Carbon-oxygen isotopic geochemistry of the Yangla Cu skarn deposit,SW China: Implications for the source and evolution of hydrothermal fluids |
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| Institution: | 1. School of the Earth Sciences & Resources, China University of Geosciences, Beijing, China;2. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China;1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PRChina;2. Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China;3. Zijin Mining Group Co. Ltd, Shanghang 364200, Fujian, PR China;4. Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Private Bag 79, Hobart, Australia |
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| Abstract: | The Yangla Cu deposit is the largest Cu skarn deposit in the Jinshajiang tectonic belt. Based on the detailed observation of crosscutting relationships, three mineralization stages (i.e., pre-ore, ore and supergene) have been identified in the Yangla deposit. The pre-ore stage is dominated by prograde skarn. The ore stage is characterized by the precipitation of hydrous silicate minerals, Fe-oxides, Fe-Cu-Mo-sulfides, quartz and calcite, whose mineral assemblages were formed in the early and late sub-ore stages. The early sub-ore stage is marked by retrograde alteration with the deposition of hydrous silicate minerals (e.g., actinolite, epidote and chlorite), Fe-oxides, abundant Fe-Cu-Mo-sulfides, quartz and minor calcite. Whilst, the late sub-ore stage, associated with silicic and carbonate alteration, is represented by widespread thick quartz or calcite veins with disseminated pyrite, chalcopyrite, galena and sphalerite. We present new carbon-oxygen (C-O) isotopic compositions of the ore-hosting marble and hydrothermal calcite of this deposit. The hydrothermal calcite in the Yangla deposit was precipitated from both the early and late sub-ore stages. Calcite I from the early sub-ore stage is anhedral, and occurs as spot in the skarn or locally replaces the skarn minerals. Calcite II from the late sub-ore stage is distinguished by being coarse-grained, subhedral to euhedral and its occurrence in thick veins. Calcite I contains lower δ13CPDB (?7.0‰ to ?5.0‰) and δ18OSMOW (7.2‰ to 12.7‰) than Calcite II (δ13CPDB = ?4.5‰ to ?2.3‰; δ18OSMOW = 10.7‰ to 19.4‰). In the δ13CPDB vs. δ18OSMOW diagram, the Calcite I and Calcite II data fall close to the igneous carbonatite field and between the fields of igneous carbonatite and marine carbonates, respectively. This suggests a dominantly magmatic origin for the early sub-ore fluids, and there might have been increasing carbonate wall rock involvement towards the late sub-ore stage. The ore-hosting marble (δ13CPDB = ?4.8‰ to ?0.3‰; δ18OSMOW = 10.2‰ to 23.9‰) also shows a positive δ13CPDB vs. δ18OSMOW correlation, which is interpreted to reflect the decreasing alteration intensity during the interactions between the hydrothermal fluids and ore-hosting carbonates. Simulated calculation suggests that both the Calcite I and Calcite II precipitated at 350 °C to 250 °C and 250 °C to 150 °C, respectively. We proposed that CO2 degassing and water/rock interactions were likely the two major processes that precipitated the calcite and led to the observed C-O isotopic features of the Yangla Cu deposit. |
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| Keywords: | Yangla Cu skarn deposit Hydrothermal calcite C-O isotopes Fluid source and evolution Sanjiang region (SW China) |
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