Oxygen and carbon isotope and cation geochemistry of metasomatic carbonates and fluids—Bergell aureole, Northern Italy

Abstract

Oxygen and carbon isotope geochemistry and Ca-Mg geothermometry are used to investigate temperatures, fluid compositions, and mechanisms of vein formation in dolomitic marble roof pendants of the Bergell intrusive complex, northern Italy. Ca-Mg and oxygen isotope geothermometry suggests that the mineralogically zoned clinohumite-chondrodite-calcite, forsterite-calcite, diopside-calcite, and tremolite-calcite veins formed over a range of temperature of approximately 520 to 380°C. Most veins apparently formed near 400°C from H2O-rich C-O-H fluids (XCo2 = 0.1-0.25).Abrupt 5–14%. isotopic “discontinuities” over several millimeters across the vein front are coincident with the mineralogically indicated vein fronts, suggesting that mass transport was controlled by an infiltration mechanism. Decarbonation did not play a significant role in determining the isotopic composition of the vein calcite. The isotopic data suggest a metasomatic fluid of rather constant composition and fluid-rock ratios as high as 100. Such ratios are minimum estimates, but not inconsistent with fluid/rock ratios calculated for complete reaction of the dolomitic wall rock with a fluid which provides the required silica.