GEOLOGY AND QUICKSILVER DEPOSITS OF THE NEW ALMADEN DISTRICT 



SANTA CLARA COUNTY, CALIFORNIA 



By EDGAR H. BAILEY and DONALD L. EVERHART 



ABSTRACT 



The New Almaden district, situated a few miles south of 

 San Jose in Santa Clara County, Calif., has yielded nearly 40 

 percent of the quicksilver produced in the United States. The 

 area mapped as the district for this report includes about 80 

 square miles, extending south from the flat Santa Clara Val- 

 ley across the moderately low foothills containing the mines 

 to the more rugged crest of the California Coast Ranges. 



The rocks underlying about three-fourths of the district, in- 

 cluding all of the mineralized area, are assigned on the basis 

 of lithology to the Franciscan group of Late Jurassic and Cre- 

 taceous age. The only diagnostic fossils found indicate that 

 the age of a part of the group is lower Upper Cretaceous 

 (Cenomanian). The group consists of lithic and feldspathic 

 graywaeke, siltstone, dark altered volcanic rocks, chert, lime- 

 stone, and a subordinate amount of metamorphic rocks, in- 

 cluding glaucophane schists an assemblage regarded as a typi- 

 cal eugeosynclinal suite. The thickness of the group cannot 

 be determined accurately because of structural complexities, 

 but the part present in the New Almaden district is believed 

 to be at least 10,500 feet thick and may be much thicker. The 

 sedimentary rocks are believed to have been derived from the 

 rapid erosion of a rising landmass, and deposited in a sub- 

 siding trough which filled to wave base only in a few places 

 near the end of the depositional period. The accompanying 

 igneous activity included local outpourings of lava onto the 

 sea floor and eruption of fragmental material of similar com- 

 position which accumulated as pyroclastic beds. The chert Is 

 abundant only in the part of the Franciscan group that con- 

 tains altered volcanic rocks, and it is believed to owe its origin 

 to the reaction of hot lava with sea water. 



The post-Franciscan sedimentary rocks, which do not occupy 

 large areas, range in age from Upper Cretaceous to Recent. 

 Two sedimentary units, differing in both lithology and degree 

 of deformation, have been assigned on the basis of a few fos- 

 sils to the Upper Cretaceous. The next younger sedimentary 

 rocks are dated as middle Eocene by fossils occurring in lime- 

 stone lenses near the base of a sequence of sandstones and 

 shales. Rocks of early, middle and late Miocene age contain- 

 ing abundant fossils are divided into two formations, the rocks 

 of which grade from sandstone through clay shale to diatoma- 

 ceous shale. The sandy sedimentary rocks below the lowest 

 diatomaceous bed have been assigned to the Temblor forma- 

 tion, and this bed and the overlying rocks have been assigned 

 to the Monterey shale. Some included salic volcanic material 

 indicates igneous activity during the middle Miocene. The 

 younger sedimentary rocks in the district are all gravel de- 

 posits, which fill the larger valleys and occur as perched rem- 

 nants on some of the lower foothills. They have been divided, 



largely on the basis of their dissection and topographic posi- 

 tion, into the Pliocene and Pleistocene Santa Clara formation 

 and Quaternary alluvium. 



Tabular masses of serpentine that have been intruded into 

 the rocks of the Franciscan group are of particular interest 

 because altered parts of them contained the quicksilver ore 

 bodies. Some of these masses are conformable with the rocks 

 of the group and are therefore sills ; others are intruded along 

 faults that dip more steeply than the bedding. The larger 

 masses consist of blocks of unsheared serpentine embedded in 

 a matrix of sheared serpentine; the smaller masses and the 

 borders of the larger ones consist entirely of sheared serpen- 

 tine. The internal structures and details of the contacts, to- 

 gether with theoretical considerations, suggest that the ser- 

 pentine masses were intruded as serpentine, not as peridotitic 

 magma. The age of the serpentine cannot be closely placed 

 by the evidence available in the district, but by comparison 

 with other occurrences in the Coast Ranges it is believed to 

 be Late Cretaceous. 



Silica-carbonate rock the host rock for the quicksilver ore 

 bodies has been formed locally by hydrothermal alteration of 

 the serpentine. On the thicker serpentine masses it commonly 

 occurs as a thin peripheral shell, but some of the thinner 

 masses are entirely converted to silica-carbonate rock. As the 

 silica-carbonate rock was formed by replacement, it shows 

 abundant relict textures inherited from the serpentine. The 

 dominant minerals of the silica-carbonate rock in the New 

 Almaden district are quartz and magnesite. The process of 

 hydrothermal alteration that formed the rock consisted chiefly 

 in bulk substitution of carbon dioxide for water, but a little 

 magnesium was removed. The alteration took place in late 

 Tertiary, probably early Pliocene time. Because of the differ- 

 ence in age between the serpentine and the silica-carbonate 

 rock, the hydrothermal solutions causing the change cannot 

 be genetically related to the serpentine or its primary magma, 

 but they may represent the early stages of the quicksilver 

 mineralization. 



The structures within the district trend northwestward or 

 westward, and are dominated by the structures in the rocks 

 of the Franciscan group. As these rocks nearly everywhere 

 show evidence of flowage, folding, or shearing, only the more 

 continuous coarse structures can be traced through them. The 

 Franciscan rocks dip in general to the north, but an anticlinal 

 flexure helped to localize many of the large ore bodies of the 

 New Almaden mine. The post-Franciscan rocks that antedate 

 the alluvial gravels occupy synclinal troughs that are nearly 

 everywhere separated from the rocks of the Franciscan group 

 by faults, but they are less crumpled than the Franciscan. 

 The gravels are locally tilted and cut by normal faults of rela- 

 tively small displacement. 



