DIVERSITY OF PRECIOUS-METAL MINERALIZATION IN FOOTWALL CU-NI-PGE DEPOSITS, SUDBURY, ONTARIO - IMPLICATIONS FOR HYDROTHERMAL MODELS OF FORMATION

The magmatic origin of contact orebodies of the Sudbury structure, in Ontario, is widely accepted. The remobilization of some of the metals from the magmatic environment to the footwall by hydrothermal processe s is not as widely accepted as a possible mode of formation of Cu-Ni-P GE orebodies in the footwall. However, precious-metal minerals (PMM) o ccur most commonly in chalcopyrite near, or at, the contact with alter ation minerals such as actinolite, quartz and epidote in footwall Cu-N i-PGE mineralization of the Sudbury Structure. Study locations include the Barnet Property, Deep Copper and Copper zones of the Strathcona m ine, McCreedy West mine (Cu stringers), and the Fraser mine (Epidote Z one). The most common PMM are Pt-, Pd- and Ag-rich tellurides and bism uthides. Each deposit has a characteristic assemblage of PMM, with dis tinct compositions of these minerals. Among Cu-Ni-PGE mineralized zone s, distinct Pt, Pd, Pi and Sb variations are most common in sobolevski te, michenerite, merenskyite, moncheite and melonite. Comparison of wh ole-rock concentrations of Cu, Ni, Lr, Ph, Pt, Pd and Au for the Barne t Property with that of other Cu-rich footwall deposits further illust rates the individuality of each deposit. The close spatial association of PMM with hydrous, locally Cl-bearing alteration and other rare min erals (including halides) is compatible with the influence of a H-O-Cl -S-Fe-Cu-Ni-Te-Se-Bi-As-bearing fluid in deposition of the PMM. The as semblages of the PMM and geochemical distinctions of each Cu-rich foot wall deposit are a function of the interaction of contact magmatic sul fide assemblages with fluids whose compositions were buffered by rock compositions in the footwall. The metals scavenged from these primary base-metal sulfides were transported into the footwall and subsequentl y deposited in hydrothermal cells.