Barite–silica mineralization at the inter-Ordovician unconformity in southwestern Sardinia (Italy): a fluid inclusion study

Summary ?Data from secondary fluid inclusions in barite and quartz occurring in strongly silicified lithologies (“Quarzite”) along the inter-Ordovician unconformity in SW Sardinia suggest that two distinct fluids are recorded in the mineral phases. (A), a high temperature (250–360 °C) – low salinity (0–6% NaCl equiv.) fluid, measured in quartz, (B), a high salinity (6–20% NaCl equiv.) – low temperature (<80–200 °C) fluid, measured in barite. Fluid (B), though less abundant than A, has also been recorded in quartz, whereas traces of fluid (A) were found in barite. The characteristics of fluid (B) make it akin to the well known saline formation waters responsible for the deposition of the post-Variscan barite and galena ores in SW Sardinia. It is inferred that at least two distinct post-depositional hydrothermal events took place, with fluids circulating along the inter-Ordovician unconformity. Fluid (A) is related to the last phases of Variscan tectonics, whereas the timing of the second hydrothermal event (fluid B) spans the interval from Permian to Mesozoic. Due to the lack of measurable primary inclusions and to the ubiquitous evidence for the stratigraphic control of the silicification, as well as to the marked deformation of the “Quarzite” horizon, it cannot be excluded that at least part of this hydrothermal event could have taken place in pre-Variscan (Silurian?) times. An alternative model is to consider the deposition of the “Quarzite” to be unrelated to pre-Variscan hydrothermal events, but to fluid circulation along Late-Variscan structures. Received July 12, 2001; revised version accepted March 11, 2002