1917] Clark: Geology and Ore Deposits of the Leana Rhyolite 375 



A second generation of pyrite occurs in a very fine, dense deposit 

 in fracture zones in the first generation. With this later pyrite small 

 stringers of chalcopyrite and sphalerite have been deposited contem- 

 poraneously. 



The second type of pyrite is much harder in character than the 

 type just described, for it contains a considerable amount of silica 

 between the partially developed crystals and between the irregular 

 masses of the ore. Where the crystals are not in contact they are 

 very securely cemented together by a siliceous matrix. Chalcopyrite 

 of two generations appears in this pyrite ; some was deposited with 

 the pyrite and some has since been deposited along cracks, partially 

 replacing the pyrite. Sphalerite and bornite are associated with the 

 second generation of chalcopyrite and were probably deposited at the 

 same time. One or two small stringers of chalcocite were observed 

 along very small fractures in the chalcopyrite. 



The highly silicified country rock, containing a large number of 

 irregular masses of pyrite of varying size, seems to represent the first 

 stages of metasomatic replacement in the formation of the ore body. 



Genesis of the Ore. — Two general hypotheses may be advanced to 

 account for the origin of the pyrite at Leona Heights : 



(1) It was deposited as a metasomatic replacement in the rhyolite 

 by magmatic waters along a fracture zone. 



(2) It was formed as a metasomatic replacement in the rhyolite 

 by cold descending solutions carrying iron sulphates derived from the 

 oxidation of the disseminated pyrite. 



The first of these processes does not seem to have had any part 

 in the production of the ore body, for if the sulphides were deposited 

 by magmatic waters there would be evidence in the mine of hydro- 

 thermal alteration, together with minerals that are thought to be the 

 products of ascending thermal solutions ; but nowhere in the mine has 

 such alteration been observed or such minerals found. 



The second hypothesis appears to account for the ore at Leona 

 Heights. Small deposits of pyrite formed by meteoric waters at 

 ordinary temperature have been reported from numerous localities, 

 the iron in many cases being supposed to have been derived from the 

 ferromagnesian minerals, and the deposition of the pyrite in the 

 present instance has been probably brought about by similar processes. 



It is thought that the ore at Leona Heights was formed in a manner 

 analogous to that known as the process of secondary sulphide enrich- 

 ment. The disseminated pyrite when oxidized at the surface formed 



