5:15 PM - 6:30 PM
[HRE12-P03] The significance of geology and sulfur isotope compositions of sulfates and sulfides at the ca.3.4 Ga Big Stubby Kuroko deposit in Pilbara, Western Australia
The Big Stubby deposit is located at the Pilbara district in Western Australia. The age of this deposit is ca. 3.4 Ga and considered as the oldest Kuroko deposit. On the other hand, it has been unknown if Big Stubby and Miocene Kuroko deposits were formed by the same geological conditions (problem 1). The seawater sulfate had an important role for the formation of Miocene Kuroko deposits. On the other hand, the sulfate-poor ocean model is popular for the Archean age, and it is unknown if Big Stubby deposit could have formed under “sulfate-poor” conditions, differing from the Miocene Kuroko deposits (problem 2). In order to approach above problems, comparative studies between the Big Stubby and Miocene Kuroko deposits are performed in the present study.
Paleo-geological structures of the Big Stubby deposit was reconstructed in the present study combining pre-existing geological information and detailed examination of core samples. As a result, massive and Keiko (silicified) ores developed at the interface between rhyolite dome and tuff. Distal massive or disseminated sulfates overlain altered rhyolite dome. Those geological, mineralogical and geochemical information suggest that the Big Stubby deposit was formed by the similar mechanism as the Miocene Kuroko formed (answer to problem 1).
The δ 34 S values for sulfate and sulfide minerals in the Big Stubby deposit range from +11 to+16‰ (V-CDT) and -1 to +1‰ (V-CDT), respectively. The isotope fractionation factor between sulfate and sulfides at the Big Stubby deposit are interestingly the identical to those at the Miocene Kuroko deposits. Such commonness suggests the circulation of sulfate-rich seawater in deep footwall rocks was responsible to generate ca. 3.4 Ga black smokers, accompanied with high temperature sulfate reduction in depth (answer to problem 2). The Δ 33 S values indicate that the sulfate and sulfide minerals in the Big Stubby deposit are surprisingly following the MDF trend. This finding is different from the MIF signature found in other Archean sulfides and sulfates. This further rises possibility that MIF of sulfur was not global but local at the middle Archean age.
Paleo-geological structures of the Big Stubby deposit was reconstructed in the present study combining pre-existing geological information and detailed examination of core samples. As a result, massive and Keiko (silicified) ores developed at the interface between rhyolite dome and tuff. Distal massive or disseminated sulfates overlain altered rhyolite dome. Those geological, mineralogical and geochemical information suggest that the Big Stubby deposit was formed by the similar mechanism as the Miocene Kuroko formed (answer to problem 1).
The δ 34 S values for sulfate and sulfide minerals in the Big Stubby deposit range from +11 to+16‰ (V-CDT) and -1 to +1‰ (V-CDT), respectively. The isotope fractionation factor between sulfate and sulfides at the Big Stubby deposit are interestingly the identical to those at the Miocene Kuroko deposits. Such commonness suggests the circulation of sulfate-rich seawater in deep footwall rocks was responsible to generate ca. 3.4 Ga black smokers, accompanied with high temperature sulfate reduction in depth (answer to problem 2). The Δ 33 S values indicate that the sulfate and sulfide minerals in the Big Stubby deposit are surprisingly following the MDF trend. This finding is different from the MIF signature found in other Archean sulfides and sulfates. This further rises possibility that MIF of sulfur was not global but local at the middle Archean age.