Abstract

The Au-Ag-Cu Brosman prospect is located in the Line Lake area on the northern limb of the Chibougamau anticline, in the northeast part of the Archean Abitibi greenstone belt. This prospect comprises a broad N-S, subvertical mineralized envelope formed in a synvolcanic fault-related environment. This envelope is at least 300 m long, up to 30 m wide, and is known to a depth of 950 m, where it remains open. It exhibits two contrasting mineralized zones that are representative of the varying styles of mineralization in the area. The Auriferous zone consists of both ubiquitous pyrite disseminations and Au-bearing silicate-sulfide veins in strongly fractured intrusive, effusive, and volcaniclastic rocks showing propylitic and chloritic alteration assemblages. The Polymetallic zone, on the other hand, is characterized by discrete, Cu-Ag-Au–rich magnetite-sulfide veins and lesser pyrite disseminations within distal propylitic and proximal potassic to sodic alteration zones. The Polymetallic zone is hosted by a chaotic assemblage of gabbro and tuff bodies interpreted as a talus megabreccia.

Several lines of evidence, including the spatial distribution pattern and nature of the alteration and mineralization assemblages, metal associations, mass-balance calculations, pyrite composition, and crosscutting relationships are indicative of a preregional deformation, synvolcanic origin for the hydrothermal activity. Crosscutting relationships, alteration characteristics as well as whole-rock and LA-ICP-MS pyrite analyses reveal two stages of hydrothermal alteration and mineralization. Stage I consists of both ubiquitous disseminated pyrite, found in the Polymetallic and Auriferous zones, and contemporaneous magnetite-sulfide veins, restricted to the Polymetallic zone. Stage II is restricted to the Auriferous zone and corresponds to Au-bearing silicate-sulfide veins with characteristic vein-halo chloritic alteration overprinting Stage I alteration and mineralization.

This hydrothermal activity is interpreted to have occurred concurrently with the development of the fertile, deeper magmatic-hydrothermal system related to the Chibougamau Pluton. Stage I hydrothermal activity, related to the early build-up of the volcanic sequence, formed a continuum of mineralization from discrete massive magnetite-sulfide veins to ubiquitous pyrite disseminations in a submarine talus megabreccia generated along synvolcanic faults. Afterward, a late Au-rich contribution to the hydrothermal fluids, possibly related to intermediate to felsic magmatism (the 2707.6 ± 1.4 Ma Line Lake stock), produced Stage II silicate-sulfide veins overprinting mineralization and alteration in favorably oriented and sustainably permeable fracture zones of the Auriferous zone.

Overall, this study suggests that the Brosman prospect is a rare example of Archean polymetallic mineralization produced by a shallow subaqueous epithermal-style subsystem developed in a broader, fertile magmatic-hydrothermal arc setting.

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