148 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



nature of the veins and the corresponding strike and dip warrant the above sug- 

 gestion, for the faulting is so complicated in this region that in any space actually 

 undeveloped by mining operations, little more than guesses can be made in many 

 cases. 



TONOPAH RHYOLITE-DAC1TE. 



The Fraction No. 2 workings lie mostly on the 400-foot level, and besides a 

 connection with the No. 1 shaft there is a drift running nearly 600 feet to the 

 north-northwest and more than 200 feet in the opposite direction. Only small 

 quartz veins, of no importance, occur in these workings. The rock encountered 

 is a rather dark-colored earlier andesite, sometimes considerably kaolinized, like 

 that encountered in the No. 1 workings. In the south drift from the shaft, 

 however, a white rock is encountered. This is solid at the end of the south drift, 

 and Ijetween this point and the shaft occurs as fragments and large bowlders up 

 to several feet in diameter in the darker andesite. The geologic features here 

 indicate that the breccia is due to movement in the rock, and this conclusion is 

 corrroborated by microscopic study. In this breccia are encountered several 

 strongly marked slip planes, which strike N. 30 or 40 E., and dip southeast at an 

 angle of 40 or more. These correspond in altitude to the northeast-striking and 

 southeast-dipping faults in the Fraction No. 1 workings, and it appears probable 

 that the hard white rock at the south end of the drift has been brought against 

 the darker and softer andesite of the north drift by means of this faulting. Some 

 perplexity has arisen concerning the nature and relation of these two rocks. 

 After study, however, the author is of the opinion that the latter rock is a phase 

 of the earlier andesite, while the white rock is a coarse-grained phase of the glassy 

 Tonopah rhyolite-dacite. 



Microscopic examination shows that this hard, white rock is considerably 

 altered. The phenocrysts are of altered feldspar, in part andesine-oligoclase and 

 in part orthoelase; the}' arfe now largely changed to muscovite (sericite) and 

 adularia. Small original biotite crystals are thoroughly bleached. The glassy 

 groundmass contains veinlets of calcite and abundant pyrite. The chemical 

 analysis of the rock, by Mr. George Steiger, is as follows: 



Analysis of altered Tonopah rhyolite-dacite. 

 [Specimen 2V9.J 



SiO, 68.19 



A1,O, 15.13 



FeA 1.31 . 



FeO 42 



MgO 29 



CaO 1.19 



Na/) 3.13 



K,O .66 



TiO, 32 



PA 15 



