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

CHEN Feng;YAN DanPing;QIU Liang;YANG WenXin;TANG ShuangLi;GUO QingYin;ZHANG YiXi;School of Earth Sciences and Resources,China University of Geosciences;State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences;China Nuclear Geology;  
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( ~ 820 Ma)( D1) with nearly EW-trending folding and syn-tectonic magmatic intrusion,Calidonian top-to-NW thrusting( 453 ~ 426 Ma)( D2),post-Calidonian NE-trending normal ductile shearing( 426 ~ 295 Ma)( D3),Late Yanshanian-Himalayan brittle-ductile extension( 87 ~ 47 Ma)( D4) and tectonic uplift and erosion since Himalayan( 47 Ma 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.
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