Geochemistry of fluid inclusion brines from Earth's oldest Mississippi Valley-type (MVT) deposits, Transvaal Supergroup, South Africa

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• 作者：Stephen E. Kesler ; Martin Reich ; Marlon Jean
• 关键词：MVT ; Precambrian ; Fluid inclusion ; Basinal brine ; Fluorite
• 刊名：Chemical Geology
• 出版年：2007
• 出版时间：5 March 2007
• 年：2007
• 卷：237
• 期：3-4
• 页码：274-288
• 全文大小：1644 K

文摘

Mississippi Valley-type Zn–Pb–(F) deposits are common in Phanerozoic rocks and rare in Precambrian rocks. By far the oldest known deposits are hosted by carbonate rocks in the Neoarchean–Paleoproterozoic Transvaal basin in South Africa, which were deposited before the 2.4 to 2.0 Ga Great Oxidation Event (GOE). Low sulfate concentrations in the world ocean before the GOE should have impeded formation of MVT deposits by the widely accepted model for Phanerozoic deposits involving sulfate-bearing basinal brines. As a test of this possibility, fluid inclusion studies were carried out to determine characteristics of brines associated with the Transvaal MVT deposits. Sphalerite in Zn–Pb MVT deposits (Bushy Park) and fluorite in F–(Zn–Pb) MVT deposits (Zeerust) are dominated by liquid–vapor (L + V) inclusions with homogenization and freezing temperatures generally similar to those of Phanerozoic MVT brines. A second, more saline brine that contains daughter minerals (L + V + D) is present in Zeerust fluorite and probably reflects contributions from the Bushveld Complex, which intruded the Transvaal sequence. Leachate and decrepitate compositions indicate that liquid–vapor inclusions from both areas are similar in composition to Phanerozoic brines and that they fall just above the Phanerozoic seawater-evaporation trend in Na/Br–Cl/Br space, possibly along a trend typical of Precambrian Ca-rich seas. These results indicate that MVT-forming processes typical of Phanerozoic basins were active in the Transvaal basin prior to or during the GOE suggesting that sufficient marine sulfate was available at least in this environment.