Australian Museum Journal The Tumbarumba Basaltic Gem Field, New South Wales: in relation to sapphire-ruby deposits of eastern Australia

Shortform:
Sutherland et al., 2002, Rec. Aust. Mus. 54(2): 215–248
Author(s):
Sutherland, F. L.; Graham, I. T.; Pogson, R. E.; Schwarz, D.; Webb, G. B.; Coenraads, Robert R.; Fanning, C. M.; Hollis, J. D.; Allen, T. C.
Year published:
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 package:
EndNote file
Title page:
Title page (14kb PDF)
Complete work:
Complete work (1036kb PDF)

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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