A palaeomagnetic study of hydrothermal activity and uranium mineralization at Mt Painter, South Australia

Abstract

Palaeomagnetic measurements indicate that the uraniferous Radium Ridge Breccias near Mt Painter in the northern Flinders Ranges of South Australia have been magnetized twice, both times in the Permo‐Carboniferous. The palaeomagnetic south pole for one remanence was estimated as 165.1°E, 65.7°S (A95 = 11.5°). The direction for the other remanence was not well defined; however samples collected from diamictite bodies within the breccia gave 133.9°E, 33.1°S (A95 = 6.9°) as the probable pole for the overprint. The latter is similar to overprint poles published from central Australia, which are generally attributed to the Alice Springs Orogeny. Magnetization directions interpreted as Permo‐Carboniferous were obtained also from the Mt Gee Sinter (a quartz‐hematite‐rich chemical sedimentary rock which overlies the Radium Ridge Breccias) and from U‐mineralized hematitic ironstone bodies within the breccia. The magnetizations of both units were probably coeval with the younger magnetization of the Radium Ridge Breccias. A positive fold test demonstrates that the remanence of the sinter is primary, indicating a major hydrothermal event in the Permo‐Carboniferous. It is not clear by how much this event post‐dates the deposition of the underlying Radium Ridge Breccias and hematitic ironstone. The latter may be Ordovician or older if earlier monazite U‐Pb data are correct, and may have been formed by granite‐related hydrothermal fluids. In that case, the older remanence in the breccia is also an overprint. Epithermal sinter formation and chemical (rather than purely thermal) resetting of the remanence in the underlying breccia were probably due to deep circulation of oxidized fluids during Permo‐Carboniferous tectonic activity. Chemical reaction of these fluids with pre‐existing magnetite‐bearing ironstone may have been responsible for uranium mineralization during the Permo‐Carboniferous. This interpretation is consistent with published textural, isotopic and fluid inclusion data, but an older age for the uranium concentration, as a primary part of the ironstone formation, cannot be excluded. Alternatively, if the monazite data are discarded, the entire hydrothermal process including iron, uranium and silica deposition could have occurred in the Permo‐Carboniferous.