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  • Special demagnetising apparatus was constructed to study the stability of several samples of basic igneous rocks from three localities in eastern Australia, particular emphasis being placed on the reliability of the directions of NRM. The direction of primary magnetisation acquired when the rocks first cooled was determined for samples at all three sites. Mesozoic dolerite from Red Hill Dyke in southern Tasmania has little or no secondary magnetisation and the mean direction of NRM is representative of the Jurassic in Tasmania. There is no evidence of systematic error due to stress or shape, and therefore the direction of NRM is a reliable estimate of the direction of the geomagnetic field at the time of intrusion. Devonian Nethercote basalt from southern New South Wales can be divided into two distinct groups, one in which the NRM is completely unaffected in either direction or intensity by demagnetisation in peak alternating fields of up to 1000 oersteds, and the other in which secondary magnetisation completely masks any primary magnetisation that may be present. Tertiary basalts from southern New South Wales show a wide range of stability. The NRM consists of primary TRM and varying proportionate amounts of secondary magnetisation, which is almost certainly viscous and which was probably acquired in the present Earth's field. The stability shown by the three rock types makes it more probable that previous palaeomagnetic results, which span a long period from Devonian to Tertiary, form a reliable record of the geomagnetic field in Australia.The general effects of alternating demagnetising fields are also discussed and a comparison made between the theoretical predictions and the data obtained.

  • This report presents the results of geochemical investigations in the Mount Isa district, northwest Queensland. Samples, mainly from cores, represent Group 2 Shales (comprising Kennedy Siltstone and Spear Siltstone), Urquhart Shale, Native Bee Siltstone, greenstones, and local basic igneous rocks. These have been used to study element distributions in mineralized and unmineralized localities. It was found that primary element dispersions are associated with the 1100 Cu orebody, but not with the Ag-Pb-Zn bodies. This, together with the different modes of occurrence of the orebodies, suggests that the mineralization at Mount Isa took place in two separate events. The Ag-Pb-Zn deposits are considered to be syngenetic whereas the Cu deposits appear to be, in part at least, epigenetic. The chemical evidence suggests strongly that much of the Cu in the silica dolomite bodies was derived from the underlying greenstones. In addition, an attempt has been made to differentiate the Urquhart Shale from the other units on the basis of chemical composition. Of the elements analysed, Ca appears to be the most diagnostic and it may be possible to define the upper limit of the Urquhart Shale using this element.

  • During 1969, the Mundaring Geophysical Observatory collected seismic refraction data from explosions used by the Bureau of Mineral Resources No. 2 seismic party in the southwest of Western Australia. The seismic party exploded 37 charges up to 4,500 kilograms on a traverse from Balladonia through Kalgoorlie to Perth. Two mobile Willmore seismographs and permanent seismographs at Mundaring and Kalgoorlie recorded the resultant seismic waves. Raw data for this survey are available on request from clientservices@ga.gov.au - Quote eCat# 76503

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