Repetitive hydraulic fracturing and shear zone inflation in an Archean granitoid-hosted, ribbon banded, Au-quartz vein system, Renabie area, Ontario, Canada

Abstract

Economically significant tonalite (∼2722 Ma)- and trondhjemite (∼2728 Ma)-hosted Archean Au-quartz vein mineralization in the Renabie area, Ontario, located in a marginal part of the 10–15 km thick Wawa Domal Gneiss Terrane, shows strong structural control. Major ore zones occur in ∼E/W and ∼NW–NNW striking ductile–brittle high strain zones that are superimposed on an earlier regional foliation. Mineralized zones are up to ∼20–30 m wide, mineable to a depth of ∼1300 vertical metres, and dip typically at 60°–80° angles. Production and reserves for the Renabie mine totalled ∼6 million tonnes at 6.6 g/t Au and 2 g/t Ag. Net shear, from kinematic indicators, is oblique reverse; south side-up, sinistral on ∼E/W structures (e.g., Nudulama; Renabie main zone, depth >2475 level) and west side-up, sinistral on ∼NW–NNW structures (e.g., Braminco #21 zone). The strain features of the shear zone-hosted mineralization, together with evidence of reaction-enhanced ductility associated with hydrothermal fluid/wallrock interaction, indicate that mineralization and hydrothermal alteration occurred synchronously with shear deformation. The characteristic `banded' appearance (book/ribbon structure) of the Renabie veins, which is typical of many Archean and Cordilleran Au-quartz vein systems, is interpreted to be the result of inflationary vein formation by repeated increments of hydraulic fracturing and mineral deposition (∼250–750) associated with hydrothermal fluid overpressuring within and below the actively deforming ductile–brittle shear zones (crack–seal type process). Fracture orientation was largely controlled by the anisotropy in rock tensile strength associated with the approximately planar shear fabric. The element/mineral association of Renabie is typical of oxidized Archean Au-quartz vein systems containing anhydrite, hematite, carbonate and tourmaline with a chalcophile association consisting of MoS 2, Cu/Pb sulphides, native Au, Ag and Pb–Bi–Au–Ag tellurides. Key factors in the development of significant mineralization at Renabie, as compared with other usually small, low grade, Archean granitoid-hosted Au-quartz vein systems, include the repetitive nature of the mineralizing process, and the focusing of Au-bearing fluids within the actively deforming shear zones. A specific and sufficiently large reservoir of overpressured hydrothermal Archean Au-bearing fluid was generated at some location, not yet identified, in the Archean crustal structure represented by the Kapuskasing section.