154 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



years, from ores that probably averaged more than 

 5 percent quicksilver. During this early period the 

 ores were obtained from stopes reached through the 

 Main, Nestor Gil, Frederica and higher adits; the 

 connecting San Andreas and Esperanza shafts served 

 largely for ventilation. The ores were treated in a 

 nearby furnace with a reported capacity of "2,500 

 flasks per month,'' but unless a large amount of pro- 

 duction remains unrecorded the furnace was seldom 

 operated at full capacity. 



In 1863, in order to prospect for downward ex- 

 tensions of the ore bodies found in the upper levels, 

 the Eldredge tunnel was begun some 130 feet below 

 the lowest level then existing. By 1875 this tunnel 

 had been driven at a high cost, through hard silica- 

 carbonate rock for 875 feet, but it failed to reach any 

 ore. Other reasons combined with this to bring opera- 

 tions at the Enriquita to a standstill. First, the 

 operators of the New Almaden mine had great dif- 

 ficulty in preventing the theft of quicksilver that was 

 recovered at the Enriquita mine, in these early days, 

 because of the remoteness of this mine from the main 

 camp; second, the discovery of large ore bodies in the 

 Randol area of the New Almaden mine left little in- 

 centive to develop any of the "outside" mines. Con- 

 sequently, after 1875 the Enriquita mine lay idle until 

 the known ore bodies of the New Almaden mine were 

 virtually exhausted. 



In 1892, when there was an urgent need to find new 

 ore on the New Almaden property little capital was 

 available for exploration; but the old Enriquita mine 

 appeared to offer more promise than most other parts 

 of the property, and a new attempt to find the con- 

 tinuation of the old ore shoots was begun. As the 

 old furnace near the Enriquita mine had been de- 

 stroyed, it was decided to treat the ore at the 

 Hacienda (pi. 1.), to which the ore could be taken 

 most cheaply by hoisting it through a new shaft from 

 the lower levels of the Enriquita mine to a road on 

 Los Capitancillos Ridge. A shaft, called the R. R. B., 

 was accordingly put down and connected with the 

 Eldredge tunnel. Exploratory work was done on the 

 tunnel level and on the 300 and 350 levels above, but 

 no new ore was found even the attempt to find the 

 old stope proved unsuccessful. In 1896 the develop- 

 ment was stopped for lack of funds, but in 1900-1902 

 and in 1909-10 other apparently unsuccessful attempts 

 to find the old stope were made. 



Finally, in 1923, a raise put up from the end of the 

 Eldredge tunnel found the old stope, but apparently 

 little fill or new ore was taken from it because of 

 the low price of quicksilver. The following year some 

 drilling was done in the mine, which revealed some 



cinnabar but no ore of sufficient extent to repay the 

 cost of mining it. Between 1927 and 1935 little min- 

 ing was done, even though nearly all the workings 

 remained open ; some quicksilver was recovered, how- 

 ever, from ore on the old dumps. In 1935 the Gua- 

 dalupe Dam was built, and with the subsequent flood- 

 ings the low-level tunnel caved near its portal. The 

 widening of a road from the dam to the top of Mine 

 Hill resulted, moreover, in the caving of the R. R. B. 

 shaft. In 1942 the old dumps, which were found to 

 average 4 pounds of quicksilver to the ton, were hauled 

 to the modern furnace on Mine Hill. In 1948 the mine 

 was idle and largely inaccessible. It had yielded more 

 than 10,000 flasks of quicksilver before 1865, but since 

 then, despite repeated attempts to revive it, the mine 

 seems to have produced only a few hundred flasks of 

 quicksilver, recovered from ore in the old dumps. 



Geology 



Little detailed geologic information about this 

 largely inacccessible mine is available, but because of 

 the apparent simplicity of the structure the limited 

 data give a general idea of the geologic setting of 

 the ores. The mine is close to the northern margin 

 of a sill of serpentine that appears on the surface to 

 be 800 feet thick. (See pi. 1.) The northern contact 

 dips steeply north, and the southern contact where 

 exposed in the workings of the nearby Providencia 

 mine dips steeply south. The sill occupies the axial 

 part of the anticline that contains the ore bodies of 

 the main New Almaden mine. In the Enriquita area, 

 however, the fold is tight, nearly isoclinal, and we 

 do not know whether the sill itself is isoclinally folded 

 or intruded after the folding. 



Both to the north and south of the main still are thin- 

 ner satellite sills, which are largely converted to silica- 

 carbonate rock. The deep-level Eldredge tunnel be- 

 gins in one of these thin sills north of the main 

 serpentine mass, and most of the workings are re- 

 ported to be in hard silica-carbonate rock. Several 

 crosscuts to the north reach the alta hanging wall: 

 diamond-drill holes and the long 400-level crosscut to 

 the southwest reach an alta footwall. (See fig. 96.) 

 According to the available records the sill is com- 

 pletely converted to silica-carbonate rock on the 

 Eldredge tunnel level, but on the 300 level above it 

 contains a medial core of serpentine. Unfortunately 

 the ore bodies are all above the levels for which some 

 riMilogic information is available, and projecting the 

 known geology upward would indicate that they :iro 

 in the central part of the sill rather than adjacent 

 to a contact, as is more usual in the district. It seems 

 likely tluit they lie along included septa of Franciscan 



