Resumen
A multidisciplinary study that includes processing of Landsat ETM+ satellite images, chemistry of gas
condensed, mineralogy and chemistry of fumarolic deposits, and fluid inclusion data from native sulphur deposits, has
been carried out in the Lastarria Volcanic Complex (LVC) with the objective to determine the distribution and characteristics
of hydrothermal alteration zones and to establish the relations between gas chemistry and fumarolic deposits.
Satellite image processing shows the presence of four hydrothermal alteration zones, characterized by a mineral assemblage
constituted mainly by clay minerals, alunite, iron oxides, and more subordinated ferrous minerals and goethite.
Hydrothermal alteration zones present in the Lastarria sensu stricto volcano are directly related to the recent fumarolic
activity. Geochemistry of fumarolic gas condensed, obtained from two fumaroles at temperatures between 328 and 320 °C,
has allowed detecting 37 diverse elements corresponding to halogens, chalcophiles, siderophiles, alkali metals, alkali
earth metals and Rare Earth Elements (REE), with concentrations that vary widely between 5,620 ppm (chlorine) and
0.01 ppm (Mo, Ag, Sn, Pb, Se, Mg and Cr). Logarithm of Enrichment Factor (log EFi
) for each element present values
between 6.35 (iodine) and <1 (K, Na, Ca, Fe and Al). Those elements are originated primarily from a magmatic source,
whereas at shallow level a hydrothermal source contributes typical rock-related elements, which are leached from the
wall rock by a strong interaction with hyperacid fluids. Mostly of elements detected are transported to the surface in the
fumarolic emissions as gaseous species, while very few elements (Mg, Ca and Al) are transported in silicate aerosols. A
wide spectrum of minerals are present in the fumarolic deposits, which are constituted by sublimates and incrustations,
and the main minerals phases are distributed in six mineral families, corresponding to sulphates, hydrated sulphates,
sulphides, halides, carbonates, silicates and native element minerals. The sublimate/incrustation minerals are dominated
by the presence of sulphate, sulphur, chlorine and diverse rock-related elements, which are formed by processes that
include a. oxidation of gaseous phase; b. strong rock-fluid interaction; c. dissolution of silicate minerals and volcanic
glass; d. gas-water interaction; e. deposition/precipitation of saline bearing minerals; f. oxidation of sublimates/incrustations
to form secondary minerals and g. remobilization of sulphur deposits by meteoric water. Despite that sublimate/
incrustation minerals are dominated by rock-related elements, its chemistry shows high contents of high-volatile elements
as As, Sb, Cd, among others. Fluid inclusions studies carried out in thin pseudobanded native sulphur from fumarolic
deposits, by use of Raman and infrared spectroscopy combined with microthermometry analyses, provided evidence of
H2
O, CO2
, H2
S, SO4
, COS bearing fluids, homogenization temperatures around 110 °C and salinities varying from ~11
to ~7 wt% NaCl. Fluid inclusions data show also evidences of a mixing (dilution) between hot and saline fluid with a
cooler fluid (cold groundwater or a steam-heated water) as the main process.