The genesis of pegmatite and rare metal mineralization of the Longgu deposit, western Sichuan, China Available to Purchase

Granite pegmatite lithium deposits, characterized by high mineralization and easy exploitation, serve as a significant source of lithium resources. Currently, there remains widespread controversy over whether the genesis of this deposit type should be attributed to differentiated crystallization, immiscibility, or anatexis. Determining the petrogenesis and rare metal mineralization mechanisms, followed by establishing a metallogenic model based on these findings, holds crucial significance for guiding field prospecting activities. The Longgu deposit, a recently discovered deposit within the Ke’eryin ore field, is located in the Triassic central Songpan-Ganze fold belt (SGFB) of western Sichuan Province, China. Detailed research on the complete diagenesis and mineralization process, from the genesis of granitic magma to the formation of mineralized pegmatite, is scant. In this context, we carried out systematic cassiterite U-Pb dating, mineralogical, and geochemistry analyses. The cassiterite U-Pb lower intercept age is 205.45 ± 4.34 Ma, and these findings are consistent (within error) with those of other rare metal deposits in the SGFB. The pegmatite is closely associated with the Ke’eryin granite and is likely the product of highly fractionated crystallization of granitic magma. Cassiterite has relatively low Zr/Hf values (1.98−6.57) and high concentrations of Fe and W (969.10−42,751.03 ppm and 1.36−1217.20 ppm, respectively) suggesting that the ore-forming fluid may be mainly derived from the magmatic system related to highly evolved granite. On the basis of mineralogical characteristics, mica can be divided into three types: primary magmatic mica (MS), transitional mica (TM), and hydrothermal mica (HM). MS is characterized by large flakes, euhedral grains, and no component zoning. During the magmatic stage, mica consists of a muscovite-phengite series, and the concentrations of Li, Rb, and Cs increase continuously with magma differentiation crystallization. TM has bright patches and irregular boundaries, and their types are mainly zinnwaldite and lepidolite. Rare metals underwent significant enrichment during this stage, representing the primary mineralization phase. HM is usually fine-grained and exists at the edge of other mineral particles. Compared with the MS and TM, it is distinctly depleted in Li, Nb, Ta, and F with higher K/Rb ratios, which is also opposite to the chemical evolution trends of micas with magma evolution. Integrating previous studies, we propose that pegmatitic melts originate from large-volume magma chambers formed by partial melting of deep-seated mica schist, which undergo prolonged fractional crystallization and ascend to shallow crustal levels to form a highly fractionated granitic magma chamber. Subsequently, this magma chamber differentiates through crystallization to produce two-mica granites and muscovite granites. During this period, pegmatitic melts continuously segregate and migrate to distal areas, forming pegmatites with diverse mineralization types. Synthetically, the formation of both granites and pegmatites is attributed to magmatic fractional crystallization, while the significant mineralization during the transitional stage should be ascribed to melt-fluid immiscibility driven by differentiation-crystallization processes.