Petrogenesis and Metallogenesis of the Ultramafic Layered North Lac Des Iles Intrusion in the Lac Des Iles Complex, Ontario, Canada
Ngue Djon, Moise
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A detailed petrologic and metallogenic study was conducted on the eastern flank of the Northern Ultramafic Centre (NUC) of the North Lac des Iles Complex (North LDIC) to understand the petrogenesis of cyclically layered ultramafic cumulates and the processes involved in the formation of the PGE mineralization. Additional investigation of historical PGE-Cu-Ni prospects in the North LDIC provides insights into the styles of mineralization and how these styles compare with the stratiform PGE mineralization of the NUC and the Pd deposits hosted by the mafic Mine Block Intrusion of the South Lac des Iles Complex (South LDIC). Field relationships, mineralogical, textural, mineral chemical, and lithochemical data from the NUC reveal the presence of two distinct paragenetic sequences of cumulate rocks. Cyclic Unit A (CUA) is characterized by an upward progression of Ol+Cr-SpOl+Opx+Cpx±Cr-SpCpx+Opx Pl+Opx±Cpx. Cyclic Unit B (CUB) exhibits a cumulate stratigraphic progression of Ol+Cr-SpOl+Cpx. CUA sequences are interpreted to have formed by fractional crystallization of more siliceous ultramafic magmas relative to CUB parent magmas. Stratiform PGE mineralization of the Sutcliffe Zone is typically hosted within four different CUA units. PGE mineralization is commonly associated with >2% disseminated Cu-Ni sulfide, and is characterized by mantle-like S/Se ratios, Pd tenors in sulfide <1000 ppm, and Pd/Pt ratios < 5. The Sutcliffe Zone is interpreted to have formed by a combination of fractional crystallization and dynamic recharge and mixing of CUA and CUB parental magmas. The majority of PGE-Cu-Ni prospects in the North LDIC share similar host rocks, sulfide and platinum group mineral compositions and textures, trace element signatures, Pd tenors, S/Se and Pd/Pt ratios as the stratiform PGE mineralization at Sutcliffe Zone, similar genesis. Similarities between stratiform PGE-Cu-Ni mineralization in the North LDIC and the magmatic breccia-style Pd mineralization in the South LDIC include the prevalence of orthopyroxene, widespread magmatic alteration, low sulfide content, Pd-rich PGE mineralization and the dominance of Pd-bismuthotellurides. Despite these similarities, the two styles of mineralization remain distinct in terms of structural controls on mineralization, host rocks, base metal sulfide and PGE mineralogy, PGE tenors of sulfide, and the source of parental magmas.