100 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



Fioum 67. Silica-carbonate rock cut by typical hllo, or quartz-dolomite 

 vein. Vein walla and Included silvers are partly replaced by cinnabar, 

 although the vein Itself Is barren. 



Other dolomite masses, formed relatively late in the 

 period of mineralization in places where the silica- 

 carbonate rock is brecciated, show all gradations be- 

 tween loosely cemented breccia to sharply bounded 

 veins with only a small proportion of fragments. In 

 most places the dolomite has partly replaced the frag- 

 ments and the vein walls, so that it forms patches and 

 discontinuous areas having a spotted appearance and 

 very irregular margins. These breccia veins are not 



known to contain cinnabar, but they locally contain 

 sphalerite, galena, and chalcopyrite. 



Lastly, dolomite forms fine-grained cavity fillings, 

 with a faint banding that is horizontal or concave up- 

 ward in some of the coarser vuggy dolomite veins. 

 This material in most places closely resembles fine- 

 grained limestone, and it is believed to have been 

 deposited by nearly stagnant solutions. 



The dolomite is widely varied in appearance: it 

 shows differences in texture due to depositional fac- 

 tors and differences in color due to small amounts of 

 compositional iron or included minute grains of other 

 minerals. It is mostly snow white, but some of it is 

 tan or buff. Some of the dolomite is colored pink by 

 finely divided cinnabar, and some that contains stib- 

 nite needles is gray or black. Most of the dolomite 

 has a bladed texture, but a great many of the veins 

 contain vugs lined with curved crystals shaped like the 

 blade of an axe. 



The chemical composition of the dolomite from sev- 

 eral kinds of veins is shown in table 15. These analy- 

 ses indicate a slight decrease in the CaCO 3 /MgCO 3 

 ratio through the long period of deposition, but they 

 fail to show any significant difference between the 

 dolomite of the barren veins and that of the veins 

 containing either cinnabar or stibnite. 



TABLE 15. -Composition and maximum indices of refraction of 

 carbonates from the New Almaden district 



[Analyses by Israel Warshaw: Index determinations by .1. J. Fahey, U.S. Geological 

 Survey. All analyses based on portion of vein soluble In 1:1 II Cl; total FeO, \H'i>. 

 CaO, and MnO determined, calculated as carbonates, and recast toeqiml 100 

 percent) 



All iron Is not present as FeCOi, for 1, 2, and 3 contain small amounts of pyrite. 



Arranged from 1 to 7 In probable order of deposition: 



1. White fine-grained dolomite vein in altered tuff. pre-ore(?). 



2. Pale honey- yellow coarsely bladed dolomite In thick vein, which In earliest 



part contained cinnabar. 



3. Pale-tan medium-grained dolomite coating pyrltc, which, In turn, coats a vein 



like 2. 



4. Snow-white coarsely bladed dolomite coating 3. 



5. Fine-grained white dolomite colored gray by ir mute needles of stlbnlte. 



6. Snow-white very coarsely bladed dolomite \.-in. 



7. Oray fine-grained horizontally banded dolomite deposited In vug In vein. 



Majrncslte (MgCOi) 



Magnesite is the dominant replacing carbonate in 

 the silica-carbonate rock in the district, but so far as 

 known it does not occur in any of the younger veins. 

 If, as appears probable, the quicksilver ore- were all 

 deposited after ilie formation of the hilo fractures, the 

 magnesite is all older than the quicksilver ores. 



