CHAPTEE VI. 



HOCK ALTERATION. 



ALTERATION OF THE EARLIER ANDESITE. 



The alteration of the earlier andesite by thermal waters has been profound, 

 indicating that these solutions were present in large quantity and were very 

 active. 



ALTERATION OF EARLIER ANDESITE CHIEFLY TO QUARTZ, SERICITE, AND 



ADULARIA. 



On Mizpah Hill the andesite is entirely altered and has a siliceous, light- 

 colored rhyolitic appearance nearly everywhere, except in depth, where the 

 Mizpah shaft on the 700-foot level shows earlier andesite altered largely to 

 chlorite, separated from the quartz-sericite alteration by a fault, and, so far as 

 yet explored, marked by the absence of veins. 



ALTERATION OF HORNBLENDE AND BIOTITE. 



Various stages in the alterations are observable. The ferromagnesian minerals, 

 hornblende and biotite, have usually been completely destroyed. Their areas are 

 marked by liberally sprinkled pyrite crystals, by siderite, and often by some 

 sericite. Frequently the grouping of the iron minerals, which follows with more 

 or less clearness the well-known outlines of an original hornblende or biotite, 

 affords the only evidences of the former existence of these phenocrysts, at the 

 same time plainly showing the demarcation of the pyrite and siderite from the 

 original ferromagnesian minerals. In further stages of alteration the pyrite and 

 siderite have escaped from the confines of the original crystal and are scattered 

 through the rock; in this case they are usually less abundant, showing a leaching 

 of iron out of the rock as the silicification increases. It has been determined by 

 assay that the pyrite in these rocks does not contain appreciable amounts of gold 

 and silver, even close to the veins. 



In other phases the ferromagnesian minerals have been entirely altered to 

 fine muscovite (sericite) and quartz. 



The alteration of biotite has been sometimes not so complete as just sketched, 

 the mineral having been bleached and the separated iron represented by pyrite and 



siderite. 



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