HYDROGEOLOGY OF THE ZUCCALE DETACHMENT FAULT, ELBA ISLAND, ITALY

Elba Island consists of a west-dipping stack of east-directed pre-extension thrust sheets invaded by 6-8 Ma Miocene syn-extension plutons emplaced at depths ranging from 6-3 km. The plutons, now at sea level, are younger and emplaced at shallower depth in the west than in the east. The space-depth relationship is opposite the relationship expected if exhumation were by the east-vergent Zuccale detachment fault (ZF). The ZF has a strongly domed shape in eastern Elba Island, and that shape has been interpreted as having formed at depth during plutonism, an interpretation also at odds with pluton exhumation by subsequent ZF displacement. These relationships warrant consideration of alternative modes of uplift. The ZF separates contrasting paleohydrologic regimes; strong oxidative alteration, dissolution, and iron and carbonate replacement in upper-plate rocks vs. sulfide mineralization and silicification in lower-plate rocks. Excellent exposures in numerous open-pit iron-mine excavations in altered upper plate rocks reveal wholesale destruction of primary schistose fabric and widespread secondary srtuctures formed by dissolution and collapse. Those relations, together with the form and contact relations of sub-grade iron-bearing masses remaining in the pits, support a hydrogeologic model of iron ore formation as residues of protracted karst activity. Karst volume loss could have kept pace with uplift protracted over a few Ma. In this open-system model, diapiric uplift during thermal relaxation fed the pluton and its country rock into a corrosive zone of interaction between hot acidic lower-plate waters and oxidative cooler upper-plate waters. Corrosion was aided by sulphuric acid derived from the breakdown of pyrite in the presence of carbonic acid streaming upward from the mantle, a process currently active beneath central Italy. Exposures of the ZF in the Punta di Zuccale and Spiaggia Nere area mark the hydrogeologic boundary zone between the two fluid systems, a zone from which the loss of several km³ of predominantly silicic rock by karsting is postulated.