A new genetic interpretation for the Caotaobei uranium deposit associated with the shoshonitic volcanic rocks in the Hecaokeng ore field, southern Jiangxi, China

摘要

Combined with in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon UPb geochronology, published and unpublished literature on the Caotaobei uranium deposit in southern Jiangxi province, China, is re-examined to provide an improved understanding of the origin of the main ore (103?Ma). The Caotaobei deposit lies in the Hecaokeng ore field and is currently one of China's largest, volcanic-related uranium producers. Unlike commonly known volcanogenic uranium deposits throughout the world, it is spatially associated with intermediate lavas with a shoshonitic composition. Uranium mineralization (pitchblende) occurs predominantly as veinlets, disseminations, and massive ores, hosted by the cryptoexplosive breccias rimming the Caotaobei crater. Zircons from one latite define four distinct 206Pb/238U age groups at 220–235?Ma (Triassic), 188?Ma (Early Jurassic), 131–137?Ma (Early Cretaceous), and 97–103?Ma (Early-Late Cretaceous transition, hereafter termed mid-Cretaceous). The integrated age (134?±?2?Ma) of Early Cretaceous zircons (group III) is interpreted as representing the time of lava emplacement. The age data, together with the re-examination of literature, does not definitively support a volcanogenic origin for the generation of the deposit, which was proposed by the previous workers based mainly on the close spatial relationship and the age similarity between the main ore and volcanic lavas. Drill core and grade-control data reveal that rich concentrations of primary uranium ore are common around the granite porphyry dikes cutting the lavas, and that the cryptoexplosive breccias away from the dikes are barren or unmineralized. These observations indicate that the emplacement of the granite porphyries exerts a fundamental control on ore distribution and thus a genetic link exists between main-stage uranium mineralization and the intrusions of the dikes. Zircon overgrowths of mid-Cretaceous age (99.6?±?5.7?Ma) in the shoshonitic volcanic rock is broadly coeval with main-stage U mineralization, which is probably attributable to a tectonothermal event related to the intrusion of the granite porphyries and further supports our genetic reinterpretation. It is thus concluded that the granite porphyry intrusions and associated magma may provide the fluids, ore components, and the thermal energy for U mineralization. However, some other types of fluids and metal sources (e.g., meteoric-derived fluids, which are yet to be identified) could have been substantially involved in the mineralization process. Our new genetic explanation may point to significant potential for mid-Cretaceous granite-related hydrothermal U deposits in Jiangxi and other parts of Southeast China.