Notice of a peculiar mode of the occurrence of Gold and Silver in the Foot-Hills of the Sierra Nevada, and especially at Whisky Hill, in Placer County, and Quail Hill, in Calaveras County, California.

BY B. SILLIMAN.

In the search for ores of copper which occurred in California in what is now known as the “Copper Belt” of the Lower Sierras, deposits of “Iron Rust,” as they were called by the miners, were observed at numerous points far below the range of the main gold belt of the Sierras. Several of these ochraceous deposits had been previously “located” by prospecting miners for gold, before there was any knowledge, or suspicion even, of the existence of ores of copper in connection with them. It was a matter of common observation that certain gulches, or watercourses in the neighborhood of these rusty deposits, were rich in placer gold, having been worked for gold from an early date. The search for copper in this kind of deposit was not commercially successful, although there were shipments of green and blue carbonates of copper, red oxide and metallic copper, to a limited extent from both the localities here referred to, the metal from which was known to contain a notable value of gold and silver, stated to be about fifty dollars to the ton of ore. This search for copper has, however, opened up these deposits so as to display their character in a conspicuous manner.

The rocks appear to have been originally talcose and chloritic schists, sometimes micaceous, inclosing masses of argillite, and of quartz which appears to have been massive enough at certain points to assume the character of a vein, and parallel to the stratification which has the usual north-western strike and easterly dip of the region. All this mass of material which at Quail Hill is certainly three hundred feet wide, and possibly twice that, and with a linear extent exceeding one thousand feet, appears to have been very highly impregnated or mineralized by sulphurets, chiefly of iron, with a portion of copper, zinc and lead. The sulphurets have undergone almost total decomposition throughout the entire mass, leaving soft ochraceous deposits of a rusty red and yellow color, and staining the rocks with brilliant colors, a peculiarity which the miners have characterized by the name of “Calico rocks.” This decomposition or oxidation of the sulphurets, has extended to a point as low as atmospheric influences extend, or probably to a point where water is permanently found, which at Quail Hill is assumed to be about 170 feet below the outcrop of the mass, or crest of the hill. Dykes of porphyry and of other rocks, commonly called intrusive, are seen dividing these great ore channels in a direction conformable to the line of strike. But the decomposition which has affected other portions of the ore channel, appears also to have changed them, for they are found to be reduced completely to the condition of kaolin and lithomarge, or kindred alterations of feldspathic rocks. The outlines of the feldspar crystals are still easily distinguished, although the mass of the dykes is completely friable.

The zinc blende which is found in small quantities at Whisky Hill, and the vitreous copper also to some extent, appear to have escaped decomposition. The copper ores appear to have been confined to a portion of the deposit, as is indicated in the section exhibited, while the auriferous sulphuret of iron has been co-extensive with the ore channel, the cubical cavities left by the decay of its crystals being found in all the outcrops both in the quartz and in the ‘calico rocks,’ resulting from the decomposition of feldspathic and talcose or chloritic constituents.

Accompanying the entire mass of decomposition at both localities, occur both gold and silver, disseminated with remarkable uniformity in all parts of the orey ground. At Whisky Hill, films of metallic silver are visible upon the talcose masses stained green by malachite or chrysocolla; the gold is rarely seen in situ, being mostly obscured by the very rusty and highly-stained character of the associated materials. But it is rare that on washing a small quantity of any of the contents of these great deposits, gold is not found in angular grains or small ragged masses, from the size of a few grains’ weight, to impalpable dust. Nuggets of several pennyweights occur occasionally. This gold has evidently accompanied the sulphurets and been left in its present position and condition by their decomposition. There can be little doubt that the gold of the gulches adjoining these deposits has been derived from them. At Whisky Hill, the gulch gold ceases to be found as soon as the limits of this deposit are passed, and the same is true at Quail Hill. The occurrence of deposits of this nature throughout the range of the foot-hills, seems to offer the best solution which has suggested itself of the origin of the placer gold which is found in situations so far removed from the gold belt of the upper Sierras, and away from sources usually recognized as those to which placer gold may be referred.

Experiments made by myself and by others on a considerable scale, the details of which will appear elsewhere, show that the amount of the precious metals disseminated in the average mass of vein stuff and decomposed materials of every name at Quail Hill, is considerably in excess of the general average tenor of gold veins in California. The mean of my own trials gave to the ton of 2,000 lbs. by assay:

Gold$35.14
Silver15.08
$50.22

While from the working of carefully prepared averages in considerable quantity by milling process, the tenor of the precious metals was:

Gold$29.18
Silver5.91
$35.09

The extremely friable condition of the entire mass of these auriferous materials renders their extraction and treatment easy and comparatively inexpensive.

At Whisky Hill, a mill of forty stamps has been set up which is now running with satisfactory returns. The cost by contract of delivery of the ores to the mill, being stated at forty cents (40c.) per ton, the cost of mining and treatment in mill being considerably less, it is said, than one dollar per ton, the amount treated being five tons to each stamp.

The chemical results of the extensive decomposition of metallic sulphids which has in former times occurred at these localities, offer an interesting problem in chemical geology. The sulphur has been removed chiefly as sulphuric acid beyond doubt, which has combined with iron and copper to form sulphates of these metals. These have for the most part disappeared, being washed out by the atmospheric waters, and have followed the drainage of the country. At Whisky Hill I found the sulphate of iron, (Coquimbite) sulphate of copper, (Cyanosite) and alum. The water of the shaft contains copper enough to redden the iron tools. At Quail Hill considerable masses of heavy spar are found, formed probably from the action of soluble sulphates upon witherite. No gypsum was observed at either locality.

The mineral species observed at Whisky Hill, are as follows:

The list of species is about the same for the deposit at Quail Hill.

The line of division between the ore-bearing ground in these great ore channels, and the country rock is quite distinctly seen on both the eastern and western outcrop at Quail Hill, and on the western at Whisky Hill. At the former place it is a dark bluish porphyritic rock, probably metamorphic, of a sandstone or silicious sediment. The outcroppings resemble those of many quartz veins, and I find the moss-covered portions of this quartzose matter full of cavities, resulting from the decay of pyrites, and yielding, by assay, three to five dollars to the ton in bullion.

From all the evidence presented, we seem justified in regarding these remarkable metallic deposits as segregated veins, holding a pretty uniform and high tenor of gold and silver, associated with and derived from the decomposition of extended masses of metallic sulphurets and quartzose matter, and carrying at times, ores of copper, the commercial value of which is, however, entirely subordinate to that of the precious metals which are found to characterize these veins or ore channels.

San Francisco, April 15th, 1867.

Mr. Falkenau read a communication “On the Spirit of the Age and its Influence in the Department of the Natural Sciences.”

Mr. Bolander exhibited specimens of the Apocynum found in Round Valley, on moist land subject to overflow. The Indians make extensive use of it for fish-lines and other purposes. The specimens presented were collected by Mr. J. S. Silver, of Humboldt Valley, Nevada.

A field excursion of the Academy was made, on Saturday, April 20th, to the hills near the Twelve-Mile House, on the San José Railroad. By the courtesy of Richard P. Hammond, Esq., General Superintendent of the road, free passes to go and return were furnished to the members participating in this excursion.