Igneous architecture and implications for diverse Cu-PGE mineralization styles in a conduit system: an example from the Area 41 Cu-PGE occurrence, Coldwell Complex, Canada

Abstract

The Area 41 copper-platinum-group element (PGE) occurrence was discovered in 2006 at the northwestern margin of the Coldwell Complex. The discovery was significant because of the sizable mineralized intersections, including 1.4 ppm Pd + Pt and 0.32% Cu over 43 m. Since then, exploration along the northern margin has resulted in the discovery of several other Cu-PGE zones with similar grades and host rocks. Field relationships, drill-core logging, petrography, and lithogeochemistry indicate that the rocks formed from several extrusive and intrusive events related to at least three magmatic series that are from oldest to youngest: Metabasalt, the Layered Series, and the Marathon Series. Three types of Cu-PGE mineralization are hosted within separate intrusions of the Marathon Series. Type 1 is hosted by coarse-grained subophitic gabbro and exhibits high Cu/Pd (> 100,000) and low PGE grades, formed at R-factors of  105) and low S/Se ratios (800–2000) that suggest S loss. Three-dimensional modeling of the occurrence shows a north-south, keel-shaped trough at the western end of mineralization and a systematic spatial variation in the proportions of various Marathon Series units, including pegmatitic subophitic gabbro, breccia units, apatitic clinopyroxenite, and oxide melatroctolite, that correlate with decreasing thickness of mineralization, eastward and away from the trough. These observations suggest that multiple intrusions were emplaced along a feeder channel, some of which flowed laterally outwards to form sills. Taken together, these characteristics are consistent with a conduit model for deposit formation and are thus important vectors for exploration.