Characteristics and Geological Significance of CO2-Rich Fluid Inclusions in Dakalasu No. 1 Pegmatite Dyke, Altay

Abstract

The fluids in of pegmatite rare metal deposits are generally rich in rare metal elements and volatiles (B, P, F, H2O, CO2, etc.), and they have a high capacity for dissolving and migrating rare metals. The Dakalasu No. 1 rare metal pegmatite vein is located in northwest China’s Altay orogenic belt. Previous studies have indicated that it is a small- to medium-sized beryllium-niobium-tantalum deposit. It showed significant mineral assemblage zonations from the rim to the core, and the mineralizing fluids define a volatile-rich NaCl-H2O-CO2 ± CH4 system. In this contribution, beryl and quartz, which are widely developed in each mineral association and textural zone, were selected for fluid inclusion research through detailed petrographic investigation, microthermometry, and LA-ICP-MS analysis. Petrographic results show that at least three types of fluid inclusions are developed in each mineral textural zone. They are CO2-rich inclusions (type I), gas-liquid two-phase inclusions (type II), and daughter mineral-bearing inclusions (type III), respectively. Additionally, minor melt inclusions (type IV) are visible in the beryl from the rim zone. Microthermometric measurements showed that the homogenization temperature of fluid inclusions in the rim zone was concentrated between 242 °C and 293 °C, with an average of 267 °C, and the salinity was between 7.2–10.3 wt% NaCleqv, with an average of 8.6 wt% NaCleqv. In comparison, the temperature of the core zone was in the range of 225–278 °C, with an average of 246 °C, and the salinity focused between 6.0–7.7 wt% NaCleqv, with an average of 7.1 wt% NaCleqv. The quantitative analysis of individual inclusions by LA-ICP-MS revealed that Li, B, K, Zn, Rb, Sb, Cs, and As were relatively enriched in the rim zone. In contrast, the core zone showed a decreasing trend in trace elements such as Li, B, K, Rb, and Cs. The CO2 content in the fluid exhibited the same decreasing trend from the rim to the core zone, indicating that volatile components such as CO2 played an essential role in the migration and enrichment of rare metal elements. The melt-fluid immiscibility is likely to be a necessary mechanism for significantly enriching rare metals in the Dakalasu No. 1 pegmatite dyke.