Abstract

The Golden Giant mine is developed in the Archean Hemlo gold-molybdenum deposit hosted within the Hemlo-Heron Bay greenstone belt in northwestern Ontario, Canada. The ore occurs in steeply dipping tabular zones within metasedimentary rocks and within quartz-eye muscovite schists in association with microcline-mica-pyrite alteration, and a variety of other minerals.The Hemlo area has been intruded by quartz-feldspar porphyries (the Golden Giant porphyry and Sceptre porphyry). These porphyries are altered, mineralized, and highly deformed. The calc-alkalic granodioritic Cedar Creek stock and Cedar Lake pluton occur near the deposit. Feldspar porphyry sills and dikes generated from the stock and pluton cut the earlier porphyries, mineralized rocks, and many of the fabric elements. The Golden Giant rocks have been regionally metamorphosed to amphibolite grades. This early, M 1 metamorphisin (T = 500 degrees -600 degrees C, P = 5-6 kbars) probably accompanied the intrusion of the Cedar Lake pluton and produced mineral assemblages in metasedimentary rocks containing hornblende, kyanite, staurolite, garnet, and micas. A second, M 2 metamorphic event (T = 300 degrees - 400 degrees +C, P = 1-3 kbars) produced mineral assemblages containing actinolite, fibrolite, andalusite, calcite, garnet, epidote, chlorite, and muscovite. The Golden Giant mine rocks are strongly foliated, and commonly exhibit mylonitic and transposed fabrics, isoclinal folds, and boudinage in both mineralized and nonmineralized rocks.It is proposed that the primary Au-Mo mineralization occurred before the M 1 metamorphic event, possibly related to emplacement of the Golden Giant quartz porphyry in a premineralization structural zone. The mineralization was modified by the M 1 event, and severely deformed during mylonitization. Dikes from the Cedar Creek stock intruded the deposit sequence and overprinted the M 1 event with an M 2 metamorphism.

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