Stable isotope and fluid inclusion studies of carbonate deposits from the Tolfa Mountains mining district (Latium, central Italy)

Abstract

Carbon and oxygen isotope analyses were made of representative samples of calcite and quartz from the carbonate deposits in the Tolfa Mountains mining district. Measurements were also made of hydrogen isotope compositions, filling temperatures and salinities of fluid inclusions in these minerals. There are three stages of mineralization at Tolfa. In stage I, characterized by calc-silicate hornfels, the carbonates have relatively high δ ¹⁸O values of 14.5 to 21.6 suggesting a rather low water/rock ratio. δ¹³C values of −0.3 to 2.1 indicate that appreciable decarbonation or introduction of deep-seated carbon did not occur. Stage II is marked by phanerocrystalline carbonates; δ ¹⁸O values of 13.1 to 20.0 and δ¹³C values of 0.7 to 5.0 identify them as hydrothermal veins rather than marbles. δ D values of −56 to −50 for inclusion fluids suggest a possible magmatic component to the hydrothermal fluid. Filling temperatures of coarse-grained samples of Calcite II are 309° to 362° C with a salinity range of 5.3 to 7.1 weight percent NaCl. Calculated δ¹⁸O values of 11–12 for these fluids are again indicative of low water/rock ratios. The sparry calcites of stage III have δ¹⁸O and δ¹³C values of 8.1 to 12.9 and −1.7 to 3.2, respectively. δ D values of inclusion fluids are −40 to −33, clearly heavier than in earlier stages and similar to values of modern local ground waters. A salinity measurement of <0.1 weight percent NaCl in a sample of Calcite III is compatible with a relatively unaltered ground water origin for this fluid. Precipitation of the sparry calcite took place at much lower temperatures, around 160° C. For quartz, δ¹⁸O values of 9.3 to 12.4 and δ D values for inclusions of −53 to −28 are consistent with its late occurrence and paragenetic link with associated carbonates.