Geochemical characterization of sedimentary organic matter and hydrothermal petroleum in the black shale-hosted zince-lead deposit at Red Dog Mine, western Brooks Range, Alaska

Organic matter in the Mississippian black shale-hosted Zn-Pb deposit at the Red Dog Mine in northwest Alaska participated both as an active chemical reductant and as a passive migrating hydrothermal petroleum in the ore-mineralization process. Removal of organic matter from the center of Red Dog mineralization through generation and expulsion of hydrothermal petroleum is indicated by blebs of solid bitumen entrained in exhalite facies and by higher organic carbon (C_org) contents (7.7 to 18.4%) in proximal shales on the perimeter of the deposit. A contrast between the mean $d13Corg$ values of sulfide-hosted bitumen (−30.1‰) and organic matter in distal shale (−29.5‰) is inferred to reflect 13C-enrichment of the residual organic matter (−29.1‰) during generation of hydrothermal petroleum with a conversion efficiency of 44%.; Gas chromatography-mass spectrometry (GC-MS) analyses demonstrate the dominance of polycyclic aromatic (PAH) and polyaromatic organosulfur compounds (PAOSC) characteristic of hydrothermal petroleum within the deposit. Established maturity parameters such as the Methylphenanthrene Index (MPI-1) and the Methyldibenzothiophene Ratio (MDR) indicate elevated thermal maturity within the deposit compared to distal and outlying shales. Other, unconventional maturity parameters calculated from ratios of various PAH and PAOSC help define an elongate thermal halo including Red Dog.; Sphalerite and galena in sulfide rock, silica rock, and barite rock have $d34S$ values consistently near 0‰, suggesting a deep homogenous source of reduced sulfur. The specific enrichment of Zn and Pb in the deposit, however, implies a sedimentary source of metals. A trend toward more positive $d34S$ values of sphalerite, galena, and pyrite in perimeter shales implies partial reduction of descending seawater sulfate and abiogenic sulfide precipitation, whereas isotopic values lower than about −15‰ imply bacterial sulfate reduction and early diagenetic precipitation of biogenic sulfides. Geothermometry calculations based on $D$_{sphalerite-galena} values suggest sulfide mineralization over a range of temperatures from 74° to 207°C. A sedimentary source of metals and igneous source of heat and sulfur imply that the Red Dog deposit hydrothermal mineralization resulted from ongoing accumulation of a metalliferous black shale followed by localized elevation of the thermal gradient and initiation of hydrothermal convection.