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The Genesis of Chromite Cumulates in Layered Mafic Intrusions and Related Ore Forming Processes - an Experimental High Temperature Centrifuge Project

English title The Genesis of Chromite Cumulates in Layered Mafic Intrusions and Related Ore Forming Processes - an Experimental High Temperature Centrifuge Project
Applicant Manoochehri Shahrzad
Number 129181
Funding scheme Marie Heim-Voegtlin grants
Research institution Institut für Mineralogie und Petrographie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Mineralogy
Start/End 01.03.2010 - 30.06.2012
Approved amount 116'862.00
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Keywords (4)

chromite cumulates; layered mafic intrusions; high-temperature centrifuging; ore mineralisations

Lay Summary (English)

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Lay summary
Cr, Pt, Pd and various other transition and noble metals are mainly mined from sub-meter thick chromite+sulphide layers in so-called Layered Mafic Intrusions (LMIs). The latter are giant magma chambers that may extend over hundreds of km and are distinguished by layered structures and cumulate textures. Cumulates are crystal accumulations which contain more of a crystal species than could form from the parent liquid in a closed system. As an example, the UG2-reef cumulate in the Bushveld complex (the hugest layered intrusion, South Africa), contains about 75-90% modal chromite, is typically 70 cm thick and extends for nearly 400 km. These layers are thought to be mainly formed through gravitational settling and subsequent pressure driven chemical compaction at the bottom of a magma chamber. The first purpose of this project is to understand the timescales and the mechanical processes leading to the formation of these mineralogically uniform layers, this by simulation through high temperature and pressure centrifuge experiments. To overcome the experimental drawbacks, gravitational forces will be amplified by employing the centrifuging piston cylinder (a worldwide unique experimental apparatus) at controlled oxidation potential. This project will also investigate the links between chromite and sulphide crystallization, the latter carrying the economically very important platinum group elements (PGEs). This will be done through investigating the co-settling of chromite with sulphide minerals or with immiscible sulphide melts to study the near liquidus phase relations of, and trace elements partitioning in suited parental magmas. The final results will be cast into a physical and thermo-chemical model of settling velocities and chemical compaction of chromite cumulates as a function of grain size, composition, density of suspension, cumulate height and temperature.
Direct link to Lay Summary Last update: 21.02.2013

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Title Type of contribution Title of article or contribution Date Place Persons involved
14th International Conference on Experimental Mineralogy, Petrology and Geochemistry (EMPG) 05.03.2012 Kiel, Germany


Abstract

Cr, Pt, Pd and various other transition and noble metals are mainly mined from sub-meter thick chromite+sulphide layers in so-called layered mafic intrusions that extend over several 100 km. These layers form through settling and compaction of chromite crystals at the bottom of cooling giant magma chambers, but quantitative models for this process do not exist. This project will study the formation time scales and mechanisms of such layers through experimental high temperature and pressure centrifugation. It will also experimentally study the links between chromite and sulphide crystallization, the latter carrying the platinum group elements, to understand and be able to predict sulphide mineralisations in such layers. The results will be cast into a theoretical model of chromite layer formation, and experimental phase equilibria and modeled time scales for the formation of such layers will then allow understanding the replenishment and cooling process/heat budget of the giant parent magma chambers.
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