The Kabanga Ni sulfide deposits, Tanzania: II. Chalcophile and siderophile element geochemistry
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The Kabanga deposit constitutes one of the most significant Ni sulfide discoveries of the last two decades (indicated mineral resource 23 Mt of ore at 2.64% Ni, inferred resource 28.5 Mt at 2.7% Ni, November 2008). The sulfides are hosted by predominantly harzburgitic and orthopyroxenitic intrusions that crystallized from magnesian basaltic and picritic magmas. However, compared with other sulfide ores that segregated from such magmas (e.g., Jinchuan, Pechenga, Raglan), most Kabanga sulfides have low Ni (<1–3%), Cu (∼0.1–0.4%), and PGE contents (≪1 ppm), high Ni/Cu (5–15), and low Ni/Co (10–15) and Pd/Ir (2–20). Sulfides with higher metal contents (up to ∼5% Ni, 0.8% Cu, 10 ppm PGE) are found in only one unit from Kabanga North. The observed metal contents are consistent with segregation of magmatic sulfides from fertile to strongly metal-depleted magmas, at intermediate to very low mass ratios of silicate to sulfide liquid (R factors) of approximately 10–400. Sulfide saturation was triggered prior to final emplacement, by assimilation of up to 50% of the total sulfur in the intrusions from sulfide-bearing metasedimentary country rocks. Immiscible sulfide liquid was entrained by the magma and ultimately precipitated in dynamic magma conduits that formed tubular and sill-like mafic–ultramafic bodies characterized by abundant magmatic breccias, highly irregular layering, and frequent compositional reversals. The unusually large degree of crustal contamination and the low R factors render Kabanga an end-member in the spectrum of magmatic Ni sulfide ores.
KeywordsSulfide Massive Sulfide Picrite Sulfide Liquid Magmatic Sulfide
We thank the staff of the Kabanga Ni company, Barrick Gold Company, and Xstrata for providing financial support for the analytical work, transport to site, and logistics. Additional analytical support was provided by NSERC (to SJB). Without the enthusiasm of Tim Livesey (exploration manager) and Carina Acland (exploration geologist), this project would not have been possible. Tafadzwa Gomwe and Xstrata staff provided feedback on an earlier version of the manuscript. We thank PC Lightfoot, J Scoates, and R Sproule for constructive reviews.
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