ORE DEPOSITS 



101 



FIGURE 68. Post-ore dolomite vein from Deep Gulch, New Almaden mine area, showing result of repeated brecciation and filling. 



Calotte (CaCOi) 



Calcite occurs locally in small veins and irregular 

 veinlets in many of the varied rocks of the Franciscan 

 group ; but since these veins are as abundant in places 

 far removed from the mines as they are near the ore 

 bodies, they probably are not genetically related to 

 the mineralization that formed the quicksilver ores. 



Quartz (SlOj) 



Quartz, though not so abundant as the carbonates, 

 is as widely distributed. It is an original constituent 

 of the silica-carbonate rock, has been deposited with 

 cinnabar in the ores formed by replacement, locally 

 replaces cinnabar in some of the richest ores, accom- 

 panies dolomite in the hilos, and occurs in small 

 amount in the postmineral veins. It was thus depos- 

 ited before, during, and after the main period of cin- 

 nabar deposition. 



The quartz in the unmineralized silica-carbonate 

 rock occurs partly in veins, but the greater part is in 

 microcrystalline patches distributed throughout the 

 rock. The vein quartz is readily recognized, for it is 

 comparatively coarse-grained and clear to milky white 

 in color; but the disseminated quartz cannot be recog- 

 nized megascopically, except where it is especially 

 abundant. In thin sections it appears as small irregu- 

 lar areas of minute interlocking anhedral grains scat- 

 tered among larger areas of coarser-grained magnesite. 



The quartz that was deposited with cinnabar in the 



replacement of silica-carbonate rocks to form the richer 

 ores is recognized by its distribution rather than its 

 character, for it does not differ even in thin section 

 from the quartz originally present in the unmineral- 

 ized silica-carbonate rock. Occasionally one can see 

 a somewhat wider zone of microcrystalline quartz ex- 

 tending along a replacement vein of cinnabar. And 

 in some replacement ores quartz is present almost to 

 the exclusion of all carbonates, so that quartz is known 

 to have been introduced with the cinnabar even though 

 no criteria by which it can be distinguished from the 

 earlier quartz have been discovered. 



The quartz that replaces some of the rich ores can 

 be seen in thin section to occur chiefly in irregularly 

 bulbose areas, imparting a worm-eaten appearance to 

 an otherwise solid mass of cinnabar, and it forms iso- 

 lated euhedral crystals in the cinnabar. The irregular 

 areas of quartz show their relation to the cinnabar by 

 their general shapes, and some of the euhedral crystals 

 contain small inclusions of cinnabar and thin films of 

 it extending along lines of growth. 



The quartz in veins cutting the ores commonly ex- 

 hibits in thin section a mosaic central part and a flam- 

 boyant border. In some places the quartz veins con- 

 tain borders that appear, in plane-polarized light, to 

 be colloform like chalcedony, but under crossed nicols 

 these borders are seen to be recrystallized to quartz in 

 optical continuity with adjacent grains of coarser 

 quartz coating the colloform vein walls. Some spher- 



