The Kabanga Ni sulfide deposits, Tanzania: II. Chalcophile and siderophile element geochemistry |
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| Authors: | Wolfgang D Maier Sarah-Jane Barnes |
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| Institution: | 1. Department of Geology, University of Oulu, Oulu, 90014, Finland
2. Sciences de la Terre, Université du Québec à Chicoutimi, Chicoutimi, Québec, G7H2B1, Canada
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| Abstract: | 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. |
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