[BBG02-P04] Geochemical study on the sulfide and organic matter at the Potterdoal deposit in Abitibi Greenstone Belt in Canada
Keywords:Archean, Hydrothermal activity, Volcanogenic Massive Sulfide (VMS) deposit, Methanotroph
At the Potterdoal VMS deposit, massive sulfides were dominantly composed of CuFeS2, ZnS and FeS2. Massive sulfides showed layered structures suggesting that sulfide precipitation on the ocean floor. Chemical compositions of ZnS suggest that all sulfides were precipitated at constant fS2 conditions (around 10-10) under thermodynamically equilibrium conditions. Chert and sandstone appeared in the distal area from VMSs and those sedimentary rocks were often brecciated by veining by of the hydrothermal fluids following VMS. The sulfur isotope composition (d34S) values of the bulk VMSs ranged from -0.1 to +2.0 ‰ (CDT) and those values of pyrite in chert and sandstone were from +5.0 to +6.9 ‰ (CDT). So total variation of d34S values was 7 ‰ and this range suggests the sulfur source of VMSs was seawater sulfate. Since d34S values of sulfate cannot be lower than those of pyrite, they were at least larger than +6.9‰ (CDT) at that time.
The organic carbon concentration of examined bulk samples were 0.0 to 0.5 wt. %C for VMSs, and 0.2 to 1.0 wt. %C for sedimentary rocks, respectively. Raman spectroscopic analyses on isolated kerogen from VMSs and sedimentary rocks showed constant metamorphic grades of all samples and the thermal alteration temperature was estimated as 291 ± 30 °C based on the full widths of half maximum of the D1 band in the Raman spectra of kerogen from VMSs, which indicates less altered features for the Archean rocks.
The carbon isotope composition of organic matter (d13Corg.) values of all Potterdoal samples ranged from -43.1 to -31.8 ‰ (PDB). Those values suggest the presence of methanotrophs in the ambient ocean water. d13Corg. values of Potter samples ranged from -48.6 to -39.6 ‰ (PDB), which are lower and relatively constant than those of Potterdoal samples.
The estimated total mass of VMS at the Potter deposit is about three times as large as the Potterdoal deposit . This probably suggests that a larger hydrothermal system existed at the Potter site. Such difference may have affected thermal degradation of organic matter at both sites. The organic matter at the Potter site suffered additional degradation by thermal decomposition of it, producing more methane to the ocean than the Potterdoal site. Such process may have promoted activities of methanotrophs resulting more 12C-rich feature of organic matter at Potter samples.
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