Abstract

This paper describes the mineral changes associated with the thermal demagnetization of sideritic jasper mesobands in the ~3.25 Ga Manzimnyame Iron Formation Bed of the Fig Tree Group in the Barberton Greenstone Belt. The study was initiated to determine the timing of hematite formation using paleomagnetic fold and conglomerate tests. If the hematite could be proven to be of early diagenetic sedimentary rather than late post-depositional in origin, it could hold important clues about redox conditions in Early Archaean depositional basins. However, the stepwise demagnetization of samples was characterized by an increase of magnetic intensity moment from heating steps above 400°C, with erratic directions related to cooling down of samples in a magnetically-shielded room. Follow-up thermal rock-magnetic studies combined with X-ray diffraction identification of mineral phases in unheated samples and samples that were cooled down after heating to 700°C, indicated that the siderites reacted to form new magnetic iron-rich spinels during heating. Some of the siderites are Mg-poor and gave rise to the formation of magnetite with a Curie temperature (TC) of ~575°C. In contrast, Mg-bearing siderites apparently formed magnesioferrite with a TC of ~550°C. Hematite, with a TC of ~680°C, persisted as a stable mineral during the entire heating cycle. Maghemite is another spinel that formed apparently at the expense of magnetite during cooling of samples from below 575°C. These newly-formed iron spinels acquired strong erratic magnetizations that concealed the pre-existing remanence of hematite. The timing of formation of the hematite nor its in situ magnetic remanence could thus not be determined. Our results can, however, be used to develop alternative techniques to establish the remanence of hematite not only in the Manzimnyame jaspilite but also in other Precambrian iron formations in general.

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