GEMOC ARC National Key Centre

Belousova, E.A¹, Griffin, W.L^1,2 and O'Reilly, S.Y.¹

1GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia.

2CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113, Australia

The main target of this project is to determine the relationship between the chemical composition of zircon and apatite that occur in wide range of igneous rocks and mineral deposits, and the igneous systems from which they formed. These minerals concentrate many trace elements, and provide a record of the chemical environment through different stages of crystallisation (Nash, 1984; Shnukov et al, 1989; Evans and Hanson, 1993). The GEMOC laser-ablation ICPMS microprobe has allowed analysis of about 30 trace elements (including REE, Y, Sr, U, Th, Pb, Fe, Mn) from 30-50 µm spots on single zircon or apatite grains. This has provided fundamental trace element information on the chemical composition of those accessory minerals and their relation to rock-forming processes. Definition of discriminants will allow the recognition of zircons and apatites from specific rock types and styles of mineralisation, so that these grains might be recognised in the heavy mineral concentrates used in geochemical exploration for mineral deposits.

Representative samples of apatite have been selected from granites (from Australia and Norway), larvikites and pegmatites (Norway), diabases (Ukraine), as well as apatites from less common rock types such as carbonatites (Fen, Norway; Palabora, S.Africa; Kovdor, Russia; Mud Tank, Australia), jacupirangite (Kodal, Norway), and iron ore deposits (Kiruna, Sweden and Durango, Mexico). Trace-element signatures specific for apatite of different origins have been defined. The results indicate that the distribution of trace elements, especially REE, Y, Mn, Th, in apatite depends not only on the mineral structure, but on the abundance of these trace elements and the chemical characteristics of the melt or fluid reservoir where the apatite crystallised.

Particular attention has been paid to the study of primary magmatic apatites from granitoid rocks with the goal of determining how the chemical composition of this mineral reflects granite fractionation. The subsequent task was to determine whether apatite major and trace element geochemistry can be used to characterise granite suites related to Cu-Au mineralisation. The study is focused on Australian Proterozoic granite suites of the Mount Isa Inlier using the extensive AGSO granite collection. Preliminary results suggest that apatite is a sensitive indicator of the crystallisation environment and that the distribution of trace elements in apatite could be used as an additional tool to recognise highly fractionated and highly oxidised granitoids related to Cu-Au mineralisation.
 

REFERENCES

Evans, O C, and Hanson, G N, 1993, Accessory-mineral fractionation of rare-earth element (REE) abundances in granitoid rocks., Chemical Geology, 110, 69-93.

Nash, W P, 1984, Phosphate minerals in terrestrial igneous and metamorphic rocks. In Nriagy, J. O. and Moore, P.B. (Ed.), Phosphate minerals, Berlin Heidelberg: Springer-Verlag, pp. 442.

Shnukov, S E, Cheburkin, A K, and Andreev, A V1989, Geochemistry of wide-spread coexisting accessory minerals and their role in investigation of endogenic and exogenic processes, Geological Journal, 2, 107-114 (in Russian).
 

Acknowledgments: We would like to thank Dr. Steve Walters for stimulating discussions and helpful advice during this work and Dr Lesley Wyborn for providing invaluable assistance with samples collection.