Compositional variation and zoning of epidote supergroup minerals in the Campi Flegrei geothermal field, Naples, Italy

Abstract

Authigenic epidote supergroups are an abundant accessory mineral in the calcium–aluminum silicate and thermometamorphic hydrothermal zones of the Campi Flegrei (Phlegraean Fields) geothermal field located west of Naples, Italy. Geothermal exploration for high-enthalpy fluid produced drill core and cuttings to ∼ 3 km depth in the Mofete (MF1, MF2, MF5) and San Vito (SV1, SV3) wells, where measured down-hole temperatures of epidote-bearing samples range from 270–350 ∘C and from 285–390 ∘C for the Mofete and San Vito areas, respectively. Two epidote group (epidote, clinozoisite), some rare earth element (REE)-bearing, and two allanite group (allanite-(Ce), ferriallanite-(Ce)) minerals were identified by electron microprobe. The allanite group is light rare earth element (LREE, La–Gd) enriched, Ce dominant, with REE + Y that varies from 30.59 wt %–14.32 wt %. Complex compositional variation such as oscillatory, sector, and complex (mixed) zoning is a ubiquitous feature observed in the epidote group, which occurs as veins, in vugs, as various size masses, and as isolated single crystals. Compositional zoning is caused by variable Fe ↔ Al3+ substitution and XFe [(Fe3+) / (Fe3++ Al)] ranges from 0.06–0.33 (Fe3+=0.185–0.967 apfu). XFe tends to decrease with increasing temperature in the Mofete wells, but its distribution is more complex in the San Vito wells, which records recent fault displacement. The variety and complexity of the epidote supergroup zoning suggest rapid fluid composition and/or intensive parameter fluctuations in the local hydrothermal system.