Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion,SW China |
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Authors: | Clément Ganino Nicholas T Arndt Mei-Fu Zhou Fabrice Gaillard Catherine Chauvel |
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Institution: | 1.Laboratoire de Géodynamique des Cha?nes Alpines,Université Joseph Fourier de Grenoble, CNRS,Saint Martin d’Hères,France;2.Department of Earth Sciences,HKU,Hong Kong,China;3.Institut des Sciences de la Terre d’Orléans, CNRS,Orléans Cedex 2,France |
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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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Keywords: | Magnetite ore Oxygen fugacity Skarn Panzhihua Emeishan LIP China |
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