Iron ore deposits in the central Southern Alps (Lombardy, Italy): lithostratigraphic and metallogenic reassessment

The central Southern Alps host a very extensive metallogenic province including different metalliferous districts which are characterised by the prevailing presence of Fe, Pb-Zn ores, fluorite, and barite. The dominant iron district showing siderite-ankerite and minor hematite and sulphides is strictly space- and time-controlled. It runs from east to west, between Lake Lugano and Lake Iseo (Giudicarie Line), and includes several iron deposits, both of syngenetic-stratiform and epigenetic vein and dissemination type (fig. 1; tabs. 1, 2, 3). Such deposits were formed during a metallogenic epoch involving the Ordovician-Silurian upper portion of the crystalline basement and the volcano-sedimentary cover formations defined, from bottom to top, by: (i) the Carboniferous-Lower Permian Basal Conglomerate Fm, (ii) the Lower Permian Collio Fm, (iii) the Upper Permian Verrucano Lombardo Fm, (iv) the Lower Triassic Servino Fm. Concerning ore genesis (the depositional environment, transport of hydrothermal fluids, generation of hydrothermal ore fluids and constituents) of the syngenetic-strati form iron mineralization in the different lithostratigraphic sequences, the following observations can be deduced: (a) the presence of a common sub-littoral marine coastal to more distal depositional environment which was CO2-rich and locally hypersaline. This was set in a shallow carbonate-producing shelf zone with contributions of terrigenous as well as magmatogenic material. Local environments include lacustrine and evaporitic (upper member of the Collio Formation), together with a continental alluvial environment for the placer deposit of the <<Conglornerato Basale>>; (b) with respect to iron concentration in the depositional sites, there is evidence for a cyclic transport mechanism by hydrothermal systems, strictly connected to synsedimentary transtensive tectonics; (c) the generation of hydrothermal iron-enriched fluids can be referred to the dissolved content of both accessory and essential iron-bearing minerals of pre-existing rocks, leached and transported under suitable Eh-pH conditions. The genetic points common to epigenetic vein and disseminated mineralization are: (a) fault systems and fractures and permeable lithologies such as conglomerates, sandstones, cataclasites can act as suitable sites of deposition; (b) because of the spatial relationships existing between the epigenetic deposits and the late-Hercynian heat engines, the main mobilisation of the metal solutions (of meteoric, metamorphic and diagenetic origin) can be assumed to be hydrothermal <<per ascensum>>; a minor remobilization <<per descensum>> of meteoric waters and chiefly of syn- to late-diagenetic hydrothermal fluids entirely affects every iron formation, especially the Servino Fm, which marks the upper limit of the iron metallogenesis; (c) the iron source can be envisaged in the leaching of any kind of pre-existing rocks, including the iron formations occurring within the sequence. On the question of iron source, the metal zoning of the basement of the Southern Central Alps can be taken into account (BRIGO & OMENETTO, 1979). Such basement is defined, from west to east, respectively, by a Au-zone (Strona Ceneri Zone), a Fe-zone (central Southern Alps), a pyrite-zone (Trentino Alto Adige Region), and a Cu-pyrite-zone (Cadore-Belluno Province). With respect to a general picture of the iron metallogenesis in the Alps, it is important to notice (BRIGO, 1998) that the deposits in the central Southern Alps are correlated with the iron deposits of the <<Steirischer Erzberg at Eisenerz>>, Styria, in the Northern Grauwacken Zone (THALMANN, 1979; SCHULZ et alii, 1997). On taking the recent investigations of the Authors into account, together with a critical literature revision of geology and of ore geology (JERVIS, 1873; CURIONI, 1877; CORNELIUS, 1916; STELLA, 1921; PENTA, 1952; MICHELETTI, 1954; GILLIERON, 1959; ASSERETO & CASATI, 1965; OMENETTO & BRIGO, 1974; FRIZZO & OMENETTO, 1974; FUMASOLI, 1974; BRIGO & OMENETTO, 1979; BERTOTTI et alii, 1993; CASSINIS el alii, 1997; SCIUNNACH et alii, 1999; SPALLA & GOSSO, 1999; SPALLA et alii, 1999; JADOUL et alii, 2000; SPALLA et alii, 2000), as well as an important re-evaluation of the extensive series of scientific works of historical, technological and archaeological nature (MAIRONI DA PONTE, 1803; ARRIGONI, 1840; CESA BIANCHI, 1874; FRUMENTO, 1952-1963; SQUARZINA, 1960; COLOMBO, 1976; TIZZONI, 1994-1995, 1997), it is possible to attain considerable evidence for a general theory concerning the origin of stratiform iron-bearing mineralizations or stratigraphic markers in different geological units especially occurring in the crystalline basement. As a consequence, the extension of this iron-bearing district in the Alps consisting of a dense distribution of small concentrations attains about 1900 km(2); such a pattern accounts for the presence of intense mining, metallurgical and artisanal activities which have taken place beginning from a remote origin until the first half of the XXth century, accounting for the present world-wide important social and economic position of Lombardy region.