132 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



ore was followed downward and explored laterally be- 

 tween the 600 and 800 levels. In 1875 the Grey shaft 

 was sunk, on the line of the Deep Gulch tunnel, and 

 by 1878 drifts on the 900, 1000, and 1100 levels had 

 explored the contact down the plunge of the ore body 

 found on the higher levels, but without revealing any 

 ore. 



Meanwhile an 8- by 8-foot adit, known as the Haci- 

 enda tunnel or Bottom tunnel, had been started in 

 1867 from a point in Almaden Canyon about 0.3 mile 

 upstream from the furnaces at the Hacienda. (See fig. 

 87.) This adit was intended to provide drainage for 

 the entire hill to the 1200 level and to afford under- 

 ground transport to the furnaces. Only 6 months 

 after it was begun it had been abandoned as too costly, 

 but in 1874, upon the discovery of the rich Cora 

 Blanca ore bodies, the driving of the tunnel was re- 

 sumed in order to drain the area and explore it at 

 depth. During the next 5 years the Hacienda tunnel 

 was alternately extended and abandoned until, 2,800 

 feet from its portal, it intersected the contact that at 

 higher levels formed the back of the Cora Blanca ore 

 bodies. Then, as no ore had yet been found in the 

 Cora Blanca mine below the 850 level, the tunnel was 

 abandoned without ever having served any useful pur- 

 pose. By the end of 1879 the Cora Blanca ore bodies 

 between the 600 and 800 levels had been virtually ex- 

 hausted; the mine had then produced 7,500 flasks, re- 

 covered from ore averaging 4.5 percent quicksilver. 

 All production from the Cora Blanca mine since 1880 

 has been accomplished by gouging thin veins in the 

 upper levels, removing fill from stopes, reworking old 

 dumps, and mining low-grade ore in the Los Angeles 

 opencut. The production from these later operations 

 is not recorded separately from that of the New Al- 

 maden mine as a whole, but it probably did not equal 

 the recorded early production from the high-grade ore. 



Geology 



The most important structural feature explored by 

 the Cora Blanca workings is the upper surface of a 

 serpentine body overlain by rocks of the Franciscan 

 group, which in this area are largely mafic tuffs. (See 

 pi. 12.) Although only rocks of the Franciscan group 

 are exposed at the surface, the serpentine body is so 

 well delineated by mine workings that it is known to 

 be a continuation of the sill that contains the Harry, 

 Velasco, and Randol stopes to the northwest. (See 

 fig. 81.) The rocks of the Franciscan group are poorly 

 exposed on the surface and considerably faulted in the 

 upper levels of the mine, but they apparently strike 

 northwestward and dip southwest, and are probably 

 overturned. In the accessible lower levels of the mine 



these rocks have a general northerly strike and easterly 

 dip, although in places they exhibit radically different 

 attitudes. The upper surface of the underlying ser- 

 pentine is irregular, but is broadly conformable with 

 the bedding of the adjacent Franciscan rocks. The 

 lower surface of the serpentine has not been reached 

 by any of the Cora Blanca workings, but north and 

 west of the Cora Blanca area the serpentine forms a 

 sill-like body with a thickness of more than 400 feet. 

 A part of the upper surface of the sill bulges outward 

 to form broad ridges and domes which have been im- 

 portant in localizing ore bodies, and in some places 

 apophyses and thin outlying sills extend above its main 

 hanging-wall contact. Beneath this contact in most 

 places the serpentine has been hydrothermally altered 

 to silica-carbonate rock, some of which is as much as 

 50 feet thick, and most of the apophyses and outlying 

 sills have been similarly altered. 



Ore bodies 



The ore bodies mined in the Cora Blanca area were 

 alike in that they all contained cinnabar as the only 

 ore mineral, but the cinnabar in altered tuff was dif- 

 ferently distributed than that in the silica-carbonate 

 rock beneath. The ore bodies mined from the upper 

 levels of the Cora Blanca mine, and from the Los 

 Angeles opencut and connected shallow workings, con- 

 sisted of either single veins or a few parallel veins, 

 generally less than half an inch thick, in altered mafic 

 tuff. Some of the veins contained only cinnabar, but 

 many contained dolomite gangue, and some also con- 

 tained quartz and hydrocarbons. Most of the veins 

 followed faults of small displacement striking a few- 

 degrees east of north. In the highest workings the 

 veins dip steeply to the west, but near the underlying 

 sill they steepen to vertical and then roll to dip east 

 ward and join the serpentine contact at a low angle. 

 The tuff bordering the veins is bleached and partly 

 converted to kaolinite-halloysite, and around one of 

 the small stopes it is extensively carbonati/.ed. Mo-t 

 of the veins lie directly above known ore bodies in 

 silica-carbonate rock and represent "leakages": as yet, 

 however, no ore bodies have been found below the 

 cinnabar veins mined in the southern part of the Los 

 Angeles workings. 



The more productive of the ore bodies in the Cora 

 Blanca area were those formed in the silica-carbonate 

 rock just below the upper contact of the sill. The 

 only large body of this type was mined in n nearly 

 continuous stope extending upward from the S.V) level 

 to a point above the 600 level. Between the Too and 

 750 levels, the stope branches to follow two diverging 

 ore shoots; the northern shoot extends upward along 



