Iron disulfide minerals occur in anomalously high concentrations in uranium-bearing zones of roll-type uranium deposits and are thus important components of the geochemical system that governs formation of these uranium orebodies. These high concentrations have been interpreted by previous workers as resulting from an ore-stage generation of sulfide superimposed upon a pre-ore generation. We have examined petrographically over 500 polished sections from five roll-type deposits, three from Texas and two in Wyoming, and have recognized both pre-ore and ore-stage generations of iron disulfides in each deposit. The two generations may be distinguished by textural relations. In each deposit, marcasite is present as an ore-stage sulfide; in

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two Texas deposits (one in Webb County, the other in Live Oak County), it is the dominant ore-stage sulfide. This ore-stage marcasite occurs as intergrowths within and overgrowths on uranium-bearing phases and in close association with ferroselite near the redox boundary. Ore-stage marcasite occurs commonly as overgrowths on pre-ore sulfides that are dominantly pyrite both in ore and in a more extensive halo around the altered tongue. In two of the Texas deposits, ore-stage marcasite is present as much as 200 m (Webb County) and 400 m (Live Oak County) downdip from the roll front. Because of the close association of marcasite and uranium mineralization, understanding the conditions that lead to marcasite precipitation enables clearer determination of the geochemical environment of ore deposition. Kinetic factors are shown to favor marcasite over pyrite and we suggest that undersaturation with "monosulfide"-type phases such as mackinaurite and greigite are a prerequisite for marcasite formation.

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