Island, Alaska, large dikes of albite 

diorite intrude greenstones and schist, and the shattered diorite has 

been extensively replaced by mineralizing solutions and cemented 

by low-grade gold ore. The minerals include quartz, albite, rutile, 

chlorite, epidote, calcite, siderite, pyrite, pyrrhotite, magnetite, 

chalcopyrite, and molybdenite. Manganese minerals are not reported.

The mines have been developed for 2,000 feet down the dip. According to Spencer,3 the ore shows no progressive change in appearance 

or value with increasing depth. In the lowest level it is quite as

1 Spurr, J. E., Ore deposits of the Silver Peak quadrangle, Nevada: Prof. Paper U. S. Geol. Survey 

No. 55,1906.

2 Emmons, W. H., A reconnaissance of some mining camps in Elko, Lander, and Eureka counties, Nov.: 

Bull. U. S. Geol. Survey No. 408,1910, pp. 75-80. 

»Spencer, A. C., The Juneau gold belt, Alaska: Bull. U. S. Geol. Survey No. 287,1906, pp. 32,115.
REVIEW OP MINING DISTRICTS. 219

rich as in the upper workings, and it is evident that changes on the 

dip are no greater than along the strike. The best level of the 

Alaska Treadwell mine, according to Hershey,1 is 1,300 feet below 

sea level. Nothing in the character of the ore indicates any important 

concentration of values by oxidizing waters. The fact that extensive 

placers were not formed is not opposed to the view expressed by 

Spencer that the gold has not been transferred; the country has been 

recently glaciated and surface accumulations have been scattered. 

The gold accumulated at the apex since glacial time was, indeed, 

recovered by sluicing.

BERNERS BAY REGION, ALASKA.

The Berners Bay region lies along the Lynn Canal, 45 miles northwest of Juneau, Alaska. The country is an area of slates and graywackes of Jurassic or Cretaceous age and basic lavas and quartz 

diorites considerably altered by dynamic metamorphism. The 

Jualin diorite, intruding the sediments and lavas, is also later than 

the metamorphosed quartz diorite.2

The gold deposits, which are mainly in the Jualin diorite, are 

stockworks and fissure veins. The ores are composed of quartz, 

some calcite and pyrite, and minor amounts of chalcopyrite, galena, 

and sphalerite. The alteration of the wall rock was attended by the 

production of albite. Evidence of enrichment is lacking.

Knopf 3 says:

Any enriched surface ores that may have existed within this region have been 

swept away by the powerful glacial erosion to which the region was subjected in the 

recent geologic past. The ores exposed at the surface are therefore of primary origin, 

modified to an unimportant extent by postglacial oxidation, and the outcrop of any 

ore deposit will furnish a true index of the value of the lode as a whole, depending 

on whether the distribution of values in the ore is or is not uniform.

BLACK HILLS, SOUTH DAKOTA.

The principal gold deposits of the Black Hills 4 are in pre-Cambrian 

schists which, like the ore bodies, are cut by Tertiary intrusives. 

Since the Cambrian conglomerates contain placer gold,6 some of the 

ores must have been deposited in pre-Cambrian time. The most 

important deposits are comprised in the Homestake belt, about 3 

miles long and 2,000 feet wide. The principal minerals are quartz,

1 Hershey, 0. H., Geology at Treadwell mines: Mln. and Sci. Press, vol. 102,1911, pp. 396-300, 334-335; 

also in Bain, H. F. (ed.), Types of ore deposits, San Francisco, 1911, pp. 157-171.

2 Knopf, Adolph, Geology of the Berners Bay region, Alaska: Bull. U. S. Geol. Survey No. 446,1911.

8 Idem, p. 36.

«Irving, J. D., Economic resources of the northern Black Hills: Prof. Paper U. S. Geol. Survey No. 26, 

1904.

 Devereux, W. B., The occurrence of gold in the Potsdam formation, Black Hflls, S. Dak.: Trans. Am. 

Inst. Min. Eng., vol. 10,1882, p. 469.
220 THE ENRICHMENT OF SULPHIDE OEES.

dolomite, calcite, pyrite, arsenopyrite, pyrrhotite,1 and gold, with 

which are associated the minerals of the schist quartz, orthoclase, 

hornblende, biotite, garnet, tremolite, actinolite, titanite, and 

graphite.2 The ores, though uniformly of low grade, are very profitable. Some of the ores at the surface were below the average tenor, 

while other surface ores were two or three times as rich as the average. 

The valuable minerals extend downward as far as exploration has 

 gone and are fairly uniform to 1,000 feet or more below the surface. 

In general, according to S. F. Emmons,8 enrichment by surface 

leaching has had relatively small importance.

HEDLEY DISTRICT, BRITISH COLUMBIA.

The Hedley district, British Columbia, is in the valley of Similkameen River, some 30 miles north of the international boundary. 

The principal deposits are about 5,000 feet above sea level, and the 

whole district was covered with ice in the glacial epoch. The oldest 

rocks 4 are of Paleozoic age, probably Carboniferous, and consist of 

limestones, quartzites, and argiUites, interbedded with which are 

subaqueous volcanic tuffs and breccias. These are intruded by 

Mesozoic diorite and gabbro, by Tertiary granodiorite, and by dikes 

of aplite, rhyolite, and andesite.

According to Camsell, the ore bodies are contact-metamorphic 

deposits, which occur mainly in limestone near intrusive diorite or 

gabbro. Metamorphism has been greatest along contacts with 

gabbro.6 The sulphides carry abundant arsenopyrite, with pyrrhotite, chalcopyrite, pyrite, and zinc blende; the gangue minerals 

include garnet, epidote, tremolite, diopside, and axinite, with some 

calcite and quartz. No manganese gangue mineral is mentioned and 

but small amounts are shown in analyses of the igneous rocks.

There is a thin capping of iron oxide, which extends in general

some 8 or 10 feet below the surface. Leaching is deeper in ores composed chiefly of pyrrhotite than in ores composed mainly of arsenopyrite. There is no zone of enriched sulphides; but gold, which is the 

principal metal, has been transported downward to some extent, 

according to Camsell, resulting hi an enrichment near the surface 

and to depths perhaps several hundred feet below. In ore bodies 

having a low dip there is a notable concentration of values along the 

gabbro footwall.

i Sharwood, W. J., Analyses of some rocks and minerals from the Homestake mine, Lead, S. Dak.: Econ. 

Geology, vol. 6, 1911.

«Irving, J. D., op. cit., p. 90.

»Idem, p. 79.

« Camsell, Charles, The geology and ore deposits of Hedley mining district, British Columbia: Mem. 

Geol. Survey Canada No. 2,1910.

6 Idem, p. 25.
EEVTEW OF MINING DISTRICTS. 221 

APPALACHIAN DISTRICTS.

The gold deposits of the southern Appalachians are among the 

oldest gold deposits of the United States and were probably formed,1 

in tke main, 3 or 4 miles below the surface at the time of deposition. 

Many of them are in mica schist and other crystalline rocks, and some 

are closely associated with granitic intrusions. Some are cut by 

diabasic intrusives, presumably later than the ore. The deposits have 

yielded considerable placer and lode gold. The minerals, according 

to Graton,2 include quartz, sericite, biotite, fluorite, gold, pjTite, 

, galena, blende, pyrrhotite, chalcopyrite, and magnetite. Manganese 

minerals are not mentioned. In Becker's tabulation of the minerals 

of the gold mines of the southern Appalachians, compiled from all 

previous descriptions and including some mines not described by 

Graton, pyrolusite is mentioned as occurring in only three mines and

rhodochrosite in one.3

Few of these deposits have been extensively explored in depth, and 

data respecting the vertical distribution of the gold are therefore 

meager. Many of them are profitable near the surface, partly by 

reason of the rotten condition of the rock, which renders it more 

easily worked, and partly because gold is accumulated or enriched 

by the removal of valueless material. In general there is, according 

to Graton, very little evidence for or against the theory of the migration of gold; but such migration, if it has taken place, has been 

extremely slow, for areas which have probably been exposed since 

Tertiary time show a marked concentration at and near the surface. 

Possibly some gold has been transferred to lower levels at the Haile 

mine, South Carolina, where, according to Graton, the limit of profitable mining is in general less than 200 feet below the limit of complete 

oxidation. In this zone scales of pyrite and free gold are found in 

joint cracks, indicating a relatively recent age. The deposits are cut 

by basic dikes. Prior to Graton's work many thought that the 

primary deposition of gold was genetically related to the dikes,4 for 

the workable ore appears to be limited to the area cut by them. If 

the basic dikes (like most basic rocks) carry manganese, it would 

appear probable that gold had been dissolved and the ore enriched 

near the dikes. Graton's tables show zinc blende hi but one mine, the 

Kings Mountain mine, and pyrrhotite in but three others, the Haile, 

the Colossus, and the Asbury (which is not described). Both minerals 

are in the ore of the Dahlonega district described by Lindgren.8 All

i Lindgren, Waldemar, The gold deposits of Dahlonega, Ga.: Bull. U. S. Geol. Survey No. 293,1908, p. 124. 

« Graton, L. C., Reconnaissance of some gold and tin deposits ol the southern Appalachians: Idem, p. 62.

* Beeker, G. F., Reconnaissance of the gold fields of the southern Appalachians: Sixteenth Ann. Kept. 

U. S. Geol. Survey, pt. 3, 1895, p. 277.

*Maclaren, J. M., Gold, its geological occurrence and geographical distribution, London, 1908, pp. 57,59.

* Lindgren, 'Waldemar, op. cit., p. 119.
222 THE ENRICHMENT OF SULPHIDE OEES.

these deposits had relatively rich ore near the surface and in none 

of them did it extend to great depths.

Certain ore deposits of Alabama recently described by McCaskey 1 

comprise fissure veins in granite and lenticular bodies in schists. The 

principal minerals are quartz, pyrite, and gold. Some garnet is found 

in the vein quartz at Pinetuckey. Weathering extends to water 

level, which lies 40 to 80 feet below the surface. The ores are oxidized 

above this level and are generally free milling, but below this level 

the ore is not profitably amalgamated so far as explored in depth. 

The ores are fairly regular in width and values, and no evidences of 

enrichment below the water level are recorded.

ANNIE LAURIE MINE, UTAH.

The Annie Laurie mine,2 175 miles south of Salt Lake., Utah, is in an 

area of dacite, rhyolite, and rhyolite tuff and probably belongs to the 

later Tertiary group. The vein is poorly exposed at the surface, 

being largely covered by morainal material. Lindgren says:

The quartz forms an almost continuous sheet along the vein, rarely less than 3 feet 

in thickness and often expanding to a width of 20 feet or more. As a rule, the walls 

are poorly defined and slickensides indicating motion are rare. In places it contains, 

parallel to the walls, streaks of iron oxides and black, sooty manganese ores. * * *

The mine workings have not penetrated below the zone of oxidation, and neither 

the quartz nor the country rock seems to contain any unoxidized sulphides.

In the absence of extensive postmineral fracturing one would 

suppose that the conditions for migration of gold were not particularly favorable. Since the workings had not penetrated3 sulphide 

ore at the date of Lindgren's report, direct evidence was lacking.

MANHATTAN, NEVADA.

The gold deposits at Manhattan, Nev., although inclosed in schists, 

are in an area of Tertiary volcanic activity and should be classed 

with the deposits formed in Tertiary time. Although the schists 

contain stringers of gold of uncertain genesis, the principal deposits 

are steeply dipping lodes of quartz and calcite, stained with iron and 

manganese oxides. Some placers are developed. Rich ore was found 

very near the surface, but it was richer a few feet below the outcrop 

than at the surface. Some fracturing has taken place since the 

deposits were formed. The gold of many of the pockets of rich 

ore is intimately associated with iron and manganese oxides. 3 In 

view of the fact that the unaltered sulphides had not been encoun1 McCaskey, H. D., Notes on some gold deposits of Alabama: Bull U. S. Geol. Survey No. 340,1908, p. 36.

2 Lindgren, Waldemar, The Annie Laurie mine, Piute County, Utah: Bull. U. S. Geol. Survey No. 

285,1906, pp. 87-90.

8 Emmons, W. H., and Garrey, G. H., Notes on the Manhattan district: Bull. U. S. Geol. Survey No. 

303, 1907, pp. 87-90.
EEVIEW OF MINING DISTRICTS. 223

tered when the mines were visited, the character of the primary ore 

is unknown to me, nor is it certain that all the deposits have 

decreased in value below the oxidized zone.

BULLFROG, NEVADA.

In the Bullfrog district, Nevada,1 the principal deposits are fissure 

veins in rhyolite. The minerals include pyrite, quartz, and manganiferous calcite. Enough manganese is present in the calcite to 

stain much of the oxidized ore chocolate-brown or black. No placers 

are developed. The outcrops .were comparatively poor, but good 

ore was encountered within a few feet of the surface, and some of 

the deposits were worked by open cut. Some of the ore deposits 

decrease in value below the 400-foot level, where ore carrying less 

than $5 a ton is encountered. Since the ore above this level carried many times this value, it appears that there has been a secondary

concentration by surface waters and that the rich ore is related to 

the present topographic surface.

In these ores, as in other manganiferous gold ores in calcite gangue, 

secondary deposition of gold has not been carried far below the 

surface.

GOLD CIRCLE, NEVADA.

The deposits of the Midas or Gold Circle district, Nevada,2 are in 

late Tertiary rhyolites. The lodes are replacement veins and sheeted 

zones and carry considerably more gold than silver by value. In the 

oxidized zone some of the ore is rich, but the sulphides are comparatively regular in value and give no evidence of extensive enrichment. Some oxidized ore shoots appear to have been increased 

in value by the removal of substances more soluble than gold. The 

minerals are chiefly quartz and pyrite. The oxidized zone contains 

seams of very rich gold ore, composed of manganese, limonite, kaolin, 

and soft hydrous silica.

DELAMAR MINE, NEVADA.

The Delamar mine, in southeast Nevada, is in quartzite which is 

cut by acidic porphyry dikes. The ore body, described by S. F. Emmons,3 is related to a strong zone of fracturing, which strikes with the 

quartzite but dips about 75°, or nearly at right angles to the dip of 

the quartzite. The ore is in shoots or zones of crushed quartzite. 

The chief ore body, which is, roughly speaking, a long and compara1 Ilansome, F. L., Emmons, W. H., and Garrey, G. H., Geology and ore deposits of the Bullfrog district, 

Nevada: Bull. U. S. Geol. Survey No. 407,1910.

2 Emmons, W H., A reconnaissance of some mining camps in Elko, Landeir, and Eureka counties, Nov.: 

Bull. U. S. Geol. Survey No. 408, 1910, pp. 48-57

8 Emmons, S. F., The Delamar and the Horn Silver mines: Trans. Am. Inst. Min. Eng., vol. 31,1901, 

pp. 668-675.
224 THE ENKICHMENT OF SULPHIDE ORES.

tively thin, nearly upright cylinder, is divided into four parts by a 

dike of quartz porphyry and a more basic dike, which cross nearly 

at right angles in the ore body. The ore follows the line of intersection of the two dikes rather closely. The ore at the bottom of the 

mine consists of quartz and pyrite, which fill fractures in the altered 

quartzite. Where the dikes cross in the ore body the light-colored 

dike appears to be continuous, but notwithstanding this the line 

of the dark dike across the light one is generally marked by a slight 

stain of manganese dioxide, which, as stated by Emmons, is characteristic of the "black" dike and perhaps gives it that name.

Oxidation extends as far down as the tenth level. The ore that 

has been found below that level is too low in grade to pay for mining. 

The gold ore carries silver and some copper. The tenor in gold 

increased from the surface downward to about the seventh level, 

although the values were not evenly distributed. Some lots of ore 

ran as high as 30 ounces per ton, and the richer parts of the mine 

averaged from $30 to $70 per ton. At the tenth level they had 

decreased to $4 or $5 per ton.

EXPOSED TREASURE MINE, CALIFORNIA.

The Exposed Treasure mine, 1 near Mohave, Cal., has produced 

considerable gold and silver. It is in an area of granitic rocks cut 

by quartz porphyry and capped by rhyolite. The lodes are probably 

Tertiary. The Exposed Treasure vein dips about 45° E. and is a 

sheeted brecciated zone. Considerable fissuring has taken place 

since the ore was deposited.

The lodes are continuous and many of them are of great width, 

some being 40 feet and more from wall to wall, but the pay streaks, 

from 4 to 15 feet in width, lie in well-defined shoots and overlapping

sheets or lenses. It is noteworthy that only those shoots or lenses 

which now reach the surface contained important quantities of calcite and manganese dioxide.

The oxidized ores contain much manganese dioxide, the concentrates carrying 12 per cent. In the altered oxidized ore are kernels 

of ore containing pyrite, chalcopyrite, galena, and sphalerite, and 

these are richer in the precious metals than the altered friable ore. 

As observed by De Kalb 

The altered ore bore manifest signs of extensive leaching, and where it had become 

almost completely decolorized by the removal of iron the precious-metal contents 

had nearly disappeared, and such ore never contained copper except in the form of 

chrysocolla.

The absence of sulphides in all the [oxidized] ores, except in the cherty skeletons 

and in the undecomposed kernels of hard ore, was very complete. The mill concentrates (150 into 1) had an average composition of Si02, 30; FeO, 37; * * * and

i De Kalb, Courtenay, Geology of the Exposed Treasure lode, Mojave, Cal.: Trans. Am. lust. MW. 

Eng., vol. 38, 1908, pp. 310-319.
REVIEW OF MINING DISTRICTS. 225

Mn02) 12 per cent. These concentrates never contained more than 1.5 per cent of 

sulphur. * * *

In the lower friable siliceous ores the ratio of gold to silver was as 1 to 12, while in 

the upper mangano-calcitic ores the ratio was as 1 to 72. Assays of gold scale and of 

coarse gold panned out from all parts of the mine showed a remarkably uniform, alloy 

of 1 part of gold to 0.461 part of silver. The silver in the upper portion of the mine 

was present almost wholly in the form of silver chloride.

On the assumption, from the evidence, that the abundance of chlorides would 

prevent the leaching out of silver and its reconcentration below water level, and that 

the ferric and cupric sulphates would have abstracted large quantities of the gold, 

which would be redeposited lower down, together with the copper in the form of 

secondary enrichments, it was natural to predict an ore below permanent water rich 

in these metals and relatively lean in silver. It would be difficult to conceive a 

nicer justification of theory than that which was afforded when development at length 

extended below water level. The ore consisted of a hard bluish-gray mass of original 

chert-cemented breccia, recemented by quartz, with partial replacement of the 

granite and quartz porphyry by silica, heavily impregnated with sulphides, among 

which were considerable quantities of chalcopyrite, some bornite, and some covellite. 

The gold content of the ore had increased 150 per cent above the average in the 

friable siliceous ores on the upper levels, and the ratio of the gold to silver was as 1 to 2.

BODIE, CALIFORNIA.

The deposits of Bodie, Cal., are east of the Sierra, near the State 

line. They are not of the California type but are associated with 

andesite and belong to the late Tertiary group so extensively developed in Nevada. R. P. McLaughlin 1 has described the most important mines. The lodes are fissure veins in andesite. Nearly all strike 

northward and are approximately parallel. The ore carries about 

equal amounts of gold and silver. The deposits are developed extensively to a depth of 500 feet below the surface. One shaft is 1,000 

feet, another 1,200 feet deep. Outcrops of encouraging value are 

rare. Almost without exception the veins have failed to carry 

pay ore beyond 500 feet below the surface; but above this depth 

occur large, rich ore bodies, which, according to McLaughlin, carry 

ore worth as much as $400 a ton. Faulting and displacement are 

probably of later date than the period of vein formation. Some of 

the oxidized ore carries manganese dioxide. It is "loose and clayey 

in texture and carries some silver to the exclusion of gold."

LITTLE BTJBBO MOUNTAINS, NEW MEXICO.

The Little Burro Mountains, just north of Tyrone, N. Mex., consist 

of pre-Cambrian granitic rocks, overlain by Cretaceous sediments and 

later eruptive rocks. These are partly buried by Tertiary (?) lavas 

and intruded by rhyolitic or trachytic stocks.2 Some gold viens in

1 McLaughlin, R. P., Geology of the Bodie district, Cal.: Min. and Sci. Press, vol. 94,1907, p. 796. 

8 Paige, Sidney, Metalliferous ore deposits near the Burro Mountains, Grant County, N. Mex.: Bull. 

U. S. Geol. Survey No. 470,-1911, p. 132.

72293° Bull. 529 13 15
226 - THE ENBICHMENT OF SULPHIDE OEES.

this region carry abundant manganese dioxide and chlorides. The 

ore minerals include chalcopyrite, pyrite, sphalerite, and galena.1 

Zinc blende is conspicuous, and the ores assay 10.2 per cent zinc. 

According to Paige ]

The Wyman vein has been worked over a distance of about 500 feet and not below 

a depth of 110 feet. The richest returns came from the part above a depth of 40 feet. 

Silver chloride and gold furnished the values in these upper levels. Zinc and copper 

are reported to have increased as greater depth was attained and gold and silver to 

have fallen to $3 to $5 a ton. In the upper portion of the vein much of the ore is 

reported to have assayed $200 to the ton.

MARYSVILLE, MONTANA.

Marysville, Mont., about 18 miles northwest of Helena, is in an 

area of pre-Cambrian sandstones, shales, and limestones that are 

intruded by quartz diorite and by later porphyry dikes.2 The deposits 

are mainly in fault fissures in sedimentary rocks. Placers are 

developed. The Drumlummon mine has been developed for about 

3,000 feet horizontally and to a depth of 1,600 feet, but no ore was 

found below the 1,000-foot level.

According to Weed 3

In general, it may be said that all the veins of the district carry rich ores in bonanzas 

and ore shoots within the first 200 feet from the surface, but that in depth the ores 

rapidly decrease in value until the vein is no longer workable. It may also be said 

that the ore shoots were well defined and the intervening vein matter barren and 

unworkable. The pitch of the ore shoots conforms to the usual habit, dipping to the 

right when looking down the dip of the vein. The ores consist of sulphides and 

eulphantimonides of silver, with gold aggregating 60 per cent of the total value. In 

the upper levels the ore is somewhat oxidized and in the ore shoots of the Drumlummon mine carried extremely high values.

Mr. Adolph Knopf 4 informs me that the gangue in depth is 

highly calcitic and that the rich ores of native gold that are found 

at and near the surface are associated with abundant manganese

oxides.

SUMMIT DISTRICT, COLORADO.

The Summit district is southwest of Alamosa, Colo., near the Rio 

Grande-Conejos county line. According to R. C. Hills,5 the metalbearing rocks are near the middle of the Tertiary eruptive series of 

south and southwest Colorado. The associated rocks are andesites, 

trachytes, rhyolites, and other eruptives; but, unlike the eruptives

1 Paige, Sidney, Metalliferous ore deposits near the Burro Mountains, Grant County, N. Mex.: BulL 

U. S. Geol. Survey No. 470, 1911, p. 136.

2 Barrell, Joseph, Geology o- the Marysville mining district, Montana: Prof. Paper U. S. Geol. Survey No. 57,1907.

»Weed, W. H., Gold mines of the Marysville district, Montana: Bull. U. S. Geol. Survey No. 213, 

1903, p. 70.

4 Oral communication.

6 Hills, R. C., Ore deposits of Summit district, Rio Grande County, Colo.: Proc. Colorado Sci. Soc., 

vol. 1,1883, pp. 20-36,
EEVIEW OF MINING DISTRICTS. 227

of most Tertiary districts in this province, these rocks appear to have 

been closely compressed and form a series which, as shown in Hills's 

sketches, is probably isoclinal. Some features of the ore deposits 

are puzzling, but, whatever their .genesis, they illustrate very clearly 

the theory of enrichment a fact which was fully recognized by Hills 

as long ago as 1883.

The ore bodies, so far as exposed, are rudely tabular and approximately vertical. The ore is chiefly quartz and pyrite but contains 

some enargite, galena, sphalerite, and other minerals.

Although Hills mentions brown oxides at several places, he does 

not say that they are manganiferous. Raymond 1 states that the 

oxides include those of purplish hue.

Placers appear to be of subordinate importance. The mineralized 

matter may be separated into three divisions (1) the impoverished 

zone near the apex, (2) the zone of rich and partly oxidized ore, and 

(3) the zone of low-grade sulphides. The zone of impoverishment 

includes the outcrops of all but two of the lodes and it extends 

downward to a depth of 50 feet or more. The zone of incompletely 

oxidized ore extends to a depth varying from a few feet to 300 feet. 

In this zone the quartz is colored dark brown by oxides and the more 

highly auriferous material is characterized by an abundance of brown 

oxide. The gold in this ore carries only about 0.025 silver. According to Hills, all the bonanzas were confined to this zone. In some 

places gold appears in disseminated form, in innumerable small 

grains so aggregated as to resemble a continuous sheet of metal. 

Locally the grains unite and form flat nuggets 1 ounce or more 

in weight. According to Hills, the occurrence of this richer material 

is confined to the immediate vicinity of a central channel which has 

been filled with earthy matter, fragments of rock, and iron oxides. 

Some of the rich seams of gold powder have been introduced into 

fractures which cut barite. Below the rich and partly oxidized ore 

the primary sulphides appear to have been unworkable under conditions then existing. There is, however, in three mines a concentration of silver at greater depth than that of the gold bonanzas. Hills 

ascribes the two rich outcropping ore bodies, which are exceptional 

in this district, to intense kaolinization on either side of them, 

causing the country rock to be much more readily eroded than 

the extremely hard quartz. This consequently remained considerably above the general surface, forming a precipitous ridge 

that was, as he explains, protected from solution, which went on 

more vigorously below, in the places where snow and water accumulated.

1 Raymond, R. W., Mines and mining west of the Rocky Mountains, vol. 10,1875, p. 329.
228 THE ENRICHMENT OF SULPHIDE OEES. 

BOHEMIA MINING DISTRICT, OREGON.

The Bohemia mining district of Oregon is an area of andesitic lavas 

and tuffs, which are cut by dacite porphyry and probably by basalt.1 

The deposits are fissure veins composed of sphalerite, pyrite, a little 

galena, and very little chalcopyrite, with a gangue of quartz, altered 

country rocks, and some calcite. According to MacDonald 2

Some of'the veins were brecciated after they were filled, and as a result oxygenated 

surface waters were able to percolate downward along the fractured zone. The ores 

were thus oxidized and sulphides leached out to depths of 100 to 300 feet, the 

depth depending on the degree of brecciation and the rate of erosion. The gold 

occurred as threads and filaments included in the pyrite. The pyrite was leached 

away, leaving the relatively insoluble gold and some iron oxide occupying a part of the 

email cavity left in the vein material. This process brought about an association of free 

gold with iron-stained spongy quartz and enriched the ore by leaching out the valueless sulphides. It also rendered the ore soft and porous, so that it is much more cheaply 

mined and milled than the unaltered ore.

Small local enrichments of free gold occur at the junctions of fissures, pyrite being 

abundant at these junctions, as shown by the mass of iron oxide left. It is probable 

that the smaller particles of gold were dissolved from the upper parts of the vein 

* * * and were precipitated' by the local masses of pyrite below.

Some secondary sulphides were observed, but these are of no commercial value. 

They consist of pyrite crystals deposited in cracks in primary pyrite and of very small 

masses of sphalerite and galena. Other secondary minerals noted were calcite and, 

rarely, cerusite.

SAN JUAN REGION, COLORADO.

The gold deposits of the San Juan region, Colorado, including those 

near Telluride, Silverton, and Ouray, are, as shown by Ransome,3 

of varied character. They are mainly Tertiary, probably middle 

Tertiary, but have nevertheless undergone very extensive erosion. 

The region is moist, its altitude is 8,000 to 14,000 feet above sea level, 

and much of it has been glaciated. The prevailing rocks are volcanic 

tuffs, breccias, and flows and intrusive igneous rocks of intermediate 

or acidic composition.

In this elevated area the ground is frozen much of the year, and 

the rapid erosion is due largely to mechanical disintegration. Secular 

decay or oxidation of the ores, according to Ransome, is not as a rule 

very extensive and is at some places negligible. Purington has 

pointed out, however, that the outcrops of the San Juan lodes, are, in 

general, of lower grade than the ore a few feet below the surface, 

possibly by reason of the migration of gold in suspension. Many of

1 Diller, J. S., The Bohemia mining region of western Oregon: Twentieth Ann. Kept. U. S. Geol. Survey, 

pt. 3,1900, pp. 7-64. MacDonald, D. F., Notes on vthe Bohemia mining district, Oreg.: Bull. U. S. Geol. 

Survey No. 380,1909, p. 80.

2 MacDonald, D. F., op. cit., p. 82.

8 Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull. U. S. 

Geol. Survey No. 182,1901. Purington, C. W., Preliminary report on the mining industries of the Telluride 

quadrangle, Colorado: Eighteenth Ann. Kept. U. S. Geol. Survey, pt. 3, 1898, p. 745. Purington, C. W., 

Woods, T. H., and Doveton, G. D., The Camp Bird mine, Ouray, Colo.: Trans. Am. Inst. Min. Eng., vol. 

33,1903, pp. 499-550.
REVIEW OF MIKIKG DISTRICTS. 229

the lodes are tight and do not appear to offer favorable conditions 

for downward migration of waters. The country is well drained and 

chlorine is probably not abundant in the mine waters. The conditions for deep-seated enrichment are therefore not particularly favorable, although some concentration has taken place locally by the leaching and removal of the less valuable materials from the ore. The 

workable ore appears to be mainly of primary origin.

At some places the gangue includes manganiferous minerals. 

There is some evidence that gold was transported to a moderate extent. 

As Ransome points out,1 black oxide of manganese occurs (in 1901) in 

the deepest workings in the Tomboy and Camp Bird mines and 

usually indicates good ore. These little sheets of rich, dark inanganiferous ore, which fill postmineral fractures, Ransome regards" as 

later than the general mass of the ore. It is reasonable to suppose 

that they represent deposition from solutions which dissolved gold in 

the upper portion of the lode, where ferric salts prevail, and which, 

in the presence of manganese, were able to transport their load to 

greater depths, but which, coming into contact with pyrite, were 

ultimately reduced and forced to give up their gold when, through 

the oxidation of pyrite, ferrous sulphate had been formed.

In the Yankee Girl mine, between Silverton and Ouray, the ores 

down to about 200 feet from the surface were mainly silver and lead, 

galena and pyrite being the principal minerals. The ore below this 

zone is composed mainly of stromeyerite, bornite, chalcocite, and some 

gray copper and barite, yielding about 30 per cent of copper and little 

or no lead. In depths below 600 or 700 feet the ore in this and 

adjoining mines is said to have gradually turned into a low-grade 

pyritous ore, which was of too low grade to work at a profit, so that 

the mines were closed. The ores in the copper-bearing zone had 

been exceptionally rich, carrying several thousand ounces of silver 

to the ton in carload lots.2

CRIPPLE CHEEK, COLORADO.

The gold deposits of Cripple Creek, Colo., are fissure veins and 

replacement deposits in volcanic breccia, in alkali-rich and other 

Tertiary intrusive rocks, and in granite. The fissures, according to 

Lindgren and Ransome,3 were formed at about the time of the 

intrusion of the associated basic dikes and represent a late phase of 

volcanic activity. The deposits are probably of middle or late 

Tertiary age and were formed relatively near the surface by hot 

ascending waters. Calaverite is the chief primary constituent;

i Ransome, F. L., op. cit., p. 101.

8 Emmons, S. F., The secondary enrichment of ore deposits, in PoSepny, Franz, The genesis of ore 

deposits, 1902, pp. 451-452.

 Lindgren, Waldemar, and Ransome, F. L., Geology and gold deposits of the Cripple Creek district, 

Colorado: Prof. Paper U. S. Geol. Survey. No. 54,1906, pp. 167-168.
230 THE ENRICHMENT OF SULPHIDE OEES.

native gold is rarely present in the unoxidized ores. Pyrite is widely 

distributed; tetrahedrite, stibnite, sphalerite, and molybdenite are 

sparingly present. The gangue is quartz, fluorite, adularia, carbonates (including rhodochrosite), some sulphates, and other minerals. 

Some of the deposits were workable at the surface, but the placers 

formed are relatively unimportant. Although rhodochrosite is 

subordinate in amount, the highly fractured country rock contains 

appreciable manganese (0.2± per cent.). According to Lindgren 

and Ransome, the processes of oxidation were attended by the formation of kaolin, hydrous silica, and oxides of iron and manganese. 

Manganese oxides are commonly present in the oxidized zone 1 and, 

according to Penrose, form nodules in the Pharmacist and Summit 

mines. They result from the alteration of rhodochrosite, manganiferous calcite, or other minerals and are generally distributed in the 

oxidized zone as stains in cracks and fissures.3 During oxidation 

manganese is greatly concentrated in the seams of the rock. In 

general, the lower part of the zone of oxidation is above water level 

and is usually less than 200 feet below the surface. In some places 

silver has been completely leached from the oxidized ores. Horn 

silver has not been noted.

Whether a slight enrichment of gold has taken place in the oxidized 

zone is not easy to demonstrate. Lindgren and Ransome are inclined 

to the belief that the oxidized zone as a whole is somewhat richer than 

the corresponding telluride zone. 3 The trivial enrichment in this 

zone may have resulted from the removal of some constituents of 

the primary ore.

If gold was dissolved in the Cripple Creek deposits it was precipitated again at practically the same horizon, for in these deposits 

the zone in which solution takes place is rich. The ground is open, 

providing paths for downward-circulating waters, but although the 

ore-bearing complex is very pervious to water it is surrounded by 

impervious rocks. After the volcanic rocks had been drained in 

mining the flow of water was comparatively small. Lindgren and 

Ransome have compared the volcanic complex to a "sponge in a cup." 

As shown by them, the conditions were unfavorable for the circulation 

of atmospheric water a fact which had an important bearing on 

their conclusion that the ores had been formed by magmatic waters. 

In the absence of a circulation the gold could not be transported. 

A check to this reasoning with respect to a downward circulation is the 

fact that in the porous, brecciated mass, filled with stagnant water, 

the oxidation extended downward to a depth generally less than 200 

feet, and even in this zone residual sulphides are present. If the

i Penrose, R. A. F., Mining geology of the Cripple Creek district, Colorado: Sixteenth Ann. Kept. U. 8. 

Qeol. Survey, pt. 2,1895, p. 123.

s Lindgren, Waldomar, and Ransome, F. L., op. cit., p. 123. 

*Idom, p. 203.
REVIEW OF MINING DISTRICTS. 281

solutions did not carry oxygen downward it would be supposed that 

they could not carry gold; and even if the gold had been dissolved at 

the higher levels, in the absence of a circulation it could not descend. 

There is some evidence which may be interpreted as an indication 

that the gold migrated laterally, or possibly that it was precipitated essentially in place from cold solutions. Richard Pearce 1 

has recorded analyses of oxidized and unoxidized ore. The material 

for the analyses was taken from a section drawn clear across the two 

different portions of the specimen. The analyses show that the 

oxidized ore carries 14.58 ounces of gold per ton, or 2.34 ounces more 

gold than the unoxidized ore, and that all the silver has been leached 

out. In ore so rich such a concentration may result merely from 

leaching out of the substances other than gold; but, on the other 

hand, the analyses of the altered rock indicate that little leaching 

of the silicate minerals has taken place and that the oxidized portion 

was originally richer than the unoxidized, or else that some gold 

had been added. Since 0.27 per cent of manganese dioxide is present 

in the oxidized ore, while none is reported in the unoxidized ore, 

it appears that manganese dioxide was added in the process of 

secondary alteration, and it is possible that the same solutions added 

gold and iron; J. W. Finch 2 regards some of the gold as secondary.

If the gold was dissolved in the Cripple Creek "sponge," it may 

have been precipitated in the stagnant solutions where they were in 

contact with pyrite. In the absence of a downward circulation of 

water such lateral migration would be possible.

The results of oxidation processes are described by Lindgren and 

Ransome 3 as follows:

Thorough oxidizing decomposition will destroy the original structure of this vein. 

In sheeted lodes with many small parallel fissures and joints the latter may become 

effaced and the lode appears as a homogeneous brown soft mass. In other cases a 

central seam may be retained and usually appears as a streak of soft, more or less 

impure kaolin; in other cases it may be filled by white compact alunite, more rarely 

by jasperoid or opaline silica. Crusts of comb quartz, if originally present, lie included 

in the clayey seams, but neither the original fluorite nor the carbonates are ordinarily 

preserved. Very rich oxidized ore sometimes fills the central cavities of the lode 

like a thick brown mud of limonite, kaolin, and quartz sand and easily flows out 

when the vein is opened.

It should not be inferred, however, that where channels are large 

and open the rich, gold-bearing brown mud is necessarily a deposit 

from solution. It may have been carried down in suspension; for 

similar rich mud, with 2 ounces of gold per ton, was found on the

i Pearce, Richard, Further notes on Cripple Creek district: Proc. Colorado Sci. Soc., vol. 4,1896, pp. 11-16. 

1 Finch, J. W., The circulation of underground aqueous solutions and the deposition of lode ores: Proc. 

Colorado Sci. Soc., vol. 7,1904, pp. 193-252. 

8 Lindgren, Waldemar, and Ransome, F. L., op. cit., p. 199.
232 THE ENRICHMENT OP SULPHIDE OKES.

floor of the twelfth level of the Gold Coin mine, after it had been 

filled with water and allowed to stand.

It thus appears that the conditions at Cripple Creek, which are 

somewhat puzzling, may be rationally explained if it is noted 

that the downward migration of gold requires not only solution and 

precipitation but also circulation, and that the conditions for circulation here were peculiarly unfavorable. The facts show also that 

a relatively rapid circulation is required to carry dissolved gold far 

below the zone of mixed oxides and sulphides.

GEORGETOWN QUADRANGLE, COLORADO.

The auriferous deposits of the Georgetown quadrangle, Colorado, 

are mainly at Idaho Springs and in the Empire district, although 

some are developed near Georgetown, in the area of the silver-lead deposits. As shown by Spurr and Garrey, 1 the gold lodes are probably 

of later age than the silver-lead deposits. They cut -the crystalline 

schists and the Tertiary porphyries but are genetically related to 

alkali-rich intrusive rocks o£>middle or late Tertiary age. They carry 

pyrite, chalcopyrite, chalcocite, quartz, adularia, and gold and 

minor amounts of barite, fluorite, telluride, and other minerals. 

Carbonates of iron, magnesium, lime, and manganese occur but are 

relatively rare. The deposits generally contain some galena and 

sphalerite. In many of the mines the ore averages 1 to 2 ounces of 

gold and 20 to 40 ounces of silver per ton. The lodes are usually 

oxidized at the surface and from 15 to 70 feet downward. They 

have yielded some moderately productive placers. In several mines 

the oxidized ore is much richer than the average ore. Below the 

zone of oxidation secondary chalcopyrite and chalcocite prevail for 

several hundred feet from the surface but decrease at greater depth. 

There is an important enrichment of gold and silver, coincident with 

the occurrence of the copper minerals. As stated by Spurr and 

Garrey 2

In the mines mentioned a portion of the copper which has contributed to the enrichment of the original sulphides has been derived from the oxidized zone, but it seems 

unlikely that this has been the case with the gold and silver, which, like the enriched 

superficial portions of the argentiferous veins, must have been derived from the overlying portions of the lodes which are now eroded. * * *

On the whole, the strongest evidence of the reworking of the ores by surface waters 

is afforded by markedly cupriferous ores. * * * Apart from this, however, and 

from the probable partial concentration of galena near the surface in some mines, the 

evidence of rearrangement of the ores by descending waters is in general not nearly 

BO great as in the Georgetown district, and such reworking has probably taken place 

to a considerably less extent.

1 Spurr, J. E., and Garrey, G. H., Economic geology of the Georgetown quadrangle, Colorado: Prof. 

Paper U. S. Geol. Survey No 63,1908, pp. 99-101. '

2 Idem, p. 149.
REVIEW OF MINING DISTRICTS. 233 

GOLDFIELD, NEVADA.

The ledges of Goldfield are in middle or late Tertiary rocks and, 

according to Ransome,1 were probably deposited within 1,000 feet 

of the Tertiary surface. Ransome states convincingly the hypothesis that these deposits were formed by hot ascending solutions 

which mingled with descending sulphate water that contained 

oxygen derived from the air. Although the deposits are probably 

the most remarkable bonanzas of native gold ores carrying little 

silver which have yet been discovered, it does not appear that they 

have been very greatly enriched since they were deposited, for, as 

remarked by Ransome, it is difficult to harmonize the extent and 

intensity of alunitization in the gold ores with the hypothesis that 

the ores were formed by the oxidation and enrichment of lean deposits during erosion. The mine waters are rich in sulphates and, judging from the geographic position of the deposits, probably carry 

chlorides. Manganese dioxide is practically unknown in these ores, 

which in this respect differ from the ores at Tonapah and from a 

great many other Tertiary deposits of the Great Basin province. 

No workable placer deposits have been discovered; yet, notwithstanding the fact that several hundred feet of vein matter may have 

been removed from these deposits since they were formed, there is 

little reason to suppose that much gold has migrated into the existing 

bonanzas from above. The gold is very finely divided and could 

easily have been scattered if it had been eroded with the ledges. 

Analyses of deposits elsewhere that were formed close to the surface by ascending hot waters show that few of them carry much 

gold. The zone of maximum deposition is lower down, for as soon 

as the ascending hot waters are contaminated by ferrous sulphate 

from the surface the gold they contain must be precipitated.

The evidence offered at Goldfield is not out of harmony with the 

conclusion that in the absence of manganese gold is not readily 

transported in mine waters.

PHILIPSBTJRG, MONTANA.

The Philipsburg quadrangle is an area of sedimentary rocks ranging in age from pre-Cambrian to late Cretaceous, with intrusions, of 

granodiorite and related rocks, probably belonging to the same 

period of intrusion as that of the Butte quartz monzonite and other 

batholiths in Montana. The most important ore deposits in this 

quadrangle are those of the Granite-Bimetallic and Cable mines.

The Granite-Bimetallic mine 2 is working a strong fissure vein in 

granodiorite, which carries chiefly silver but also an important

1 Ransome, F. L., The geology and ore deposits of Goldfield, Nov.: Prof. Paper U. S. Geol. Survey 

No. 66, 1909, p. 174.

2 Emmons, W. H., and Calkins, F. C., Geology and ore deposits of the Philipsburg quadrangle, 

Montana: Prof. Paper U. S. Geol. Survey No. 78 (in press).
234 THE ENRICHMENT OP SULPHIDE ORES.

amount of gold. There is conclusive paragenetic. evidence of the 

enrichment of silver below the water level, and the rich silver ore 

carries also more gold than the low-grade silver ore in the bottom 

of the mine. The outcrop of this deposit carried some silver but 

very little gold, and after the discovery the location was allowed 

to lapse by reason of the small assay returns from the gossan. Richer 

ore with secondary cerargyrite, native silver, and ruby'silver in 

cracks across the older sulphides appeared in considerable amount 

200 to 400 feet below the surface and extended to depths of 800 or 

900 feet. The shoot of high-grade ore, which extended for about a mile 

along the strike of the deposit, followed, in a broad way, the present 

rugged surface. The gangue is rich in manganese. No pyrrhotite 

was noted, but zinc blende is abundant at several places in the 

primary ore below the richer sulphides. Some migration of gold 

has undoubtedly taken place. No associated placers have been 

developed.

At the Cable mine 1 the deposits are included in a long, thin block 

of limestone, in contact on either side with quartz monzonite. The 

principal minerals are calcite, quartz, pyrrhotite, pyrite, magnetite, 

and chalcopyrite, with chlorite, muscovite, and other silicates. At 

one or two places small traces of manganese dioxide have been noted 

in the oxidized ore, but it is very much less abundant than in the 

deposits of the Granite-Bimetallic type. This deposit yielded important placers. Good ore was found at or very near the surface 

and, according to the best obtainable data, the tenor increased 

somewhat for a short distance below the surface. Some concentration has taken place by the removal of calcite and other valueless 

material more rapidly than gold, but there is no evidence of enrichment in gold below the water table. The conditions indicate that 

the gold has not been extensively transported since the deposit 

was formed.

NEmABT, MONTANA.

The Neihart district, Montana,2 is a region of Archean granite and 

pre-Cambrian quartzite with intrusive diorite and porphyry. The 

deposits are silver-bearing fissure veins, which in depth carry considerable sphalerite, pyrite, and galena. Superficial alteration is 

not marked, and there are no great zones of carbonates and oxidized 

ore. The Broadwater vein is partly oxidized to a depth of 170 feet 

and in pipes and fissures to even greater depths. Below the oxidized 

ore were large bodies of secondary silver sulphides, consisting of 

polybasite, pyrargyrite, pearceite, and "sooty sulphide ores," much

i Emmons, W. H., and Calkins, F. C., Geology and ore deposits of the Philipsburg quadrangle, Mont.: 

Prof. Paper U. S. Geol. Survey No. 78 (in press).

s Weed, W. H., Geology of the Little Belt Mountains, Montana: Twentieth Ann. Kept. U. S. Geol. 

Survey, pt. 3,1900, p. 271.
REVIEW Of MINING DISTRICTS. ~ 235

of this ore running 200 to 1,000 ounces per ton. The rich silver 

minerals occur in cracks in the primary ore or coat fragments of that 

ore. Manganese oxides abound.

The zones of impoverishment, of enrichment, and of unaltered primary sulphides 

recognized in the case of the copper veins are clearly present here, though the uppermost is of limited extent, and the zones are not so sharply or definitely separated 

from one another as they are in copper deposits, owing to the later fissuring of the 

vein filling allowing the secondary enrichment to be mixed with the unaltered sulphides. * * * In the Neihart ores polybasite seems to show an alteration to 

pyrargyrite and pyrite, and the former in turn changes to native silver in the upper

zone. 1

TINTIC DISTRICT,'UTAH.

The Tintic district 2 is in central Utah, in a mountainous area 

that rises some 8,000 feet above the sea and nearly 4,000 feet above 

the plains. The temperature is moderate, the climate is dry, and most 

of the streams that drain the area disappear in the loose material of 

the desert. The area is occupied by a thick series of Paleozoic quartzites, slates, limestones, and sandstones, which are overlain by Tertiary rhyolite and andesite and intruded by monzonite and basalt. The 

andesite flows are intruded by great masses of monzonite and by basalt 

dikes, and an andesite equivalent to the monzonite caps the rhyolite. 3 

These rocks are folded and extensively fractured and faulted. The 

late history of the region may be briefly stated as follows: *

1. Elevation of the region, with folding of the Paleozoic sedimentary rocks.

2. Erosion, which began with the Mesozoic uplift and continued 

into the Tertiary, producing a surface with greater relief than that 

of to-day.

3. Tertiary volcanic activity, the earlier rhyolitic lava filling deep 

canyons, on the slopes of which talus was cemented by the lava and 

the later andesite lava flows, largely rejuvenating the deeply eroded 

mountain range.

4. Fissuring and ore deposition in the more compact igneous and 

sedimentary rocks.

5. Erosion, by which great masses of igneous rock have been 

removed, with only slight changes in the topography of the limestone ridges, which had been buried by the lavas.

The ore deposits are (1) large fractured zones in sedimentary rocks, 

chiefly in the limestone; (2) fissure veins in igneous rocks; and (3) 

contact-metamorphic deposits in sedimentary rocks near intrusive 

igneous rocks, mainly in the limestone near monzonite.5

iWeed, W. H., The enrichment of gold and silver veins: Trans. Am. Inst. Min. Eng., vol. 30, 1901, 

p. 446.

2 Tower, G. W., Jr., and Smith, G. O., Geology and mining industry of the Tintic district, Utah: Nineteenth Ann. Kept. U. S. Geol. Survey, pt. 3,1899, pp. 603-767.

8 Idem, p. 657.

«Smith, G. 0., written communication.

* Tower, G. W., jr., and Smith, G. O., op. cit., p. 722.
236 THE ENRICHMENT? Otf SULPHIDE ORES.

In igneous rocks the water level is encountered 200 to 700 feet 

below the surface; above this the ore is almost completely oxidized. 

In limestone the water level is much deeper and oxidized ore extends 

1,800 feet or even 2,000 feet 1 below, the surface.

The primary ore minerals of the deposits include pyrite, galena, 

enargite, chalcopyrite, and tennantite. Sphalerite is rare in this 

district,2 and .pyrrhotite is not mentioned. The gangue includes 

quartz, barite, carbonates, chalcedony, and gypsum. Oxidation 

products include limonite, hematite, anglesite, cerusite, minium, 

melaconite (?), native sulphur, jarosite, copper carbonates, cuprite, 

native copper, and a large.number of rare arsenic compounds which 

have resulted from the decomposition of enargite. Argentite, 

stephanite, and native silver are somewhat rare. Cerargyrite is 

more abundant. Chalcocite and bornite become increasingly abundant in the lower parts of the oxidized zone. Some quartz is clearly 

secondary.3

The metals, named in the order of then- importance up to 1898, are 

silver, lead, gold, and copper. With increasing depth silver and lead 

have decreased relatively and copper has become more abundant, 

especially in the Centennial Eureka mine. In 1909 the yield of 

copper was 6,000,000 pounds, the district ranking in copper production the thirteenth in the United States. According to Graton,4 secondary sulphide deposition seems to have played but little part in 

the distribution of values.

A remarkable feature of the district is the great depth at which 

the oxidized ores are found. They are deeper, indeed, than the 

submerged oxidized ore at Bisbee, Ariz., where the deep secondary 

zone is related to a pre-Comanche erosion surface rather than to 

the present one. At Tintic, however, the ores are probably early 

Miocene.5 Since they were formed, great masses of igneous rocks, 

according to Smith,6 have been removed by erosion.

GEORGETOWN, COLORADO (SILVER DEPOSITS).

The Georgetown district, Colorado, is a rugged area of gneisses and 

schists, which are cut by Tertiary intrusives of varied composition. 

Several thousand feet of overlying rocks, according to Spurr, Garrey, 

and Ball,7 have been eroded since the veins were formed. Some of

1 Lindgren, Waldemar, oral communication.

'Tower, G. W., jr., and Smith, G. 0., op. cit., pp. 686, 703.

 Idem, p. 693.

«Graton, L. C., Mineral Resources U. S. for 1906, U. S. Geol. Survey, 1907, p. 407; idem for 1907, pt. 1, 

U. S. Geol. Survey, 1908, p. 618.

* Smith, G. 0., written communication.

«Smith, G. 0., The mineral crest, or the hydrostatic level attained by the ore-depositing solutions in 

certain mining districts of the Great Salt Lake basin (discussion of W. P. Jenney's paper): Trans. Am. 

Inst. Min. Eng., vol. 33,1903, pp. 1060-1062.

'Spurr, J. E., Garrey, G. H., and Ball^S. H., Economic geology of the Georgetown quadrangle, Colorado: Prof. Paper U. 8. Geol. Survey No. 63,1908, p. 145.
EEVIEW OF MINING DISTEICTS. 237

the valuable minerals of the eroded portions have been redeposited 

in the portions remaining.

The zone of complete oxidation is 5 feet to 40 feet deep. In the 

silver-lead deposits the minerals on the lower levels are chiefly zinc 

blende and galena, with pyrite, chalcopyrite, and a little silver and 

gold. Rich soft sulphides are found, especially along cracks and 

watercourses, and are of secondary origin, having evidently been concentrated from the leaner ore by descending waters. They occur down 

to considerable depths from the surface but in decreasing quantity. 

The older and typically more massive ores in which they have formed 

contain usually very much less silver and also less gold. For example, 

secondary sulphides which contain 200 to 300 ounces of silver have 

been formed by this concentration process from primary ore which 

carries only 20 to 30 ounces.

Below the zone where soft secondary sulphides occur and irregularly overlapping 

the lower portion of this zone the rich ores contain polybasite, argentiferous tetrahedrite, and ruby silver, better crystallized and more massive than the pulverulent 

sulphides but also subsequent in origin to the massive galena-blende ore. These 

richer ores diminish in quantity as depth increases, though gradually and irregularly, 

so that the lower portion of the veins contains relatively less silver and lead. The 

best ore in most veins has been found in the uppermost 500 feet, although good ore 

extends locally down to 700 or 800 feet, and in the Colorado Central and to a minor 

extent in other veins down to a thousand feet or more. 1

BBECKENBIDGE, COLORADO.

The fundamental rocks in the Breckenridge region, Colorado, as 

stated bj Ransome,2 are granites, pegmatites, gneisses, and schists of 

pre-Cambrian age. The oldest sedimentary rocks, which rest directly 

on the pre-Cambrian, are red sandstones and shales, supposed to be 

of Triassic or of Permian age. Apparently conformable above them 

is the Dakota quartzite with some gray shale, which is overlain by a 

thick formation of Upper Cretaceous shales. The sediments and the 

pre-Cambrian rocks are intruded by monzonitic porphyries, which 

occur mainly as sills.

The primary deposits, according to Ransome, include veins of a 

zinc-lead-silver-gold series, stockworks and veins of a gold-silver-lead 

series, and the gold veins of Farncomb Hill. The placers of the 

district have been important producers of gold.

The Wellington veins afford the chief examples of the zinc-leadsilver-gold series, the filling of which consists mainly of sulphides, 

quartz in notable quantity being absent from most of them. In the 

Wellington mine the principal constituents of the ore are galena, 

sphalerite, and pyrite in various proportions. Even along the outi Spurr, J. E., Garrey, O. H., and Ball, S. H., Economic geology of trie Georgetown quadrangle, Colorado: Prof. Paper U. S. Geol. Survey No. 63, 1908, p. 144.

3 Ransome, F. L., Geology and ore deposits of the Breckenridge district, Colorado: Prof. Paper U. S. 

Geol. Survey No. 75,1911, pp. 25-26.
238 THE ENRICHMENT OF SULPHIDE ORES.

crops of the veins of the Wellington group most of the deposits carry 

galena in the claylike product resulting from thorough oxidation, and 

the change to essentially sulphide ore generally takes place at depths 

of less than 300 feet. The depth of the oxidized zone, however, 

varies, being greatest in general near the crest of the ridge in which 

the ore bodies occur and least along the lower slopes.

The normal sequence from the surface down appears to be (1) a soft, heavy yellowish claylike ore consisting largely of earthy cerusite and containing residual nodules of 

galena; (2) a lead-silver ore in which the galena is only in part oxidized, while the 

pyrite has been for the most part changed to limonite and the sphalerite altered to 

smithsonite and limonite, with removal of much of the zinc in solution; and finally 

(3) a lead-silver-zinc ore in which galena predominates and in which the early stages 

of oxidation are indicated by the formation of a little spongy smithsonite, or "dry 

bone," as the miners call it, at the expense of the zinc blende. 1

A notable feature of the oxidized ores is their general high content 

of lead and silver as compared with the sulphides beneath. In some 

mines this difference was so great that their owners after extracting 

ore profitably to the base of the oxidized zone found the sulphides of so 

low a grade that they abandoned work. Here and there the oxidized 

ores also show a noteworthy concentration of gold even where the 

sulphide ores below contain only negligible quantities of that metal. 

Thus in the Helen mine, on the south side of French Gulch, some 

gold ore was found near the surface, although the latest and deepest 

workings have exposed nothing but a little sphaleritic zinc ore. In 

the Juventa mine, which produced some good oxidized ore to a depth 

of 200 feet and was then abandoned, wire gold is said to have been 

found.

It is believed that a large proportion of the galena is the result of 

downward concentration by atmospheric water, which, after percolating with comparative rapidity through the oxidized zone to the 

.local ground-water level, thence moved more slowly down through 

the sulphides, emerging finally along the bottoms of the main valleys. 

Although some sphalerite is younger than other sphalerite, it is not 

surely a deposit of sulphate waters. Iron is deposited as an impure 

siderite in veinlets traversing the sulphide ores or as the lining of 

vugs in these ores. Additional iron issues in springs after performing 

its work of enrichment. Silver, according to Ransome, generally 

keeps close to the lead, and gold appears to accumulate in the zone 

of oxidation rather than at greater depths. In general, profitable 

operations did not extend below 300 or 350 feet in depth.

The Farncomb Hill veins immediately below the zone of oxidation 

consist of pyrite, chalcopyrite, sphalerite, galena, calcite, and gold. 

Some wire gold has been found in the unoxidized vein material but

iRansome, F. L., Geology and ore deposits of the Breckenridge district, Colorado: Prof. Paper U. S. 

Geol. Survey No. 75,1911, p. 134.
BEVIEW OF MINING DISTRICTS. 239

not far below the zone of general weathering, and the principal concentration of the native metal is connected with oxidation. 

SaysRansome: 1

Two intimately related processes appear to have been effective in enriching these 

veins. These, in the order of their action at one place, were (1) enrichment by solutions depositing calcite, galena, gold, and perhaps sphalerite below the zone of oxidation and (2) enrichment in the zone of oxidation by solution and redeposition of the 

gold.

It is clear that during the weathering of these veins the gold was acted on by some 

very efficient solvent, for otherwise it would be impossible to account for the large 

crystalline masses of gold characteristic of the hill. These could not have been 

deposited in the veins as part of their original fillings, for they are limited to the 

Oxidized zone, and the once very productive placers below the hill show that this 

zone can not coincide with originally rich upper portions of the veins. Evidently 

the original tops of the veins have been eroded away and their contained gold in part 

has been strewn along the ravines and down the main valleys and in part has seeped 

down in solution along the fissures and been deposited in segregated masses. Active 

as solution must have been, erosion apparently was overtaking it; at least the richness 

of the placers proves that the gold was not carried down and redeposited fast enough 

to escape the forces of mechanical disintegration.

Ransome states that a manganiferous carbonate occurs in the 

Wire Patch mine of Farncomb Hill, and sphalerite from the Silver 

vein showed some manganese, but some oxidized material from the 

Reveille showed none. It is not known whether the mine water 

carried appreciable chloride, but the experiments of Brokaw (p. 126) 

indicate that under some conditions a faint trace of chlorine is effective in the solution of gold. In connection with the abrupt impoverishment in depth, attention should be called also to the mineral 

association of these ores and to experiments previously cited showing 

the rapidity with which acid solutions are reduced by sphalerite and 

auriferous chloride solutions are neutralized by calcite. With both 

calcite and sphalerite the conditions for rapid precipitation of gold 

are very favorable.

CBEEDE, COLORADO.

The deposits of Creede, Colo.,2 are strong fissure veins in rhyolites and rhyolite breccias. One of them extends for nearly 2 miles 

along the strike and is developed from 1,000 to 1,400 feet below 

the surface. The rocks show great postmineral fracturing. The 

primary deposits consist of pyrite, zinc blende, galena, and other 

minerals in a gangue of manganiferous amethystine quartz with 

barite, chlorite, and some adularia. The most valuable deposits of 

silver, the most important metal in this district, were found 200 to 

700 feet below the surface, but wire silver was mined at depths below

i Ransome, F. L., Geology and ore deposits of the Breckenridge district, Colorado: Prof. Paper U. S. 

Geol. Survey No. 75,1911, p. 169.

' Emmons, W. H., and Larsen, E. S., A preliminary report on the geology and ore deposits of Creeda 

Colo.: Bull. U. 8. Geol. Survey No. 630,1913, pp, 42-65.
240 THE ENRICHMENT OF SULPHIDE ORES.

1,000 feet. Some of the richest silver ore is evidently secondary 

and consists of red jasper with abundant native silver.

Stringers of black manganese oxide cutting unoxidized sulphide 

ore extend downward to depths of nearly 1,000 feet. In the Happy 

Thought mine, on the Amethyst vein, between levels 6 and 7, a body 

of partly oxidized ore composed of galena, zinc blende, copper carbonates, cerusite, and anglesite carries a conspicuous amount of 

manganese dioxide, which coats the copper sulphides and occurs in 

fractures cutting the partially oxidized ores. A considerable body 

of this ore yielded $20 a ton in gold, which indicates notable enrichment, since the average content of gold in the mine is about $2 a ton.

RICO, COLORADO.

The rocks of Kico, Colo.,1 include Paleozoic limestones, sandstones, 

and shales, which are cut by dikes and laccolithic sheets of monzonite 

porphyry. The ore deposits include fissure veins and ribbon-like 

masses, which make out in limestone from the fissure veins where the 

latter cross the contact of limestone with overlying shale. In the 

lower levels of the mines, some 200 feet below the "contacts," the ore 

is mainly quartz, pyrite, and chalcopyrite. In raising on the veins 

rhodochrosite, galena, sphalerite, and tetrahedrite become prominent. 

Upward, toward the "contact," the proportion of metallic minerals 

steadily increases, and the ore becomes much richer in gold and 

silver. Irregular nuggets and sheets of argentite, polybasite, and 

stephanite are found. Occasionally pyrargyrite, proustite, and 

native silver appear. Rhodochrosite, on the other hand, is almost 

never found in the zone immediately below the shale, although it is 

abundant in the vein lower down.

LAKE CITY, COLORADO.

The silver-lead deposits of Lake City,2 in southwestern Colorado, 

are in an area of Tertiary flows and tuffs of the Silverton volcanic 

series, which consists of andesites,. rhyolites, latites, and basalts. 

These are cut by intrusions of rhyolite and quartz latite and by quartz 

monzonite porphyries.

The deposits are narrow fissure veins and some fill conjugate fissures. Their vertical range, according to Irving and Bancroft, is 

over 5,000 feet, and in their lower levels the primary minerals are 

chiefly quartz, galena, zinc blende, and pyrite. The ores formed at 

shallower depths include also tetrahedrite, rhodochrosite, barite, and 

jasperoid. The mineralization was probably effected by solutions

i Ransome, F. L., The ore deposits of the Rico Mountains, Colo.: Twenty-second Ann. Rept. U. S. Geol. 

Survey, pt. 2,1901, pp. 229-398. Farish, J. B., The ore deposits of Newman Hill: Proc. Colorado Sci. Soc., 

vol. 4, 1892, pp. 151-164.

'Irving, J. D., and Bancroft, Howlaud, Geology and ore deposits near Lake City, Colo.: Bull. U. 8. 

Geol. Survey No. 478,1911.
. EEVIEW OF MINING DISTRICTS. 241

connected genetically with the quartz monzonite intrusion or with 

some closely related deeper rock.

Many of the lodes are greatly fractured, and the conditions in this 

rugged country are favorable to deep circulation. Erosion is rapid, 

however, and the oxidized zone is not deep, extending generally not 

more than 100 or 200 feet below the surface. This zone contains iron 

oxides and sulphates, copper carbonates, and considerable anglesite, 

with some native copper and silver. Cerusite is not abundant.

The secondary silver minerals are chiefly pyrargyrite and galena 

but include some chalcocite and possibly proustite. Native gold, 

though not common in the oxidized ore, is present in the upper part 

of the zone of sulphide enrichment. Of the total production of all 

metals having, a value above $7,000,000, about one-sixth is gold. 

The table showing the metal production of the district indicates a 

fairly constant proportion in the annual production of gold and 

silver. The abundance of sphalerite in the primary ore would seem 

to favor relatively rapid precipitation of the metals by descending 

solutions. Some of the secondary ore shoots, however, have a wide 

vertical range, but such a distribution of the richer ore has been 

greatly favored by the very rugged, topography. At the Moro mine 

the secondary sulphides are found nearly 500 feet vertically below 

the surface, but the slope is between 30° and 40° and the circulation is presumably vigorous. The reactions precipitating the metals 

seem to have been relatively rapid. As stated by Irving and Bancroft/ zinc blende has very generally exerted an extensive effect in 

reprecipitating the downward-moving sulphates in the form of secondary sulphides.

ASPEN, COLORADO.

The Aspen district, Colorado, 2 is an area of granite overlain by 

Paleozoic limestones, sandstones, and shales, which are intruded by 

dikes and sills of diorite porphyry and quartz porphyry. Structurally the district is a fault mosaic of folded beds and the principal 

ore deposits are replacements of limestones in and along fault fissures. 

The primary ore deposition was effected by ascending magma.tic 

waters and took place in a relatively brief period, but according to 

Spurr* it had three successive stages (1) barite veins, (2) silver 

sulphides, sulphantimonites, and sulpharsenites, (3) galena and 

blende each stage being preceded by slight fracturing of the rocks. 

The maximum deposition was below shale beds.

Near the surface the ores occur as oxides, sulphates, and carbonates, mixed with sulphides, from which they are evidently

'Irving, J. D., and Bancroft, Howland, Geology and ore deposits near Lake City. Colo.: Bull. U. S. 

Geol. Survey No. 478,1911, p. 64.

' Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Survey, vol. 31, 1888. 

'Spurr, J. E., Ore deposition at Aspen, Colorado: Econ. Geology, vol. 4, 1909, p. 303.

72293° Bull. 529 13 16
242 THE ENBICHMENT OF SULPHIDE OKES.

derived. The principal ore in these upper zones consists of earthy 

carbonates and sulphates, chiefly of lead (cerusite and anglesite). 

Among the oxides hematite and limonite are very common, and the 

red oxides of copper and lead (cuprite and minium) occur in blotches 

in the oxidized ores, usually indicating the presence of silver. With 

increase in distance from the surface the oxides, sulphates, and carbonates disappear and give place to pure sulphides. Argentiferous 

galena and blende are abundant in the deeper ore and other sulphides 

are of less frequent occurrence. Pyrite and chalcopyrite, with occasional bornite, are also found. Tetrahedrite and tennantite are 

common and contain a large proportion of silver sulphide. The 

gangue is quartz and barite.

In the Molhe Gibson and Smuggler mines there is much polybasite, 

which generally occurs in flesh-colored barite, the color being due to 

a small amount of iron oxide. Along watercourses the polybasite is 

reduced to native silver, so that the ore consists of pink and gray 

barite bound together by irregular wires and masses of silver. As 

this process is attended by some loss of bulk, the ore also becomes 

much jointed and loses cohesion.

Spurr states that native silver is abundant to depths of at least 900 

feet in deposits where the water level stood about 300 feet. Generally the native silver is found near silver sulphides and in fractures 

that are later than primary mineralization. Nearly all the larger 

bodies of native silver are on the side of the vein which is formed by 

shale and some of them are wholly within the shale wall.

LEADVILLE, COLORADO.

Leadville, Colo.,1 is in an area of Paleozoic limestones, quartzites, 

and shales that are intruded by dikes and sills of acidic porphyries. 

Normal faulting has taken place on an extensive scale. The climate 

is moist, the altitude high. The most important deposits are found 

in Carboniferous limestone at or near its contact with an overlying 

porphyry, which is generally the "White porphyry." Thus the ores 

constitute a sort of sheet, the upper surface of which, being formed 

by the base of the porphyry sheet, is comparatively regular, while 

the lower surface is ill denned and irregular, there being a gradual 

transition from ore to limestone, the ore extending to varying depths 

below the surface, occupying in places the entire thickness of the 

Leadville ("Blue") limestone. Other deposits include, however, 

steeply dipping veins, some of them in fault fissures, and irregular 

masses or sheets in limestone near the "Gray" or other porphyries.

The ore that is most abundant and that is economically by far the 

most important is argentiferous galena and its secondary products,

' Emmons, S. F., Geology and mining industry of Leadville, Colo.: Mon. U. S. Geol. Survey, vol. 12,1886. 

Emmons, S. F., and Irviag, J. D., The Downtowq district of Leadville, Colo,: Bull. U. S. Geol. Survey 

Np. 320,1907.
REVIEW OP MINING DISTRICTS. 243

cerusite, or carbonate of lead, and cerargyrite, or chloride of silver. 

Lead is also found as anglesite and pyromorphite and occasionally as 

oxide in the form of litharge or, more rarely, of minium. Silver 

occurs frequently as chloroiodide and very rarely in the native state. 

The gangue minerals include quartz, chert, barite, and clay, the 

clay being frequently charged with iron and manganese oxides or 

with sulphates.

A common alteration product of mixed pyrite and galena, a 

product that is associated in considerable quantity with the ore bodies, 

is " basic ferric sulphate," an ocherous substance of somewhat uniform 

appearance but of varying composition, mainly a mixture of jarosite, 

or yellow vitriol, and hydrated basic ferric sulphate, with more or less 

anglesite and pyromorphite.

Gold occurs in the native state, generally in extremely small flakes 

or leaflets. It is also said to have been found in the filiform state in 

galena.

Accessory minerals are zinc blende, calamine, arsenic (probably as 

sulphide), antimony (probably as sulphide), wulfenite, copper 

(carbonate and silicate), and bismuth sulphide.

In depth the ores consist of pyrite, sphalerite, and galena in fairly 

equal amounts, with some chalcopyrite and other minerals.

Nodules of galena surrounded by lead carbonates are locally 

numerous in the oxidized zone. Several of these nodules have been 

assayed, and the sulphide has been found to carry about six times as 

much silver as the surrounding carbonate shell. 1

Emmons 2 states that gold exists in the limestone ores only in 

traces. In certain veins of sulphide ores below the porphyry contacts, according to G. Montague Butler,3 some small masses of 

sphaleritic manganiferous ores are very rich in gold. A picked 

sample assayed 8 ounces per ton. Silver is found in the oxidized 

ores mainly as chloride. It is a generalization of the miners of the 

region, according to Emmons,4 that rich silver-chloride ores are 

likely to accompany manganese. As suggested by him and also on 

a previous page of this paper (p. 120), it is possible that the agency 

of manganese in the generation of free chlorine is important in connection with the precipitation of silver chloride under some conditions.

In general, silver values diminish with depth. The upper contact 

bodies as a whole were richest in silver; the "second contact" bodies 

were slightly lower in tenor; and at lower horizons the ore is of distinctly low grade.8

i Ricketts, L. D., The ores of Leadville, Princeton, 1883, p. 37.

*Emmons, S. F., op. cit., p. 545.

'Butler, G. M., Some recent developments at Leadville: Econ. Geology, vol. 7,1912, p. 318,

<Emmons, S. F., op. cit., p. 562.

'Emmons, S. F., and Irving, J. D., op. cit., p. 34,
244 THE ENRICHMENT OF SULPHIDE OEES.

Very recently large bodies of iron-stained smithsonite have been 

found in the oxidized zones below lead-carbonate ores. Some of 

these were formerly supposed to be iron-stained limestone. A. A. 

Blow 1 maintained that sphalerite is deposited by downward-moving 

waters just in advance of oxidation, and in this S. F. Emmons 2 

appears to concur. Some small veinlets of galena are found also 

in sulphide ore just below the oxidized zone.

Of considerable interest are some small fractures in the quartzite 

at a lower horizon, wjiich, as Mr. Emmons informed me, very commonly carry small amounts of high-grade manganiferous .gold ore. 

This ore he regarded as a deposit from descending waters.

EUREKA, NEVADA.

The Eureka district,8 in eastern Nevada, is an area of Paleozoic 

quartzites, limestones, and shales, which were intruded, probably 

in late Mesozoic time, by .granite, granite porphyry, and quartz 

porphyry. Subsequently, probably in the Tertiary period, the sedimentary rocks were intruded by great igneous bodies of andesitic 

composition and covered in places by rhyolite and basalt. The 

beds are thrown into open folds and the dominant structure is a 

fault mosaic, the principal faults being of the normal type.

The ores occur in sedimentary rocks belonging to the Cambrian, 

Silurian, and Devonian periods. Through a section involving 17,000 

feet of deformed strata they have been deposited in sufficiently large 

bodies to encourage mining exploration.

According to Curtis,4 the ore bodies are chiefly replacements of 

fractured limestones and include lodes, stocks, and bedding-plane 

deposits. The elevation of the region is from 6,000 to 7,500 feet 

above the sea, but neighboring peaks are higher. The climate is arid 

and the water level deep. On a section by Curtis through the Jackson, Eureka Consolidated, and Richmond shafts 5 the water level is 

shown at a depth of 500 feet in the Jackson and from 1,000 to 1,200 

feet in the Richmond shaft.

The larger ore bodies, according to Curtis, are capped by caves or 

are in some way connected with caves or fissures, developed by 

solution, and the fall of rock into the openings has caused further 

fissuring. Since this action took place the ore has in many places 

been redistributed by the flow of underground waters. The ore 

above the water level is composed principally of the minerals galena,

ifilow, A. A., The geology and ore deposits of Iron Hill, Leadville, Colo.: Trans. Am. Inst. Min. Eng., 

vol. 18,1890, p. 172.

» Emmons, S. F., The secondary enrichment of ore deposits, in PoSepny, Franz, The genesis of ore deposits, 1902, p. 440.

 Hague, Arnold, Geology of the Eureka district, Nevada: Mon. U. S. Geol. Survey, vol. 20, 1892. 

«Curtis, J. S., Silver-lead deposits of Eureka, Nov.: Mon. U. S. Geol."Survey, vol. 7,1884.

 Idem, PI. in.
EEVIEW OF MINING DISTRICTS. 245

anglesite, cerusite, mimetite, and wulfenite, with very little quartz 

and calcite, the gangue being for the most part hydrated oxide of 

iron. The ore carries also considerable gold and silver and some 

zinc, which occurs probably as carbonate and silicate. The ore 

below the water level is composed chiefly of pyrite, arsenopyrite, 

galena, zinc blende, and a few other sulphides, as well as silver and 

gold. According to Curtis the oxidized ore grades into the sulphide 

ore. At some places altered ore is found below the water level, and 

Curtis supposed that the water level was recently elevated. The 

development of mimetite, a cMoro-arsenate of lead, as stalactites in 

the oxidized zone, is of interest. Zinc blende is found to some 

extent in upper parts of the mines, according to Curtis, and is of 

common occurrence in the lower workings in connection with pyrite 

and galena. The deposits had produced over $60,000,000 in silver 

and gold and 225,000 tons of lead in 1882. More recently enormous 

bodies of low-grade ferruginous gold ore have been mined in the 

shattered and altered limestones that surround the old silver stopes.

PIOCHE, NEVADA.

The deposits of Pioche, Nev./ are in a faulted area of quartzites, 

limestones, and shales which are cut by an acidic porphyry intrusive. 

The most important deposits, some of which have been very productive, are near the intrusives. They fill fissure veins in the quartzite 

and limestone and the ores are more abundantly developed in the 

quartzite. They were stoped at the surface and averaged about $150 

a ton. Ores consisting of silver chloride and sulphide and lead carbonate extended to water level, where the oxidized ore gave place to 

sulphides and large amounts of zinc also entered. At this depth, 

according to Pack,2 the vein was generally strong and persistent, but 

no valuable deposits were in sight when the property was abandoned, 

even though prospecting had been very extensive.

In recent years some other large productive deposits have been 

developed. Of these I have no exact knowledge.

TONOPAH, NEVADA.

The deposits at Tonopah, Nev., are silver-gold replacement veins 

in andesite. They are of middle or late Tertiary age but possibly 

somewhat older than the Comstock lode. Placers are not developed. 

The primary ore, according to Spurr, 3 is composed of quartz, adularia, sericite, and carbonates of lime, magnesia, iron, and manganese, 

with argentite, stephanite, polybasite, chalcopyfite, pyrite, galena,

» Pack, F. J., Geology of Pioche, Nev., and vicinity: School of Mines Quart., vol. 27, 1906, pp. 285-312, 

365-386.

»Idem, p. 372. 

1 Spurr, J. E., Geology of the Tonopah raining district, Nevada: Prof. Paper U. S. Geol. Survey No. 42, 

1905, p. 90.
246 THE ENRICHMENT OF SULPHIDE ORES.

silver selenide, and gold in an undetermined form. Pyrrhotite is not 

listed. According to Spurr, a little zinc sulphide is present, but Burgess 1 does not list sphalerite as a vein mineral.

The zone of oxidation extends to greater depth in the more highly 

fractured places, and for this reason the brittle and more broken 

lodes are more deeply oxidized than the wall rock. The Mizpah 

vein is for the most part oxidized to a depth of 700 feet. Standing 

ground water is lacking. The oxidized ore contains limonite and 

manganese dioxide, with plentiful horn silver and some bromides 

and iodides of silver. The so-called oxidized ore from the outcrop 

down is, according to Spurr, a mixture of original sulphides (and 

selenides), together with secondary sulphides, chlorides, and oxides. 

At a depth of 500 feet (in the Montana Tonopah mine) good crystals 

of argentite, polybasite, and chalcopyrite have been formed freely in 

cracks and druses of the sulphide ore. These minerals are later than 

the massive ore, but it can not be shown that they were not deposited 

upon it by ascending waters. The deposit of dark ruby silver 

(pyrargyrite) is different, however, for it is formed in cracks in the 

oxidized ore, and some argentite fringes minute particles of horn silver 

as if secondary to it. The evidence, therefore, as stated by Spurr, 

favors the view that these secondary sulphides in the oxidized zone 

originated from descending surface waters, and probably part of the 

sulphides in druses in the sulphide ore have a similar origin.

The waters that descend through the oxidized zone carry sulphates 

and chlorides, and "wad" is plentiful; but judging from the fairly 

constant proportion of gold to silver (about 1 to 100 by weight) 

there has been little selective migration of gold and silver during 

oxidation, although the vein has been enriched to some degree by 

downward penetration of minerals leached from the outcrop as it was 

eroded. The rich ore shoots, though partly oxidized, seem to be in 

the main original without thorough rearrangement. According to 

Spurr, this may be ascribed in part to the relatively scanty supply of 

water in this arid region.

COMSTOCK LODE, NEVADA.

The Comstock lode 2 is a broad fault zone in late Tertiary rocks. 

The ore shoots occur here and there in this zone, which is developed 

more than 4,000 feet below the surface.

1 Burgess, J. A., The halogen salts of silver and associated minerals at Tonopah, Nov.: Econ. Geology, 

vol. 6,1911, pp. 13-21.

2King, Clarence, The Comstock lode, in Hague, J. D., Mining industry: U. S. Geol. Expl. 40th Par., 

vol. 3,1870, pp. 11-96. Church, J. A., The Comstock lode, New York, 1879. Becker, G. F., Geology of the 

Comstock lode and the Washoe district: Mon. U. S. Geol. Survey, vol. 3,1882. Reid, J. A., The structure 

and genesis of the Comstock lode: Bull. Dept. Geology Univ. California, voL 4,1905, pp. 177-199.
REVIEW OP MINING DISTRICTS. 247

King * says:

Quartz forma the only gangue in the Comstock lode. Those small masses of carbonate of lime which occur, intermingled with quartz, in the Gold Hill and Hale & 

Norcross lower levels are rather to be considered an included mineral of accidental 

occurrence than as a true gangue. With the exception of small quantities of silver 

minerals contained in the clay sheets where they are placed in close contact with the 

bonanza, the whole silver tenor of the lode is contained in the bodies of quartz. The 

ore itself is composed of native gold, native silver, silver glance, stephanite, rich galena, 

occasional pyrargyrite, horn silver, and, with extreme rarity, sternbergite. Intimately 

associated with these occur iron and copper pyrites and zinc blende. Of these, pyrargyrite and horn silver are rarities; polybasite and sternbergite, in recognizable crystals, 

occupy a few scattered localities; stephanite, in denned crystallizations, has been 

found in nearly every bonanza, but the main body of the ore is a confused semicrystallized association of native gold and silver, vitreous silver ore, rich galena, copper 

and iron pyrites, and zinc blende.

In general the bonanza ore is pretty uniformly disseminated through 

the quartz. Large, solid accumulations are rarely found. The silver 

minerals ordinarily lie in masses about the size of a hen's egg. In the 

central portions of bonanzas there is usually a somewhat denser 

arrangement of ore. It is evident, from the manner in which the ore 

itself is broken and dislocated, that the dynamic action which 

powdered the quartz occurred after it was charged with ore.

Since the period of crushing additional charges of quartz and ore 

have been introduced into the fissure to a small extent. In a few 

places, as in the 800-foot level of the Yellow Jacket mine, broken 

fragments of quartz, themselves containing ore, have been recemented by sheets of stephanite which have penetrated the cracks, 

and over the stephanite a secondary growth of quartz crystals has 

taken place, and these quartz crystals themselves are again coated 

with a fine varnish of silica.2

Analyses of ore from the lower workings of the Savage and Kentuck 

mines (1869) gave zinc sulphide 1.75 and 0.13 per cent, respectively. 

Ores from "middle depths" of the California and Ophir mines gave 

12.85 and 14.45 per cent of zinc.

Like King, Becker notes that there were clearly two periods of 

movement, one before the deposition of the primary ore and one 

after it. The later movement, mainly parallel to the lode, gave conditions for an active circulation of water after the primary deposition. 

According to Becker,3 "it is possible that the seams of rich ore in the 

great bonanza represent a deposition posterior to the final cessation 

of movement," and "it is also by no means impossible that some of 

the richer ores have been redeposited, forming at the expense of 

surrounding bodies of lower grade." As already remarked, analysis 

of the vadose water of the Comstock shows that it contains both gold

i King, Clarence, op. cit., p. 79. 2 Idem, p. 81. 8 Becker, G. P., op. cit., p. 273.
248 THE ENRICHMENT OF SULPHIDE OEES.

and silver. It is noteworthy that this water contains much manganese, presumably as sulphate. Some associated placers were developed, but they are of very subordinate value compared with that of 

the lode. Oxidation extended downward to a depth of 500 feet. 

According to King,1 "a zone of manganese oxide occupies the entire 

length of the lode from the outcrop 200 feet down." The upper part 

of this manganif erous zone was not of high grade in general, especially 

in its uppermost portions. The longitudinal projections 2 show that 

many of the stopes carried from below stop some distance below the 

surface.

Von Richthofen (quoted by Becker) says that "the proportion of 

gold to silver decreased during the early period of working the lode 

but is now (1865) on the increase again." Presumably silver at the 

very surface wasleached more rapidly than gold. The vadose waters, 

as shown by Reid,3 are rich in sulphate, and his analyses, as well as 

others, show the presence of chlorides in appreciable amounts. The 

conditions appear to have been favorable for the migration of both 

silver and gold in the upper levels, even in the comparatively short 

tune that has elapsed since the primary ores were deposited. The 

bonanza ore below consisted largely of stephanite, polybasite, argentite, and other minerals.

I have no detailed descriptions of the character and position of the 

richest ores, but Eliot Lord 4 states that the "Big Bonanza," an enormously rich ore body in the lode, extended below the fifteenth level. 

Possibly some of the richer ore bodies extended still deeper, but it is 

well known that rich ore was more abunckmt in the upper than in the 

lower levels. The deposits in the upper levels yielded, according to 

Richthofen, from $70 to $107 a ton, whereas in later years the average 

value of the ore was not more than $37 a ton.

It thus appears that the evidence of the Comstock lode, from the 

surface down, is favorable to the hypothesis that extensive solution 

and deposition of gold and silver have taken place.

The proportion of gold to silver was presumably higher near the 

surface and in the lower part of the lode than in the middle part. 

When Richthofen made his report he estimated that to the close of 

1865 the lode had produced $15,250,000 in gold and $32,750,000 in 

silver (gold equals 47 per cent of the silver); whereas Becker reports 

the amount recovered from 1865 to 1881 as $87,121,988 in gold and 

$105,548,157 in silver (gold equals 83 per cent of the silver).

The relation of "horn silver" to the surface is different from that 

shown in "chloride mines." According to King,5 silver chloride is

1 King, Clarence, op. cit, p. 75.

sBecker, G. F., op. cit., atlas sheets X-XII.

>Reid, J. A., op. cit.

«Lord, Eliot, Comstock mining and miners: Mon. U. S. Geol. Survey, vol. 4,1883, p. 311.

* King, Clarence, op. cit., p. 82.
REVIEW OF MINING DISTRICTS. 249

accidental, although rare small crystals were found at the outcrop of 

the Gold Hill group and elsewhere.

ST. EUGENE MINE, MOYIE, BRITISH COLUMBIA.

The St. Eugene mine is in the mountainous country near Lake 

Moyie, British Columbia. This region is situated at a moderately 

high latitude and is therefore of particular interest, since the workable ore shows a relation to the present topography that appears 

to indicate appreciable secondary concentration. The deposit affords 

one of the best examples of sulphide enrichment that I have noted in 

Canada. The following data, from a report by S. J. Schofield,1 are 

supplied through the courtesy of Mr. R. W. Brock, director of the 

Geological Survey of Canada.

The claims operated by the principal company include two veins, 

which strike east and dip 70° S. They are about 600 feet apart and 

the developments extend through a vertical range of 2,100 feet. 

The lower workings are over 100 feet below the level of Lake Moyie. 

The two main veins are connected by a series of important cross 

veins, most of which meet the main veins at acute angles. These 

cross fissures, the spaces between which are not uniform, are locally 

termed "avenues." Most of the ore bodies are in the fractured and 

folded area along and between the main veins, and in places large 

ore shoots occur near or at the places where the avenues meet the 

main veins. Very little displacement was observed along the mam 

fissures, although the strata are slightly folded or bent near the fissures. 

The country rocks consist of argillaceous quartzites and purer heavybedded quartzites of the pre-Creston, the oldest known subdivision of 

the Purcell series, which here form the axis and eastern limb of an anticline. The ore bodies are associated with the massive purer quartzites of the pre-Creston formation. The ore consists of galena, both 

fine and coarse grained, associated in places with zinc blende. The 

gangue, which is small in amount, consists of garnet, anthophyllite 

(a variety of amphibole), and a little quartz, the last mineral being 

very prominent where the vein pinches in the argillaceous quartzites. 

Locally the wall rock in the immediate vicinity of the ore bodies 

shows strong metamorphism by the development of garnet and 

anthophyllite.

A vertical projection or stope sheet supplied by Mr. Brock shows 

workings along the hill for 4,900 feet. The slope of the hill is about 

25°. Thirteen tunnels, one above another, are driven to the ore 

zone. Some of these tunnels are 2,000 to 3,000 feet long, but all 

the stopes appear to be within 1,500 feet of the surface, as measured 

on a level, or somewhat less than 800 feet from the nearest points

1 Schofield, 8. J., Reconnaissance In east Kootenay: Summ. Kept. Geol. Survey Canada for 1911,1912, 

pp. 158-164.
250 THE ENRICHMENT OF SULPHIDE OEES.

on the surface. The distance from the surface to which most of the 

ore shoots were followed is practically uniform. Below the level of 

the lake the workings did not extend so deep. At depth the fissures 

tighten, the ore becomes less concentrated, and zinc blende becomes 

relatively more abundant, so that the ore is no longer of commercial 

grade. At the junction of the " avenues" and main vein and nearer 

the surface were large bodies of clean shipping ore. Elsewhere the ore 

was concentrating ore. Mr. Brock informs me that a similar relationship between present surface and the ore bodies is noticeable at 

many points in the Slocan.

COBALT, ONTARIO.

Cobalt,1 in the Nipissing district, northern Ontario, is on the great 

ancient peneplain which extends over much of Ontario and the 

surrounding region. The country is hilly, but the relief is not great. 

The climate is cold and moderately moist. The recent glaciation is 

clearly evident, but drift is generally thin or absent.

The basement rocks are the Keewatin series, a complex of metamorphosed basic igneous rocks, usually known as greenstones, 

which includes also some rock of sedimentary origin. The eroded 

surface of the Keewatin is overlain by Huronian conglomerates, 

graywacke, and other metamorphosed sedimentary rocks. A quartz 

diabase sill some 500 feet thick was intruded into both Huronian 

and Keewatin rocks. This dips southward at an angle of about 17°.

The deposits are short, narrow fissure veins, very numerous and 

rich. They are found in the Huronian, in the diabase, and in the 

Keewatin, but the more productive deposits are in the Huronian 

near the diabase sill, or they were below the footwall of the sill before 

the sill was eroded. The deposits are probably genetically related 

to the diabase intrusive, and the fractures have been assumed to 

represent cooling cracks formed in connection with the intrusion of 

the diabase. Postmineral fracturing and faulting have taken place 

extensively.

The principal sulphides of earlier age include smaltite, cobaltite, 

chloanthite, and bismuth sulphide, with some arsenopyrite and tetrahedrite. Pyrite, galena, and sphalerite are present in the wall rock 

near the vein. The silver occurs as native metal, proustite, pyrargyrite, dyscrasite, and argentite. The gangue minerals include calcite 

and quartz.

The zone of oxidation is exceedingly shallow or altogether absent, 

but certain exceptionally rich superficial deposits, a few feet thick, 

are directly connected with the zone of weathering. This is called 

the "nugget horizon," and in it the smaltite and cobaltite have been

i Miller, W. G., The cobalt-nickel arsenides and silver deposits of Temiskaming, 3d ed.: Kept. Ontario 

Bur. Mines, voL 16, pt. 2, 1908.
BEVIEW OF MINING DISTRICTS. 251

largely altered to secondary minerals or leached out altogether. In 

this zone "cobalt bloom" and "nickel bloom" are characteristic 

minerals.

Extending downward 200 or 300 feet or more below the surface 

are rich silver minerals, largely in veinlets in earlier sulphides. The 

minerals of the veinlets include native silver, argentite, and calcite. 

The change from rich to low-grade ore is very abrupt, both in depth 

and on the strike. Van Hise l and Emmons 2 have attributed these 

richer silver ores to processes of sulphide enrichment, but Miller 3 is 

inclined to the belief that this feature of the genesis has been too 

much emphasized.

BOSSLAND, BRITISH COLUMBIA.

Rossland, British Columbia, is in the Trail Creek district, a short 

distance north of the international boundary. The country is 

glaciated and the altitude of the principal deposits is about 3,400 

feet above sea level. The rocks exposed 4 include Carboniferous 

limestone, quartzites, and shales with interbedded tuffs, ash beds, 

and lavas. Above this series are volcanic agglomerates and lavas. 

Intruding these rocks are masses of monzonite, granodiorite, nepheline 

syenite, etc.

The principal deposits are fissure fillings and replacement veins, 

fractured zones, and impregnations. The most important lodes have 

steep dips. These lodes are intricately faulted and many of the faults 

follow or are followed by basic dikes. The deposits carry commercial 

amounts of copper and silver.

The gangue minerals are biotite, quartz, calcite, tourmaline, 

amphibole, chlorite, and garnet; the sulphides include pyrrhotite, 

chalcopyrite, pyrite, arsenopyrite, marcasite, gold, and other minerals.

In certain gold-bearing quartz veins the gold and chalcopyrite are 

intimately related and there is a notable concentration of gold near 

the surface.

The ore shoots in the cupriferous precious-metal lodes extend downward 50 to 500 feet. One shoot averaged 150 feet long and 56 feet thick 

and was worked downward 500 feet. There is very little oxidation 

above these deposits and no secondary chalcocite zone. The values 

of shipments decrease as greater depths are reached, but this decrease 

may be due to improvement of conditions which permits profitable 

extraction of lower-grade material.

i Van Hise, C. R., The ore deposits of the Cobalt district, Ontario- Jour. Canadian Min. Inst., vol. 10, 

1907, pp. 45-53.

' Emmons, S. F., Cobalt district, Ontario, in Bain, H. F., and others, Types of ore deposits, San Francisco, 1911, pp. 140-156.

'Miller, W. G., Notes o» the Cobalt area: Eng. and Min. Jour., vol. 92,1911, pp. 645-649.

 Brock, E. W., Preliminary report on the Rossland, B. C., mining district, Qeol. Survey Canada, 1900.
252 THE ENRICHMENT OF SULPHIDE OEES.

CCETJR D'ALENE, IDAHO (LEAD-SILVER DEPOSITS).

The geology and ore deposits of the Creur d'Alene mining district, 

Idaho, are described by Ransome and Calkins.1 The district is an 

area of pre-Cambrian quartzites and siliceous slates, which are intruded 

by large masses of monzonite and related rocks. Some contact metamorphismhas taken place and certain of the ore bodies are of the garnet 

sulphide type, but the principal deposits are wide lead-silver lodes free 

from garnet.

The country is extensively faulted, but the lodes are not along the 

major faults. The ores were formed partly by filling open spaces, 

but largely by replacement along zones of fissuring or shearing. The 

deposits, which have a vertical range of 4,000 feet, were probably 

formed under several thousand feet of rock which has since been 

removed.

The ore minerals are galena, pyrite, chalcopyrite, sphalerite, and 

subordinate pyrrhotite, with some argentiferous tetrahedrite and 

stibnite. Siderite is the most abundant gangue mineral, with subordinate quartz and a little barite. The ores carry about 8 per cent 

lead and 4 ounces silver to the ton.

Ransome 2 says:

Although the mineralogical character of the large lead-silver deposits exhibits great 

uniformity, some of the ore bodies are known to change their composition with increase 

of depth. No evidence has been found, however, that in any way connects the 

observed changes with the descent of solutions from .the zone of oxidation. Such 

variations as occur are apparently original and not due to secondary enrichment from 

above.

In the process of 'oxidation cerusite develops from galena without any visible 

intermediate product. The change is not a direct replacement of the sulphur of carbonic anhydride, but the galena is irregularly corroded by the oxidizing solutions 

and cerusite crystals are deposited in the cavities thus formed. In other words, the 

lead itself is somewhat mobile during the change from sulphide to carbonate. The 

various minerals of the oxidized ores are still forming and make up soft earthy masses 

in which no sequence of crystallization is apparent. It is not known, for example, 

whether pyromorphite, like cerusite, forms directly from galena or whether it is 

derived from some oxysalt of lead.

The small quantity of sphalerite formerly in the ores which have undergone oxidation has left no recognizable trace of its presence.

1 Kansome, F. L., and Calkins, F. C., The geology and ore deposits of the Coeur d'Alene district, Idaho: 

Prof. Paper U. S. Geol. Survey No. 62,1908.

2 Idem, p. 112.
INDEX.

A. Page.

Acknowledgments to those aiding........... 11

Adams, F. D.,cited......................... 27

experiments of........................... 27

Adularia, occurrence of...................... 152

Alabama. See Stone Hill.

Alaska, enrichment in................. 162,164,166

oxidation in.......................... 17-18,25

See also Prince William Sound; Kasaan 

Peninsula; Douglas Island; Berners Bay; Bonanza mine. 

Albite, occurrence of......................... 152

Alien, E. T., acknowledgments to........... 12

cited............................... 59,145,149

Alien, E. T., and Crenshaw, J. L., cited..... 140

Altitude, relation of, to enrichment.......... 19

Aluminum, presence of, in mine waters...... 69

minerals of, list of....................... 151

occurrence of...................... 151-155

Alunite, occurrence of....................... 154

Analysis, qualitative, analogy of, to enrichment processes.................. 88-89

Anglesite, occurrence of...................... 137

Ankerite, occurrence of.................... 147-148

Annie Laurie mine, Utah, geology of........ 222

ores of................................... 222

Antimony, presence of, in mine waters...... 68

Appalachians, geology of.................... 221

ores in.......................... 39,103,221-222

Argentite, occurrence of................... 120-121

Aridity, relation of, to enrichment... 19,36,164,165

relation of, to oxidation................. 165

Arizona, enrichment in........... 34-35,94,164-165

mine waters from, analyses of............ 70

ore deposition in........................ 21

ores of.................... 33,44-45,98-105,108,

109,112,142,146, 153-154,157,159

oxidation in........... 10,32,33,34,36-40,42,45

See also Bishee; Globe; Miami; Ray;

Morenci; Jerome; Silver Bell. 

Arsenic, presence of, in mine waters......... 68

Arscnopyrite, occurrence of.................. 150

Aspen, Colo., geology of..................... 241

ores of............. 118,123,134,159,160,168,241

Atacamite, occurrence of.................. 100-101

Aurichalcite, occurrence of................... 143

Australia, enrichment in................167,169-170

ores of...................._............ 97

Azurite,occurrence of..................... 101-102

B. 

Ball, S. H., Spurr, J. E., and Garrey, G. H.,

cited............... 23,149,232,236-237

Bancroft, H.,and Irving, J. D., cited........ 85,92,

109,122,124,139,156,160

Page. 

Bard, D. C., cited.......................... 98,176

Barite, occurrence of....................... 159-160

Barlow, A. E., cited......................... 18

Base-leveled regions, enrichment in.......... 19-20

See also Erosion. 

Bauxite, occurrence of..................... 154-155

Becker, G. F.,cited....................... 247-248

Berners Bay, Alaska, geology of............. 219

ores of................................. 166,219

Bingham, Utah, copper ores of............... 177

geology of............................... 177

ores of... 33,36,38,39-40,97,99,100,103,104,105,

109,111, 113,124,134,135,148,159,177-179

enrichment of........... 40,44,164,165,177

oxidationat...................... 3.'J, 36,38,177

silver and gold from................... 177,179

Bisbee, Ariz., copper ores of............... 180-182

description of............................ 182

geology of............................. 179-180

mine waters from........................ 182

ores of................... 10,23,32,36,38,39,99,

100,101,102,104,105,108,112,153,180-182 

enrichment of....... 34-35,417,165,181-182

oxidation at............ 10,32,34,36,39,181-182

Black Hills, geology of....................... 219

mine waters from, analyses of............ 70

ores of................................. 219-220

enrichment of..................... 106,220

Blind lodes, enrichment of................... 24

Blow, A. A., cited..................... 139,142,244

Bodie, Cal., ores of.......................... 225

ores of, enrichment of................... 166,225

Bohemia district, Oreg., geology of........... 228

ores of............................... 29,37,228

enrichment of....................... 228

Boleo, Lower California, Mex., geology at.... 203

ores of................................... 203

Bonanza mine, Chitina belt, Alaska, geology

of..................................... 215

ores of.................... 17-18,25,107,215-216

Bornite, occurrence of..................... 110-111

Boundary district, B.C., geology of......... 209

ores of......................... 162,163,209-210

oxidation in............................. 209

Boutwell, J. M., cited....................... 38,

104,105,109, 111, 124,159,177-178 

Braden mines, Chile, deposits of............ 18,

23,101,162,163,203-204

geology of............................... 203

Breckenridge, Colo., geology of.............. 237

ores of................. 139,142,148,162,237-239

enrichment of................. 168,238-239

oxidation at............................. 238

Brewer, W. M., cited......................... 201

253
254 INDEX.

Page. 

British Columbia, enrichment in............ 18,25,

162-163,167-169 

oresof............................. 108,110,112

See also Vancouver; Boundary; Hedley;

Moyie; Rossland. 

Brochantite, occurrence of................... 100

Brock,B. W.,cited....................... 209,249

Brokaw, A. D., experiments of........ 126-127,128

Buckley, E. K., cited....................... 150

Buehler, H. A., and Gottschalk, V. H., experiments of............... 49-51,76,91

Bullfrog, Nev., ores of....................... 223

ores of, enrichment of............. 166,167,223

Burgess, J. A., cited......................... 116

Burro Mountains, N. Mex., geology of....... 193

oresof................................. 193-194

Butler, B. S., acknowledgments to.......... 12

cited........................ 38,85,109,161,192

paper by, on San Francisco region..... 197-199

Butler, G. M.,cited......................... 243

Butte, Mont., copper ores of................. 172

geology of............................. 170-172

mine waters at........................ 173-176

analyses of........... 60,63,67,68,69,70,87

ores of... 33,37,38,85,92,97,99,100,102,103,104,

105-106, 110, 111, 112, 113, 120-121,

124, 125, 128, 141, 153, 159, 171-177

enrichment of... 40,94,164,165,168,174-175

oxidation at......................... 33,37,174

silver ores of........................... 172-173

C.

Calamine, occurrence of................... 142,155

Calcium, presence of, in mine waters......... 68-69

California, enrichment in.......... 162,164,166,168

mine waters from, analyses of... 62,65,71,72,74 

ores of..... 106,109,111,112,120,124,130,142,159

sulphides in............................. 18

See also Shasta County; Copperopolis; 

Mother Lode; Nevada City; 

Grass Valley; Ophir; Exposed 

Treasure mine; Bodie. 

Calkins, F. C., and Eansome, F. L., cited.... 252

Campo Morado. See La Reforma mine.

Camsell, Charles, cited...................... 220

Canada, oxidation in........................ 18,25

Cananea, Mex., geology of................... 194

mine waters from, analyses of... 61,64,69,72,90 

oresof................... 33,98,103,157,195-196

enrichment of.................. 44,164,196

oxidation at........................ 33,195-196

Carbonates, deposition of.................. 161-162

occurrence of............................ 162

presence of, in mine waters.............. 67-68

solution of............................... 161

Catherinet, Jules, cited...................... 110

Celestite, occurrence of...................... 160

Cerargyrite, occurrence of................. 119-120

Cerusite, occurrence of....................... 138

Chalcanthite, occurrence of.................. 100

Chalcedony, occurrence of................... 156

Chalcocite, absence of............... ... 162-163

deposition of, chemistry of............... 52

depthof............................... 164,165

effect of, on depth of secondary zone..... 93-94

on gold enrichment................ 133-134

Page.

Chalcocite, occurrence of....... 25,104-109,164-165

origin of......................... 10,42,104,108

relation of, to glaciation............... 162-164

togold.and silver.................... 169

Chalcopyrite, occurrence of........;....... 111-113

relation of, to occurrence of Chalcocite.... 163,

164,165 

Chemical equilibria, data on................. 57-59

Chemistry of enrichment, experimental data

relating to....................... 48-60

See also Mine waters; Oxidation; Solution; Precipitation; Enrichment; 

Sulphides; particular minerals; 

etc. 

Chert, occurrence of......................... 156

Chile. See Braden mines.

Chitina belt, Alaska, geology of.............. 215

oresof.................... 17-18,25,107,215-216

Chlorine, presence of, in mine waters........ 66-67

Chrysocolla, occurrence of............. 102-103,155

Circulation. See Underground circulation. 

Clark, Donald, cited......................... 167

Clarke, F. W., acknowledgments to......... 12

Clifton-Morenci district. See Morenci.

Climate, relation of, to enrichment........ 16-19,25

Cobalt, Ontario, geology of.................. 250

oresof.......................... 18,119,250-251

enrichment of...................... 25,251

oxidation at............................. 250

Co3ur d'Alene, Idaho, geology of............. 252

ores Of. 105,109,119,122,134,135,136,147,159,252 

enrichment of..................... 165,252

oxidation at............................. 252

See also Snowstorm lode. 

Cold solutions, d eposition by............. 10,13,16

Collins, A.'L., cited......................... 54

Colorado, enrichment in... 22,26,34,35,166,168,169 

mine waters from, analyses of............ 62,

63,67,69,70,71,72,73,74 

ores in.................................. 81-82,

98, 102, 109, 118-125, 128, 129, 130,

137-139,141-142,148,153,157,159-162

analyses of.......................... 82

oxidation in.................... 35)36,37,81-82

underground water in................... 29,30

See also Summit; San Juan; Cripple Creek; 

Georgetown; Creede; Breckenridge; Rico; Lake City; Aspen; 

Leadville; Silverton; Custer 

County. 

Comstock lode, Nev., geology of............. 246

mine waters from, analyses of......... 68,69,70

ores of......... 119,121,122,123,134,169,247-249

enrichment of.................. 22,168,248

 oxidation of............................. 248

Contact-metamorphic deposits, deposition

of............................... 13,14

Cooke, H. C., cited................ 115,123,158-159

Copper, leaching of, from gold copper ores... 97 

minerals of, chemical relations of....... 95-113

listof............................... 95

occurrence of....................... 97-113

solution of........................... 95-96

oresof, deposition of..................... 13,16

deposits of......................... 162-165

enrichment of.................. 9,10,22-23

See also particular districts.
INDEX. 255

Page. 

Copper, precipitation of............... 54-56,95,96

presence of, in mine waters.............. 70

Copper, native, occurrence and formation of. 98-100 

Copper mines, waters of, analyses of......... 60-61

Copper-nickel ores, occurrence of............ 14

Copperopolis, Cal., geology of................ 202

ores of......................... 162,163,202-203

enrichment of..................... 202-203

oxidation at........................... 202-203

Copper pitch, occurrence of.................. 104

Cotunnite, occurrence of..................... 137

Covellite, occurrence of.................... 109-110

Creede, Colo., geology of..................... 239

mine waters from, analyses of. 63,67,71,72,73,141 

ores of......................... 36,119,120,128,

130,134,141,159,160,161,169,239-240 

enrichment of.............. 26,166,168,240

Crenshaw, J. L., and Alien, E. T., cited..... 140

Cripple Creek, Colo., geology of.............. 229

ores of................................... 37,

81-83,102,124,154,156,159,160,229-232

analyses of.......................... 82

enrichment of.............. 22,166,230-232

oxidation at............................ 37,231

Cuba, enrichment in......................... 164

See Santiago; Pinar del Rio.

Curtis,J.S.,cited......................... 244-245

D.

Deep-vein zone, deposits in............... 13,14-15

De Kalb, Courtenay, cited................. 224-225

Delamar mine, Nev., geology of............. 223

ores of................................. 223-224

oxidation at............................. 224

Diaspore, occurrence of.................... 154-155

Dickson, C. W., cited........................ 213

Diller, J. S., and Kay, G. F., cited........... 204

Dioptase, occurrence of...................... 103

Don, J. R,.,cited............................ 126

Douglas, James, cited....................... 34

Douglas Island, Alaska, geology of........... 218

ores of.......................... 17,166,218-219

Ducktown, Tenn., geology of................ 205

mine waters from, analyses of........... 60-61,

64,67,70,71,74,89-90,110,141,149 

ores of......................... 33,38,44,77,79,

84,97,99-100,103,104,109,158,168,169,205-207 

analyses of.......................... 79

enrichment of...................... 19-20,

33,39,112-113,163,164,168,169,208-207 

oxidation at.............. 33,37,38,39,44,78-79

solution at............................... 77,84

E.

p'arth's crust, openings in................... 26-27

Edgemont, Nev., geology of................. 218

ores of................................. 166,218

Electromotive potentials of sulphides, relative

powerof................... 50-51,76,91

Ely, Nev., geology of........................ 189

mine waters from, analyses of............ 71

ores of............. 108,109,134,157,159,190-191

enrichment of.................. 44,164,191

Ely, Vt., ores of........................... 207-208

enrichment of...,.,.,..,.,,,.,..,..... 164,208

Page. 

Embolite, occurrence of..................... 120

Emmens, S. H.,cited....................... 56

Emmons, S. F., acknowledgments to........ 11

cited.................... 54,92,153,243-244,251

Enargite, occurrence of...................... 113

Encampment district, Wyo., geology of..... 204

ores of....................... 38,39,109,204-205

enrichment of.................. 39,164,205

Enrichment, chemistry of............ 10,11,48-162 .

chemistry of. See also Chemistry; Precipitation; Oxidation.

maximum depth of...................... 27

method of, analogy of, to laboratory

methods......................... 88-89

theory of>............................... 9-11

applicability of, nonuniversality of.. 10 

Epigenetic deposits, classes of............... 13

Erosion, rate of, relation of, to enrichment... 19,21

Erosion, ice. See Glaciation.

Eureka, Nov., geology of.................... 244

ores of................................. 244-245

Evaporation, precipitation due to........... 43-44

Exposed Treasure mine (Mohave), Cal., geology at........................... 224

ores of............................. 120,224-225

enrichment of................. 166,168,225

oxidation at............................. 224

F. 

Ferguson,H. G.,cited.................... 166-167

Ferric sulphate, reduction of................ 144

Famatinite, occurrence of................... 113

Finch, J.W.,Cited......... 27,29,30-31,43,211,231

Finlayson, A. M., cited................ 139,142,167

Fissures, relation of, to enrichment....... 20-21,92

Flowage, zone of............................. 26-27

Fracture, zone of............................ 20-27

zone of, enrichment in................... 27

Fuchs,Edmond,cited....................... 203

G. 

Galena, occurrence of........................ 138

Gangue minerals, genesis of................ 150-151

list of.................................... 151

See also Aluminum; Silica; Sulphur;

Carbonates. 

Garrey, G. H., Ball, S. H., and Spurr, J. E.,

cited............... 23,149,232,236-237

Genth, F. A., cited.......................... 85

Georgetown, Colo., geology at......... 232,236-237

mine waters from, analyses of...... 69,70,71,74

ores of..... 119,120,121,123,149,169,232,236-237

enrichment of............. 166,168,232,237

oxidation at............................. 237

silver of............................... 236-237

Georgia, ores of.............................. 128

Germany, mine waters from, analyses of..... 63,

71,73,74 

Gibbsite, occurrence of.................... 154-155

Glaciation, relation of, to enrichment...... 24-26i

162-164,166 

Globe, Ariz., copper ores of................ 183-185

geology of............................... 183

mine water in........................... 184

ores of............... 21,101,102,108,112,183-185

enrichment of....... 40,45,164,166,184-185
256 INDEX.

Page. 

Globe, oxidation at......................... 184

Gold,concentration of,in oxidized zone. 132-133,166 

manganese and, relations of........... 128-132,

166-167 

migration of........................... 165-169

ores of, deposits of..................... 166-170

enrichment of........... 9,132-133,166-170

relation of, to chalcocite....... 133-134

relation of, to glaciation......... 166

See also particular districts. 

placers of, enrichment of................. 131

precipitation of....... 95,97-98,128-129,166-167

presence of, in mine waters.............. 70

solution of........ 95,96-97,125-127,134,166-168

telluridesof............................. 135

Gold Circle, Nev., geology of................. 223

ores of................................. 166,223

Goldfleld, Nov., geology of.................. 233

ores of..... 146,149,153,154,157,159,160,161,233

enrichment of.................. 22,166,233

Gold placers, manganese, and relations of... 167 

Gold mines, waters of, analysis of........... 62-63

Goldschmidt, V. M., cited................... 106

Gordon, C. H., Lindgren, W., and Graton,

L. C., cited................ 34,35,36,99

Goslarite, occurrence of...................... 141

Gossan, analyses of.......................... 78-79

poverty of............................... 31,33

Gossan lead, Va., geology of................. 207

ores of................................. 112,207

enrichment of....................... 164

Gottschalk, V. H., and Buehler, H. A., experiments of............... 49-51,76,91

Grass Valley, Oal., geology of.............. 216-217

mine waters from, analyses of........... 68

ores of................................. 166,217

Graton, L. C., acknowledgments to.......... 12

cited........ 22,99,104,106, 111, 131,142,175,176,

178,188,191-192,199,221-222,236 

Graton, L. C., Gordon, C. H., and Lindgren,

W., cited..................... 34,35,36

Great Basin, enrichment in................. 19,22

Gregory, J. W., cited........................ 153

Ground water, level of...................... 27-28

Grout, F. F., experiments of................ 49

Gypsum, occurrence of...................... 159

H. 

Hanover district, N. Mex. See Santa Rita

and Hanover districts. 

Hedley, B. C., geology of.................... .220

ores of................................... 220

Hematite, occurrence of..................... 146

Hershey,O.H., cited....................... 219

Eickling, George, cited...................... 152

Hill, R.T.,cited.. ......................... 208

Eillebrand, W. F., analyses by.............. 71

cited.................................... 74

Hills, E.G., cited.......................... 226-227

Hot solutions. See Thermal waters.

Hunter, W. H., acknowledgments to........ 12

Hydrogen sulphide, generation of...... 87-88,91-92

precipitation of sulphides from....... "88-89,170

Hydrozincite, occurrence of.................. 142

Hynes, D. P.,cited......................... 46,176

Page. 

Idaho, enrichmentin........................ 165

ores of..... 105,109,119,122,137,139,148,149,159

oxidation in............................. 36

See also Coeur d'Alene; Snowstorm lode. 

Information, sources of...................... 11-12

Ireland, mine waters from, analyses of....... 70

Iron, minerals of, chemical relations of..... 143-145

minerals of, list of....................... 143

occurrence of...................... 145-150

solution of........................... 143

. presence of, in mine waters.............. 69-70

sulphides of, oxidation of....... 9,32,41-42,144

precipitation of...................... 89,96

Irving, J. D., cited.......................... 45

Irving, J. D., and Bancroft, H., cited........ 85,

92,109,122,124,139,156,160

J.

Jaquet, J. B., cited.......................... 169

Jarosite, occurrence of..................... 147,161

Jasper, occurrence of......................... 156

Jerome, Ariz., geology of..................... 192

gold at.................................. 193

mine waters from, analyses of............ 70

ores of.............................. 97,192-193

enrichment of..................... 164,193

oxidation at............................. 193

K. 

Kaolin, occurrence of...................... 153-154

Kasaan Peninsula, Alaska, geology of........ 214

ores of...................... 17,162,163,214-215

Kay, G. F., and Diller, J. S., cited..:........ 204

Kemp, J. F., acknowledgments to........... 11

Cited............ 27,29,101,102,108,110,112,210

King, Clarence, cited.............. 119,123,247-248

King, L.V., cited........................... 27

Kirk, C.T., cited.......................... 153,175

Knopf, Adolph, cited...................... 219,226

L.

Laboratory, analogy in, to sulphide enrichment ............................ 88-89

LaDicha, Mex., ores of...................... 208

ores of, enrichment of.................. 164,208

LakeCity,Colo., geology of................. 240

ores of ............................ 92,109,121,

122,124,134,139,156,160,169,240-241 

enrichment of.................. 85,168,241

Laney, F. B.,cited..................... 46,106-107

experiments of writer and............... 64

Lang, S. S.,cited............................ 194

La Eeforma mine, Mexico, geology at....... 211

ores of...................... 18,162,163,165,211

Latouche Island, Alaska, ores of............ 17,214

Lawson, A. C.,cited ...................... 157,191

experiments of............................ 51

Lead, minerals of, list of..................... 135

ores of, deposition of.................... 10

enrichment of....................... 9,35

occurrence of...................... 136-139

oxidation of......................... 82

solution of......................... 135-130

Lead, native, precipitation of................ 136

Leadhillite, occurrence of.................... 137

Leadville, Colo., geology of.................. 242
INDEX. 257

Page. 

Leadvllle, Colo., ores of................... 120,134,

135,139,142,154,242-244 

enrichment of.......... 34,166,168,243-244

oxidation at............................. 35

Leith, C. K., and Van Hise, C. R., cited..... 23

Lepsius, B.,cited........................... 64

LeRoy, O. E.,cited........................ 209

Lewis, J.V., cited........................... 106

Limestone, oxidized copper ores in.......... 165

Limonite, occurrence of................... 145-146

Lincoln, F. C.,cited......................... 214

Lindgren, Waldemar, acknowledgments to.. 12 

cited.............. 39,40,42,55-56,77,83,85,92,

99,100,103-105,108,109,112,154,

157,188-189,193-194,217,220,222

Lindgren, W., and Ransome, F. L., cited .. 81-82,

229-231 

Lindgren, W.; Graton, L. C., and Gordon,

C.H.,cited............... 34,35,36,99

Little Burro Mountains, N. Mex., geology of. 225 

oresof...........................'.. 166,225-226

Lodes, erosion of............................ 24,47

Lord, Eliot, cited........................... 248

Lower California. See Boleo.

M.

McCaskey, H. D., acknowledgments to...... 12

cited.................................... 222

McCaughey, W. J.,cited .................... 126

MacDonald, D. F.,cited.................... 228

McLaughlin, R. P., cited.................... 225

Maddren, A. G.,andMoffit,F. H.,cited.. 17-18,216 

Magmatic segregations, deposition of......... 13-14

Magnesium, presence of, in mine waters...... 69

Magnetite, occurrence of................... 146-147

Malachite, occurrence of...................... 101

Manganese, minerals of, list of............... 130

minerals of, source of, in gold deposits. 130-131 

oxides of, effect of, in gold precipitation. 128-129 

presence of, in mine waters.............. 69

sulphides of, occurrence of............... 131

Manhattan, Nov., geology of................. 222

oresof............................. 166,167,222

Marcasite, occurrence of................... 149-150

Maryland, ores of............................ 158

Marysville, Mont., geology at................ 226

ores of................................. 124,226

enrichment of................. 166,169,226

Melantcrite, occurrence of.................... 147

Metasomatic replacement, sequence of....... 84-86

Meteoric water, depth of penetration of...... 28-30

stagnation of............................ 30-31

Mexico, enrichment in..................... 162-165

mine waters from, analyses of............ 61

64,69,72,90 

oresof.................... 23,33,98,103,122,157

oxidation in............................. 33

sulphides in............................. 18

See also Cananea; Nacozari; Lower California; La Dicha; San Jose; La Reforma 

mine; Velardefla. 

Miami, Ariz., geology of..................... 185

mine water in........................... 184

oresof........................ 10,43,44,185-186

enrichment of............. 164,165,185-186

Mine waters, analyses of..................... 60-63

analyses of, discussion of................. 63-70

72293° Bull. 529 13 17

Page. 

Mine waters, composition of................. 60-74

changes in, with depth........ 70-73,89-90

precipitates from........................ 73-74

Mining districts, review of................. 170-252

Missouri, ores of............................. 138

Mofflt,F.H.,and Maddren, A. G.,'citcd.. 17-18,216 

Mohave, Cal. See Exposed Treasure mine. 

Montana, chalcocitein....................... 94

enrichment in........... 33-34,164-166,168-169

erosion of lodes in....................... 47

mine waters from, analyses of... 60,63,64,69,70 

ores of............... 33,99,100,102-106,110-113

119-125,128,130,137,139,141,145,153,159 

oxidation in....................... 33,37,38,40

See also Butte; Marysville; Philipsburg;

Neihart. 

Morenci, Ariz., geology of......... ^.......... 187

oresof...................... 33,38-39,42,77,83-

84,85,92,98-99,100,102,103,104-105,108,

109,112,142,146,153,154,157,159,187-189

enrichment of........... 40,44,164,188-189

oxidation at...................... 33,36,38,189

Mother Lode district, Cal., geology of........ 216

ores of................................. 166,216

Mowbray, J. M.,cited....................... 18

Moyie, B. C.,geology of...................... 249

oresof...................... 18,168,169,249,250

N. 

Nacozari, Mex., geology of................... 196

oresof...................... 46,125,162,163,196

Nantokite, occurrence of................... 100-101

Neihart, Mont., geology of................... 234

 ores of............. 120,121,123-124,139,234-235

enrichment of..................... 168,235

oxidation at............................. 234

Nevada, enrichment in............. 22,164,166-169.

oxidation in............."................ 36-37

mine waters from, analyses of............. 60,

62,63,65,69,70,71 

ores from........................... 22,80-81,

102, 108, 109, 116, 119-123, 130, 134,

146,148,149,153, 154,155, 157,159,160

analyses of.......................... 79-80

oxidation in............................. 36

See also Ely, Nov.; Yerington; Silver 

Peak; Edgemont; Manhattan; 

Bullfrog; Gold Circle; Delamar 

mine; Goldfleld; Eureka; Pioche; 

Tonopah; Comstock lode. 

Nevada City, Cal., geology of............... 216-217

mine waters from, analyses of... 62,71,72,74,87 

oresof............................. 1.24,166,217

New Hampshire, oxidation in............... 25

New Jersey, ores of........................ 106,142

New Mexico, enrichment in......... 34,35,164-166

ores of.................... 34-36,99,103,120,142

oxidation in.......................... 34,35,36

See also Santa Rita; Hanover; Burro 

Mountains; Little Burro Mountains. 

New Zealand, enrichment in................ 167

Nickel-copper ores, occurrence of............ 14

Nitrates, presence of, in mine waters......... 68

Nodules, source of........................... 33-35

North Carolina. See Virgilina district.

Norway, oresof............................. 106
258 INDEX.

0. Page. 

Oklahoma, ores of........................... 107

Ontario, enrichment of.................... 162-163

ores of.................................. 14,119

oxidation in............................. 18,25

See also Sudbury; Cobalt. 

Ophir, Cal., geology of....................... 217

ores of............................. 166,217-218

Ore deposits. See Primary deposits; Secondary deposits. 

Oregon, enrichment in....................... 164

See also Queen of Bronze mine; Bohemia. 

Organic matter, precipitation by............ 16

Ostwald,W., cited.......................... 115

Oxidation, changes due to................... 78-83

Chemistry of...... 9,15,16,21,32-33,41-42,75-86

depth of............................. 36-37,165

relation of, to occurrence of chalcocite............................. 163,165

relation of, to ancient topography..... 10,23-24

replacement due to, order of............. 84-86

See also Weathering. 

Oxidized zone, characteristics of............. 31-33

depth of................................. 36-39

mine waters in.......................... 70

ores of, texture of........................ 44

relation of, to water table................ 37-38

segregation of metals in.................. 33-35

solution and precipitation in............. 32-33

transition of, to secondary sulphide zone. 38-39

P. 

Pack, F. I., cited............................ 245

Paige, Sidney, cited....................... 194,226

Paige, S., and Wright, C. W., cited........ 214-215

Paleotopography. ' See Topography, ancient. 

Bearce, Richard, cited..................... 126,231

Eearceite, occurrence of.................... 123-124

Pegmatite veins, deposition of............... 13,14

deposits in.............................. 14

Eenfield, S. L., cited........................ 124

Pennsylvania, ores of........................ 1.58

Penrose, A. F., jr., acknowledgments to..... 11

cited.................................. 138,230

Permeability, relation of, to occurrence of

chalcocite..................... 163,105

relation of, to enrichment............ 20-21,170

Ste also Pore space. 

Philippines, enrichment in................ 166-167

Philipsburg, Mont., geology of............... 233

ores of................................... 112,

120,121,122,128,130,134,145,169,233-234 

enrichment of.............. 47,166,168,234

Bhillips, J. A., cited......................... 169

Phosphates, presence of, in mine waters.... 68

Physical- conditions, relation of, to enrichment.. .......................... 16-26

See also Climate; Relief; Altitude; Permeability; etc. 

PinardelRio, Cuba, ores of................. 209

oresof; enrichment of.................. 164,209

P.joche,.Nev., geology of..................... 245

ores of............................. 168,169,245

Pisanite, occurrence of....................... 100

Pogue, J. E,, thanks to..................... 59

Polybasite, occurrence of.................... 123

P-ore space, deposition in.................... 21

See also Permeability.

Page. 

Potassium, presence of, in mine waters...... 68

Precipitation, chemistry of.................. 9,15,

16,21,32-33,43,46,48-60 

order of.............................. 57,84-86

Primary deposits, age of, relation of, to enrichment.!...................... 22-23

alteration of............................. 13

classification of.......................... 13

definitionof.............................. 12

composition of, as affecting depth of

secondary zone.................. 92-94

genesis of................................ 13

See also Cold solutions; Thermal waters; 

Syngenetic deposits; Epigenetio 

deposits. 

Primary zonje, ores of........................ 31

Prince of Wales Island. See Kasaan Peninsula. 

Prince William Sound, Alaska, geology of.... 214

oresof........................... 17,162,163,214

enrichment of...................... 164,214

Proustite, occurrence of.................... 122-123

Pseudomorphous replacement, occurrence of. 45 

Purington, C. W., cited...................... 228

Pyrargyrite, occurrence of................. 121-122

Pyrite, effect of, on depth of secondary zone.. 93-94 

occurrence of.......................... 148-149

oxidation of, chemistry of............. 48-52,56

relation of, to occurrence of chalcocite.... 163

Pyromorphite, occurrence of............... 136-137

Pyrrhotite, effect of, on depth of secondary

zone......................... 93-94,165

occurrence of............................ 150

relation of, to occurrence of chalcocite.. 163-165 

to gold and silver migration......... 168

Quartz, deposition of...................... 155-156

Queen of Bronze mine, Orcg., ores of....... 164,204

oxidation in............................. 204

R.

Rainfall, relation of, to enrichment.......... 18-19

Ransome, F. L., acknowledgments to........ 12

cited..... 23,32,35,36,37,38,43,45,100,101,105,

108, 112, 122, 129, 138, 142, 146, 148, 149,

153,162,200-201,216,228-229, 233, 237-239

paper by, on Bisbee district........... 179-182

on Globe and Miami districts...... 183-186

on Jerome district................. 192-193

on Ray district.................... 186-187

Ransome, F. L., and Calkins, F. C., cited.... 252

Ransome, F. L., and Lindgren W.> cited....81-83,

229-231 

Ray, Ariz., copper ores of.................. 186,187

geology of................................ 186

oresof.................................. 43,187

enrichment of.................. 164,165,187

Raymond, R. W., cited..................... 227

Reduction, rate of........................... 91-92

Reid, J. A., cited.................. 116,134,202,248

Relief, relation of, to enrichment............. 19-20

Rhodesia, sulphides in....................... 18

Rhodochrosite, occurrence of........... 130-131,162

Rhodonite, occurrence of.................. 130-131

Rickard, T. A., cited....................... 32,126
INDEX. 259

'Page. 

Rico, Colo., geology of....................... 240

ores of......................... 122,123,154,240

 Rosier, H., cited............................. 153

Rossland, B. C., geology of.................. 251

ores of......................'............ 251

Russell, Clarence, aid of....................; 66

S.

St. Eugene mine. See Moyie.

Sales, Reno, cited........................... 37,

38,77,105-106,110, 111, 113,174,175 

San Francisco district, Utah, geology of...... 197

ores of.................. 85,109,143,162,197-199

enrichment of..................... 198-199

San Jose, Hex., geology of................... 210

ores of............................. 162,163,210

San Juan, Colo., geology of................ 228-229

ores of............................. 124,128,229

enrichment of....................... 229

Sankowsky, N., aid of....................... 65

Santa Rita and Hanover districts, N. Mex.,

geology of....................... 191

ores of....................... 44,99,103,191-192

enrichment of................. 164,105,192

Santiago, Cuba, ores of...................... 208

ores of, enrichment of.................. 164,208

Schofleld, S. J., cited........................ 249

Schuermann, E., experiments of....... 57-59,84-85

Scope of paper............................... 10-11

Secondary deposits, definition of............. 12-13

Secondary sulphide zone, characteristics of.. 31,

39-40 

depth of, dependence of, on composition

of primary ore................... 92-94

dependence of, on fracturing......... 92,94

marooning of, above water table......... 42

ores of, texture of........................ 44-46

position of............................ 31,39-40

precipitation in.......................... 42-44

relation of, to water table............. 39,41-42

sulphur in, source of..................... 87-88

transition to, from oxidized zone......... 38-39

Sedimentary beds, deposits in............. 13

Seidell, Atherton, cited...................... 143

Sericite, occurrence of..................... 152-153

Shasta County, Cal., geology of.............. 199

ores of......... 106,111,112,131,142,159,199-200

enrichment of..................... 164,200

Shearer, H. K., aid of....................... 65

Siderite, occurrence of..................... 147-148

Siebenthal, C. E., acknowledgments to...... 12

Silica, deposition of........................ 156-157

presence of, in mine waters.............. 68

solution of.............................. 155

.Silver, bromide of, solution of..... ........ 115

chlorides of, movement of................ 116

solution of..................... 116,134,168

Iodide of, solution of..................... 115

manganese and, relations of.............. 168

migration of........................... 168-170

minerals of, chemical relations of.....'... 115

. Ustof................................ 114

occurrence of...................... 118-125

relations of........................ 114-118

 ores of, deposits of..................... 168-170

enrichment of....................... 9

See also particular districts.

Page.

Silver, precipitation of. 64,55,95,97-98,116-118,168 

presence of, in mine waters.............. 70

pyrrhotite and, relations of............ 168-169

solution of................... 95,97,114-116,134

sulphate of, solution of................... 115

sulphides of, enrichment of.............. 40

precipitation of...................... 85

tellurides of.............................. 135

Silver, native, occurrence of............... 118-119

Silver Bell, Ariz., geology'of................. 197

ores of................................... 197

Silver mines, waters of, analyses of.......... 62-63

Silver Peak, Nov., geology of................ 218

ores of................................. 166,218

Simpson, J. F., cited...................... 106,175

Smith, Alexander, cited..................... 115

Smithsonite, occurrence of................. 141-142

Snowstorm mine, Idaho, ores of.... 36,105,165,202

oxidation at............................. 202

Sodium, presence of, in mine waters......... 68

Solution, chemistry of....................... 9,

15,16,21,32-33,43,46,48-60,75-86 

factors in................................ 75-76

order of............................ 57,59,76-78

rate of................................... 75-77

in oxidizing environment............ 76-78

in reducing environment............ 83-84

zone of. See Oxidized zone. 

Sommerlad, H., experiments of.............. 123

Sonora, Mex. See Nacozari.

South Carolina, enrichment in............... 167

South Dakota. See Black Hills.

Spain, chemistry of leaching in.............. 53-54

enrichment in........................... 164

mine waters from, analyses of............ 70

. oresof.................. 97,111,112,125,134,142

Spencer, A. C., acknowledgments to......... 12

cited........................... 52,108,218,219

paper by, on Ely, Nov., district....... 189-191

Sphalerite. See Zinc, sulphide of.

Spurr, J. E., cited.......................... 79--81,

118,158,218,241-242,245-249 

Spurr, J. E., Garrey, G. H., and Ball, S. H,,

cited............... -23,149,232,236-237

Stagnant water, depth to..................*. 30-31

Stephanite, occurrence of.................... 123

Stevens, H. T., cited.............. 193,203,209-210

Stokes, H. N., cited..... 78,90,108-109,126,133,158

Stone Hill, Ala., oresof...................... 207

ores of, enrichment of................. 164,207

Structure, relation of, to enrichment......... 24

Sudbury, Ont., geology of................... 212

oresof................ 10,17,18,162,163,212-213

enrichment of....................... 212

Sullivan, E.G., experiments of........... 54-56,100

Sulphates, deposition of...................... 9

formation of.............................. 65-66

migration downward of.................. 9,32

occurrence of.......................... 158-161

presence of, in mine waters.............. 65-66

solubility of........................ 53,157-158

Sulphate solutions, acidity of................ 65

acidity of, decrease of, with depth.. 70-73,89-90 

deposition by............................ 9,13

oxidation of............................. 90-91

Sulphide deposits, progressive alterations of,

with depth...................... 31

zone of................................... 31-47
260 INDEX.

Page.

Sulphides, enrichment of. See Enrichment, 

enrichment of, zone of. See Secondary

sulphide zone, 

oxidation of........................ 9,65,75-86

chemical changes during............. 78-83

factors in............................ 75-76

reduction of, relative rate of............. 91-92

relative electromotive force of...... 50-51,76,91

relative precipitation of.................. 57-59

relative solubility of............... 57,59,76-78

replacement of, by secondary sulphides,

order of....................... 57,84-86

solution of, factors in.................... 75-76

rate of............................... 75-77

in oxidizing environment........ 76-78

in reducing environment........ 83-84

See also Secondary sulphides. 

Sulphide zone. See Secondary sulphide zone. 

Sulphur, occurrence of.................... 158-161

solution of............................ 157-158

Sulphur, native, occurrence of............. 158-159

Sulphuric acid, formation of................. 65-66

presence of, in mine waters.............. 65-66

Summit, Colo., geology at................. 226-227

ores of................................... 227

enrichment in....................... 227

Syngenetic deposits, classes of............... 13

T. 

Tarr, W. A., cited........................... 107

Temperature, relation of, to enrichments. - -. 16-18 

Tenuantite, occurrence of................. 113,125

Tennessee. See Ducktown.

Teuorite, occurrence of...................... 104

Tetrahedrite, occurrence of............ 113,124-125

Thermal waters, deposition by.............. 13,15

deposits of.............................. 15-16

Tiutic, Utah, geology of...................... 235

ores of............................... 32,36,37,

77,113,121,156,157, 159, 161, 165, 235-236 

oxidation at............................. 36,37

Tolraan, C. P., cited........................ 189

Tonopah, Nov., mine waters from, analyses

of............................... 63

oresof.......... ........... 36,79-81,116,119,

120,121,122,123,148,159,169,245-246 

analyses of.......................... 79

enrichment of.............. 12,168,245-246

oxidation at......................... 36-37,246

Topography, ancient, relation of, to oxidation 10,

15,23-24,37,42-43,182

Tovote.W.L., cited....................... 196

Treadwell. See Douglas Island.

Tutton, A. E. H., cited..................... 159

Tyson, J.W., cited......................... 208

U.

Underground circulation, relation of, to enrichment ........................ 26-31

Utah, enrichment in...................... 162,165

mine waters from, analyses of........... 73

oresof................................. 33,38,

99,103-105,109, 111,113,121,124,

134,138,143,148,156,158,159,161

oxidation in.................... 33,36,37,38-40

See also Bingham; San Francisco; Annie 

Laurie mine; Tintic.

V. 

Vadose water, definition of.................. 28

depth of................................. 28-29

Page. / 

Vadose zone, oxidation in................... 42 ^

sulphides marooned in.................. 36,42

Vancouver, B. C., geology of................ 201

oresof................................... 201^

Van Hise, C. R., acknowledgments -to....... 11 '

cited................................... 26,251

Van Hise, C. R., and Leith, C. K., cited..... 23

Veinlets, secondary, occurrence of........... 45-46

Velardefla, Mex., geology at............... 211-212

oresof....................... 18,23,162,163,212

Vermont. See Ely, Vt.

Virgilina district, Va.-N. C., ores of.......... 46,

106-107,112,164

oresof, enrichment of.................... 46,87

Virginia. See Virgilina district; Gossan lead. * 

Vogt, J. H. L., cited.... 26,115,118-119,139,158,169 ;

experiments of.......................... 52

W.

Water, underground. See Vadose water; \ 

Meteoric water; Ground water; 

Stagnant water. 

Water table, level of......................... 27-28 :

ores oxidized beneath.................... 37-38 ,

Watson, T.L.,cited........................ 207 '

Weathering, effect of, on sulphide ores...... 9

See also Oxidation. 

Weed, W. H., acknowledgments to.......... 11

cited........... 43,92,109,111,113,123-124,139, ,

174-176, 203, 207, 208, 226, 235 I 

Weigel, 0., acknowledgments to............. 12.

experiments of....................... 57-59,84

Wells, R. C., acknowledgments to........... 12'

cited.................................... 128

experiments of. 58,59-60,83-84,88,91-92,109,136 

WiJlemite, occurrence of................... 142,155

Winchell, H. V., acknowledgments to....... 12

cited.............................. 108,113,175

experiments of....................... 48,51-52

Winslow, Arthur, occurrence of.............. 150

Wright, C. W., and Paige, S., cited........ 214-215

Wurtz, Henry, cited......................... 125

Wurtzite, occurrence of...................... 143

Wyoming, enrichment in.................... 1G4

ores of............................... 38,39,10!)

See also Encampment district.

Y. 

Yates, W. S.,cited.......................... 09

Yeatman, Pope,cited....................... 23

Yerington, Nev., geology of................. 250

oresof............................. 109,200-201

enrichment of....................... 201

Z. 

Zinc, minerals of, list of..................... 140

minerals of, occurrence of.............. 141-143

solution of......................... 140-141

oresof, deposition of..................... li>

enrichment of....................... 9,35

oxidation of.......................... 41-42,82

presence of, in mine waters.............. 70

sulphide of, effect of, on depth of secondary zone......................... 93-94

occurrence of.............. 142-143,163-165

precipitation of..................... 89,141

relation of, to gold and silver...... 169-170

solution of......................... 140-141

Zinc blende. See Zinc, sulphide of.

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