62 



GEOLOGY AXD QUICKSILVER DEPOSITS, NEW ALMADEX DISTRICT. CALIFORNIA 



FUJI-HP: 46. Photomicrographs of silica-carbonate rock derived from 

 beared serpentine. Magnesite (M), Quartz (Q), and magnetite 

 (mgt). Section also contains minute needles of mlllerite too small 

 to show In the photomicrographs. Upper, Plane light. Lower, 

 Crossed nlcols. 



dolomite have been reported in other quicksilver dis- 

 tricts. This, however, is probably uncommon, for 

 these rocks are derived, by simple carbonntization. 

 from serpentines containing little or no calcium. 



Considerable dolomite is present in the "dolomitic 

 transitional rock'' of tables 11 and 12. This rock, 

 which is coarsely crystalline and relatively incoher- 

 ent, occurs in the New Almaden mine only between 

 the unaltered serpentine and the normal variety of 

 silica-carbonate rock, and because of this relationship 

 it was regarded as representing an intermediate stage 

 in the alteration and was mapped as a "transitional 

 rock." The chemical analyses, however, show that the 

 rock is not intermediate in chemical composition, as 



was first supposed, for it has an abnormally high con- 

 tent of lime and low content of silica. It was probably 

 formed in a late stage of rock alteration, during which 



TABLE 11. Analyses of rocks from tin AW- Almaden 



showing change from serpentine to cinnabar-bearing siln-n- 

 carbonate rock 



Major elements determined chemically 



[All analyses by K. A. Qonyer except those marked ('), which are by J. J. F;ih-y 

 U.S. Geological Sir 



Minor elemenU determined spertrographically 



[K. J. Murata, U.S. Oeolocical Survey, analyst. Elements soueht but not found: 

 Au, AR, Pt, Pd, Be, Zn, Cd, Oe, In, Tl, Pb, Su. l.a. Y] 



Location of samples referred to mining company coordinates: 



. 1535X.-5130\V. 



2. 1BIO X.-5100 W. 



3. 1650X.-50MI W. 



4. 1680 X. -5075 W. 



5. 1685 X. -5035 W. 



alt 1,183 ft. 

 alt 1,183ft. 

 alt 1,183 ft. 

 alt 1,183ft. 

 alt 1,183ft. 



the open fractures in the silica-carbonate rock were 

 filled with dolomite. As it is present only locally and 

 nowhere contains any ore, it has been included with 

 the serpentine on the composite geologic maps of the 

 New Almaden mine (pis. 5-10). 



Origin 



The process whereby serpentine is converted to 

 silica-carbonate rock has already been partly implied 

 in the description of the microscopic' features of these 

 rocks. It is chiefly a replacement proces>. wherein the 

 structures of the serpentine are retained during the 

 conversion of serpentine to a mixture of quart/, and 

 magnesite. The process is not, however, entirely a 

 simple molecular replacement: for at an early -tage 

 the crystals of both magnetite and quartz become too 

 large to preserve the lines! serpentine Mruc-iiiiv.- that 

 can be seen in thin sections, and, moreover, n limited 



