Abstract

Located in the Matagami mining district of the Abitibi greenstone belt, Quebec, the Bell River Complex is a >5-km-thick tholeiitic layered gabbro and/or anorthosite body (2724.6 +2.5/−1.9 MaU-Pb), which likely acted as the heat source that drove hydrothermal convection and volcanogenic massive sulfide (VMS) hydrothermal mineralization in the area.

Abundant fractures and veins crosscutting the western lobe of the Bell River Complex formed over a range of temperatures from 250° to 700°C. The 250° to 400°C assemblage, quartz-epidote ± sericite ± chlorite ± plagioclase, is the most widespread and occurs as orthogonal, anastomosing, and random vein sets. Vein densities average between 15 and 25 veins per m2, locally reaching as high as 40 to 60 veins per m2. These veins, typically 1 to 3 mm wide, are interpreted to represent thermal cracking associated with hydrothermal fluid mineralization in the district. Furthermore, they crosscut earlier higher temperature pyroxene-plagioclase (>600°C) and magnetite-rich (300°–600°C) veins (1–10 mm wide; densities commonly ~0–5 veins per m2).

Detailed field measurements of quartz-epidote vein geometries coupled with permeability tensor theory have produced a first-order approximation of the maximum model permeability structure (veins unfilled) of the hydrothermal cracking zone. Representative district-wide values indicate maximum model bulk permeabilities of 10−10 to 10−8 m2 for the hydrothermal cracking zone; similar to permeabilities calculated for the fractured sheeted dike complexes of the Semail and Troodos Ophiolites. However, a high-flow zone located within the central parts of the hydrothermal cracking zone is characterized by a maximum model permeability of 10−7 m2. Locally (<1 m2), the high-flow zone reaches maximum model permeability values as high as 10−6 to 10−5 m2 where two or more veins occur with apertures in excess of 2 cm.

Mapping has shown that the hydrothermal cracking zone is confined to an ~350-m-thick interval located within a strongly layered gabbro and/or anorthosite horizon of the Layered zone, upper Bell River Complex. The base of this interval is 1,000 m below the top of the Bell River Complex. Furthermore, in and around the town of Matagami, the dominant orientation of quartz-epidote veins parallels the orientation of the layering (110° ± 15°). This parallelism is further reflected in the orientation of the calculated quartz-epidote vein permeability tensors (94° ± 30°, 1σ, n = 71). The parallelism suggests that heterogeneities associated with the layer contacts provided planes of low tensile strength along which the hydrothermal cracks preferentially developed. Such an interpretation further explains why the hydrothermal cracking zone appears to be restricted to the strongly layered zone; there are many more low tensile strength planes.

Zones of quartz-epidote veining approximately orthogonal to layering are also present. The orthogonal veins are usually less continuous and commonly truncated (but not crosscut) by layer-parallel veins. The orthogonal subset of veins is interpreted to represent short pathways that allowed fluids to travel between adjacent layer-parallel veins. However, locally, longer layering-orthogonal quartz-epidote veins, showing offsets of 10 to 30 cm normal to the layering of the Bell River Complex, may represent initial conduits that allowed fluids to travel into overlying stratigraphy and through to the paleosea floor.

Although volumetrically small in comparison with the permeability structure of the bulk sea floor, the significantly higher permeability of the hydrothermal cracking zone indicates its importance in controlling the flow paths and fluxes of fluids deep within the hydrothermal system.

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