Abstract
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 H2O, CO2, H2S, 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.
Highlights
The global comprehension of the fluids circulation in an active volcanic system allows determining the origin, nature and evolution of those fluids, where is especially important to detect and understand the different processes involved in the evolution of fluids from its deep original sources to the surface
Some of those processes include gas-rock, gas-water, gasgas and gas-atmosphere interaction, condensation and sublimation of the gaseous phase, which play fundamental roles in the gaseous phase composition emitted to the surface, and in the products associated, like composition and extension of the hydrothermal alteration zones, mineralogical assemblage related to fumarolic deposits, among others
In this work we present a detailed multidisciplinary study based on processing of Landsat ETM+ satellite images, chemistry of gas condensed, mineralogy and chemistry of fumarolic deposits, and fluid inclusion data from native sulphur deposits, with the objective to determine the distribution and characteristics of hydrothermal alteration zones and to establish the relations between gas chemistry and fumarolic deposits
Summary
The global comprehension of the fluids circulation in an active volcanic system allows determining the origin, nature and evolution of those fluids, where is especially important to detect and understand the different processes involved in the evolution of fluids from its deep original sources to the surface Some of those processes include gas-rock, gas-water, gasgas and gas-atmosphere interaction, condensation and sublimation of the gaseous phase, which play fundamental roles in the gaseous phase composition emitted to the surface, and in the products associated, like composition and extension of the hydrothermal alteration zones, mineralogical assemblage related to fumarolic deposits, among others.
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