A PALEOMAGNETIC STUDY OF HYDROTHERMAL ACTIVITY AND URANIUM MINERALIZATION AT MT PAINTER, SOUTH-AUSTRALIA

Palaeomagnetic measurements indicate that the uraniferous Radium Ridge Breccias near Mt Painter in the northern Flinders Ranges of South Aus tralia have been magnetized twice, both times in the Permo-Carbonifero us. The palaeomagnetic south pole for one remanence was estimated as 1 65.1-degrees-E, 65.7-degrees-S (A95 = 11.5-degrees). The direction for the other remanence was not well defined; however samples collected f rom diamictite bodies within the breccia gave 133.9-degrees-E, 33.1-de grees-S (A95 = 6.9-degrees) as the probable pole for the overprint. Th e latter is similar to overprint poles published from central Australi a, which are generally attributed to the Alice Springs Orogeny. Magnet ization directions interpreted as Permo-Carboniferous were obtained al so 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 rema nence of the sinter is primary, indicating a major hydrothermal event in the Permo-Carboniferous. It is not clear by how much this event pos t-dates the deposition of the underlying Radium Ridge Breccias and hem atitic ironstone. The latter may be Ordovician or older if earlier mon azite U-Pb data are correct, and may have been formed by granite-relat ed hydrothermal fluids. In that case. the older remanence in the brecc ia is also an overprint. Epithermal sinter formation and chemical (rat her than purely thermal) resetting of the remanence in the underlying breccia were probably due to deep circulation of oxidized fluids durin g Permo-Carboniferous tectonic activity. Chemical reaction of these fl uids with pre-existing magnetite-bearing ironstone may have been respo nsible for uranium mineralization during the Permo-Carboniferous. This interpretation is consistent with published textural, isotopic and fl uid inclusion data, but an older age for the uranium concentration, as a primary part of the ironstone formation, cannot be excluded. Altern atively, if the monazite data are discarded, the entire hydrothermal p rocess including iron, uranium and silica deposition could have occurr ed in the Permo-Carboniferous.