Abstract
Alteration in magmatic-hydrothermal systems leads to distinct changes in rock texture and mineralogy, and a strong redistribution of elements between fluid and rock. Here, we experimentally interacted andesite scoria with hyperacidic, high-sulfidation style fluids from Kawah Ijen volcano (Indonesia) at 25 and 100 °C, seeking to reproduce the textures observed in natural samples from this volcano, and to understand the element fluxes that accompany alteration. The susceptibility to alteration in the experiments is Cu–Fe-sulphide > calcic plagioclase > pyroxene > titano-magnetite > sodic plagioclase, with complete preservation of glass. Silicate minerals alter to opaline silica, and gypsum, barite and a Zr-phase precipitate. The selective alteration of the phenocryst minerals results in a preferential release of compatible elements, as the glass is the main incompatible element host. The experiments reproduce the alteration textures of the natural samples, including the preservation of glass, but the predicted compatible over incompatible element enrichment in the alteration element flux is not observed in the natural setting. This suggests that alteration at Kawah Ijen is dominated by lithologies that lack abundant glass, in particular lava flows where the glass has devitrified, despite these lava flows having a lower surface area compared to scoria.
Highlights
Water–rock interaction is a powerful mechanism for element segregation that can lead to formation of economic element enrichments by both selective leaching and residual enrichment.Igneous systems are interesting in this respect as these can host large-scale hydrothermal systems with development of multiple distinct alteration zones radiating out with the temperature gradient [1,2]
The outcome is highly sensitive to the fluid composition and system parameters including T, P, pH, S activity, and f O2, which control to solubility of elements in the fluid, the susceptibility of different minerals to alteration, and the secondary mineralogy [6,7]
The starting andesite is similar in composition to Kawah Ijen andesites reported earlier and falls on the Kawah Ijen magmatic fractionation trend [11,14,15,16]
Summary
Igneous systems are interesting in this respect as these can host large-scale hydrothermal systems with development of multiple distinct alteration zones radiating out with the temperature gradient [1,2]. These systems are hosts to several types of major mineralisations, including porphyry and epithermal deposits [2,3], testifying to the efficacy of element segregation during water-rock interaction. The outcome is highly sensitive to the fluid composition and system parameters including T, P, pH, S activity, and f O2 , which control to solubility of elements in the fluid, the susceptibility of different minerals to alteration, and the secondary mineralogy [6,7]. The common incongruent dissolution of minerals during alteration and the non-attainment of equilibrium owing to kinetic factors are further complications which can make interpretation of natural alteration products ambiguous
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