SECOND AUSTRALIAN CONFERENCE ON INIS-mf—12772 GEOCHRONOLOGY 27-29 September, 1989 WORKSHOP PROGRAM AND ABSTRACTS 1989 SYDNEY Unedited abstracts - not to be cited without permission of the author SPONSORS: Centre for Isotope Studies, CSBRO Division of Exploration Geoscience Selby Anax, Sydney Advanced Analytical P/L, VG Isotech, UK Sydney ACOG 89 PROGRAM WEDNESDAY 27 SEPTEMBER 9.45 Brian Gulson - Welcome and introductory remarks. SESSION I. NEW DEVELOPMENTS IN ISOTOPE STUDIES METHODOLOGY, INSTRUMENTATION Chairman; Brian Gulson 10.00 Steve Clement and Bill Compston (RSES): SIMS at high mass resolution. 10.25 Peter Kinny (RSES): New SHRIMP applications. 10.50 Chuck Douthitt and Jim Bearpark (Finnigan MAT): Recent advances in thermal ionization mass spectrometry. 11.15 Soey Sie (CSIRO): Preview of the CSIRO-AMS facility. 11.40 John Cantle (VG Instruments): A comparison of multicollector methods plus a discussion of factors relating to optimum precision in a thermal ionisation mass spectrometer. 12.05 Masahiko Honda, Ian McDougall, A. Doulgeris and Des Patterson (RSES): Analytical techniques for measurement of noble gases in terrestrial samples. 12.30 LUNCH Chairman: Bill Compston 2.00 Graham Mortimer (RSES): A new facility in isotope geochemistry at RSES. 2.25 Ian McDougall (RSES): Precision of K-Ar and 40Ar/39Ar dating. SESSION II. ISOTOPE STUDIES RELATING TO ECONOMIC GEOLOGY 2.50 Caroline Perkins, Ian McDougall, Jon Claoue-Long (RSES) and Paul Heithersay (Geopeko): ^Ar/^Ar an(j u-Pb geochronology of the Goonumbla and Gidginbung gold deposits, NSW. 3.15 Jeremy Richards and Ian McDougall (RSES): K-Ar and 40Ar/39Ar age relationships at the Porgera gold deposit, PNG. 3.40 AFTERNOON TEA Chairman: Joe Hamilton 4.05 Graham Carr and Judy Dean (CSIRO): Mantle lead signatures of Ordovician gold mineralization in the Lachlan Fold Belt of NSW. 4.30 Brian Gulson, Karen Mizon, (CSIRO), Brian Atkinson, Tony Andrew (OGS), Dave Burrows, Nick Callan, Steve Noble (U of T), Fernando Corfu (ROM): The dilem ma of the source of gold in Archean greenstone deposits. 5.30-7.30 Ice Breaker, NBTC Conference Rooms THURSDAY 28 SEPTEMBER SESSION III. ISOTOPE STUDIES RELATING TO GRANITES Chairman: Rod Brown 9.00 Rob Creaser (La Trobe): U-Pb and Sm-Nd isotopic evidence for the age and origin of mid-Proterozoic felsie magmatism of the eastern Gawler craton, South Australia. 9.25 Stirling Shaw and Dick Flood (Macquarie): Carboniferous igneous activity in the Lachlan and New England Fold Belts: parts of the same magmatic arc? 9.50 Yadong Chen (Macquarie): Sr isotopic studies on mafic enclaves and host rocks of the Glenbog, Anembo and Blue Gum plutons of the Bega Batholith. 10.15 Morning Tea Chairman; Rod Page 10.40 Shen-su Sun (BMR): Chemical and isotopic systematics of the Blue Tier Batholith, N.E. Tasmania: its bearing on the origin of the tin-bearing alkali feldspar granites. SESSION IV. FISSION TRACK STUDIES/SEDIMENTARY BASINS AND OTHERS 11.05 Andy Gleadow (La Trobe): Fission track length studies and age patterns in western Victoria, (abstract not received) 11.30 Joe Hamilton (CSIRO), Tony Fallick (SURRC), Julian Andrews (East Anglia) and Dave Whitford (CSIRO): Isotopic studies of the provenance and post depositional alteration of Jurassic clay mineral assemblages, Scotland. 11.55 Rod Brown (La Trobe): Dating and measuring regional episodes of denudation using apatite fission track analysis: an example from southern Africa. 12.20 LUNCH SESSION V. OTHER PETROLOGICAL APPLICATIONS Chairman: Stirling Shaw 2.00 Dave Whitford (CSIRO), Tony Crawford (U. of Tas.), Michael Korsch and Steve Craven (CSIRO): The Mount Read Voleanics, Tasmania: Sr and Nd isotope geochemistry. 2.25 Sue O'Reilly (Macquarie) and Bill Griffin (CSIRO): Isotopic signatures of mantle rocks from south eastern Australia. 2.50 Neil McNaughton (UWA), Dave Nelson, John de Laeter (Curtin) and Ian Fletcher (GSWA): The Bunbury Basalt: Kerguelen - Heard Island hot-spot volcanism along the continental margin of SW Australia. 3.15 Bill Griffin (CSIRO) and Sue O'Reilly (Macquarie): Sm-Nd dating of garnet- bearing rocks: empirical evaluation of constraints on resetting. SESSION VI. PRECAMBRIAN GEOCHRONOLOGY Chairman: Dave VVhitford 4.05 Li Huimin (China), Rod Page, Mick Bower (BMR) and Bill Compston (RSES): Four methods of zircon dating: case study from the Proterozoic Zhongtiao Mountains, Shanxi Province, China. 4.30 Dave Nelson (Curtin), Alee Trendall (GSWA) and John de Laeter (Curtin): Sm-Nd isotopic complexities in Archaean mafic lavas and implications for Sm-Nd geochronology. 4.55 Jon Claoue-Long (RSES), R.W. King and Rob Kerrich (Saskatchewan): Dating Archaean gold deposits with the ion-probe: a Canadian example. 3.40 AFTERNOON TEA 7.00 for 7.30 pm ACOG DINNER, MACQDARIE UNIVERSITY STAFF CLUB FRIDAY 29 SEPTEMBER Chairman: Ian McDougall 9.00 John de Laeter (Curtin) and W.G. Libby (GSWA): 500 Ma biotite Rb-Sr dates in the Yilgarn Block near Harvey. 9.25 Des Patterson (RSES): Noble gases as tracers of volatiles in subduction zones. 9.50 R.J. Ryburn, Rod Page, John Richards, V. Laynne and E.P. Shelley (BMR): 'Ozchron' - a national database of Australian geochronology. 10.05 Discussions about ICOG-7 10.30 MORNING TEA Visit stable/radiogenic/accelerator laboratories Dating and measuring regional episodes of denudation using apatite fission track analysis: an example from southern Africa. Roderick Brown Department of Geology, La Trobe University, Bundoora, 3083, Australia. The chronology and nature of landscape development in southern Africa has stimulated debate since the beginning of the present century, and yet no consensus has been reached. Major differences in viewpoint result largely from a lack of quantitative information concerning the timing and magnitude of denudation episodes affecting the sub-continent, particularly since the break, up of Gondwana, The major discrepancies between the various proposed landscape chronologies imply that there are fundamental problems associated with dating approaches used up until now, for example, in the correlation between landsurfaces and unconformities in the offshore sedimentary record. This situation suggests that the use of a more direct dating technique may help to resolve these discrepancies. Apatite fission track analysis (AFTA) is a valuable tool for evaluating the thermal history of rocks at temperatures below ~125°C and consequently for examining the thenno-tectenic development of the upper few kilometres of the Earth's crust. As a result AFTA can be used to date major periods of denudation directly . The application of apatite fission track analysis to rocks collected from a range of elevations along the western continental margin of souihem Africa indicates that the present land surface exposes rocks that have cooled rapidly from temperatures above ~125°C to temperatures below ~60°C during the early Cretaceous. This rapid cooling of the upper crust is interpreted as the result of accelerated erosion and consequent uplift of the rock column associated with the early development of the continental margin. The timing of this episode of denudation is broadly synchronous with the break up of West Gondwana and correlates with the pattern of sedimentation derived from borehole data in the adjacent offshore basin. Furthermore, the -125°C palaeo-isothermal surface for the pre-uplift crust is recorded by the apatite age-elevation profiles as a distinct break in slope. The present day elevation at which this break occurs allows an estimate of the amount of denudation to be made. For reasonable estimates of the palaeo-geothermal gradient some kilometers of denudation must have taken place. For a palaeo-geothermal gradient of 30°C/km the caJcuatcd amount of denudation is 2.5 km. In addition, the presently available AFTA data indicates that the denudation was regional in extent ABSTRACT PR J E CANTLE. VG INSTRUMENTS A COMPARISON OF MULT1OOLLECTOR METHODS PLUS A DISCUSSION OF FACTORS RELATING TO OPTIMUM PRECISION IN A THERMAL IONISATION MASS SPECTROMETER Static multicollection is the simultaneous collection of multiple ion beams where each isotope i? dlocated to a designated collector. It lias been developed as a technique which removes errors due to ion beam fluctuations which can limit the precision of single collector measurements. In the Dynamic multicollection mode, groups of isotopes are measured with a peak-jumping technique. It has been shown that dynamic mukicollection provides significant benefits in terms of reproducibilily in isotope systems that can be normalised. In static multicollection, the precision of measurement is constrained not only by ion statistics, but also by amplifier noise and amplifier drift. In particular, the stability of the relative gain between the resistors in each collector channel can have a significant effect on the ultimate precision achievable. The VG Sector 54 thermal ionisation mass spectrometer uses the Multi-2 collector which is unique in commercially-available niulticollectors in that the resistor and amplifier boards are kept within a special environmentally-sealed housing which is evacuated and cooled by a 'Peltier-effect' device to 10°C + 0.01°C. Furthermore ultra low temperature coefficient resistors are used with a specification of 200ppm/°C. The design of the Multi-2 collector and the use of ultra-low temperature coefficient resistors permits extremely stable long-term gain calibrations. Stability is such thai ultimate precision in static mode should only be limited by ion statistics. MANTLE LEAD SIGNATURES OF ORDOVICIAN GOLD MINERALIZATION IN THE LACHLAN FOLD BELT OF NSW Graham R. Carr and Judith A. Dean CSIRO Division of Exploration Geoscience Lead isotopic signatures of ore deposits are used by a number of Australian mining companies as a gcochemical discriminator in their exploration programs. In the Lachlan Fold Belt (LFB), the isotopic signatures of massive sulf ide mineralization associated with greywackes and acid volcanic rocks is well established. However there has been a dearth of Pb isotopic data on other styles of mineralization, particularly the porphyry-breccia-pipe-epithermal associations which can repre- sent major resources of Cu and Au.