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GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



Because of this fact, the detailed geology of the New 

 Almaden mine as a whole cannot readily be described. 

 Therefore the various parts of the mine are described 

 separately, and the major emphasis has been placed 

 on the ore-controlling subsurface structures exposed 

 along the underground workings. A brief preview of 

 the main structural features, however, may help the 

 reader to visualize the relation of the various parts to 

 the complete structure. 



The rocks in the mine area are those of the Fran- 

 ciscan group, serpentine, and silica-carbonate rock. 

 These have a general northwesterly strike and form 

 an anticline in the area explored by most of the work- 

 ings. The crest of the anticline passes just south of 

 the highest part of Mine Hill, and its limbs dip some- 

 what more steeply than the general slope of the to-~ 

 pography. The distribution of the serpentine in the 

 anticline can best be seen in the cross sections on plate 

 11. These show that there are two thick sills of ser- 

 pentine on the north limb of the anticline, but that 

 beneath the summit of Mine Hill these are merged, 

 forming a central mass of serpentine which extends to 

 the surface. On the north limb there are also several 

 thinner sills above the two thick ones; these extend 

 northward from the central mass and taper out down- 

 ward. On the south limb of the anticline only one 

 thick sill extends from the central mass, forming a 

 continuation of the lower of the two main sills on the 

 north limb, but above it are many thin subparallel 

 sills. 



Most of the ore bodies mined lay on the north limb 

 of the anticline, along the top of the uppermost of 

 the two thick sills. The shape of the top of this sill 

 is shown by subsurface contours in figure 81, which 

 also shows the relation of the mineralized areas to the 

 minor structural irregularities in this surface. Other 

 ore bodies lay close to the junction of the two main 

 sills, some of them on the lower margin of the upper 

 sill and some on the upper margin of the sill beneath. 

 Also on the north limb of the anticline some of the 

 smaller apophyses lying above the two principal sills 

 contained small but rich ore bodies. On the south 

 limb of the anticline, ore was found on the top side of 

 the extension of the lower sill and also along several 

 thin short apophyses that extended outward from the 

 central serpentine mass nearer the surface. 



In the following description of the New Almaden 

 mine, the workings are subdivided into units, or 

 "areas," each containing closely related ore bodies that 

 were found and mined at about the same time. Ore 

 was first discovered close to the apex of Mine Hill, 

 near the center of the area that was subsequently ex- 

 plored, and the first thing that the miners did was to 



follow this ore downward. Then, from time to time, 

 prospects were begun in adjacent areas, and generally 

 those that revealed ore were ultimately connected with 

 the ever-expanding central workings. In the early 

 days, however, many groups of workings, being in iso- 

 lated ore bodies, constituted separate mines and were 

 known under separate names, which were those first 

 applied to the original unconsolidated prospects. These 

 groups are, in the order in which they will be dis- 

 cussed, the Cora Blanca, Harry, Velasco, Central stope, 

 Victoria, North Randol, South Randol, San Fran- 

 cisco, Santa Mariana, and San Pedro-Almaden ; their 

 relative positions are shown in figure 92. For each 

 there will be given a description of the workings, a 

 brief history of the development and production, an 

 account of the local geology, a discussion of the pe- 

 culiarities of the local ore controls, and suggestion for 

 further development. 



CORA BLANCA AREA 

 Location and extent of workings 



The Cora Blanca area contains the southeasternmost 

 group of workings in the New Almaden mine, origi- 

 nally developed as an independent mine known as La 

 Cora Blanca. The workings underlie an area in the 

 upper part of Deep Gulch where the general surface 

 altitude is about 1,200 feet above sea level. The prin- 

 cipal drifts and crosscuts lie between 700 and 1,200 

 feet above sea level, and their aggregate length is 

 about 10,000 feet. (See pi. 4.) They were formerly 

 reached through the Deep Gulch, Faull. Cora Blanca, 

 and Water adits and through the Grey and Cora 

 Blanca shafts, but in 1948 the Cora Blanca adit pro- 

 vided the only access. A second group of near-surface 

 workings and an opencut lying above some of the v 

 Cora Blanca workings, known locally as the Los An- 

 geles workings, are considered here as a part of the 

 Cora Blanca workings, with which they are closely 

 related. 



History and production 



Mining in the Cora Blanca area appears to have 

 begun in 1864, on thin veins of cinnabar found in 

 altered tuff of the Franciscan group. These veins, 

 though small, were fairly continuous, and, after they 

 had been followed downward for a short distance, the 

 Deep Gulch tunnel was driven from Deep Gulch to 

 cut under them at a depth of less than 200 feet. This 

 tunnel penetrated little ore. and in 1868, after the mine 

 had yielded about 2,700 flasks of quicksilver, it was 

 virtually abandoned. In October 1873 the Cora Blanca 

 shaft was begun, and unexpectedly it struck rich ore at a 

 depth of about 50 feet. The Deep Gulch tunnel was 

 thereupon extended to connect with the shaft, and the 



