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The Mordor Alkaline Igneous Complex, Central Australia: PGE-enriched disseminated sulfide layers in cumulates from a lamprophyric magma

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Abstract

The Mordor Alkaline Igneous Complex (MAIC) is a composite intrusion comprising a body of syenite and a funnel-shaped layered mafic–ultramafic intrusion of lamprophyric parentage, the Mordor Mafic–Ultramafic Intrusion or MMUI. The MMUI is highly unusual among intrusions of lamprophyric or potassic parentage in containing primary magmatic platinum-group element (PGE)-enriched sulfides. The MMUI sequence consists largely of phlogopite-rich pyroxenitic cumulates, with an inward dipping conformable layer of olivine-bearing cumulates divisible into a number of cyclic units. Stratiform-disseminated sulfide accumulations are of two types: disseminated layers at the base of cyclic units, with relatively high PGE tenors; and patchy PGE-poor disseminations within magnetite-bearing upper parts of cyclic units. Sulfide-enriched layers at cycle bases contain anomalous platinum group element contents with grades up to 1.5 g/t Pt+Pd+Au over 1-m intervals, returning to background values of low parts per billion (ppb) on a meter scale. They correspond to reversals in normal fractionation trends and are interpreted as the result of new magma influxes into a continuously replenished magma chamber. Basal layers have decoupled Cu and PGE peaks reflecting increasing PGE tenors up-section, due to increasing R factors during the replenishment episode, or progressive mixing of between resident PGE-poor magma and more PGE-enriched replenishing magma. The presence of PGE enriched sulfides in cumulates from a lamprophyric magma implies that low-degree partial melts do not necessarily leave sulfides and PGEs in the mantle restite during partial melting.

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Acknowledgments

We thank Tanami Gold NL for financial support to SJB and for analytical work, and for permission to publish. The manuscript was greatly improved following reviews by Marco Fiorentini, Wolfgang Maier, Michael Styles, and Larry Meinert. Greg Hitchen provided assistance with microprobe analyses. We thank Marcus Burnham and staff of GeoScience Laboratories, Sudbury, and the staff of Genalysis Laboratories, Perth, for analytical data.

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Authors and Affiliations

  1. CSIRO Exploration and Mining, 26 Dick Perry Ave., Kensington, Western Australia, 6151, Australia

    Stephen J. Barnes

  2. Tanami Gold NL, P.O. Box 1892, West Perth, Western Australia, 6872, Australia

    J. A. C. Anderson & T. R. Smith

  3. Centre for Exploration Targeting, School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia

    L. Bagas

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  1. Stephen J. Barnes
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Correspondence to Stephen J. Barnes.

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Barnes, S.J., Anderson, J.A.C., Smith, T.R. et al. The Mordor Alkaline Igneous Complex, Central Australia: PGE-enriched disseminated sulfide layers in cumulates from a lamprophyric magma. Miner Deposita 43, 641–662 (2008). https://doi.org/10.1007/s00126-008-0187-1

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