Abstract

More than half of the giant Ni-Cu-platinum-group element (PGE) sulfide ores of the Sudbury Igneous Complex are associated with a discontinuous unit at the base of the main mass known as the "sublayer." The sublayer is comprised of two fragment-rich members: (1) a metamorphic-textured footwall breccia, and (2) an igneous-textured contact sublayer. The contact sublayer occurs as a thick unit in depressions at the base of the Sudbury Igneous Complex termed "embayments"; it is associated with disseminated to massive sulfides and contains a range of inclusion types such as diabase, melanorite, olivine melanorite, metamorphosed melanorite, wehrlite, and dunite. We present data for a case study of the Whistle mine embayment and show that the igneous-textured sublayer matrix is geochemically unlike the main mass norites, quartz gabbros, and granophyres. For example, the igneous-textured sublayer matrix in the Whistle mine has La/Sm = 4.0, La/ Nb = 5.1, and Th/Zr = 0.02, and the main mass norites, quartz gabbros, and granophyres have ratios of 5.5 to 7, 2.8 to 4.2, and 0.04 to 0.05, respectively. This matrix contains partially digested hornfels diabase fragments and ghost textures of magnetite remaining from the melting of diabase. The igneous-textured sublayer matrix at the Whistle mine can be modeled with small amounts of assimilation of local country-rock granitoids ( approximately 10%), large degrees of assimilation of diabase that are not derived from the immediate country rocks ( approximately 70%), and small contributions from the main mass magma type ( approximately 20% mafic norite). The amount of diabase assimilation would be too large for traditional assimilation models controlled by the available heat of the mafic magma; even with a superheated magma, 70 percent assimilation is too large. A model is proposed where the melting of the target rocks initially produces a felsic melt sheet which is laden with mafic fragments. These fragments sink to the base of the melt sheet in the crater and are concentrated and further melted to produce the mafic sublayer. There are significant differences in the composition of the igneous-textured sublayer matrix between different embayments which may reflect differing degrees of digestion of compositionally different protolith fragments. The melanorite inclusions in the sublayer at the Whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous-textured sublayer matrix, but they have similar high incompatible element concentrations (e.g., olivine melanorites have 20-65 ppm Ce in rocks with 15-21 wt % MgO), 1 to 10 percent interstitial sulfide, up to 0.5 percent apatite, 1 to 15 percent biotite, and 1.85 Ga age zircon and baddeleyite. The mafic-ultramafic inclusions are interpreted to be the broken-up remnants of an earlier cumulate formed at depth from a main mass magma; this parental magma has compositional traits which suggest that crystal accumulation took place from a magma which contains a large contribution from the diabase. Olivine compositional data for the mafic inclusions (Fo (sub 71-79) ; 500-3,800 ppm Ni), in the absence of reequilibration, indicate that olivine crystallization both predated and postdated sulfur saturation of the magma. Cr-rich spinels from these rocks confirm that the parental magma was especially Cr rich, and based on the olivine compositional data, had a tholeiitic Mg/Fe ratio.

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