Montbrayite from the Svetlinsk Gold-Telluride Deposit (South Urals, Russia): Composition Variability and Decomposition

A rare gold-telluride montbrayite from the large Svetlinsk gold-telluride deposit (South Urals, Russia) was comprehensively studied using optical microscopy, scanning electron microscopy, electron microprobe analysis, reflectance measurements, electron backscatter diffraction, and Raman spectroscopy. Significant variations in the composition of the mineral were revealed (in wt%): Au 36.98-48.66, Te 43.35-56.53, Sb 2.49-8.10, Ag up to 4.56, Pb up to 2.04, Bi up to 0.33, Cu up to 1.42. There are two distinct groups with much more-limited variation within the observed compositional interval (in wt%): (1) Au 36.98-41.22, Te 49.35-56.53, Sb 2.49-5.57; (2) Au 47.86-48.66, Te 43.35-44.92, Sb 7.15-8.10. The empirical formula calculated on the basis of 61 apfu is Au16.43-23.28Sb1.79-6.09Te32.01-38.89Ag0-3.69Bi0-0.14Pb0-0.90Cu0-1.96. Two substitution mechanisms for antimony are proposed in the studied montbrayite grains: Sb -> Au (2.5-5.6 wt% Sb) and Sb -> Te (7-8 wt% Sb). The dependence of the reflection spectra and Raman spectra on the antimony content and its substitution mechanism, respectively, was found in the mineral. The slope of the reflectance spectra decreases and the curve in the blue-green region of the spectrum disappears with increasing Sb content in montbrayite. Raman spectra are reported for the first time for this mineral. The average positions of the peak with high-intensity are similar to 64 cm(-1) and similar to 90 cm(-1) for montbrayite with Sb -> Te and Sb -> Au, respectively. Two grains of montbrayite demonstrate decomposition according to two schemes: (1) montbrayite (7 wt% Sb)-> native gold + calaverite +/- altaite, and (2) montbrayite (5 wt% Sb)-> native gold + tellurantimony +/- altaite. A combination of melting and dissolution-precipitation processes may be responsible for the formation of these decomposition textures.