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

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

doi:10.4401/ag-3202

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 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.

Highlights

Non-sulfide ores, mainly the supergeneformed types, are rapidly becoming an important source of metallic zinc as well as lead (Large, 2001; Boni, 2003; Hitzman et al, 2003)
We report on a first attempt made to use paleomagnetism for constraining the age of the non-sulfide Zn-Pb ore in the IglesienteSulcis district (SW Sardinia, Italy), where the oxidation of primary sulfides has been related to paleoweathering episodes dating back to Cenozoic and even Mesozoic times (Boni et al, 2003)
The Alternating Field (AF) followed by thermal demagnetization protocol was designed as the most efficient demagnetization treatment, which could provide information on magnetic mineralogy, so that the Characteristic Remanent Magnetization (ChRM) components carried by high-coercivity minerals having low and high unblocking temperatures could be identified

Summary

Introduction

Non-sulfide ores, mainly the supergeneformed types, are rapidly becoming an important source of metallic zinc as well as lead (Large, 2001; Boni, 2003; Hitzman et al, 2003). These authors claim, though not without debate (Kesler et al, 2004), that a Characteristic Remanent Magnetization (ChRM), acquired during the ore minerals forming processes, can be recognized in most ore deposits. In these studies, magnetic susceptibility, demagnetization of the natural remanent magnetization, acquisition of artificial remanences and other magnetic measurements were performed on several thousand specimens of mainly Mississippi Valley Type (MVT) mineralization and gangue, in order to identify their magnetic carriers. The main conclusion is that the paleomagnetic ages of most sulfide deposits seem to be coeval with the later stages of major orogenic events in the same region

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