AbstractsEarth & Environmental Science

Graphite-bearing and graphite-depleted basement rocks in the Dufferin Lake Zone, south-central Athabasca Basin, Saskatchewan

by Marjolaine Pascal

Institution: University of Saskatchewan
Year: 2014
Keywords: Athabasca Basin; graphite; pelitic schists; carbon consumption; carbon precipitation; CH4-N2 fluids; basinal brines
Record ID: 2045458
Full text PDF: http://hdl.handle.net/10388/ETD-2014-07-1628


Unconformity-type uranium deposits from the Athabasca Basin are considered to be the result of mixing between oxidized basinal brines and basement-derived reduced fluids/gases, and/or reduced basement rocks. Graphite and/or its breakdown products are suggested to be responsible for uranium mineralization by acting as a reductant that could trigger deposition of uranium. Also, graphite is considered to be indicative of basement structures; being often concentrated along structures which can be identified as electromagnetic (EM) conductors. Thus, exploration for uranium deposits is often focused on the search for EM conductors. Underlying the sedimentary rocks of the basin in the Dufferin Lake zone are variably graphitic pelitic schists (VGPS); altered to chlorite and hematite (Red/Green Zone: RGZ), and locally bleached equivalents near the unconformity during paleoweathering or later fluid interactions. These altered zones are texturally similar rocks within “graphite-depleted zones” as the unconformity is approached. Both zones are characterized by a lower concentration of carbon and sulfur, with the bleached zone showing higher concentrations of uranium and boron, the latter corresponding to high dravite content. The major element composition of the graphite-bearing pelitic schists and altered equivalents (RGZ) are similar. Raman analyses indicate that well-ordered carbon species (graphite to semi-graphite) are present in the pelitic schists, with both types more common within shear zones. In contrast, only rare low-ordered carbon species (carbonaceous matter) were detected in the graphite-depleted samples within the RGZ. This variation is interpreted to be the result of graphite consumption by oxidizing fluids migrating downward from the Athabasca Group. This graphite consumption may have resulted in the production of a mobile reductant (gas or fluid), which may have played a subsequent role in the deposition of uranium mineralization. Secondary fluid inclusions (FI) examined in different quartz vein generations using microthermometry and Raman analysis, provide an indication of the fluids that have interacted with these rocks. Monophase vapor are the dominant type of fluid inclusions in the VGPS, whereas aqueous two-phase (L+V) and three-phase (L+V+Halite) FI occur in the RGZ. CH4-dominant and N2-dominant FI identified using Raman could be the result of fluid(s) interaction with the graphitic lithologies. This would have generated the breakdown of graphite to CH4 and associated feldspars/micas to NH4/N2. CH4, N2 and H2 (resulting from the decomposition of NH4+) represent possible reductants of uranium-bearing brines. Two brines in the RGZ: a regional basinal fluid and an evolved fluid possibly related to U mineralization; similar to other nearby deposits, are observed. These suggest that the basinal brines have circulated in the basement rocks and have been able to evolve by interaction with the basement rocks to possibly be related to uranium mineralization.