Hydrothermal alteration at the Tolfa volcanic district (Latium, Italy): texture, mineral and isotope chemistry

The Tolfa volcanic district (TVD, Latium, Central Italy) is an extinct hydrothermal setting where the effect of fluid-rock interactions can be studied also as a key to unravel the processes taking place in active volcanic systems. TVD is characterized by a sulfate alteration associated with a pervasive deposition of opaline and/or microcrystalline silica, consisting of mineral replacements, veins and agate druses, and argillic facies in the pre-existing volcanic host rocks (Conte et al., 2022; Lombardi and Sheppard 1977; Marchesini et al., 2023).

The TVD hydrothermal alteration products have received attention in the previous literature with some petrographic and isotopic studies of sulfates and clay minerals, as well as scattered and incomplete rock and fluid geochemistry (Cinti et al., 2011; Lombardi and Sheppard 1977). These early studies, however, suggest that TVD is of particular interest in terms of both alteration and mineralization features.

This study is part of a bigger project on the Allumiere-Tolfa district and focuses on the aluminium sulfate and kaolinite-rich deposits of the Tolfa hydrothermal alteration area, deepening the investigation of the processes that generated the alteration materials. We conducted a field campaign at Tolfa and Allumiere and in several surrounding localities up to the Santa Severa mountains. Various collected samples have been analyzed by integrating different techniques: X-ray diffraction (XRD), scanning electron microscopy with microanalysis by energy-dispersive spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FT-IR), whole-rock geochemistry and stable O- and S- isotope geochemistry.

The mineralogical analyses show a wide morphological and compositional variability of the sulfates mineralization. An alunite group mineral occurs as solid solutions. We identified natroalunite as well as walthierite, usually characterized by lamellar/tabular and pseudocubic habits; instead, alluminium-phosphate–alunite (APA) is present as pseudocubic crystals. Kaolinite group minerals have been also detected and, in particular, kaolinite is widespread, occurring in well-shaped grains, often in vermicular packages. Quartz is abundant in most of the samples, associating to alunites. Fe-Ti oxides contain Cr, V, Ni. Alunite can be characterized for high Ba or Sr content and a distinct P-rich core. The ẟ³⁴S varies between 1.2 and 10.3‰ and the ẟ¹⁸O between -5.7 and 11.7‰, depending on mineral type. The preliminary data collected are useful (i) to improve the knowledge of mineral chemistry of these deposits and to characterize in detail alunite and kaolinite minerals; (ii) to constrain the sulfate mineralizations by the hydrothermal fluids, (iii) to have a background for the investigation of mineralization zones and processes, and (iv) to compare this case study with hydrothermal alteration and processes from extinct and quiescent volcanic districts, such as Campi Flegrei and Ischia in Campania.

Cinti D et al. (2011).  Chemical Geology 284, no. 1-2: 160-181.

Conte A. et al. (2022). Physics and Chemistry of Minerals, 49(10), p.39.

Lombardi G., Sheppard S.M.F. (1977) Petrographic and isotopic studies of the altered acid volcanics of the Tolfa-Cerite area, Italy: the genesis of the clays. Clay Miner 12(2):147–162.

Marchesini B. et al. (2023) Structural control on the alteration and fluid flow in the lithocap of the Allumiere-Tolfa epithermal system. Journal of Structural Geology, 105035.