Fluid evolution and physicochemical constraints of the Nasigatu greisen-type Be deposit in Inner Mongolia

Abstract

The Nasigatu greisen-type beryllium (Be) deposit, located at the western slope of the southern Great Xing’an Range (GXR), within the Central Asian Orogenic Belt (CAOB), exhibits a complex ore-forming history involving both magmatic and hydrothermal processes. The deposit consists of lenticular or vein-shaped orebodies hosted in alkali-feldspar granite, and two types of mineralization are observed: disseminated beryl mineralization in greisen and quartz–beryl vein mineralization cutting through the greisen or into surrounding rocks. The entire ore-forming process can be divided into two periods: magmatic (alkali-feldspar granite, M_I) and hydrothermal (I–IV) periods. The hydrothermal period is divided into four stages according to its mineral assemblage characteristics: (I) greisen, (II) quartz–muscovite–beryl ± fluorite veins in greisen, (III) quartz–beryl ± phenakite ± fluorite pegmatite veins, and (IV) quartz–fluorite veins.

Melt inclusion (M−type), vapour-rich two-phase aqueous (LV-type), pure vapour (V-type), and daughter mineral-bearing three-phase (SVL-type) fluid inclusions (FIs) are developed in M_I quartz, and the coexistence of melt inclusions and brine fluids indicates that they are captured in the magmatic-hydrothermal transition stage. At stage I, liquid-rich two-phase aqueous (VL-type), V-, LV-, and SVL-type FIs (525–585 °C, 6.0–42.7 wt% NaCl eqv) existed, and VL-type FIs exhibited a relatively high vapour/liquid ratio, ranging from 30 % to 40 %. From stage Ⅱ to stage IV, only VL-type FIs existed, with temperatures of 336–395, 287–326, and 180–225 °C and salinities of 6.6–8.5, 5.8–8.0, and 4.6–5.8 wt% NaCl eqv, respectively. The isotopic data (H-O) indicate that the ore-forming fluid initially consisted of magmatic water, which later interacted with meteoric water. Based on P-T diagrams and the physicochemical conditions delineated by the logaSiO₂–logaAl₂O₃ and µHF–µKF relationships, the mineralization of the Be deposit is estimated to have occurred within a pressure–temperature regime of 50–210 MPa and 308–585 °C. In contrast, the P-T conditions required for pegmatite vein formation were relatively lower. Furthermore, the formation of phenakite demands lower activities of SiO₂ and Al₂O₃ and higher HF activity compared to beryl. The elevated HF activity suggests that phenakite crystallization is favored in a relatively more acidic environment, which is consistent with the fluid’s increasing fluorine concentration during the later stages of mineralization.

It can be concluded that the initial ore-forming fluid evolved following the intrusion of Early Cretaceous magmatic rocks, with the fluid being directly derived from the crystallizing silicate melt under conditions characteristic of a two-phase zone. Beryllium (Be) became concentrated in the residual magma during the crystallization of alkali-feldspar granite. As temperature and pressure declined, the fluid exsolved, incorporating Be along with Cl⁻ and F⁻, leading to the formation of disseminated beryl mineralization within the greisen. Subsequent fluid cooling, induced by temperature reduction and mixing with meteoric water during stages II and III, facilitated the transition of the fluid from the two-phase to the single-phase region. The precipitation of fluorite, coupled with meteoric water interaction, is likely to have been a key mechanism controlling the formation of the Nasigatu Be deposit. With progressive fluid evolution, the decrease in activities of SiO₂, Al₂O₃, and HF contributed to a gradual transition in the Be mineral assemblage, from beryl to a beryl + phenakite association.