QUANTITATIVE BORATE ANALYSIS OF FLUID INCLUSIONS IN QUARTZ THROUGH RAMAN SPECTROSCOPY

Borate contents in magmatic fluids are largely unknown, even though boron is an important constituent in granitic magmas and, potentially, in their ore fluids. Salinity expressed as NaCl_eq derived via microthermometry may be inappropriate for fluid inclusions from Li-rich pegmatites, which are typically Li, B, F-rich aqueous solutions. Instead of NaCl_eq, borate may be used as an internal standard for Laser-Ablation-ICP-MS analysis of inclusions in pegmatites, if borate can be measured by Raman spectroscopy, a direct and non-destructive technique. Our focus is to validate and optimize the quantification of borate concentrations in fluid inclusions using Raman spectroscopy.

We evaluated the effects of the addition of NaCl (1.1-12 % g/g), LiCl (1.5-16), Li₂CO₃ (0.05-0.6), NaF (0.6-1.7), and LiF (0.006-0.06) on Raman borate analysis. We used standard solutions with H₃BO₃ ranging from 0.2 to 1.4 %, placed in glass-covered cuvettes. The error induced by the additional solutes was less than ±5 %, for all but the solutions with the lowest borate concentrations, which had errors of up to ±30 %. We also tested the influence of the fluid-inclusion host by rotating standard solutions covered by quartz plates of known crystallographic orientations, relative to the polarization plane of the laser. We observed a sinusoidal variation of borate concentration with a maximum of ±18 % deviation occurring at a 45° angle from the 0° extinction position. The effects of quartz orientation are eliminated if the quartz hosting the natural fluid inclusions and quartz cuvettes holding the calibration standards are placed at extinction.

The improved technique was applied to aqueous fluid inclusions in quartz from San Diego County pegmatites, CA, which have been mined for gemstone-quality tourmaline. We used microthermometry, crush-leach chemistry of the bulk fluid, including boron analysis by ion chromatography, Raman, and LA-ICP-MS to comprehensively characterize the fluid chemistry in quartz from miarolitic cavities. At the Cryo-Genie mine, B (up to 1 %) and Li (up to 0.3 %) are the dominant species in the late pegmatitic fluid that deposited quartz, tourmaline, lepidolite, beryl, etc. Furthermore, the distinct chemical signatures of fluids in gem-bearing and gem-barren pockets can be used as a gemstone exploration indicator.