Abstract

Recent study of the platinum-group elements (PGE) in the Strathcona Ni-Cu sulfide deposit has shown that their distribution is zoned. Pt, Pd, Au, and Cu increase progressively, from the Hanging-wall ore zone through the Main zone into the Deep ore zone; Rh, Ru, Ir, and Os show a reverse trend (Naldrett et al., 1982).The Copper and Deep Copper zones at Strathcona are the farthest away from contact with the Sudbury Igneous Complex and consist of veins of massive sulfides up to 6 m in width, which narrow down to thin veins (<10 cm); branching from the veins are thin apophyses which are poor in sulfides and rich in quartz. New data from the veins indicate that they fall at one extremity of the compositional variation. They are marked by the highest abundances of PGE and ratios of Cu/(Cu + Ni) and (Pt + Pd)/Ir.PGE are not distributed uniformly within the Deep Copper zone. Pt, Pd, and Au increase with (Cu + Ni) in areas where Fe decreases in abundance. The following zonation is observed: (1) across the vein system as a whole, (2) outward from the center of a vein into its halo of dispersed sulfides within the wall rock, and (3) the transition from the massive sulfide veins to the quartz sulfide veins.The Cu-rich stringers of the Copper and Deep Copper zones are best explained as the result of the expulsion, into the footwall, of a Cu-rich sulfide liquid that had formed at the base of the Sudbury Igneous Complex as a result of fractional crystallization of monosulfide solid solution from an original sulfide magma. Much of the Pt, Pd, and Au fractionated into the Cu-rich liquid. The crystallization of a PGE-poor intermediate solid solution from the Cu-rich liquid resulted in a further increase of Cu, as well as Pt, Pd, and Au, in the residual sulfide liquid. A Cl-bearing aqueous fluid is believed to have separated from the residual liquid as it cooled and crystallized and to have caused the hydrothermal alteration that is observed at the margins of the sulfide veins, thus giving rise to quartz-chalcopyrite veins. The amount of Pt, Pd, and Au is thought to have increased in this fluid, but these elements appear not to have been transported very far from the magmatic sulfide source. The deposition of PGE as discrete minerals was subsequently controlled by Te, Bi, Sn, As, and Cl in the residual hydrothermal liquid.

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