Geologic and geochemical investigations of the White Pine sediment-hosted stratiform copper deposit, Ontonagon County, Michigan.
Mauk, Jeffrey Lathrop
1993
Abstract
Within the White Pine mining district, there is evidence for three episodes of faulting: (1) synsedimentary faulting, (2) subsequent high-angle, dominantly normal faulting, and (3) thrusting. Two stages of copper mineralization occur at White Pine. The first, main-stage mineralization, formed a sediment-hosted stratiform copper deposit during diagenesis. Main-stage copper sulfides formed by replacement of precursor diagenetic pyrite which was relatively enriched in $\sp{34}$S and, more speculatively, via abiogenic reduction of hydrothermal sulfate which was relatively depleted in $\sp{34}$S. Second-stage mineralization was synchronous with thrusting, introduced additional copper and sulfur to the White Pine ore body, and may correlate with native copper mineralization in the Keweenaw district. Second-stage sulfides have $\delta\sp{34}$S values of $-$6 to $-$13 permil, and appear to be roughly in thermal isotopic equilibrium. Thrust faults and cogenetic tear faults provided conduits for second-stage mineralizing fluids. Water-rock interactions within the basin formed fluids that evolved to more $\sp{18}$O-enriched compositions. Indigenous and exogenous organic matter were involved in precipitation of copper-bearing minerals during main- and second-stage mineralization. Biomarker data demonstrate that bitumens from the White Pine area have significantly higher levels of thermal maturity than bitumens from unmineralized strata outside the mine area. Therefore, main- and/or second-stage copper mineralization at White Pine coincides with a thermal anomaly. Bitumen from the 'domino' shale is altered significantly because of mineralization and/or biodegradation. Inclusions of liquid petroleum and solid pyrobitumen in veins that fill fractures associated with compressional faults record migration of petroleum into the White Pine district from deeper portions of the rift penecontemporaneous with second-stage fluids. Some of this petroleum may have acted as a reductant during precipitation of native copper in basal ore horizons. Inclusion oils show variable but extensive depletion of light hydrocarbons, consistent with published laboratory simulations of water-washing. Relative abundance of n-alkanes indicates that inclusion oils are not biodegraded, and relative depletion of cyclic and aromatic hydrocarbons suggests that phase separation was not the dominant cause of alteration. Water/petroleum ratios necessary to produce the alteration range from 200:1 to very much greater than 7000:1. White Pine inclusion oils therefore represent one of the best end-member examples of water-washing in natural systems.Other Identifiers
(UMI)AAI9319583
Subjects
Geology Geochemistry
Types
Thesis
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