86 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



PRIMARY ORES. 

 LOCALITY. 



The contents of veins lying near the surface have been transformed more or 

 less into new minerals minerals that are more stable under surface conditions; 

 the materials originally deposited from the mineralizing solutions must therefore 

 be sought in the unoxidized lower region. The Montana Tonopah veins carry 

 solid sulphide ores, primary and contemporaneous with the original quartz gangue 

 and very slightly altered, presenting strong contrast with the oxidized ores of 

 the Mizpah mine. Similar sulphide ores have been found in the North Star, the 

 Tonopah Extension, the Midway, and the Tonopah and California. 



COMPOSITION. 



MINERALS. 



Quartz. In these veins the chief gangue mineral is quartz, frequently well 

 crystallized and translucent, but more usually rather fine-grained and dense, and 

 mixed with more or less aluminous material. This material, which will be described 

 later on, is a residue of the least soluble material of the earlier andesite. Under 

 the microscope the quartz has a characteristic structure, distinct from that of 

 ordinary crystalline vein quartz. Instead of the coarse interlocking grains com- 

 monly displayed by vein quartz, these veins usually show a mosaic in which the 

 grain varies enormously in size, ranging from very fine cryptocrystalline to very 

 coarse. Under the microscope the aluminous material proves to be very fine 

 muscovite (sericite). The quartz holds numerous fluid inclusions, which contain 

 bubbles, showing that the included material was in a state of vaporous tension 

 at the time of its inclusion or at the time of the vein formation, and that it has 

 contracted so as to fill only part of its original chamber upon the lowering of 

 the temperature. The inclusions are frequently densely packed and curiously 

 arranged. In some cases the interior of the crystals is clear, while the marginal zone 

 is packed with inclusions. Frequently the quartz has the rough retiform structure 

 which is due to the intergrowth of idiomorphic crystals starting from independent 

 crystallization centers, and which is often characteristic of quartz formed by 

 replacement." There are also coarser veinlets of quartz, later than the bulk of 

 the vein, which were introduced along cracks, and these in places show comb 

 structure. 



Adularia. The nearly pure potash feldspar, adularia, a purer variety of 

 orthoclase, is a common gangue mineral. It is frequently very abundant, usually 

 in more or less idiomorphic crystals that show the characteristic rhombic cross- 

 section. It is intercrystallized with the quartz, which often incloses isolated 



aSpurr, J. E., Mon. U. 8. Oeol. Survey, vol. 81, p. 218. 



