Boron-rich ore-forming fluids in hydrothermal W-Sn deposits from South China: Insights from in situ Raman spectroscopic characterization of fluid inclusions

Boron (B) is an important volatile component in many ore-forming fluids of hydrothermal deposits related to granites. The Maoping and Piaotang W-Sn deposits are typical magmatic-hydrothermal deposits located in South China. In this study, boron (i.e., H 3 BO 3 )-rich fluid inclusions are discovered in the above two deposits through in situ Raman spectrosocpic analyses. Based on careful experimental calibrations, we calculated the ratio of relative molar scattering factors between water and H 3 BO 3 ( J H 2 O / J H 3 B O 3 $J_{{\mathrm{H}}_2\mathrm{O}}/J_{{\mathrm{H}}_3{\mathrm{B}\mathrm{O}}_3}$ ). In addition, the relationship between salinity and J H 2 O / J H 3 B O 3 $J_{{\mathrm{H}}_2\mathrm{O}}/J_{{\mathrm{H}}_3{\mathrm{B}\mathrm{O}}_3}$ was constructed. Then, Raman spectroscopic quantification of the NaCl-H 3 BO 3 -H 2 O solutions was proposed. Because corrections for the response function of the spectrometer and the influences of the wavenumber and temperature on the spectral intensity were applied, our Raman spectroscopic quantifications allow comparison across different laboratories. The salinity [ m NaCl in m (mol/kg H 2 O)] and molality of B ( m H 3 BO 3 ) for topaz-hosted fluid inclusions at the Maoping W-Sn deposit were determined to be 0.29–1.48  m and 0.06–0.46  m , respectively. A large quartz crystal was collected from a wolframite-bearing quartz vein of the Piaotang W-Sn deposit. The H 3 BO 3 concentration of fluid inclusions in this crystal was determined using Raman spectroscopy. From the core to the rim, m H 3 BO 3 and m NaCl do not show monotonic changes; m H 3 BO 3 fluctuates between 0.11 and 0.33  m , while m NaCl ranges from 0.11 to 1.41  m . Further thermodynamic calculations revealed moderately acidic ore-forming fluids with pH ranging from 4.9 to 5.1 at 23 °C. The fluctuation of m H 3 BO 3 during the growth of a single quartz crystal indicates the episodic replenishment of magmatic-hydrothermal fluids. In addition, microthermometric measurement can result in non-negligible uncertainties in the fluid compostion determination for such complex NaCl-H 2 O-H 3 BO 3 -CO 2 -N 2 -CH 4 fluid system. Alternatively, Raman spectroscopy can provide non-destructive and fast quantification of such fluid inclusions based on careful experimental calibrations.