FAULT ARCHITECTURE, FLUID-FLOW, AND SILICA NANO-PRECIPITATES ASSOCIATED WITH MINERALIZATION ZONES, KIGGAVIK REGION, NUNAVUT

The Kiggavik region is a major uranium district within the Thelon Basin of central Nunavut, Canada. Basement units consist of Neoarchean metagreywackes and extrusive volcanics (Woodburn Gp.) overlain by Paleoproterozoic strata comprising a characteristic basal orthoquartzite (Ketyet River Gp.) intruded by the 1.83 Ga Lone Gull granite. Both Archean and Paleoproterozoic units exhibit a subhorizontal regional fabric resulting from thrusting, ductile deformation, and transposition on which the Thelon basin is imposed. Crustal-scale dextral transcurrent faults (e.g. Thelon and Judge Sissons) were active from at least 1.75-1.5 Ga with periodic reactivation throughout mineralization episodes. Multiple fluid events, commonly seen as quartz veining and most notably as intense silicification and/or hematization of host rock, are demonstrated by mesoscopic structures within the major fault zone. U-mineralization occurs along brittle cross-structures associated with intense, sequential alteration. Two stages of uraninite have been recognized with alteration to coffinite; each is associated with formation of distinctive illite at approximately 225°C and 160°C and have respective formation ages of 1120 Ma and 1040 Ma. These thermometric and geochronology data support the longevity of fault activity and constrains the deformation environment of the faults. Silicification and hematization of the large fault zones precedes the first stage of mineralization and is concomitant with brecciation and microcataclasis of host units with repeated cannibalization of fault clasts. The end product is a micrometer or smaller grain size aggregate of quartz with nanometer-scale hematite distributed throughout silica zones. Absence of U-mineralization within large movement zones argues that these faults have the primary role of establishing zones of low-permeability. The latter faults in combination with subhorizontal, low-permeability zones formed by quartzite layers and/or shallow silica breccias generate three-dimensional fluid-flow ‘cells’ within the basement. In this architecture, altered cross-faults act as fluid conduits while the silicified faults limit the spatial migration of fluids, having the overall effect of constraining and focusing fluid re-circulation.