142 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



In 1893 it was abandoned and the lower Randol work- 

 ings were allowed to fill with water. 



Failing to find new ore bodies at depth, the manage- 

 ment explored the North Randol area laterally to the 

 east on the 1100 level in 1888 and 1889, to the south 

 on the 1800 level in 1889, and to the north on the 

 1500 level in 1890. This development work revealed 

 some cinnabar but no ore, and in 1892, owing to the 

 exhaustion of the deeper ore bodies, the Randol shaft 

 was hoisting ore from the upper levels during only 

 one shift a day. In January 1896, when the Randol 

 shaft needed retimbering from the 1000 to the 1400 

 level, it was closed down completely, and the workings 

 below the 800 level were allowed to fill with water. 

 Since that date the shaft has never been reopened, and 

 the North Randol workings and ore bodies have been 

 completely inaccessible. 



The amount of quicksilver obtained from the huge 

 North Randol ore body cannot be accurately deter- 

 mined from the available records, but from 1879 to 

 1895 inclusive this single ore body probably yielded 

 nearly 200,000 flasks of quicksilver from ores aver- 

 aging more than 2 percent quicksilver. At various 

 times since the closing of the shaft attempts have been 

 made to recover quicksilver from the dump at the 

 mouth of the Randol tunnel, but the total amount re- 

 covered has been insignificant compared with the ear- 

 lier production. 



Geology 



Most of the North Randol workings extend along 

 the upper contact of the same serpentine sill that con- 

 tained the Cora Blanca, Harry, and Velasco ore bodies, 

 and the great central ore bodies of the New Almaden 

 mine. The general strike of the sill in this area is 

 about east-west, and the average dip about 50 N. This 

 attitude persists westward into the South Randol area, 

 but east of the North Randol stope and below the 800 

 level the sill swings abruptly southeastward and 

 steepens to nearly vertical. This sharp flexure, as 

 shown by the abrupt change in direction of several 

 of the lower levels near their eastern ends, may be the 

 result of the sill being offset by a steep fault. Above 

 the 650 level the sill arches over into the broad dome 

 of the Velasco area, below the 1900 level it steepens 

 to vertical, and at greater depth it is overturned and 

 dips steeply to the south. 



The upper surface of the sill is highly irregular; it 

 has sharp rolls in both strike and dip, and two thin 

 finlike apophyses rise from it. One of these apophyses 

 branches from the main sill along a line extending 

 between a point above the middle course of the Great 

 Eastern tunnel and a point southeast of the Randol 

 shaft on the 1200 level. The other, which assumes 



much greater importance in the discussion of the 

 South Randol ore bodies, branches from the first near 

 the 1000 level, and is separated from the main sill by 

 a narrow wedge of graywacke down to a point about 

 midway between the Randol and Santa Isabel shafts 

 on the 1800 level. These relations are shown in figure 

 81, which is a map showing contours drawn on the 

 upper surface of the sill, and perhaps are even better 

 shown in figure 93. The serpentine making up these 

 narrow apophyses, and also that along the periphery 

 of the main sill, was nearly everywhere converted by 

 hydrothermal solutions to silica-carbonate rock, which 

 is the host for the ore. The thickness of the shell of 

 silica-carbonate rock is ascertainable in only a few 

 places, but it apparently reaches a maximum of about 

 40 feet in the upper levels and is somewhat thinner 

 in the lower levels. 



The rocks above the sill and its apophyses are domi- 

 nantly clastic sedimentary rocks of the Franciscan 

 group. On the north 1500 level, however, a thin sill 

 of serpentine with some barren silica-carbonate rock 

 was cut 350 feet above the sill, and 30 feet higher was 

 a layer of greenstone 120 feet thick. The main sill 

 is a little more than 350 feet thick where penetrated 

 by the Randol shaft, and according to surveyors' rec- 

 ords no silica-carbonate rock was found along its lower 

 border at this point. It is separated from another 

 thick sill lying below it by about 350 feet of clastic 

 sedimentary rocks and greenstone. This lower sill is 

 totally unexplored between the 1800 level and the Day 

 tunnel (800 level), but it was prospected for a short 

 distance on the 1800 level south, where it had a thin 

 shell of silica-carbonate rock. According to the sur- 

 veyors' records the silica-carbonate rocks were trav- 

 ersed at this point by "stringers of calcite containing 

 traces of cinnabar." (See section B-B', pi. 11.) 



Ore bodies 



As the North Randol workings have been totally 

 inaccessible for more than 50 years, all information 

 about the ore bodies must either be gleaned from 

 sketchy reports of geologists and surveyors who ob- 

 served them or else be inferred from the shapes and 

 positions of the workings shown on the old company 

 maps. The plan of the ore body mined in the great 

 North Randol stope is known with certainty, but its 

 third dimension is less well known. The stope V length 

 is about 1,300 feet and its average width about L'OO 

 feet; its height apparently varied from 25 feet in the 

 upper levels to less than 10 feet in the lower levels. 

 The single ore body mined from it must have been 

 remarkably persistent, for only a few pillars were left 

 in spite of the fact that the ground was exceptionally 

 "heavy," requiring huge, costly timbers to support the 



