https://orcid.org/0000-0002-7723-3537
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Abstract
Minerals consisting of the platinum-group elements iridium, osmium and ruthenium (IPGE) are widely dispersed in alluvial deposits derived from ultramafic igneous complexes originating in ophiolite belts. Rock types, such as cumulate dunite–pyroxenite–harzburgite, and their serpentinite derivatives are especially fertile sources of IPGE minerals, which are colloquially termed ‘osmiridium’. IPGE minerals have been recorded from five localities in Victoria—beach sands at Waratah Bay, and stream concentrates from Turtons Creek, Stockyard Creek and Horseshoe Creek in South Gippsland. A grain of IPGE was recently found in a gully near the Dolodrook River further north. These occurrences are all poorly documented by comparison with the northwest Tasmanian deposits, and only Waratah Bay has provided enough material to enable full characterisation. Microprobe data show that all four minerals currently defined in the Os–Ru–Ir system occur at Waratah Bay, including rutheniridosmine. Gold and chromite are also present in the alluvial samples from Waratah Bay and Turtons Creek. Attempts to correlate compositions of alluvial chromite with those from likely proximal serpentinite source rocks proved to be inconclusive due mainly to inadequate representative sampling. However, the chromite compositions are broadly characteristic of those found in the early Paleozoic ultramafic ophiolite complexes in northwestern Tasmania, in which IPGE minerals and chromite are abundant. Regional aeromagnetic trends across Bass Strait suggest that the Victorian IPGE occurrences represent disaggregated and metamorphosed extensions of the Tasmanian sequences.
KEY POINTS
IPGE minerals (alloys of platinum-group elements osmium, iridium, and ruthenium) have been recorded from five Victorian placer deposits, but only those in beach sands at Waratah Bay, in southern Victoria, have been preserved for study.
The primary sources for most occurrences can be tracked to nearby Cambrian mafic–ultramafic rocks (‘greenstones’) sequences.
The IPGE minerals range in composition across all currently defined species in the Os–Ir–Ru system and are typical of occurrences in Tasmania and elsewhere in the world.
Aeromagnetic trends across Bass Strait suggest a linkage between the Victorian and Tasmanian occurrences.
Compositions of chromite in the serpentinites cannot be correlated with those in associated placer deposits.
Acknowledgements
I am grateful to the following people for their invaluable assistance with this investigation: Graham Hutchinson, from the Electron Microprobe Laboratory in the School of Geography, Earth & Atmospheric Sciences, University of Melbourne; Ross Cayley at Geoscience Victoria for providing specimens and information on Waratah Bay; Andy Tomkins at Monash University School of Earth and Environmental Sciences for information on the Dolodrook River occurrence; Alan Carroll at Geoscience Victoria’s Drill Core Library for access to the FOS-1 core; and Oskar Lindenmayer at Museums Victoria for photography. The manuscript also greatly benefited from critical comments made by Reid Keays. For other assistance, I should also mention and thank Clive Willman, Louis Cabri and Museums Victoria librarians. Two reviewers are also acknowledged for their suggestions.
Disclosure statement
The author has no conflict of interest in this research.
Notes
1 Letter to the Editor, Foster Mirror and South Gippsland Shire Advocate, from James Dewar, on 4 March 1915.
2 Quarterly Reports of the Mining Surveyors and Registrars, September 1881.
3 The Argus, 17 September, page 9. Other newspapers picked up the report.
4 Gazette Notices in the Gippsland Times, 18 July 1892 and 17 April 1893. The lease holders were ‘A Pilfoot and another’. The voidance notice also appeared in the Morwell Advertiser, 21 April 1893.
5 The Sands of Pactolus; gold washed from the sea, untold wealth on the beach. The Sun (Sydney), 21 January 1917. According to legend, the sand banks along the Pactolus River in the ancient kingdom of Lydia in Asia Minor (now in modern Turkey) were rich in gold.
6 Discovery of Gold at Waratah. South Gippsland Shire Echo, 26 February 1915; The Waratah Gold Discovery. Toora and Welshpool Ensign and South Gippsland Observer, 5 March 1915.
7 Assays of Waratah Bay sands were made by Mr W.W. Dods in Melbourne. Weekly Times, 20 March 1915. Dods was an assayer and metallurgical chemist, formerly chief chemist and assayer for the Australian Smelting Company Pty Ltd, Dry Creek, South Australia. Advertisement in The Argus, 12 October 1901.
8 Gold found in beach sand. The Age, 3 March 1915; Gold at Waratah. Gippsland Standard and Alberton Shire Representative, 17 March 1915.
9 The letter was from Charles Ashe, of Romsey. His reference to the sample being on display in the Mineralogical Museum in the Public Library suggests that it was in the collections of the Industrial and Technological Museum.
10 The Herald (Melbourne), 5 August1931.
11 The Advocate (Burnie, Tasmania), 17 July 1931.
12 Letter in the Geological Survey archives ‘osmiridium’ file.
13 Gold found in beach sand. The Age, 3 March 1915; Waratah gold discovery. Every Week (Bairnsdale), 25 March 1915.
14 This sample appears to have been lost.
15 Osmiridium as a ‘clean’ sample and in sand was donated to the Industrial and Technological Museum by the Mines Department Laboratory on 23 March 1893, but without further details.
16 Progress Report No. 2. An entry to this effect is in the Mines Department Catalogue of rocks, minerals and fossils (1873–1875), with the sample given the number 2826. No details other than the mineral name and donor were recorded, and the entry is annotated ‘sample retd 6/3/74’.
17 The Age, 21 January 1926; Gippsland Times, 21 January 1926; The Argus, 30 January 1926; Gippsland Times, 1 February 1926; Bairnsdale Advertiser and Tambo and Omeo Chronicle, 2 February 1926.
18 Discovery of Gold at Waratah. South Gippsland Shire Echo, 26 February 1915.