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Carbon Isotope Geochemistry of Archean Carbonaceous Horizons in the Timmins Area

By
F.-W. Wellmer
F.-W. Wellmer
Federal Institute for Geosciences and Natural Resources, 30631 Hannover, Germany
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U. Berner
U. Berner
Federal Institute for Geosciences and Natural Resources, 30631 Hannover, Germany
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H. Hufnagel
H. Hufnagel
Federal Institute for Geosciences and Natural Resources, 30631 Hannover, Germany
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H. Wehner
H. Wehner
Federal Institute for Geosciences and Natural Resources, 30631 Hannover, Germany
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Published:
January 01, 1999

Abstract

Two types of Archean kerogenic-graphitic facies associated with base metal mineralization, and considered to be coeval with them, can be observed at three localities in the Timmins area: the Kidd Creek deposit, the Chance deposit, and the Rusty Hill occurrence. The type A facies normally has S13C values between -18 and -33 per mil and shows little or no correlation with values for total organic carbon; the S13C values for type B facies are more negative and lie between -29 and -48 per mil and show a positive, nonlinear correlation with values for total organic carbon. Kidd Creek argillites belong to type A. They resemble normal Archean argillites and have S13C values ranging from -19.3 to -26.4 per mil. Type B horizons occur close to the Chance deposit and the Rusty Hill sulfide occurrence and are also associated with base metal mineralization at other locations. Type B facies grades laterally into type A. Facies of both types also occur at stratigraphically different horizons. Both facies cover the same range of H/C ratios. Low H/C ratios <0.02 are observed over the whole range of S13C values, indicating that alteration during metamorphism cannot explain the differences in isotopic compositions. The large range of S13C values is interpreted to reflect primary differences during deposition. The more negative S13C values are thought to be associated with hydrothermal activity in small, restricted basins, in which methane recycling in the anoxic bottom waters is the principal pathway for bacterial biomass production. The isotopic signature is believed to result from organic matter synthesized by several different pathways, including H2S chemosynthesis, methylotrophy, and photosynthesis.

In contrast, type A carbonaceous argillites, such as those at Kidd Creek, are believed to indicate larger basinal settings in which organic matter is synthesized by assimilation of atmospheric and/or hydrothermal CO2 by C3 photosynthetic pathways in oxic surface waters with only minor methane recycling. Carbon isotope anomalies indicating mineralization were observed only in type B horizons, such as those at Chance. The smaller, isolated basins better preserve a record of methanogenesis associated with submarine volcanic exhalations. Despite the different carbon isotope signatures, the narrow range of the sulfur isotope data for the Chance mineralization, as well as inorganic geochemical data for the argillites, is consistent with the Chance deposit being part of the much larger Kidd Creek hydrothermal system. No kerogenic-graphitic horizons in large sulfide systems resulting in economic ore deposits have been studied in enough detail to understand the primary controls on the distribution of carbon isotopes fully. In large sulfide systems, the relationship between sulfides, methane recycling, and S13C value distribution is more complex and may reflect mixing of carbon sources and alternating successions of bacterial biomass, generated in the deeper part of the water column, and phytoplankton from the euphotic zone.

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Economic Geology Monograph Series

The Giant Kidd Creek Volcanogenic Massive Sulfide Deposit, Western Abitibi Subprovince, Canada

Mark D. Hannington
Mark D. Hannington
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C. Tucker Barrie
C. Tucker Barrie
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490052
Publication date:
January 01, 1999

GeoRef

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