17:15 〜 18:45
[HRE13-P09] Importance of interaction between organic matter and hydrothermal fluids for the genesis of ca. 15 Ma Kuroko deposits
The sulfur cycle in the submarine hydrothermal circulations has been modeled, and the model is applied to explain the ore genesis of volcanogenic massive sulfide deposits. Recently, the importance of hydrothermal/carbonaceous sediment interaction has been emphasized (e.g., Wang et al., 2023). This raises the question of whether organic matter in sediments may contribute to ore genesis and sulfur cycle in submarine hydrothermal circulations.
Here, we investigated Shinsawa and Kowarizawa deposits, which are 15 Ma Kuroko deposits in the Hokuroku district of Akita, Japan. Ores were developed in organic matter-rich mudstones in both deposits. Bulk rock chemistries of mudstones disseminated in ore (such as Al2O3 and Pb+Zn concentrations) indicate assimilation or replacement of sediments into ores. Besides typical sulfides in Kuroko ores, barite and carbonate are found in Shinsawa and Kowarizawa ores.
The 87Sr/86Sr values of barites from the Shinsawa and Kowarizawa are from 0.7060 to 0.7075, respectively. They are intermediate values between Miocene seawater (0.7083 to 0.7090; McArthur et al., 2020) and typical oceanic crusts (~0.704). The δ88/86Sr values of these barites, measured for the first time by this study, are -0.1 to +0.03‰ (relative to NIST SRM987). These Sr isotope data are also explained by fluid/rock interactions. Petrographic observations, chemical composition, and Sr isotope data indicate that the hydrothermal fluids and seawater were mixed in the sediments, and those three components contributed to the ore genesis.
The δ13C and δ18O values of carbonate minerals from the Shinsawa were varied from -6 to -3 ‰ (V-PDB) and +15 to +20 ‰ (V-SMOW), respectively. The δ13C and δ18O values of some carbonate minerals were lower than those of typical marine carbonate minerals. This suggests that a part of the carbon source of carbonates was organic matter in mudstones (-26 to -22‰, V-PDB), and δ18O values of carbonates were equilibrated with circulating hydrothermal fluids (most likely, -2‰, V-SMOW).
δ34S sulfide values in disseminated ores from the Shinsawa and Kowarizawa were 0 to +7‰ (V-CDT). δ34S values of sulfate in these rocks was +21 to +25‰ (V-CDT), which is heavier than the δ34S value of sulfate in Miocene seawater (+21‰, V-CDT). Such a shift to heavier δ34S values is most likely caused by Rayleigh fractionation in mudstones. Sulfur and carbon isotope compositions indicate that thermochemical sulfate reduction in a closed system (i.e., in mudstone) occurred in the sediments of the Shinsawa and Kowarizawa. Furthermore, the Δ33S (= δ33S – 0.515×δ34S) values of sulfide and sulfate minerals disseminated in mudstones from the Shinsawa and Kowarizawa were from -1.1 to +1.1‰. It is interpreted that thermochemical sulfate reduction with organic matter caused Mass-Independent Fractionation of sulfur (MIF), as Watanabe et al. (2009) reported.
Here, we investigated Shinsawa and Kowarizawa deposits, which are 15 Ma Kuroko deposits in the Hokuroku district of Akita, Japan. Ores were developed in organic matter-rich mudstones in both deposits. Bulk rock chemistries of mudstones disseminated in ore (such as Al2O3 and Pb+Zn concentrations) indicate assimilation or replacement of sediments into ores. Besides typical sulfides in Kuroko ores, barite and carbonate are found in Shinsawa and Kowarizawa ores.
The 87Sr/86Sr values of barites from the Shinsawa and Kowarizawa are from 0.7060 to 0.7075, respectively. They are intermediate values between Miocene seawater (0.7083 to 0.7090; McArthur et al., 2020) and typical oceanic crusts (~0.704). The δ88/86Sr values of these barites, measured for the first time by this study, are -0.1 to +0.03‰ (relative to NIST SRM987). These Sr isotope data are also explained by fluid/rock interactions. Petrographic observations, chemical composition, and Sr isotope data indicate that the hydrothermal fluids and seawater were mixed in the sediments, and those three components contributed to the ore genesis.
The δ13C and δ18O values of carbonate minerals from the Shinsawa were varied from -6 to -3 ‰ (V-PDB) and +15 to +20 ‰ (V-SMOW), respectively. The δ13C and δ18O values of some carbonate minerals were lower than those of typical marine carbonate minerals. This suggests that a part of the carbon source of carbonates was organic matter in mudstones (-26 to -22‰, V-PDB), and δ18O values of carbonates were equilibrated with circulating hydrothermal fluids (most likely, -2‰, V-SMOW).
δ34S sulfide values in disseminated ores from the Shinsawa and Kowarizawa were 0 to +7‰ (V-CDT). δ34S values of sulfate in these rocks was +21 to +25‰ (V-CDT), which is heavier than the δ34S value of sulfate in Miocene seawater (+21‰, V-CDT). Such a shift to heavier δ34S values is most likely caused by Rayleigh fractionation in mudstones. Sulfur and carbon isotope compositions indicate that thermochemical sulfate reduction in a closed system (i.e., in mudstone) occurred in the sediments of the Shinsawa and Kowarizawa. Furthermore, the Δ33S (= δ33S – 0.515×δ34S) values of sulfide and sulfate minerals disseminated in mudstones from the Shinsawa and Kowarizawa were from -1.1 to +1.1‰. It is interpreted that thermochemical sulfate reduction with organic matter caused Mass-Independent Fractionation of sulfur (MIF), as Watanabe et al. (2009) reported.