ARE CRUSTALLY DERIVED METALS OF SIGNIFICANCE IN THE GENERATION OF MAGMATIC CU-NI-PGE DEPOSITS?

The contamination of mafic to ultramafic magmas by country rocks has come to be a reasonably well-accepted tenant with respect to the formation of sulfide-rich, world –class Cu-Ni- (PGE) deposits. Many large deposits such as those associated with intrusions of Noril'sk and Pechenga, Voisey's Bay, the Duluth Complex, the Kabanga area of Tanzania, plus komatiite-related deposits in Canada and Australia are characterized by a close spatial proximity to S-bearing country rocks. In many cases the country rocks are also enriched in organic C or graphite. Sulfur isotopic data have provided corroborating geochemical evidence for the importance of the assimilation of country rock derived S in ore genesis. In contrast, S isotopic data from S-poor, reef-style PGE deposits such as those in the Bushveld Complex, Great Dyke, and Stillwater Complex are not supportive of crustal contamination as a key ingredient in their formation. However, Os isotope data from the Merensky Reef has been shown to be strongly non-chondritic, but the timing of contamination by crustal Os is widely debated. At both Voisey's Bay and Duluth, Os isotope data indicate that crustally derived Os in excess of 40% was involved in the sulfide mineralization.

Os isotopic data continue to foster questions related to the origin of PGEs in magmatic deposits. Organic-rich shales are well-known as a repository of many metals, including PGEs. In the case of the Proterozoic Virginia Formation which occurs as part of the footwall sequence to the Duluth Complex, kerogen contains up to 1.2 ppm Pd and 140 ppb Pt. Re and Os contents of kerogen and graphite are as high as 770 ppb and 30 ppb, respectively. The Re/Os ratios of kerogen and graphite are identical to those of the massive sulfides in the Duluth Complex. There is no question that C-rich sedimentary rocks or graphitic metamorphic rocks could supply high concentrations of PGEs to mafic magmas. Externally derived metals reduce the requirement of sequestration from questionably large volumes of magma in mixing models, and large thicknesses of solidifying crystals in models involving upward moving fluids. Anomalous Os isotopic data from igneous rocks should not be ignored and, where appropriate country rocks are present, potential increases in PGE concentrations due to magma-country rock interaction should be critically evaluated.