ORE DEPOSITS 



105 



: 7 ^ 



FIGURE 72. Polished surface on ore specimen from Randol workings of the New Almaden 

 mine. Dark veins are cinnabar with a little quartz ; rest of specimen Is silica-carbonate 

 rock, which has replaced sheared serpentine that contained a few larger unsheared rounded 

 fragments of serpentine. This specimen is unusual in that the cinnabar is largely confined 

 to the veins, being disseminated out from them only enough to give the vein borders a 

 fuzzy appearance. U.S. National Museum specimen. 



divided for description into two groups, depending 

 upon whether the cinnabar is accompanied by other 

 minerals in veins or occurs unaccompanied in vugs. 

 The cinnabar in veins is generally accompanied by 

 quartz or dolomite and takes a variety of forms. Some 

 cinnabar is fine grained and is dispersed through the 

 other vein minerals; some forms botryoidal clusters 

 deposited along the vein walls or on previously de- 

 posited vein minerals, and this is generally itself en- 

 crusted; and some forms small crystals erratically 

 distributed between the quartz spheres in the hydro- 

 carbon-bearing "froth veins." Many of the veins that 



686-671 O 63 8 



contain disseminated cinnabar have irregular selvages 

 in which cinnabar replaces the wallrock, and it is often 

 difficult to tell just where replacement of wallrock 

 stops and vein filling begins. In other veins, in which 

 the cinnabar is separated from the wallrock by a layer 

 of dolomite or quartz, the margins are sharp and are 

 not replaced. Such veins, consisting largely of coarsely 

 bladed dolomite, probably formed the greater part of 

 the ore of the Senator mine, which averaged only about 

 10 pounds of quicksilver to the ton. 



The cinnabar occurring by itself in vugs or open 

 cracks is generally coarse grained, and as it does not 



