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Abstract
Investigation of coarse (>2 mm) heavy-mineral concentrates from the White Hills Gravel near St Arnaud in western Victoria provides new evidence for the age and provenance of this widespread palaeoplacer formation. A prominent zircon – sapphire – spinel assemblage is characteristic of Cenozoic basaltic-derived gemfields in eastern Australia, while a single diamond shows similar features to others found in alluvial deposits in northeastern Victoria and New South Wales. Dating of two suites of zircons by fission track and U – Pb (SHRIMP) methods gave overlapping ages between 67.4 ± 5.2 and 74.5 ± 6.3 Ma, indicating a maximum age of Late Cretaceous for the formation. Another suite of minerals includes tourmaline (schorl – dravite), andalusite, rutile and anatase, which are probably locally derived from contact metamorphic aureoles in Cambro-Ordovician basement metapelites intruded by Early Devonian granites. U – Pb dating of rutile grains by laser ablation ICPMS gave an age of 393 ± 10 Ma, confirming an Early Devonian age for the regional granites and associated contact metamorphism. Other phases present include pseudorutile, metamorphic corundum of various types, maghemite and hematite, which have more equivocal source rocks. A model to explain the diverse sources of these minerals invokes recycling and mixing of the far-travelled basalt-derived suite with the less mature, locally derived metamorphic suite. Some minerals have probably been recycled from Mesozoic gravels through Early Cenozoic (Paleocene – Eocene) drainage systems during various episodes of weathering, ferruginisation and erosion. Comparison between heavy-mineral assemblages in occurrences of the White Hills Gravel may allow distinction between depositional models advocating either separate drainage networks or coalescing sheets. Such assemblages may also provide evidence for the present-day divide in the Western Uplands being the youngest expression of an old (Late Mesozoic – Early Cenozoic) stable divide separating north- and south-flowing streams or a much younger feature (ca 10 Ma) which disrupted mainly south-flowing drainage.
Acknowledgements
Nigel Grigg discovered the Carapooee diamond and made it available to Museum Victoria. Ken Neilsen, formerly with Kinex Mining Pty Ltd, kindly allowed the Museum to retrieve the concentrates from the company's workings and also provided geological background on the occurrence. Dermot Henry and Julian Hollis assisted with transport of the concentrates to Buninyong and also with their processing. Martin Brown and Michael McRae-Williams arranged for equipment set-up at Buninyong, and Chris Buchanan (volunteer) and James Steinhauser (work-experience student) helped with the sieving of the concentrates and their transport to Melbourne. Pat Kelly provided access to electromagnetic separation facilities at Geotrack International and with SEM imaging. Sorting of the concentrates was commenced by Chris Buchanan and has been continued by Judy Rowe. Lin Sutherland and Dietmar Schwartz coordinated the EDXRF analyses of sapphires. Rolf Schmidt assisted with the diagrams, and Ben Healley took the mineral photographs. Comments on the draft manuscript were provided by Martin Hughes, and the final version benefited from constructive reviews by David Taylor and Paul Carr.