Iridium and palladium as discriminants of volcanic-exhalative, hydrothermal, and magmatic nickel sulfide mineralization

Abstract

Neutron activation of Pd, Au, and Ir in samples from (1) economically significant Ni sulfide deposits of Western Australia, (2) the host rocks of these deposits, (3) hydrothermal and/or metasomatic Ni sulfide (mainly millerite) mineralization in peridotite at Nullagine, Western Australia, (4) hydrothermal replacement (volcanic-exhalative?) Ni-Cu sulfide mineralization at Sherlock Bay, Western Australia, and (5) volcanic-exhalative sulfides in inter flow sediments at Kambalda are used to show that Ir and Pd can be used to discriminate between the various origins (magmatic, volcanic-exhalative, hydrothermal/metasomatic, and metamorphic) ascribed to Ni sulfide deposits.All the economically significant Ni sulfide deposits in Western Australia are strongly enriched in the precious metals relative to other crustal rocks and have similar Pd/Ni and Ir/Ni ratios to each other. These ratios persist regardless of whether the nickel sulfides occur at the base of volcanic ultramafic piles (e.g., Kambalda) or as centrally disposed interstitial sulfides in intrusive dunite bodies. The strong similarities of the Pd/Ir ratios of the Ni sulfide ores and of the spinilex-textured komatiites, which represent ultramafic liquids from which the Ni sulfide liquids were derived, together with other features of the precious metal geochemistry of the ores and host rocks of the Ni sulfide deposits, are used to show that the deposits had magmatic origins.In contrast, the Pd/Ir and Ir/Ni ratios of the hydrothermal and/or metasomatic Ni sulfide mineralization at Nullagine are distinctly different from those of the major Ni deposits. The former contains high Au, moderate but variable Pd, and exceptionally low Ir. Similarly, the volcanic-exhalative sulfides at Kambalda and Sherlock Bay contain negligible quantities of Pd and Ir. All the Ir in both the interflow sediments and the hydrothermal metasomatic mineralization is contributed by the host rocks and not by the sulfides. The Western Australian data, together with precious metal data for other examples of hydrothermal Ni-Cu sulfide mineralization, strongly suggest that although Ni, Au, and Pd may be transported in aqueous solution, Ir cannot. Hence, the relative abundance of Ir can be used as a discriminant of magmatic sulfides.