VIMP Report 52 - Test drilling on the southern margin of the Murray Basin
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Product description:Download The downloadable version of this report is supplied in PDF format. Abstract This report gives the results from the Victorian Initiative for Minerals and Petroleum (VIMP) drilling program carried out in western Victoria in the first half of 1997. It also places these results into the context of what is presently understood about the geology and tectonic setting of the region. The program aimed to increase the understanding of the Palaeozoic geology and mineral resource potential of the southern margin of the Murray Basin. The Palaeozoic basement has received only preliminary investigation, but exploration in the region has established that it is prospective for gold, base metal and diamond deposits. Eighteen holes were completed, mostly in the Horsham 1:250 000 map sheet area, where the Murray Basin cover is almost complete but is less than 300 m thick, with large areas of cover less than 100 m thick. The drilling mostly targeted strategic stratigraphic units in the Stawell Zone and the Dimboola, Miga and Ozenkadnook Subzones of the Glenelg Zone, identified in Moore's (1996) interpretation of airborne magnetic and ground based gravity surveys over the area. In the Stawell Zone, two drill holes intersected highly strained metasedimentary rocks, probably from the hangingwalls of significant faults. Both holes contained minor amounts of quartz veining and traces of gold. A third hole intersected similar dolerite to those at the Stawell Gold Mine and was extensively altered and cut by late quartz-calcite veins. It bottomed in a fine grained rhyolitic intrusion that was U-Pb zircon dated at 411±7 Ma, the same age as the felsic dykes at Stawell. Drillholes in the Dimboola Subzone intersected basalt, andesite to dacite, ?harzburgite and Glenthompson Sandstone. The tholeiitic basalt was deposited in a shallow water, oxidising environment. The low-titanium andesite and dacite are similar to the Mt Dryden oceanic island arc rocks. The results support the hypothesis that the Dimboola Subzone was an Upper Cambrian oceanic island arc. The serpentinised ?harzburgite is probably on the boundary between the Stawell Zone and the Glenelg Zone. Drillholes in the Miga Subzone intersected andesite and granite. Andesite is similar to the Mt Dryden rocks, and is altered and contains traces of gold. Sheared granite was zircon dated at 504±8 Ma. A drillhole in a late phase of the Duchembegarra Granite, the largest intrusion on the Horsham 1:250 000 map sheet area, intersected a mafic tonalite. The zircons gave a 404±6 Ma age, which places the intrusion amongst the furthest west of Lachlan Fold Belt-associated granite. The holes in the Ozenkadanook Subzone intersected non-magnetic sediments, gabbro and migmatite. Two holes targeted a magnetic mafic volcanic package but intersected non-magnetic metasediments, one with a possible volcanic protolith. Both showed extensive epidote-clinozoisite±garnet±carbonate alteration. Low amphibolite facies biotite gave a K-Ar age of 501±4 Ma. The gabbro gave a 524±9 Ma zircon age. It had intruded a sequence interpreted to be ocean floor basalts and had been metamorphosed to greenschist facies. The intrusion gives a minimum age for the host sequence. The migmatite is derived from metamorphosed high-boron sediments, possibly from a sequence containing evaporites. Metamorphic zircon rims gave an age of 589±14 Ma, and protolith cores ages of at least 1000 Ma. This 600 Ma event is the oldest high-grade metamorphic event known east of the Tasman Line on mainland Australia. K-Ar dating of the same rock gave an age of 490±4 Ma. The results add further weight to the correlation of the western Victorian 500 Ma Mt Stavely Volcanic Complex with the Mt Read Volcanics of Western Tasmania. In both areas 500 Ma island arc volcanics occur with Upper Proterozoic basement. Four holes intersected Grampians Group sandstones. None penetrated the contact with the underlying Cambrian rocks, so Cayley and Taylor's (1997) hypothesis of a flat faulted contact between the two remains untested. The subhorizontal cataclasite zones at the base of VIMP17 are consistent with their interpretation. Bibliographic reference Maher, S., Moore, D.H., Crawford, A.J., Twyford, R. & Fanning, C.M., 1997. Test drilling on the southern margin of the Murray Basin. Victorian Initiative for Minerals and Petroleum Report 52. Department of Natural Resources and Environment. Related products:
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