210 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



ALTERATION OF EARLIER ANDESITE, CHIEFLY TO CALCITE AND CHLORITE. 



In the earlier andesite at points sufficiently remote from the important veins, 

 calcite and chlorite appear as distinct alteration products, which do not occur in 

 the rock nearer the veins and which take the place, partly or wholly, of the quartz 

 and sericite of the phases described above. This phase has a green color, growing 

 in depth of shade as the proportion of chlorite increases, and the rock has no 

 resemblance to the light-colored quartz-sericite alteration phases. Iron in the form 

 of pyrite and siderite is common to both phases, but while in the quartz-sericite 

 alteration it is characteristically in small quantity and diminishes with increasing 

 alteration, in the chlorite-calcite alteration it is abundant and remains so when 

 the rock is completely altered. 



In this process of alteration the feldspar is usualry largely altered, chiefly to 

 calcite with a little quartz. Rarely the alteration is to quartz and epidote. 

 Original hornblende and pyroxene are always completely altered, usually to chlorite 

 (ripidolite) pseudomorphs. Biotite has been observed altered to sericite, with a 

 little calcite and hematite. 



The groundmass is similarly altered to chloritic material, intermixed with 

 secondary quartz, etc. 



TRANSITIONS BETWEEN ALTERATION PHASES OF EARLIER ANDESITE. 



There are all transitions between the typical quartz-sericite alteration phase, 

 in which calcite and chlorite are always absent, and the typical calcite-chlorite 

 phase, in which quartz, and especially sericite, are decidedly subordinate. Thus 

 in a specimen from the 700-foot level of the Siebert shaft (from the same rock 

 mass as some of the typical calcite-chlorite phases) the feldspars are chiefly altered 

 to sericite, with a little chlorite; the hornblende and biotite crystals are altered 

 chiefly to chlorite; and while calcite is present, it is not prominent. 



DIFFERENT ALTERATIONS THE EFFECT OF THE SAME WATERS. 



The conclusion is thus reached that the chemical effects of the same mineralizing 

 waters became continually different as they penetrated to a greater and greater 

 distance from the circulation channels. Along these channels, which became veins, 

 the transformation or replacement of the rock by the addition of silica and the 

 sulphides of silver, antimony, etc., with gold and selenides, and by the complete 

 leaching out of soda and magnesia and the partial leaching out of lime and iron, 

 was profound. In the siliceous phase of the altered andesite near the veins a 

 similar alteration, though weaker, is recorded. The metals did not penetrate here, 

 but the partial replacement of lime, iron, magnesium, and soda by silica and potash 

 is present in all its stages. In the rock more remote from the vein channels the 



