Petrology, geochemistry, sulphide and platinum-group element mineralization of the Geordie Lake intrusion, Coldwell Complex, Ontario / by Thomas Mulja

Abstract

PGE-bearing Geordie Lake intrusion is located in the north-central region of the Coldwell comples near the town of Marathon.

The Geordie Lake Intrusion (GLI) consists of alternating zones of layering-free troctolite and olivine gabbro outcropping in the north-central part of the Coldwell alkaline complex, northwestern Ontario. The troctolite exhibits harrisitic texture in which dendritic olivine (F044-56), plagioclase (An48-57), and skeletal magnetite are the main constituents. Small amounts of clinopyroxene (Di36Hd60Ae4-Di30Hd66Ae4) are present in the troctolite. The ophitic olivine gabbro consists of clinopyroxene (Di4iHd54Ae5 Di29Hd66Ae5), plagioclase (An46-54), altered olivine, and skeletal magnetite. Some gabbros contain high-alumina clinopyroxenes (Ti-Px4CATS48Ae48-Ti-PxgCATS42Ae5o). Mineral chemistry and whole-rock geochemistry indicate that the troctolite and gabbro are not related by differentiation and their parent magma is a relatively evolved low-alumina tholeiite. The GLI is characterized by high Sr, Rb, Ba, Th, Ta and light rare-earth element (LREE) content but low in Ni and Cr content. Europium anomalies are absent. Fe-Ti oxide geothermometry and geobarometry of the troctolite and olivine gabbro give an average equilibration temperature of 603° ± 35°C and oxygen fugacity of bar. Disseminated chalcopyrite is the dominant style of Cu-sulphide mineralization in the GLI. Massive chalcopyrite aggregates are rare.