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Organic geochemistry of Devonian and Pennsylvanian metalliferous black shales of Indiana
234
ABSTRACTSFROMIGCP 1991ANNUALMEETING
forms of the metals. Oxidation and consequent alterations of the character of the organic matter would result. The 1.1 billion-year-old Nonesuch Formation of northern Michigan hosts deposits of elemental copper and silver and copper-iron sulfides in a lacustrine black shale setting. The deposits are best developed at White Pine, Michigan, where they provide an opportunity to investigate the geochemical processes which have contributed to formation of metal sulfide ores. We have collected a suite of several hundred rock sampies from within the White Pine mine and from boreholes from outside the mineralized section of the Nonesuch Formation. Total organic carbon (TOC) concentrations typically are low, averaging less than 0.1%0by weight, but range up to 1.5%. Comparison of TOC and copper concentrations in these samples has corrected an earlier impression that TOC has been destructively oxidized in copper-rich zones. Instead, higher levels of TOC and of copper correlate. Many samples contain evidence of petroleum migration in the past; petroleum is still present in some parts of the mineralized zone. Most mineralized rocks, however, contain little extractable hydrocarbon material, whereas rocks from outside the mineralized zone have full petroleum-like distributions of aliphatic hydrocarbons. Organic carbon stable isotope contents of rocks from within and outside the mineralized sections are typically depleted in 13C, having ~13C values in the - 31 to - 34%0 range. Attempts to assess incorporation of oxygen or sulfur as functional groups into organic matter in mineralized rocks by FTIR spectroscopy have not been very informative; kerogen appears to be thermally mature and to contain no functional groups in these rocks. ORGANIC GEOCHEMISTRY OF DEVONIAN AND PENNSYLVANIAN METALLIFEROUS BLACK SHALES OF INDIANA N.R. Shaffer and E.M. Ripley (Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA)
Previous studies have documented the presence of thin but highly metalenriched shales in southern Indiana. Metals that are enriched include Zn, Mo, Ni, V, and U. Isotopic and microscopic studies have demonstrated early formation of metal sulfides and stratigraphic and paleogeographic patterns in metal enrichment and organic matter. New gas chromatography and pyrolysis-gas chromatography data have been used to refine details of organic materials associated with metal-rich zones. Indiana black shales are immature and variations in organic materials are more related to original organic input and to conditions of sedimentation than to degree of maturity. CPI values range from 0.6 to 1.3 and probably reflect proportions of terrestrial input with slightly higher values in metal-rich beds. Pristane:n-C17 ratios are lower (0.9) in metalliferous beds than in the rest of the shale ( 1.5 ) as are phytane:n-Cls values of 0.6 and 0.9 respectively. These
ABSTRACTS FROM IGCP 1991 ANNUAL MEETING
235
counter-intuitive findings may argue for veryexcellent preservation in which chlorophyll precursor is less degraded. Porphyrins are common in the metalrich bed. Pristane to phytane values in the metal-rich beds are only slightly higher (2.3) than in the rest of the shale (2.2). More oxidizing conditions occurred during phosphate deposition just prior to the onset of metal enrichment. Evidence for later oxidizing (ore-type?) fluid was not observed. Pennsylvanian shales showed much greater variability in part related to paleogeography. The ratio V : V + N i has been used to determine paleoredox conditions. In our shales very high values ( > 0.84) of this ratio are observed in metal-rich zones implying very anoxic conditions during their deposition. However, the overall correlation of metal to V / V + Ni is not good. As suspected, phosphaterich beds related to metalliferous beds indicate less severe anoxic conditions just prior to deposition of metal enriched shale. Recently discovered sphalerite-rich siderite nodules in shale lying just above Devonian metal-rich beds and multigeneration banded sphalerite and dendritic galena from just below that shale suggest that some mobilization or leakage of certain metals may have occurred. These hint that black shales can sequester metals from anoxic sedimentary environments to form source beds for ore deposits as well as for petroleum. The co-occurrence of extremely good preservation of organics, metal enrichment, and phosphate in black shales throughout the world implies a common, widespread but as yet poorly understood mechanism. SEDIMENTSFROM THE BLACKSEAAND PERU MARGINAS ANALOGUESFOR NON-METALLIFEROUSBLACKSHALES W.E. Dean (U.S. GeologicalSurvey,Federal Center, Denver, CO. 80225, USA) and M.A. Arthur (Department of Geosciences,PennsylvaniaState University,UniversityPark, PA 16802, USA) Black shales that are described as "metalliferous" typically contain thousands of parts per million (ppm) of several metals, especially Zn and V, and hundreds of ppm of several other metals, commonly Ni, Cu, Mo and (or) Cr. For black shales without an obvious hydrothermal source, trace-metal enrichment may have been the result of chemical sorption onto organic detritus, clays, etc. settling through the water column, and (or) by coprecipitation with sulfides in an anoxic water column, at the sediment-water interface, or during early diagenesis. Laminated Holocene sediments, deposited in anoxic waters to depths of up to 2200 m in the Black Sea, and within an oxygen minimum zone on the highly productive, shelf and upper continental slope on the Peru margin often have been used as examples of modern analogues for interpreting the environments of deposition of ancient black shales. Once buried into the geologic
ABSTRACTSFROMIGCP 1991ANNUALMEETING
forms of the metals. Oxidation and consequent alterations of the character of the organic matter would result. The 1.1 billion-year-old Nonesuch Formation of northern Michigan hosts deposits of elemental copper and silver and copper-iron sulfides in a lacustrine black shale setting. The deposits are best developed at White Pine, Michigan, where they provide an opportunity to investigate the geochemical processes which have contributed to formation of metal sulfide ores. We have collected a suite of several hundred rock sampies from within the White Pine mine and from boreholes from outside the mineralized section of the Nonesuch Formation. Total organic carbon (TOC) concentrations typically are low, averaging less than 0.1%0by weight, but range up to 1.5%. Comparison of TOC and copper concentrations in these samples has corrected an earlier impression that TOC has been destructively oxidized in copper-rich zones. Instead, higher levels of TOC and of copper correlate. Many samples contain evidence of petroleum migration in the past; petroleum is still present in some parts of the mineralized zone. Most mineralized rocks, however, contain little extractable hydrocarbon material, whereas rocks from outside the mineralized zone have full petroleum-like distributions of aliphatic hydrocarbons. Organic carbon stable isotope contents of rocks from within and outside the mineralized sections are typically depleted in 13C, having ~13C values in the - 31 to - 34%0 range. Attempts to assess incorporation of oxygen or sulfur as functional groups into organic matter in mineralized rocks by FTIR spectroscopy have not been very informative; kerogen appears to be thermally mature and to contain no functional groups in these rocks. ORGANIC GEOCHEMISTRY OF DEVONIAN AND PENNSYLVANIAN METALLIFEROUS BLACK SHALES OF INDIANA N.R. Shaffer and E.M. Ripley (Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA)
Previous studies have documented the presence of thin but highly metalenriched shales in southern Indiana. Metals that are enriched include Zn, Mo, Ni, V, and U. Isotopic and microscopic studies have demonstrated early formation of metal sulfides and stratigraphic and paleogeographic patterns in metal enrichment and organic matter. New gas chromatography and pyrolysis-gas chromatography data have been used to refine details of organic materials associated with metal-rich zones. Indiana black shales are immature and variations in organic materials are more related to original organic input and to conditions of sedimentation than to degree of maturity. CPI values range from 0.6 to 1.3 and probably reflect proportions of terrestrial input with slightly higher values in metal-rich beds. Pristane:n-C17 ratios are lower (0.9) in metalliferous beds than in the rest of the shale ( 1.5 ) as are phytane:n-Cls values of 0.6 and 0.9 respectively. These
ABSTRACTS FROM IGCP 1991 ANNUAL MEETING
235
counter-intuitive findings may argue for veryexcellent preservation in which chlorophyll precursor is less degraded. Porphyrins are common in the metalrich bed. Pristane to phytane values in the metal-rich beds are only slightly higher (2.3) than in the rest of the shale (2.2). More oxidizing conditions occurred during phosphate deposition just prior to the onset of metal enrichment. Evidence for later oxidizing (ore-type?) fluid was not observed. Pennsylvanian shales showed much greater variability in part related to paleogeography. The ratio V : V + N i has been used to determine paleoredox conditions. In our shales very high values ( > 0.84) of this ratio are observed in metal-rich zones implying very anoxic conditions during their deposition. However, the overall correlation of metal to V / V + Ni is not good. As suspected, phosphaterich beds related to metalliferous beds indicate less severe anoxic conditions just prior to deposition of metal enriched shale. Recently discovered sphalerite-rich siderite nodules in shale lying just above Devonian metal-rich beds and multigeneration banded sphalerite and dendritic galena from just below that shale suggest that some mobilization or leakage of certain metals may have occurred. These hint that black shales can sequester metals from anoxic sedimentary environments to form source beds for ore deposits as well as for petroleum. The co-occurrence of extremely good preservation of organics, metal enrichment, and phosphate in black shales throughout the world implies a common, widespread but as yet poorly understood mechanism. SEDIMENTSFROM THE BLACKSEAAND PERU MARGINAS ANALOGUESFOR NON-METALLIFEROUSBLACKSHALES W.E. Dean (U.S. GeologicalSurvey,Federal Center, Denver, CO. 80225, USA) and M.A. Arthur (Department of Geosciences,PennsylvaniaState University,UniversityPark, PA 16802, USA) Black shales that are described as "metalliferous" typically contain thousands of parts per million (ppm) of several metals, especially Zn and V, and hundreds of ppm of several other metals, commonly Ni, Cu, Mo and (or) Cr. For black shales without an obvious hydrothermal source, trace-metal enrichment may have been the result of chemical sorption onto organic detritus, clays, etc. settling through the water column, and (or) by coprecipitation with sulfides in an anoxic water column, at the sediment-water interface, or during early diagenesis. Laminated Holocene sediments, deposited in anoxic waters to depths of up to 2200 m in the Black Sea, and within an oxygen minimum zone on the highly productive, shelf and upper continental slope on the Peru margin often have been used as examples of modern analogues for interpreting the environments of deposition of ancient black shales. Once buried into the geologic