Polyphase deformation-controlled giant Renli Nb–Ta deposit, South China: Constraints from systematic structural analysis

Abstract

• Renli and adjacent areas have experienced polyphase deformation. • D 2 represented orogen exhumation and produced a domal structure. • D 3 formed extensional fractures and was accompanied by the Nb–Ta mineralization. The Early Cretaceous Renli Nb–Ta deposit was recently discovered near the southern margin of the Late Jurassic–Early Cretaceous Mufushan composite granitic batholith, central Jiangnan Orogenic Belt, and represents the largest known rare metal deposit in South China. Substantial breakthroughs have been made in prospecting the pegmatite-dike-type of Nb–Ta mineralization in the deposit, especially from the perspective of geochemical exploration. However, there is still a lack of systematic structural studies in this area which has seriously hindered the in-depth understanding of the emplacement of pegmatite dikes and the distribution patterns of Nb–Ta ore bodies. In order to shed new light on this key issue, this study presents new and detailed structural data from the Mufushan region. The results indicate that the Renli and adjacent areas have experienced several deformational events, including D 1 Neoproterozoic NE–SW contraction and upright-folding, D 2 latest Late Jurassic to Early Cretaceous regional extension accompanied by granitic magma intrusion, and D 3 Early Cretaceous brittle normal faulting and fracturing with granitic magma emplacement and Nb–Ta mineralization. The D 1 event, corresponding to the Jiangnan Orogeny, formed regional-scale NW–SE-striking upright folds with ubiquitous subvertical S 1 cleavages and subhorizontal L 1 crenulation lineations. The D 2 event represented orogenic exhumation and produced a domal structure cored primarily by gneissic/mylonitic rocks and granites and mantled by schists, which dominated the bulk structural architecture of the Mufushan area. This was followed by the D 3 event which controlled the formation of numerous extension fractures in the study area. The Nb–Ta-bearing magmatic-hydrothermal fluids migrated along the pre-existing D 2 shear zone and subsequently filled in the widespread D 3 extension fractures to form pegmatite dikes. The D 2 to D 3 phases can be considered as a progressive exhumation event from plastic to brittle regimes. The D 2 and D 3 structures dictate the spatial distribution of Nb–Ta ore bodies.

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