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i i “Main” — 2006/8/13 — 12:21 — page 1 — #1 i i JOURNAL AND PROCEEDINGS OF THE ROYAL SOCIETY O F NEW SOUTH WALES Volume 137 Parts 3 and 4 (Nos 413–414) 2004 ISSN 0035-9173 Published by the Society Building H47 University of Sydney NSW 2006 Australia Issued December 2004 i i i i i i “Main” — 2006/8/13 — 12:21 — page 98 — #2 i i THE ROYAL SOCIETY OF NEW SOUTH WALES OFFICE BEARERS FOR 2003-2004 Patrons His Excellency, Major General Michael Jeffery, AC, CVO, MC, Governor General of the Commonwealth of Australia. Her Excellency Professor Marie Bashir, AC, Governor of New South Wales. President Ms K. Kelly, BA (Hons) Syd Vice Presidents Mr D.A. Craddock, BSc(Eng) NSW, Grad.Cert. Management UWS. Prof. W.E. Smith, MSc Syd., MSc Oxon, PhD NSW, MInstP, MAIP. Mr C.F. Wilmot Mr J.R. Hardie, BSc Syd., FGS, MACE. one vacancy Hon. Secretary (Gen.) Prof. J.C. Kelly, BSc Syd., PhD Reading, DSc NSW Hon. Secretary (Ed.) Prof. P.A. Williams, BA (Hons), PhD Macq. Hon. Treasurer Dr R.A. Creelman, BA, MSc, PhD Hon. Librarian Ms C. Van Der Leeuw Councillors Dr Eveline Baker Dr Anna Binnie, PhD Dr M.D. Hall Dr M.R. Lake, Ph.D Syd A/Prof. W. Sewell, MB, BS, BSc Syd., PhD Melb., FRCPA Mr M.F. Wilmot, BSc Mr R. Woollett Ms M. Haire Ms J. Rowling BEng UTS Ms R. Stutchbury Southern Highlands Rep. Mr H.R. Perry, BSc. The Society originated in the year 1821 as the Philosophical Society of Australasia. Its main function is the promotion of Science by: publishing results of scientific investigations in its Journal and Proceedings; conducting monthly meetings; awarding prizes and medals; and by liason with other scientific societies. Special meetings are held for: the Pollock Memorial Lecture in Physics and Mathematics, the Liversidge Research Lecture in Chemistry, the Clarke Memorial Lecture in Geology, Zoology and Botany, and the Poggendorf Lecture in Agricultural Science. Membership, as an Ordinary, Associate or Absentee Member, is open to any person whose application is acceptable to the Society. An application must be supported by two members of the Society. Subscriptions for the Journal only are accepted. The Society welcomes, from members and non-members, manuscripts of research and review articles in all branches of science, art, literature and philosophy for publication in the Journal and Proceedings. ISSN 0035-9173 Copyright The Royal Society of New South Wales does not require authors to transfer their copyright. Authors are free to re-use their paper in any of their future printed work and can post a copy of the published paper on their own web site. Enquiries relating to copyright or reproduction of an article should be directed to the author. i i i i i i “Main” — 2006/8/13 — 12:21 — page 99 — #3 i i Journal & Proceedings of the Royal Society of New South Wales, Vol. 137, p. 99–122, 2004 ISSN 0035-9173/04/020099–24 $4.00/1 Gemstone Characteristics, North-East Barrington Plateau, NSW d.l. roberts†, f.l. sutherland‡⋆, j.d. hollis†⋆, p. kennewell† and i.t. graham⋆. Abstract: Corundum and zircon concentrates from gravel deposits in two northern Barring- ton plateau prospects were studied for their features, to investigate potential gemstone sources within the catchment zone. The studies utilized field research, colour sorting, gemmological microscopy, EDAX and PIXE trace element analysis and zircon fission-track dating. The re- sults showed statistical differences in corundum suites from different gravel horizons, which suggested different source areas. Anorthoclase and ilmenorutile were identified as inclusions in Barrington corundums for the first time. Zircon fission-track results showed that several episodes of volcanic eruptions were involved (66–4 Ma), reinforcing and adding to previous eruption ages. This suggests multiple corundum delivery both in space and time into drainage systems. Keywords: Corundum, ruby, gravels, terraces, eruption, trace elements, mineral inclusions, zircon, gemstone sources. INTRODUCTION 2. Detailed fission-track dating on zircons concentrated from the northeastern Barrington The Barrington volcanic plateau, 150 km north- field that helps to clarify the extent of gem bear- northwest of Newcastle (Figure 1, page 117) ing eruptive episodes in this area. is one of the many basalt lava fields in east- ern Australia (Mason 1989, O’Reilly and Zhang GEOLOGICAL SETTING 1995, Sutherland and Fanning 2001). The field is particularly noted for its association with The general geology of the Barrington Plateau gemstones (ruby, sapphire, zircon), brought up region, its economic deposits and literature are in multiple basaltic eruptions between 60 and outlined in Gilligan et al. (1987) and Suther- 4 Ma and concentrated by erosion into adja- land and Graham (2003). The general geology cent alluvial deposits (Sutherland et al. 1998, of the study area in the North-East Barrington Sutherland and Fanning 2001). The presence Plateau is summarised below. of gem quality ruby in the Barrington deposits Basement geology consists of Late Devonian (Webb 1997) has promoted investigations into to Early Carboniferous mudstone and siltstone the economic potential of the gem deposits by with minor interbedded conglomerate, lithic the Australian exploration and mining company sandstone and limestone. Most of this mate- Cluff Resources Pacific NL. Exploration and rial was deposited in a deep marine environ- testing programs have been concentrated in the ment. The sheet-like deposits of greywacke northeastern part of the province, particularly within the mudstone were probably deposited in the terraces and recent alluvial deposits of the by turbidity currents, while some of the con- upper Manning River at Gummi Flats. Plates 1 glomerates may have been deposited as sub- to 3 show examples of rubies and sapphire from marine fans. A Permian hornblende-biotite the Gummi Flats deposits. granodiorite pluton and associated dykes in- This study presents: truded through the folded Devonian, Carbonif- 1. The first detailed comparison of corun- erous and Early Permian sedimentary rocks at dums found in the Gummi Flats terraces, to as- Gummi Flats. These sequences are capped sist in evaluating their origin and distance trav- mostly by Cenozoic basaltic flows. elled from source regions. This volcanic activity most likely extended i i i i i i “Main” — 2006/8/13 — 12:21 — page 100 — #4 i i 100 ROBERTSetal. from around 60Ma to perhaps < 5 Ma. Alkali This grey gravel horizon ranges in thickness basaltic flows and tuffaceous and fragmented up to 0.5 m and overlies the lower gravel hori- volcanics are common, and form the resistant zon for approximately 55 m strike-length in the plateau region. Underlying these massive basalt test trench. It is moderately to well sorted flows is a basaltic horizon containing highly and exhibits mostly rounded to sub-rounded altered basalt bombs. The sequence includes lithic fragments with sizes from sub-mm up to ankaramitic basalt that may mark an intrusive 250 mm. There is an upward fining in the se- sheet. quence, and a grey clay matrix makes up ap- This elevated basalt plateau has been sub- proximately 45% volume of this horizon. Lithic jected to radial drainage patterns over time, types include, igneous, metamorphic and meta- cutting into the basalts and underlying Palaeo- sedimentary rocks. Generally more granodior- zoic sediments and granodiorites. Local stream ite fragments are present than in the underlying and drainage patterns have accumulated clasts gravel horizon. of these rocks as well as gem stones in many (B). Lower gravel horizon of the Upper Terrace river flats and catchment areas. Changes in (Section 18-B; Figure 3). stream patterns and velocities have created This gravel contains rounded to sub-angular, river terraces in localised areas. igneous, metamorphic and meta-sedimentary lithic fragments, with sizes ranging from sub- STUDY AREA mm to > 256 mm. The horizon is up to 2.6 m thick and has a reddish fine sandy clay matrix, Two study sites were investigated in this study with occasional sand and silt lenses up to 20 cm in the northern Barrington Plateau. in length and 10 cm thick. There is indistinct upward fining, but generally the horizon is very Gummi Flats Area poorly sorted. This horizon is overlain in part by the grey gravel horizon (Section 18-A), and The main sampling sites for corundum and zir- a medium grey sand or a fine silty clay. Hu- con included sections of older terraces and the mus topsoil some 0.5 m thick caps the sequence. present alluvial terrace deposits of the upper Weathered granodiorite forms the underlying Manning River. These lie below the eastern basement. edge of the basalt field both upstream and (C). Middle Terrace horizon downstream of Backwater Creek junction (Fig- (Section T4A; Figure 4, page 103). ure 2, page 101). The deposits were excavated The horizon consists of sub-rounded to sub- during bulk testing and sampling programs by angular, igneous, metamorphic and meta- Cluff Resources Pacific NL, during the period sedimentary rocks. Rock sizes vary from sub- 2000–2003. mm to > 256 mm. An orange clay matrix, Suites of corundums were recovered from with sporadic black clay lenses up to 20 cm heavy mineral concentrates from four specific thick underlies a grey gravel in some places. gravel horizons: The upper (A) and lower (B) Some fragments show heavy manganese stain- horizons of the Upper Terrace deposit (Trench ing. The horizon ranges up to 2.5 m thick, is 11); the Middle Terrace deposit (C; Trench 4A) poorly sorted but shows a slight fining upwards. and Recent alluvial deposit (D; Trench 2A). Basement is weathered granodiorite, and large The zircons were recovered from heavy mineral sporadic fresh granodiorite boulders appear at concentrates from the Upper Terrace deposit the base of the gravels.