158 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



these years "rich ore" is said to have been hauled to 

 the Hacienda furnaces (pi. 1). The average quick- 

 silver content of the ore treated in the New Almaden 

 furnaces in 1899 was about 1 percent, and the ore 

 from the San Mateo mine must have been considerably 

 richer to have justified the long haul to the furnaces. 

 After 1901 the mine was apparently abandoned until 

 1908, when the lower adit was reopened, but after 

 new workings of considerable extent had been run 

 without finding ore it was again abandoned. 



Between 1915 and 1917, under Landers' supervision, 

 the old adit was reopened and several small ore bodies 

 were found. The ore from these was apparently piled 

 at the portal with the intent of hauling it through 

 the 260-level Senator tunnel to the Senator furnace, 

 but eventually this ore also was hauled around to the 

 Hacienda furnace. In 1917, when Landers left, the 

 mine was abandoned. During the period between 1935 

 and 1940 the long adit from the west was driven by 

 P. S. Schneider and associates, and some ore was 

 burned in a retort constructed near its portal. Since 

 1940 the mine had not been worked, though the most 

 recent tunnel is still open and some cinnabar is visible. 

 No accurate production records for the San Mateo 

 mine are available, but it seems likely that at least 

 1,000 flasks has been recovered from its small but rich 

 ore bodies. 



Geology and ore bodies 



The San Mateo mine is unique in the district in that 

 its ores and all its workings lie in rocks of the Fran- 

 ciscan group. These rocks are dominantly graywacke 

 and shale, but they include a little greenstone, and 

 in the easternmost accessible workings some black 

 limestone. The rocks are in great part highly sheared 

 parallel to their bedding planes, which dip north- 

 west, and in places they resemble alta because of the 

 intercalation of layers and pods of greenstone. They 

 are cut by many steep faults, of which the most 

 conspicuous strike N. 55 W. and dip steeply north- 

 east. Along the faults, and locally elsewhere, some 

 of the rocks are silicified and others are argillized. 

 The principal ore bodies lay along the northwest- 

 trending faults. The cinnabar was partly dissemi- 

 nated as small crystals, but part of it occurred in 

 irregular minute veinlets, most of which were in 

 graywacke but some of which were in other types of 

 rock. Narrow veinlets of dolomite accompany the 

 ore, and considerable pyrite is disseminated in the 

 rocks or forms small euhedrnl crystals in vugs. 



for further development 

 The ores of the San Mateo mine were in rocks of 

 the Franciscan group rather than in silica-carbonate 



rock like nearly all the other ore bodies in the district, 

 and it may be for this reason that the mine does not 

 seem to have been developed to as great an extent as 

 most of the other productive workings in the district. 

 Judging by the occurrence of large and valuable ore 

 bodies in sandstone of the Franciscan group elsewhere 

 in the Coast Ranges, as, for example, at Oat Hill 

 (Yates and Hilpert, 1946, p. 260-265), this fact alone 

 should not discourage further development. Because 

 of the topographic position of the mine, exploration 

 at depth would have to be carried on by means of 

 drifts from a shaft, which should be located in the 

 midst of the mineralized areas. It would be reason- 

 able to expect that ore bodies of the type already 

 mined would be found along the downward extension 

 of the favorable northwest-trending faults. By com- 

 paring the ores with those in rocks of the Franciscan 

 group above the main ore bodies of the Cora Blanca 

 workings in the New Almaden mine, one might infer 

 that there is mineralized silica-carbonate rock below 

 the San Mateo ore bodies, but the lack of any nearby 

 outcrops of silica-carbonate rock or serpentine argues 

 against this speculation. 



SENATOR MINE 

 Location and extent of workings 



The Senator mine has been the most productive of 

 the "outside" mines on the New Almaden property, 

 and the only one extensively exploited since the turn 

 of the century. The mine area extends across Los 

 Capitancillos Ridge 3i/ 2 miles northwest of Mine 

 Hill (sec. 29, T. 8 S., R. 1 E.), and to the west it 

 adjoins the Guadalupe mine property. (See pis. 1, 

 14.) The workings, which were totally inaccessible 

 in 1947, are on 15 main levels and are about 21,000 

 feet in aggregate length. (See pi. 15.) They under- 

 lie an area measuring about 2,000 by 1,200 feet, and 

 they explore a vertical interval of about 1,290 feet, 

 from the surface at the crest, of the ridge to about 

 600 feet below sea level. The highest extensive level 

 in the mine, the 100 level, is reached through an iso- 

 lated adit driven southwest from the northeast slope 

 of the ridge near the Guadalupe property line. The 

 260 level, which is the most extensive, is in part a real 

 tunnel, with portals on both sides of the ridge: it is 

 reached by the Nones shaft, which extends downward 

 256 feet from the ridge top. This shaft is also con- 

 nected with the 125 and 225 levels. Two internal 

 shafts, known as the Working shaft (Old Vertical 

 winze) and the Senator incline, were sunk from the 

 260 level. The two-compartment Working shaft, 

 which is the older, extended about 263 feet to the 500 

 level; the Senator incline, which served as the main 

 hoisting shaft of the mine, was sunk 886 feet at an 



