Industrial mineral occurrences associated with Cenozoic volcanic rocks of Sardinia (Italy): Geological, mineralogical, geochemical features and genetic implications

摘要

The most important Industrial Mineral (EVt) deposits associated with the Cenozoic volcanics of Sardinia include kaolin, bentonite, and zeolites. Though of less significance, perlite and potassium feldspar, are also important in local and national markets. Bentonite, and to a lesser extent kaolin and perlite, form economically viable deposits and zeolites appear as very important industrial mineral commodities for future economic development. The market situation for potassium feldspar is still under evaluation for use in ceramics. All these deposit types occur as alteration products of Cenozoic volcanic rocks, which were emplaced during three main volcanic cycles: in the Eocene (EC, 55 to 51 Ma), Oligocene-Miocene (OMC, 32 to 13 Ma), and Pliocene-Pleistocene (PPC, 5 to 0.1 Ma). Kaolin, bentonite, zeolites, and potassium feldspar occur as alteration products of the OMC volcanites; perlite occurs as an alteration product of the PPC volcanites. All deposits have generally derived from the alteration of the most acidic members of the Cenozoic volcanic rocks (rhyolites to rhyodacites), but differences among the associated ore-forming phenomena are recognized. Hydrothermal sensu stricto, deuteric alteration and weathering are the most important processes involved in ore formation. Weathering, although of minor importance, can also contribute to enhance the alteration of the deposits formed by hydrothermal and deuteric processes. The extent and type of occurrences are mainly controlled by the local geological and structural features, and the environmental geochemical conditions. Kaolin occurs in three main districts: at Romana and Tresnuraghes it is hosted within ignimbritic sequences, while at Serrenti-Furtei it is found in pyroclastic flows and ash-flow tuffs. Kaolin formed by hydrothermal processes occurs in high-sulfidation {Romana and Serrenti-Furtei} and low-sulfidation (Tresnuraghes) type epithermal systems. The alteration is proximal to the feeder conduits and structural setting controls kaolin formation. Bentonite is widespread throughout Sardinia. Deposits occur as stratiform mineral bodies (in situ-altered) within poor welded pumice-rich and ash-rich layers and also as distal fall-out deposits, as lenticular bodies interlayered within the Miocene sedimentary sequences. The former originated by deuteric alteration and formation is mainly controlled by lithology, whereas the second type originated by weathering, which altered the volcanic ash-lenses deposited in lacustrine and/or fluvial-deltaic environments. Zeolites, mainly clinoptilolite and mordenite, are widespread in northern and central Sardinia, and deposits are distal from the vents. Zeolites form in situ-altered and epiclastic mineralization. The former shows higher zeolite contents than the latter, and zeolitization phenomena pervasively affect the ash-rich and pumice-rich layers. Principally deuteric processes gave rise tophreato-magmatic deposit-types and deposits formed in open hydrologic systems. Epiclastic mineralization originated from weathering of the volcanic ash-products. The perlite of Monte Ard (central-southern Sardinia) is associated with the PPC, which consists of a subalkaline sequence (from subalkaline basalts to rhyolitic lavas). Obsidian rhyolitic lavas, affected by weathering, host the mineralization. The weathering process involves complex interactions between non-hydrated volcanic glasses (obsidian) and meteoric water, which was heated circulating below the paleo-surface and formed hydrated glasses (perlite). Potassium feldspar mineralization is associated with low-sulfidation type epithermal systems, and was originated by hydro-thermal phenomena sensu stricto. The K-feldspar-rich bodies occur preferentially within the most acidic members (rhyolitic-rhyodacitic pyroclastites) of the volcanic sequence.