Structure, stability and geochemical role of palladium chloride complexes in hydrothermal fluids
Résumé
In situ X-ray absorption spectroscopy (XANES and EXAFS) was applied to study palladium speciation in model (Na,Li)Cl-HCl-HNO3-HClO4-H2O solutions and to measure the solubility of PdO(s) and PdS(s) in NaCl-HCl-H2O solutions up to 450 degrees C and 600 bar. In HNO3 and HClO4 solutions, the square planar Pd(H2O)(4)(2+) cation with an average Pd-O distance of 2.00 +/- 0.01 angstrom is the dominant form at 30-100 degrees C. At T > 100 degrees C, this cation undergoes hydrolysis resulting in Pd precipitation. In (Na,Li)Cl-HCl solutions Pd forms square planar PdCln(H2O)(4-n)(2-n) complexes with Pd-O and Pd-Cl distances of 2.00-2.10 and 2.26-2.31 angstrom, respectively. At 30-250 degrees C our data are consistent with a mixture of PdCl(H2O)(3)(+), PdCl2(H2O)(2 (aq))(0) , PdCl3(H2O)(-), and PdCl42-, but at T > 250 degrees C PdCl3(H2O)(-) and PdCl42- become dominant over a wide range of chloride concentration, from similar to 0.03 to at least 9 mol/kg H2O. XANES and EXAFS analyses of these species distribution allowed derivation of the stability constant of the reaction PdCl3- + Cl- = PdCl42- from 300 to 450 degrees C and 600 bar. These results, coupled with in situ PdO(s) and PdS(s) solubility measurements of this study and a revision of thermodynamic data for these solid phases, allowed generation of a self-consistent thermodynamic data set of the system PdS(s)-PdO(s) PdCl3―PdCl42-. Our data indicate that Pd can be significantly mobilized as chloride complexes only in sulfide-free oxidizing geological settings (e. g., Chudnoe deposits in Russia, Waterberg deposits in South Africa, Jacutinga-type deposits in Brazil). By contrast, at typical pH (4-8), chlorinity (<10-15 wt% NaCl) and H2S contents (0.001-0.1 wt%) of most hydrothermal fluids, the absolute concentration of Pd-Cl complexes is too low to explain Pd enrichment in volcanogenic massive sulfide, modern seafloor sulfide, and porphyry Cu-Au-Mo deposits. Complexes with S-bearing ligands, very likely other than H2S/HS-, such as S-3(-) and other polysulfide anions, are required to account for Pd hydrothermal mobility in these geological settings.