PLATINUM GROUP ELEMENT MINERALIZATION OF THE MESOPROTEROZOIC MIDCONTINENT RIFT, NORTHWESTERN ONTARIO, CANADA

Four 1106–1124 Ma mafic to ultramafic intrusions, and a series of mafic sills, were emplaced into the <1670 Ma Sibley Group and underlying Archean metasedimentary rocks of the southern portion of the Nipigon Embayment, NW Ontario. The intrusions are cut by diabase sills of the 1110-1113 Ma Nipigon Sill Complex. All four intrusions are composed of a pyroxene peridotite core predominately cumulate textured with an irregular olivine gabbro to olivine melagabbro border zone. Present geometry of the intrusions is sill-like covering areas ranging between 20 and 85 km² and 130 to 750 m thick. The Seagull Intrusion is the largest intrusion with a central saucer-shaped portion that may be a product of post-intrusion faulting or a result of emplacement into a pre-existing structure, but an underlying, or peripheral, feeder structure has not yet been identified for this, or any of the other intrusions. The four intrusions are geochemically similar, with relatively uniform REE and HFSE contents comparable to ocean island basalts.

The Seagull Intrusion contains ultramafic hosted PGE mineralization located in discrete zones with PGE contents that vary laterally from anomalous to very PGE enriched (e.g., 3399 ppb Pt, 1198 ppb Pd). PGE, Ni and Cu mineralization is found at the base of the ultramafic section of the intrusion and in a zone 100-150m above the basal contact. The upper zone is continuous over 700m and contains three distinct horizons separated by at least 2m of unmineralised ultramafic rock. The mineralization is interpreted to be a result of sulfur saturation of the magma during initial stages of emplacement (lower horizon) and also as a result of at least two later influxes of less evolved magma (upper horizons).

The similarities in the geochemistry of the mafic sills and the intrusions suggest that the sills may be part of the same magmatic event, and could be indicators of other large, unexposed intrusions in the Nipigon Embayment with a similar potential to host PGE mineralization. The ultramafic intrusions of the Nipigon Embayment may be part of a regional scale early Keweenawan magmatic event that has also resulted in the emplacement of the PGE-bearing Mesoproterozoic Yellow Dog peridotite located about 400 km to the south in northern Michigan.