Genesis of Sublayer, Footwall Breccia, and Associated Ni-Cu-Platinum Group Element Mineralization in the Sudbury Igneous Complex

Abstract The Ni-Cu-platinum group element (PGE) sulfide mineralization in the 1.85 Ga Sudbury impact structure occurs two discrete environments: (1) along or near basal contact, including disseminated to semimassive an inclusion-rich sublayer, fragmental underlying anatectic footwall breccia, and veins disseminations pseudotachylitic (2) inclusion-bearing quartz diorite phase of radial concentric offset dikes that extend up 20 km from contact. inclusion diorite, breccia all contain abundant mafic-ultramafic inclusions—local xenoliths derived nearby country rocks, exotic unexposed upper-middle crustal target rocks and, locally, anteliths olivine melanoritic early border Igneous Complex—but margins very few local inclusions no ultramafic inclusions. similar populations indicate a genetic relationship between sublayer interaction breccia. But is characterized by cumulate noritic matrix, noncumulate dioritic felsic-intermediate matrix. overlying main mass norite homogeneous terms Hf isotopes, indicating melt sheet was well mixed, but ores, lesser degree norites vary widely their Pb-S-(Os) isotope compositions. majority inclusions, except for anteliths, exhibit signature Ni-Cu-PGE depletion caused prior saturation, which indicates association attributable primarily refractory nature hydrodynamic behavior as secondarily, derivation sulfides same sources large event volatilized much Pb-S-Zn-Cd-Se-Bi significant amounts Sb-Ag-Cu-Au-As rocks. It generated superheated only minor debris during compression excavation stage, when inclusion- sulfide-poor marginal emplaced. Large debris, sulfide-bearing Huronian basalts, Nipissing East Bull Lake intrusions, Archean mafic gneisses, were isostatic rebound, formation central uplift, collapse uplift crater walls, internal Convective gravity flow aided horizontal transport residual xenomelts into embayments funnels form protosublayer. Olivine-saturated melts, thermomechanical erosion olivine-bearing locally crystallized mass, disrupted preserved some North Range dikes. Continued enlarged dike exploited other fractures generate geochemical heterogeneities. As rate decreased heat conducted increased, contact ores fractionally melts vein systems.