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The Geology and Ore Deposits of Goldfield, Nevada. By Frederick 

 Leslie Ransome, assisted in the field by W. H. Emmons and 

 G. H. Garrey. U.S. Geol. Surv. Professional Paper 66. 



Aside from the importance of a careful and thorough description of a 

 region so rich in mineral deposits as that of Goldfield, the present volume 

 is especially interesting to students of economic geology in that it describes 

 a practically new type of wall-rock alteration. Hitherto the primary 

 deposition of deposits of gold, silver, and copper have been generally 

 ascribed to the agency of hot ascending alkaline solutions; the minerals 

 associated with the ores at Goldfield show, however, that the waters at 

 the time of deposition were highly acid in character. To this type of 

 deposition the name "alunitic and kaolinitic gold-quartz veins" is applied. 



The geology of the district is comparatively simple: a basement com- 

 plex of metamorphic rocks, intruded by masses of alaskite, probably in 

 early Cretaceous, and overlain by a succession of Tertiary lava-flows and 

 lake-beds. Of the lava-flows there are six rhyolites, a latite, three ande- 

 sites, four dacites, and two basalts. The lake-beds are probably Miocene. 

 All of these rocks have received a careful petrographical description. 



Some ore-deposits occur in andesite, but most of them are in dacite. 

 They are complex mineralogically, occurring in irregular tabular deposits 

 which are approximately defined by the term metasomatic fissure veins. 

 The most notable features of the ore-bodies are their remarkable richness 

 and their equally remarkable irregularity. Three distinct types of wall- 

 rock alteration are recorded; the first results in craggy outcrops of silici- 

 fied volcanic rock, which form a marked feature of the topography; the 

 second is a soft, light-colored mass of quartz, alunite, kaolinite, and pyrite ; 

 the third type is propylitic with the formation of calcite, epidote, chlorite, 

 and pyrite. The ores are closely associated with the first two types of 

 alteration, though the alteration is much more extensive than the ore 

 deposits. It is the extensive formation of sulphate minerals, alunite, and 

 kaolinite, in close association with primary ores, that gives to the deposits 

 their chief genetic interest. The changes in the dacite have been care- 

 fully studied both qualitatively and quantitatively, as this method prob- 

 ably furnishes the best clue to the nature of the depositing solutions. The 



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