1917] Clark: Geology and Ore Deposits of the Leona Rhyolite 377 



known fact that crystals of pyrite, according to Ransome, 18 are found 

 in and associated with kaolin, for kaolin is very abundant in the 

 oxidized zone at Goldfield, and is also found in the sulphide zone, 

 associated with quartz, alunite, and crystals of pyrite. He also states 19 

 that pyrite forms a considerable part of the altered rocks, particu- 

 larly the dacites and latites that have been altered to alunite and 

 kaolin rock. In the Butte district Weed 20 reports pyrite, sericite, and 

 quartz as the chief minerals in the altered zones. Mr. Lindgren 21 

 gives pyrite as a mineral formed in the zone of secondary sulphide 

 enrichment by meteoric waters. 



The Relation of Fracturing to the Ore Deposit. — The rhyolite in 

 the vicinity of the ore deposit is intensely fractured, as shown in the 

 mine workings. Along most of the fracture planes the fact of move- 

 ment is shown by slickensides or gouge. The intensity of the move- 

 ment along fractures has been doubtless due to the larger faults that 

 traverse this region. The major fault traverses the valley to the west, 

 while a minor fault, as shown on the geologic map, extends through 

 the hill near the pyrite deposit. The development of this complex 

 fracture zone has facilitated the passage of the meteoric waters through 

 the acid porphyry in such a way as to cause the localization of the ore 

 at this point. It is also possible that the two types of pyrite are the 

 products of a continuous period of mineralization interrupted by 

 faulting movement. 



The Reducing Effect of the Rhyolite. — The oxygenated meteoric 

 waters descending through the intensely fractured acid porphyry 

 would react on the feldspars in the wall rock, producing sulphates, 

 kaolin, and silica, and it seems probable that by the time these de- 

 scending solutions had traveled vertically and probably for some dis- 

 tance horizontally through the rhyolite, which was then much thicker 

 and wider than at present, they would have become neutral, or even 

 slightly alkaline. According to Emmons, 22 acid solutions may become 

 neutral, and even alkaline, in a relatively short distance from the 

 surface. He says: "There is considerable evidence that acidity de- 

 creases below the water-level. Analysis of two samples of water taken 

 from a column of water in the Callaway shaft, Ducktown, Tennessee, 

 indicate a decrease in acidity of more than 50 per cent within a ver- 



i* Prof. Paper, U. S. Geol. Surv., no. 66, p. 127, 1909. 

 » Ibid., p. 113. 



2" Prof. Paper, U. S. Geol. Surv., no. 74, p. 90, 1912. 



21 P>onomic Geology, vol. 2, p. 105, 1907. 



22 Oil. cit., p. 89, 1913. 



