104 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



FIGURE 71. Large polished slab of ore from the New Almaden mine, showing cinnabar replacing silica-carbonate along a fracture which Is 

 no longer apparent. Neutral-gray band crossing the central part of the specimen longitudinally In cinnabar. Note the preservation of 

 the serpentine texture In the silica-carbonate rock. Light patches resembling phi'iim-rysts arc ri-plmvcl l>:istiti<- p-ii-inlinnnrph!. ; liglit- 

 colored coarse transverne veins are largely magnesite that has replaced veins of chrysotile. U.S. National Museum specimen. 



fact that most of this ore contains so much cinnabar 

 that the possibility of it having been deposited in 

 minute openings or fractures is untenable. 



In the ores replacing silica-carbonate rock sheared 

 textures inherited from sheared serpentine are gener- 

 ally pronounced, and in some ore that replaces silica- 

 carbonate rock which has itself replaced relatively 

 unsheared peridotite, relict bastitic pseudomorphs are 

 easily recognized. The magnesite of the silica-carbon- 

 ate rock is the most susceptible to replacement by 

 cinnabar, and quartz is also extensively replaced, but 

 the serpentine minerals seem to be resistant to replace- 

 ment. Where the silica-carbonate rock contained vein- 

 lets or seams of serpentine minerals, these remain to 

 mark the serpentine texture, even though most of the 

 rock has been replaced by cinnabar. (See figs. 73-77.) 



The formation of ore in silica-carbonate rock gener- 

 ally began with replacement along fractures, which in 

 many places were later filled by quartz or dolomite to 

 form hilos, and the ore extended out from these hilos 

 for a distance of an inch to as much as a foot. The 

 substitution was generally so complete that the ore 

 contains from 35 to more than 90 percent cinnabar; 

 only in zones less than half an inch wide between the 

 rich ore and the barren silica-carbonate rock does the 

 cinnabar content amount to only a few percent. Ex- 

 tensive ore bodies were formed where the feeding frac- 

 tures were closely spaced, but in some places the re- 

 placement along even a single fracture was extensive 

 enough to form a minable ore body. Because of 'the 



sharp transition between ore and country rock, the ore 

 bodies of the district differ from those in many Cali- 

 fornia mines in two important respects: they are gen- 

 erally surrounded by rock containing only a trace of 

 cinnabar rather than by submarginal ore, and they are 

 exceptionally amenable to hand sorting. 



Ore formed mainly by replacement of shale and 

 graywacke was mined in the Yellow Kid workings in 

 the southern part of the Harry area of the New Alma- 

 den mine, and it also constituted the ore bodies of the 

 San Mateo mine. Very few specimens of such ore 

 were available for study, and these showed only a few 

 scattered crystals and patches of cinnabar. Most of 

 the cinnabar in these specimens is judged from its 

 distribution to have been deposited by replacement, 

 but some cinnabar deposited in open spaces is also 

 present. 



Ore containing cinnabar deposited in open spaces 

 is neither as abundant nor as rich in the New -Alma- 

 den district, for the ore formed by replacement. Nev- 

 ertheless, open-space filling forms a minor part of 

 many of the ore bodies ami it apparently was domi- 

 nant in the large ore bodies of the Senator mine and 

 possibly in the near-surface ore mined from openents 

 on Mine Hill during the period from 1941 to 1945. 

 Most ore of this character was found in .silica-carbon- 

 ate rock or in veins cutting this rock, but some has 

 been found in graywacke and shale of the Franciscan 

 group. Although these ores all resulted from deposi- 

 tion of cinnabar in open spaces, they may be readily 



