Abstract
Megacrystalline uraninite (up to one centimeter in size) represents one of the most important discoveries in uranium mineralogy in the western margin of the Yangtze Block and even in China in recent years. However, the genesis of megacrystalline uraninite remains controversial. In this study, the megacrystalline uraninite found in the felsic and quartz veins in the Haita area is examined for the first time. The study examined the geochemical characteristics of uraninite in the two veins and resulted in two primary findings. (1) The genesis of the uraninite was likely intrusive and was closely related to partial melting. (2) The quartz vein and feldspar vein are cogenetic and have a simple differentiation evolution relationship. Therefore, the partial melting of felsic materials during migmatization may be the most important mechanism of uranium mineralization in the study area. Furthermore, further simple fractional crystallization may be another important mechanism for the formation of megacrystalline uraninite. This study enriches the REE database of uraninite in uranium deposits worldwide, which is meaningful for studying the genesis of megacrystalline uraninite.
Highlights
Uraninite is commonly found in uranium deposits formed in a variety of geological environments
At a temperature of >350 ◦C, the dilatational nature of the uraninite structure allows the incorporation of large amounts of rare earth element (REE) without fractionation (ΣLREE/ΣHREE ≈ 1, intrusive and synmetamorphic deposits), resulting in “flat” REE patterns and a strong negative Eu anomaly reflecting early fractionation of plagioclase in the silicate melt for intrusive deposits
The above features indicate that the uraninite from the quartz and felsic veins in the study area originated from intrusive rock. We suggest that such uranium mineralization be classified into the subtype of Pegmatite-Alaskite [4] according to the IEAE (International Atomic Energy Agency) classification
Summary
Uraninite is commonly found in uranium deposits formed in a variety of geological environments. These include high-grade metamorphic conditions as well as plutonic, metasomatic, hydrothermal, and basin diagenetic environments. These deposits may be found in volcanic to sedimentary and superficial environments [1,2,3,4,5]. Uraninites of different genesis have significant differences in their chemical composition [2], which are controlled by the respective forming conditions, such as temperature, pressure, fluid composition, and oxygen fugacity [8]. The composition of uraninite (e.g., ∑REE, U/Th ratio, Y and Zr contents, and LREE/HREE ratio) is useful for investigating its genesis [7,12,13,14,15,16,17]
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