A FIRST ATTEMPT TO PALEOMAGNETIC DATING OF NON-SULFIDE ZN ORES IN SW SARDINIA (ITALY)

Non-sulfide ores, whose economic focus lies on the supergene-formed types (the "Calamine"), are rapidly becoming an important source of metallic zinc. However, time constraints for the deposition of these ores are still unclear, due to multiple oxidation events through time, which control their Zn-silicates/carbonates paragenesis. Because the timing and evolution of weathering profiles might have important implications for exploration of non-sulfide zinc deposits, it is a priority to obtain a reliable estimate for the age of the oxidation phenomena, either by use of radiogenic isotope systems, or by other unconventional methods.

We carried out a first attempt to use paleomagnetism for constraining the age of the Calamine ore in the Iglesiente district (SW Sardinia, Italy), where the oxidation of primary sulfides could be related to paleoweathering episodes dating back to Tertiary and even Mesozoic times. We collected 37 oriented cores in Cambrian carbonate hosted ore deposits from 3 different sites with (a) non-oxidised primary sulfides and (b) oxidation ore (calamine). Reliable paleomagnetic data were obtained from 18 samples, all from the carbonates hosting the deposits. Three of such samples (same site) show a scattered characteristic 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, that was acquired after the last phase of counterclockwise rotation of Sardinia (that is after 15 Ma), in a time interval long enough to span at least one reversal of the geomagnetic field. In fact, the data show both normal (10 samples) and reverse (5 samples) polarities and pass the reversal test of McFadden and McElhinny, with a classification of Rb. Hematite is the main magnetic carrier in the limestone, whereas (hydrothermal) dolomite contains goethite or a mixture of both. The recognition that goethite and hematite carry the same ChRM also implies that after the last oxidation episode, related to the acquisition of the ChRM, the host rocks were not heated to temperatures higher than 120°C (Neél temperature of goethite).