NOTE Andrea Dini Æ Gianfranco Di Vincenzo Giovanni Ruggieri Æ Jeffrey Rayner Pierfranco Lattanzi Monte Ollasteddu, a new gold discovery in the Variscan basement of Sardinia (Italy): first isotopic ( 40 Ar 39 Ar, Pb) and fluid inclusion data Received: 18 January 2005 / Accepted: 28 July 2005 / Published online: 11 October 2005 Ó Springer-Verlag 2005 Abstract The Monte Ollasteddu deposit represents a major gold discovery in the Variscan basement of southeastern Sardinia. Gold occurs in late-Variscan extensional brittle structures hosted by meta-volcanic, and subordinately meta-sedimentary, rocks. The vein mineralogy is dominated by quartz; arsenopyrite is the main sulphide. Reconnaissance 40 Ar– 39 Ar dating gives ages around 260 Ma on K-feldspar from mineralized veins, whereas metamorphic white mica from the host rock gives ages clustering at 307 Ma. The best age estimate for biotite from a nearby leucogranite body is 286.3±2.2 Ma. The Pb isotope signature of ore and gangue minerals is entirely consistent with literature data for Variscan deposits of Sardinia, and for Euro- pean Variscan gold deposits. Fluid inclusion data point to the presence of both CO 2 -bearing and CO 2 -free fluids, with homogenization temperatures ranging from 220 to 415°C, with low-to-moderate salinities (0.4–6.2 wt% NaCl equivalent). Monte Ollasteddu shows several fea- tures similar to European Variscan gold deposits; how- ever, the age of mineralization might post-date granitoid intrusion by as much as 30 Ma, being instead coeval with very late calc-alkaline basaltic dykes, marking the transition to a post-orogenic, pre-Tethyan geodynamic setting. Introduction Recent exploration in the Variscan basement of Sardinia (Italy), carried out by Sardinia Gold Mining S.p.A. (SGM) and by its successor Medoro Resources Ltd., has resulted in the discovery of several new gold occurrences of possible economic relevance. These include the Monte Ollasteddu and Brecca prospects, located in the south- eastern part of the island (Rayner and Manis 2001; Rayner and Farci 2003; Garbarino et al. 2003; Naitza and Tocco 2004; Funedda et al. 2005). They were de- scribed as belonging to the ‘‘orogenic gold’’ class of deposits (Goldfarb et al. 2001). Such findings represent a new gold target in Sardinia. Previous exploration and exploitation during the 1990s were focused on Tertiary epithermal gold deposits (see Lattanzi 1999; Rayner and Manis 2001; and references therein). Gold represents a recent addition to the metal inventory of Sardinia, the most important mining region for base metals in Italy for several centuries (see e.g. Marcello et al. 1994; Boni et al. 2002; and references therein). This note is a preliminary geochronologic, fluid inclusion and Pb isotopic study of the Monte Ollasteddu deposit, the most prominent discovery among the gold occurrences hosted in the Variscan basement of south- eastern Sardinia. Geological setting and gold mineralization Regional geology Sardinia represents a segment of the Southern European Variscan belt showing a complete sequence from the high-grade metamorphic axial zone in the north to the Electronic Supplementary Material Supplementary material is available for this article at http://dx.doi.org/10.1007/s00126-005- 0021-y Editorial handling: V. Bouchot A. Dini (&) Æ G. D. Vincenzo Æ G. Ruggieri Istituto di Geoscienze e Georisorse–CNR, Via Moruzzi 1, 56124 Pisa, Italy E-mail: a.dini@igg.cnr.it J. Rayner Medoro Resources Ltd., Furtei, 09040 Cagliari, Italy P. Lattanzi Dipartimento di Scienze della Terra, Universita` di Cagliari, Via Trentino 51, 09127 Cagliari, Italy Mineralium Deposita (2005) 40: 337–346 DOI 10.1007/s00126-005-0021-y