江南造山带西南段摩天岭穹隆脆韧性剪切与铀成矿作用

在造山带构造体制转换环境下,铀元素的活化、迁移和沉淀富集过程多与脆韧性剪切带的递进发育过程紧密相关。鉴于此,详细的区域和矿田构造解析是正确理解铀矿床成因的关键。华南江南造山带及其周缘分布着大面积低温铀矿床,既有晚古生代铀矿床,又有后期叠加的新生代铀矿床,均受脆韧性剪切转换带控制。华南铀矿床成矿作用具有多期叠加成矿特点,以往对单个铀矿床成矿地质和赋矿特征的研究较多,但对铀成矿作用与区域地质、构造体制及其剪切转化关系的系统研究则较为薄弱,造成对矿床成因认识很不统一,制约了对这一区铀成矿作用的深入理解。为深入了解这类铀矿床的形成机制,本文选取了摩天岭穹隆376矿床和374矿床两种不同地质特征的代表性铀矿床开展研究。通过详细地构造解析,我们认为摩天岭穹隆至少发育了五期构造变形,分别为新元古代(～820Ma)(D1期)近东西向褶皱与同构造岩浆侵位、加里东期顶部向NW的逆冲(453～426Ma)(D2期)、后加里东期NE走向的正向韧性剪切(426～295Ma)(D3期)、燕山晚期-喜马拉雅期的脆-韧性伸展(87～47Ma)(D4期)及喜马拉雅期以来的构造隆升与剥蚀(47Ma～至今)(D5期);结合显微构造与电子探针分析,认为D3期和D4期为关键铀成矿期。通过系统野外调查,以构造地质解析为主线,对新元古代三防花岗岩体及其周缘主要含矿构造与典型铀矿床的关系进行了详细解析,提出了摩天岭两种类型铀矿床由脆韧性递进变形控制的统一铀成矿模式,以期对华南铀矿勘查工作提供借鉴。

The brittle-ductile shearing and uranium metallogenesis of the Motinaling dome in the southwestern Jiangnan Orogenic Belt

Leaching, transportation, and enrichment of uranium in orogenic belts are closely associated with tectonic transition and progressive deformation of brittle-ductile shear zones. As such, detailed structrural analysis on shear zones of regional and district scales are of crtical importance in better understanding uranium ore genesis. The giant low-temperature uranium metallogenic belt in South China is distributed along and around the Jiangnan Orogenic Belt. Most uranium deposits within that belt have formed in the Late Paleozoic and Cenozoic, both being controlled by the brittle-ductile shear zones. Previous studies have focused on characteristics of mineralization of individual uranium deposits, the relationship between uranium mineralization and regional tectonic transition remain poorly understood. This has led to ambiguous interpretation of the ore genesis. In this paper, we present geological and structural data of two representative uranium deposits (No.376 and No.374 deposits) within the Motianling dome in the southwesternmost Jiangnan Orogenic Belt, aiming to provide a genetic linkage betweent the regional tectonics and uranium mineralization. We suggest the Motianling dome experienced at least five structural stages, including Neoproterozoic (~820Ma) (D1) with nearly EW-trending folding and syn-tectonic magmatic intrusion, Calidonian top-to-NW thrusting (453~426Ma) (D2), post-Calidonian NE-trending normal ductile shearing (426~295Ma) (D3), Late Yanshanian-Himalayan brittle-ductile extension (87~47Ma) (D4) and tectonic uplift and erosion since Himalayan (47Ma to present) (D5), of which the D3 and D4 are the pivotal period of the uranium metallogenesis combined with microstructure and electronic probe microanalysis. Based on geological and structural analysis, relationship between progressive deformation of the brittle-ductile shear zones cutting through the Sanfang granite and formation of the No.376 and No.374 uranium deposits is established. The results suggest that the ductile to brittle transformation of the Motianling dome had fundamental control on the release, migration and enrichment of uranium along the brittle-ductile shear zone. A new tectonic model with two types of uranium deposits in the Motianling dome highlighting the control of progressive brittle-ductile deformation on uranium mineralization is proposed. This model has implications for the exploration of uranium deposits in the South China.