Constraints on the timing of magmatism and rare-metal mineralization in the Fangzheng Rb deposit, Altai, NW China: Implications for the spatiotemporal controls on rare-metal mineralization

Abstract

Amazonite granites are potential Rb resources given the abundance of Rb-bearing minerals they contain. The Altai Orogen, located in the southwestern portion of the Central Asian Orogenic Belt, is the world's most famous rare-metal metallogenic belt, containing numerous Li–Be–Nb–Ta–Cs deposits (e.g., the Koktokay No. 3 pegmatite deposit). The Fangzheng deposit, however, is the only known independent Rb deposit in this belt, with a proven Rb2O reserve of ∼12,940 tons at an average grade of 0.07%. The deposit is hosted by the Jiangjunshan pluton, which is composed largely of two-mica alkali feldspar granite and lesser amazonite granite on the peripheries, with amazonite pegmatite veins cutting the pluton and its wall-rocks. Although the pluton has been dated using various geochronological techniques, the magmatic and the rare-metal mineralization ages remain ambiguous given the complexity in zircon genesis and evolution, and their high Th and U contents. In this contribution, U–Pb geochronology of zircon, monazite, cassiterite, and columbite-group minerals, as well as their trace-element characteristics, are utilized to constrain the timing and evolution of magmatism and mineralization in the Fangzheng Rb deposit. Based on morphological and geochemical features, and cathodoluminescence images, zircon grains from different lithofacies are classified into three types — magmatic zircon, hydrothermally overprinted magmatic zircon, and hydrothermal zircon. The first two types of zircon grains are characterized by low Th/U ratios (0.22–4.00, with an average of 0.84) and ΣLREE contents (8.66–466, with an average of 90.6), indicative of a magmatic origin, whereas the third type is characterized by high Th/U ratios (0.22–7.26, with an average of 2.26) and ΣLREE contents (1154–4944 ppm, with an average of 2905 ppm), indicative of a hydrothermal origin. The magmatic and hydrothermally overprinted magmatic zircon grains from the two-mica alkali feldspar granite and amazonite granite yield U–Pb ages of 272.7±1.4 Ma and 272.5±1.7 Ma, respectively. Therefore, the earliest age of magmatism of the Jiangjunshan pluton is approximately 272 Ma (early–middle Permian). Based on the internal structure and geochemical characteristics of accessory minerals (monazite, cassiterite, and columbite-group minerals) selected from the ore bodies (amazonite granite and amazonite pegmatite), it is suggested that they crystallized during the magmatic–hydrothermal transition. The U–Pb ages of hydrothermal zircons (253.2±17.0 Ma), monazite (254.4±0.9 Ma), and columbite-group minerals (254.9±1.4 Ma) from the amazonite granite, as well as the U–Pb ages of monazite (253.3±0.7 Ma), cassiterite (250.4±1.4 Ma), and columbite-group minerals (254.5±0.8 Ma) from the amazonite pegmatite indicate that the age of rare-metal mineralization in the Fangzheng deposit is approximately 255–250 Ma (i.e., close to Permian/Triassic boundary). Combined with previous geochronological results and interpretations of tectonic evolution, it is evident that i) the middle Permian to Late Triassic is an important period of granitic magmatism and related rare-metal mineralization in the Chinese Altai, and ii) the rare-metal deposits formed in an intracontinental extensional setting after the tectonic transition from subduction to extension.