ORE DEPOSITS 



99 



FIGURE 66. Stibnite (st.) in dolomite vein cutting earlier dolomite veins. Senator mine dump. 



Mlllerlte (N18) 



Millerite is nowhere abundant in the district, but it 

 is apparently widespread; it forms minute curved 

 needlelike crystals and "hairs" in the silica-carbonate 

 rock. It seems to be as abundant in places far re- 

 moved from any cinnabar ores as in the ore bodies 

 themselves, and is therefore thought to have formed 

 during the conversion of serpentine to silica-carbonate 

 rock rather than during the period of quicksilver 

 mineralization. 



GANGUE MINERALS 

 Dolomite [CaMg(CO3)j| 



Dolomite is the most abundant vein-forming car- 

 bonate in the district, where it has at least four modes 

 of occurrence worthy of description. In two of these 

 it is closely related to the ore deposits and may con- 

 tain cinnabar, whereas in the other two, which are 

 of later origin, it is barren. 



White dolomite accompanied by quartz forms the 

 principal filling for the northeastward-trending veins. 

 1/2 to 2 inches thick, which occur as swarms in the 

 silica-carbonate rock of most of the large ore bodies. 

 These veins, or hilos, are generally composite and 

 sharply bounded, and contain slivers of wallrock. 

 Where the hilos traverse ore bodies, the included 

 slivers are generally fragments of cinnabar ore, and 



the hilos themselves may be veined or partly replaced 

 by cinnabar. (See fig. 67.) Conversely, many hilos 

 not adjacent to ore bodies contain virtually no cin- 

 nabar. 



In some places both the silica-carbonate rock and 

 the other rocks in the mineralized area are cut by 

 prominent veins of northwesterly trend, which consist 

 largely of white dolomite but locally contain some 

 quartz. These veins are generally much thicker than 

 the hilos; they range in thickness from a few inches 

 to as much as several feet. Some of the largest and 

 best exposed are shown on the geologic map of the 

 New Almaden mine area. (See pi. 3.) Most of these 

 veins are composite and show repeated filling, either 

 simply by the layering of dolomite of different pale 

 tints or by less obvious textural discontinuities. Some 

 others, however, have been brecciated one or more 

 times during their formation and show complex inter- 

 nal structures, resulting from filling along new frac- 

 tures or coating of brecciated fragments. (See fig. 

 68.) In some places, as, for example, in the Senator 

 mine, these thick northwest-trending veins contain 

 enough cinnabar deposited intermittently with the 

 dolomite to constitute ore, but in most places they are 

 barren or contain only scattered needlelike crystals of 

 cinnabar. 



