Abstract
Natural zeolite occurrences have been recognized in several Cenozoic pyroclastic deposits in central Sardinia. This study concerns the mineralogical and geochemical characterization of the zeolitized tuffites in the Asuni area (Oristano province) and aims to complement information regarding the zeolitization processes developed in the nearby Allai deposits. Optical and scanning electron microscopy, X-ray powder diffraction, qualitative vs. quantitative microanalyses and bulk-rock geochemistry were performed. Analytical results allow defining the mineral distribution, textural relationships and geochemical features of the zeolite-bearing rocks. The most abundant secondary minerals are Ca-Na mordenites. Contrarily to the most common worldwide clinoptilolite + mordenite paragenesis, mordenite is dominant and occurs in different morphologies, rarely coexisting with clinoptilolite in the studied volcanic tuffites. Glauconite and dioctahedral smectite complete the authigenic assemblages. The primary volcanic components mostly include plagioclase, quartz and glass shards, roughly retaining their original appearance. The tuffites range in composition from dacite to rhyolite. The collected dataset shows that zeolitization is most abundant in coarser-grained deposits and points to a genetic process that mainly involves an open hydrothermal environment governed by aqueous fluids with significant marine component, in post eruption conditions.
Highlights
Sardinia has been a very important mining region of the Mediterranean area with a large variety of metalliferous deposits, among which the Pb-Ag-Zn (Ba-F) ores were the most important exploited resources in the past centuries [1,2,3,4]
Planargia [23],Here, Sarcidana-Marmilla, Nureci-Fordongianus andtheAllai areas [30], little is we focus on the Asuni area, central Sardinia, in[25,26,27,32,36,37]
Sequences consist of a series of coarse sandy-to-ashy layers with variable thickness of down to few ca. few centimeters, showing planar-to-medium angle lamination andand locally characterized by greenish
Summary
Sardinia has been a very important mining region of the Mediterranean area with a large variety of metalliferous deposits, among which the Pb-Ag-Zn (Ba-F) ores were the most important exploited resources in the past centuries [1,2,3,4]. The general overview of the mining industry on the island has changed profoundly, with the closure of the Pb-Ag-Zn districts and the focus on industrial minerals (i.e., feldspars, silica sands, granites, marbles, etc.) [5,6,7,8,9,10], gold-bearing ores (Cenozoic porphyry/epithermal Au-Cu-Te and Au-Ag, and Paleozoic mesothermal “orogenic” Au deposits [11], albitite belt (central Sardinia [12]) and zeolite minerals deposits [9,13]. Natural zeolites have a peculiar framework structure characterized by channels and cages among the oxygen atoms linking the constituting SiO4 and AlO4 tetrahedra. These cavities have molecular-sizes and are usually occupied by H2 O molecules and extra-framework cations [14], resulting in selective properties for circulating substances on dimensional and charge basis. Due to the above characteristic structure, are of great industrial interest for their well-known use in several application fields [15], such as filtering [16], chemical sieve [17], odor removal [18], water softening and gas absorption [19], in cement and brick production [20], in agriculture [21], and in medicinal purposes (i.e., gastroenterology [22]).
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