Abstract

The dominant facies of Cu mineralization at White Pine, Michigan, is characterized by a distinct relationship of Cu to sedimentary features and by restriction to beds at the base of the Precambrian Nonesuch Shale formation. A second minor facies of mineralization, however, is fracture-controlled. The fracture-controlled mineralization is of 2 types: 1) Fracture-filled veins. The distribution of fractures in the White Pine mine is controlled by the dominant White Pine fault. These fractures were mineralized with calcite, quartz and chalcocite during or shortly after faulting. Native Cu, native Ag, blue chalcocite, covellite, bornite, chalcopyrite, and pyrite also occur in certain veins and probably formed by alteration of the earlier chalcocite. 2) Halos. Halos are chalcocite-bearing zones in wall rock adjacent to the veins and symmetrical with respect to the vein. Halos are mineralogically and chemically distinct from the adjacent wall rock in that native Cu is absent, and the average Cu content is lower. The amount of chalcocite varies systematically across the halos, decreasing away from the veins and becoming sparse in the outer parts of the halos. The over-all deficiency of Cu in the halos is apparently compensated by the occurrence of chalcocite in the veins; halos are confined to chalcocite-bearing veins, and the width of each halo appears to be proportional to the amount of chalcocite in the central vein. The fracture-controlled mineralization is interpreted to have originated from low temperature (less than 105 degrees C.), hypogene, solutions. These solutions deposited calcite and quartz in the fractures. The distribution of Cu in the veins and halos is inferred to be due to mobilization of Cu by solutions diffusing into the walls. The process involved is: 1) oxidation of metallic Cu along a diffusion front, 2) migration of Cu ions toward the vein, and 3) deposition of chalcocite in the veins and adjacent wall rock. There is no evidence for the introduction of Cu into the fractures from outside the cupriferous sequence.

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