SULFIDE MORPHOLOGY AND DISTRIBUTION AT THE THREE CROW ROLL-FRONT URANIUM DEPOSIT, NEBRASKA

Roll-front uranium deposits represent a principal uranium resource in the United States. However, compared to other deposit types little research has been carried out on these uranium deposits over the past decades. Constraining the processes associated with uranium enrichment in the near surface low-temperature environment is an important step in further understanding the mineralizing processes in roll-front deposits and to derive new exploration guidelines.

Representative sampling of the Oligocene Chamberlain Pass Formation was carried out at the Three Crow deposit, Nebraska, to test whether the mineralogy of the heavy mineral fraction of the host sandstones varies across the roll-front. Heavy mineral separates were examined by quantitative mineral analysis using Qemscan technology in conjunction with reflected light and scanning electron microscopy. The results of these investigations indicate that iron sulfides and oxides are highly variable in type and morphology across the roll-front, with gradients that broadly correspond to the oxidation state of the samples. Red to yellow oxidized sandstone samples contain little iron sulfide but significant amounts of iron oxide. When iron sulfides are present, pyrite is generally more abundant then marcasite. Marcasite occurs as blocky crystals or intergrown with pyrite. In contrast, reduced gray and dark green sandstone samples contain no iron oxides but substantial amounts of iron sulfide. Marcasite is more abundant than pyrite in the samples proximal to the mineralization. Several generations of iron sulfide precipitation were observed in the reduced samples. Pyrite framboids, interpreted to be diagenetic and probably biogenic in origin, can have an overgrowth of euhedral pyrite or bladed marcasite. These grains can then be coated with another generation of marcasite or pyrite respectively. The second overgrowth is typically the other iron sulfide polymorph than the first overgrowth. Bladed marcasite overgrowths occur in significant amounts only on the downstream side of the mineralization. As marcasite is metastable with respect to pyrite, ore-stage formation of marcasite is probably related to kinetic factors such as the rate of crystal growth or variation in the sulfur geochemistry of the groundwater.