310 University of California Publications in Geology [Vol. 8 



westerly trend. Following this event, monzonite and monzonite- 

 porphyry were intruded along an east-west belt through the center of 

 the district in a series of bosses. Then followed extensive erosion, 

 which was succeeded in Tertiary time by the extrusion of rhyolitic 

 lavas and tuffs. Normal faulting of an easterly and northwesterly 

 trend followed, and then continuous erosion to the present time. 



The copper ores now being mined are disseminations of secondary 

 sulfids through monzonite-porphyry. On this account, from the point 

 of view of economic geology, interest centers in this intrusive rock, 

 its relations, and its history. 



THE ORE-PORPHYRY , 



The forms of the masses of ore-porphyry, in plan, are irregular and 

 elongated on east-west axes. In cross-section they appear to be nearly 

 vertical, with irregular, branching walls, giving them in some cases 

 the appearance of laccoliths. In the lowest mine workings, about 300 

 feet in Eureka Pit (pi. 31, fig. 1) and 500 feet at the bottom of 33 W 

 winze in the Veteran Mine, where the rock is least altered, it shows 

 an hypidiomorphic groundmass containing large porphyritic pheno- 

 crysts of orthoela.se. The rock has been much altered, but an exami- 

 nation of the least altered portions with the aid of a microscope shows 

 the groundmass to consist essentially of plagioclase and hornblende. 

 These minerals are generally altered to a brown mica, sericite, kaolin 

 and secondary quartz, which are accompanied usually by cupriferous 

 pyrite. and by the secondary sulfids of copper. 



ORE-PORPHYRY INTRUDED BY A LATER MONZONITE-PORPHYRY 



It was suggested by Lawson 2 that the ore-bearing porphyry was 

 later than the monzonite of Weary Flat which had penetrated the 

 deformed Carboniferous sediments, and probably belonged to the same 

 period as the post— Jurassic granites of the Sierras. But the writer 

 has found that a monzonite-porphyry, probably the correlative of the 

 Weary Flat Mesozoic monzonite, was intruded into the ore-porphyry, 

 probably after the latter had undergone some alteration. Evidence 

 of this was found in the workings extending from the bottom of 33 W 

 winze in the Veteran Mine just north of Weary Flat. It consists of 

 fifty feet of contact between the ore-porphyry and the Veteran mon- 



2 Op. cit., p. 289. 



