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GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



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FIOCRK 76. Photomicrograph of rich ore formed by replacement of 

 Mica-carbonate rock by cinnabar. Light areas granular quartz (Q), 

 gray areai magneslte (M), and dark areas cinnabar (C) with a little 

 magnetite. The lennlform structure and the magnetite are inherited 

 from sheared serpentine. Cinnabar baa mainly replaced quartz, but 

 Iti embayed boundaries suggest that in a late stage there was a little 

 replacement of cinnabar by quartz. 



FIGURE 77. Photomicrograph of high-grade ore formed by the replace- 

 ment of silica-carbonate rock by cinnabar. Quartz (Q), magneslte 

 (M), and black Is chiefly cinnabar with a few small residual grains 

 of magnetite. Although textures inherited from the sheared serpen- 

 tine are still visible, there has been considerable migration and re- 

 depoaltlon of the quartz and magnexlte, and the magneslte-quarti 

 ratio has been diminished. In places where the replacement Is most 

 advanced, the ore contains only cinnabar and quartz. 



221), is nearly everywhere barren near ore bodies in 

 this district; but in outcrops on the top of Church 

 Hill, half a mile east-northeast of the summit of Mine 

 Hill, it contains some cinnabar, though not enough to 

 constitute ore. Unaltered serpentine contains a little 



cinnabar about 70 feet east of the Santa Rita shaft 

 on the 800, or Day tunnel, level, but this occurrence 

 seems to be unique in the district. 



An isolated occurrence of cinnabar in rocks younger 

 than the Franciscan group, which aids in dating the 

 mineralization period, was found on a low ridge 2 

 miles S. 43 E. of Lone Hill. Here cinnabar coats 

 fractures in a silicified volcanic rock of late Miocene 

 age, but it is so scarce in the surface exposures that 

 any search for ore in this area would probably be 

 fruitless. 



STRUCTURAL CONTROL 



The ore bodies in the silica-carbonate rock have in 

 the aggregate an extremely small volume compared 

 with the volume of silica-carbonate rock in the dis- 

 trict; the ratio as observed in existing exposures is 

 probably less than 1 to 1,000. Study of the structural 

 setting of the ore bodies that have been found permits 

 further limitation of the areas favorable for the find- 

 ing of new ore bodies in this rock, and knowledge of 

 this structural control of ore bodies is of the utmost 

 importance in planning the search for ore. Insofar as 

 quicksilver ore bodies are concerned, two radically dif- 

 ferent views have been held regarding the importance 

 of structural controls. In the New Almaden district 

 adherence to these views has in the past led either to 

 disregarding favorable places where there might be 

 ore bodies or to virtually useless prospecting in un- 

 favorable places. The two prevalent views, which 

 contradict each other, are: cinnabar ore occurs, like 

 gold, "where you find it," and cinnabar ore occurs 

 only in structural traps beneath cappings of shale, 

 alta, or other relatively impervious rocks. There is 

 some justification for each of these beliefs, but a 

 thorough study of the district shows that the loca- 

 tion of an ore body is likely to have been determined 

 by several mappable, and for the most part predict- 

 able, structures that have operated together. 



linn fractures 



Most of the ore bodies in the silica-carbonate rock 

 have been formed where the rock is traversed by 

 swarms of closely spaced and steeply inclined narrow 

 fractures, which, where filled with quart/. carbonate 

 veins, are termed "hilos." These hilos appear to be 

 about as abundant on upper borders of sills as on 

 lower borders, but they an- less common away from 

 the borders. Almost everywhere in the district they 

 trend between North-South and N. 40 E., regardless 

 of the direction of the strike or the dip of the altered 

 serpentine sill. The hilos are generally from 0.5 to 2 

 inches thick, being widest in the silica-carbonate rock 

 near the alta contact. How far they extend from the 



