The Roby Zone of the Lac des Iles Pd-Pt-Ni-Cu-Au mine, located 90 km north-northwest of Thunder Bay, Ontario, was the initial locus of mining activity in the deposit. A striking aspect of the deposit in the early stages of mining was the coincidence of the ore zone with hydrothermal alteration. Based on this an investigation of fluid inclusions was undertaken to determine if any correlation existed between the Roby Zone mineralization and hydrothermal activity existed.
From a suite of samples collected during pre-and early-mining stages at the Roby Zone hand specimen were selected. Hand specimen were selected based on the presence and abundance of transparent minerals. The selected samples were used in the resulting fluid inclusion work. Four distinct types of fluid inclusions were observed in the study: Type I - single-phase liquid inclusions,Type II - two-phase inclusions (liquid + vapour), Type III - polyphase inclusions (liquid + vapour + one or more solid phases) and Type IV - CO2 rich polyphase inclusions.
The study shows that fluids with temperatures occurring between 350°C and 480°C, and with a composition of NaCl-CaCl2-H2O and/or NaCl-H2O and salinities of 11 to 20 equivalent wt% NaCl are associated with the ore zone of the Roby Zone. It is believed that the distribution of ore is the result of hydrothermal activity from these fluids remobilizing and concentrating the base and precious metals.