No abstract

Lead isotope analyses were applied to the major Pb-Zn ore-deposits hosted in Lower Cambrian carbonates of SW Sardinia. The isotopic composition of lead in barren Cambrian carbonates is shown to be less radiogenic than in ores and in adjacent wall-rocks. A minimum 60 Ma time gap between sediment deposition and ore emplacement can be deduced. Moreover, Pb isotope ratios in Cambrian-hosted deposits are identical with those in Ordovician-hosted ores and in Ordovician conglomerates. These results are interpreted to indicate an epigenetic origin for these deposits during a Middle-Upper Ordovician event probably connected to the "Sardic distensive phase s./.".

An attempt has been made at correlation between the Lower Palaeozoic Iglesiente domain of southwest Sardinia and the southern Cévennes-Montagne Noire Variscan domain of France, with particular regard to the Cambrian-hosted economic lead-zinc concentrations and the spatially associated precious-metal (Au) occurrences. Geological, lithogeochemical and lead isotope investigations led to the following conclusions. (1) The major lead-zinc deposits of Iglesiente and southern Montagne Noire have a Mississippi Valleytype origin, which involved remobilization of pre-existing Cambrian syngenetic mineralization during the Ordovician distensive 'Sardic phase'. Lead isotope evidence suggests mixed crustal sources for the Iglesiente lead. (2) The southern Cévennes stratiform Minerai Zéro of the Malines district is not syngenetic with the Cambrian host rocks. For this ore type a synto late-tectonic Variscan formation model is proposed, similar to the model indicated for the genesis of the Salsigne gold deposit in the Montagne Noire. A reevaluation of the Vigan gold occurrences, regionally associated with the Minerai Zéro economic lead-zinc bodies, is therefore suggested. (3) The newly discovered Tertiary epithermal gold province of Sardinia is genetically quite distinct from the Palaeozoic lead-zinc province of Iglesiente.

Log correlations, biostratigraphical results and seismic data were combined to show that from Late Triassic Norian to Early Cretaceous Aptian times, the Euphrates area (Eastern Syria) was part of a huge saddle-like northeast-trending ridge (the Hamad Uplift) characterized by a prolonged stratigraphic hiatus. This uplift, developed in the Late Triassic, was multiply reactivated during the Mesozoic, particularly in the Early Cretaceous Aptian-Albian times, during a major reorganization phase of the Neo-Tethyan rift system. This uplift marked the separation between two regions with distinctive tectono-sedimentary evolutions: an eastern isolated and starved region (Euphrates Graben) and a western region that was mainly influenced by the sedimentary dynamics of the westerly Bishri Trough, linked to the Palmyrides Basin. The Hamad Uplift broke-up into a N140°E-oriented graben system in late Albian times. This early NE-trending extensional stage was accompanied by volcanic activity and introduced the main phase of horst-and-graben development within the Euphrates Graben in the Late Cretaceous Senonian times.