98 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



Pyrite 



Disseminated 

 cinnabar and , 

 pyrite 



Hematite 

 Pyrite 



Disseminated 

 pyrite 



Pyrite 

 Cinnabar 

 Cinnabar 

 Cinnabar 



Pyrite coated 

 with cinnabar 

 at base 



FIGURE 65. Photomicrograph ot section across banded dolomite vein containing some cinnabar, pyrite, and hema- 

 tite. Such veins, which are a little younger than most of the quicksilver ores, were mined as ore in the 

 Senator mine, but elsewhere la the district they contained too little cinnabar to be of commercial grade. 



rock formed by the weathering of silica-carbonate 

 rock is probably ferroan magnesite rather than pyrite. 



Stibnite L-.II.S i 



Minute needles of stibnite occur sparingly in parts 

 of the Senator and New Almaden mines. It is most 

 abundant in the former, where it occurs in banded 

 dolomite veins containing a little quartz, pyrite, and 

 cinnabar. (See fig. 66.) Where both stibnite and 

 cinnabar occur in a single vein, they alternate in thin 

 layers but are not intermingled ; although both min- 

 erals were deposited in the same period of mineraliza- 

 tion, they were not strictly contemporaneous. In the 

 New Almaden mine, stibnite was seen on the 800 level 

 in two places in the Day tunnel and in the Cora 

 Blanca workings and in both places it appears to 

 have been deposited after the major period of cinna- 

 bar mineralization. 



S|ilinli-rl 1 1- i 



The deep-brown or black ferroan variety of sphaler- 

 ite known as "black jack" occurs in ;i few places in the 

 New Almaden mine. Its dark color suggested that if 

 might contain some mercury, but specimens subjected 

 to the sensitive fluorescent screen test showed not even 

 a trace. The sphalerite tested was found in irregular 

 breccia veins on the 800 level, in a crosscut extending 

 westward from the Day tunnel, about 3,000 feet from 



the portal. It formed a coating on quartz, and was 

 coated in turn with dolomite. Other accompanying 

 minerals included galena and pyrite. No cinnabar was 

 found in the vicinity, but throughout the mine this 

 type of breccia vein generally is younger than the 

 quicksilver ores. Silica-carbonate rock on the dump 

 by the St. George shaft is traversed by quartz-dolomite 

 veins containing sphalerite, galena, and chalcopyrite, 

 and some of these cut cinnabar-quart/, veins. The 

 sphalerite, therefore, was probably all deposited after 

 the main period of cinnabar deposition. 



Galena (PbS) 



Galena occurs with sphalerite in very small amount, 

 and is likewise younger than the cinnabar ores. 



Cnalcopyrtte (CuFoSi) 



Since chalcopyrite is associated with sphalerite and 

 galena on the dump of the St. (ieorge shaft, it is also 

 believed to have been deposited after the main period 

 of cinnabar deposition. 



Bornlte << -iiiivs,) 



Bornite is indicated on one of the maps made by 

 S. B. Christy as occurring on the 2100 level of the 

 New Almaden mine a short distance south of the 

 Buena Vista shaft. Although none lias been seen by 

 the writers. Mr. ( 'hri.-ly's other statements have proved 

 so reliable that this one is believed to be authentic. 



