Abstract
In southwestern China, several large magmatic Fe–Ti–V oxide ore deposits are hosted by gabbroic intrusions associated with the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite deposit at its base, concordantly intrudes late-Proterozoic dolostones. Mineralogical and chemical studies of the contact aureole in the footwall dolostones demonstrate that the metamorphism was largely isochemical but released large quantities of CO2 as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H2O-poor Emeishan basalt as parent magma, shows that under normal conditions, Fe–Ti oxides crystallize at a late stage, after the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO2-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations show that when magma with the composition of Emeishan basalt is fluxed by a CO2-rich gas phase, its equilibrium oxygen fugacity (fO2) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO2, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit.
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Acknowledgements
We express our gratitude to Fernando Tornos, Chris Harris and Michael Toplis for their constructive comments and their help with the preparation of the manuscript. We thank J. Gregory Shellnutt for interesting and useful discussions, Professor Yuxiao Ma for providing assistance during our fieldwork and Adeline Besnault for her assistance with the geochemical analyses. Constructive and helpful reviews were provided by Bernd Lehmann, Peter Lightfoot, Steve Prevec and an anonymous reviewer. The project was initiated during a PROCORE exchange program between France and the Hong Kong Research Grants Council. Financial support for the research was obtained from the French CNRS (DyETI programme).
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Ganino, C., Arndt, N.T., Zhou, MF. et al. Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion, SW China. Miner Deposita 43, 677–694 (2008). https://doi.org/10.1007/s00126-008-0191-5
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DOI: https://doi.org/10.1007/s00126-008-0191-5