Geochemical zoning around the McClean uranium deposits, Saskatchewan, Canada

Some 3400 drill core samples of the Athabasca Group and underlying regolith have been obtained by sampling 8-m sections from 55 diamond drill holes in the McClean Lake Area, Saskatchewan. Forty eight holes are from sections over six areas known to contain uranium mineralization and 7 are from unmineralized areas. The uranium zones are covered by 30 to 180 m of Athabasca sandstones and conglomerates.The uranium mineralization of the McClean deposits can be described as belonging to two different facies; a more reduced sulphide-arsenide facies and a more oxidized hematite-“bleached” facies, superimposed on any one of three host rocks.The sulphide-arsenide facies is probably older and, except in one deposit, it forms a patchy distribution of relicts. The principal pattern is an upwards increase in sulphur/arsenic ratio. The hematite-“bleached” facies appears to be younger and may represent a reworking of the earlier sulphide-arsenide facies mineralization under more oxidizing conditions.The trace metals can be grouped according to their redox behaviour. Vanadium, Mo and U, occurring as oxides, form one group while Ni, Co, Zn, Cu and As, occurring as sulphides and/or arsenides, form intermediate and most reduced groups, respectively. Iron is concentrated in the most oxidized facies as hematite or goethite and in the most reduced facies as pyritebravoite, siderite and chlorite. Manganese follows the distribution of siderite.The ratio of oxidized to reduced minerals can be represented by the ratio of U/Ni. This ratio can be used to estimate the variation of redox potential in the deposit at the time of deposition or alteration. The principal pattern is one of systematic increase in U/Ni ratio or oxidation potential both upward and to the northeast in each of the hematite and bleached facies.A generalized Eh-pH diagram is used to qualitatively describe the significance of each mineral facies.The U/Ni ratio of the transition between the hematite and “bleached” facies increases upwards. The phase diagram suggests that a possible cause is an upward decrease in pH and increase in Eh.The overall distribution is compatible with the origin of the deposits due to reduction at the interface between a plume of reductant emanating from fractures in the sub-Athabasca basement and an oxidizing aquifer moving southwestward in the sandstone. This model does not accurately predict the distribution of elements among the seven McClean Lake pods. However, it explains the overall pattern. Local variations in metal ratios in solution either in time or reflecting local provenance could be adduced to explain local deviations from the overall pattern.Clay alteration, apparently largely contemporaneous with the earliest mineralization, is largely illitic but with sporadic occurrences of xenotime and goyazite. Thus K and to some extent P, Y and light REE's are also enriched to some degree in the deposits.Uranium analysis of the drill core shows that there is little movement of U into the overlying sandstones from basement rocks and regolith that contain no uranium deposits. Uranium in the Athabasca sandstone from these areas averages less than 1 ppm. However, where uranium zones have been found in the basement rocks, regolith and lower Athabasca sandstone, U values greater than 2–3 ppm consistently occur in the overlying sandstones at or near surface.There appear to be two types of U anomalies, those that flank the deposits and those that directly overlie them. Both types exhibit vertical and lateral continuity. They are associated with vertical to steeply dipping fracture systems that traverse the Athabasca sandstone from basement to surface. The fractures are thought to be related to diagenetic processes from compaction of the sedimentary rocks over basement irregularities as well as tectonic processes (i.e. re-adjustment along old basement structures).These results suggest that target areas containing deeply buried uranium deposits could be defined by U analyses of the Athabasca sandstone from quite widely spaced holes of limited depth. This conclusion can be compared to XRD and chemical studies of clay mineral distribution within the Athabasca sandstone about the deposits.