1914] 



Whitman: Notes on the Copper Ores at Ely, Nevada 



315 



PEECIPITATION OF SULFIDS ABOVE WATER TABLE 

 Pyrite has been precipitated in kaolin in the laboratory 3 under 

 conditions similar to those of vadose circulation ; and the writer 

 believes that either sericite or kaolin is capable of producing the 

 same effect in nature, without reference to the ground-water level. 

 If this is true, then meteoric waters carrying the soluble products of 

 oxidation of cupriferous pyrite, in percolating down through a porous 

 and much altered rock like the ore-porphyry, would here and there 

 have precipitated from it grains of pyrite which in turn would pre- 

 cipitate, by a process of metasomatism, chalcopyrite, and this in turn 

 chalcocite. 



That this process might have occurred and that these ores might 

 have originated above the ground-water level is a possibility which 

 cannot be ignored. The writer regards it as a plausible hypothesis. 

 If the massive cupriferous pyrite deposits of Shasta County, Cali- 

 fornia, can be regarded as typical of ores formed by the regrowth 

 of sulfid bodies at ground-water level, from sulphate solutions descend- 

 ing from the zone of oxidation, then this Ely ore is a notable exception 

 which challenges explanation. Certainly a water-table intersected by 

 a vertical downward circulation constitutes a rather definite locus for 

 the precipitation and massive growth of sulfid bodies, whether the 

 sulphate solutions originate from the oxidation of disseminated sulfids 

 or from a massive sulfid body ; but the Ely sulfids are diffused through 

 a vertical range of over six hundred feet, being of essentially the 

 same character and richness throughout, save where concentrated 

 along gouge-seams or impervious contacts, which fact in itself indicates 

 not stagnation but active circulation such as characterizes the vadose 

 region. 



The c-uestion then arises as to the mode of reduction and precipita- 

 tion of sulfids from sulphate solutions in the zone of oxidation. The 

 answer to this involves a subdivision of that zone into (1) an upper 

 region of general and complete oxidation, characterized by the 

 universal oxidation of iron compounds to limonite or hematite result- 

 ing in the formation of gossan or in the discoloration of the rock, 

 and (2) a lower region of localized oxidation along channels of more 

 active circulation, such as fractures and joints. In this region the 

 general mass of rock not immediately adjacent to such channels is not 

 completely oxidized, but is undergoing a slow alteration under the 

 influence of diffused moisture. 



s Econ. Geol., vol. 8, no. 5, pp. 455^68, August, 1913. 



