The Xiwukou Rb deposit in the Jiangnan uplift belt, Ningguo City, Anhui Province: Rb occurrence and ore-forming mechanisms

doi:10.13745/j.esf.sf.2021.8.4

Abstract

Abstract:

The Xiwukou rubidium (Rb) deposit—a granite-related ore deposit—is the first discovered Rb deposit in the Jiangnan uplift belt, Ningguo City, Anhui Province. Two spatially distinct orebodies, type-Ⅰ and type-Ⅱ, developed in the Xiwukou deposit. The type-Ⅰ orebody is hosted in granite porphyry, and the type-Ⅱ is at the top of the granite porphyry. The mineral combinations are all phengite + quartz, but ore grade is higher in type-I (0.25%) than in type-Ⅱ (0.21%) orebody. Previous researches had focused on the mineralization characteristics, chronology, petrogenesis, and magmatic source, but the mineral occurrence status of Rb and the metallogenic mechanism in the Xiwukou deposit are still unclear. In this study, we used electron probe analysis and LA-ICP-MS to characterize the granite porphyry-hosted plagioclase, K-feldspar and biotite and phengite in the type-Ⅰ, Ⅱ orebodies. We found that the average Rb contents in plagioclase, K-feldspar and biotite, formed in the early stage, were 3.10 × 10^-6, 872 × 10^-6 and 1 328 × 10^-6, respectively; while in phengite, formed in the late stage, the Rb contents were 2544 × 10^-6 and 2686 × 10^-6 in type-I and type-II orebodies, respectively. The stoichiometric calculation results show that Rb⁺ is incorporated into phengite through isomorphism replacing K⁺ and Na⁺. Combining the geological characteristics of the deposit with comparative analysis of regional igneous rocks, we propose the following ore-forming mechanism for the Xiwukou Rb deposit: The Rb background value and fluorine content in the ore-forming magmatic fluids was fairly high, and mineral crystallization led to further Rb enrichment in the residual magma. During the transition stage from magma to hydrothermal fluid, the high fluorine condition caused a structural change in silicate, from a framework to layered structure, which facilitated large-scale crystallization of Rb-rich phengnite to form Rb-rich orebody.

Key words: Jiangnan uplift belt, Xiwukou deposit, rubidium, occurrence status, metallogenic mechanism

CLC Number:

CHEN Xuefeng, FAN Yu, ZHOU Taofa, YU Jianghua, QIAN Shilong, LU Zhongqiu, YANG Zhangyi, HONG Jianmin. The Xiwukou Rb deposit in the Jiangnan uplift belt, Ningguo City, Anhui Province: Rb occurrence and ore-forming mechanisms[J]. Earth Science Frontiers, 2022, 29(1): 65-80.

Figures/Tables 16

Fig.1 Tectonic location (a) and regional geological map (b) of the Jiangnan uplift belt (Anhui section) showing the distribution of mineral deposits. Modified after [20-23])

Fig.2 Simplified geological map of the Xiwukou deposit. Modified after [20-21].

Fig.3 Schematic cross-section of the Xiwukou deposit showing the locations of the Type-I, II orebodies. Modified after [20-21].

Fig.4 Photo (a) and microphotograph (b) of hand specimen of the Xiwukou granite porphyry

Fig.5 Photos and microphotographs of hand specimen of the Type-I (a, b) and II (c, d) orebodies from the Xiwukou deposit

Table 1 Chemical compositions of plagioclase and K-feldspar from the Xiwukou deposit

Table 2 Representative chemical compositions of plagioclase and K- feldspar from the Xiwukou deposit by EMPA

Table 3 Results of concentrations for trace element in plagioclase and K- feldspar determined by LA- ICP-MS from the Xiwukou deposit

Fig.6 Ternary classification diagram for feldspars from the Xiwukou magmatic rocks

Table 4 EPMA analysis results of mica from the Xiwukou deposit

Fig.7 Classification diagram for micas from the Xiwukou deposit

Table 5 Trace element compositions of mica from the Xiwukou deposit by LA-ICP-MS

Fig.8 Statistical chart for Rb contents in minerals from the Xiwukou deposit

Fig.9 Covariations of K, Na and Rb (in apfu) in mica from the Xiwukou deposit

Fig.10 Covariation of Rb and (K + Na) (in apfu) in mica from the Xiwukou deposit

Fig.11 Relationships between F and Rb, K or Na contents in mica from the Xiwukou deposit

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