The geology, petrology, geochemistry, sulphide and platinum-group mineralization of the Quetico intrusions, northwestern Ontario

Abstract

The Quetico Intrusions are a group of small, geochemically primitive, locally Cu-Ni-PGE-rich, mafic to ultramafic intrusions closely associated with the margins of the Quetico Batholithic Complex. They are composed of distinctively textured, sometimes layered, hornblende-rich, cumulate rocks that range from hornblende wehrlite to quartz diorite. The ultramafic rocks characteristically contain numerous large, interstitial, poikilitic hornblendes up to 3 cm in diameter. The mafic lithologies are often coarsely hornblende-phyric. All rock types contain texturally distinct intermediate to mafic patches and veins that exhibit large, ragged, cored, prismatic to acicular hornblendes referred to as appinites. This study examined fifteen of the Quetico Intrusions, concentrating on those containing mineralized zones. Three intrusions were mapped in detail. Petrographic examination suggests a crystallization order of: olivine, olivine + clinopyroxene, clinopyroxene (locally orthopyroxene), clinopyroxene + hornblende, hornblende, hornblende + plagioclase. Biotite, apatite and titanite are present within most rock types. Alteration and the cumulus nature of the rocks precludes the use of whole rock data to show definitive fractionation trends. Instead, emphasis was placed on the quantitative mineral chemistry of olivine and clinopyroxene. Olivine is rich in MgO and exhibits a narrow forsterite range of Fo77.55-80.89 Clinopyroxene is generally diopsidic with a compositional range of Wo44.60-50.53' En39.03-49.02' Fs5.35- 11.31. Mineral chemistry data, plus the presence of myrmekitic intergrowths of magnetite with clinopyroxene, suggests that the Quetico clinopyroxenes crystallized from a magma of sub-alkaline, possibly calc-alkaline, affinity. Nine intrusions host mineralized zones containing copper, nickel and platinum-group element mineralization. This mineralization consists of chalcopyrite, pyrrhotite, pentlandite, and magnetite with accessory pyrite and nickeliferous cobaltite, minor cubanite, altaite, empressite, breithauptite, and rare bismuthinite. Violarite and covellite occur as minor alteration products of pentlandite and chalcopyrite, respectively. The major sulphide and accessory phases probably crystallized from a PGM-bearing Ni- and Cu-rich monosulphide phase trapped interstitially to surrounding silicates. The presently observed textures are mainly due to subsolidus reequilibration and to the formation of lower temperature phases as the intrusions slowly cooled. The platinum-group minerals (PGM) were identified using semi-quantitative EDS techniques and are generally less than 5µm in diameter. They include michenerite, hollingworthite, froodite, sobolevskite, and sperrylite. Complex PGM intergrowths are common. Palladian gold is the only PGE­ bearing alloy observed. A few PGM are interstitial to, or contained within, silicate minerals, suggesting some form of late-stage remobilization. A comparison of the Quetico lithologic, textural, petrographic, whole rock, and mineral chemistry data with that of the appinite suite and the Alaskan-type Complexes suggests that the Quetico Intrusions are Archean equivalents of the Caledonian-age appinite suite of Great Britain, Ireland, and elsewhere. They may have crystallized from a water-rich sub-alkaline olivine basalt liquid, possibly of calc-alkaline affinity, at a relatively high PHW and P . They must have crystallized under a relatively high PHW in order for hornblende to remain stable over such a wide range of magmatic conditions. Assimilation of metasedimentary wall-rock may have resulted in the hybridization of some of the magma.