58 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



sections contain considerable fine-grained chlorite. The 

 genetic relation between the gabbroic rocks and the 

 serpentine remains an unsolved problem, but the two 

 rocks do not appear to have formed by any sort of 

 segregation in place. 



SILICA-CARBONATE ROCK 



"Silica-carbonate rock" is the term applied in this 

 report to a rock that is derived from serpentine by 

 hydrothermal alteration and is composed principally 

 of silica (quartz, or chalcedony, or opal) and a car- 

 bonate (generally ferroan magnesite). This rock is of 

 special importance in the New Almaden district, as in 

 many other quicksilver districts in the Coast Ranges, 

 because it is the host rock of all the more productive 

 bodies of quicksilver ore. The different varieties that 

 occur in California are variously referred to by min- 

 ers as "vein rock," "ledge matter," "quicksilver rock." 

 "ore rock," "opalite," "opaline," and "silica-carbonate 

 rock." Because many of these terms have been used 

 to apply to other kinds of rocks, it is fortunate that 

 the most acceptable term, "silica-carbonate rock," is 

 also the most widely used. The term generally has 

 been applied only to rocks derived from serpentine, 

 but in recent years, "silica-carbonate rock" has also 

 been applied elsewhere (Faust and Callaghan, 1048. 

 p. 11-74) to rocks of different origin and mineral 

 composition. 



Distribution 



The distribution of the silica-carbonate rock in the 

 New Almaden district is much more restricted than 

 that of the serpentine bodies from which it is derived. 

 (See pi. 1.) Most of the outcrops of silica-carbonate 

 rock are scattered along the Los Capitancillos Kidge, 

 in a zone that includes Mine Hill and extends north- 

 westward 1 mile beyond the Guadalupe mine. A sec- 

 ond zone of outcrop, less continuous and partly cov- 

 ered, diverges from the first one east of the Senator 

 mine and extends eastward across the valley of Ala- 

 mitos Creek into the Santa Teresa Hills. The first of 

 these zones contains all the highly productive mines 

 of the district; the second, although it contains some 

 cinnabar, is little prospected. A third zone, contain- 

 ing only small pods of silica-carbonate rock, extends 

 along the north side of the Santa Teresa Hills, where 

 it has yielded a little quicksilver at the Santa Teresa 

 and Bernal mines. 



Not only is the silica-carbonate rock restricted to the 

 serpentine bodies lying in these zones, but it is even 

 further restricted to certain parts of them. Although 

 some small serpentine bodies that are thin and sheared 

 are completely replaced, the thicker, more massive ones 



are generally replaced only along their sheared mar- 

 gins and have, in effect, an armorlike shell of the hard 

 silica-carbonate rock. There is no relation between the 

 size or thickness of the serpentine bodies and the 

 thickness of the shell of silica-carbonate rock devel- 

 oped around them, nor is there any relation between 

 the thickness or extent of the shell and the occurrence 

 of quicksilver ore. The largest exposed body of silica- 

 carbonate rock the one that lies north and west of 

 the Guadalupe mine is perhaps 1 mile long and a 

 few hundred feet wide, but it is sparsely mineralized 

 only here and there. On the other hand, many small 

 bodies of silica-carbonate rock, as well as some of in- 

 termediate size, contain extensive bodies of minable 

 quicksilver ore. It must therefore be concluded that 

 small bodies are as likely to contain ore as larger 

 ones. Furthermore, the distribution of the silica- 

 carbonate rock from the surface downward does not 

 show any marked change within the depths explored 

 by the mines. In the New Almaden mine a remark- 

 ably large amount of silica-carbonate rock is found 

 in the upper levels, where the serpentine sills are 

 fairly flat, but, according to the company records, 

 large masses of "vein rock" were also cut in workings 

 lying as much as 500 feet Mow sea level, or 1.7.M> 

 feet directly below the present erosion surface. 



Megascopic features 



The silica-carbonate rock varies widely in appear- 

 ance, but most of it is readily recognized because of 

 its pseudomorphic textures inherited from serpentine 

 and because of its areal relation to serpentine masses. 

 The variations in the silica-carbonate rock result 

 partly from original differences in the mineralogy and 

 texture of the parent serpentine and partly from vari- 

 ation in the kind and grain si/.e of the replacing silica 

 and carbonate minerals. In the silica-carbonate rock 

 of the New Almaden district the silica is nearly all 

 quartz; opal and chalcedony are so uncommon that in 

 a general description they can be disregarded. In ad- 

 dition, the source rock is mainly peridot ite. and silica- 

 carbonate rocks derived from dunite are too uncom- 

 mon to merit more than a brief mention. The princi- 

 pal variations in the rock in this district, therefore, 

 depend chiefly on the quantity of shearing in the origi- 

 nal serpentine, on the coarseness of the component 

 minerals, and on the relative abundance of quart/ and 

 carbonate. 



Most of the silica-carbonate rock is derived from 

 the sheared serpentine, and in many places it retains 

 the sheared structure and al><> contains residual un- 

 altered minute crystals of chromite or picotite. Where 

 fresh, silica-carbonate rock of this origin has a lenticu- 



