168 



UNITED STATES MINERAL RESOURCES 



(Bogdanov, 1968), and Ehrlich and Vogel (oral 

 commun., 1972) suggested that some of the copper 

 in the Nonesuch Shale was carried in this way. 

 Because sea water contains only 0.003 ppm copper 



(Green, 1969), the discharge of an estimated 1x10" 

 tons new copper by streams each year must be bal- 

 anced against various mechanisms that remove it. 

 In reducing environments, copper is probably pre- 

 cipitated as the sulfide by sulfur-reducing bacteria 

 or adsorbed on decaying humic organic matter. Cop- 

 per also accumulates in some oxidizing environ- 

 ments, such as in manganese nodules and in pelagic 

 muds where precipitation may occur as the oxide. 

 The fate of copper in an evaporite environment is 

 not understood. Some copper may be concentrated 

 in residual brines and eventually squeezed into ad- 

 jacent porous sediments and precipitated by reaction 

 with hydrogen sulfide. Copper may also be trapped 

 in connate pore waters in basins of nonmarine depo- 

 sition. Precipitation would eventually occur by re- 

 action with hydrogen sulfide or organic matter, or 

 as a carbonate, sulfate, silicate, or phosphate. The 

 regional and stratigraphic association of strata- 

 bound copper ores with evaporites was described by 

 Davidson (1965). 



Prior to the evolution of a modern atmosphere 

 containing about 20 percent oxygen some 1.2-1.4 



billion years ago (Cloud, 1971), the geochemical 

 cycle for copper may have been significantly differ- 

 ent. Primary copper sulfide minerals were probably 

 sufficiently stable to survive weathering and redepo- 

 sition as heavy minerals in detrital rocks. Subse- 

 quent exposure of buried sediment to oxygenated 

 waters could have resulted in redistribution of the 

 copper in chemically reactive environments within 

 the sedimentary sequence, for example in beds rich 

 in organic matter. 



MINERALOGY 



Copper occurs in at least 160 minerals. Table 38 

 gives the name, composition, and general mode of 

 occurrence of 20 of the more common ones. 



Chalcopyrite is by far the most abundant copper 

 sulfide ; bornite and chalcocite are next in abundance. 

 The sulfarsenides enargite and tennantite and the 

 sulfantimonides tetrahedrite and famatinite are 

 generally rare, but each makes up major parts of 

 at least one large ore body. The mineral digenite 

 (Cuo-j^S) is not shown in table 38, but it may be a 

 major ore mineral. It is detected only by careful 

 petrographic work or X-ray analysis and is gener- 

 ally reported as chalcocite. Native copper is abun- 

 dant in certain types of deposits. 



Malachite, azurite, and chrysocolla are the most 



Table 38. — Minerals of copper; composition and environment of origin 



[o, major ore mineral; x, minor ore mineral] 



Secondary 



enrichments 



and secondary 



deposits 



§1 



Major minerals 



Cu 



Cu.S 



CuS 



CujFeS, X 



CuFeS. 



CuaAsS. o 



Cu^O 



Cu=(0H)2(C03) 



Significant supplementary minerals 



(Cu,Fe),2Sb,Si3 



(Cu,Fe)i.As4Si3 



CusSbS, 



CuzFeSnSi 



Cuo(0H)3Cl 



CuO 



C\iAOB.)ACO^), 



CuSi03'2H-0 



CU,(S04) (OH)a 



Cu3(S0,)(0H). 



CuSG.'SHoG 



Na2Cu(SO,)2-2H=0 



Native copper 



Chalcocite 



Covellite 



Bornite 



Chalcopyrite 



Enargite 



Cuprite 



Malachite 



Tetrahedrite 



Tennantite 



Famatinite 



Stannite 



Atacamite 



Tenorite (melaconite) 



Azurite 



Chrysocolla 



Brochantite 



Antlerite 



Chalcanthite 



Kroehnkite 



(?) 



