Catastrophic sulfide saturation and genesis of Ni-Cu-PGE mineralization in the Sudbury Igneous Complex

Many models for the formation of the Ni-Cu-PGE ores in the 1850 Ma Sudbury Igneous Complex assume that the sulfides exsolved from the melt sheet during slow cooling, that the ore-localizing embayments represent original topographic features, and that the sulfide ores "passively" collected in the embayments. It is more likely, however, that the Ni-Cu-PGE ores in the SIC exsolved almost instantaneously as the superheated melt sheet was catastrophically driven to the liquidus following incorporation of inclusions derived from footwall rocks during crater rebound, injection of footwall dikes, and incorporation of fragmented footwall rocks during the formation of embayments and troughs. This explains the very strong correlation between inclusions, mineralization, and ore-localizing embayments. The equally rapid increase in effective viscosity explains the absence of subsequent settling of semi-massive pods of sulfides in the dikes and coarse disseminated sulfides in Sublayer norite. Lateral variations in chalcophile element enrichment/depletion trends and Pb isotopic contents of norites overlying and away from ore-localizing embayments, and in the S-Os-Pb isotopic compositions and As contents of the various ore zones reflect preservation of these signatures within relatively small (similar to 15 km(3)) convection cells.