122 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



OENEHIH 



AOE OF THE QUICKSILVER ORES 



If it be assumed that the quicksilver was all de- 

 posited during one period of mineralization, that pe- 

 riod fell between the late Miocene and the Pliocene 

 and Pleistocene. This age assignment agrees with 

 others that have been made for quicksilver mineraliza- 

 tion in the California Coast Ranges (Ransome and 

 Kellogg, 1939, p. 365; Eckel and others, 1941, p. 544; 

 Bailey and Myers, 1942, p. 420; Yates and Hilpert, 

 1945, p. 23; and Yates and Hilpert, 1946, p. 252), 

 although at Sulphur Bank (Everhart, 1946, p. 139- 

 140) the deposition of quicksilver has persisted until 

 Recent time. 



The quicksilver ores cannot have been formed later 

 than the perched gravel lying on the north slope of 

 Blossom Hill, 6,000 feet S. 20 E. of the Union School, 

 for this gravel contains detrital cinnabar. The gravel, 

 which is one of several isolated remnants of an old 

 stream deposit, lies at an altitude of 700 feet above 

 sea level, or about 400 feet above the floor of the ad- 

 jacent Santa Clara Valley, and it has been correlated 

 with the rather heterogeneous assemblage of marine 

 and continental sediments that constitutes the Santa 

 Clara formation. In view of the physiographic 

 changes and the amount of erosion since the deposition 

 of the stream gravel, this formation must be at least 

 as old as earliest Pleistocene and is probably Pliocene. 



The quicksilver ores cannot be older than the silici- 

 fied volcanic rock of middle or late Miocene age crop- 

 ping out on a small knoll 2.0 miles S. 43 E. of Lone 

 Hill, for this rock is cut by quartz veins containing 

 cinnabar. Indirect supporting evidence is given by 

 dolomite veins which cut rocks of late Miocene age on 

 the north slope of the ridge by the Guadalupe mine. 

 These veins do not contain cinnabar, but similar veins 

 in the Guadalupe mine are so closely related to the 

 ores that they are believed to represent the last stages 

 of the mineralization. Additional evidence that the 

 mineralization is later than late Miocene is based on 

 the fact that the cinnabar occurs along sharply de- 

 fined fractures in the silica-carbonate rock, the age of 

 which is believed to be later than late Miocene. A 

 secondary line of evidence indicating that the quick- 

 silver ores were deposited fairly recently is the general 

 lack of faulting of either the silica-carbonate rock or 

 the ore bodies observed in the mines. 



CHARACTER OF THE MINERALIZING AGENTS 



From the relations of ore bodies to structural fea- 

 tures, we conclude that the ore-forming agent traveled 

 upward, chiefly along zones of open fractures, espe- 

 cially those which were later filled to form hilos. It 



also tended to follow contacts between silica-carbonate 

 rock and rocks of the Franciscan group, and it appears 

 to have been retarded and spread out beneath domal 

 and troughlike structures. It was not confined, how- 

 ever, to open fractures, for it also penetrated the seem- 

 ingly unfractured part of the silica-carbonate rock 

 bordering the fractures for distances of several inches 

 to a foot, depositing cinnabar and some quartz, and 

 simultaneously dissolving other substances, chiefly 

 magnesium carbonate. Considering the large amount 

 of material deposited, and the equally large amount 

 taken into solution, the mineralizing agent must have 

 been a fluid rather than a gas. 



The nature of the ore-forming fluid can be inferred 

 only from the minerals that it deposited or took into 

 solution. The most effective solvents of mercurial salts 

 and silica are alkaline solutions, from which cinnabar 

 can be deposited directly, and, as such solutions are 

 also capable of dissolving magnesium carbonate, the 

 ore-forming fluid may have been an alkaline hydrous 

 solution. 



DEPTH OF DEPOSITION OF THE QUICKSILVER ORES 



Quicksilver deposits are classed as epithermal (Lind- 

 gren, 1933, p. 469), meaning they are deposited at rela- 

 tively shallow depths. As the New Almaden mine is 

 the deepest quicksilver mine in the world, it offers an 

 unparalleled opportunity to estimate the vertical range 

 through which quicksilver ore bodies may form ; and, 

 if the amount of erosion since their deposition can be 

 established, it provides an opportunity t<> establish the 

 maximum depth at which they are known to have 

 formed. 



The deepest workings in the New Almaden mine are 

 on the 2450 level, which is 643 feet below sea level, 

 and geologic reports contain many statements to the 

 effect that ore was found at this depth. There is no 

 record in the company surveyors' notes, however, to 

 show that cinnabar was found on this level or on 

 either the 2300 or 2200 level. The deepest opening 

 reported to contain ore is a shallow win/.e sunk from 

 the 2100 level, which lies 275 feet below sea level. A> 

 good ore is reported to have cropped out on the apex 

 of Mine Hill at an altitude of l.T.M) feet, the vertical 

 range through which the cinnabar ores of the district 

 as a whole were deposited was at least L'.O-J.'I feet. But 

 the 2100 level lies only 1,275 feet below the ground 

 surface just overhead, and ore found in a crosscut 

 south of the Randol shaft of the New Almaden mine 

 on the 1800 level actually lies deeper below the pres- 

 ent overlying surface, which is 1,430 feet above it. 



To estimate the maximum depth and vertical range 



