Deposition of metals and metalloids in the fumarolic fields of Guallatiri and Lastarria volcanoes, northern Chile

A detailed study on the mineralogy and chemistry of fumarolic deposits with a focus on metals and metalloids is presented. Two active volcanoes in northern Chile, Guallatiri and Lastarria, are characterised by vigorous and persistent fumarolic activity and the formation of zoned fumarolic deposits. Twelve minerals were identified in the studied deposits by X-ray diffraction: anhydrite, barite, gypsum, rhomboclase, orpiment, pyrite, galena, cristobalite, quartz, magnetite, sassolite, and native sulphur. Chemical analyses revealed the presence of 30 chemical elements, including metals and metalloids such as As, Hg, Pb, Sb, Te, and Tl. Backscattered-electron imaging revealed four types of rock textures in the fumarolic deposits: i) massive and poorly vesiculated, ii) highly vesiculated, iii) clastic-like, and iv) flow-like. Micro-chemical analyses showed the presence of cinnabar nataliyamalikite and unidentified mineral assemblages of Assingle bondS, Tesingle bondS, Pbsingle bondS, Hgsingle bondS, Tlsingle bondI, and Tl-I-Pb compositions. Based on colour, outlet gas temperature, mineralogy, and chemistry, four distinct zones of fumarolic deposits are identified. (1) Grey zone: sulphide and silicate dominant with minor native elements, high concentrations of Pb and As, and a clastic-like texture (gas outlet temperature > 265 °C). (2) Red zone: sulphide and silicate dominant with minor oxides and native elements, small As, Pb, Tl, I, and Te minerals, and predominance of a flow-like texture (200–280 °C). (3) Yellow-orange zone: dominated by native sulphur and minor borates and sulphur with small As-, Sb-, Hg-, and I-containing minerals, massive and vesiculated texture (80–200 °C). (4) Yellow-white zone: formed by native sulphur, minor borates and As-rich zones, usually displaying a vesiculated texture (<100 °C).