96 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



EVIDENCE FAVORING SECONDARY DEPOSITION OF SULPHIDES BY DESCENDING WATERS. 



On the other hand, the formation in the oxidized zone of limonite from pyrite 

 and of cerargyrite from sulphides affords evidence that the metallic minerals of 

 the ores have actually been dissolved and reprecipitated by surface waters, and in 

 several cases the occurrence of rub3 r silver (pyrargyrite) in cracks in these partially 

 oxidized ores shows beyond a doubt that it also is due to descending surface 

 waters. Moreover, some of the ores, when studied microscopically, show argentite 

 fringing cerargyrite, as if secondary to it. The iron sulphate necessary to the 

 solution of the silver sulphide has been present (as is shown in the alteration of 

 calcite to gypsum) and the silver has actually been dissolved, and such occurrences 

 of secondary sulphides as have been described would be the natural result. The 

 evidence therefore favors the view that these secondary sulphides in the oxidized 

 zone originated from descending surface waters and probably part, but not all, of 

 the sulphides in druses in the sulphide ore have a similar origin. 



The characteristics of the superficial alteration of the ores are those which 

 naturally result from the climatic and topographic conditions." In all of the 

 mines discussed (yielding ores) standing ground water is lacking; at least, none 

 has been encountered up to the considerable depths attained (over 1,100 feet). 

 Therefore the alteration is spotty and incomplete, but extends irregularly to very 

 considerable depths in various places. 



No definite secondary sulphide zone has been noted, the secondary sulphides 

 being associated with the predominant oxides, chlorides, etc., in the oxidized 

 zone and coating crevices in the primary sulphides. 



VEINS OF THE TONOPAH BHYOLITE-DACITE PERIOD. 



In many mine workings there are quartz veins of a certain class which are 

 large and may carry values, but which are to be separated from the principal ore- 

 bearing system. These are easily confounded with the veins of the earlier 

 andesite, just as the silicified Tonopah rhyolite-dacite, in which they usually occur, 

 may be confounded with certain highly silicified phases of the earlier andesite. 

 Such veins have been encountered in the Belle of Tonopah, the King Tonopah, 

 the Mizpah Extension, the Desert Queen, North Star, Montana Tonopah, Mizpah, 

 Midway, MacNamara, West End, Tonopah Extension, and Ohio Tonopah, and are 

 described in the detailed account of these mines. On account of their resemblance 

 to the earlier andesite veins they have been the object of a good deal of exploration 

 and development work, which, on the average, has been decidedly unprofitable. 



In connection with the occurrence of such veins, which are described elsewhere 

 in the report in the mine descriptions, another occurrence, somewhat different 

 from the rest and having considerable interest, maj r be described. 



"Spurr, J. E., Geology Applied to Mining, pp. 275-276. 



