SECONDARY SULPHIDES. 95 



secondary. In the Mizpah, ruby silver is rare, but it has been noted in the 

 250-foot level, where, from microscopic examination, it appeared that both ruby 

 and horn silver are secondary to the original black silver sulphide. 



ARGENTITE, POLYBASITE, AND CHALCOPYRITE IN DRUSES. 



In the Montana Tonopah, at a depth of about 500 feet, were found specimens 

 showing good crystals of argentite, polybasite (in part, perhaps, stephanite), and 

 chalcopyrite, often sitting free in cracks and little druses in the solid rich sulphide 

 ore. Evidently these minerals were formed subsequent to the solid ore, and the 

 silver seems to have been concentrated from the main mass and to have been 

 precipitated in the crevices. Secondary pyrite has also been noted, for example, 

 in the Fraction mine, sitting free upon quartz crystals which line druses in the 

 vein. 



COMPARISON OF SECONDARY SULPHIDES AT NEIHART AND TOSOPAH. 



At Neihart, Mont., Mr. W. H. Weed* has described polybasite and pyrar- 

 gyrite (ruby silver) incrusting impure galena, blende, pyrite, quartz, and 

 barite. These crusts are now forming in vugs and watercourses filled by 

 sluggish descending surface waters. The polybasite seems to be an alteration 

 product of galena, and in some cases pyrargyrite is undoubtedly derived from it. 

 Blende is also in some cases secondary. Argentite is probably present. Mr. 

 Weed explains the secondary precipitation by lixiviation of the ores by iron 

 sulphate, formed by oxidation of iron sulphide (pyrite). 



The Tonopah occurrence is analogous, except that here satisfactory evidence 

 of the manner of deposition has not been found. There is little doubt that the 

 pyrargyrite and argentite found along cracks were formed subsequently to and 

 are probably derived from the primary ore. This primary ore is, however, 

 richer than that at Neihart; indeed, it consists largely of silver sulphide, in part 

 antimonial. For this reason the mode of occurrence of polybasite and argentite 

 in druses in the rich Montana Tonopah ore is not of such plain import. In the 

 Montana Tonopah it has been shown that during the period of primary deposition 

 the vein, after being filled, was crushed and reopened, and again cemented by 

 similar rich sulphides, somewhat richer apparently than those of the first deposition 

 (see p. 172); and the polybasite, argentite, and chalcopyrite in druses may mark 

 a third and final stage in the primary deposition. Also, chalcopyrite occurs in 

 the bulk of the ore as more or less definite seams, apparently somewhat later than 

 the rest, but not clearly of different origin. 



a Probably argentite (see p. 92). STrans. Am. Inst. Min. Eng., vol. 30, p. 434. 



