OEE DEPOSITS 



103 



FIGURE 70. Polished specimen of rich ore from the New Almaden mine, showing cinnabar (gray) replacing 

 silica-carbonate rock. Probably the nearly straight left edge resulted from the specimen breaking free 

 along the margin of a quartz-dolomite hilo, for typically the cinnabar replacement extended outward from 

 hilos for a distance of a few inches and terminated abruptly, as In this specimen. 



bar, is of prime importance, the following discussion 

 is largely confined to a description of its mode of oc- 

 currence. The greater part of the ore mined in the 

 district is of primary origin, but one alluvial deposit 

 containing transported ore has been mined. In this 

 alluvial deposit the nuggets of ore are like the primary 

 ore in all respects; thus, they require no further de- 

 scription. 



The cinnabar ores in the New Almaden district were 

 formed by both replacement and filling of open spaces, 

 and, although the ore in some specimens and even in 

 a few ore bodies was all formed by one of these proc- 

 esses, most of the ore bodies contain cinnabar deposited 



in part by one process and in part by the other. Con- 

 sidering the district as a whole, however, far more 

 cinnabar was deposited by replacement of the country 

 rock than was deposited in open spaces. 



Ore formed in silica-carbonate rock by replacement 

 is the common variety in the district, but small quan- 

 tities of ore formed by replacement of graywacke and 

 shale of the Franciscan group have been mined. There 

 is compelling evidence that these ores were formed by 

 replacement. The nearly perfect preservation of rock 

 textures by fine-grained cinnabar and the absence of 

 distinct veins or veinlets of cinnabar both indicate re- 

 placement (figs. 70-72). Still more conclusive is the 



