关键词: 橄榄玄粗岩, 铀矿床, 火山口, 草桃背, 火山岩

Abstract:

The Huichang faulted basin is located in southern Jiangxi, where appear shoshonite rock series of Cretaceous along Shicheng-Xunwu deep fault. The shoshonitic rocks host a variety of significant mineral deposits, such as iron, copper, gold, lead, zinc, silver and uranium deposits in southeastern China, including hydrothermal uranium deposits at Xiangshan and Caotaobei respectively in northeastern and southern Jiangxi Province. The Caotaobei deposit lies in the Hecaokeng district (ore field). The host rocks for ore of the Caotaobei uranium deposit are primarily medium-to coarse-grained porphyritic biotite monzogranites of the Dafuzu intrusion and Cretaceous shoshonitic volcanic rocks. The Dafuzu pluton corresponds to calc-alkaline S-type granites, with high content of SiO₂ (averaging 74.67%), strongly peraluminous (（Al₂O₃)>(CaO+Na₂O+K₂O)), high total alkalis (average (K₂O+Na₂O) content 7.995%), high content of CaO content (average CaO content 0.54%). The shoshonite series are characterized by: rich SiO₂ (varying 45.78 % to 59.78 %)，high K₂O/Na₂O (average 1.02), high total alkalis (average (K₂O+Na₂O) content 7.37%)，low TiO₂ content (average 0.86%), low iron content (average ∑Fe 7.09%). Uranium mineralization occurs predominantly as veinlets and massive and disseminated replacement ores in shoshonites, granite and cryptoexplosive rocks, among them, the chemical composition of uranium ore in granite and in cryptoexplosion rocks are similar with host granite. Whereas the Fe₂O₃+Fe₂O，CaO and MgO leached out and SiO₂ enriched in the ores hosted in the shoshonitic volcanic rocks, indicating U precipitated from uraniferous solution by cations (such as Fe,Mg,Ca) leached out during shoshonite series metallogenesis. Uranium mineralization within Caotaobei deposit are simultaneously with shoshonitic rocks in the Caotaobei crater, and they are closely related both in space and in genesis. Affected by thermal activities from shoshonitic magma, uranium elements activated and transferred to form uranium ores at the late stage of Early Cretaceous to early stage of Late Cretaceous. Based on petrographic, geochemical and ore-host features studies, an integrated model of the entire mineralized system is summarized, in which uranium mineralization is related to cryptoexplosive breccias caused by overpressured fluids exsolved from crystallizing shoshonitic magma below the Caotaobei caldera and simultaneously leached ore metals from the host volcanic rocks and the caldera basement (the Dafuzu granite batholith). Several potential targets such as Banling, Shangliao, Xiaofuzu are delineated for Caotaobei type uranium deposit.

Key words: shoshonite, uranium deposit, volcanic crater, Caotaobei, volcanic rocks