Roberto Emanuele Rizzo and David Healy (University of Aberdeen) organised a six days field trip (1st – 6th September 2014) to Elba Island (Tuscany Archipelago, Italy) on behalf of TSG (the Tectonics Studies Group). The excursion involved Postgraduates and Researchers (thirteen people) from different European Universities.
The aim of this trip was observing and understanding the relationships between regional tectonics and emplacement of magmatic bodies, which enhanced the location of hydrothermal fluids with the formation of a variety of ore deposits (especially Fe-ore bodies). These features have been related with the extensional phases, which affected the Northern Apennines Chain during the Early, Middle Miocene to present.
Nine tectonic units represent the geological frame of the island, all pertaining to the Tuscan and Ligurian (including Ligurian-Piedmontese units) paloegeographic domains. During a time interval between 8.5 and 5.4 Ma some of the Elba’s tectonic units were intruded by two major acidic plutons (the Monte Capanne and the La Serra-Porto Azzurro monzogranites), and by minor dyke swarms of variable composition.
The excursion days have been divided following the geography of the Island from West to East.
On the first day of the trip (2nd of Sept.) we went to observe the main geological feature of Western Elba, represented by the Monte Capanne monzogranitic pluton with its associated dykes swarm, and then its thermometamorphic aureole consisting in different types of hornfels.
The Monte Capanne pluton (about 6.8 Ma) intrudes and ophiolite succession including Jurassic oceanic basement and volcanics, and their Upper Jurassic to Cretaceous pelagic sedimentary cover rocks.
The main facies of the Monte Capanne pluton locally exhibits a marked inequigranular texture due to the occurrence of large Karlsbad-twinned K-feldspar megacrysts (up to 10 cm along the c-axis). We observed how on the margins of the intrusive body the K-feldspar megacrysts may display flow structures and accumulations in the monzogranite, usually in associations with fluidal whirling structures, which can indicate a rapid cooling of the margins of the pluton. Another peculiar feature of this intrusive body is the presence of abundant mafic microgranular enclaves with ellipsoidal shapes.
On the 3rd September we moved to the Central-East Elba, where we visited the best outcrops (Punta di Zuccale, Terra Nera) of the lowermost tectonic units, in order to recognise their tectonic relationships ant the setting of the hosted Fe-ore bodies.
The structural setting of this part of the island is characterised by a pile of eight structural units, separated by a low-angle tectonic surfaces (thrusts and detachments), which lays onto the lowermost unit by a low-angle detachment fault marked by a decametric cataclastic horizon (Zuccale Fault ant its related cataclasite). The period during which the Zuccale fault explicated its movement postdate the emplacement of the La Serra-Porto Azzurro pluton (5.9 Ma). At ‘Punta di Zuccale’ the detachment juxtapose the Cretaceous Flysch (hangingwall) over the Verrucano Triassic sediments and the Paleozoic rocks. Interposed between these two portions there is a large cataclastic horizon, which consists of fragments deriving both the foot- and the hangingwall. It was possible to observe how this level is characterized by strongly foliated structures, which define a top-to-the-east sense of shear.
Similar features have been observed in the ‘Terra Nera’ area, however here the structural setting appears complicated by a complex system of high angle normal faults, which are often associated with Fe-oxides and Fe-hydroxide fills. At ‘Terra Nera’ is evident how these high-angled normal faults have played an important role in the formation of the large Fe-ore deposits.
On the fourth day of our trip (4th September) we went look at the northeastern part of Elba (‘Rio Marina’ area) which host some of the main iron mines of the Island.
The iron deposits occurring at Rio Marina and northwards of it, are constituted by stratiform, massive, or veined bodies, which are preferentially hosted at the contact between the Permo-Carboniferous phyllites and the overlying calcareous levels. According to some authors (cf. Gillieron, 1959) the setting of the ore-bodies is markedly controlled by tectonic lineaments, produced by the Apenninic deformation event. All these deposits includes hematite as main ore mineral.
The aim of the last day in the field (5th of September) was the ‘Monte Calamita’ Promontory (southeastern Elba) and its iron ore deposits. In this area of the Elba is almost exclusively made up by the Elba deepest tectonic unit. This unit was intruded by the Late Miocene-Lowermost Pliocene granitoids and mainly by acid dykes. Here the relationships between the emplacement of the plutonic bodies and the final deformations of the tectonic stack are easily detectable: e.g. the low-angle Zuccale fault that directly superimposes the Cretaceous Flysch unit above the Paleozoic unit.
Iron exploitation at the ‘Calamita’ probably started in pre-Roman times. The northern sector of the ‘Calamita’ mines, ores were strictly associated with skarns, and consisted of lenses and massive bodies of magnetite. Moreover, masses of Fe-Cu sulphides (pyrite, chalcopyrite ± malachite, azurite) were locally exploited at the contact between the garnet skarn and the magnetite lenses.
Review by Roberto Emanuele Rizzo