Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion, SW China

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-Proter...

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Bibliographic Details
Published in:Mineralium deposita 2008-08, Vol.43 (6), p.677-694
Main Authors: Ganino, Clément, Arndt, Nicholas T., Zhou, Mei-Fu, Gaillard, Fabrice, Chauvel, Catherine
Format: Article
Language:English
Subjects:
Basalt
Carbon dioxide
Crystallization
Earth and Environmental Science
Earth Sciences
Geochemistry
Geology
Magma
Metamorphism
Mineral Resources
Mineralogy
Rocks
Sciences of the Universe
Sediments
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Summary: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 CO 2 as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H 2 O-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 CO 2 -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 CO 2 -rich gas phase, its equilibrium oxygen fugacity (fO 2 ) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO 2 , such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit.
ISSN:0026-4598
1432-1866
DOI:10.1007/s00126-008-0191-5