RENDICONTI DELLA SOCIETA ITALIANA DI MINERALOGIA E PETROLOGIA, 1988, \bI. 4'.2, _pp. 44~·462 Variscan Skarn Ores in South·West Sardinia: their relationships with Cambro·Ordovician stratabound deposits FABIO AroNTE, GruSEPPINA BALASSONE, MAJuA BONl, LUCA CoSTAMAGNA, GroVANNl DI MAID Dipartimc:nto Scienzc della Terra dell'Universit1l di Napoli, Largo 5. Marcellino 10,80138 Napoli AI.ISTltACT. - In South-West Sardinia, in addition to sediments (BoNJ, 1985), other types of the Lower Paleozoic matabound sedim.entary-diagenetic mineralization, smaller in size and therefore ore deposits, some mineralized skarn occurrences exist, associated with pall-tectonic Variscan intrusions of of lesser economic importance, have been leucogranites. Their metal content ranges from Fe-Zn­ exploited near major intrusive bodies of Pb·Ba (already ptesent in the Lower Paleozoic Variscan age (Fig. 1). The mineralizations associations) to F and Cu (with minor As, Mo, Wand consist of high temperatW'e ores, ranging from Bi). The metamorphic·metasomatic association consists skarn to hydrothermal deposits, containing of garnets, clinopYroxenes and wollastonite, often replaced by amphiboles, epidotes ard, lastly, chlorites. Cu, F, As with minor amounts of W, in Thermometamorphic evolution, calculated at pressures addition to Fe, Zn, Pb and Ba which are of about 1 Kb, may be subdivided into three stages: normally present in the mineralization of metamorphic (T > :;:;O°C), metasomatic {T > 4OQ°Cl, Cambrian and Ordovician age. The type of hydrothermal (T < 375°C). The miIll:ralization is linked to the skarn !ithotypes, whi£h acted as a S1tuetural guide mineralization discussed is generally hosted and a trap. We can distinguish in each area between in rocks which were affected by high-grade metamorphic (along stratigraphic contacts) and vein contact- metamorphism. Calc-silicate layers, skarns (KERRlCK, 1'J77). Most of the ore minerals, typical of skarn elsewhere in the world, are consisting of several generations of pyrite, magnetite, pyrrhotite, sphalerite, chalcopyrite and galena, each present in these rocks, and they have showing about the same paragenetic order of deposition recognizable sedimentary and/or diagenetic in all the studied areas, are related to the metasomatic structures, as fwnd in the older sedimenrs and and hydrothermal stages. associated diagenetic ores. It is interesting to Ore genesis has been envisaged through a remobilization of metals contained in the Lower note that, with only few exceptions, this Paleozoic stratabound deposits, with a minor mineralization can be found in the same contribution of elements from the intntding magmatic srratigraphic position as the lower Paleozoic bodies and/or intruded host rocks. stratabound deposits. Key words: South·West Sardinia, Variscan, Skaro, Ore On the other hand, the sedimentary fades deposits. hosting the non-metamorphic stratabound ores, and those affected by metamorphism, Introduction are quite different, so that any meaningful comparison between the ores is of necessity In South-West Sardinia, in addition to the incomplete, whereas their affinity can be only well-known stratabound ore deposits in the casual. Furthermore, ther~ are zones, like that Lower Cambrian and Upper Ordovidan of SE Sulcis, where the mineralization only 446 E APONTE, G. BALASSONE. M. "BQNI, L. COSTAMAGNA. G. DI MAlO occurs where the host rocks were changed to fragmentary, mineralized as well as skam, whereas in the stf'atigraphically unminera1ized, were reconstructed, particular corresponding horizon no mineralization was attention being focused on the developed. In other llI"fiS. such as the sedimentological characteristics. These Fluminese region, the chief constraints of the sequences have been tentatively compared and ores and hosting skarns are tectonic, as well carefully sampled. Studies of both polished as the change of permeability betw«n and thin sections and X-ray analyses were adjacent lithotypes. carried out. Fluid inclusion studies and The overall genetic framework of the high­ microprobe analyses of the skam minerals are temperature ores of SW Sardinia is therefore planned for future work. not simple, and is open to different interpretations. Some authors (e.g. ClAMPI, 1909; DRAS, 1957; VIOW & ZueCHINI, 1965) Geological setting of the Lower Paleozoic consider the ores of complete epigenetic terrane origin. Others (e.g. BENZ, 1964; DRAS et al., Within the autochtonous Cambrian 1965) have suggested that the skam ores are sequence of SW Sardinia (COCOZZA, 1979; products of metamorphism, which combines BECHSTAEDT et al., 1985; 1988) (Fig. 2a) thick the elements of the pre-existing mineralization carbonates (GoDnesa Formation) developed with new elements scavenged by hot fluids on top of carbonate-dastic sediments (Nebida from the hosting sedimentary rocks, with Formation), both being of Lower Cambrian perhaps an additional contribution from the age. The carbonate sequence is overlain by nearby magmatic intrusives. nodular limestones (Campo Pisano In our approach to the problem, we Formation), followed by deeper water clastics undertook a careful examination of the host (Cabitza Formation), comprising the time­ rocks and the mineral paragenesis of the ores span from Middle Cambrian to Lower in various districts. These studies were Ordovician. During the Lower Cambrian SW complemented with sedimentological work Sardinia was an epicontinental region. and integrated with the accepted Deposition took place in shallow waters, paleogeographic setting of the Lower bordered by an erosional area from the E to Paleozoic terranes. The genetic models were SE. With time a shallow ramp developed into then discussed in the light of the acquired a carbonate platform, due to its isolation from data. the source of the clastics. This isolation was caused by tensional tectonics, resulting in the formation of a basin between the platform and Methods of .tudy the (still exposed?) basement (BECHSTAEDT et The area of study was subdivided into al., 1988). Tensional tectonics were active three districts: Fluminese, Oridda and SE from late Nebidan times onwards, as shown Sulcis (Fig. 1), each showing different for instance by slump structures. Crustal characteristics. stretching was especially active in Gonnesan Accurate mapping on a scale of 1:10.000 times, as indicated not only by slope sediments was carried out in the above-mentioned with their breccias and mass roovements, but districts, in areas underlain by non­ also by ruptural events, which affected the metamorphic and high-temperature platform, and its margins. metamorphic rocks. In places, the latter areas Numerous stratabound Pb-Zn and Ba lie close to granitic outcrops. 10 othen, the deposits of Lower Cambrian age are known granitic rocks do not outcrop, and the and have been partially exploited (BRuSCA & distribution of skarns and homfe1s are the DESsAu, 1968). These deposits occur within only evidence for an intrusive body at depth. the carbonate sequence of the Gonnesa Fm, Where possibile, underground mapping and locally also within the upper parts of the was also carried out, e.g. in the Su Zurfuru Nebida Fm (Fig. 2b). They were defined by mine. Stratigraphic sequences, in places Bom (1985) as of Mississippi Valley type. VARISCAN SKARN ORES IN SOUTH·WEST SARDINIA; THEIR RELATIONSHIPS WITH ETC. 447 CD~+b" +++ + "" S rd inia \ ~ CA, l IA--::-::"l.-.o :~'~ Pecor. """""",, ";1> -R.P1 I o N DDA y SAD " o km 10 1 I r:t+'l ~ 1 2 3 4 5 F18. 1. - Geological map of SW Sudinia: in the squares are the investi••cd areas. 1) Upper Ordovician; 2) Cambrian I.mver Ordovician; J) Post.ltynematic: p-anitcS ~tes); 4) Synkynernatic: granites;.5) Main (auIl lines. 448 F. APONTE, G. BAtASSONE, M. BONI, L. COSTAMAGNA, G DI MAID ---'--. '. '. ;....;.:.:... --- ---,-' '---..:. • '. • ••• • '00 CD z .. --.~ CD Om A B c A. SEE TEXT AND BECHSTAEDT ET AL. (in press.l ~ B. SEE: BONI (19851; 1---1 SARlTE:; ~ Pb - Zn - Ft SULPHIDES. I·::··I:··::;~I <:..0':"'"-' r LEUCOGRANITES; I'i.·.~r·.J"''''1 VEIN SKARN; METAMORPHiC AND VEIN SKARN; G) SKARN OCCURRENCES. Fig. 2. - Idealized profiles of the Camb-o-Ordovician in SW Sardinia. A) General profile with sedimentary fades; Bl Enlarged profile of the mineralized interval with the location of the stratabound ores; C) Same profile with the location of the skarn ores: 1) Chia, 2) Mt. Flacca, Is Carillus, S. Luda (Teulada), Fuone Morettu, Sideriu Boi, 3) Sa Palma, 4) S. Luda (Fluminimaggiore), :5) Su Zurfuru, 6) Gutturu Pala, Reigraxius, Barraxiuua, Perdu (ana, 7) Arenas p.p., Pcrda Niedda, Tiny, 8) Arenas p.p., 9) Roja Piccioru. Recently, and a[ leas[ for some of these ores, They consist of barhe and layers of galena and a complex genedc history is envisaged sphaleri[e. Locally these deposits are (BECHSTAEDT &. BONI, in press). Only sparse associated with massive pyrite-marcasite, mineralized occurrences are known in [he showing evidence of colloforrn structures and uppermost carbonate levels of [he Nebida Fm. diagenedc breociation. In Southern Sulds VARISCAN SKARN ORES IN SOUTH·WE.ST SARDINIA; THEIR REU.TIONSHIPS WITI'! ETC. 449 these mineral occurrences are also associated phases), magmatism (Variscan granites) and with the upper carbonate lenses, hut are different stages of fluid circulations (BONJ, completely metamorphosed and also contain 1985). copper minerals. Some stratiforrn barite bodies and massive sulfides are found intercalated in the dolomite sequences of the Variscan tectonics and metamorphism in lower Gonnesa Fm (Dolomia Rigata), whereas South-West