ORE DEPOSITS 



97 



forms, and many are two-fold twins. Such crystals 

 were described by Melville and Lindgren (1890, p. 22). 

 Needlelike crystals of cinnabar occur sparingly in some 

 of the youngest dolomite veins. Most of the cinnabar, 

 however, forms complex aggregates of minute crystals 

 exhibiting a multiude of sparkling crystal faces and 

 cleavage planes. 



Interesting, but uncommon, are small hemispherical 

 crystalline groups showing no apparent radial struc- 

 ture. These are believed by local miners to have 

 formed directly from globules of native mercury, and 

 all examples of these observed by the writers do ap- 

 pear to be accompanied by native mercury. Other 

 hemispherical aggregates of pulverulent cinnabar, 

 characterized by smooth surfaces, concentric banding, 

 and an orange- to brick-red color, were found only 

 locally in near-surface workings and may be of second- 

 ary origin. 



Metaclirnabar (HgS) 



Metacinnabar, the black tetrahedral mercuric sul- 

 fide, was not found in the course of this study, though 

 it has been said to occur in the district. It was first 

 reported by Melville (1890, p. 293-296; 1891, p. 80- 

 83), who gives an analysis of impure material and a 

 list of interfacial angles measured on minute tapering 

 conical crystals, cross sections of which are equilateral 

 triangles. He assigned these crystals to the hexagonal 

 system, and as his report fails to state their color, they 

 may have been of a deep-colored variety of cinnabar. 

 His original material is apparently no longer avail- 

 able for reinvestigation. Other metacinnabar speci- 

 mens labelled "collected by F. L. Ransome from the 

 lower levels of the New Almaden mine, definite point 

 unknown" were loaned the writers by the U.S. Na- 

 tional Museum; but as the specimens consist in part 

 of a dense black silica-carbonate rock, which is not 

 known to occur in the New Almaden mine, and as they 

 contain a large amount of jarosite, which also is un- 

 common in the mine, it seems probable that the speci- 

 mens are incorrectly labelled. 



Metacinnabar is also reported in the bulletins on 

 "Minerals of California" (Eakle, 1923, p. 94; and 

 Pabst, 1938, p. 56) as occurring in both the New 

 Almaden and Guadalupe mines, Melville's article be- 

 ing cited as authority. No specimens of metacinnabar 

 from the district were found, however, in the museums 

 of the California State Division of Mines, of Stanford 

 University, of the University of California, or of the 

 University of California at Los Angeles. The occur- 

 rence of metacinnabar in the district therefore needs 

 to be verified, although there is no reason for believing 

 that it could not occur there. 



Mercury (Hg) 



Native mercury did not occur in much of the ore 

 in the district, though in some it occurred in consider- 

 able quantity. All the native mercury seen by the 

 writers was accompanied by cinnabar. Most lay in 

 fractures and vugs in silica-carbonate rock, but some 

 of -that in the Cora Blanca workings of the New Alma- 

 den mine was in greenstone tuff, and in the adjacent 

 Harry workings it occurred in graywacke of the 

 Franciscan group. Although native mercury is be- 

 lieved by some to be a supergene mineral, no relation 

 between its abundance and depth is apparent in the 

 New Almaden district. It was found near the surface 

 in the opencuts developed at the New Almaden mine 

 during World War II; but it was also found at a 

 depth of about 500 feet in the same mine near the 

 Santa Rita stope and in the Far West stope, and it 

 was reported {Christy, 1879, p. 455) to have "run out 

 of shattered alta" on the 1500 and 1600 levels in the 

 Randol part of the mine. Specimens from the Guada- 

 lupe mine containing a little native mercury have been 

 seen, but their exact source is unknown. No native 

 mercury has been observed in ores from the Senator 

 mine. 



Tlemannlte (HgSe) 



Tiemannite, the gray mercuric selenide, is said to 

 have been found in ores from the Guadalupe mine 

 (Eakle, 1923, p. 64), but none was found in this study. 



ACCOMPANYING SUIFIDES 

 Pyrlte (FeSi) 



Pyrite is generally scarce in the quicksilver ores of 

 the district, and in none of them was it so abundant 

 as to necessitate special precautions in furnacing. As 

 pyrite is more widespread than cinnabar and is not 

 particularly concentrated in or near the ore bodies, it 

 cannot be used as a guide to ore. Many specimens of 

 high-grade cinnabar ore contain no visible pyrite, but 

 others equally rich contain several percent. Con- 

 versely, some barren silica-carbonate rock, especially 

 the black chalcedonic variety common in the western 

 part of the district, contains as much as 5 percent of 

 pyrite. In some places the pyrite was deposited be- 

 fore the cinnabar; elsewhere it accompanies dolomite 

 veins that were formed after the ore bodies. Some 

 specimens from the Senator mine show alternations 

 of pyrite and cinnabar in banded dolomite veins as is 

 shown in figure 65. Other specimens from the same 

 mine show cinnabar crystallizing selectively on pyrite, 

 but such relationship is rare. Pyrite found near the 

 surface is generally altered to hydrous iron oxides, 

 but the major source of the iron stains in the ocherous 



