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University of California Publications in Geology 



[Vol. 8 



it occurs in its freshest state at points deep below the surface, as for 

 example, the bottom of Eureka Pit and the deepest workings of the 

 Veteran Mine. The alteration is also least where the ore is poorest, 

 in general. 



The progress of alteration, as traced from the least to the most 

 altered portions, is, first, kaolinization of the feldspars ; then a develop- 

 ment of sericite and of a brown mica, accompanying the disappearance 

 of the hornblende ; and. lastly, a more or less complete replacement 

 of the whole by silica. At the stage where kaolinization is the char- 

 acteristic alteration, the original structure of the rock is vaguely pre- 

 served in a mottled appearance, with here and there the form of a 

 large orthoclase phenocryst remaining. This mottling gradually dis- 

 appears with the increase of sericite and quartz, until finally nothing 

 remains but a bluish-white, soft friable mass, of mixed sericite and 

 silica, blotched in places with brown mica and speckled with chalcocite, 

 chalcopyrite, pyrite, and molybdenite. 



The alteration appears to have extended outward from anastomoz- 

 ing, more or less vertical fractures, thus leaving central lenses of less 

 alteration in which the mottling is quite conspicuous. However, no 

 continuous channels of dominant sericitization or of silicification are 

 distinguishable, and it appears that the alteration could not have been 

 effected at distinct periods in which first one and then the other 

 process was dominant, but that the different phases of alteration must 

 have been performed by the same solutions according to varying local 

 conditions. Also, since the quartz of the veinlets contains secondary 

 sulfids, the two must have been formed simultaneously. 



DEPOSITION OF SECONDAKY SULFIDS 



The secondary sulfids are disseminated through all the altered 

 phases of rock, but are most abundantly associated with sericite or 

 silica. This applies particularly to the chalcocite, which is commonly 

 found as an original constituent of the anastomozing veinlets of silica. 

 Chalcocite is the dominant copper sulfid, but pyrite is probably in 

 greater proportion, in most of the ore, in the form of small cubes or 

 dodecahedrons or irregular masses. The pyrite contains variable 

 amounts of copper, and the massive pyrite grades so imperceptibly 

 into chalcopyrite as to point strongly to the derivation of the latter 

 from the former by secondary enrichment in copper. It also appears 

 that the chalcocite had a similar origin, for in many cases grains which 



