102 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



ules of similar material replacing the cinnabar in the 

 rich ores are likewise converted to radial groups of 

 quartz. 



The quartz in the late postmineral veins is generally 

 somewhat clearer than the older quartz, and forms 

 rather spectacular groups of stubby well-developed 

 crystals in many of the vugs of the dolomite veins. 



Chalcedony <si< >.> 



Chalcedony is rare in the ores in the New Almaden 

 district. Some of the silica deposited almost contem- 

 poraneously with the cinnabar doubtless was chalced- 

 ony originally, for it shows spherulic and colloform 

 textures, but nearly all of it has recrystallized to radial 

 groups of flamboyant quartz. 



Opal (SlOinHiO) 



Opal is not a constituent of the silica-carbonate rock 

 or of the ores in this district. The only opal found 

 during the study was in the Guadalupe mine, where 

 it formed the filling in a late fracture. 



Barlte (Huso,) 



Barite was found only as small euhedral crystals 

 coating silica-carbonate rock in the old dump near the 

 Buena Vista shaft, and its relation to the ores is un- 

 known. 



Apopliylllte | K I < H,<si i > >, M I i >| 



Apophyllite crystals, saturated with hydrocarbons, 

 are said to have been found in a vein in the New 

 Almaden mine (Clarke, 1890, p. 22-23), but no details 

 concerning their relation to the ores have been given. 

 This occurrence was confirmed by the finding, in the 

 new opencut on Mine Hill, of a loose piece of silica- 

 carbonate rock containing a quartz vein studded with 

 quartz pseudomorphs after apophyllite crystals. 



Uyrollte (HiCai8liOi-9HiO) 



Gyrolite was described by Clarke (1890, p. 23) as 

 forming a fibrous layer between apophyllite crystals 

 and wallrock in a specimen from the New Almaden 

 mine. 



Hydrocarbons (compounds of H and C) 



Hydrocarbons, both tars and oils, are common in 

 and near the ore bodies in the district, where they 

 occur in thin quartz-dolomite veins. The prevalent 

 variety is a deep-brown, nearly black, tar, which is 

 hard enough to break under a sharp blow, but plast ic 

 enough to flow, at an extremely slow rate, down the 

 walls of mine workings. All gradations from this 

 dark tar to light-colored thin oils have been observed. 

 In one unusual type of occurrence, hydrocarbons fill 

 spherical shells of quartz, which are aggregated into 

 what the writers have termed "froth-veins." ( See 

 fig. 69.) Such veins are interpreted as having formed 



FIOURI 69. Photomicrograph of section of "froth vein." These veins 

 formed by crystallization of quartz at the interface between two Im- 

 miscible liquids, one of which was a thin oil and the other a hydrous 

 fluid like that which deposited the quicksilver ores. I 'MNT. Plane 

 light. Locally there were open spaces between the loosely packed 

 quartz spheres, and in a few of these dolomite (ill was ile|iositiil. 

 The thin black layer Inside spheres is a carbon residue left when the 

 specimen was roasted to remove the oil before thin sectioning. Lower, 

 Crossed nlcols. Note that each quartz shell is double owing to the 

 quartz having grown both ways from the oil-water interface. 



by quartz crystallizing first :it the interface bet worn 

 droplets of oil and hydrous vein solutions, and then 

 growing toward both the oil and the vein solution. Tin- 

 spaces between the two-layered shells in some of the 

 veins are voids resulting from tin- incomplete filling 

 of the vein. Another hydrocarbon encountered during 

 development of the deep levels of tin- New AlnmoYn 

 mine is referred to in tin- surveyors' records us "in 

 flammable gas"; this was doubtless chiefly nit-thane. 



CHARACTER OF THE ORKN 



The character of the ores is dependent upon the way 

 the ore and gangue minerals that have been described 

 fire distributed through the minerali/ed rock: but be 

 cause the distribm ion of the chief ore mineral, cinna- 



