Abstract

Copper sulfides, in interesting amounts, have recently been discovered in the Keweenaw Peninsula of northern Michigan, a district long famous for its deposits of native copper. The sulfides occur extensively in the vicinity of Mount Bohemia, chiefly as open-space fillings and replacements in amygdaloidal flow tops, a traditional mode of occurrence of native copper in the region. An andesite dike (or dikes), almost invariably associated with the mineralized flow tops, typically carries at least trace amounts of sulfides.The present study indicates that both copper and sulfur have been added to dikes and flow tops in the Mount Bohemia area, probably by hydrothermal solutions moving upward along zones of structural weakness, and outward along relatively permeable flow tops and broken dike margins. The sulfur and most of the copper are most likely of direct magmatic origin, although some copper may have been derived at depth from pre-existing flows.The chronologic sequence developed in the Mount Bohemia area is: dike emplacement and alteration, faulting and fissuring, and sulfide mineralization. At least some of the faulting is related to regional deformation thought to have occurred in late Upper Keweenawan time. Regionally, native copper deposition is also believed to have followed this period of deformation and preceded copper sulfide mineralizationMicroscopic studies, supplemented by X-ray fluorescence analyses, have defined a zonal pattern of total copper, sulfur, and sulfide minerals developed about the Bohemia Fault, a northwest-trending break on the northeast flank of Mount Bohemia. Chalcocite predominates nearest the fault, with bornite, chalcopyrite, and finally pyrite becoming increasingly abundant with increasing distance from the break.Mineralization in the Mount Bohemia area followed several chemical trends during the period of sulfide deposition, with each point in a mineralized dike or flow top undergoing at least part of a generalized mineralization sequence which included: (1) introduction of solutions whose initial Cu/S ratios were relatively low; (2) progressive increase in the Cu/S ratios of the ore-forming solutions; and (3) progressive increase in the f (sub O 2 ) of the solutions.

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