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 (Fo44-56), 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 (Di41Hd54Ae5-Di29Hd66Ae5), plagioclase (An46-54), altered olivine, and skeletal magnetite. Some gabbros contain high-alumina clinopyroxenes (Ti-Px4CATS48Ae48-Ti-Px3CATS42Ae50). Mineral chemistry and whole-rock geochemistry indicate that the troctolite and gabbro are note 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 equilibrium temperature of 603°± 35° and oxygen fugacity of 10-17 bar.
Disseminated chalcopyrite is the dominant style of Cu-sulphide mineralization in the GLI. Massive chalcopyrite aggregates are rare. Bornite exsolved from chalcopyrite. The other sulphides present are pyrite, millerite, siegenite with exsolved pentlandite, galena, and supergene chalcocite. The tellurides and PGM occur as small inclusions (5 – 10 mm) in, and as coarser sunhedral to anhedral grains on the margins of, the disseminated chalcopyrite. Small amounts of tellurides and PGM are also present in the massive chalcopyrite and silicate minerals. The tellurides are melonite, hessite, unnamed Ag3Te2, and altaite. The PGM are kotulskite, merenskyite, michenerite, sopchiete, palladium bismuthotelluride, paolovite, palladium arsenide, guanglinite, and palladium antimonide. Sperrylite is the only platinum mineral present in the GLI. Electrum forms small discrete and replacement minerals. Two probable new minerals are Pd1.6As1.5Ni and AgSb4. Textural evidence from the sulphides, tellurides, PGM, and host rock silicates favour a late stage magmatic origin. Cu-sulphide mineralization was induced by precipitation of magnetite and other Fe-bearing minerals, and subsequent mineralization resulted from cycles of sulphide and magnetite deposition. Conditions of deposition for the tellurides and PGM associated with the disseminated chalcopyrite have been estimated to be approximately from 550°C to between 450° and 400°C, and at log P(Te2) from -2.4 ± 0.13 bar to between -6.4 and -9.7 bar, and at log P(S2) from between -3.7 and -0.8 to between -4.2 and -2.3 bar. The predominance of palladium over platinum minerals reflects the relatively evolved nature of the GLI.
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