34 GEOLOGY OK TOJSOPAH MINING DISTRICT, NEVADA. 



chlorite, quartz, siderite, frequently calcite, and sometimes sericite, kaolin, and 

 zeolites. Small apatite crystals occur, in part as inclusions in the phenocrysts. 



Magnetite and specular iron occur as primary minerals, often abundantly. In 

 several cases an isotropic cloudy material of a brilliant green color, suggesting 

 chromium or nickel, was observed in thin sections; and to this some of the rocks 

 owe, in part at least, their peculiarly vivid color. At times this secondary 

 substance seemed to be derived from the augite, but in one section it was 

 plainly derived from the magnetite, for it formed rims around the magnetite 

 crystals. As analysis showed a trace of nickel, it is probable that the magnetite 

 contains some nickel oxide." Siderite also occurs as rims around the magnetite 

 and as pseudomorphs after it. 



Siderite and pyrite are more abundant than in the early andesite. They are 

 usually intimately associated, and their relations are interesting. Frequently they 

 seem to have been contemporaneous in origin, and to have formed side by side 

 without inconvenience. As stated above, however, the siderite is more intimately 

 disseminated through the mass of the primary ferruginous mineral (biotite, 

 augite, or hornblende) whence it is derived than is the pyrite. Occasionally the 

 pyrite is altered to siderite along its margins, but in many more cases the siderite 

 has unmistakably altered to pyrite along its borders. A delicate set of changes 

 is thus indicated. The intimate association of the siderite with the primary 

 minerals, its frequent replacement by pyrite along the borders, and the evident 

 alteration of the carbonate to the sulphide show that in general a period of 

 pyritization succeeded one of carbonization, or, if both were contemporaneous, 

 the period of pyritization was longer. 



The groundmass when fresh is brown glass, sometimes spherulitic, or it is 

 microlitic with brown glass cement. Feldspar, pyroxene, and magnetite microlites 

 may sometimes be recognized. The groundmass alters, like the phenocrysts, to 

 quartz, chlorite, serpentine, siderite, pyrite, calcite, sericite-like aggregates, and 

 occasional zeolites and epidote. 



In general the decomposition products of the rock are typically quartz, chlorite, 

 calcite, pvrite, and siderite, but occasionally portions altered chiefly to quartz and 

 sericite-like aggregates* may be found. 



Location. The later andesite outcrops in only the northeastern portion of the 

 area mapped, for in the southwestern portion, as a result of relative subsidence 

 attendant upon faulting, only higher beds are exposed. It occurs in depressions 



<iln magnetite some of the ferrous iron is rarely replaced by nickel; thus a variety from Pregratten, in the Tyrolese 

 Alps, in a schistose serpentine, gave 1.76 per cent nickel oxide (NiO), together with traces of the oxides of manganese, 

 chromium, and titanium. 



t For some information on the real nature of these sericite-like aggregates see p. 240. It appears probable that 

 hydrargillite and talc form a large part of these masses. 



