Abstract

A subvertical gold-bearing Quartz-Fuchsite vein at the Dome mine, Timmins area, is located in a highly carbonatized rock at the contact with a unit of slate. The chemical composition (anhydrous basis) of the wall rock of the vein and diagnostic interelement relations indicate that its protolith is a komatiite. Intense carbonatization of the ultramafic rock was accompanied by gains in K 2 O, CaO, Rb, Sr, Ba, Au, As, and Sb, and by loss of H 2 O. Two distinct alteration stages can be recognized in the wall rock: an initial pervasive carbonatization of the komatiitic protolith, and a later, more intense fuchsite-pyrite alteration centered on a quartz-feldspar porphyry lens located in the zone of the carbonatized rock. Apart from shallower levels, no distinct alteration envelope is developed about the Quartz-Fuchsite vein, which actually lies in the least altered komatiite. This suggests that the vein-forming fluid was essentially at thermal and compositional equilibrium with local wall rocks. The Quartz-Fuchsite vein is interpreted as an integral part of a progressive wall-rock alteration sequence generated during regional compressive deformation. Early carbonate replacement reaction resulted in a drastic reduction of permeability in the komatiite. As a consequence, the vertical hydrothermal fluid flow became progressively confined to the less reactive quartz-feldspar porphyries, and a high pore fluid pressure regime was built up in the carbonatized komatiite, creating favorable conditions for hydraulic fracture and eraplacement of the Quartz-Fuchsite vein near the slate-komatiite contact. This metasomatically and structurally induced conduit provided a favorable drain for pumping fluids from depth and from adjacent wall rocks. Gold introduction is genetically related to the late fuchsite-pyrite alteration event.

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