Abstract
The Keerqin rare metal ore field is located in the middle section of the Songpan-Garzê orogenic belt on the eastern edge of the Qinghai-Tibet Plateau. It has been confirmed to contain over 4 million tons of Li2O resources, making it a newly discovered ultra-large pegmatite-type rare metal resource base. However, research on the magmatic-hydrothermal evolution processes and indicator mineral exploration methods in the Keerqin ore field is still relatively weak, and further studies are needed to better understand its genesis mechanisms and effective exploration indicators, which will guide the exploration and evaluation of similar ore fields in the region.
Muscovite is one of the most common rock-forming minerals in pegmatites, and its spectral characteristics and mineral chemistry can effectively indicate the magmatic-hydrothermal evolution process, providing important evidence for ore field exploration. This study focuses on muscovite from the Dangba, Lijiagou, Longgu, and Yelonggou pegmatite deposits in the Keerqin ore field. Microstructural observations, short-wave infrared spectroscopy measurements, and electron probe analyses were conducted.
Based on the petrographic and short-wave thermal infrared spectroscopic characteristics of muscovite, three types of muscovite have been identified in the pegmatites of this region:
- Primary Muscovite (MS): Formed during the magmatic stage, characterized by relatively low silicon (6.07 apfu), iron (0.11 apfu), and high aluminum (5.80 apfu) content, with lithium content ranging from 0.03 apfu to 0.10 apfu. The Pos2200 peak is concentrated between 2200.2 nm and 2207.6 nm (average wavelength 2202.5 nm), and the lepidolite spectral maturity (IC value) ranges from 0.15 to 0.72.
- Lithium-Rich Muscovite (LM): Formed during the magmatic-hydrothermal transition stage, characterized by relatively high silicon (6.16 apfu), iron (0.36 apfu), and low aluminum (5.48 apfu) content, with lithium content ranging from 1.63 apfu to 4.31 apfu. The Pos2200 peak is concentrated between 2197.6 nm and 2199.2 nm (average wavelength 2198.8 nm), and the IC value is greater than 4.3.
- Hydrothermal Muscovite (HM): Formed during the hydrothermal alteration stage, characterized by relatively high silicon (6.21 apfu) and low aluminum (5.38 apfu) and iron (0.18 apfu) content, with lithium content ranging from 0.01 apfu to 0.26 apfu. The Pos2200 peak is concentrated between 2195.6 nm and 2197.0 nm (average wavelength 2196.1 nm), and the IC value ranges from 2.99 to 3.82.
Comprehensive analysis reveals that the sequence from MS to LM to HM represents a continuous geological evolution series, with the Si, Al, and Fe element contents closely related to the changes in the Pos2200 peak. As the degree of magmatic evolution increases, the Si content gradually increases, while the Fe and Al contents and the Pos2200 peak show a decreasing trend. Simultaneously, the Li content shows an initial increase followed by a decrease. The formation temperature of primary muscovite (MS) ranges from 309.44 °C to 448.78 °C, with pressure greater than 6.0 GPa; the formation temperature of lithium-rich muscovite (LM) ranges from 248.16 °C to 288.44 °C, with pressure between 5.61 GPa and 5.77 GPa; and the formation temperature of hydrothermal muscovite (HM) is lower, approximately 209.84 °C to 219.14 °C, with slightly reduced pressure ranging from 5.56 GPa to 5.78 GPa. This further indicates that these three types of muscovite underwent a coherent and orderly evolution process. When the Pos2200 of muscovite is between 2198.2 nm and 2199.2 nm and the IC value is greater than 4.3, it can serve as an important indicator for lithium mineralization in the Keerqin ore field.
