Geochemical Evolution of the Epidote Zone, Fraser Mine, Sudbury, Ontario: Ni-Cu-PGE Remobilization by Saline Fluid

Exploration & Mining Geology, Vol. 5, No. 1, 1996

CATHARINE E.G. FARROW, Ontario Geological Survey, Ministry of Northern, Development and Mines, Willet Green Miller Centre, 933 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 6B5. DAVID H. WATKINSON, Ottawa-Carleton Geoscience Centre, Carleton University, Ottawa, Ontario, Canada, K1S 5B6

The Ni-rich Epidote Zone of the Fraser Mine at Sudbury is characterized 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 Breccia below the Sudbury Igneous Complex. The assemblages grade from epidotized Footwall Breccia matrix, through actinolite-bearing rocks with few vestiges of original Footwall Breccia textures, into the most intensely altered rock, a magnetite-rich facies. Pentlandite, pyrrhotite and pyrite are the dominant sul?de minerals; chalcopyrite is minor. The strongest geochemical differences compared to original Footwall Breccia occur where epidote and actinolite dominate the alteration assemblage (EPDZact facies). The most intense alteration facies are typi?ed by low Cu/(Cu+Ni) values of <0.1. Strong positive correlations exist 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 all elements from the least to the most intensely altered facies except for Au and Cu. As the proportion of actinolite increases in the EPDZact facies, the chondrite-normalized pro?les ?atten considerably due to low Au and Cu contents. Abundances of Ni, Ir, Ru and Rh almost ubiquitously increase with degree of alteration. In contrast, Cu, Zn, Ag and Au contents are low in almost all facies. It is suggested that saline hydrothermal ?uids moved through the Epidote Zone formation and became more oxidized as they reacted with the Footwall Breccia and with footwall units, including ultrama?c rocks. As this trend continued, magnetite-rich zones eventually developed. The occurrence of calcite locally in these magnetite-rich zones suggests that the pH of the ?uid increased, causing Cu solubility to drop as activity in the hydrothermal system waned. Sudden changes in ?uid composition such as this may have been due to an in?ux of ?uid from bounding ultrama?c rocks. Hydrothermal activity was focused along the margins of footwall ultrama?c units. Individual hydrothermal cells may have formed in the Footwall Breccia along pre-existing structures or sul?de concentrations. Metals may have been scavenged from sul?de-bearing Footwall Breccia and possibly Sublayer rocks. Nickel and some PGE were deposited in the Epidote Zone, whereas Cu, Au and Ag continued outward into the footwall where they were deposited as Cu-rich footwall veins.
Keywords: Geochemistry, Geochemical evolution, Frasier Mine.