2 University of California Publications in Geology. [Vol. 7 



the silver haloids. The latter author collected a good suite of the 

 minerals found in the veins and sent them to the writer for 

 further study and report, and it is from a study of this material 

 that the following notes on the minerals have been prepared. 



Origin of the deposits. — The silver deposits occur in a trachyte 

 which Spurr first designated as "earlier andesite, " and were 

 formed by ascending solutions carrying the sulphides and the 

 gold and depositing them at a period immediately following the 

 eruption. The typical ore consists of a gangue of massive white 

 quartz and white feldspar containing blotches and bands of 

 finely granular, black silver minerals, small amounts of pyrite, 

 chalcopyrite, galena, and sphalerite, and occasionally free gold. 

 The dark silver bands are mainly argentite, with which are 

 intimately mixed polybasite and perhaps stephanite and tetra- 

 hedrite. An analysis of the primary sulphide concentrates of 

 the Montana-Tonopah mine, made by Hillebrand and quoted by 

 Spurr, indicates that the simple sulphide, argentite, predom- 

 inates, but has probably mixed with it small amounts of silver 

 sulphantimonites and sulpharsenites and some selenide of silver, 

 or of lead and silver. These sulphide minerals are all primary 

 constituents of the veins, yet most if not all of them also occur 

 secondary, and some of the crystals in the cavities of the gangue 

 are doubtless of a later generation. 



Oxidation of the veins. — The deposits at Tonopah lie in an 

 arid region and, like most desert deposits of sulphide minerals, 

 have their oxidized zones characterized by a variety of rare sec- 

 ondary minerals not found where simple hydration only is pos- 

 sible. This is due to the fact that surface waters penetrating to 

 the vein matter below and bringing about its alteration become 

 charged with salts leached from the overlying and adjacent 

 strata, and complex oxidation with less usual mineral precipita- 

 tions result. The descending solutions which brought about the 

 main oxidation of the silver minerals at Tonopah contained the 

 alkali bromide and iodide as well as the much more abundant 

 chloride, and the complexity of the depositing solutions is evi- 

 denced by the coatings of haloids, phosphates, arsenates, man- 

 ganates, and sulphates which line the walls of fissures and cavities 

 in the gangue. 



