Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt

M. Boni, J. Dinarès-Turell, L. Sagnotti


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 onward.
Paleomagnetic analyses were performed on 43 cores from 4 different localities, containing: a) non-oxidized
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 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, 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 paleomagnetism
can be used to constrain the timing of oxidation in supergene-enriched ores.



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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X