The sandstone-hosted Pb occurrence of Rio Pischinappiu (Sardinia, Italy): a Pb-carbonate end-member

The discovery of a new type of Pb-Ba deposit in a elastic complex near Monti Ferru, Sardinia, Italy is discussed with respect to source of metals and conditions of deposition, in a near-shore oxidizing and high-pH environment, as well as conditions of diagenesis and metal concentration. Unlike most red-bed-type Pb deposits, this occurrence is characterized by oxidized minerals, without sulphides, with both elastic material and metals coming from a Tertiary volcanic edifice. The main anions, comprising carbonate, sulphate and chloride species, are of both continental and marine origin. The reconstruction of the mineralizing phenomena in this coastal environment is based on evidence of: transport of metals, composition of the pore waters, pH-Eh conditions, and differences in solubility of the possible species accounting for the predominance of barite and cerussite. The main mineralized stratum was originally an ash tuff bed transformed into an aggregate of authigenic K-feldspar and cerussite during diagenesis. In this less-permeable 'trap-level', intercalated in the normal sedimentary series, the mobility of Pb was restricted, resulting in deposition of ore-grade quantities of cerussite. (C) 1998 Elsevier Science B.V. All rights reserved.