Hydrothermal alteration, fumarolic deposits and fluids from Lastarria Volcanic Complex: A multidisciplinary study

Abstract

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 EF i ) 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 H 2 O, CO 2 , H 2 S, SO 4 , COS bearing fluids, homogenization temperatures around 110 °C and salinities v...

New insights into the magmatic-hydrothermal system and volatile budget of Lastarria volcano, Chile: Integrated results from the 2014 IAVCEI CCVG 12th Volcanic Gas Workshop

“…Because the basement rocks within the Lastarria region are mostly composed of Paleozoic intrusive rocks and Tertiary-Quaternary volcanic rocks (Naranjo, 1992;Naranjo and Cornejo, 1992;Mamani et al, 2008), we can assume that the carbonate and organic volatile signatures are derived from the subducted slab, indicating that the subducted slab is the main volatile source at Lastarria. This interpretation is consistent with the findings of Aguilera et al (2016), who infer a subducted sediment source for condensed gases sampled from Lastarria in 2014.…”

Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes

“…The proposed NaCl content is realistic given the composition of Uturuncu dacites, phase relations in the system NaCl-H 2 O, and the NaCl contents (10%-50%) measured at other volcanoes (e.g., Aguilera et al, 2016). The dacite magma prior to eruption, as shown by melt inclusions (Muir et al, 2015), contained ≤3.9 wt% H 2 O and 0.23 wt% chlorine.…”

“…Samples from the Lastarria hydrothermal system show evidence for mixing of (1) saline fluids superheated, by a deep, magmatic source that was ultimately derived from mantle melting ( 3 He/ 4 He ratio of 4-6 times the average atmospheric composition) with (2) cooler, shallower, less saline groundwater (Aguilera et al, 2012(Aguilera et al, , 2016Lopez et al, 2018). The emission rate of SO 2 was measured by mini-differential optical absorption spectroscopy (DOAS) by Tamburello et al (2014) in 2012, and in 2014 using several techniques by Lopez et al (2018), who measured gas composition with the MultiGAS instrument to calculate the total emission rate of all volatiles of ~12,400-13,500 t/d.…”

“…7. Gas Emissions: At Lastarria, fumarolic gases were sampled and analyzed by Aguilera et al (2012) and Lopez et al (2018), and vapor condensates and fumarolic solid samples (including fluid inclusions) were analyzed by Aguilera et al (2016).…”

Volatile Chalcophile Elements in Native Sulfur from a Submarine Hydrothermal System at Kueishantao, Offshore NE Taiwan

“…The abundance of these elements in the mantle could also be indicative of several planetary processes such as the formation of the Earth's core and the Late Veneer process, meaning the late accretion of asteroidal or cometary material to terrestrial planets [3][4][5]. Moreover, studying the geochemical behavior of these chalcophile elements in hydrothermal deposits could facilitate a greater understanding of the magmatic-hydrothermal processes that occur in seafloor hydrothermal systems [6][7][8]. Notably, the distributions and concentrations of chalcophile elements in arc-related hydrothermal deposits and systems can be used for deciphering provenance, indicating magmatic-hydrothermal processes such as fluid-rock interaction and mineral precipitation during the ascent of hydrothermal fluids from the mantle to the seafloor, and reflecting the physicochemical conditions of the hydrothermal system [9][10][11][12].…”