Abstract

The Tolfa Mts. Volcanic Province (central-western Italy) has extensively been studied by a geological, petrological and geochemical point of view (e.g. Conforto, 1956; Negretti & Morbidelli, 1963; Lauro & Negretti, 1969; Fazzini et alii, 1972; Civitelli & Corda, 1982; Calderoni et alii, 1984, 1985; 1986; De Rita et alii, 1997). The studied area has suffered the Apenninic orogenesis in the Tortonian. During this compressional phase the allochthonous geological units ("Ligurides") thrusted the "Tuscan Series", the latter forming the backbone of the Apenninic belt. During the Pliocene a distensive tectonic regime took place and originated several NW-SE oriented sedimentary basins that characterises central and southern Italy (e.g. De Rita et alii, 1997 and references therein). The magmatic events, that occurred between 2 and 6.4 Ma (Devoto & Lombardi, 1977), mainly consisting of qz-latites and rhyolites (e.g. Pinarelli, 1991), can be referred to this distensive phase. Successively, the Tolfa Mts. area has experienced a relatively strong hydrothermal activity with the formation of Pb-Cu-Fe-Zn ore deposits (Lombardi & Mattias, 1979). This study presents new data on the geochemical distribution of Cu, Pb, Zn, Ag, Au and Hg from fresh (29 samples) and altered (17 samples) volcanic rocks, hydrothermal veins (13 samples) and sedimentary rocks (52 samples) from the Tolfa Mts. area. On the basis of the chemical results obtained, the Tolfa Mts. region can be regarded as a geochemically enriched area in Ag and Au. Depletion in Zn and enrichments in Cu, Ag and, particularly, Hg have been observed in the altered volcanic rocks. Such enrichments, prevalently concentrated along the main fault systems and associated to the hydrothermal veins, suggest that the Tolfa Mts. area has been affected by an intense circulation of mineralising fluids. The relatively high concentration of Hg (up to 284 ppm in some quartz-calcite-rich veins) implies that such a circulation acted up to low-temperature hydrothermal phases. Thus, the Tolfa area suffered an extensive low-temperature hydrothermal circulation, as also suggested by enrichments in Hg and to a minor extent, Cu and Ag in some carbonate rocks close to the main fault systems. The sedimentary rocks from the Tolfa Mts. area, excepting the above mentioned enriched samples, have contents of Cu, Pb, Zn, Ag, Au and Hg that well agree with those reported in literature for similar rock types.

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