In situ Raman spectroscopic investigation of the hydrothermal speciation of tungsten: Implications for the ore-forming process

摘要

Knowledge on hydrothermal tungsten (W) species is vital towards a better understanding of tungsten transport and mineralization mechanisms. In this study, in situ Raman spectra of a 0.005 - 0.1 mol/kg (m) K_2WO_4 solution containing CO_2, HCl, and NaHCO_3 were collected at 50-400 °C and 20-60 MPa. The spectra for the symmetric stretching vibration mode of the W]O bond, v_1(W]O), were analyzed to investigate the hydrothermal tungstate species. Results showed that carbonate/bicarbonate do not associate with tungstate to form carbonic tungstate species. Nevertheless, the presence of CO_2 can increase the fluid acidity, which favors the formation of polymeric tungstate species at<300 °C. Above about 300 °C, monomeric tungstates (e.g., WO_4 ~(2-), HWO_(4-), H_2WO_4 and alkali tungstate ion pairs) are responsible for the hydrothermal transport of tungsten, and the v_1(W] O) modes of these species are centered at ~930 cm~(-1) and 950 cm~(-1). Based on the above observations, we simulated the mineralization process in the context of fluid-rock interactions using tungstate and alkali tungstate ion pairs as the only aqueous W species. The thermodynamic simulations showed that (a) the timing of mineralization mainly depends on the W concentration in the initial mineralizing fluid and the availability of Ca~(2+), Fe~(2+) and Mn~(2+), with higher W concentrations generally favoring higher temperature mineralization; (b) highly W-enriched fluid is not essential for W mineralization, while extremely low contents of Fe, Mn and Ca in the magma are useful to maintain the mobility of aqueous W until favorable host rocks are encountered; and (c) a "hydrogen reservoir" effect was identified for dissolved CO_2. The presence of CO_2 can promote the extraction of Fe(II) from the pelitic host rocks, thereby facilitating a high-grade vein-type W mineralization. At<~300 °C, polytungstate species, whose v_1(W]O) modes are centered at ~965 - 995 cm~(-1), are important hydrothermal W species