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NOVEMBER 1868.

XLII. Notes on the Chemical Geology of the Gold-fields of California. By J. ARTHUR PHILLIPS*.

DURING three separate visits to the Pacific coast of North America I have had numerous opportunities of studying the geology of the Californian gold-fields, and of investigating the circumstances attending the chemical and physical changes which have formerly occurred and which, to a certain extent, are still taking place in those regions. Whilst carrying out these researches, various facts have come under my notice which appear to throw some light on the formation of auriferous veins, as well as on the distribution of the precious metal in the rocks in which it is found.

I have endeavoured to embody in the following paper some of the results of my inquiries, in the hope that, should the conclusions to which I have arrived be shown to be fallacious, the facts which have been collected may nevertheless assist other investigators in arriving at a correct interpretation of the phenomena attending the formation of auriferous veins. In doing this, I propose to first give a short description of the gold regions, and subsequently to treat of their quartz veins, alluvial deposits, hot springs, and salt lakes, all of which are intimately connected with the chemical geology of the districts in which they occur.

Rocks of the Gold-Region of California.

The great sedimentary metalliferous belt of California lies on the western slope of the Sierra Nevada, beginning in the neigh* Communicated by the Author, being the substance of a paper read before the Royal Society, March 12th, 1868.

hil. Mag. S. 4. Vol. 36. No. 244. Nov. 1868.

Y

bourhood of the Téjon Pass, and extending through the State to its northern limit. In consequence, however, of various local circumstances, different portions of this band are of very unequal importance as gold-producing districts.

The principal auriferous region may be said to occupy the western portions of the several counties of Mariposa, Tuolumne, Calaveras, Amador, El Dorado, Placer, Nevada, Sierra, and Plumas, with portions of the eastern sides of Yuba and Butte counties.

The apparently limited extent of the auriferous belt towards the north, in the counties of Plumas and Butte, is, according to the State Geological Survey, not owing to the thinning out of the gold-bearing formation in these localities, so much as to its being here, as well as in Shasta and Siskiyou counties, in a great measure covered by large masses of lava of very recent origin. This has been poured forth from Lassen's Butte and other volcanic cones in its vicinity; and, overflowing the older slates, it has covered them to a great depth with a non-metalliferous and almost indestructible capping.

This

Beyond Mariposa, in the southern portion of the gold-region, the slates are narrower and subject to interruption, and, from being more frequently and more extensively encroached on by the granite, they almost cease to form a continuous belt. gradual decrease in the width of the auriferous formation from north to south, and the continuously increasing amount of metamorphism displayed, are very marked, since the granite progressively occupies a relatively larger portion of the Sierra, and by degrees descends lower down its flank.

The slates of the auriferous belt of California have been satisfactorily shown by Professor Whitney to belong, for a great extent, to the Jurassic period, although the occurrence of numerous Triassic fossils in the gold-bearing rocks of Plumas county and elsewhere renders it more than probable that no incons derable portion of the slates in the heart of the gold-region are of that age. The sedimentary rocks of the great auriferous belt lying on the western slope of the Sierra Nevada principally consist of various slates and schists, sometimes containing nodules of white felspar, which is generally more or less decomposed. Among them are also found sandstones of various degrees of fineness (often transformed into quartzites), black talcose schists, with slates exhibiting a well-defined cleavage and silky structure, together with bands of crystalline limestone.

The rock constituting the principal mass of the Sierra Nevada is a granite containing only a small proportion of quartz, and in which but one species of felspar (oligoclase) is generally found. Geological Survey of California.

*

In addition to quartz, felspar, and mica, the latter of which is well crystallized and often of a brilliant black colour, this rock encloses titaniferous iron, and occasionally crystals of sphene of a light yellow colour. Magnetic oxide of iron is also present in notable quantities, and consequently imparts this property to the enclosing granite. The granite of the Sierra appears to retain its peculiar characteristics throughout a great portion of its extent; but towards the southern extremity of the chain hornblende begins to make its appearance as one of the constituents of the rock; the mica at the same time becoming less plentiful, and losing its brilliancy and dark colour.

Lying between the band of metamorphic slates and the great central granitic mass forming the more elevated portions of the chain, are found various eruptive rocks, such as syenites, diorites, and porphyries. The first of these pass into granites by almost imperceptible gradations, and appear to belong to the same geological period. The diorites, on the contrary, are of more recent origin, and contain neither sphene nor titaniferous iron, although they enclose a considerable amount of magnetic oxide of iron. In many places diorite is observed to traverse the granite in the form of distinct dykes. The porphyries are comparatively of small extent, and contain crystals showing the usual striæ, indicating the presence of plagioclastic felspars.

Quartz Veins.

All the crystalline rocks in the vicinity of the sedimentary deposits contain numerous veins of quartz, which traverse the granite and diorites near their point of junction with the slates, and, in addition to gold, enclose crystals of iron pyrites and other metallic sulphides.

These quartz veins of the crystalline rocks are comprised within a narrow zone running from south to north, along the western flank of the mountains above the great band of metamorphic slates forming the most productive portion of the goldregion, and extend, in the vicinity of the line of junction, nearly throughout its whole extent.

The quartz veins of the band of metamorphosed slates occupying the western slope of the Sierra, and lower on its flank than the crystalline rocks above described, are numerous and important. They are not, however, by any means equally distributed throughout the region of slates, but are chiefly concentrated in a belt having a width from east to west of some twelve or fifteen miles, and extending from south to north throughout the whole length of the formation. These veins, for the most part, follow the general direction of the strata in which they are enclosed. This parallelism, however, is not

absolute, since in many instances a vein, besides having a somewhat different direction from that of the bedding of the enclosing rock, throws off branches cutting the slate at considerable angles.

One of the most remarkable gold veins in California is that extending from Mount Ophir, in Mariposa county, to Mokelumne Hill in Calaveras, a distance of over seventy miles. This lead*, which frequently crops boldly out above the surface of the ground, and varies in thickness from six to sixty feet, may in some places be traced for many miles across the country, and often presents an outcrop like an immense white wall. Although by no means continuous, this may be considered as an axis with regard to the other veins of the region, which have generally an almost similar direction, and are most frequently grouped at no very considerable distances from it.

The gangue of the auriferous veins of California is invariably quartz, which is generally crystalline in its structure, or partially vitreous and semitransparent. In the majority of cases the quartz constituting an auriferous veinstone is ribboned in such a way as to have the appearance of a succession of layers parallel with the walls of the lead; and some one or more of these laminæ are not unfrequently far more productive of gold than the others.

In some instances these parallel bands are separated from each other by a thin layer of quartz, slightly differing, either in colour or structure, from that forming the seams themselves; or they may be only distinguished by a difference of the colour or structure of two adjoining members of the series.

In many cases, however, laminæ of the enclosing slates divide the vein into distinct bands; and in such instances it will be observed that the thickness of the interposed fragments of slate is sometimes not greater than that of a sheet of the thinnest paper. Cavities or druses containing crystals of quartz sometimes occur in all the auriferous veins of the country; and a certain amount of crystallization may also not unfrequently be remarked along the lines of junction of the several bands of which a vein is composed. In such cases crystallization appears to have been set up on the surface of the last-deposited stratum, which has induced the formation of a similar crop of crystals on the layer subsequently formed on its surface. Quartz crystals, however, rarely occur in notable quantities in any of the most productive veins; and when the structure of a lead is highly crystalline, and the quartz more than ordinarily transparent, it is con

* In California a quartz vein is called a "lead," and in Australia a "reef." In the gold-regions of the latter country the term lead is applied to the deposits of the "deep placers."