Hydrothermal Alteration, Fluid Inclusion and Stable Isotope Studies of the North Roby Zone, Lac des Iles PGE Mine, Ontario, Canada

The Archean mafic-ultramafic complex of Lac des Iles, Ontario, Canada, hosts economic platinum group elements (PGE)-Au-Cu-Ni mineralization in the Roby Zone. All lithologies in the North Roby Zone have been affected by hydrothermal alteration. The alteration products include talc (the most dominant mineral), anthophyllite, serpentine, actinolite, tremolite, chlorite, hornblende, zoisite, clinozoisite, epidote and sericite. In the altered rocks, light rare earth elements (La, Ce, Nd, Sm), Pb, Rb, Ba, Cs, S and possibly Y have been added by hydrothermal solution whereas Eu and heavy rare earth elements (Yb, Gd, Dy, Er) remained immobile. There are five types of fluid inclusions in the pegmatitic plagioclase with homogenization temperature and salinity ranging from 240 degrees C to 445 degrees C and 15.37 to 48.52 wt% equivalent NaCl, respectively. The delta(18)O and delta D of talc range form 6.2 parts per thousand to 6.9 parts per thousand and -28 parts per thousand to -48 parts per thousand, respectively. delta(18)O and delta D water in equilibrium with talc during the hydrothermal alteration suggest a modified source for the hydrothermal solution. Microthermometry and stable isotope studies suggest that high temperature-high salinity fluid was diluted by, and mixed with, low temperature-low salinity meteoric solution. This mechanism precipitated the hydrothermal assemblage and redistributed trace elements during and after pegmatite formation in the North Ruby Zone.