72 ECONOMIC GEOLOGY 



metal therefore is found most abundantly in what may be only 

 the lower portions of the argentiferous veins. 



Sierra Region : Goldfield is situated in the southwestern part 

 of Nevada in Esmeralda County. (See Fig. 51.) According 

 to F. L. Ransome the base of the geological series consists of pre- 

 Cambrian metamorphics. These suffered denudation until the 

 close of the Jurassic Age, when the intrusive alaskite and granite 

 were introduced. This was followed in Tertiary time by eruptives 

 ranging from rhyolite to basalt. The dacite is the most produc- 

 tive extrusive, while some rich ores are found in the andesites. 

 The ore bodies are noted for their remarkable richness and irregu- 

 larity. The fissures are usually irregular, small and intersecting 

 fracture-flows passing into brecciated material. Faulting seems 

 to be absent. After the dacite lode solidified unknown stresses 

 developed this intricate fracturing. The ores are free gold and 

 auriferous pyrite associated with silver, copper, antimony, arsenic, 

 bismuth and tellurium minerals. Magmatic waters contributed 

 the gold to the fissures. These ascending solutions bore H 2 S 

 and CO 2 Near the surface the hydrogen sulphide was in part 

 oxidized to H 2 SO4- The downward trend of these acid solutions 

 through the shattered dacites and andesites and their subsequent 

 mingling with ascending solutions caused the precipitation of 

 their metallic contents. The gold would be precipitated by alka- 

 line carbonates, as native gold. The freshly formed pyrite would 

 serve as a reducing agent upon its encased gold. A second stage 

 of fracturing and mineralization seems to have occurred in this 

 field (Figs. 52 and 53). 



Comstock Lode : The Comstock Lode is situated in the south- 

 western part of Nevada on the eastern flank of Mt. Davidson in 

 the vicinity of Virginia City, Washoe County. The geology of 

 this region has been a matter of much study on the part of able 

 scientists like Becker, Von Richthofen, Hague and Iddings. Von 

 Richthofen considered the ore body as filling a fissure on the 

 contact between a syenite foot-wall and an eruptive propylite 

 hanging wall (Fig. 54). 



Clarence King considered that the vein filled a fissure between 

 a syenite and the Tertiary eruptives poured out upon the flank 

 of Mt. Davidson. Arnold Hague and J. P. Iddings, from an exten- 

 sive study of the rock masses, concluded that the Comstock Lode 

 occupies a line of faulting rocks of the Tertiary age and cannot be 

 considered as a contact vein between two different rock masses. 



