17:15 〜 19:15
[HRE12-P08] Significance of thermochemical sulfate reduction for forming sediment-hosted volcanogenic massive sulfide deposits in the Archean.
Thermochemical sulfate reduction (TSR) is common in ore deposits formed within sedimentary rocks. This phenomenon is important for generating hydrogen sulfide, an essential component for ore formation. On the other hand, although most Archean ore deposits are associated with organic-rich sedimentary rocks, the primary sulfur source of the ore formation has been attributed to magmatic sources due to sulfate-poor Archean seawater. Therefore, TSR plays a role in forming Archean ore deposits is still not demonstrated.
Here, we investigated the Potter mine in the Abitibi greenstone belt in Canada. The Potter mine hosts ~2.7 Ga sediment-hosted volcanogenic massive sulfide (VMS) deposits. Sulfide minerals formed within black shales were primarily pyrrhotite and chalcopyrite. Additionally, barite, carbonate minerals such as calcite and siderite, and pyrobitumen (solidified hydrothermal petroleum) were identified. The maximum metamorphic temperature of the pyrobitumen, estimated by Raman spectroscopy, was approximately 280°C, consistent with the greenschist facies metamorphic temperature. This mineral assemblage suggests that TSR occurred within the sedimentary environment.
The δ13CV-PDB values of calcite and siderite in the examined samples ranged from -20 to -4‰, significantly lighter than those of marine carbonates. Such negative δ13CV-PDB values indicate carbonate ions derived from the oxidation of organic matter. Therefore, sulfate ions in this hydrothermal system likely acted as an oxidant, leading to TSR.
The δ88/86SrSRM987 values of calcite ranged from -0.03 to +0.33‰, while black shale exhibited a δ88/86SrSRM987 value of +0.20‰. Furthermore, the sample with the lowest δ13C value in calcite also exhibited the lowest δ88/86Sr value. These results suggest that TSR in a closed system influenced stable Sr behavior.
The δ34SV-CDT and Δ33S values of sulfide minerals showed a positive correlation, indicating that magmatic and seawater sulfur contributed to ore formation. Additionally, a negative correlation was observed between the Δ33S values of sulfide minerals and the δ13CV-PDB values of kerogen, while a positive correlation was found between the Δ33S values and the Eu anomaly. These findings suggest a strong relationship between mass-independent fractionation of sulfur and TSR formation processes.
Thus, our study demonstrated that TSR driven by the interaction of seawater sulfate, organic matter, and hydrothermal fluids contributed to ore formation in even the Archean sediment-hosted VMS deposits.
Here, we investigated the Potter mine in the Abitibi greenstone belt in Canada. The Potter mine hosts ~2.7 Ga sediment-hosted volcanogenic massive sulfide (VMS) deposits. Sulfide minerals formed within black shales were primarily pyrrhotite and chalcopyrite. Additionally, barite, carbonate minerals such as calcite and siderite, and pyrobitumen (solidified hydrothermal petroleum) were identified. The maximum metamorphic temperature of the pyrobitumen, estimated by Raman spectroscopy, was approximately 280°C, consistent with the greenschist facies metamorphic temperature. This mineral assemblage suggests that TSR occurred within the sedimentary environment.
The δ13CV-PDB values of calcite and siderite in the examined samples ranged from -20 to -4‰, significantly lighter than those of marine carbonates. Such negative δ13CV-PDB values indicate carbonate ions derived from the oxidation of organic matter. Therefore, sulfate ions in this hydrothermal system likely acted as an oxidant, leading to TSR.
The δ88/86SrSRM987 values of calcite ranged from -0.03 to +0.33‰, while black shale exhibited a δ88/86SrSRM987 value of +0.20‰. Furthermore, the sample with the lowest δ13C value in calcite also exhibited the lowest δ88/86Sr value. These results suggest that TSR in a closed system influenced stable Sr behavior.
The δ34SV-CDT and Δ33S values of sulfide minerals showed a positive correlation, indicating that magmatic and seawater sulfur contributed to ore formation. Additionally, a negative correlation was observed between the Δ33S values of sulfide minerals and the δ13CV-PDB values of kerogen, while a positive correlation was found between the Δ33S values and the Eu anomaly. These findings suggest a strong relationship between mass-independent fractionation of sulfur and TSR formation processes.
Thus, our study demonstrated that TSR driven by the interaction of seawater sulfate, organic matter, and hydrothermal fluids contributed to ore formation in even the Archean sediment-hosted VMS deposits.