174 



UNITED STATES MINERAL RESOURCES 



per sulfide minerals are persistent for many miles. 

 The ore occurs in dark shales of the lower Roan 

 Formation of the Katanga Supergroup. Most of the 

 lower Roan Formation is made up of arkosic con- 

 glomerates and eolian sandstones overlying a gra- 

 nitic basement, but the dark shales of the main 

 ore horizon also contain algal reefs. Thus, the ore 

 horizon was probably deposited in a shallow re- 

 stricted basin surrounded by algal reefs. An im- 

 portant feature suggestive of a syngenetic origin 

 is the zonal arrangement of ore minerals within the 

 ore formation parallel to the shoreline as it mi- 

 grated back and forth with transgressions and re- 

 gressions of the sea (Garlick, 1961, p. 155-160). 

 The sequence proceeds from barren sediment along 

 the shoreline through disseminated chalcocite, born- 

 ite, and chalcopyrite to pyrite in the same direction 

 as the general flow of currents, as determined from 

 crossbedding. However, the mineralized zone cuts 

 stratigraphically upward through the succession 

 from conglomerate to sandstone to argillite to im- 

 pure dolomite. 



The later discovery by Malan (1964) that chal- 

 copyrite occurs in narrow argillaceous interstices 

 between stromatolite columns in algal reefs is also 

 cited as evidence that copper was deposited at the 

 time of sediment deposition, perhaps by sulfate- 

 i-educing bacteria, or as a colloidal sulfate gel. Metal 

 deposition in the copper, iron, and cobalt-bearing 

 Chibuluma West ore body, Zambia, was syngenetic 

 with sands and muds, according to Whyte and 

 Green (1971) and could have been by (1) biogenic 

 deposition of sulfides, (2) precipitation especially 

 of iron and cobalt hydroxides by changes of pH 

 where river waters entered the lake or branch of 

 the sea, (3) detrital deposition of metalliferous 

 minerals, or (4) by a combination of these methods. 

 Many of the deposits in Zaire have been altered by 

 supergene enrichment, and they produce secondary 

 copper minerals and a variety of other elements 

 from dolomitic beds. 



The very large resource of copper in the African 

 copper belt may result from a combination of sev- 

 eral important processes. If we assume that the 

 deposits are syngenetic, they may have formed dur- 

 ing the interval of Precambrian time when oxygen 

 first became available in suflScient quantity to mo- 

 bilize copper. Moreover, these deposits are at or 

 near the base of a marine transgression which could 

 mobilize copper in the underlying crystalline and 

 nonmarine rocks in a manner suggested by Wede- 

 pohl (1971, p. 272) for the Kupferschiefer. Common 

 features of the African copper belt and the Kupfer- 

 schiefer were discussed by Putzer (1972). 



The current rate of copper production from the 

 African copper belt would suggest a known reserve 

 of well over a billion tons of ore. Speculative re- 

 sources may be several times as great. 



PRECAMBRIAN SEDIMENTARY COPPER DEPOSITS 

 IN THE UNITED STATES 



At the White Pine district in Ontonogan County, 

 Mich., copper occurs in the Nonesuch Shale of Pre- 

 cambrian Y age, especially in the basal parting 

 shale bed (Ensign and others, 1968). About 5 per- 

 cent of the U.S. copper production comes from this 

 single bed which is 1-8 meters thick and contains 

 anomalous copper over a distance of at least 250 

 kilometers (160 miles) in the Upper Peninsula of 

 Michigan and northern Wisconsin. Chalcocite is the 

 principal sulfide mineral in the mineralized zone 

 whereas pyrite is the characteristic sulfide in the 

 upper part of the Nonesuch Shale. The richest cop- 

 per occurs in the darkest carbonaceous siltstones, 

 shales, and sandstones, where chalcocite grains 

 ranging in size from 2 to 20 microns are dissemi- 

 nated interstitial to the framework grains. Although 

 the mineralization is persistent in certain beds 

 throughout the district, a crosscutting relation be- 

 tween ore and bedding on a regional scale and the 

 absence of a characteristic copper-bearing lithologic 

 facies distinguish this deposit from the African 

 copper belt. Consequently, an epigenetic, but not 

 magmatic, source for the copper seems most likely. 

 However, Robert Ehrlich and T. A. Vogel stated 

 (oral presentation at Institute on Lake Superior 

 Geology, Houghton, Mich., May 4, 1972) that accord- 

 ing to their interpretation, the copper in the None- 

 such Shale is syngenetic, with some diagenetic 

 modification of the distribution. 



Strata-bound sedimentary copper deposits occur 

 in Precambrian Y rocks of the Belt Supergroup in 

 western Montana and adjacent parts of Idaho. Cop- 

 per sulfides occur as disseminations, blebs, and 

 veinlets in persistent beds of white fine-grained 

 quartzite and siltite of the Revett Formation along 

 a belt parallel to a broad domal feature formed 

 before the close of Belt time. Anomalous amounts 

 of copper are more widely known in green strata 

 throughout the Belt basin. Most of the Belt rocks 

 are feldspathic and contain variable amounts of 

 mica (illite and sericite) plus chlorite in amounts 

 inversely related to grain size. Anomalous amounts 

 of silver and mercury are known to be associated 

 with copper locally, but lead is known only in sepa- 

 rate beds. The apparent relation between minerali- 

 zation and a late Precambrian broad domal struc- 

 ture is suggestive of a diagenetic or early epigenetic 



