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Variations in platinum group element concentrations in the Alexo mine komatiite, Abitibi greenstone belt, northern Ontario

  • Sarah-Jane Barnes (a1) and A. J. Naldrett (a2)

The noble element (Os, Ir, Ru, Rh, Pt, Pd, Au) patterns from sulphides associated with the komatiites at the Alexo mine show variable degrees of fractionation. Massive sulphides at the contact between underlying intermediate volcanics and overlying komatiites have the least fractionated patterns (Pd/Ir = 44). Net-textured sulphides which immediately overlie the massive sulphides have extremely fractionated noble element patterns (Pd/Ir = 171). The disseminated sulphides in the overlying komatiite exhibit an intermediate degree of fractionation (Pd/Ir = 110). The variations in noble element patterns are complemented by variations in the Ni, Cu and Co concentrations. The massive sulphides are depleted in all three of these elements relative to the net-textured sulphide ore. The disseminated sulphides have intermediate values of Ni and Co, but are enriched in Cu relative to the other two sulphides.

Processes that could have affected the composition of the sulphides include: (a) fractionation of monosulphide solid solution (Mss) from a sulphide liquid during initial cooling of the komatiite, and (b) mobilization of Pt, Pd, Au, Cu, Ni and Co from the massive sulphide into footwall veins. A mass balance calculation indicates that the sum of the massive and net-textured sulphide agrees with the disseminated sulphide for all elements except Au. Thus the massive sulphide cannot have lost significant amounts of Pt, Pd, Ni and Co to footwall veins and significant mobilization of these elements does not appear to have occurred. The crystallization of Mss from a sulphide liquid will account for the enrichment of Ir, Os, Ru and Rh in the massive sulphide relative to the net-textured sulphide, and enrichment of Pd, Pt, Au, Co and Ni in the net-textured sulphide relative to the disseminated sulphide.

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S.-J. Barnes 1985. The petrology and geochemistry of komatiite flows from the Abitibi Greenstone Belt, Canada, and a model for their formation. Lithos 18, 241–70.

S.-J. Barnes , A. J. Naldrett & M. P. Gorton 1985. The origin of the fractionation of platinum-group elements in terrestrial magmas. Chemical Geology 53, 303–23.

W. E. Ewers & D. R. Hudson 1972. An interpretive study of a nickel iron sulphide ore intersection, Lunnon Shoot, Kambalda, Western Australia. Economic Geology 67, 1075–92.

E. L. Hoffman , A. J. Naldrett , J. C. Van Loon , R. G. V. Hancock & A. Mason 1978. The determination of all the platinum group elements and gold in rocks ores, etc. Analytica Chimica Acta 102, 157–66.

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R. R. Keays , E. H. Nickel , D. I. Groves & P. J. Mcgoldrick 1982. Iridium and palladium as discriminants of volcanic-exhalative, hydrothermal, and magmatic nickel sulphide mineralization. Economic Geology 77, 1535–47.

P. C. Lightfoot , A. J. Naldrett & C. J. Hawkesworth 1984. The geology and geochemistry of the Waterfall Gorge section of the Insizwa Complex with particular reference to the origin of the nickel-sulfide deposits. Economic Geology 79, 1857–79.

A. J. Naldrett , D. Innes , M. P. Gorton & J. Sowa 1982. Compositional variations within and between five Sudbury ore deposits. Economic Geology 77, 1519–34.

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Geological Magazine
  • ISSN: 0016-7568
  • EISSN: 1469-5081
  • URL: /core/journals/geological-magazine
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