Abstract
Apatite is a nearly ubiquitous accessory phase in igneous rocks that crystallizes during the entire magma evolution process and has great implications for geochronology and petrogenesis. Previous studies suggested that Nb-Ta mineralization in the giant Renli deposit was genetically related to Late Jurassic two-mica monzogranite or Early Cretaceous muscovite monzogranite. Moreover, the magmatic–hydrothermal evolution of these two stages is poorly understood. In our study, we confirm that the muscovite monzogranite, biotite monzogranite, and two-mica monzogranite are all spatially associated with Nb-Ta pegmatites. We present new apatite U-Pb ages to constrain the timing of Nb-Ta mineralization and related magmatism. The results show that apatite from the two-mica pegmatite yield a lower intercept age of 130 ± 2 Ma (2σ), and apatite grains from two two-mica pegmatite samples yield a lower intercept age of 135 ± 8 Ma (2σ) and 134 ± 3 Ma (2σ), respectively. Apatite and whole-rock geochemistry suggest the oxidation degree of the Nb-Ta mineralization increases from north (RL-6) to south (RL-16) in the giant Renli deposit. This study demonstrates that a combination of apatite composition and U-Pb ages can be used to constrain the magmatic–hydrothermal evolution of granite and pegmatite-type Nb-Ta deposits.
Highlights
Before apatite U-Pb dating and analysis of major and trace elements, samples were polished into thin sections (50 μm thickness) for microscopic observation; backscattered electron (BSE) and cathodoluminescence (CL) imaging of the apatite grains was undertaken at the Guangzhou Tuoyan Analytical Technology Co., Ltd., Guangzhou, China
The levels of loss on ignition (LOI) of RL-6, RL-10, and RL-16 are less than 1 wt.%, and they do not have porosity, which indicates that post-magmatic alteration or weathering is not obvious [44]
The apatites have a subhedral to anhedral structure and the surface of backscattered electron (BSE) images are homogeneous without fractures (Figure 4g–i)
Summary
The Nb-Ta deposits represent an important source of critical metals and minerals, and they play an important role in the national defense industry, the aerospace industry, the nuclear industry, super-hard materials industry, and other industries [1,2]. The giant Renli Nb-Ta deposit is located at the southwestern margin of the Mufushan composite granite, which belongs to the central Jiangnan Orogen in South China (Figure 1a). It is considered a continent–continent collisional belt between the Yangtze Block and Cathaysia Block [16–19]. The Mufushan composite granite in the middle of the Jiangnan orogenic belt was formed during the Yanshanian stage and is closely related to rare-metals mineralization (Figure 1) It consists of complex intrusions, such as monzogranite, biotite granodiorite, biotite monzogranite, and two-mica monzogranite, which are NeoproterozoicLate Mesozoic rocks [27]. Abundant rare-metal deposits have been discovered to be associated with the Mufushan composite granite, including the Renli Nb-Ta, Chuanziyuan Li-Nb-Ta, and Baishawo Be-Nb-Ta deposits [20,28–30]
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