Abstract

The Bousquet 2 mine is a deformed pyritic polymetallic Archean gold deposit located within a zone of intense ductile-brittle deformation composed of highly sheared and altered metamorphic tectonites of contrasting rheological properties. The ore occurs in a metamorphosed zone of aluminous alteration and is composed of foliation-parallel massive and semimassive pyritic lenses overprinted by late en echelon extensional veinlets containing Au-Ag-Cu mineralization.Structural and textural analyses show that the gold-bearing sulfide bodies and enclosing host rocks contain discrete tectono-metamorphic features resulting from two principal episodes of metamorphism, M 1 and M 2 . The earlier prograde event, M 1 , and coeval regional deformation, D 1 , were accompanied by overall ductile deformation of sulfides and by recrystallization and remobilization of the mineralization parallel to foliation. M 1 caused development of a pervasive east-west-trending metamorphic layering of the pyritic mineralization, transposition and elongation of the pyritic bodies parallel to mineral stretching L 1 lineations, meter-scale folding of sulfides, sulfide brecciation, coarsening and annealing recrystallization of pyrite, and ductile remobilization of incompetent sulfides and gold into interstices and triple-point junctions of juxtaposed pyrite grains. Extensive differential remobilization of ductile sulfides (mainly sphalerite, pyrrhotite, galena, chalcopyrite, chalcocite, and bornite) and gold took place during M 1 and gave rise to planar zones enriched in Au-Ag-Cu mineralization along the interfaces of pyritic lenses and their adjacent silicate rocks.M 2 metamorphism is chronologically related to a late brittle incremental stage of the progressive deformation and is characterized by high fluid influx, pervasive retrogression of peak metamorphic minerals, and remobilization of sulfides and gold into transgressive structures. The retrograde mineralization displays a wide range of small-scale structures resulting from fluid-assisted remobilization of ductile sulfides, including piercement and injection structures, as well as foliation-oblique extensional veinlets. Metamorphic remobilization of ore constituents took place mainly at ore-wall rock interfaces. Fe-, Pb-Zn-, and Au-Ag-Cu- bearing extensional veinlets generally occur in foliation-parallel protores of similar mineralogical composition and are interpreted as having resulted from in situ metamorphic remobilization.Gold and copper show a corelationship throughout the deposit; the highest gold and copper values occur within extensional veins overprinting previously Au-Cu-enriched planar zones. Mineralogical zoning in the sulfide masses is structurally controlled and can be related directly to abundant Au-Cu-bearing veinlets that represent mineralized segments of a systematic joint system.Structural and textural evidence suggests that gold mineralization with its present textural relations formed during M 1 and M 2 , but that a large proportion of the structurally controlled vein mineralization resulted from in situ remobilization rather than from additions from an external source.

First Page Preview

First page PDF preview
You do not currently have access to this article.