Geochemistry and mineralogy of the alteration halo associated with the Three Crow roll-front uranium deposit, Nebraska, USA

Abstract

Roll-front uranium deposits are a significant source of uranium and a growing target for exploration. Despite their importance, there has been comparatively little research on roll-front deposits over the past decades. The present study refines the genetic model of roll-front uranium deposits and establishes new geochemical and mineralogical exploration vectors to ore by examining the mineralogy and geochemistry of the alteration halo associated with the Three Crow roll-front uranium deposit, Nebraska. The deposit is hosted by arkosic sandstones of the late late Eocene Chamberlain Pass Formation. Uranium enrichment occurs along a redox boundary within the sandstone aquifer. The study of the accessory mineralogy of the sandstone showed that several distinct morphological types of pyrite and marcasite occur downstream of the roll-front. Framboidal diagenetic pyrite represents the only pre-ore sulfide mineral. Ore deposition was associated with extensive formation of ore-stage marcasite, implying a change in pH conditions during roll-front mineralization. In the oxidized zone upstream of the ore zone, goethite is predominant. There is little evidence for the oxidation of previous ore-stage pyrite or marcasite upstream of the current roll-front location. Cathodoluminescence petrography showed that framework quartz grains of the sandstone were affected by radiation damage, causing extensive damage in the quartz crystal structure. The occurrence of widespread damage halos was found to be restricted to samples with elevated uranium contents (>100 ppm). Dosage calculations confirm that radiation damage visible in cathodoluminescence microscopy could not have developed from U-bearing groundwater, but record the presence of uranium in the sandstone matrix. The lack of extensive radiation damage upstream of the roll-front suggests that the now oxidized sandstone in the altered tongue has never hosted ore. The framework grain mineralogy does not alter significantly across the roll-front with the exception of a slight increase in feldspar alteration in the oxidized zone. The mineralogical changes identified in this study are shown to influence the whole-rock geochemistry. Geochemical trends include elevated As and Mo proximal to the ore zone related to enrichment in ore-stage sulfides, depletion of C and S in the oxidized zone from the destruction of organic matter by oxidized groundwater, and some redistribution of iron from the oxidized zone to the reduced zone from the alteration and dissolution of pre-ore sulfides. The mineralogical and geochemical changes in turn are a reflection of the Eh gradient from the oxidized upstream portion of the deposit to the reduced downstream zone. The results of this research indicate that the ore-roll at Three Crow is not continuously moving within the host aquifer and has either been stagnant for a pronounced period of time or only moved periodically. Continuous movement of the ore-roll as required by the current deposit model does not appear to be a characteristic feature of all roll-front deposits.