136 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



the Franciscan group and the underlying serpentine 

 sill explored by the Harry workings to the southeast 

 and the Randol workings to the north. In addition, 

 a higher sill, lying nearly parallel to the surface and 

 in most places extending down to a depth of less than 

 100 feet, is explored by some of the higher adits. The 

 lower margin of the upper sill is altered to silica- 

 carbonate rock and contained some scattered ore, but 

 the larger, more productive Velasco ore bodies were 

 found in the upper margin of the underlying sill, 

 which is even more extensively altered to silica-car- 

 bonate rock. (See section C-C', pi. 11.) 



Ore bodies 



The character of the ore in the Velasco ore bodies 

 must largely be conjectured, but if the ore bodies 

 averaged more than 15 percent quicksilver, as reported, 

 the cinnabar cannot all have been in hilos; some of 

 it must have replaced the silica-carbonate rock. In 

 places where the ore bodies were small the cinnabar 

 is said to have formed "small seams making up against 

 the alta"; but where the bodies were larger it "ex- 

 tended back into the 'vein' [silica-carbonate rock]." 

 Little is really known regarding the structural en- 

 vironment and control of the ore bodies. For exam- 

 ple, detailed information about the geology of the 

 small but very rich ore body lying at an altitude of 

 1,430 feet and mined through the Robles and Upper 

 Velasco tunnels is lacking, although, judging from 

 what can be seen on the surface, it lay in silica-car- 

 bonate rock above the upper contact of the upper sill. 

 The more important of the ore bodies mined in the 

 Theatre and Velasco stopes and extending southwest 

 to the Santa Rita East stope lay in silica-carbonate 

 rock along the crest of a domal structure on the upper 

 surface of the main serpentine sill. Although it is 

 possible to contour the general upper limit of the ser- 

 pentine (fig. 81), the contact must be exceedingly com- 

 plex in detail with several apophyses and included 

 wedges and septa of rocks of the Franciscan group. 

 The irregularities of the contacts in this area are indi- 

 cated in an annual report of the Quicksilver Mining 

 Co. for 1866, which describes the Velasco ore bodies 

 as "extremely irregular in their mode of occurrence, 

 being in some instances contorted to a degree to defy 

 the most scrutinizing investigation"; and this irregu- 

 larity seems to be reflected by the exploratory work- 

 ings, which apparently do not follow the contacts in 

 this area as faithfully as in most other parts of the 

 New Almaden mine. 



Suggestions for further development 



The Velasco area seems to have been thoroughly 

 explored, and no further exploration to find new ore 



shoots in the area seems to be justified. However, as 

 most of the workings have been inaccessible for many 

 years, lessees might find considerably more ore suit- 

 able for retort operation remaining in the Velasco 

 stopes than in other parts of the New Almaden mine 

 that have been repeatedly opened during the last 40 



years. 



CENTRAL STOPE AEEA 



I .oral Ic MI and extent of workings 



The term "central stope area" is applied to that 

 part of Mine Hill which was explored by a compact 

 group of workings clustered around the internal main 

 shaft (pi. 5) directly under the highest part of the 

 hill; it also includes the Santa Rita stope. which ex- 

 tends northward from the cluster, and the northern 

 part of the Day tunnel, which served as a haulageway. 

 (See fig. 92.) The central cluster of workings honey- 

 combs the hill from the surface down to the 600 level, 

 and on this level the Santa Rita stope extends north- 

 ward, following a nearly flat contact to and beyond 

 the Santa Rita shaft. The Day tunnel, which is on 

 the 800 level and whose portal is about 1,700 feet 

 northeast of the Santa Rita shaft, extends beneath the 

 central cluster of stopes and continues southwest w;inl 

 to connect with workings in the San Francisco area. 

 From it branch off various crosscuts, one of which 

 connects with the Santa Rita shaft. The two deepest 

 workings in the area are the 900-level drift from the 

 bottom of the Santa Rita shaft, and a short drift 

 from the bottom of a winze that lies at the intersec- 

 tion of the Day tunnel and the crosscut to the Santa 

 Rita shaft. Two access adits on the 300 level formerly 

 served as haulageways. One of these, called the Main 

 tunnel, is 800 feet long and has its portal southeast 

 of the area, and from it the important internal Main 

 shaft extended down to the 600 level. The other ac- 

 cess adit was the Juan Vega tunnel, whose portal is 

 about 1,000 feet north of the stope area. The area 

 also contains many other levels driven as haulageways 

 or for exploration, and the combined length of all the 

 level workings is about 25,000 feet. (See pi. 4.) 



History and production 



The history of production from the central stope 

 workings begins in 1846, with the first first mining of 

 the ores exposed on the surface of Mine Hill, and 

 extends through 1945. Most of the ore, however, was 

 taken out before 1870, and subsequent operations have 

 consisted largely in stripping the walls of old stopes 

 and removing stope fill. Although the total produc- 

 tion from this comparatively small area cannot be 

 exactly determined, it is known to exceed 500,000 

 flasks (19,000 tons) of quicksilver. To follow in de- 



