Fluid evolution in the southern part of the Proterozoic Vazante Group,Brazil: Implications for exploration of sedimentary-hosted base metal deposits |
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| Institution: | 1. Instituto de Geociências, Universidade de Brasília, 70.910-900 Brasília, DF, Brazil;2. Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K 7L 3N6, Canada;3. Instituto de Geociências, Universidade de Brasília, CEP 70.910-900 Brasília, DF, Brazil;4. Votorantim Metais, Engenheiro Luís Carlos Berrini Avenue 105, 6 floor, Itaim Bibi 04.571-010, São Paulo SP, Brazil;1. Isotope Geosciences Unit, SUERC, Rankine Avenue, East Kilbride G75 0QF, Scotland, UK;2. Norwegian Geological Survey, Leif Eirikssons vei 39, N-4791 Trondheim, Norway;3. School of Geosciences, Meston Building, University of Aberdeen, Old Aberdeen AB24 3UE, Scotland, UK;4. Department of Earth and Environmental Sciences, St. Andrews University, St. Andrews, Fife KY16 9AL, Scotland, UK;1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;2. Unconventional Natural Gas Institute, China University of Petroleum, Beijing 102249, China;3. Unconventional Gas Collaborative Innovation Center, China University of Petroleum, Beijing 102249, China;4. The 4th Producing Plant, The Southwest Oil & Gas Company of SINOPEC, Chengdu, Sichuan, China;5. No.1 Oil Production Plant, Daqing Oilfield Co. Ltd., PetroChina, 163000, China |
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| Abstract: | The Proterozoic Vazante Group carbonate rocks were submitted to multiple stages of fluid circulation from diagenesis to orogenesis as documented by detailed mineralogical, fluid inclusion and isotopic studies of the Upper Morro do Pinheiro and Lower Pamplona members from the Serra do Poço Verde Formation in the southern part of the paleo-basin. These units are the main hosts for the hypogene, structurally-controlled zinc silicate deposits in the Vazante Zinc District, including the Vazante mine, which is considered to be the largest willemitic (Zn2SiO4) deposit in the world, with estimated total resources of 40–60 Mt at 20% Zn. Five hydrothermal alteration types were identified in the Southern Extension of the Vazante Group. (I) Early stage alteration comprises dolomite substitution bands and nodules, associated with moderate salinity H2O-NaCl-CaCl2 fluids, with temperatures around 90 °C, interpreted as late-diagenetic. The calculated C and O isotopic compositions of the fluids suggest meteoric and/or connate origin and interaction with organic carbon. (II) Pre-ore stage alteration is evidenced by dog-tooth dolomite and quartz with minor Fe-oxi/hydroxides which fills dissolution voids formed by H2O-NaCl-CaCl2 ± MgCl2 fluids at temperatures around 100° to 150 °C. The C isotopic data also indicate interaction with organic carbon. The two first stages are poor in ore-related elements. (III) Ore stage encompasses four phases of mineral precipitation. The first is distal and characterized by red stained dolostones due to disseminated hematite and red dolomite. The second is the main phase of the ore stage, composed of massive red dolomite, massive hematite and willemite with enrichment mainly in Fe2O3, SiO2, Ag, As, Cd, Cu, Ni, Pb, Se and Zn. The third phase comprises white dolomite, hematite and traces of willemite with enrichment in MnO, Cd, Ni and Pb. The calculated C and O isotopic compositions of the fluids (at 180 °C) associated with pervasive alteration yielded values heavier than the diagenetic stage and lower than the host rock. The fourth phase corresponds to Zn-chlorite and quartz which are associated with H2O-NaCl fluids of variable salinities and distinct temperatures (90–140 °C and 170–190 °C). (IV) Pyrite-bearing vein characterized by pyrite, sphalerite, white dolomite, fine hematite and late quartz, with C and O isotopic and fluid inclusion compositions that are similar to the three phases of dolomite of the ore stage. These data suggest a co-genetic relationship of these two alteration styles. (V) The late sulfide stage characterized by rare galena-bearing stringers with sphalerite, chalcocite, greenockite, covellite and white dolomite that cross-cut the main ore stage phases. The ore-related fluid compositions associated with the silicate zinc mineralization in the southern part of the Vazante Group are also similar to the fluids reported in previous work for the sulfide zinc-lead deposits in the northern part, indicating favorable conditions for metal transport during the Brasiliano Orogeny. Sulfide ore would have precipitated in zones where sulfur was available and silicate zinc in structures where mineralizing fluids interacted with evolved meteoric water. This finding implies that carbonate sequences in other districts with sulfide lead-zinc deposits may also host hypogene silicate zinc deposits. |
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