GEOCHEMICAL EVOLUTION OF THE EPIDOTE ZONE, FRASER-MINE, SUDBURY, ONTARIO - NI-CU-PGE REMOBILIZATION BY SALINE FLUIDS

The Ni-rich Epidote Zone of the Fraser Mine at Sudbury is characterize d by alteration assemblages rich in epidote, actinolite and magnetite which are roughly concentrically zoned and up to hundreds of meters in length. These zones form discrete areas within the igneous Footwall B reccia below the Sudbury Igneous Complex. The assemblages grade from e pidotized Footwall Breccia matrix, through actinolite-bearing rocks wi th few vestiges of original Footwall Breccia textures, into the most i ntensely altered rock, a magnetite-rich facies. Pentlandite, pyrrhotit e and pyrite are the dominant sulfide minerals; chalcopyrite is minor. The strongest geochemical differences compared to original Footwall B reccia occur where epidote and actinolite dominate the alteration asse mblage (EPDZact facies). The most intense alteration facies are typifi ed by low Cu/(Cu+Ni) values of <0.1. Strong positive correlations exis t between Ni and Co, Cu and Au, and among Ir, Os, Ru and Rh. Chondrite -normalized Ni - PGE - Au - Cu plots display an overall increase in al l elements from the least to the most intensely altered facies except for Au and Cu. As the proportion of actinolite increases in the EPDZac t facies, the chondrite-normalized profiles flatten considerably due t o low Au and Cu contents. Abundances of Ni, Ir, Ru and Rh almost ubiqu itously increase with degree of alteration. In contrast, Cu, Zn, Ag an d Au contents are low in almost all facies. It is suggested that salin e hydrothermal fluids moved through the Epidote Zone formation and bec ame more oxidized as they reacted with the Footwall Breccia and with f ootwall units, including ultramafic rocks. As this trend continued, ma gnetite-rich zones eventually developed. The occurrence of calcite loc ally in these magnetite-rich zones suggests that the pH of the fluid i ncreased, causing Cu solubility to drop as activity in the hydrotherma l system waned. Sudden changes in fluid composition such as this may h ave been due to an influx of fluid from bounding ultramafic rocks. Hyd rothermal activity was focused along the margins of footwall ultramafi c units. Individual hydrothermal cells may have formed in the Footwall Breccia along pre-existing structures or sulfide concentrations. Meta ls may have been scavenged from sulfide-bearing Footwall Breccia and p ossibly Sublayer rocks. Nickel and some PGE were deposited in the Epid ote Zone, whereas Cu, Au and Ag continued outward into the footwall wh ere they were deposited as Cu-rich footwall veins. Copyright (C) 1996 Canadian Institute of Mining, Metallurgy and Petroleum.