Geological sketches of the precious metal deposits of the western United States

Chapter I.—GEOLOGICAL SKETCH OF THE PACIFIC DIVISION

By George F. Becker.

It is the purpose of this chapter to present such au outline of the geology of each of the states and territories west of the Rocky mountains as will serve to assist those unfamiliar with the country in forming an idea of the character and distribution of its mineral resources, and to convey such rudimentary information concerning the relations which the ore deposits bear to the larger features of the geological structure as is necessary to an intelligent perusal of the statistical chapters. The information available for this purpose is far from ample. The government geological explorations of the last twenty years have done a large amount of extremely valuable work, some of which has made a permanent mark on the history of the science; but the territory is so vast that many decades must elapse before even the preliminary explorations are completed. The collections and the data gathered by the census experts are also very valuable. Few mines, however, can be properly understood without a somewhat extended examination of the surrounding country, for which the experts had no time, and the information is therefore rather fragmentary.

The order adopted in sketching the states and territories is not that which would have been chosen had the information been more complete. Washington territory and Oregon are placed after California, because little is known of them directly, while certain inferences maybe legitimately drawn from the analogous territory embraced in the last-named state, and Idaho is described after Nevada and Utah for similar reasons.

The regularity of the distribution of ores in the Pacific division and its relations to the singularly uniform topography long ago drew the attention of writers to the resources of this region. Mr. W. P. Blake (a) first published a note on the subject in 1866, and his statement was accepted and enlarged upon by Mr. King (b) in 1870. The more detailed technical and scientific investigations of later years have greatly increased our knowledge of the distribution and extent of the ores, and it will now scarcely be maintained that there are more thaurfour well-defined and continuous ore belts west of the Rocky mountains. Beginning at the east, the first is that at the western foot of the Wahsatch and the southwestern continuation of that range. With the exception of the Leeds (Silver Reef) district, all the important ore deposits of Utah lie in the foot-hills of this range, bearing a very definite relation to the main line of crests. The gold and copper belt of California stands for a long distance in a similar relation to the Sierra Nevada. The quicksilver belt in the California coast ranges is not quite so regular in its occurrence, yet its direction is very nearly parallel to the coast, and it is very persistent, though nowhere broad, for some 300 or 400 miles. The Arizona belt is less known than any of the others, but no one can glance at a map of the territory showing the mining districts without perceiving that these lie in a northwestern and southeastern line diagoually across the country. The mining districts in Nevada are extremely numerous, so much so indeed that some grounds could be given for assuming a belt to run in almost any desired direction, but they are scarcely close or regular enough in any one line to compel the observer to regard them as connected.

These four distinct belts appear to have an intimate connection with the four great orograpliieal changes which the region west of the Rocky mountains has undergone during its geological history. The last of these was post- Miocene, and resulted in the uplift of the Pacific Coast ranges and the great interior valley of California, with a large part of Oregon and Washington territory. The disturbing force seems to have been most powerful to the north and south of San Francisco, or approximately in the region marked by the quicksilver deposits. A post-Cretaceousupheaval raised the whole western central portion of the continent now occupied by the complex system of the a Annotated catalogue of the principal mineral speciea hitherto recognized in California, etc.

b Exploration of the 40th Parallel, iii, p. 6.

Report to state hoard of agriculture,.

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Eocky mountains. The Wahsatch forms the western edge of this uplift, and the dislocation took place on an old fault coincident with the present western foot of that range. Here also lie the numerous mines of Utah. The Sierra Nevada and the ranges of the Great Basin were raised by a post-Jurassic uplift. The line of most intense disturbance coincided with the Sierra, and the greatest dislocation occurred along its western foot, in the gold belt, though it also extended to the south of that wonderful series of deposits. The earliest disturbance in the far west was that which raised the Palaeozoic strata of eastern Nevada, western Utah, and a portion of Arizona above the surface of the ancient sea. The western limit of this Palaeozoic area has been traced in detail across the belt surveyed by the exploration of the fortieth parallel, and in that latitude it trends nearly north and south in longitude 117°30'. "To the south the limit has been fixed at a considerable number of points, though it has not been followed in detail. South of Austin the course of the western edge of the Palaeozoic is somewhat west of south, and it enters California a little north of Owen’s lake. In this region it is deflected toward the southeast, crosses the Colorado river in Virgin canon, passes by Prescott, and on through Arizona to the neighborhood of Tombstone. The main Arizona belt of deposits has the same trend as the border of the Palaeozoic, and nearly coincides with it in position. In short, though the relation still requires much investigation, the Arizona mineral belt appears to stand in nearly the same relation to the western edge of the post-Carboniferous upheaval as do the belts of Utah and California to the other uplifts.

That these relations exist as a matter of fact is beyond question, though it is possible that they may be accidental. In any case, however, the uplifts, as such, are not to be regarded as the cause of the formation of the mineral belts; these must rather be due to the Assuring of the rocks and the dislocations attending the orographical changes. There is evidence that the post-Carboniferous uplift in the state of Nevada and in southeastern California was comparatively gentle, and that it was not attended by any considerable crumpling of the strata. This would account for the fact that the number of ore deposits at its edge in these states is not very large. Nevertheless, the lead deposits of Battle Mountain, the Austin mines, the Candelaria district (which includes the famous Northern Belle mine), Panamint, and Cerro Gordo all occur at or close to the western edge of the Palaeozoic. Taken in connection with the geological similarity of their position to that of the Arizona mines, these deposits may perhaps fairly be regarded as the rudiments of a belt. In Arizona the area in which the contact occurs has been too little investigated to allow of any statement as to the violence of the uplift, but, all things considered, it would be remarkable if it should not ultimately prove to have been attended by much disturbance.

This theory of a relation between the ore-belts and the lines of uplift is, of course, not to be understood as equivalent to the assertion that the deposits are to be found only along a single line representing the actual main fissure of the uplift. One is apt to think of the dislocation attending an orographical change as confined to a single vertical or highly inclined surface, but every geologist is aware that this is not an exact view. Simple fissures in the earth’s crust are very rare, and parallel sets of fissures, with cross fractures and stringers into the surrounding country, are the rule even in the case of insignificant cracks. In disturbances such as those of the great uplifts a considerable belt of country is almost necessarily crushed and torn, and innumerable rents and cracks standing in most complex relations to one another penetrate the rocks in many directions. The breadth of such a zone must usually be measured in miles.

It may be that some of the ore deposits of the Pacific division are independent of volcanic action, but the association of eruptive rocks with ores is a rule with comparatively few apparent exceptions, and in many cases the agency of solfatarie action (a) is manifest. This has long been recognized by observers.

That there are relations between the rocks inclosing ore deposits and the character of the ores has been known to miners for centuries, but the study of the nature of this dependence is comparatively new. It is far too complex a subject to be discussed in this chapter, but it may at least be stated that the census collections and data appear to confirm, emphatically, the existence of such relations. Lead ores are almost invariably accompanied by limestone, and veifis in granite present only a very small number of associations of minerals, which are possibly reducible to a single one. Deposits in metamorphic rocks, too, though more varied than the others, appear to represent but a few types. It was not practicable, however, for the experts to make such minute examinations of the mines as would have been necessary to furnish material for a conclusive investigation of this subject.

With some hesitation most of the determinations of the ore and gangue minerals, the country rocks, and the kind of deposit, arc introduced county by county. There can be no doubt that the list of ore minerals is often imperfect. The determination of the wall rocks is subject to some uncertainty without a thorough examination in the field as well as in the study, and the nature of a deposit is in many cases not to be decided by a single visit. It is probable, however, that the determinations of rocks and minerals are nearly always correct as far as they go, and in the cases in which the character of the ore deposits was not clear as much may generally be inferred from the statement regarding them. The tables, therefore, contain much information of value, and many suggestions to such geologists and miners as are careful to remember that they are not exhaustive statements. Except in a few cases, in which I happened to have visited the localities, the determination of the character of the deposits rests on the authority of the experts. The determinations of rocks and minerals inclosed in parenthesis are also due to the experts, a As originally employed, the term solfatarie action denoted only the effect of gaseous emanations front volcanic vents. In use, however, it has gradually come to include the action of heated waters charged with these gases or holding them in solution, and ia so employed in this chapter.

GEOLOGICAL SKETCH OF THE PACIFIC DIVISION.

while the remainder I have made from the specimens collected by my assistants. The difficulties met with in making these determinations were considerable, for rocks near ore deposits are usually much altered, and the ore minerals need much closer attention than in ordinary specimens of merely mineralogical interest. A few minute particles of such minerals in a hand specimen often make the difference between waste rock and rich ore and the inspection of the samples needed to be correspondingly searching.

Maps of the states and territories of the Pacific division accompany the sketches. These are necessarily on a small scale, but present the leading features sufficiently well to assist the reader in following the descriptions. On them are entered signs indicating the distribution of gold, silver, and quicksilver. These are not designed to represent every spot where precious metals have been detected, but to indicate at a glance their general distribution.

GEOLOGICAL SKETCH OF CALIFORNIA.

The mineral resources of California are extremely varied, but are also of very unequal importance. Its gold production is an essential factor in determining the relations of the mediums of exchange throughout the world, its total value since 1849 exceeding $1,200,000,000. Its quicksilver production, also, though of far less value, exceeds that of any other country. The total known product of California from 1850 to the close of 1880 was a trifle less than 91,000,000 pounds, of an average value of 71 cents, and has yielded above $65,000,000. It is important as an adjunct to the precious-metal industries no less than as an independent source of profit. The silver product yields above a million per year ; and the coal-fields, though not of the best, furnish a large part of the supply necessity for home consumption. Asphalt anti petroleum are obtained in small quantities, and some sulphur and borax are extracted, while lead is reduced only as an incident of the silver industries, and copper and iron, though their ores are plentiful, are worked on a small scale only. Tin, chromic iron, black oxide of manganese, and other useful minerals also occur in the state, but as yet contribute little to its commercial prosperity.

The great industrial importance of the gold production of California has drawn the attention of many geologists and engineers to the geology of the state, and the literature on the subject is comparatively extensive. The Pacific railroad survey, Mr. J. A. Phillips’ work on gold and silver, the reports of the mining commissioners, the proceedings of the California Academy of Sciences, and the scientific journals of America and Europe, all contain contributions to the subject; but the chief source of authority is the volumes of the state geological survey of California, conducted under the charge of Professor J. D. Whitney, who had the assistance of Messrs. Clarence King, W. H. Brewer, W. M. Gabb, ‘William Asliburner, W. H. Pettee, and others. Unfortunately, the legislature ceased to appropriate funds for the survey iu 1874 before a single geological map had been issued. Professor Whitney, however, has continued to work up the material collected, and has issued a number of volumes during the last eight years. The census reports and collections also furnish some information of value from a geological point of view, but the following sketch owes most to the data collected by Professor Whitney and his assistants or recorded in the volumes published under his supervision.

The interior of California forms a long, oval valley. Its greater diameter is parallel to the coast, and extends from the neighborhood of Fort Tejon to Mount Shasta, a distance of 450 miles, while the average width is about 40 miles. This valley is surrounded by mountains, except at a single point, where San Francisco, San Pablo, and Suisun bays afford an outlet for the drainage gathered by the Sacramento river from the north and the San Joaquin from the south.

Though the mountain ranges inclosing this basin unite at its extremities, the Great Valley is not a mere undisturbed area between different ranges of a complex chain; on the contrary, the Sierra Nevada to the east and the Coast ranges to the west represent upheavals of different characters and widely distant eras. The Sierra Nevada is a single range forming the western rampart of the elevated plateau of the Great Basin, and was raised in post-Jurassic times. The Coast ranges consist largely of detritus from the Sierra; they were uplifted for the most part at the end of the Miocene, and constitute a mountainous belt of country to which even the name of chain can scarcely be applied. No term answering to the Coast ranges was used by the Spanish settlers of the country, but they gave special names to a considerable number of small ranges within the Coast belt, and these are still in use. The elevation of the Coast ranges is greatly inferior to that of the Sierra, a number of peaks of the latter exceeding 14,000 feet, while none of the culminating points of the Coast ranges appear to rise more than 6,000 feet above sea level. *

Both the Sierra Nevada and the Coast ranges are greatly metamorphosed and contain extensive deposits of useful minerals, and the alteration of the strata and the deposition of ore are probably in each case related phenomena; but the metamorphosis and ore-deposition of the Coast ranges occurred long after the cessation of similar activity in the Sierra, and led to widely different results. The more remarkable deposits of the Coast ranges are cinnabar, chromic iron, coal, asphalt, and mineral oil, while gold and copper are characteristic of the western slope of the Sierra, lead and more or less auriferous silver occurring very extensively on the eastern slope, of which only a portion lies within the limits of the state.

The backbone of the Sierra is granitic, the higher summits and a large part of the western slope of the range being of this rock, except in the northern portion of the state, where it has been covered by basaltic and

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andesitic lavas. The granite penetrates some of the accompanying strata in dikes, and Professor Whitney regaich it as beyond question of eruptive origin, while some other geologists see in it only highly metamorphosed sedimentary material reduced to a plastic state in situ. Except at the northern end of the Great Valley, near Mount Shasta, and near Owen’s lake, no Palaeozoic strata have been identified. In the Gray mountains, Shasta county, a limited area of Carboniferous limestone occurs, amply identified by fossils. A small amount of limestone with the same external characteristics occurs farther south, and Professor Whitney regards it as not improbably of the same age. From a mining poiut of view, however, it is insignificant, carrying little gold. The principal strata on the west flank of the Sierra are Jurassic and Triassic, occurring chiefly and characteristically as slates and shales. They are highly metamorphic, contain few fossils, and have been profoundly disturbed, showing that the range was uplifted since their deposition. These are the main gold-bearing rocks, and will be more particularly.described further on. Near the foot of the range are areas of Cretaceous and Tertiary beds, chiefly marine, nearly horizontal, and resting unconfqrmably on the upturned auriferous slates. Above the Mesozoic slates lie fresh water auriferous gravels, mainly of Tertiary age, and these toward the north are in part covered by flows of Tertiary and post-Tertiary lavas. Inyo and Mono counties lie to the east of the Sierra. The metamorphic slates and limestones of this region are for the most part Triassic, though the Jurassic is probably also represented, and are covered to a great extent by volcanic rocks. Mr. Gilbert has shown that the eastern edge of Inyo county reaches the Palaeozoic area.

The Jura-Trias strata extend to the east of the Sierra about as far as longitude 117° 30'. They rest directly upon Archaean schists and granite, and the long interval of time which they represent seems to have been extremely q»iet, for no non-conformity has been detected in the series. At the close of the Jurassic, however, the whole area from the western foot of the Sierra to the middle of the state of Nevada was raised above the ocean and compressed from west to east, resulting in the formation of a number of parallel ranges, of which the most westerly were the Sierra Nevada and the Blue Mountain range of Oregon. Ore deposits occur on the eastei n as well as on the western flank of the Sierra, but their character and mode of occurrence differ from those prevailing in the gold belt.

The Coast ranges, or the western mouutaiuous belt between Mount Shasta and fort Tejon, are for the most part composed of more or less altered rocks of Cretaceous and Tertiary age. The geologists aud the paleontologists of the state survey divided the Cretaceous into lower aud upper, and, while recognizing the later divisions of the Tertiary, failed to find anything certainly corresponding to Eocene. Of late, however, it has been shown that the fossils of what had been considered as the Upper Cretaceous exhibit strongly marked Tertiary affinities, aud it seems by no means impossible that the beds in question, which are sometimes called the T6jon group, and include the Monte Diablo coal-fields, really represent the Eocene. Considering that differences of climate must always have existed, whether more or less marked than those of the present time, it is not strange that doubtful cases like those of the T6jon group, the Laramie beds, and the Australian coal-bearing rocks occur, but rather that it is so often possible to determine the correspondence of strata in widely separated areas.

Though the Coast ranges here aud there show granitic rocks, granite is of only local importance, and does not appear to form the central mass, as is the case with the Sierra. The body of these ranges is made up of crumpled and fractured strata, indicating, according to Professor Whitney, sharp and sudden elevations and depressions, extending through the Pliocene epoch. To the southward the prevailing rocks are Tertiary, but north of the bay of San Francisco these almost disappear, the Cretaceous becoming predominant. Volcanic rocks are not widelyspread, most of the known occurrences being found between San Fraucisco and Clear lake.

It appears, therefore, that the elevation of the coast as a whole was comparatively recent. While the quartz veins were forming, aud while the gravels were accumulating on the west flank of the Sierra, the region of the Coast ranges and the Great Valley were wholly or partly under a gulf or sea, shallow in parts and surrounding more or less extensive islands. The existence of this shallow sea must have had an important influence oh the climate of the Sierra, for, supposing the evaporation to have been the same, nearly the whole amount of moisture now distributed through the Coast ranges and the interior of California would have fallen on the Sierra in addition to its present rainfall. But evaporation is considerably more rapid from shallow seas than from deep ones, and the rainfall on the Sierra must consequently" have been enormous. The chief uplift of the Coast ranges took place at the close of the Miocene, and the great metamorphism aud ore deposition are probably for the most part referable to the same period, though it is likely" that the still later volcanic eruptions induced a portion of them. The Pliocene or i>ost-Pliocene disturbances were comparatively gentle, but Professor Whitney regards the break at the Golden Gate, the prevalence of volcanic rocks from that point north to Clear lake, and the disturbances of the Pliocene south of San Francisco bay, as connected phenomena.

The region south of fort T6jon has been much less investigated than the central portion cf the state. It appears to possess some extremely interesting geological features, but also to present unusual difficulties. The San Gabriel range north of Los Angeles has a granitic axis, and it is possible to trace this granite ridge uninterruptedly" through Los Angeles, San Bernardino, and San Diego counties into Lower California, and along the peninsula to within a few miles of the old mission of Santa Gertrudis. (a) The sedimentary- rocks accompanying this granite ridge are for the most part highly- metamorphosed, and are frequently penetrated by dikes of granite. They are nevertheless considered by both Professor Whitney and Mr. Gabb as of Cretaceous and Tertiary age, and the uplift is referred, like that of the Coast ranges proper, to the close of the Miooene.

a Geological Survey of California, ii, 137.

GEOLOGICAL SKETCH OF THE PACIFIC DIVISION.

Besides the bitumen springs of Ventura and Los Angeles counties, there are gold mines in this southern California range, but few details have been published as to their occurrence, and their geological relations are still to be studied.

The character of the rocks of the Coast range shows that the Cretaceous and Tertiary sea near the present coast was shallow, but there is evidence that the Great Valley represents a former depression of immense depth. This, however, would not prevent the gulf at the foot of the Tertiary Sierra from being as warm, for example, as the Gulf of Mexico, for the temperature of the water of a land-locked basin depends on the depth of the inlet to it, and if this is small the water of the basin will be warm.

In Russia and Australia the Silurian is the gold-bearing formation, and Sir Roderick Murchison enunciated the somewhat rash generalization that gold was to be looked for only in the Palaeozoic. In California it is amply proved by rare but characteristic fossils that the gold-bearing sedimentary rocks are Mesozoic. Generalizations similar to Murchison’s have been attempted with reference to ores of other metals, but the simple fact seems to be that eruptive activity or metamorphism is usually a concomitant of the concentration of ores in veins and other allied deposits, and that the older the rocks the greater the general probability that they will have been subjected to action of this description. Iu the search for coal the fact that the important deposits of the best character are confined to one formation has been of great economical value. The geological indications accompanying the occurrence of veins are to be sought, not in the age of the rocks, but in evidences of disturbance and of certain kinds of decomposition of the surrounding country. The decomposition or alteration of rocks in the neighborhood of ore deposits has been but little studied by geologists until lately, for very sufficient reasons; but of the fact of a connection between it and the deposition of ore California affords excellent examples. The “bed-rock” or auriferous slates of the gold belt is characteristically altered, and the metamorphic stratum in which cinnabar occurs are at once recognized by those familiar with them as “quicksilver rock”.

The belt of metamorphic rocks which incloses the greater part of the gold-quartz veins of California is insignificant in width and of little industrial importance south of the southern boundary of Mariposa county. To the north of that line, however, it suddenly widens. Passing northward, the breadth of the l>elt is stated at about 25 miles hi Tuolumne county, 24 miles in Calaveras, 12 in Amador, and 30 in El Dorado. In Placer it is not well exposed, being covered by gravel and volcanic rocks. North of Placer county the metamorphics occupy most of the western slope of the range for a considerable distance, with occasional irregularly distributed patches of granite, but in Butte county the edge of the great lava fields, which occupy much of the surface of northeastern California, are encountered, and cut off the central mining region. The same gold-bearing series seems to reappear in the north¬ western counties, but its character and relations are less well understood, and its industrial importance is smaller than in central California.

As illustrative of the structure of the gold belt, Professor Whitney describes iu some detail the important portion lying between the Merced and the Stanislaus rivers. Starting from the west, or at the bottom of the Sierra, the first rock encountered is horizontally stratified and undisturbed Tertiary sandstone. To this succeeds the belt of Mesozoic metamorphics in nearly vertical strata. The lower edge is composed of talcose and chloritic slates, weathering irregularly, and locally known as u grave stone ” slates. Next comes a wide belt of a dark-grayish green, somewhat porpliyritic, material, which shows a sheeted structure, though not the tine lamination of clay slates. This was known to the state survey as “ porpliyritic green slate”, but Professor Whitney and Mr. Wadsworth are inclined to regard it as a metamorphosed diabasitic tufa. This belt incloses another of argillaceous slate, carrying Jurassic fossils, with which is associated the “mother lode”, or the “great quartz vein”. Accompanying the argillaceous slate and the mother lode is a band of serpentine, (a) In the southern portion of this section the serpentine is confined to the northeast side of the argillaceous slate, but near the Stanislaus river it widens out, occurring in irregular patches and on both sides of the slates.

The strike of the metamorphosed rocks is, as a whole, parallel to the trend of the Sierra, but there are many sharp deflections. The dip of the slates in the southern and central portion of the gold belt is nearly vertical, and usually to the northeast; but in the northern portion, where the belt widens out, the dip becomes irregular, and over wide areas is to the west, becoming flatter as the distance from the crest of the range increases.

Though not confined to the argillaceous slates, or even to the metamorphic strata, the gold-quartz veins of California are more frequent and richer in the argillaceous slates than elsewhere, many fine veins beside the “mother lode” occurring in it. The veins are usually parallel to the stratification, as the following quotation shows: ( b)

A very heavy quartz vein passes a little south of Big Oak Flat, Tuolumne county, cutting the strata of slate in which it is contained at a small angle, the lines of bedding of the wall-rock appearing to run nearly northwest and southeast, while the vein of quartz has a strike of N. 30° \V.; it dips to the east at an angle of 30°, the slates themselves standing nearly vertical. This is, perhaps the most marked instance hitherto observed in the state of a heavy quartz vein differing essentially both in dip and strike from the inclosing rocks.

a The origin of serpentine is a disputed point. If it is a fact, as eminefrt mineralogists have maintained, tha t it occurs as an alteration of hornblende and pyroxene as well as of olivine, there appears to be no difficulty in accounting for its presence in metamorphic rooks. Chlorite and serpentine, however, are occasionally confounded. b Geological Survey of California : Geology, vol I, p. 237.