Gem sapphires and zircons in a basalt diatreme, northeastern Tasmania

Bottrill, Ralph1, Everard, John1, Duncan, David4, Sutherland, Lin5, Meffre, Sebastien2 and Matchan, Erin3

1Geological Survey, Mineral Resources Tasmania, Rosny Park, Australia; 2School of Earth Sciences, University of Tasmania, Hobart, Tasmania, Australia; 3School of Earth Sciences, University of Melbourne, Parkville, VIC; 4Kingston, Tasmania; 5Australian Museum, Sydney, NSW

Many notable occurrences of gem sapphire are known in Tasmania, mostly in the extensive alluvial tin-bearing deposits in the northeast (Yim et al., 1985, McGee, 2005, Sutherland & Webb, 2007, Bottrill & Baker, 2008 ). The alluvial sapphires are typically associated with coarse-grained , ferroan (pleonaste) spinel and zircons, but except for the spinel, these are rarely seen within any host rock, although a basaltic source is commonly suspected (Sutherland & Webb, 2007).

Source rocks for these sapphires have recently been determined at a site on Logans Road in the Priory sapphire field, east of St Helens, where a small diatreme or breccia pipe has been emplaced within Devonian granite. The diatreme consists mainly of basanite clasts containing abundant, partly disaggregated, spinel lherzolite xenoliths, but also contains altered granite xenoliths within the diatreme. This area is undisturbed by tin mining.

Nearby, sapphires occur as  detrital grains in the surrounding headwater drainage together with   pleonaste spinel, zircon, pyroxenes and olivine, which mostly appear to derive from the diatreme. The mineral grain shapes are rather irregular and angular with little or no transport abrasion and thus obviously adjacent to source. The creeks also contain variably weathered basanite boulders and pebbles, with soft brown weathered surfaces revealing many emergent mineral grains.  A single subrounded/subangular blue to grey sapphire, 7 mm across with a distinct parting with a distinct parting, was found partly exposed in an a weathered, inclusion-rich pebble by a prospector, Michael Lloyd (Duncan & Lloyd, 2013). It was confirmed with gemmological tests, indicating at least some of the Tasmanian sapphires have a basaltic source.

Fresh volcanic glass from the basanite diatreme was separated and dated using Ar/Ar multi-collector mass spectrometry to reveal a 42.0 (± 0.1) – 47.2 (± 0.1) Ma (2σ) age for this basanite diatreme.

Coarse red-brown gemmy zircons are present with the alluvial sapphires, although they were not identified within the basanite itself, and these were dated by the U-Pb method using laser ablation inductively coupled mass spectrometry  (LA-ICPMS). Two populations were found, one  between ~42-44 (± 4) Ma and one between 233-247  (± 4) Ma.

The younger zircon date probably records partial resetting by the basanite host, as it closely matches its Ar/Ar date ; the older date probably records an unknown, underlying Triassic intrusive source.  

There is a 233 ± 5 Ma (K/Ar) alkali basalt flow near St Marys (Calver & Castleden, 1981) and a 214 ± 1 Ma (K/Ar) age from “felsic tuff” near Bicheno (Bacon & Green, 1984). Rhyolite clasts occur in the upper Triassic near St Marys, and may have a proximal source (Forsyth, 1989). No in situ Triassic felsic lavas or intrusives have been found in Tasmania, however.

References

Bacon, C.A. & Green, D.C., 1984. A radiometric age for a Triassic tuff from eastern Tasmania. Tasmanian Geological Survey, Unpub. Rept. 1984/29

Bottrill, R.S. & Baker, W.E., 2008, A Catalogue of the Minerals of Tasmania. Bull. 73. Tasmanian Geological Survey

Calver, C. R.; Castleden, R. H. 1981. Triassic basalts from Tasmania. Search 12:40-41

Duncan, D.McP. and Lloyd, M., 2013. The discovery and exploration of the Priory Sapphire Field. Unpublished Report for RG Prospecting.

Forsyth, S.M. 1989. Upper Parmeener Supergroup. Geology and Mineral Resources of Tasmania. Geol. Soc. Aust., Spec. Pub. 15.

Khin Zaw, Sutherland, F. L. Dellapasqua, F., Ryan, C.G., Yui, Tzen-Fu, Mernagh, T.P. and Duncan, D.McP, 2006. Contrasts in gem corundum characteristics, eastern Australian basaltic fields: Trace elements, fluid/melt inclusions and oxygen isotopes. Mineralogical Magazine 70(6):669-687

Matchan, E. 2020. 40Ar/39Ar step-heating analysis of basanite groundmass sample R013378/NJ449 Report UM20-0801.

McGee, B. M., 2005, Characteristics and origin of the Weldborough sapphire, NE Tasmania. Unpub Hons thesis, U.Tas.

Sutherland, F. L., Webb, G.B., 2007. Australian Sapphires and Rubies. Rocks & Minerals · March 2007. 82.2. p116-139

Tasmania Department of Mines, 1970 Occurrences of gemstone minerals in Tasmania. Mineral Resources Tasmania

Yim, W.W.-S., Gleadow, A.J.W. & Van Moort, J.C., 1985: Fission track dating of alluvial zircons and heavy mineral provenance in North-east Tasmania. J. Geol. Soc. Lond. 142: 351-356.


Biography

Ralph is a geologist, mineralogist, petrologist. I work for Mineral Resources Tasmania (Mines Dept of Tas) where I run the geology/ mineralogy/ petrology/ geochemistry labs and rock collection; study various Tasmanian mineral deposits, rocks and minerals. An associate curator with both the Tasmanian Museum and the Queen Victoria Museum

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