Annals 48, 2, 2005defdef 301 ANNALS OF GEOPHYSICS, VOL. 48, N. 2, April 2005 Key words paleomagnetism – Sardinia – non-sul- fide ores – calamine 1. Introduction Non-sulfide ores, mainly the supergene- formed types, are rapidly becoming an important source of metallic zinc as well as lead (Large, 2001; Boni, 2003; Hitzman et al., 2003). How- ever, time constraints for the deposition of these ores are still unclear, due to multiple oxidation events through time, which control their sili- cates/carbonates paragenesis. Because the tim- ing and evolution of weathering profiles might have important implications for the exploration of non-sulfide deposits, it is a priority to obtain a reliable estimate for the age of the oxidation phe- nomena, either by use of radiogenic isotope sys- tems, or by other unconventional methods. In this paper, we report on a first attempt made to use paleomagnetism for constraining the age of the non-sulfide Zn-Pb ore in the Iglesiente- Sulcis district (SW Sardinia, Italy), where the ox- idation of primary sulfides has been related to pa- leoweathering episodes dating back to Cenozoic and even Mesozoic times (Boni et al., 2003). 2. Paleomagnetism and ores In the last twenty years, a major effort has been made in dating stratabound, disseminated and massive sulfide ore deposits, as well as their Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt Maria Boni (1), Jaume Dinarès-Turell (2) and Leonardo Sagnotti (2) (1) Dipartimento di Geofisica e Vulcanologia, Università degli Studi di Napoli «Federico II», Napoli, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy Abstract A first paleomagnetic investigation aimed at constraining the age of the non-sulfide Zn-Pb ore deposits in the Iglesiente district (SW Sardinia, Italy) was carried out. In these ores, the oxidation of primary sulfides, hosted in Cambrian carbonate rocks, was related to several paleoweathering episodes spanning from the Mesozoic on- ward. Paleomagnetic analyses were performed on 43 cores from 4 different localities, containing: a) non-oxi- dized primary sulfides and host rock, b) oxidized Fe-rich hydrothermal dolomites and (c) supergene oxidation ore («Calamine»). Reliable data were obtained from 18 samples; the others show uninterpretable results due to low magnetic intensity or to scattered demagnetization trajectories. Three of them show a scattered Characteris- tic Remanent Magnetization (ChRM), likely carried by the original (i.e. Paleozoic) magnetic iron sulfides. The remaining 15 samples show a well defined and coherent ChRM, carried by high-coercivity minerals, acquired after the last phase of counterclockwise rotation of Sardinia (that is after 16 Myr), in a time interval long enough to span at least one reversal of the geomagnetic field. Hematite is the main magnetic carrier in the limestone, whereas weathered hydrothermal dolomite contains goethite or a mixture of both. The results suggest that pale- omagnetism can be used to constrain the timing of oxidation in supergene-enriched ores. Mailing address: Dr. Leonardo Sagnotti, Istituto Na- zionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy; e-mail: sagnotti@ingv.it 302 Maria Boni, Jaume Dinarès-Turell and Leonardo Sagnotti host rocks with paleomagnetic methods (Symons et al., 1996, 2002; Symons and Stratakos, 2000; Leach et al., 2001; Bradley and Leach, 2003). These authors claim, though not without debate (Kesler et al., 2004), that a Characteristic Rema- nent Magnetization (ChRM), acquired during the ore minerals forming processes, can be recog- nized in most ore deposits. In these studies, magnetic susceptibility, de- magnetization of the natural remanent magnetiza- tion, acquisition of artificial remanences and oth- er magnetic measurements were performed on several thousand specimens of mainly Mississip- pi Valley Type (MVT) mineralization and gangue, in order to identify their magnetic carriers. Ther- mal demagnetization and specific rock magnetic analyses show that the ChRM in most deposits, as well as in monomineralic specimens, is carried by single or pseudosingle domain pyrrhotite and/or magnetite. Almost all the paleomagnetic studies used several stability tests (conglomerate, fold, contact and reversals test) to constrain the acqui- sition age of ore remanence (Symons et al., 1996). The main conclusion is that the paleomag- netic ages of most sulfide deposits seem to be co- eval with the later stages of major orogenic events in the same region. Few investigations have been undertaken so far to date non-sulfide deposits, with the excep- tion of few economic laterites and bauxites (mainly aimed to broader geodynamic and/or paleoclimatic studies) and of Proterozoic man- ganese deposits (Evans et al., 2001). 3. Geological setting of SW Sardinia and the non-sulfide zinc-lead deposits 3.1. Geological setting The Iglesiente-Sulcis area in SW Sardinia (fig. 1) is one of the oldest mining districts in the world, where exploitation started in pre-Ro- man times, initially for silver-lead-copper and later for zinc and barium deposits. Due to in- creasing economic difficulties, the last produc- ing mine in the area was closed in 1998. SW Sardinia is dominated by Paleozoic lithotypes, of sedimentary as well as igneous origin, followed by Cenozoic and Quaternary volcanics and sedimentary rocks. Among the Paleozoic rocks, Cambro-Ordovician litholo- gies (limestones and dolomites of the Gonnesa Group) predominate (Bechstädt and Boni, 1994). The Cambrian sediments are the prefer- ential host for both primary and secondary (ox- idated) Zn-Pb ores. Most of the primary ores are stratabound (Sedex + MVT): they consist of Zn-Pb sulfides locally associated with barite, with hydrothermal alteration limited to local dolomitization and silicification (Boni, 1985; Bechstädt and Boni, 1994; Boni et al., 1996). A strongly pervasive hydrothermal dolomiti- zation («Geodic Dolomite»: Boni et al., 2000) of possible Permo-Triassic Age affected large vol- umes of the Cambrian carbonate rocks across the whole district (more than 500 km2). These dolomites are Fe-rich, both replacive and saddle- shaped, frequently with zebra structures. The Late Variscan uplifts in Sardinia were followed by the inset of small porphyry stocks and sever- al pulses of extensional tectonics, which caused repeated circulation of hydrothermal fluids dur- ing the Mesozoic (Boni et al., 1992, 2002), ac- companied by deep karstification of the Cambri- an carbonate rocks, which continued during the Cenozoic. The development of karstic networks in the Cambrian limestone, was favored in all periods by the aggressive character of the circu- lating waters and by the high sulfide content of the carbonate rocks undergoing dissolution. A phase of widespread tensional tectonics in the Middle Oligocene produced a large rift system locally accompanied by the deposition of continental sediments. This was followed by a major episode of Oligocene-Miocene volcan- ism, correlated to a drift-related ∼50° counter- clockwise rotation of the Corsica-Sardinia mi- croplate (Gattacceca, 2001). The spreading in the Liguro-Provençal Basin and the rotation of Corsica-Sardinia, ended no earlier than 16 Myr (Early Langhian; see Speranza et al., 2002) in response to the retreat of a westward dipping subducting slab (i.e. Faccenna et al., 2004). During Miocene and Pliocene, SW Sardinia was subjected to several emersion-erosion phas- es, alternating with marine transgressions, dra- matic climatic changes (Messinian evaporites) and volcanic episodes of calc-alkaline to alka- line basalts. A last Plio-Pleistocene tensional 303 Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt tectonic phase was responsible for the differen- tiated uplift of the Paleozoic basement, which was fragmented into a series of stepped fault blocks in both the Iglesiente and Sulcis areas. 3.2. Non-sulfide deposits The «Calamine»-type ore is considered to be the result of supergene oxidation of primary carbonate-hosted sulfide mineralization, with subsequent remobilisation of zinc and partly of lead, followed by redeposition of secondary ox- idized minerals either in situ as gossans, or as reaction products within the hosting carbonates. In the latter case, they not only replace substan- tial areas of the fractured and karstified carbon- ate rocks, but fill as internal sediments their dis- solution vugs and karst cavities, producing sec- ondary enrichments also at greater distances from the primary sulfide bodies (Large, 2001; Hitzman et al., 2003). Historically, the «Calamines», consisting of a mixture of zinc carbonates and hydroxy-car- bonates and silicates (with minor Pb), capping in the whole area the primary sulfide bodies, were the principal source of zinc of the Igle- siente-Sulcis district (Moore, 1972; Boni et al., 2003). The most important mines (fig. 1), where non-sulfide ores have been exploited for several hundred of meters in depth from the present sur- face, were: San Giovanni, Monteponi and Cam- po Pisano in the Iglesias valley, as well as the Nebida-Masua mineralized complex along the western coast. Smithsonite, hydrozincite, and hemimorphite were the principal zinc-bearing economic minerals. Cerussite, anglesite and the rare mineral phosgenite were also common, Fig. 1. Geological sketch map of the Iglesiente mining district, with the location of the sampled sites (modified from Boni et al., 2003). Abbreviations: 1 – overthrust; 2 – normal fault; 3 – Cenozoic; 4 – Mesozoic; 5 – Variscan granites; 6 – Palaeozoic (allochthonous); 7 – Ordovician to Devonian succession; 8 – Iglesias Group (Middle Cambrian-Lower Ordovician); 9 – Gonnesa Group (Lower Cambrian); 10 – Nebida Group (Lower Cambrian). 304 Maria Boni, Jaume Dinarès-Turell and Leonardo Sagnotti generally associated with nodules and lenses of residual galena and a range of fairly exotic species (Billows, 1941; Moore, 1972; Stara et al., 1996; Aversa et al., 2002). A complex as- semblage of iron and manganese oxy-hydrox- ides characteristic for its red-brown staining (goethite, lepidocrocite, hematite) and residual clay minerals accompany the non-sulfide ore. An interesting parallel effect of the oxidation of the sulfide ore, is the supergene alteration of the ferriferous hydrothermal dolomites («Geod- ic Dolomite»), which are the host rock of most sulfide deposits in the Iglesiente district. In fact, these dolomites, are not only Fe-rich (some of them approach ankerite composition), but con- tain small pyrite grains. When affected by su- pergene processes, the «Geodic Dolomite» turns reddish-brown, its disseminated pyrites alter to goethite and hematite and a patchy de-dolomiti- zation process starts locally. These effects in the dolomite host rocks can be followed deep down, at least until the same levels in the mines where the «Calamine» mineralization has been exploit- ed (e.g., level −100 of the Monteponi mine). We emphasize that we consider the weathering of the «Geodic Dolomite» not related to recent sur- ficial weathering only, but contemporary with the formation of «Calamine» ore. When investigating the timing of the non- sulfide deposits in a given area, one must con- sider the ages of the various stages of tectonic uplift and peneplanation. To document the ma- jor constraints on the oxidation and preserva- tion of the deposits, it is also necessary to relate them to climatic oscillations both at a global and local scale. What makes this task difficult in SW Sardinia, is the lack of precise geologi- cal constraints on the development of the nu- merous karstification phases that are closely re- lated to formation of the «Calamine» ores. Karstic dissolution related to uplift of the Pale- ozoic carbonates, occurred repeatedly: 1) dur- ing Permo-Mesozoic, 2) several times during Cenozoic, and 3) after the Plio-Pleistocene block tectonics until the Present. However, the most reliable time span we can envisage from the limited geological constraints, in which both tectonic and climatic conditions were favorable for the formation of the non-sul- fide deposits ranges from Middle Eocene to Plio-Pleistocene. The latter was the time when the differentiated uplift of distinct sectors of the Paleozoic basement displaced at very different levels the base of the weathering profiles in the various areas of the Iglesiente-Sulcis. Our aim was to use paleomagnetism to pro- vide constraints on the age(s) of the oxidation process, evaluating the characteristic remanent magnetization carried by various iron minerals, like hematite, goethite and, in case, of the re- maining as well as of newly formed magnetic iron sulfides. Sardinia has been the subject of several paleomagnetic studies since the 1960’s and a reliable dataset of paleomagnetic results is available (see van der Voo, 1993; Speranza, 1999; Speranza et al., 2002 and references there- in), referring to a wide age range (Paleozoic to Present), to provide a suitable reference for com- parison with the new data. 4. Sampling strategy Samples were collected using a portable gasoline-powered drill and were oriented in situ with a compass mounted on an inclinometer. 43 oriented cores were collected from 4 sites in the surroundings of Iglesias, either on outcrops (sites IG01 and IG04) or inside two abandoned mines (sites IG02 and IG03). Site IG01 - Weathered «Geodic Dolomite» along the «Strada Camionabile» (Nebida) – The sampling took place shortly east of the village of Nebida, in a small valley cut in the carbonates (39°20′N-8°27′E), where patchy outcrops of hy- drothermal dolomite («Geodic Dolomite») on the Cambrian limestone are clearly exposed. The contacts with non-dolomitized host rock are sharp, locally marked by a slight silicification. Vertical schistosity planes mostly control the dolomite bands; the control is rarely made by fractures. The crystals are typically saddle- shaped, lining concretionary voids and small horizontal cavities. The weathered hydrothermal dolomite is brown-reddish due to its high Fe-oxide content. Locally, the dolomitization affects a post- Variscan breccia, whose carbonate clasts show schistosity and traces of primary sulfide crys- 305 Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt tals, that have also undergone weathering and oxidation processes. However, some of the breccia clasts have not been dolomitized and still consist of white Cambrian limestone. 21 cores were sampled at this site, in vary- ing lithologies: 6 samples represent the Cam- brian limestone, with oxidized sulfides, 9 sam- ples were collected from the weathered «Geod- ic Dolomite» and 6 samples were drilled in the carbonate clasts, with traces of primary sulfides crystals, composing a brecciated patch. Site IG02 - «Zona Ossidati Piombiferi», lev- el + 150, San Giovanni mine (Iglesias) – The sampling was performed underground in the ox- idation area of Idina-Sant’Anna of the San Gio- vanni mine (39°15′N- 8°27′E). These are the up- per levels of the mine and contain only local rem- nants of sulfides (galena), prevailing Fe-oxides- hydroxydes, clay-infillings and limited concen- trations of Zn- and Pb-carbonates. It is probable that in this area of the mine, the primary orebod- ies consisted not only of the stratabound Zn-rich Pre-Variscan mineralizations, but also of the Post-Variscan (Ag)-galena bodies in paleokarst breccias. Host rocks are the Cambrian limestones and the weathered hydrothermal dolomites. 7 cores were sampled at this site: 5 of them represent the mineralized dolomite breccias, 1 was drilled in a clast of Cambrian limestone and 1 in the weathered hydrothermal dolomite. Site IG03 - Zona «Calaminari», level +92, Nebida mine (Nebida) – The sampling has been performed underground at the +92 level of the Nebida mine (39°25′N-8°25′E). In this level, sit- uated well above the present water table, there are almost no sulfides left, whereas in the lower levels, a continuous transition to Zn-Pb sulfides takes place. The direction of the oxidized ore bodies, which replace the primary sulfides as well as part of the carbonate host rocks, is N-S, which is the general tectonic trend of the whole area. The carbonate host rock is heavily dolomi- tized («Geodic Dolomite» replacing hydrother- mally Cambrian limestone), but also karstified and altered to a mixture of de-carbonated mate- rial, Zn-carbonates and hydrocarbonates, Zn-sil- icates, Pb-carbonates and sulfates, and Fe ox- ides/hydroxides. Beyond the described occur- rences of non-sulfide mineralization at the level +92, allochthonous concentrations of ferrugi- nous, «earthy» smithsonite and hemimorphite- rich clays, fill a maze of interconnecting karst cavities and open conduits in this, as well as in the other upper levels of the mine. 9 cores were sampled at this site, all in the dolomite hosting the non-sulfide Zn ores. Site IG04 – San Giovanni mine (Iglesias) – In this site, behind the old building of the mine direction (39°15′-8°27′), only the white lime- stone belonging to the upper stratigraphic part of the Lower Cambrian succession (Gonnesa Group) was sampled. In this outcrop the lime- stone is strongly deformed by Variscan tecton- ics (penetrative schistosity and recrystalliza- tion), not dolomitized neither mineralized. In this site 6 cores were sampled. 5. Methods In the laboratory two to three standard cylin- drical specimens of 2.54 cm in diameter and 2.20 cm length were cut from each sample. Nat- ural Remanent Magnetization (NRM) was meas- ured on a 2G Enterprises DC SQUID cryogenic magnetometer operating in a magnetically shielded room at the Istituto Nazionale di Geo- fisica e Vulcanologia (INGV) in Rome, Italy. A Pyrox oven located in the shielded room was used for thermal demagnetization. Alternating Field (AF) demagnetization was performed with three orthogonal coils installed inline with the cryogenic magnetometer. We first analysed the behaviour of 10 couples of pilot «sister» speci- mens (i.e. standard paleomagnetic cylinders cut from the same core). Of each couple, one speci- men was subjected to stepwise AF demagnetiza- tion up to 100 mT, the other to thermal demag- netization up to 650ºC. On the basis of the pilot study results, the remaining specimens followed an hybrid AF and thermal demagnetization pro- tocol. In this case, stepwise AF demagnetization was first applied and consisted in 9 steps at in- crements of 5 mT in the interval 5 to 30 mT and then increments of 10 mT up to 60 mT. Samples were subsequently demagnetized at 60°C, 80°C, 100°C, 120°C, 200ºC, 300°C, 400°C, 500°C, 306 Maria Boni, Jaume Dinarès-Turell and Leonardo Sagnotti 580°C, 610°C and 640°C. Magnetic susceptibil- ity was measured after each heating step in order to monitor alteration and/or neoformation of magnetic minerals during the thermal treatment. The AF followed by thermal demagnetization protocol was designed as the most efficient de- magnetization treatment, which could also pro- vide information on magnetic mineralogy, so that the ChRM components carried by high-co- ercivity minerals having low (goethite) and high (hematite) unblocking temperatures could be easily identified. Low-coercivity minerals like magnetite and magnetic iron sulfides (pyr- rhotite, greigite), instead, would have been most- ly demagnetized below 60 mT upon the AF rou- tine. Orthogonal vector demagnetization plots (Zijderveld, 1967) were used to represent de- magnetization data, and principal component analysis (Kirschvink, 1980) was used to identify the characteristic remanent magnetization. The mean directions were calculated using Fisher (1953) statistics. The reversal test of McFadden and McElhinny (1990) was applied to assess the antipodality of the normal and reverse means. 6. Results The AF demagnetization behaviour indicated that for most samples the main magnetic carriers are high-coercivity minerals only (i.e., the NRM intensity shows very low to null decrease in AF up to 100-150 mT). For such samples, therefore, magnetic components were retrieved after ther- mal demagnetization (figs. 2a-c to 5a-c). The measurements of magnetic susceptibility after Fig. 2a-c. Stepwise demagnetization data for sample IG01-01A (non-dolomitized brecciated limestone, Nebi- da «Camionabile»). From left to right: a) normalized intensity plot, the intensity of the Natural Remanent Mag- netization (NRM) at the beginning of the treatment is also indicated; b) equal area projection in geographic co- ordinates (open circles – upper hemisphere, reverse polarity); c) orthogonal vector diagrams in geographic co- ordinates (open circles – vertical projection; full circles – horizontal projection). The AF demagnetization treat- ment (upper diagrams) essentially did not affect the NRM of the sample, whereas the sharp drop in NRM in- tensity during the thermal treatment between 610°C and 640°C (lower diagrams) indicate that the characteristic component of the NRM is of reverse polarity and is carried by hematite. a b c c a b 307 Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt Fig. 3a-c. Stepwise demagnetization data for sample IG01-03B (non-dolomitized brecciated limestone, Nebi- da «Camionabile»). Plots and symbols as in fig. 2a-c (full circles in the equal area projection (b) indicate pro- jection on the lower hemisphere, normal polarity). The sample shows essentially the same behavior of sample IG01-01 (see fig. 2a-c), but its characteristic component of the NRM is of normal polarity. Fig. 4a-c. Stepwise demagnetization data for sample IG02-07A (San Giovanni mine, level +150). Plots and symbols as in fig. 2a-c. The NRM of the sample is essentially not affected by AF treatment and shows a sharp drop in intensity just after the first heating step (60°C) and is almost fully demagnetized at 120°C, indicating that the characteristic component of the NRM is of reverse polarity and is carried by goethite. a a b b c c a b c a b c 308 Maria Boni, Jaume Dinarès-Turell and Leonardo Sagnotti each heating step indicate that neoformation of magnetic minerals occurs above 500°C. In any case, no significant change in paleomagnetic di- rection was detected in association to such ther- mal alteration. Reliable paleomagnetic data were obtained from 18 samples out of a total of 43 (table I). No tectonic correction was applied to the paleo- magnetic data. The white-coloured, non-miner- alized limestones (samples 13-18 taken in the carbonate clasts with traces of primary sulfides crystals at site IG01-Nebida and all samples from site IG04-San Giovanni), show very low NRM intensities (of the order of a few 10–5 A/m) and irregular behaviour during progressive de- magnetization. No reliable paleomagnetic data could be obtained from these samples. The weathered hydrothermal dolomites sam- pled at Nebida, site IG01, are generally reddish and show demagnetization behaviours indicating that their main magnetic carriers are high-coer- civity minerals only (the NRM intensity shows very low to null decrease in AF up to 150 mT). The behaviour during thermal demagnetization indicates that dolomite samples contain a mix- ture of goethite (indicated by a significant loss of the NRM intensity at 80-120°C; i.e. Dekkers, 1988, 1989) and prevailing hematite (as indicat- ed by complete demagnetization at temperatures of 640-680°C; i.e. Dunlop, 1971). The preva- lence of hematite is evident in the non-dolomi- tized limestones containing weathered crystals of former pyrite, sampled in the same locality (i.e. IG01-01 and IG01-03, figs. 2a-c and 3a-c). The narrow range shown by the unblocking tempera- ture of hematite is possibly related to its pseudo- morphic nature on precursor pyrite crystals. The mineralized limestones lacking sulfides sampled in both mines (sites IG02 and IG03) are also characterized by high-coercivity magnetic Fig. 5a-c. Stepwise demagnetization data for sample IG02-02A (San Giovanni mine, level +150). Plots and symbols as in fig. 2a-c. The NRM of the sample is reduced at ca. 50% of its initial value after AF treatment in peak field up to 100 mT. The following thermal treatment pointed out a sharp drop in intensity between 300°C and 400°C, after which the sample is essentially demagnetized, indicating that the characteristic component of the NRM is likely carried by intermediate-coercivity magnetic iron sulfides (pyrrhotite ?). The direction of the characteristic remanence is however aberrant. a b a c cb 309 Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt Table I. Summary of reliable paleomagnetic data. Specimen Lithology Decl ChRM Incl ChRM MAD Mag. Min. (°) (°) (°) IG01-01 Cambrian limestone 179.6 − 53.9 4.9 Hematite IG01-02 Cambrian limestone 12.9 44.5 3.1 Hematite IG01-03 Cambrian limestone 3.1 50.5 2.8 Hematite IG01-04 Cambrian limestone 17.6 47.5 4.0 Hematite IG01-05 Cambrian limestone 356.8 50.7 5.8 Hematite IG01-07 Weathered dolomite 347.7 48.2 7.0 Magnetite IG01-09 Weathered dolomite 182.9 − 67.9 3.2 Hematite IG01-12 Weathered dolomite 3.4 52.0 8.5 Goethite + hematite IG01-20 Weathered dolomite 352.4 51.1 5.3 Goethite + hematite IG01-21 Weathered dolomite 8.7 33.1 4.4 Goethite IG02-01* Mineralized dolomite breccia 160.9 − 26.1 5.9 ? Fe-sulfides IG02-02* Mineralized dolomite breccia 222.1 47.3 3.3 ? Fe-sulfides IG02-03 Mineralized dolomite breccia 169.7 − 58.8 2.7 Goethite IG02-04 Mineralized dolomite breccia 169.6 − 46.4 6.3 Goethite IG02-06* Cambrian limestone 228.8 50.6 6.0 ? Fe-sulfides IG02-07 Weathered dolomite 170.2 − 44.5 4.3 Goethite IG03-02 Weathered dolomite 0.9 52.3 6.8 Hematite IG03-06 Weathered dolomite 357.0 41.4 7.1 Hematite Mean (N == 15) 359.7 49.9 α 95 = 4.7 Decl – declination; Incl – inclination; ChRM – Characteristic Remanent Magnetization; MAD – Maximum An- gular Deviation from principal component analysis (Kirschvink, 1980); Mag. Min. – main magnetic minerals carrying the ChRM (see text); * – discarded samples, whose ChRM is likely carried by magnetic iron sulfides; α 95 – half-angle of the 95% confidence circle about the mean direction (Fisher, 1953). carriers, with a very low to null NRM intensity loss up to 150 mT AF. Their behaviour during thermal demagnetization indicates that their main magnetic minerals are hematite (that is, most of the remanence is lost between 580°C and 640°C) and/or goethite (i.e. sample IG02- 07, fig. 4a-c). Generally, however, it has been observed that the prevailing magnetic carrier is goethite at site IG02 (San Giovanni mine) and hematite at site IG03 (Nebida mine) (table I). Three samples among those cored at the site IG02 (San Giovanni mine, level +150) show a Characteristic Remanent Magnetization (ChRM) likely carried by magnetic iron sulfides (thermal demagnetization achieved at 200-350°C; i.e. sample IG02-02, fig. 5a-c), whose directions were not coherent among the 3 samples (fig. 6 and table I). The «anomalous» paleomagnetic directions observed in such sulfides-dominated samples are likely due to their acquisition be- fore tilting of the host-rocks (i.e. magnetic iron- sulfides are of Paleozoic Age). Structural com- plexity of the area and lack of precise elements to define a simple tectonic correction to apply to the data prevent further speculation about pa- leomagnetic data from those three samples. Paleomagnetic data from all the remaining 15 samples, indicate that the high-coercivity minerals provided a well defined and coherent ChRM that was acquired after the rotation of Sardinia (that is after 16 Myr) and in a time in- terval long enough to span at least one reversal of the geomagnetic field. In fact, 10 samples dis- play a normal polarity ChRM and 5 specimens a reverse polarity ChRM (fig. 6). Assuming that, by chance, the oxidation event started just be- 310 Maria Boni, Jaume Dinarès-Turell and Leonardo Sagnotti fore a reversal and ended soon after it, a lower limit of the order of 5-10 kyr can be placed for the duration of such oxidation event. No upper limit can be set in view of the paleomagnetic data. The mean value calculated on such high-co- ercivity ChRM, after transforming all polarities to normal is: declination = 359.7°, inclina- tion = 49.9°, α 95 = 4.7°, k = 68.2, N = 15 (fig. 7 and table I). The data pass the reversal test of Mc- Fadden and McElhinny (1990), with a classifica- tion of «B» (i.e. the observed angle between the mean of the normal polarity set and the mean of the reverse polarity set is 8.9° and the critical an- gle for the reversal test is of 9.3°). Such ChRM was recognized as carried by hematite (or a mixture between goethite and pre- vailing hematite) in 10 samples, by only goethite in 4 samples (3 of which with reverse polarity, all from the San Giovanni mine, site IG02, where hematite was not recognized as a main magnetic carrier) and by magnetite in 1 sample (from the reddish dolomites at site IG01). 7. Conclusions Despite the limited dataset due to the scarci- ty of accessible «exposures» or mining sites, Fig. 6. Equal area projection in geographic coordinates (open circles – upper hemisphere, reverse polarity; full circles – lower hemisphere, normal polarity,) of reliable paleomagnetic data for sites IG01, IG02 and IG03. El- lipses around individual directions indicate the maximum angular dispersion on the mean paleomagnetic direc- tion computed by principal component analysis (Kirschvink, 1980) on the demagnetization data. At site IG01 the Characteristic Remanent Magnetization (ChRM) are carried by hematite and goethite and define two antipodal clusters of normal and reverse polarity along the N-S axis. At site IG02 the ChRM are carried either by goethite (3 samples) or magnetic iron sulfides (3 samples). The first group defines a cluster of reverse polarity close to the N-S directions, the ChRM of the second group are dispersed and far from the N-S direction. At site IG03 only two reliable ChRM were obtained, which are similar and of normal polarity close to the N-S axis. Fig. 7. Equal area projection and statistical param- eters (Fisher, 1953) of all the reliable paleomagnetic data carried by high-coercivity oxi-hydroxides for sites IG01-IG03. Geographic coordinates, all data flipped to normal polarity. Ellipses around the indi- vidual directions indicate maximum angular disper- sion on the mean paleomagnetic direction computed by principal component analysis (Kirschvink, 1980) on the demagnetization data. The star indicates the direction of the geocentric axial dipole field in the study area. 311 Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt some interesting conclusions can be drawn from the paleomagnetic data obtained in this study. First of all, in both the non-sulfide ore bod- ies and in their host rocks consisting of deeply weathered ferroan dolomites, a complex mag- netic mineralogy could be detected. The most common magnetic carrier is hematite, followed by goethite; in a few samples it was even possi- ble to detect primary (?) iron sulfides. These different magnetic mineralogical phas- es could be related to distinct weathering episodes, which took place repeatedly during the geological evolution of the area. However, the last measura- ble oxidation event happened for sure after the ro- tation of the Corsica-Sardinia continental block (Early Miocene, ca. 16 Myr), as indicated by the clustering of the paleomagnetic directions on the present-day geocentric axial dipole field. Because some of the measured samples show inverse polarity, the last oxidation episode must have started before ca. 780 kyr (age of the Brun- hes-Matuyama boundary; see Tauxe et al., 1996). However, it could have been possible that the last oxidation episode lasted much longer in time and encompassed one or more polarity re- versals of the geomagnetic field. Based on geological reasoning only, we can argue that the most probable age for the bulk of supergene non-sulfide Zn-Pb ore in SW Sar- dinia, as well as for the weathering and «redden- ing» of the hydrothermal «Geodic Dolomite» host rocks, was comprised in a time span be- tween Middle Eocene and Plio-Pleistocene (Boni et al., 2003). By paleomagnetic methods, we could not narrow this interval, but only con- firm that at least part of the oxidation episodes were younger than the Early Miocene (Early Langhian) and likely extending back quite far from the Present. The observed reactivation of the alteration profiles in the whole mining dis- trict, which caused the replacement of earlier- deposited supergene mineral phases by others adapted to new hydro-geochemical conditions, could also have caused the precipitation of sev- eral generations of magnetic minerals presently coexisting in the same oxidized lithotype. Fur- thermore, the recognition that the goethite-dom- inated samples from site IG02 show reverse po- larity implies that the host-rocks were not sig- nificantly heated (the 120°C Neél temperature of goethite drops radically in goethites substituted with diamagnetic elements, see Mathé et al., 1999) during at least the last 780 kyr. Even though a precise age determination could not be reached for the genesis of non-sul- fide Zn-Pb ores in the Iglesiente district, the pale- omagnetic method, together with radiogenic iso- tope systems and fission tracks, could be a pow- erful tool to establish the timing of the weathering profiles (Théveniaut et al., 2002) associated to non-sulfides in other mineralized areas with a less complex geological history than SW Sardinia. Acknowledgements The authors wish to thank the IGEA Com- pany (Iglesias, Sardinia) for having granted ac- cess to their properties and R. Sarritzu for his help during the sampling campaign. We thank F. 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