Palaeomagnetism of Australian Hematite Ore Bodies--II Western Australia

Abstract

Summary Iron ore deposits of the Lake Superior type within the Archaean and Proterozoic banded iron formations of Western Australia show consistent and stable directions of remanent magnetization. The palaeomagnetic poles of the ores do not correlate with poles of Post-Cambrian Australian rocks, which suggests a Pre-Cambrian age for the ore formation. Magnetic anisotropies of the ores are low and the distribution of the principal susceptibility axes tends to reflect the magnetic fabric of the host rocks. In a previous publication (Chamalaun & Porath 1967) we reported the results of a palaeomagnetic study on the hematite ore bodies of the Middleback Ranges in South Australia. It was found that these were consistently and stably magnetized with a Pre-Cambrian direction. We therefore extended the study to the Western Australian ore bodies, the results of which are reported here. A large number of high-grade hematite ore bodies are known to occur in the Archaean and Proterozoic banded iron formations of Western Australia, but in order to obtain fresh subsurface samples we have sampled only those that are actively mined. The localities of the ore bodies sampled are shown in Fig. 1. The Mt Goldsworthy and Koolyanobbing deposits occur in Archaean banded iron formations, whilst the host rocks for the Mt Tom Price and Mt Newman ore bodies are Proterozoic iron formations. 2. Results (a) Mt Goldsworthy. The Mt Goldsworthy deposit is located at the northern extremity of the Western Australian Archaean shield. A folded and faulted banded iron formation, which consists of alternating layers of silica and hematite, forms the host rock of a number of high-grade hematite ore bodies of varying origins. Brandt (1964, 1966) makes a distinction between lode ore and crust ore deposits. Lode ores are deep lenses of massive finely crystalline hematite (average grain size about 20p). The major deposit at Mt Goldsworthy consists of lode ore and is controlled by a steep angled transcurrent fault. Pebbles of massive hematite ore occur in the basal Proterozoic conglomerate indicating that the ore formed in Pre-Proterozoic times (Brandt 1964). Brandt (1966) favours a hypogene origin of the lode ore along the lines suggested by Dorr (1965) for the Brazilian ores. Crust ores form shallow cappings of hematite and goethite on the upturned