Compositional evolution of the muscovite of Renli pegmatite-type rare-metal deposit, northeast Hunan, China: Implications for its petrogenesis and mineralization potential

Abstract

The Renli deposit is the largest known pegmatite-type Ta–Nb deposit in south China. Its rare-metal pegmatites show regular zoning of pegmatite types and rare-metal mineralization assemblages from northeast to southwest. One hundred and forty major outcropping pegmatite dikes throughout the deposit can be approximately divided into four zones outward from the Mufushan batholith: microcline pegmatite zone (P1); microcline–albite pegmatite zone (P2); albite pegmatite zone (P3); and albite–spodumene pegmatite zone (P4). The sizes of the dikes decrease with distance from the batholith, and their rare-metal mineralization type changes as follows: Be → Nb–Ta (–Be) → Li (–Nb–Ta). Field geological characteristics, chronological and Lu–Hf and Sr–Nb–Pb isotopic studies indicate that these rare-metal pegmatites have a close petrogenetic relationship with the Mufushan complex granitic intrusions. However, because of the similarities in geochemical characteristics, magma source properties, and emplacement ages of these different granitic intrusions, identifying the parental rock of rare-metal pegmatites is challenging.Muscovites are common throughout the granitic intrusions and four types of pegmatites in the Renli deposit, and are intimately associated with rare-metal mineralization. The geochemistry of muscovite obtained from granites and pegmatites can be used to demonstrate the parental relationships of rare-metal pegmatites and Mufushan granites, allowing pegmatite evolution trends and the mineralization potential of the Renli deposit to be evaluated. The K/Rb versus Cs, Rb, Li, Nb, and Ta ratios and Ta/Nb versus Li, Ta, and TiO2 ratios in muscovites from the granites and pegmatites show that they form an association with the following sequence of evolution: biotite monzogranite → two-mica monzogranite → microcline pegmatite → microcline–albite pegmatite → albite pegmatite → albite–spodumene pegmatite. The above complete evolution sequence represents the magmatic stage of the deposit and the lepidolite-quartz core in pegmatite with well internal zoning represents the fluid-rich stage of the deposit. The Al4Si–3□–1 exchange vector (where □ represents a vacancy) is shown to have played an important role in the first magmatic stage of the Renli deposit when compositional changes in Li-poor muscovite formed in the magmatic stage involved this substitution. During the late fluid-rich stage formed by highly differentiated magma at the end of the magmatic evolution of the deposit, Li was incorporated into micas by Li3Al–1□–2. Many pegmatite dikes with transitional boundaries can be observed in the outcrops of two-mica monzogranite (RLG2), and geochemical characteristics of muscovite suggest that this monzogranite show the highest differentiation degree in Renli granites, whose average K/Rb and Ta/Nb ratios are close to those of the Renli pegmatites. This indicates that the two-mica monzogranite (RLG2) has the closest petrogenetic relationship with the Renli rare-metal pegmatites. Combined with the data from this and previous studies, the K/Rb ratio, Li and Cs contents of muscovite can be used to distinguish the barren pegmatite and different types (Be, Nb–Ta, or Li type) of rare-metal pegmatites to a certain degree. Therefore, muscovite can indicate the metallogenic type and economic potential in a pegmatite-type rare-metal deposit.