Skarn development in calcareous-pelitic succession affected by thermal metamorphism and fluid flow at Acquarilli beach (Elba Island, Italy)

Skarn deposits represent one of the most significant ore types occurring throughout the world (Meinert et al. 2005 and references therein). Their importance derives from the variety of elements that could be enriched during their formation (Meinert et al. 2005 and references therein; Zharikov et al. 2007). Skarn mineralization is formed as a consequence of the emplacement of magmatic bodies at relatively shallow depth which trigger metamorphic and metasomatic reaction involving both intrusive host rocks in the presence of magmatic- and/ or metamorphic-derived fluid. In the Elba Island (Tuscan Archipelago, Italy), two magmatic bodies intruded into the upper crust (c. 6 km depth): the Monte Capanne (western side) and the Porto Azzurro (eastern side) monzogranite plutons, dated at c. 7.2 and 6.5 Ma, respectively (Gagnevin et al. 2011). Their emplacement occurred during the extensional tectonics that, since early Miocene, affected the inner NorthernApennines giving rise to the northern Tyrrhenian Sea (Carmignani et al. 1994; Bartole 1995). Acquarilli beach (near Norsi locality, north–west of Capoliveri) can be considered one of the best geosites highlighting how metamorphic and metasomatic processes, linked to the emplacement of the Porto Azzurro magmatic body, affected selected beds of an original sedimentary succession made up of siliceous limestone alternated with shale (Argille a Palombini Fm; early Cretaceous; Bortolotti et al. 2001). In detail, the protolith succession consisted of rhythmic alternation of (Fig. 1): (1) centimetre- to decimetre-thick beds of fining-upward siliceous limestone, calcarenite and calcilutite; and (2) centimetre- to metres-thick marl and siliceous siltstone and claystone beds. Rocks exposed in the Acquarilli area were subjected to a thermal perturbation that activated an intense geothermal fluid circulation enhancing the development of metamorphic and metasomatic processes. Considering the lithological heterogeneity and the nature of the genetic processes affecting these rocks, two different skarn types can be recognised at outcrop scale. A first type occurred in correspondence of the transition from siliceous marble to metapelite layers (Fig. 2a). In this case, the metamorphism produced the formation in the marble beds of outstanding spherulitic wollastonite aggregates up to 3 cm in size (Fig. 2b), whereas boundary metasomatism (Zharikov et al. 2007) produced at the contact with the metapelite 2 cm-thick reaction zones due to twoway diffusion of different components (Zharikov et al. 2007; Fig. 2c): (1) a discontinuous garnet-rich zone (close to the wollastonite-rich marble), and (2) a clinopyroxene rich and garnet-bearing zone (close to the contact with the metapelite). The first reaction zone shows yellowish to orange garnets with grossular composition. The second zone displays a contrasting pale green to greyish colour and is dominated by hedenbergite and grossular. A second type of skarn can be also observed in correspondence of the former marly layers. It is characterised by centimetric green vein both parallel and nearly orthogonal (NW–SE trending) to the beds and developed along the pre-existing primary (e.g. stratigraphic) or secondary (i.e. tectonic) discontinuities. They are filled by fibrous amphibole crystals (of ferro-actinolitic and actinolitic composition) and minor amounts of quartz. In the marl, on both sides of the vein, whitish thin bands enriched in anorthite were almost symmetrically developed. This situation is a clear evidence of elements diffusion processes related to near-vein metasomatism (Zharikov et al. 2007).