Geochronology and source of the rare-metal pegmatite in the Mufushan area of the Jiangnan orogenic belt: A case study of the giant Renli Nb–Ta deposit in Hunan, China

Abstract

The Jiangnan orogenic belt hosts many rare-metal deposits that exhibit close ore-forming ages and similar geological backgrounds and is an important Li–Be–Nb–Ta concentration belt located in eastern China. However, the metallogenic mechanism and regularity of these rare-metal deposits have not yet been reported in detail because of the lack of accurate geochronological data and systematic studies on the sources of metallogenic materials. The giant Renli rare-metal deposit, located at the south margin of the Mufushan batholith in the central Jiangnan orogenic belt, is one of the few high-grade granitic-pegmatite-type Nb–Ta deposits in China. Columbite–tantalite U–Pb dating has been developed as a more robust method when compared with the traditional dating methods to constrain the timing of rare-metal mineralization. Six columbite grains, obtained from the microcline–albite pegmatite of the Renli deposit, exhibit a concordant laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb age of 133.0 ± 2.6 Ma, which denotes the Nb–Ta mineralization age. The zircon grains from the columbite-bearing microcline–albite pegmatite subjected to different degrees of alteration by the hydrothermal fluids yield a concordant LA–ICP–MS U–Pb age of 131.2 ± 2.4 Ma, which can be considered to be the age of newly magmatic–hydrothermal growth and recrystallization under the transitional magmatic–hydrothermal conditions observed in case of the Renli deposit. Furthermore, the zircon grains from the host biotite monzogranite yield concordant LA–ICP–MS U–Pb ages of 140.7 ± 0.7 and 140.3 ± 0.7 Ma, whereas the zircon grains from the host two-mica monzogranite yield a concordant LA–ICP–MS U–Pb age of 138.3 ± 0.3 Ma, denoting the intrusion timing of two types of host Mufushan granites of the pegmatite dykes. Finally, the zircon grains from two schist samples of the Lengjiaxi Group yield concordant LA–ICP–MS U–Pb ages of 825.9 ± 5.6 and 834.4 ± 5.3 Ma, constraining the maximum age for the termination of deposition. These ages constitute a clear intrusion-cooling-hydrothermal time series with respect to the Renli deposit.Zircon can retain the original magmatic isotopic composition and is extensively used to trace the petrogenetic processes because it is quickly crystallized from magma. The Mufushan granite and rare-metal pegmatite exhibit consistent negative eHf(t) values (mean eHf(t) values of −7.2 and −7.3, respectively), and their two-stage model ages exhibit similar distribution patterns when compared with that exhibited by the Lengjiaxi Group. Furthermore, the Mufushan granite shows the typical geochemical characteristics of the crustal-derived granite. Thus, the rare-metal pegmatite and host granite of the Renli deposit were concluded to be sourced from the partial melting of the Lengjiaxi Group metasedimentary strata.The compressive torsional nature of the Gajiang fault zone, widespread occurrence of the S-type granite, and absence of basaltic rocks suggest that the central-to-eastern part of the Jiangnan orogenic belt was in a subduction setting during the Early Cretaceous. Thus, this economically important Early Cretaceous (ca. 133–129 Ma) rare-metal metallogenic belt was formed owing to the development of the Jiangnan orogenic belt, and the thick Lengjiaxi Group metasedimentary strata provided the metallogenic material source.