MINES 



151 



The ore lying between the 600 and 700 levels was ap- 

 parently never mined, and since 1888 several unsuc- 

 cessful attempts have been made to gain access to this 

 ore through either the old stopes or the Lower Amer- 

 ica tunnel. In 1943 a small quantity of low-grade ore 

 was mined in an opencut lying above the old stopes, 

 and in 1947, when the Upper America tunnel was 

 again reopened, a little ore was recovered from a thin 

 seam about 190 feet from the portal. In 1948 the 

 mine was abandoned and the Upper America tunnel 

 caved. The total production of the America mine re- 

 sulting from all these ventures is probably less than 

 2,000 flasks of quicksilver. 



Geology 



The America area contains a serpentine sill intru- 

 sive into the rocks of the Franciscan group, which 

 in this area are chiefly greenstone tuffs but include 

 a small amount of amphibolite and clastic sedimentary 

 rocks. Most of the ore found in the America mine 

 came from the lower side of this sill, which is rela- 

 tively thin and has largely been converted to silica- 

 carbonate rock. The sill is folded with the rocks 

 of the Franciscan group to such a degree that its con- 

 tacts are too irregular to be projected with any cer- 

 tainty, but it may be an extension of the sill cut in 

 the southern part of the 1400 level southwest of the 

 Santa Isabel shaft in the New Almaden mine. Its 

 course on the surface near the mine workings cannot 

 be traced, for the area in which it might be expected 

 to crop out is largely covered by dumps. Under- 

 ground, however, the lower surface of the sill, along 

 which the ore was localized, can be fairly well deline- 

 ated from an area above the Upper America tunnel 

 to a little below the 600 level. In the upper levels 

 the sill strikes east and dips about 50 N. ; on the 600 

 level its dip is about the same but the contact is more 

 sinuous, striking northwestward where it is mineral- 

 ized and northeastward elsewhere. Little is known 

 of the upper surface of the sill, but the sill is more 

 than 60 feet thick on the upper levels and only about 

 30 feet thick on the 700 level. (See pi. 13.) 



Elsewhere in the America mine there are other 

 bodies of silica-carbonate rock, which apparently con- 

 tain only insignificant amounts of cinnabar. These 

 include a thin sill that borders the opencut on the 

 west, a sill crossed by the upper part of the America 

 shaft, and a lens of silica-carbonate rock penetrated 

 on the 500 level about 450 feet east of the America 

 shaft. 



Ore bodies 



The America mine appears to have contained only 

 one ore body, although existing maps are so poor, 



especially with regard to elevations, that the true 

 position of the old stopes is uncertain. The cross 

 section A- A' on plate 13 was constructed on the 

 assumption that all the stopes extended along a single 

 contact the lower side of the main serpentine sill. 

 If that is correct, the ore body has been mined to the 

 600 level, or for at least 250 feet down the dip of the 

 sill. The average width of the stope was about 30 

 feet, but no information on the thickness of the ore 

 body is available. 



Even less is recorded about the character of the ore, 

 but specimens gleaned from the dumps are much like 

 specimens from the New Almaden mine. Although 

 the stope extends somewhat more nearly east-west 

 than most of the narrow quartz-carbonate veins or 

 hilos found on Mine Hill, the abundance of veins in 

 the rock on the dump suggests that the ore body 

 follows a swarm of such hilos down the dip of the 

 contact. The small quantity of low-grade ore mined 

 from above the Upper America tunnel in 1947, how- 

 ever, bore no relation to any hilos; it consisted of 

 irregular cinnabar-bearing veinlets and a few scattered 

 crystals of cinnabar in silica-carbonate rock lying be- 

 neath a thin septum of greenstone tuff. 



Suggestions for furtner development 



The lower part of the main ore body is reliably 

 reported to have been left unmined below the 600 

 level, although a winze proved that it continued down 

 for at least another 80 feet. This unmined ore prob- 

 ably has about the same tenor in cinnabar as the 

 mined ore on the upper levels about 2 percent. It 

 could probably be reached most cheaply by sinking 

 an inclined shaft down the plunge of the ore body, 

 for past experience has shown that reopening any of 

 the caved adits would be a costly procedure. Ore 

 was also found in "boulders" of silica-carbonate rock 

 at the bottom of the America shaft, but it appears 

 unlikely that exploration at depth will be worth the 

 cost of sinking a new shaft in the foreseeable future. 



PROVIDENICA MINE 

 Location and extent of workings 



The Providencia mine, which is one of the smaller 

 "outside" mines on the New Almaden property, lies 

 on the south slope of Los Capitancillos Ridge, be- 

 tween the America and Enriquita mines and about 

 8,000 feet northwest of the summit of Mine Hill. 

 (See pi. 1.) The principal workings consist of more 

 than a dozen shore adits penetrating a body of silica- 

 carbonate rock between 1,000 and 1,120 feet above sea 

 level; one other longer adit, known as the Lower 

 Providencia tunnel, extends under these workings 



