Tumbarumba Latitude -35.777243, Longitude 148.011375
Located in the Snowy Valleys Way Region of NSW, nearest town Tumbarumba
The Tumbarumba Basaltic Gem Field, New South Wales: In Relation to Sapphire-Ruby Deposits of Eastern Australia, F.L. Sutherland, I.T. Graham, R.E. Pogson, D. Schwarz, G.B. Webb, R.R. Coenraads, C.M. Fanning, J.D. Hollis and T.C. Allen firstname.lastname@example.org
Tumbarumba Shire CouncilImages: Gem concentrate, Tumbarumba, NSW and Ruby crystals, Tumbarumba, NSW photos, G.Sutherland, Australian Museum.
Commonly called sapphire or ruby in its gem varieties, corundum is a crystalline form of aluminium oxide (Al2O3). It becomes coloured with traces of iron, titanium, vanadium and chromium. In gem form it is one of the naturally clear transparent materials, but can have different colours when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colours are called sapphire. The pink to red colour of ruby stems from its chromium content, whereas the sapphire's blue results from its titanium and ferric iron content. Mauve sapphires result from their vanadium content. Yellow and green sapphires contain variable amounts of ferrous and ferric iron.
Corundum can occur as a result of direct crystallization from magma (molten rock) of appropriate composition, and by metamorphism of pre-existing rocks. It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering. Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral and is commonly used as an abrasive. In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, which is very high for a transparent mineral composed of the low atomic mass elements aluminium and oxygen.
Dating of gem minerals and basalts suggests this gemfield mainly evolved over some 10 million years from Late Oligocene to Middle Miocene time (27-15 Ma). Firstly zircon and associated corundums were discharged explosively as xenocrysts (a crystal foreign to the igneous rock in which it occurs) in undersaturated volatile-rich basaltic melts. This activity centred in the northwest region of the Tumbarumba Gemfield. It followed initial low volume mantle melting, when zircon and corundum crystallised from melted material within the mantle/crust boundary.
Eastern Australia is a prime source of gem corundums derived from basaltic eruptives. Tumbarumba gemfield in the Snowy Mountains basalt province, NSW, yields corundums, zircons and garnet, corroded by magmatic effects and abraded by alluvial transport. Research suggests present drainage profiles mimic Miocene drainage which underlies the basalt, these buried alluvial deposits are called leads.
When rocks are exposed to surface weathering they are relentlessly scoured by wind and water and subjected to alternating heating and cooling. The process of erosion moves rock fragments into streams where they tumble and are fractured, releasing any gems they contain. In places where the current naturally slows, the heavy gemstones settle into the river or streambed. Over time, sapphires become concentrated and the site becomes a type of secondary deposit called an "alluvial placer".
The gemstones in the Tumbarumba region are found in sub-basaltic leads and later alluvial redistributions along the present-day drainage, principally along Tumbarumba Creek, Buddong Creek, Ruby Creek, Paddy's River and near Kiandra. These leads and river courses were extensively worked for gold and corundums and zircons were often recovered in the process.
The best preserved zircon crystals were found at Ruby Creek. This suggests a local source, especially as pyroclastic and volcaniclastic horizons occur in the area. Gemstones in a Buddong Creek tributary include strongly abraded "early age" zircons (26Ma) shed from a small basaltic lead. This suggests derivation from a source that lay towards Rutherford Ridge in the north part of the basalt field. Gemstone deposits appear in Tumbarumba Creek 4 km south of its headwaters towards Laurel Hill and suggest a flanking rather than a northerly source.
As a general rule, fossicking in accordance with the legislation can take place on any land, provided that permission is obtained from the landowner (including government departments) or land manager, except in National Parks where fossicking is prohibited. Fossicking is permitted in NSW State forests under a special purpose permit. Individual regional permits can be obtained from Forests NSW. For more information on fossicking in state forests call 1300 655 687.
One popular local fossicking area is Parson's Gully, which is in the NSW Bago Forest (remember you must obtain a permit from forestry). You can pick up a mud map for directions to Parson's Gully from Tumbarumba Visitor Centre.
Please note that fossicking is not permitted in the Kosciuszko National Park.
Tumbarumba is situated in south eastern New South Wales in the Snowy Valleys Region. The town is situated on the Snowy Valleys Way south of Tumut. The Bago State Forest lies to the North East of Tumbarumba and the nearest forestry office is in Tumut phone (02) 6947 3911.