Epigenetic uranium deposits

Finch, W.I., 1967. Geology of epigenetic uranium deposits in sandstone in the United States. U.S. Geol. Surv. Prof. Pap., 538, 121 pp. 

The bulk of the world s uranium has been produced historically from (1) lower Proterozoic uraninite placer deposits in quartz-pebble conglomerates, (2) epigenetic uranium deposits in sandstones located in many cases at, or near, groundwater oxidation-reduction interfaces and (3) hydrothermal vein uranium deposits. These three distinctly different geologic environments provided most of the uranium that was produced from the 1940s to the early 1970s and they continue to be important exploration targets in the search for new uranium deposits. 

The largest and highest-grade epigenetic deposits are those in sandstone, but other types are important in a few areas. Epigenetic uranium deposits contain about 32% of the Western world s reasonably assured 30 uranium resources. 

Abstract strong commodity prices in the last few years have led to a remarkable renaissance of uranium exploration in Labrador, focused in a complex and geologically diverse region known as the Central Mineral Belt (CMB). Potentially economic epigenetic U deposits are mostly hosted by supracrustal rocks of Paleoproterozoic and Mesoproterozoic age, and are difficult to place in the traditional pantheon of uranium deposit types. Recent exploration work implies that structural controls are important in some examples, but the relationships between mineralization and deformation remain far from clear. Geochronological data imply at least three periods of uranium mineralization between 1900 and 1650 Ma. It seems likely that the Labrador CMB represents a region in which U (and other lithophile elements) were repeatedly and sequentially concentrated by hydrothermal processes. The current exploration boom lends impetus for systematic research studies that may ultimately lead to refined genetic models that may be applicable elsewhere. 

Uranium in epigenetic sandstone deposits is believed to have come from such varied sources as weathering of granitic rocks, siliceous tuffs or other uraniferous rocks in the source area for the sandstone devitrification of tuffaceous sediment in or interbedded with the sandstone hydrothermal solutions from nearby magmas and recycling and redistribution of earlier-formed uranium deposits. 

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