138 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



with any certainty, for they may change markedly in 

 strike, dip, or thickness within a very short distance. 

 Consequently, the larger sills, which contain most of 

 the ore bodies, can be described in some detail, but 

 others cannot. 



Three thin north-dipping sills are cut by the 300- 

 level Juan Vega tunnel, but only the lower side of 

 the northernmost of these is mineralized. Another, 

 lying only a short distance below the 300 level, con- 

 tained the Pruyn ore body. Below these thin sills lies 

 the extensive sill which to the north contained the 

 Victoria and the North and South Randol ore bodies, 

 and to the east the Velasco and Harry ore bodies; this 

 sill also contained many of the largest ore bodies of 

 the central stope area. Its general shape is shown by 

 contours drawn on its upper surface (fig. 81 and sec- 

 tions B-B' and C-C' of pi. 11). The thickness of the 

 sill where it diverges from the more massive serpen- 

 tine body to the south is about 200 feet, though in 

 places this thickness includes some pods of Franciscan 

 rocks. In the vicinity of the Junction shaft (pi. 6) 

 to the north it is only 50 feet thick, but only a little 

 farther north, near the Santa Rita shaft, it attains a 

 thickness of about 300 feet, which it maintains as it 

 dips downward to pass under the Randol ore bodies. 

 Beneath this economically important sill is still an- 

 other, which is penetrated by the southern part of the 

 Day tunnel. The thickness of this sill is uncertain, 

 because its base has not been reached at any place 

 near the central stope area, but it is believed to be 

 somewhat thinner than the other large sill. These 2 

 major sills merge east of the New Ardilla stope on 

 the 800 level, below the Santa Rosa stope at about the 

 600 level, and below El Collegio stope near the 500 

 level. 



The Day tunnel, on the 800 level, affords a good 

 opportunity to see both of these sills. The upper sur- 

 face of the higher sill is cut about 500 feet from the 

 portal because of timbering the exact distance is not 

 known and here the sill is said to have had a very 

 thin selvage of silica-carbonate rock. At 1,610 feet 

 the tunnel ran into silica-carbonate rock, through 

 which it was continued for 30 feet to the lower edge 

 of the higher sill. The tunnel then continued through 

 700 feet of tuffaceous greenstone of the Franciscan 

 group to the upper contact of the lower sill. The 

 tunnel went through either silica-carbonate rock or 

 serpentine for the next 900 feet to the point where it 

 again passed into graywacke lying above the main 

 San Francisco area sill. (See pi. 6 and section C-C', 

 pi. 11.) 



Ore bodies 



The ore bodies mined throughout the central stope 

 workings were among the richest ever found in the 

 mine, but in general character they resembled those 

 found in the rest of the mine. They all occurred in 

 silica-carbonate rock close to its contact with rocks of 

 the Franciscan group. Some of the richest bodies lay 

 on the upper borders of sills, but others that were 

 highly productive were on the lower surfaces of sills, 

 just above their contacts with rocks of the Franciscan 

 group. In many of the stopes hilos trending north- 

 eastward were prominent, but in others the most 

 prominent veins ran northwest, and in still others only 

 a few veins were present. Mineralization consisted 

 mainly in replacement of the silica-carbonate rock and 

 the filling of open spaces by cinnabar, but in at least 

 one place some native mercury was found. 



The upper margin of the large sill that contained 

 the Randol ore bodies to the north also contained the 

 richest and largest ore bodies in the central stope area. 

 Along a gentle arch in the upper part of this sill were 

 distributed the Santa Rita, North Ardilla, and part 

 of the Ventura ore bodies, and the ore body mined in 

 the 400-level stope was formed where the contact 

 steepens to nearly vertical in the southern part of the 

 area. The lower part of this sill in the same general 

 area contained the Far West, Marysville, Sacramento, 

 Buenos Ayres, La Ardilla, and El Collegio ore bodies 

 (pi. 4), all lying just above a wedge of rocks of the 

 Franciscan group. Down the dip of this contact to 

 the north lay the New Ardilla ore body, the top of 

 which was just below the 700 level and the bottom a 

 few tens of feet below the 800 level. Beneath the 

 wedge of rocks of the Franciscan group lies the 

 large sill penetrated by the southern part of the Day 

 tunnel. Although the upper margin of this sill was 

 extensively converted to silica-carbonate rock, no ore 

 bodies have been found along it except in a small area 

 underlying El Collegio stope. (See section C-C', 

 pi. 11.) 



Suggestions for further development 



This central part of the New Almaden mine was 

 developed at a time when ore was plentiful, and the 

 rich ore bodies were found chiefly by following the 

 narrow hilos northward from one ore body to the 

 next. Considering the richness of the great masses 

 of ore taken from this area, considerably more lateral 

 exploration toward the Harry area or toward the un- 

 explored area to the west seems to be justified, but the 

 complexity of the geology is such that favorable struc- 

 tures for ore localization cannot be predicted far in 

 advance of mining. 



