Jacob Montenegro, BSc
J. Montenegro1, G. Chi1, T. Foley2
1University of Regina, Regina, Saskatchewan, Canada
2Saskatchewan Geological Survey, Regina, Saskatchewan, Canada
A sulphide-bearing mafic–ultramafic unit was identified during 1:20 000-scale bedrock mapping of the Milton Island Assemblage in the Reindeer Lake region of northern Saskatchewan. Originally mapped as a “pyroxenite,” the unit hosts disseminated Fe–Cu sulphides, yet its protolith, metamorphic history, and the timing of mineralization have remained poorly constrained. This study integrates structural mapping, petrography, SEM–BSE imaging and SEM–EDS analyses, and whole-rock geochemical data to clarify the origin and mineralization history of the unit. The study area lies within the Paleoproterozoic Reindeer Zone of the Trans-Hudson Orogen, specifically the Rottenstone Domain, which experienced upper-amphibolite to granulite facies metamorphism during D₂ deformation at ~1.82–1.79 Ga. Two correlated outcrops on Amiskit Island were examined using detailed 1 x 1 m grid mapping and targeted sampling. Petrographic study reveals a peak hornblende – clinopyroxene – orthopyroxene – plagioclase assemblage defining a southwest-striking foliation parallel to the regional S₂ fabric, constraining peak metamorphic conditions to upper-amphibolite to near-granulite facies (~650–730 °C, 0.2–0.4 GPa). Whole-rock geochemical data indicate a gabbro to gabbro–diorite protolith, rather than a true pyroxenite, consistent with observed mineralogy and textures. Variations from strongly foliated to granoblastic, massive domains are attributed to heterogeneity of D₂ strain. Retrograde modification, in addition to late hydrothermal fluid-flow, includes replacement of pyroxene and hornblende with biotite and chlorite, and minor microcline, and variably developed sericitization of feldspar. Localized sulphide enrichment, spatially associated with late, rusted quartzofeldspathic veins, is characterized by disseminated pyrrhotite with chalcopyrite overprints and oxidation rims. The lack of metamorphic textures in sulphides and alignment with the regional S₂ foliation suggests Fe–Cu mineralization post-dates peak D₂ metamorphism and reflects a late hydrothermal event. These results provide new constraints on the evolution of sulphide-bearing mafic intrusions in the Reindeer Lake region. Moreover, one sample from the gabbroic unit yields an anomalously elevated uranium concentration (25,800 ppm), prompting further examination of the processes responsible for this enrichment.