Sericitic alteration at the La Crocetta deposit (Elba Island, Italy): interplay between magmatism, tectonics and hydrothermal activity

The La Crocetta mine near Porto Azzurro (Elba Island, Tuscany, Italy) is an important producer of raw material for the ceramic industry. Exploitation focuses on a pervasively sericitized porphyritic aplite of the Tuscan Magmatic Province, locally known as ‘‘eurite’’, which underwent significant potassium enrichment during sericitic alteration. Eurites are located along the hanging wall of the Elba Centrale Fault, a low-angle extensional lineament of regional significance. A later carbonatization stage, apparently associated with high-angle extensional tectonics, locally overprinted the sericitized facies. It is expressed by carbonate ± pyrite ± quartz veins, with adverse effects on ore quality. Sericitization was accompanied by addition of potassium, and loss of Na (± Ca, Fe). Rubidium was not enriched along with potassium during sericitization, contrary to what would be expected for interaction with late-magmatic fluids. New 40Ar–39Ar data from eurites provide an isochron age of about 6.7 Ma for the sericitization, whereas the age of the unaltered protolith is ca. 8.8 Ma. Field evidence indicates the Elba Centrale Fault to be the main channel for the hydrothermal fluids. On the other hand, the involvement of heat and/or fluids contributed by the Porto Azzurro pluton, which crops out in the La Crocetta area, is ruled out by field, geochemical and geochronological data (40Ar–39Ar age of Porto Azzurro =5.9 Ma, i.e. significantly younger than the sericitization event). Fluid inclusion studies suggest that sericitization was associated with a low-temperature (<250 C) hydrothermal system. Fluids were locally boiling, of variable salinity (4–17 wt% NaCl equiv.), and contained some CO2 (XCO2 £ 0.027). Their ultimate source is not unequivocally constrained; meteoric and/or magmatic contributions may be possible. Low-salinity (ca 2.6 wt% NaCl equiv.), low-temperature (<250 C) fluids are associated with the late carbonate veining. They are considered to be of dominantly meteoric nature because of their low salinity. In summary, sericitization at La Crocetta is regarded as the product of a detachment fault-related, low temperature hydrothermal system, resulting from the structurally controlled focusing of meteoric and possibly magmatic fluids. Hence, potential targets for exploration for similar resources are represented by aplitic bodies located in the hanging wall of Elba Centrale Fault.