A partial description of the copper mines of Copperopolis will be found in the “Geology of California,” Vol. I, pp. 254-257. The depth of the main shaft in the “Union” is now stated to be a little over five hundred feet, and the greatest depth reached in the “Keystone,” is said to be five hundred and sixty feet. All the deposits of ore here worked lie parallel with the strike and dip of the inclosing strata. The great ore mass of the “Union” Mine forks or divides into two branches towards the northwest; and at the lowest depth now reached, its width or thickness, after having reached a maximum, is again diminishing. In the “Keystone” Mine there have been two separate and nearly parallel bodies of ore worked to a considerable extent, and a third one was struck last spring previous to the suspension of work in the mine. The two main bodies of ore in this mine have “pinched out” or disappeared in various directions in their lines of strike and dip. They seem to have an irregular lenticular form, and together with the great mass of the “Union” appear to lie in what are called “shoots,” which pitch at an angle of 50° or 60° in the direction of the strike towards the northwest. The northwesterly prolongation of the strike of the great “Union” deposit does not coincide with either of the “Keystone” deposits, but passes east of them. There have been other and smaller deposits in the “Union” ground, more or less worked, lying west of the main body, some of which may possibly connect with the “Keystone” shoots, though the best information I could obtain leads me to think otherwise, and that they were probably isolated lenticular masses. The mass of the great deposit in the “Union” Mine consists of an intimate mixture of chalcopyrite and iron pyrites, containing on an average sixteen to seventeen per cent. of copper. Well defined selvages are not to be seen at Copperopolis, and the country rock is impregnated in all directions, sometimes to a considerable distance from the purer ore, with more or less finely disseminated copper and iron pyrites. In Europe it would pay to crush and work much of the wall rock itself for the copper which it contains; but here it is entirely worthless, as even ten to twelve per cent. ore is not worth mining and shipping at present prices.
It will be seen that the more recent and deeper developements in the Copperopolis mines have only served to confirm the opinion expressed two years ago by the State Geologist (Geol. Vol. I, p. 225) that “the deposits of copper ore in this region, like nearly all the others in California, do not appear to be included in regular fissure veins, but rather to form independent masses [the italics are mine] lying in the direction of the strike of the inclosing rocks, and dipping with them.” It seems, further, that they are here arranged in some sort en échelon. There is no evidence whatever of the existence here of a regular and continuous vein of copper ore, stretching for miles through the country, as some have supposed. (See Ross Browne’s Report, p. 144.)
The finding of “copper indications,” i.e., of small and isolated bodies of ore, distributed with some constancy through a narrow belt of country, for no matter how many miles in length, is anything but conclusive evidence of the existence beneath of a regular vein of corresponding length (which, by the way, if it existed, would be an anomaly in the mining world)—especially when all the developments of the most extensive workings hitherto made point so decidedly and strongly to the opinion that there is no true vein at all. Such “indications” are however evidences, so far as they go (and they go a good way in this direction) of the probable existence of other large bodies of ore distributed here and there along the belt in question. It is not improbable that such may be found in the future, and it would not be strange even if some of them should surpass in magnitude and value the great deposit of the “Union,” which has already yielded such enormous quantities of copper, and is yet far from being worked out.
A description of the auriferous deposit of Quail Hill, in the Gopher Range, together with a similar one at Whisky Hill (called also the “Harpending Mine”) in Placer County, by Prof. B. Silliman, was read before the California Academy of Natural Sciences, at their meeting of April 15th, 1867, and will be found in their published “Proceedings,” Vol. III, pp. 349-351. This paper describes well the particular deposits in question, as well as the general appearance and character of the formation in which they occur. Such deposits, however, are not confined to one or two localities; but there are other points in Calaveras County at which gold is known to exist in considerable quantity, and with similar mode of occurrence. Among these I may mention Quail Hill No. 2, near the Napoleon Copper Mine, two or three miles southeast of Quail Hill No. 1, and the “Plymouth Rock,” or “Austin and Hathaway” claim, at Rich Gulch, near the Calaveras River. Moreover, the geological causes and the peculiar chemical decomposition of the rock, which have been involved in the formation of the deposits in question, are by no means confined to the localities where gold is known to occur. On the contrary, they may be traced with considerable constancy through a narrow belt of country along the southwest flank of the Gopher Hills, and stretching from the Calaveras River southeast for a distance of at least fifteen miles, and perhaps farther. Towards the northwest, the same belt crosses the Calaveras; but how much farther it extends in this direction I have no present means of knowing. It is not unlikely that a similar formation may be found to exist, here and there at least, in the same general line of strike, nearly parallel with the stratification of the country, through Amador and El Dorado Counties to Placer, and perhaps beyond. The possibility of this at least is worth remembering. Throughout this belt, in the Gopher Range, surface cuts and shafts, of greater or less depth, made and sunk in prospecting for copper, are of frequent occurrence. In fact, this is the same belt that has been so often mentioned as “the second important copper-bearing belt of Calaveras County,” and located some six or seven miles southwest of the main copper belt of Copperopolis. The “importance” of this belt, on account of the copper ores which it contains, has been most grossly exaggerated. An amusing illustration of this fact is to be seen in a “map of the copper mines in Calaveras County,” published a few years since, which represents the whole region in question as literally covered for miles with highly colored “locations” or “copper claims,” the whole of which, with few exceptions—and these due not to copper but to gold—have served no further end than that of rendering their locators and owners sadder and wiser men. At one locality, indeed, viz, the “Napoleon Mine,” a body of copper ore was found which in many countries would have been remunerative, and was worked to a considerable extent; but the working here was attended only with loss, and was some time since entirely discontinued. It should be remembered, however, in speaking of the copper mines of California, that not only have they had to contend with the general ignorance of copper mining, and especially of copper metallurgy which has existed throughout the State, and with extremely high prices for labor and transportation; but also that, for a year or two past, the largely increased supply of ore from the mines of Chili in South America, and elsewhere—together with the diminished demand and consequent low price for metallic copper, reacting with increased effect upon the value of the ore—have told with crushing weight even upon the best mines. There are certainly not more than one or two, perhaps not even a single deposit of copper ore in the known world, which surpasses or equals, in magnitude and intrinsic richness combined, that of the “Union” Mine of Copperopolis; and yet it is said that even the “Union” itself, which is the only mine now active at Copperopolis, is hardly more than paying expenses at present rates. So far then as my observations extend, there is simply nothing whatever in this “second copper belt” which can for some time to come justify the expenditure of money in searching for copper here; though it is not impossible that, besides the “Napoleon” Mine, other deposits of ore may exist within the belt, which at some future time, and under more favorable circumstances of labor, fuel, and transportation, may become of value for the copper which they contain.
It has been already remarked that the zone or belt of surface decomposition in which the “Quail Hill” and other similar mines occur, may be traced with considerable constancy for at least fifteen or eighteen miles, and that it is not improbable that it is much longer than this. We cannot, however, infer from our present knowledge that the decomposed or “calico” rock is continuous throughout the belt, or even for any considerable portion of its length. On the contrary, its distribution within the belt appears capricious and local, i.e., it seems to occur in more or less detached and isolated masses, which vary largely in form and size, and are irregular and indefinite in outline; so that little more can be predicated of their occurrence in general, than that they are mostly confined within a comparatively narrow belt, and that their longest dimension exhibits a general tendency to approximate parallelism with the axis of the belt, and the stratification of the inclosing country. Sometimes, as for instance, along the northeastern side of the Quail Hill formation, this tendency is so strongly developed, and the passage from the decomposed to the undecomposed rock is so rapid, as to form for some little distance a tolerably straight and well defined “wall” or line of demarcation, parallel, or nearly so, with the stratification of the country. But the change or passage from the decomposed or “calico” rock to the surrounding undecomposed country, though sometimes rapid is always gradual, so far as I have seen; and though we cannot yet speak much from underground explorations, the surface appearances throughout the country would indicate decidedly that so regular a line of demarcation as this at Quail Hill is the exception, and not the rule. The southwestern limit of the decomposed mass of Quail Hill has been found at several points; but here the change from the decomposed to the undecomposed rock is not so rapid; and though the explorations here, being shallow and limited, are insufficient to determine this point with certainty, it is not probable that any such regularity of demarcation exists here as upon the opposite side. Most of the “calico rock” of this belt still retains distinctly the structure of the undecomposed rock from which it was formed. The crystalline hornblendic rock is thus seen to have been largely altered by the decomposing agency, and even the hornstone, which lay in its track, seems to have been more or less affected by it. The decomposition has been purely an oxidation, accompanied by such mechanical and chemical changes as filtering mineral waters might produce. It is probably superficial, both in origin and character, extending to no great depth, although the main level at Quail Hill is nearly one hundred and twenty feet beneath the summit of the hill, and the decomposition of most of the rock at this depth, so far as exploration has gone, is as perfect as at any higher level. It is certainly long subsequent in date to the metamorphism of the surrounding country, and is unquestionably largely due to the action of the products of the oxidation of metallic sulphurets (chiefly those of iron and copper) which were originally distributed through the rock. At the same time it is not easy to account for the whole of it in this way alone, since at certain localities undecomposed sulphurets are seen near the surface, and in rock which is apparently much more permeable to atmospheric influences than was much of that which has been more deeply decomposed; and again, much of the decomposed rock, though retaining well its original structure, shows far too little traces of sulphurets to readily account for so general and thorough a decomposition as has taken place. It is all indeed more or less colored by oxide of iron, but much of it is not deeply colored, and the undecomposed hornblendic rock itself, in the absence of all sulphurets, contains sufficient iron in the state of protoxide to impart a strong coloring when the rock is decomposed and the iron passes to the state of sesquioxide. Much of the iron originally present has undoubtedly been removed in a soluble form, as sulphate, etc. But in rock which preserves its original structure, as well as most of this does, pyrites, if originally present, would have left traces of its existence in the form of casts or cavities in the decomposed mass, which might or might not have been filled with ferric oxide or other matter. In certain localities the decomposed rock is in fact filled with such cavities, often cubical in form, attesting the former presence of large quantities of disseminated sulphurets. But in other localities they are few and far between, and here accordingly the decomposition can hardly be supposed to have been due to the local presence of sulphurets alone.
The exact methods by which the general and local decomposition has been effected, and those by which the rock was originally impregnated with metallic ores—as well as the manner in which certain substances, as barytes, now found as sulphate, and true porphyry, now found as kaoline or lithomarge, have found their present situation in the belt in question—all these would possess both interest and importance in a high degree, could they be more definitely known. Such questions, however, cannot be answered with certainty, and their discussion here would lead us too far into the doubtful realm of chemical geology.
But whatever may have been the agencies at work, it is evident that there is nothing in all this to remind us of a true vein formation. It appears that the zone in question is neither a vein, nor generally speaking a system of veins. On the other hand, it possesses emphatically in general the characteristics of what the Germans style an impregnation—an impregnation indeed which exhibits a certain regularity as being mostly confined within a narrow zone, and stretching through a considerable extent of country, but which within these limits shows the greatest irregularity of form, and much variety of character. Veins of quartz occur here and there within the belt; but they are not more frequent here than elsewhere, and their occurrence has probably little or no direct connection with the peculiar character of the belt itself. There is very little that deserves the name of quartz at Quail Hill, though much of the surface rock is pretty highly silicious in character.
The impregnation of the rock with metallic sulphurets, particularly with sulphurets containing copper, has in certain localities been sufficiently powerful and concentrated to assume, in greater or less degree, the characteristics of segregated veins of limited extent. This has been the case at the Napoleon mine, and also at Quail Hill, where there is, or was, a band of oxidized ores of copper traversing the decomposed rock in a direction parallel with the general stratification. This band consisted chiefly of the green and blue carbonates of copper, mingled with ferruginous and earthy matter, and accompanied by barytes. The last named mineral, so common a veinstone in other parts of the world, but hitherto so rare in California, occurs here in considerable quantity. Its form is granular compact, sometimes quite pure, but usually contaminated and intermingled with other matters. Crystallized specimens of it have not been found here to my knowledge. It is hardly probable that the barytes itself contains either gold or silver; yet it certainly occurs here in the most intimate contact with both, as I have seen respectable particles of gold in place upon the immediate surface of compact specimens of barytes—and a sample of heavy concentrated barytic sand from the tailings of the mill, of sufficient fineness to pass through a sieve of one hundred holes to the linear inch, yielded to the assay over eleven dollars per ton in gold and silver.
The thickness of the copper band varied from one to three or four feet. Its outlines were indefinite, and its original characteristics of form, etc., much obscured by the complete decomposition both of itself and the surrounding rock. It was without doubt originally a segregated mass of sulphurets; and though it seems now to have nearly or quite run out and disappeared, it may be found to come in again as such, in depth, unaltered below the line of surface decomposition.