Abstract

A late calc-silicate alteration characterizes the Archean Hemlo gold deposit in Ontario, Canada. Calc-silicate minerals, including epidote-clinozoisite, tremolite-actinolite, titanite, prehnite, pumpellyite, allanite, grossular garnet, antimonian vesuvianite, and armenite, occur as mono- or bi-mineralic veins or aggregates but are volumetrically minor in the main orebody (Golden Giant mine). Aggregates of various combinations of tremolite-actinolite, clinopyroxene, phlogopite, talc, chlorite, calcite, dolomite, microcline, tourmaline, and quartz are abundant in the diffuse westward extension (C zone of the Williams mine and North zone of the Golden Sceptre Resources Ltd. property). Mineral equilibria, calcite-dolomite geothermometry, and fluid inclusion study indicate that the late calc-silicate alteration occurred at about 200 degrees to 400 degrees C and 1 to 2 kbars in an aqueous fluid of moderate to high salinity. X (sub co 2 ) in the hydrothermal fluid increased from <0.02 in the main orebody to 0.1 in the diffuse westward extension. Compositions of phlogopite yield log (f HF /f (sub H 2 O) ) from -4.97 to -6.08 for the coexisting hydrothermal fluid at 350 degrees C. The calc-silicate alteration most likely occurred at about 2637 to 2644 Ma, at least 30 m.y. later than the peak metamorphism of the region. Homogenization temperatures of fluid inclusions in both quartz and carbonates range from 200 degrees to 400 degrees C and exhibit a good, almost linear correlation with salinity (5.2-37 wt % NaCl equiv), which is consistent with mixing of a hot, saline magmatic fluid with a cool, dilute fluid within a dilatant regional planar structure (the Hemlo shear zone). A direct association of native gold grains with calc-silicate minerals is observed in all parts of the Hemlo deposit. The late calc-silicate alteration must have been responsible for at least some of the economic mineralization in the Hemlo deposit.

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