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GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



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FIOUBI 99. Ruins of the mercury reduction plant at the Senator mine In 1947 shortly before It was dismantled. On right, brick 

 Scott furnace ; on left, rusted cylindrical structure Is the first Herresboff furnace used In reduction of quicksilver ore, and, 

 barely visible behind It, Is the top of a concrete structure which was the first Cottrell dust preclpitator used aa a quicksilver 

 plant. 



Geology 



The following description of the geology of the 

 Senator mine is based on geologic observations made 

 at the surface by our party, geologic maps of parts of 

 the 100 and 125 levels by L. S. Hilpert and G. D. 

 Eberlein, geologic data and mine maps included in a 

 private report by J. H. Farrell 1S dated October 1923, 

 and the company surveyors' records and superintend- 

 ents' reports. It was necessary to rely on second-hand 

 sources of information about the geology of the work- 

 ings, which were all inacessible during our field in- 

 vestigation. Although these sources are somewhat 

 contradictory in minor details, they agree fairly well 

 in all major features and lead to a structural inter- 

 pretation believed to be sufficiently accurate to explain 

 the distribution of the ore. 



Most of the workings of the Senator mine are in 

 clastic sedimentary rocks of the Franciscan group 

 and three sill-like serpentine bodies with borders of 

 silica-carbonate rock, although the 260 level, which 

 extends far to the southwest of the other workings, 



U Farrell, J. II., 1923, Unpublished private report on the Senator 

 mine, October. 



also penetrates greenstone tuff of the Franciscan group 

 and another major body of serpentine. (See pis. 16- 

 18.) The only sill that is economically important 

 is the third in order of position from northeast to 

 southwest. The structures of the rocks of the Fran- 

 ciscan group is not fully understood, for no reliable 

 indications of bedding have been observed on the 

 surface and only a few attitudes are recorded on the 

 available mine maps. In general, however, the strike 

 of the beds is N. 50-80 W. and their dip is 40-75 

 S"W. The four serpentine bodies which crop out in 

 the area and which are explored underground were 

 injected approximately along the bedding planes of 

 the rocks of the Franciscan group, although some 

 tongues and apophyses break across the beds. As in 

 other parts of the district, these serpentine bodies are 

 enveloped in alta. 



The most northeasterly of the intrusive bodies is 

 narrow, discontinuously exposed, and of no economic 

 importance. (See pi. 14.) It crops out as a small 

 lens of serpentine extending 1,000 feet northwestward 

 from a point 150 feet N. 35 E. of the portal of the 

 100 level. Underground it is cut only on the 260 



