Fluid inclusions in fluorite mineralizations of northwestern Sicily, Italy

Abstract

Several fluorite ± barite mineralizations, mainly controlled by post-Pliocene disjunctive faults, occur in silicified and altered Mesozoic limestones in the area of Termini Imerese (NW Sicily). In this study, we examined fluid inclusions in fluorite from the localities of Rocca Grande and Poggio Balate. Fluorite from both localities contain abundant two-phase liquid-rich aqueous inclusions. Less common types are: (a) monophase (liquid or gas); (b) two-phase, mostly gas-rich, hydrocarbon inclusions; (c) three-phase (liquid + gas + an accidentally trapped solid); and (d) three-phase (gas + two liquids: an aqueous brine and a hydrocarbonic fluid). In many inclusions, the presence of gas under pressure (most probably methane) is revealed by crushing tests. Homogenization temperatures ( T h) of aqueous inclusions spread over a large range (60° to over 300°C), being in the average higher at Rocca Grande (∼200°C) than at Poggio Balate (∼150°C). Pressure corrections to T h are presumably minimal. However, several factors concur in suggesting that most T h's are probably too high with respect to true trapping temperatures. Tentatively, the lowest T h-values (∼100°C at Poggio Balate, and ∼150°C at Rocca Grande) may be taken as indicative of trapping conditions. Temperatures of final melting of ice are, with few exceptions, higher than −3°C. The geological setting and previous geochemical studies suggest that the fluorite-forming fluids were essentially heated meteoric waters. This is in agreement with the low salinities of most inclusions. On the other hand, reconnaissance δD determinations on bulk sample inclusions ( ∼−20‰ vs. SMOW) may reflect minor contributions of seawater to the hydrothermal fluids. The presence of abundant oil and methane is ascribed to interaction with organic matter-rich beds occurring in the local stratigraphic sequence.