Ore mineralization in the Mofete and San Vito geothermal fields, Campi Flegrei volcanic complex, Naples, Italy

Abstract

The Mofete and San Vito geothermal fields, located west of Naples, Italy, are part of the Campi Flegrei volcanic complex. In the 1970s, exploratory wells were drilled to a depth of similar to 3000 m in an attempt to locate highenthalpy fluids for potential power production. Drill core samples from Mofete wells (MF1, MF2, and MF5) and from San Vito wells (SV1 and SV3) contain authigenic ore mineralization. Pyrite, pyrrhotite, and galena are abundant. Less common are chalcopyrite, sphalerite, arsenopyrite, and scheelite; rare are millerite, violarite, native bismuth, tellurobismuthite, cassiterite, molybdenite, and acanthite. Mineral chemistry was determined by electron microprobe wavelength dispersive spectroscopy aided by a scanning electron microscope equipped with energy-dispersive spectroscopy. The mineral assemblage suggests a low sulfidation environment and the absence of pyrrhotite in the MF1 well and upper part of the SV1 well indicates variable sulfur activity. Both molybdenite and scheelite were identified in samples SV1-2860 and SV3-2353 and scheelite in the SV3 well is zoned with variable Mo6+ content; low Mo6+ zones show blue cathodoluminescence, whereas, zones with high Mo6+ content are yellow to brown. Zoned scheelite and the occurrence of both Mo-bearing minerals attest to the variability of fO(2) and fS(2) in the geothermal fluid.