Evolution of the hydrothermal system at La Crocetta “feldspar” mine: fluid inclusion and stable isotope constraints on the environment of late stage veins

A significant portion (average 200,000 t/y) of the Italian production of feldspathic raw materials for ceramic industries comes from La Crocetta deposit (southern Tuscany, Elba Island). The quarried rocks (so called “eurites”) are pervasively sericitized porphyritic aplites, showing peculiar K-rich (7 to 11 wt% K2O) and Fe-Ca-S-poor compositions. The main stage of mineralization (i.e., sericitization of the aplites) was described in previous studies. This paper is focussed on the late-stage, post-sericitization veins which crosscut eurite rocks. The development of these N-S striking veins may be associated with conjugate faults related to movements along the Elba Centrale regional fault. Mineralogy of veins includes calcite with subordinate quartz and pyrite. A peculiar rock facies, locally known as “candor”, is made up by particularly abundant calcite-bearing veins, which represent the last hydrothermal event at La Crocetta. Calcite-hosted fluid inclusions show homogenization temperatures (Th) between 197° and 227° C, with salinities between 1.1 and 2.7 wt.% NaCl-equivalent. In agreement with geological background and inferred fluid circulation model, a low pressure regime of vein emplacement can be assumed. Accordingly, these Th should correspond to trapping temperatures not exceeding 250° C. The carbon (δ13CPDB) and oxygen (δ18OV-SMOW) values of calcite are -5.4 to -7.4‰, and 6.3 to 10.2‰, respectively. At 250°C, the calculated values of δ18O of water, and δ13C of CO2 in equilibrium with vein calcites are in the ranges, respectively, of 1.0 to 2.9‰, and -6.2 to -4.1‰. These data are consistent with formation of the late-stage veins from a dominantly meteoric fluid, mixed with CO2 from a deep-seated magmatic or metamorphic source.