The Uranium Mineralization of the Macusani District, Southeast Peru: Mineralogy, Geochemistry, Geochronology and Ore-Genetic Model.
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Uranium mineralization of the Quenamari Meseta, Macusani District, SE Peru, is hosted by peraluminous, sillimanite-andalusite-muscovite-biotite rhyolites of the 12.3 - 6.8 Ma Macusani Formation of the Miocene-Pliocene Quenamari Group. The main ore mineral, meta-autunite, (Ca[(UO2)(PO4)]2(H2O)6-8), occurs in fractures in association with weeksite and hydrous Mn- and Fe-mineraloids. The absence of U4+ minerals, as well as any hydrothermal alteration associated with the U ore, strongly suggests that meta-autunite is the primary U mineral. LA-ICP-MS U-Th-Pa geochronology of the meta-autunite revealed that the main ore-forming events occurred at 69 ka, 130 ka and 314-317 ka, and probably at ca. 103-113 ka, 210 ka and 400 ka, all long after the last magmatic activity in the area. The obtained ages unambiguously relate U ore formation to low-temperature Quaternary processes. The δ18O and δ2H values of the mineralizing fluids, inferred from those of the meta-autunite, range from -21.7 ‰ to –12.2 ‰ and from -181 to -123 ‰, respectively, consistent with ore formation from low-temperature meteoric fluids. The timing of ore formation, as well as the mineralogical and geochemical characteristics of meta-autunite and host rocks, such as REE patterns and alteration assemblages, suggests that U was leached from the glassy matrix of the rhyolites, transported by meteoric waters and precipitated as meta-autunite. The majority of known deposits are located on the upper walls of active fluvial canyons, which suggests that the geomorphological environment that is now focusing groundwater flow was also favourable for meta-autunite precipitation. Meta-autunite probably precipitated through the mixing of low-temperature fluids and interaction of the uranyl ion with Fe- and Mn-mineraloids.Thus, although the Macusani U deposits are volcanic-hosted, they differ radically from documented magmatic-hydrothermal systems.