Mineralogy and Geochemistry (HFSE and REE) of the Present-Day Acid-Sulfate Types Alteration from the Active Hydrothermal System of Furnas Volcano, São Miguel Island, The Azores Archipelago

Iuliu Bobos,Celso Gomes

doi:10.3390/min11040335

Abstract

Acid-sulfate alteration is comprised by clays, sulfate, sinter and native sulphur minerals crystallized as neoformation products from dissolution of primary minerals during water-rock interaction. Smectite, kaolinite, halloysite-7 Å and opal-A occur in assemblages with alunite. Smectite represents a mechanical mixture between two (propylitic and acid-sulfate) alteration types. High amounts of high-field strength elements (HFSE) and rare earth elements (REE) were measured in acid-sulfate rocks. The Nb vs. Ta and Zr vs. Hf show a positive trend and a widely scattered relationships, suggesting a large fractionation during acid-sulfate alteration. Higher ∑REE amounts (up to 934.5 ppm) were found in clay-sulfate assemblages and lower ∑REE amounts in sinter (opal-A ± sulfate, 169.05 ppm) than to fresh rocks (up to 751.2 ppm). The acid-sulfate rocks reveal a distinctive gull-wing chondrite-normalized pattern with a negative Eu anomaly and light- and heavy-REE “wings” similar to the gull-wing pattern of fresh rocks. The Eu/Eu* shows a large fractionation of acid sulfate rocks from 0.16 to 0.78 with respect to fresh trachyte products (0.10 to 0.38). Variation of (La/Sm)N and (La/Yb)N ratio show a large fractionation of light-REE and heavy-REE. The Y vs. Dy and Y vs. Ho show a very good positive correlation coefficient and a large Y fractionation in acid-sulfate rocks with respect to fresh rocks.

Highlights

Acid sulfate–type alteration is associated with geothermal systems related to volcanoplutonic activity along to convergent and divergent plate boundaries, where steam condensate forms and mixes with meteoric water encouraging intensive water-rock interaction and silicate hydrolysis, typically at high water-to-rock ratios [1,2,3,4,5,6]
Three distinct assemblages were identified by X-ray diffraction (XRD) and scanning electron microscope (SEM), namely: i) kaolinite + alunite; ii) alunite + opal-A ± halloysite-7Å ± native sulphur); and iii) opal-A ± alunite
High-field-strain elements and rare earth elements (REE) provide important insights into the active hydrothermal-meteoric system of Furnas volcano, where their confirmed mobilization within the acid-sulfate alteration was controlled by temperature, pH and solution chemistry

Summary

Introduction

Acid sulfate–type alteration is associated with geothermal systems related to volcanoplutonic activity along to convergent and divergent plate boundaries, where steam condensate forms and mixes with meteoric water encouraging intensive water-rock interaction and silicate hydrolysis, typically at high water-to-rock ratios [1,2,3,4,5,6]. Low pressure favors high SO2 /H2 S and HCl/NaCl in the emitted magmatic vapors, increasing the acidity of near-surface condensate [9,10]. The acid-sulfate alteration stage is connected with the low-enthalpy (T < 200 ◦ C) water resources, representing a much larger potential and a wider regional distribution than high-enthalpy resources [12]. Such environments are highly analogous and inseparable from the high-sulfidation environments responsible for some types of epithermal ore formation [5,13,14].

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