Abstract

Nickel sulfide mineralization in the Agnew deposit comprises matrix or net-textured ores, primary in situ massive sulfides, structurally remobilized massive sulfides, and low-grade disseminated or "cloud" suifides. Matrix and massive sulfides are also present at the laterally correlative Rocky's Reward deposit 2 km to the north. Systematic compositional contrasts are noted between the different ore types when analyses are normalized to abundances in 100 percent sulfides. Matrix ores are enriched in Ni, Co, Cu, Pt, Pd, and Au but depleted in Os, Ir, Ru, and Rh relative to adjacent massive ores, both at the Agnew deposit and at Rocky's Reward. Cloud sulfides are similar to matrix sulfides in Os, Ir, Ru, and Rh but are enriched in Ni, Co, Cu, Pt, Pd, and Au relative to both massive and matrix ores. Overall abundances of all the above elements are slightly lower at Rocky's Reward than at Agnew, and both deposits are depleted in these elements relative to ores at Kambalda. Remobilized and in situ massive sulfides are identical in composition, implying that remobilization occurred by tectonic, not hydrothermal, processes. Massive sulfides could not have originated by metamorphic upgrading of matrix ore.The nickel content of olivine within the mineralized flow at Agnew is strongly depleted relative to typical komatiitic olivines, Olivine in the cloud sulfide zone is depleted to a lesser extent. Quantitative modeling shows that the observed olivine Mg/Fe ratios and nickel contents, as well as the Ni contents of the Agnew sulfides, formed by extensive preemplacement extraction of olivine and sulfide liquid from a primitive komatiite parent. Olivine and sulfides in the cloud zone show less extensive differentiation in a model.Compositional contrasts between the Agnew and Rocky's Reward deposits are attributed to lateral differentiation within lava flows, since Agnew is closer to the site of eruption. The difference in Os, Ir, Ru, and Rh concentrations between matrix and massive ore cannot be the result of in situ fractionation of the sulfide liquid or of hydrothermal remobilization. The different ore types must have formed from discrete batches of sulfide liquid which segregated under different fugacities of sulfur and/or oxygen.

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