166 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



depth can be determined only as far down as the 200 

 level ; here the body appears to be much narrower 

 than at the surface, and in the central part, at least, 

 of the mine area it apparently pinches out completely 

 near the 300 level. To the southeast, in the Senator 

 mine, the extension of the same serpentine body also 

 pinches out, at a slightly lower level. The entire 

 northwestern part of the serpentine body is altered 

 to silica-carbonate rock, and so are parts of the mar- 

 gins of the rest of the mass. The silica-carbonate rock 

 along the southern margin is rich in carbonate like 

 that along the southern part of the entire intrusive 

 complex; but that along the northern margin is gen- 

 erally hard and flinty, apparently contains little car- 

 bonate, and is cut by only a few thin carbonate veins. 

 In general this siliceous type of silica-carbonate rock 

 seems to be unfavorable for ore deposition, though a 

 few small ore bodies have been found in it at depth. 



A landslide, containing brecciated silica-carbonate 

 rock, that lies near the crest of the ridge requires more 

 lengthy description, not because of its economic im- 

 portance, which is small, but because it contains some 

 unusual conglomerates and breccias not found else- 

 where and not previously described in this report. 

 The breccia, which covers an area of about 360,000 

 square feet, is explored by the Kelly workings, the 

 New Prospect tunnels, and other shallow cuts. On 

 the surface it is similar in general appearance to some 

 of the blocky outcrops of silica-carbonate rocks found 

 elsewhere in the district, but in cuts and underground 

 workings where it is better exposed it is clearly seen 

 to have a different structure. It is made up of frag- 

 ments that are angular to subangular and range in 

 diameter from several feet to a small fraction of an 

 inch, with all sizes about equally well represented. 

 There is no matrix, for the smaller fragments are 

 packed in solidly between the nearly equidimensional 

 larger blocks. Scattered through this breccia are a 

 few carbonate veins filling straight fractures; and 

 carbonate fills irregular cracks and cavities between 

 the angular fragments. 



A second unusual rock in the area is another type of 

 breccia, similar in texture to that just described but 

 of different composition. It was observed only in the 

 New Prospect tunnels, where it consists of angular 

 fragments of graywacke and shale of the Franciscan 

 group embedded in a matrix of buff to light-brown 

 clay. The largest fragments do not exceed 18 inches 

 greatest diameter, and most are no more than a few 

 inches long. 



A third unusual lilhologic unit is an unconsolidated 

 pebble conglomerate Ix-lieved to be of late Tertiary 

 nge. This rock is well exposed in only two places, 



both near the north edge of the area. The first is at 

 the portal of the middle New Prospect tunnel, where 

 it forms a 2-foot bed overlying arkose of the Fran- 

 ciscan group and overlain by highly sheared weath- 

 ered serpentine. For at least 300 feet west and north 

 of the adit, scattered pebbles from the conglomerate 

 may be found on the surface close to the serpentine 

 contact, but there are no real outcrops in this area. 

 The second place is in the Kelly tunnel, 800 feet north- 

 west of the New Prospect tunnels, where a 5- to 6-foot 

 bed of the conglomerate lies above greenstone and 

 below silica-carbonate rock, from which it is separated 

 by a low-angle shear. The conglomerate has a sandy 

 matrix and contains a mixture of pebbles and cobbles 

 of graywacke, siltstone, greenstone, and chert of the 

 Franciscan group, considerable serpentine, and a few 

 well-rounded pebbles of soft ocherous sandstone simi- 

 lar to the Upper Cretaceous sandstone of the Santa 

 Teresa Hills. It is poorly bedded, and in the only 

 two exposures it has widely different attitudes. We 

 believe it to be of late Tertiary age, but on no more 

 definite evidence than its unconsolidated character 

 and its geologic occurrence. 



The origin and structural relations of these un- 

 usual units could not be determined witli certainty 

 from their limited exposures. In general the breccias, 

 as exposed in the adits, dip gently southwestward into 

 the ridge, and at some places in the New Prospect 

 tunnels they are overlain by rocks of the Franciscan 

 group. Diamond-drill holes extending downward 

 from the crest of the ridge in the Kelly area have 

 reached the base of the brecciated silica-carbonate 

 rock, and they indicate that it forms a flat-lying 

 roughly lenticular body having a maximum thickness 

 of about 75 feet. The character of the material and 

 its general distribution suggest that it may be a rem- 

 nant of a late Tertiary (?) landslide which migrated 

 from the southwest when the ridge was somewhat 

 higher and overlapped a late Tertiary terrace deposit. 



Ore bodies 



The chief ore mineral of the Guadalupe mine is 

 cinnabar, though a little native mercury was found 

 on the No. 8 level of the Old Mine and possibly else- 

 where. In character and occurrence the ores were like 

 those in the other large mines in the district: the 

 cinnabar was deposited in and beside hilos cutting 

 through silica-carbonate rock, and was especially 

 abundant where the hilos were closely spaced or were 

 near the intrusive contacts. The grade of the ore 

 probably varied considerably, even within a few 

 incites, and since hand sorting in the early days re- 

 sulted in the discarding of rock that in more recent 



