The Macassa Mine Archean lode gold deposit, Kirkland Lake, Ontario; geology, patterns of alteration, and hydrothermal regimes

The only remaining gold producer of seven interconnected mines. The district has produced over 710,000 kg Au since 1913. The area is underlain by part of the Timiskaming Group. Three types of gold ore: (1) native Au in chloritic fault gouge or small quartz lenses, within a thrust-fault system, called break ore; (2) gold-bearing quartz veins in both hanging and footwalls of this fault system, termed vein ore; (3) several relatively wide zones of intensely fractured, bleached, silicified, and pyritized rock containing lenses and pods of quartz with native Au and telluride minerals, called breccia ore. The delta (super 18) O and delta D of rocks and mineral separates in these three ore types suggest that the ore was precipitated from hydrothermal fluids of delta (super 18) O = 7 to 9.6 per mil and delta D = -35 to -85 per mil, at 350 degrees to 460 degrees C. Initial hydrothermal regime was followed by downward penetration of oxidizing, sulfate-bearing fluids, delta (super 18) O = 0 to 2 per mil, delta D = -20 to -70 per mil and of probable marine and/or meteoric water origin, initially at temperatures of <200 degrees C, waning to 50 degrees C or less. A third fluid regime is indicated for quartz-magnetite-chlorite veins which have mineral pair fractionations corresponding to fluids of delta (super 18) O = -4.0 to -0.5 per mil and 210 degrees to 260 degrees C. These three hydrothermal fluid regimes are interpreted to reflect a sequence of crustal compression, relaxation, and finally uplift above sea level, probably all during the Archean. Incremental (super 39) Ar/ (super 40) Ar upper plateau age of 2,575 m.y. for vein actinolite. The deposit is interpreted to have formed during a period of major fracturing, ductile deformation, and thermal perturbation accompanying the episode of batholith emplacement.--Modified journal abstract.