42 2 REVIEWS 



covellite, some argentite, pyrite, and native silver are probably due to 

 sulphide enrichment, but a part of the argentite and pyrite and all of the 

 bornite, chalcopyrite, tetrahedrite, and gaJena are apparently primary. 



Quartz was the first mineral to be deposited and it was followed at 

 once or even accompanied by the greater part of the sulphides. The 

 later stages of vein-fiUing were marked by coarsely crystalline calcite, 

 largely manganiferous and barren of sulphides. Mineralization of the 

 wall rock is not prominent; in andesites, pyrite has penetrated the rock 

 for short distances from the veins, or the rock is cut by veinlets of quartz, 

 calcite, and copper sulphides. The dark silicates in the andesites are 

 often almost completely altered, and calcitization is common. The 

 rhyohtic country rock near the veins shows some secondary siHcification, 

 and the feldspars are frequently replaced by quartz with an apparent 

 decrease of volume. 



Although the oxidation zone is generally shallow, it may extend to 

 the 500- or 700-foot levels. Argentite appears here and there as filling 

 in minute cracks, apparently later than the veins, and pyrite clearly of 

 later origin is seen in fissures in the quartz. No sulph-arsenic or sulph- 

 antimony salts of silver, such as commonly characterize many sulphide 

 enrichment zones, are found, but chalcocite and a small amount of 

 covellite do attest the effectiveness of some enriching action. Probably 

 sulphide enrichment is appreciably active only where unusually favorable 

 conditions, such as later faulting, were present. The ground water-level 

 lies very deep and the mines are dry, except along the Queen fault. 



The ore shoots are 300 to 600 feet in drift length and about as long 

 parallel to the dip, with widths averaging 5 to 15 feet. They tend to 

 show flat bottoms, which is thought by the investigator to be suggestive 

 of enrichment. 



The peculiar localization of the productive veins within a small 

 district not over one square mile in extent is attributed to the faulting, 

 which only produced the throw necessary to bring the ore within reach 

 of the surface over a very small area; greater or lesser throws either 

 resulted in too deep an erosion or in the exposure of the barren low- 

 temperature manganiferous-calcite veins. 



The important mines are the Cooney, Little Fanney,and Last Chance; 

 the last being the largest. In the past twenty-five years the Last Chance 

 mine has yielded about $7,500,000, worth of ore. The metals obtained 

 are gold and especially silver, and a high-grade ore lies close to $20 a 

 ton in value. 



C. H. B., Jr. 



