Permeability Testing of Drill Core from Basement Rocks in the Fault-Hosted Gryphon U Deposit (Eastern Athabasca Basin, Canada): Insights into Fluid–Rock Interactions Related to Deposit Formation and Redistribution

Abstract

Fluid percolation conditions in fault zones are often inferred in geologic studies, but the porosity and permeability of rocks are rarely tested during mineral exploration drilling. Routine permeability and porosity tests on drill core, with adequate detection limits and accuracy, can provide new insights into present- and paleo-fluid flow processes and deposit formation in sedimentary basins and permeable fault zones (e.g. uranium deposits, sub-sea hydrothermal ore deposits). In this study, geological and hydrogeological observations were combined with results of over a thousand permeability tests at spot-on unconfined drill core, extracted along transects through the Gryphon U deposit and fault zone, using a pressure-decay N2 gas probe with adequate seal to the rock surface. The new seal method allowed a wide range of permeability determinations under field conditions (over 5 × 10–20 to 10–12 m2). The penetrative macropore networks were observed directly from gas discharge patterns on drill core surfaces. For the tested crystalline basement rocks below the Athabasca Basin, we inferred that the matrix permeability and porosity distribution appear to be preserved since the late Paleoproterozoic/Mesoproterozoic, under the cover of a sedimentary basin. The permeability values at the present time, and the patterns of rock alteration and deformation, offer insights to paleo-hydrogeological conditions. The gneissic and pegmatitic host rocks have relatively low permeability at present time, except fractured (discrete flow channels) and/or intensely altered to a more porous rock. The alternating zones of silicification and desilicification appear to pre-date the intense argillic alteration, and it may have been a result of earlier hydrothermal activity. The Gryphon U deposit occurs in elongated narrow “lenses” along several fault strands of a fault zone (below the unconformity surface and within basement rocks). U-ore is a cementing mineral in fault-related fractures and fault rocks, but remnant flow channels in fractures in U-ore are still preserved. The most permeable and porous fault rocks are coarse gouges (~ 40% porosity, ~ 10–15 m2 permeability) with approximately three orders of magnitude higher rock matrix permeability than the gneissic host rocks. Such fault rocks locally contain carbonaceous matter, and similar fault rocks may have been preferentially mineralized by U to form the U ore lenses, but the role of brittle-ductile and ductile fault and shear zone rocks in U mineralization is not yet clear at Gryphon. Our review of U–Pb uraninite ages at Gryphon suggests a protracted faulting and U-mineralization history. In response to numerous episodic events of brittle deformation and hydrothermal alteration, the permeability and porosity of the host rocks and U-ores have evolved, as observed by multiple cross-cutting reaction fronts. We inferred from the test results and the alteration halo at the U deposit, affecting a wide part of the fault zone, that the permeability and porosity increased by the downward ingress of U-rich acidic basinal brine that percolated into the basement-hosted (rooted) shear/fault zone and nearby crystalline basement rocks. However, the younger prominent U roll/redox-fronts caused local-scale reduction in porosity and permeability by hematite cementation.