20 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



TABLE 3. Analyse* of alia and sillstone from the Franciscan 



group, New Almaden district, Santa Clara County, Calif. 



[A. C. Vllsidis. U.S. Geological Survey, analyst] 



SiO,.. 



Fe,0,._ 



FeO 



MgO 



CaO... 

 Na,0- 



H.O-. 

 H,O+. 

 TiO, 



MnO.. 

 CO,.-.. 



8 ...... 



Organic. 



Total 



LesaO-S. 



Total. 



54.67 



14. 01 



. 15 



5.37 



3.77 



3.26 



1.93 



2.95 



.50 



3.42 



.76 



.19 



. 10 



6.22 



.23 



1.22 



99.65 

 . 12 



99.53 



62. 54 



14. 81 



2. 02 



5. 47 



3.38 



1. 40 



2. 90 

 2. 13 



. 82 

 2. 91 

 .87 

 . 15 

 .05 

 .08 

 .05 

 .96 



100.54 

 .03 



100. 51 



NOTE. Description of sample and locality as follows: 



1. Alta (NA-412) from the Relief drift, New Almaden mine, Santa Clara County, 



Calif. 



2. Black siltstone (NA-315) from Fern Peak, New Almaden district, Santa Clara 



County, Calif. 



apparently very rare or lacking in the lower part of 

 the Franciscan group exposed in the southern part of 

 the district. 



Megascopic features 



A typical conglomerate lens no more than 10 feet 

 thick is exposed on Cemetery Hill about l,4oo feet 

 southeast of the New Almaden furnace (coordinates 50 

 N., 3,500 W., pi. 3). This conglomerate consists of 

 well-rounded pebbles and boulders as much as 9 inches 

 in diameter, but averaging about 2 inches, set in a fairly 

 abundant graywacke matrix that appears similar in all 

 respects to the feldspathic graywacke of the Franciscan 

 group. The rock is cut by fractures which pass through 

 t he pebbles without deviation, and some of the fractures 

 are lined with quartz. Where the rock is weathered the 

 pebbles, which are somewhat more resistant than the 

 matrix, protrude to form a knobby surface. A count 

 of a hundred of the pebbles gave the results shown in 

 table 4. Pebbles of variously metamorphosed sedi- 

 mentary and igneous rocks are about equally abun- 

 dant. Only about 10 percent of them are pebbles 

 that could possibly have been derived by erosion of 

 slightly older strata in the Franciscan group, and it 

 is very likely that all of them were derived from pre- 

 Franciscan formations. Pebbles of such distinctive 

 rocks as glaucophane schist or amphibolite, which are 

 found elsewhere (Taliaferro, 1943b, p. 141-143) in 



TABLE 4. Pebbles of a conglomerate of the Franciscan group 

 exposed on Cemetery Hill, New Almaden district, Santa Clara 



group in having a higher content of microcrystalline 



pyrite or dark chlorite; chemical analyses were there- 



fore made to see whether any constituents had been 



introduced from the bordering serpentine. Table 3 



gives an analysis of alta taken from close to the in- 



trusive contact, where the serpentine had been con- 



verted to silica-carbonate rock; for comparison the 



table also gives an analysis of siltstone from the 



Franciscan group. These analyses indicate that per- Sedimentary rocks: 



haps the specimen of alta for analysis was poorly se niack fine-grained feldspathic quartzite.. 



, , , ., .! , " , , , ,, i Gray medium-grained silicified gravwacke ____________ 5 



lected, for it apparently was carbonated by the hy- Light-gray finegrained arkose.J 13 



drothermal solutions that formed the silica-carbonate Tan fine-grained clayey arkose.. 4 



rock. Otherwise, the only significant features are the Gray silicified (?) siltstone ........... ______________ l 



lower silica and ferric iron content of the alta, which White quartz conglomerate, metamorphosed.. 



may also be attributed to hydrothermal alteration. No Gra y to bkck chert -- 



difference that would adequately account for the Total 54 



darker color of the alta is apparent, and it seems 



likely that this darker color is due to the wider dis- Igneous rocks: 



persal of the organic matter by intimate shearing. Aplite.. 



Quartz porphyry ..... ------- 



CONGLOMERATE Lavas with quartz phenocrysts ------------- 



Tuffs and lnvrria- with quartz _____________________ 5 



True conglomerate constitutes only a very small Mnfi( , 1(r ,., n ., s . iml tuffs __ ........................ i 



part of the Franciscan group in the New Almaden Light-roinn-ci aii.Tni lavas ........... - 



district, although gr:ivw:><-kp beds containing scattered Oreewtone. mortly mHamorphowd and silicified.. 



. . . Diabase _________ ......................... - ..... - ' 



rock pebbles or pieces of shale are not uncommon. 



The conglomerate that was noted formed relatively Total... ........... ... ....... . ......... _______ i"> 



thin lenses which could nowhere be traced for more 



than a hundred feet. Most of these lenses lie in the Vl ' in <|Uartz 



central part of the district, and conglomerates are Total included in count .................. . ....... 100 



