Abstract

The Lac des Iles Complex is a late Archean, mafic to ultramafic complex that is host to 20.4 × 106 t Pt, Pd, and Au mineralization with an average grade of 6.34 g/t platinum-group elements (PGE). The 30 km2 complex, located in the Wabigoon Subprovince, consists of several coalescing mafic to ultramafic intrusive centers. The complex is emplaced into early gneissic tonalite and is contemporaneous with late granitoids.The magmatic evolution of the complex reflects the emplacement of multiple pulses of previously fractionated magma, some of which underwent subsequent in situ fractionation. Distinct magma sequences are recognized on the basis of intrusive relationships, differences in the cumulus minerals, crystallization order, and trace-element chemistry. These sequences are (1) hornblende gabbro, (2) gabbro and gabbronorite, and (3) ultramafic. Where there is evidence of intrusive relations, the most primitive sequences are emplaced late and occur toward the north end of the complex. Microprobe analyses indicate the following compositional ranges of mineral phases within the complex: olivine–Fo84–76, orthopyroxene—En82–67Fs15–30Wo2–3, and clinopyroxene—En53–46Fs3–12Wo41–51. Cyclic layering is developed only in the northern ultramafic part of the complex where several cycles of mesocumulate- to adcumulate-textured wehrlite, clinopyroxenite, and websterite are identified.Although the parental magma compositions are not well constrained, model calculations using major-element and rare-earth-element (REE) contents of cumulus phases and adcumulate rocks indicate that the parental magmas had mol MgO/(MgO + FeO) ranging from at least 0.61 to 0.54 and had fractionated REE abundances with CeN/YbN of approximately 5.0. Mineral chemistry and crystallization sequences indicate that the ultramafic and gabbroic parts of the complex had tholeiitic basalt parental magmas of picritic and high-alumina affinities, respectively.The major PGE mineralization at Lac des Iles occurs in the Roby Zone along the interface between the gabbro and gabbronorite in the southern part of the complex. This interface has been intruded by a sheet of pyroxene cumulate, which is probably part of the ultramafic sequence. The PGE sulphide mineralization in the Roby Zone is associated with disseminated chalcopyrite, pyrrhotite, pentlandite, pyrite, and altered silicates in (1) the pyroxene cumulate sheet, (2) the mixed gabbro–gabbronorite, and (3) the gabbroic pegmatite dikes and breccia zones. Cl-rich apatite and monazite are accessory phases associated with PGE mineralization PGE occurrences are also present within sulphide-bearing orthopyroxene–clinopyroxene cumulates in the northern ultramafic part of the complex. The Roby Zone mineralization is localized by mixing the PGE and sulphide-rich gabbronorite and pyroxene cumulates with the volatile-rich gabbro. The PGE are redistributed by late magmatic volatile activity, which generated mineralized gabbroic pegmatites and breccia zones.

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