70 spencer: chalcocite deposition 



great contemporary local variation in a single basin as may be 

 found in the deposits now forming along any modern coast. The 

 local character of many of the beds, the rapidity with which they 

 merge laterally into others of unlike lithologic character make 

 it necessary to use many local formation names. They should 

 also make the geologist cautious in his interpretations of the 

 absence of any particular bed or formation. When, for example, 

 a sandstone which forms the top of the marine Cretaceous sec- 

 tion in one area is absent in another area its absence may be 

 due to erosion, but its apparent absence may also be due to the 

 fact that the sandstone is there represented by a shale or by 

 non-marine deposits of a totally different character. 



GEOCHEMISTRY.— Chalcocite deposition. Arthur C. Spen- 

 cer, Geological Survey. ^ 



The most common secondary sulfide in many copper mining 

 districts is chalcocite, occurring under conditions which indicate 

 that deposition has taken place from solutions containing copper 

 sulfate. It is not' difficult to show that the direct or indirect 

 source of this dissolved copper compound is chalcopyrite which 

 may be termed primary, and it is a matter of observation that 

 the same double sulfide of iron and copper is a very effective 

 precipitant or localizer of chalcocite. Pyrite has been usually 

 regarded as the most common nucleus for secondary chalcocite, 

 but in certain districts chalcopyrite must be recognized as occu- 

 pying this role instead of pyrite. The presence and proportion 

 of chalcocite in ores now being mined on a large scale in several 

 districts determines their commercial value. This is notably true 

 of the so-called porphyry ores in which the metallic sulfides are 

 thoroly disseminated thru great masses of rock. 



In 1903 H. V. Winchell^ published the results of experiments 

 devised to indicate the probable conditions under which chalco- 

 cite has been deposited in the veins of the Butte district from 

 waters carrying cupric sulfate. The determining agent in the 

 deposition was thought to be SO2, and later^ A. N. Winchell 



^ Published by permission of the Director. 

 ' Bull. Geol. Soc. Am. 14: 269-276. 1903. 

 3 Economic Geology 2: 290-294. 1907. 



