The Pd deposits of the Lac des Iles Complex, Northwestern Ontario

The Lac des Iles Complex contains the Roby, Twilight, and High-grade zones, which make up Canada’s only primary platinum-group element (PGE) ore deposit with a grade of ~3 ppm Pd + Pt. The ores have remarkably high Pd/Pt ratios, averaging 7 in the Roby and Twilight zones and even higher, 14, in the High-grade zone. In contrast most PGE-dominated deposits have Pd/Pt ratios of 0.5 to 3. The Lac des Iles ore zones occur within a small (~2 ×y 3.5 km) concentrically zoned mafic intrusion and are approximately 0.5 km wide by 1 km long at surface. There are three major rock types present, gabbronorite, metagabbronorite, and chlorite-actinolite schist.
\nThe Roby and Twilight zones consist of magmatic breccia of gabbronorite or metagabbronorite, which contains pegmatoidal and varitextured patches. The gabbronorites have adcumulate textures and consist of deformed
\nplagioclase and orthopyroxene with minor interstitial clinopyroxene, biotite, and hornblende. In the metagabbronorites the pyroxenes and hornblende have been replaced by actinolite and the plagioclase has been partly altered to sericite. In the most altered metagabbronorites the plagioclase has been replaced by chlorite. The High-grade zone occurs between the breccia of the Roby zone and the homogeneous East Gabbro. The main rock type is actinolite-chlorite ± talc schist. Three sulfide assemblages are present: (1) pyrrhotite, pentlandite, chalcopyrite ± pyrite; (2) pentlandite, chalcopyrite, and pyrite; and (3) chalcopyrite, pyrite, and millerite. Assemblage (1) is present in all rock types and shows equilibrium textures in the fresh rocks. Assemblages (2) and (3) are present only in the metagabbronorite and chlorite-actinolite schist and show disequilibrium textures. Pentlandite is an important host for Pd in assemblages (1) and (2), the other important hosts for Pd in these assemblages are Pd tellurides. In assemblage (3) the Pd is found in a wide variety of platinum-group minerals (PGM); Pd tellurides, Pd sulfides, Pd antinomides, and Pd arsenides. The PGM in assemblages (1) and (2) are found in association with the sulfides, while in assemblage (3) they are found as isolated grains. Whole-rock geochemistry shows that the most of the rocks no matter what their texture or degree of alteration have similar compositions. Most compositions fall on plagioclase-orthopyroxene tie lines. Mantle-normalized patterns show that the rare earth element (REE) and high field strength element (HFSE) concentrations are low (0.8−2 times mantle) and similar. In this flat pattern there are positive Sr, Eu, Pb, and Sc anomalies. These observations are consistent with the rocks being plagioclase-orthopyroxene adcumulates. A small group of metagabbronorite and schist samples show MgO, CaO, and Cr enrichment, indicating the presence of some cumulate olivine, clinopyroxene, and/or chromite. The rocks no matter what their texture or degree
\nof alteration have similar incompatible element ratios, indicating that they all are comagmatic. The low normative clinopyroxene concentrations and the low HFSE content of these rocks suggest that there is very little trapped liquid component present. This observation appears to be in contradiction to the field appearance of the magmatic breccia which indicates the matrix represents a frozen magma. We suggest that the magma chamber was being deformed at the time of intrusion and the fractionated liquid was squeezed out of both the matrix and fragments during this process. The formation of pegmatite and varitextured rocks could have occurred when the magma became fluid saturated and this fluid infiltrated the partially consolidated gabbronorite causing recrystallization. The composition of the varitextured and pegmatiodal rocks is similar to that of the other rocks and thus the fluid did not appreciably change the composition of the recrystallized rocks. Processes that have been considered for forming the ores include: collection by a sulfide liquid from a silicate magma; zone refining of the sulfides during repeated injections of magmas; and collection of the metals by deuteric or hydrothermal fluids. For samples from the Twilight and Roby zones there is a strong correlation between S, Cu, and PGE, indicating that sulfide minerals control the PGE and thus collection by a sulfide liquid could have occurred. However the high Pd/Pt ratio of the ores suggests that the sulfide liquid did not segregate from a primary mafic magma. Possibly, there was a feeder chamber to the Lac des Iles intrusion. The magma in the feeder system became saturated in sulfide liquid and this collected and crystallized in a structural trap between chambers. A fresh injection of S-undersaturated magma from the lower chamber partially melted the sulfides, enriching the magma in S, Cu, Pd, and Au. The Pd-enriched magma was then injected into the Lac des Iles chamber, mixed with the partially consolidated resident magma, and Pd-rich sulfides segregated from it. In the High-grade zone there appears to have been an additionally low temperature process that added Pd, Au, As, and Sb to these rocks. Possibly these elements were added by a fluid that exsolved from the magma in underlying magma chamber and which scavenged the elements from the sulfides formed at depth. The fluid was focussed in the shear zone between the East Gabbro and the Roby zone because most of the Lac des Iles intrusion had solidified at this point.

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