Abstract

Three prominent mineralized calc-silicate occurrences (Cr-rich skarn, Fe-rich skarn, and REE-enriched calc-silicates) are present in close association with regional planar structures at the White River property of LAC Minerals Ltd., in the Archean Hemlo-Heron Bay greenstone belt of the Superior province, Ontario, Canada. The Cr- and Fe-rich skarns were metasomatically derived, respectively, from ultramafic-mafic host rocks at 400 degrees + or - 50 degrees C, 3 kbars, and an aqueous fluid (X (sub CO 2 ) <0.03), and mafic host rocks at 500 degrees to 560 degrees C, 4 kbars, and an aqueous fluid (X (sub CO 2 ) < 0.05). The REE-enriched calc-silicates are characterized by abundant epidote-prehnite veins and minor REE-rich minerals and were formed at 320 degrees C and 1.7 kbars during infiltration of an aqueous fluid of moderate salinity. These three calc-silicate occurrences collectively indicate a protracted episode of skarn development which commenced immediately after the peak of the regional thermal metamorphism.Metallogenesis correlates strongly with the respective host lithologies and is characterized by Fe-Ni-Co-Zn-Cu-Mo sulfides in the Cr-rich skarn, similar to the assemblage of Outokumpu-type ores, and Fe-Zn-Cu-Ni-Co sulfides in the Fe-rich skarn, resembling sulfide ores in calcic iron skarns. Textural evidence indicates that most of the polymetallic sulfides crystallized during the earlier stages of the skarn development, but Mo (as molybdenite) in the Cr-rich skarn was most likely introduced during the late calc-silicate alteration.Gold mineralization is spatially related to the REE-enriched calc-silicates and was most likely emplaced during the late calc-silicate alteration. It differs from most Phanerozoic gold skarn deposits by its geologic setting in a brittle-ductile shear zone and association with carbonate-free host rocks, and from Arcbean mesothermal lode gold deposits by a lack of prominent quartz veins and involvement of a hydrothermal fluid of moderate salinity. Although the brittle-ductile shear zone almost certainly provided necessary permeability for the fluid responsible for gold mineralization and the REE-enriched calc-silicates, the localization of gold deposition was probably controlled largely by a preexisting pyritic zone.

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