SECONDARY DEPOSITS OF COPPER MINERALS 195 



dicated. The geologic evidence comprises the facts showing the position 

 of the ores of the vein, their relation to structural or physical features, 

 and the alteration of the ore bodies as masses in contradistinction to 

 purely mineral alteration. 



For convenience of presentation the different ores have been grouped 

 according to the predominance of their principal ore. This treatment 

 necessarily involves a slight repetition, since silver often occurs with 

 copper and lead with zinc. 



COPPER 



For many years past there appears to have existed among mining 

 engineers and a few geologists a tacit and sometimes an openly ex- 

 pressed belief in the secondary derivation of many deposits of copper 

 glance. This statement is based on a review of the published ac- 

 counts of a number of copper deposits in which such an origin has 

 been either implied or stated, though, as a rule, no evidence has been 

 presented.* This belief, if it may be so called, has undoubtedly had its 

 origin in the descriptions of the Ducktown, Tennessee, deposits, which 

 are described in all the text-books on ore deposits. The mass of rich 

 "black copper" ore lying between the limonite gossan and the un- 

 altered pyrrhotite has been stated to be of secondary origin ever since 1856, 

 when Whitney published his now classic work on " The Metallic Wealth 

 of the United States." In accounts of these deposits by Sterry Hunt, 

 Hermann Credner, Carl Henrich, and lately by Professor Kemp the 

 secondary nature of these ores has not been questioned, and the com- 

 monly accepted theory of their origin has been the leaching of the former 

 lean sulphide ores, now altered to gossan, and the redeposition of the 

 copper to form the rich black copper ore. Recent studies of the copper 

 deposits of the West have made it desirable to review the evidence on 

 which this theory rests, and to ascertain if bodies of crystalline sul- 

 phides have been formed by such action I made a visit to this locality 

 in December, 1899. As is well known, the conditions prevailing at 

 Ducktown are peculiarly favorable for a leaching of the gossan zone. 

 Rainfall is frequent and heavy, and an altitude of 1,700 feet above sea- 

 level favors rapid and considerable changes in temperatures. Moreover, 

 the region has not been glaciated, and the gossan zone has presumab^ 

 been forming since the elevation of the Tertiary peneplain, to which the 

 district belongs. The following extracts from the more important papers 

 describing this locality will, it is believed, serve better than an account 

 by me to show the occurrence and nature of the secondary ore. 



*See Kemp : Ore deposits of the United States, 2d ed., New York, 1896, p. 164. 

 XXVIII— Bui,t,. Gkot,. Soc. Am., Vol. 11, 1809 



