Copper mineralization in the upper part of the Copper Harbor Conglomerate at White Pine, Michigan

Abstract

Native copper is irregularly disseminated in cross-stratified sandstones in the upper 85 feet of the Copper Harbor Conglomerate at White Pine, Michigan. The topmost 100 feet of this formation can be divided into an upper chloritic facies, 1 to 20 feet thick, and a lower hematitic facies, 80 to 100 feet thick. Most of the native copper occurs as lenses in the chloritic facies in the upper 7 feet of the formation, but small amounts are found in chloritic layers within sandstones of the hematitic facies. The native copper occurs mostly as rims around interstitial carbonaceous material, and the carbonaceous material controls the distribution of native copper. Both carbonaceous material and native copper are typically found along cross-laminae, but there are significant departures from exact correspondence between mineralization and cross-stratification. These departures reflect the irregular distribution of carbonaceous material along cross-laminae. This distribution is in turn related to variations in porosity, probably due to variations in sorting.The native copper is restricted to chloritic sandstone, and its presence must be related to conditions that existed in the chloritic layers. The chlorite is diagenetic and formed in a reducing environment. The reducing environment may have been created as water, squeezed out of the overlying Nonesuch Shale during compaction, circulated through the sandstone.Native copper is later than carbonaceous material but did not replace carbonaceous material. Copper therefore either replaced some other material or filled open space. There is no evidence of replacement of any other material. It seems most likely that copper ions were adsorbed onto the surface of carbonaceous material before the interstices were filled with other minerals during diagenesis. The native copper is therefore diagenetic.