Abstract

Tumbarumba gemfield in the Snowy Mountains basalt province, NSW, yields corundums, zircons and garnet, corroded by magmatic effects and abraded by alluvial transport. Sub-basaltic contours suggest present drainage profiles mimic Miocene sub-basaltic leads. Six types of corundum were identified. Blue, green, yellow (BGY) zoned sapphires (80%) contain ferrocolumbite as a main mineral inclusion and exhibit variable Fe2O3/TiO2 and low Cr2O3/Ga2O3 (<1). Two sub-types differ in colour absorption spectra, one being unusual in lacking the typical Fe2+-Fe3+ charge transfer effects found in such sapphires. Related trapiche-like corundums (5%) show higher Cr2O3/Ga2O3, possibly due to Fe-Ti oxide exsolution. Vari-coloured, diffuse-zoned and pale blue sapphires (10%) have higher Cr2O3/Ga2O3 and colour absorption characteristics intermediate between BGY sapphires and pink to red corundums with elevated Cr2O3/Ga2O3. The BGY and trapiche-like sapphires are considered magmatic, the intermediate sapphires magmatic-metasomatic (possibly through interactions with Cr-bearing serpentinite bodies) and the pink to red corundums metamorphic in origin. Zircons include low- to high-U types. The latter show [100]-[110] prism combinations (unusual in eastern Australian zircons) and suggest incompatible element enriched parental melts. The magmatic sapphires and zircons (U-Pb age 23 Ma) crystallised in deep evolved salic melts, before transport in basalt. Magmatic-metasomatic sapphires contain zircon inclusions with both older inherited U-Pb ages (up to 903 Ma) and younger magmatic U-Pb ages (2722 Ma). Basalts represent little evolved undersaturated melts (basanites and alkali basalts), and minor near-saturated transitional melts (olivine basalts). Most generated from garnet peridotite sources, but some from spinel peridotite sources. Mantle normalised incompatible multi-element patterns suggest Oceanic Island Basalt (OIB) melts interacted with amphibole (+ apatite) veined mantle. A sapphire and zircon-bearing basalt, also carries kaersutitic amphibole, apatite, alkali feldspar, titanian mica and titanian magnetite xenocrysts from a veined metasomatised source. Olivine micro-dolerite in a plug resembles the Cainozoic basalts in freshness, but its distinct trace element pattern and Early Devonian K-Ar age (400 Ma) indicate an earlier unmetasomatised spinel peridotite source. The Tumbarumba field evolved through explosive gem-bearing basaltic activity between 2715 Ma and peaked in basalt lava activity. Interactions of basaltic melts with amphibole-rich mantle, serpentinite bodies and metamorphic corundum deposits combined to generate multi-modal gem suites.

 
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Bibliographic Data

Short Form
Sutherland et al., 2002, Rec. Aust. Mus. 54(2): 215–248
Author
F. L. Sutherland; I. T. Graham; R. E. Pogson; D. Schwarz; G. B. Webb; Robert R. Coenraads; C. M. Fanning; J. D. Hollis; T. C. Allen
Year
2002
Title
The Tumbarumba Basaltic Gem Field, New South Wales: in relation to sapphire-ruby deposits of eastern Australia
Serial Title
Records of the Australian Museum
Volume
54
Issue
2
Start Page
215
End Page
248
DOI
10.3853/j.0067-1975.54.2002.1358
Language
English
Date Published
10 July 2002
Cover Date
10 July 2002
ISSN
0067-1975
CODEN
RAUMAJ
Publisher
The Australian Museum
Place Published
Sydney, Australia
Subjects
GEODIVERSITY; MINERALOGY
Digitized
10 July 2002
Available Online
10 July 2002
Reference Number
1358
EndNote
/Uploads/Journals/17941/1358.enw
Title Page
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Complete Work
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