Geochemistry of Camp Lake Zone, Lac des Iles Complex, Thunder Bay, N. Ontario

Abstract

The Archean Lac des Iles suite is located just north of the Wabigoon-Quetico boundary, approximately 90 kilometers north of Thunder Bay in Northwestern Ontario. The suite includes discrete mafic and ultramafic complexes associated with sanukitoids, which were emplaced along deep-seated regional faults. Of these, only the Lac des Iles Complex hosts an economically significant palladium deposit at the Lac des Iles mine. The complex is divided into two parts: North Lac des Iles and South Lac des Iles. The North Lac des Iles mainly comprises ultramafic rocks such as websterite, clinopyroxenite, wehrlite, lherzolite, dunite, and peridotite. In contrast, the South Lac des Iles Complex is primarily composed of mafic rocks including gabbro, gabbronorite, norites, and melanorite and is the main host of the Roby, Offset, and Camp Lake zones which are the main economically important mineralized zones. This study focused on the Camp Lake zone, the deepest part of the deposit, recently identified by exploration drilling. The host rocks of the Camp Lake mineralization primarily consist of gabbronorite and norite, exhibiting preserved cumulate textures with plagioclase and pyroxenes as the main primary phases. The pyroxenes predominantly comprise orthopyroxene with minor clinopyroxene, which are partially to completely replaced by amphiboles (chlorite, actinolite, and tremolite). The plagioclase is weakly to moderately altered by sericite and generally retains its original habit. Magmatic sulfide contents in Camp Lake rocks have modal abundances from 0.5% to 3%, and are dominated by pyrrhotite, pentlandite, and chalcopyrite, with minor pyrite. Sulfide minerals often occur as blebs or disseminated grains intergrown with silicate minerals. Palladium enrichment is interpreted as magmatic, driven by sulfide saturation and high metal-silicate partitioning efficiency, with platinum group minerals (PGMs) hosted primarily in pentlandite and chalcopyrite. A new zircon U-Pb age was acquired for the mineralized Camp Lake rocks, yielding an emplacement age of 2690.56 ± 0.80 Ma, similar to that of the Roby and Offset deposits. Geochemical signatures, including depleted incompatible elements and positive εNd values, support a mantle-derived origin with minor crustal input. The Camp Lake Zone rocks are enriched in LREE (La/Smn ranging from 1.29 to 7.75, with a median of 2.90), have unfractionated HREE (Gd/Ybn ranging from 0.56 to 1.49, with a median of 0.82), and negative Nb anomalies. These values are similar to those of the Roby and Offset zones and are consistent with a subduction zone setting. Similar to the Roby-Offset deposits, PGE values in Camp Lake range between 1.0 g/t and 3.0 g/t, increasing with Cu and Ni content. However, Camp Lake is distinguished by higher proportions of pyrrhotite compared to chalcopyrite and lower Pd/Pt and Cu/Pd ratios than the other zones. The Camp Lake Zone has low δ34S values, ranging from (-1.1‰ to +0.3‰), whereas the Roby and Offset zones show wider variations ranging from (-0.37 to +3.28‰). This suggests that the sulfur in the Camp Lake Zone is of mantle origin. Sm–Nd isotopic compositions (εNd = +0.30 to +1.12) for the Camp Lake Zone is also slightly more positive than the Roby and Offset zones (εNd = -4.1 to 0.384). Overall, when compared with the more contaminated Roby-Offset zones, Camp Lake represents a transitional domain with greater geochemical uniformity, suggesting a continuum of magmatic processes and metallogenic evolution within the Lac des Iles Intrusive Complex.