Geochemical Characteristics of the Roby and Twilight zones of the Lac des Îles Complex, Northwest Ontario, Canada, with specific reference to platinum-group elements

摘要

The Lac des iles Complex contains Canada's only primary Platinum-group element (PGE) ore deposit; this is known as the Roby Zone. The Twilight Zone is a PGE occurrence to the northeast of the Roby Zone. A remarkable feature of both the Roby and Twilight Zones is the high PdIPt ratios of the ores, as high as 20. Most PGE deposits have Pd/Pt ratios of 0.5-3. Most of the Twilight and Roby Zones consist of magmatic breccia of gabbronorite composition. Some of this breccia is now an amphibolite. There is a strong correlation between S, Cu and the PGE indicating sulphides are the phase controlling the PGE. There appears to be no difference in PGE or lithophile element contents between the amphibolite and fiesh rocks, indicating that metamorphism has not changed the composition of the rocks. There is a difference between the composition of the breccia clasts and their matrix. The clasts tend to be more leucocratic and have a lower PGE content than the matrix. But both the matrix and clasts show the same PGE and incompatible element ratios indicating that they are CO-magmatic. A small portion of the eastern section of Roby Zone has been converted to actinolite-chlorite+/-talc schist and higher grades of Pd and Pt are found in this rock than in the Breccia ore. This is referred to as High Grade ore. In the High Grade ore there is no correlation between the PGE and S. This could be because S has been lost from the High Grade ore, but most of the chalcophile elements do not show the same enrichment as Pd, suggesting that it is Pd that has been added to the rocks rather than S lost.