The mineralizations of the Allumiere-Tolfa District (Central Italy)

The Allumiere-Tolfa district is located approximately 60 kilometers northwest of Rome (Italy) and has been characterized by a long mining history for base metals (Pb, Fe), alunite and kaolinite mineralizations dating from prehistoric times (Field & Lombardi, 1972).

These deposits are genetically associated with the emplacement in arc setting of a Plio-Pleistocene volcanic dome consisting of latites and rhyolites, which intruded sedimentary carbonate and siliciclastic country rocks. The intrusive event first produced contact metasomatism of the limestones, and lately triggered hydrothermal activity, which caused a widespread epithermal alteration of the volcanic rocks, resulting in the alteration of the volcanic rocks and emplacement of polymetallic proximal association of Hg-Sb, Pb-Cu, Fe, Zn, Ag, Ba-Sr, F and distal U, Th, Ce, La, Be, Ba, Sr mineralizations (Masi et al., 1980, Della Ventura & Patanè, 2020).

The present work is part of a bigger project investigating various geological features of the Allumiere-Tolfa district (Marchesini et al., 2023) and aims to deepen previous knowledge and investigate in detail the nature of the polymetallic mineralizations of the area. Samples, collected in old pits and outcrops, were analyzed through various techniques: optical microscopy, X-ray diffraction, SEM-EDS analysis, and chemical analysis.

In the area, it was possible to identify distinct alteration facies, residual silica, advanced argillic and argillic facies, with specific ore minerals. The residual silica facies is commonly sulfide-bearing, containing pyrite, As-pyrite, cinnabar, with sphalerite, galena and chalcopyrite. In the advanced argillic facies, pyrite is disseminated in between the aggregates of planar crystals in veins and cavities. Pyrite is also associated with smectite and dickite in the argillic alteration zone. The metasomatized carbonates contain several sulfides such as pyrite and cinnabar with molybdenite, Sr-sulfates, thorutite, thorianite and fluorite. Supergene alteration affected this epithermal assemblage bringing to the formation of gossans with Fe-oxy-hydroxides and secondary sulfides. Preliminary bulk-rock chemical analyses report trace to ultra-trace concentrations of Au.

The identified alteration zones and ore minerals in outcropping rocks indicate low-temperature ore-forming conditions, similar to those occurring in epithermal ores (Simmons et al., 2005), suggesting that possibly the mineralized system extends in depth.

References

Della Ventura G. & Patanè A., 2020. Le miniere dei Monti della Tolfa-Allumiere. Memorie Descrittive Della Carta Geologica D’Italia 106, 23-32.

Field C. & Lombardi G, 1972. Sulfur Isotopic Evidence for the Supergene Origin of Alunite Deposits, Tolfa District, Italy. Mineral. Deposita (Berl.) 7, 113-125.

Marchesini B., Tavani S., Mercuri M., Mondillo N., Pizzati M., Balsamo F., Aldega L., Carminati E., 2023. Structural control on the alteration and fluid flow in the lithocap of the Allumiere-Tolfa epithermal system. Journal of Structural Geology, 105035.

Masi U., Ferrini V., O’Neil J. R. & Batchelder J.N., 1980. Stable Isotope and Fluid Inclusion Studies of Carbonate Deposits from the Tolfa Mountains Mining District (Latium, Central Italy). Minerl. Deposita (Berl.) 15, 351-359.

Simmons S.F., White N.C. & John D.A., 2005. Geological Characteristics of Epithermal Precious and Base Metal Deposits. Economic Geology 100th Anniversary Volume, 485-522.