134 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



Between 1893 and 1905 the Harry area yielded about 

 25,000 flasks of quicksilver from 150,000 tons of ore 

 averaging about 0.6 percent of quicksilver. Since 1905 

 some of the Harry clumps have been reworked, and 

 the low-grade ore overlying the Yellow Kid workings 

 has been removed by opencut mining. How much 

 quicksilver was recovered in this way cannot be de- 

 termined from the available records, but the amount 

 was probably not more than 5,000 flasks and may hnve 

 been much less. 



Geology 



Most of the underground workings in the Harry 

 area follow an intrusive contact between the upper 

 surface of a serpentine sill of unknown thickness and 

 the rocks of the Franciscan group. The Franciscan 

 rocks from the surface to within about 70 feet of the 

 contact are largely greenstones, with tuff predominat- 

 ing, but those bordering the sill are mostly graywacke 

 and siltstone with minor intercalated lenses of mafic 

 tuffs. The sedimentary rocks, in general, strike nearly 

 north and dip about 35 E. The serpentine sill is the 

 same one that contains the Velasco and Randol ore 

 bodies to the northwest and the Cora Blanca ore bod- 

 ies to the southeast. It has the same general attitude, 

 but its upper surface is diversified by terraces, domal 

 structures, and thin sill-like apophyses which were of 

 importance in localizing the various ore bodies. The 

 upper part of the sill and the overlying thin apophy- 

 ses are nearly everywhere altered to silica-carbonate 

 rock, which in this area had a thickness of from 20 

 to a little more than 50 feet. 



The thick east-dipping dolomite vein conspicuous in 

 the Cora Blanca area had a somewhat different coun- 

 terpart in the Harry workings. A north-trending 

 zone of breccia of silica-carbonate rock cemented with 

 vuggy dolomite is followed by the easternmost work- 

 ings on the Harry 550 level; this breccia extends 

 downward and northward to the 600 level, where it is 

 exposed near the Harry shaft and for about 400 feet 

 farther north. The breccia zone, which is up to 3 feet 

 wide, seems to be genetically related to the thick vein 

 of banded dolomite in the Cora Blanca workings, but 

 differs from it in that its dolomite cement contains 

 hydrocarbons and a little cinnabar. 



Ore bodies 



The principal ore bodies were formed in silica-car- 

 bonate rock close to the overlying sedimentary rocks 

 of the Franciscan group. The principal ore mineral 

 was cinnabar; but native mercury was abundant 

 enough in the silica-carbonate rocks on the 600 level 

 at 2360 N. and 4270 W. to have been hazardous to 

 the miners, and it was also noted in the graywacke 



on the 550 level near the south end of the Harry 

 stope. As the ore has all been mined, its character 

 can only be inferred. The margins of the stopes con- 

 tain innumerable thin steep north-trending hilos; in 

 places these contain some cinnabar, and the silica- 

 carbonate rock alongside them is locally replaced by 

 cinnabar. Judging by these exposures, one may con- 

 clude that the main ore bodies consisted of concentra- 

 tions of cinnabar both in the veins and in the adja- 

 cent wallrock. It should be emphasized, however, that 

 in many parts of the Harry area where the hilos are 

 exceedingly abundant there is virtually no cinnabar. 

 This condition is found east of the Harry stope be- 

 tween the 550 and 700 levels (fig. 80). As is shown 

 in the following descriptions of the individual ore 

 bodies, the structural relations and shape of the in- 

 trusive contact was as important in the localization 

 of the ore as the abundance of the hilos. 



The large ore body mined in the Harry stope was 

 800 feet long, about 45 feet in average width, and had 

 an average thickness of about 15 feet. Its north end 

 was about 100 feet southeast of the Santa Maria shaft, 

 and from there in plan it extended nearly straight 

 along a bearing of S. 20 W. From its north end 

 above the 600 level it extended downward to the 600 

 level, and south of its intersection with that level its 

 plunge reversed, so that it rose at a low angle nearly 

 to the 500 level. (See pi. 12.) Here the ore body 

 divided, one part of it extending westward to the 500 

 West Harry stope and the other part southeastward 

 to the Yellow Kid and Buzztail areas. The localiza- 

 tion of the Harry ore body was affected along a struc- 

 tural terrace on the east-dipping upper surface of the 

 serpentine sill, and it was widest in those places where 

 there were small domes or gently plunging flexures 

 on the nearly level part of the terrace. The aggre- 

 gate thickness of the ore body was also influenced by 

 thin sill-like apophyses overlying the main intrusive 

 contact, for, where these were most numerous, the 

 stope attains its greatest depth. This occurrence of 

 similarly favorable places for ore deposition beneath 

 several different septa of alta resulted in the forma- 

 tion of several thin superimposed ore bodies, but. as 

 the entire mass of ore and alta \v:is mined together, 

 the superimposed ores are here treated as part of a 

 single ore body. 



Other less extensive ore bodies include the one 

 mined at the south end of the 600 level in the Ma 

 chine stope, and another mined on the 700 level 100 

 feet east of the Harry shaft. (See fir. s-J and pi. ."..) 

 The Machine stope ore body \vas locali/ed beneath a 

 small domal structure on the intrusive contact, and 

 like the Harry ore body it was composite, owing to 



