Upgrading iron-ore deposits by melt loss during granulite facies metamorphism

• Melt loss and metamorphism is a mechanism to concentrate magnetite and hematite in the residual rock package.
• The specific degree of enrichment is controlled by the melt fertility of the rock.
• The concentration of Fe-oxides may increase by > 35 % in fertile, muscovite-rich horizons as a result of melt loss.
• Upgrading of these sub-economic horizons adds to the overall grade of the ore system.

Forward modelling of Fe-rich phyllite is used to evaluate the effects of partial melting and melt loss on the concentration of iron in the residual rock package, leading to enrichment in Fe-oxide minerals (magnetite and hematite). The effect of melt loss during prograde metamorphism to peak conditions of ~ 850 °C was modelled using a series of calculated pressure–temperature (P–T) phase diagrams (pseudosections). The results show that metapelitic rocks with lower iron content are more fertile, produce more melt and therefore show a more significant increase (up to 35%) in the Fe-oxide content in the residual (melt depleted) rock package. Rocks with primary Fe-rich compositions are less fertile, lose less melt and therefore do not experience the same relative increase in the amount of Fe-oxides in the residuum. The results of the modelling have implications for the formation of economic-grade iron ore deposits in metamorphic terranes. Fe-rich compositions that represent primary ore horizons prior to metamorphism may not experience significant enrichment. However, those horizons with lower primary iron contents may be significantly upgraded as a result of melt loss, thereby improving the overall grade of the ore system. The application of the modelling to the highly metamorphosed Palaeoproterozoic Warramboo magnetite–hematite deposit in the southern Gawler Craton suggests that melt loss during granulite facies metamorphism led to upgrading of sub-economic units within the low-grade Price Metasediments to form the economically viable granulite facies Warramboo ore system. The results of this study suggest that high-temperature metamorphic terranes offer attractive exploration targets for magnetite-dominated iron ore deposits.

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