Petrographic, REE, fluid inclusion and stable isotope study of magnesite from the Upper Triassic Burano Evaporites (Secchia Valley, northern Apennines): contributions from sedimentary, hydrothermal and metasomatic sources

Abstract

Sparry and microcrystalline magnesite are minor constituents of the Upper Triassic Burano Evaporite Formation of the northern Apennines in Italy. Petrography and geochemistry of magnesite suggest three modes of formation. (1) Evaporitic precipitation of stratified microcrystalline magnesite layers associated with sulfate and carbonate rocks. Most REE are below ICP-MS detection limits. δ18O is +20.2‰ (SMOW) and δ13C is –2.6‰ (PDB). (2) Hydrothermal infill of Fe-rich (9.78 wt% FeO) lenticular sparry magnesite. This type of magnesite is characterized by very low LREE concentrations, whereas HREEs are relatively high. The fluid inclusion composition is NaCl–MgCl2–H2O, salinity is ~30 wt% NaCl equiv., and total homogenization temperatures range from 204–309 °C; δ18O is +17.5‰ and δ13C is +1‰. (3) The partial or total replacement of dolostones by lenticular sparry magnesite. LREEs are lower in magnesite compared with the partly replaced dolostones. Magnesite yields δ18O and δ13C compositions of +17.3 to +23.6‰ and +0.5 to +1.4‰, respectively, whereas the partly replaced dolostones yield δ18O and δ13C values of +25.0 to +26.2 and +1.3 to +1.9, respectively. Complete replacement of dolostones produced massive lenticular sparry magnesite rock containing ooids and axe-head anhydrite relicts; LREEs are depleted compared to unaffected dolostones; δ18O and δ13C compositions range from +16.4 to +18.4‰ and +0.4 to +0.9‰, respectively. These data and the association between fracture-filling and replacive magnesite suggests a metasomatic system induced by hydrothermal circulation of hot and saline Mg-rich fluids. These processes probably occurred in the Oligocene–Miocene, when the Burano Formation acted as main detachment horizon for the Tuscan Nappe during the greenschist facies metamorphism of the Apuane complex. Thrusting over the Apuane zone produced large scale fluid flow focused at the Tuscan Nappe front. Sources of Mg-rich fluids were metamorphic reactions in the Apuane complex and dissolution of Mg-salts at the thrust front. Considering a maximum tectonic burial depth of 10 km, as inferred from the geometry of the chain, the pressure-corrected temperature of magnesite precipitation (380 to 400 °C) and the calculated fluid composition (δ18O=+13.3±1.2‰) are in the range of the published Apuane metamorphic temperatures (300–450 °C) and fluid compositions (δ18O=7–16‰). The results of this study support the hydrothermal-metasomatic model for the formation of sparry magnesite deposits at the expense of dolostone units involved in thrusting and low-grade metamorphism, as proposed for the Northern Graywacke Zone (Alps) and the Eugui deposit (western Pyrenees).