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

Abstract

Acid-sulfate alteration is comprised by clays, sulfate, sinter, and native sulphur minerals crystallized as neoformation products from the 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 widely scattered relationships, suggesting a large fractionation during acid-sulfate alteration. Higher ΣREE amounts (up to 934.5 ppm) were found in clay-sulfate assemblages and lower ΣREE amounts in sinter (opal-A ± sulfate, 169.05 ppm) than 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 (0.10–0.38). The variation of (La/Sm)N and (La/Yb)N ratios 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 trachyte rocks.