Montecristo Island, Portoferraio, Livorno Province, Tuscany, Italyi

Regional Level Types
Montecristo Island	Island
Portoferraio	Commune
Livorno Province	Province
Tuscany	Region
Italy	Country

Montecristo is a wild and uninhabited island, having an area of 10.39 km² and covered with Mediterranean bush, located along the western margin of the Tuscan Archipelago. All of its nearest neighbouring lands are islands: Scoglio d'Affrica 9 km to the west, Pianosa 30 km to the northwest, Giglio 50 km to the east, and then, at a distance of 60 km, Corsica and Elba Island. Administratively it belongs to the municipality of Portoferraio (Elba Island).

The island is a state nature reserve, presently part of the Tuscan Archipelago National Park, and a special authorization is needed to land on it.

Montecristo mainly consists of acid intrusive rocks dominated by a near-circular peraluminous monzogranite pluton, very similar to other plutons of the Tuscan Magmatic Province, namely the granite intrusions of Monte Capanne and Porto Azzurro on Elba Island to the north and of Giglio to the east.

Montecristo medium-grained monzogranite, emplaced at 7.1±0.2 Ma (Innocenti et al., 1997), is made of abundant K-feldspar and quartz megacrysts with subordinate phenocrysts of plagioclase and biotite. Accessory minerals include tourmaline, apatite, zircon, ilmenite, titanite, allanite, monazite, rutile, and hellandite. Fine-grained aggregates of muscovite, biotite, and chlorite after former cordierite crystals also occur. The pluton is cut by aplite and greyish porphyritic dykes. Strongly zoned plagioclases with calcic cores (An58-51) and more sodic margins (An48-40) and a fibrous pale amphibole, apparently replacing pyroxene, are found among the components of mafic microgranular enclaves sporadically occurring throughout the pluton.
The old mineralogical specimens of orthoclase, quartz, tourmaline, garnet, arsenopyrite, chalcopyrite, bornite, fluorite, kaolinite, and secondary uranium minerals were mainly found in pegmatite cavities and aplite dykes at Cala Maestra and partly also near the remains of the Monastery of St. Mamilan (San Mamiliano).

Country rocks are scarce with limited outcrops occurring near the southern shoreline (Cala S. Maria-Punta Nera-Cala Mendolina and Cala dei Ladri-Punta Rossa-Cala Corfù areas). These xenolithic blocks comprise an association of metagabbros, brown or black quartzites, and metapelites. Overall, the metamorphic association is thought to have been derived from an Apenninic ophiolite sequence, which is typically composed of mafic and ultramafic rocks overlain by a sedimentary cover of limestones, cherts, and pelagic shale.

Metagabbros are characterised by the occurrence of Mg-rich amphibole often with augite cores, partially albitised relict plagioclase, and minor talc, chlorite, and epidote.

Metasedimentary rocks are fine-grained granoblastic quartzites and albite-muscovite-epidote hornfels. These rocks are cut by numerous veinlets of lilac-coloured axinite-(Fe) (Mg/(Mg+Fe) = 0.19-0.21) and/or epidote with minor adularia; scattered scheelite is also generally present as inclusions in axinite-(Fe). Fine-grained wollastonite occurs locally along vein margins.

Metasedimentary rocks are exposed over more than 1000 m² on two cupolas above Punta Rossa and mainly consists of calc-silicate hornfels with variable proportions of calcite, garnet, diopside, and quartz, commonly in granoblastic texture; rare biotite and plagioclase are also observed. Peculiar in this sequence is the occurrence of garnetiferous layers as much as 40 cm thick which are almost completely composed of a birefringent garnet characterised by rhombic dodecahedral habit, sector twinning, and concentric zoning with grossular-rich composition (Gr59-67And38-29Alm3-4) (Innocenti et al. 1997; Rocchi et al., 2003).

Johann Jakob Ferber (1773 and 1776) was one of the first authors to report minerals from Montecristo. In fact, he mentioned "large and columnar sherl [i.e., schorl, black tourmaline]" observed in the natural history collection of Francesco Caluri, a famous physician and professor at the University of Siena. Later, in the geological report of Warington W. Smyth (1841) are recorded: large feldspar crystals in association with quartz, black and green mica, black tourmaline, and pyrite from granite; garnet, epidote, and axinite [presently axinite-(Fe)] from the dark metasedimentary rocks of Puntanera (Punta Nera). Giuli (1843) described crystals of feldspar (orthoclase), hyaline quartz, and garnet from granite at San Mamiliano (occurrence similar to those of San Piero in Campo, Elba), black prismatic crystals of tourmaline and earthy kaolinite at Cala Maestra. Then, for Montecristo Island, Pareto (1844) mentioned orthoclase, garnet, epidote, and axinite; D'Achiardi (1872) orthoclase, tourmaline, garnet, epidote, axinite, and kaolinite; Jervis (1874) orthoclase, quartz, garnet, chalcopyrite, and epidote.

In 1875 Giorgio Roster observed the presence of torbernite (“uranite” according to the original text) and arsenopyrite in granite cavities filled with feldspar crystals from Cala Maestra (Roster, 1876). This is the first report on the presence of a uranium mineral in Italy.

Later, Millosevich (1912) determined as zeunerite a similar uranium mineral, appearing as tiny apple-green to grass-green tabular crystals in association with K-feldspar, black tourmaline, fluorite, arsenopyrite, limonite, melanterite, bornite, and chalcopyrite, found in pegmatite cavities at Cala Maestra. Palache et al. (1951) more properly classified it as metazeunerite. Recent studies (Innocenti et al. 1997; Rocchi et al., 2003) on the Montecristo monzogranite have evidenced the presence of REE-Y minerals (monazite, hellandite).

Geochronology

Mineralization age: Miocene : 7.1 ± 0.2 Ma

Important note: This table is based only on rock and mineral ages recorded on mindat.org for this locality and is not necessarily a complete representation of the geochronology, but does give an indication of possible mineralization events relevant to this locality. As more age information is added this table may expand in the future. A break in the table simply indicates a lack of data entered here, not necessarily a break in the geologic sequence. Grey background entries are from different, related, localities.

Geologic Time				Rocks, Minerals and Events
Phanerozoic
	Cenozoic
		Neogene
			Miocene	ⓘ Metagabbro 7.1 ± 0.2 Ma Cala S. Maria, Montecristo Island, Portoferraio, Livorno Province, Tuscany, Italy ⓘ Monzogranite 7.1 ± 0.2 Ma ⓘ Meta-serpentinite 7.1 ± 0.2 Ma Cala dei Ladri, Montecristo Island, Portoferraio, Livorno Province, Tuscany, Italy ⓘ Metamudstone 7.1 ± 0.2 Ma Cala dei Ladri, Montecristo Island, Portoferraio, Livorno Province, Tuscany, Italy ⓘ Metaquartz-sandstone 7.1 ± 0.2 Ma Cala dei Ladri, Montecristo Island, Portoferraio, Livorno Province, Tuscany, Italy

Fossils

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Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/Montecristo
Wikidata ID:	Q203169
GeoNames ID:	3172959

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References