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Stable isotope and fluid inclusion studies of carbonate deposits from the Tolfa Mountains mining district (Latium,central Italy)
Authors:U Masi  V Ferrini  J R O'Neil  J N Batchelder
Institution:(1) Istituto di Geochimica, Università di Roma, Italia;(2) Centro di Studio per la Mineralogia e la Petrologia delle Formazioni Ignee ldquoCNRrdquo c/o Istituto di Mineralogia e Petrografia, Università di Roma, Italia;(3) U.S. Geological Survey, 94025 Menlo Park, Ca, U.S.A.
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 delta 18O values of 14.5 to 21.6 suggesting a rather low water/rock ratio. delta13C 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; delta 18O values of 13.1 to 20.0 and delta13C values of 0.7 to 5.0 identify them as hydrothermal veins rather than marbles. delta 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 delta18O values of 11–12 for these fluids are again indicative of low water/rock ratios. The sparry calcites of stage III have delta18O and delta13C values of 8.1 to 12.9 and –1.7 to 3.2, respectively. delta 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, delta18O values of 9.3 to 12.4 and delta D values for inclusions of –53 to –28 are consistent with its late occurrence and paragenetic link with associated carbonates.
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