Chemical environment of unusually Ge- and Pb-rich willemite, Tres Marias mine, Mexico

The Tres Marias carbonate-hosted Zn-Ge deposit in Chihuahua, Mexico contains willemite [Zn$_{2}$SiO$_{4}$] with unusually high concentrations of minor and trace elements (e.g., Pb, Ge, As, P, V); Pb concentrations are as high as 2 wt %, and Ge may reach 4000 ppm (average 900 ppm). Electron microprobe analyses and synchrotron X-ray fluorescence maps show that Zn and Ge, as well as Zn and Pb are negatively correlated, whereas Ge and Pb are positively correlated across zoned willemite crystals. In cathodoluminescence (CL) images, those areas of willemite having high trace element concentrations have no, or low CL intensities, whereas zones low in trace elements (except for P) display bright blue CL colors. X-ray absorption fine structure (XAFS) spectroscopy was used to characterize the chemical nature of Ge and Pb in willemite. Comparisons to reference spectra of natural and artificial substances points to the presence of Ge$^{4+}$ and Pb$^{2+}$ in Tres Marias willemite. No evidence for Pb$^{4+}$ was detected. Oscillatory zonation reflects trace element incorporation into willemite from the oxidation of primary Ge-bearing sphalerite and galena (PbS) by siliceous aqueous fluids.