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Centre for Earth and Environmental Science Research

The volcanotectonic architecture of epithermal Au-Ag mineralisation at the Waihi Epithermal System, Coromandel Volcanic Zone, New Zealand.

Executive Summary
ResearchersMr David Lawrence
Dr Peter J. Treloar
Professor Andy Rankin
Mr Paul Harbidge (RandGold Resources)
Funding Body/SourceRandGold Resources
DurationSeptember 2006- September 2009
Project SummaryThe specific aims and objectives of the project are to develop: a paragenetic sequence of mineralisation and alteration; a comprehensive data set of chemistries and PVT properties for the various fluid generations; a detailed analysis of the structural and lithological controls on fluid migration and subsequent alteration and mineralisation and a geochemical data base that constrains successive alteration haloes. These data will be used to establish structural and geochemical constraints on mineralisation; model fluid sources and chemistries; to develop and test a genetic model for mineralisation and to develop a model for future exploration.

Background

The Loulo mining district in West Africa comprises two world-class orogenic gold provinces known as Gara and Yalea, as well several satellite and smaller deposits. Orogenic gold deposits are a newly recognised form of deposit previously referred to as mesothermal deposits. Orogenic gold lodes are structurally controlled epigenetic deposits, and are the most significant and most common type of gold deposit accounting for over a quarter of the total historic gold production (Goldfarb et al., 2005). A considerable proportion of mined placer deposits are erosional products of orogenic lodes adding to their significance in terms of production. The source of the Au-bearing fluids is controversial, either fluids released from metamorphic reactions (e.g. Pitcairn et al., 2006), or crystalising granitic magmas (e.g. Burrow & Spooner, 1989). The source of the metals is also uncertain with some authors suggesting a pre-concentration of gold is needed (e.g. Bierlein et al., 1998), while others believe gold can be leached from normal crustal abundances as long as the wallrock is mafic in composition (e.g. Fyfe & Kerrich, 1984).

Fig. 1 Left: Mineralised quartz-carbonate-pyrite veins and veinlets at Gara. Right: Local panning for gold

Loulo is situated in a Paleoproterozoic inlier, known as the Kedougou-Kenieba inlier (KKI), in western Mali, close to the Senegalese border. The inlier is part of the much larger West African craton, which covers ~4,500,000 squared km's (Rocci et al., 1991) , and represents a large-scale juvenile crust-forming event at ~2.1Ga, loosely referred to as the Eburnean orogeny. The gold production in West Africa is on a similar scale to well-known gold provinces in Canada and Western Australia. As well as Loulo (11.5Moz), the KKI also contains several other world-class orogenic gold deposits, such as Sadiola (>14Moz) in Senegal and Morila (~10Moz) in southern Mali, the latter deposit also belonging to RandGold Resources. The West African craton also contains the well-known Ashanti gold province in Ghana. All these deposits are shear-hosted, situated in second-or-third splays off major terrane bounding strike-slip faults.

The aim of the project is to study the various fluid generations at Loulo by using fluid inclusion, isotopic and other analytical techniques. Loulo will provide an excellent opportunity to contribute to this current debate on the fluid and metal source of orogenic gold deposits. Detailed petrographic and fluid inclusion studies will also provide useful information on the various gold depositional mechanisms that take place in forming large orogenic gold deposits. Structural data will provide essential information on the controls involved in localsing and concentrating the ore.

A young handsome geologist standing by some sheared sediments. Senegal is on the other side of the Faleme River

REFERENCES

BIERLEIN, F.P., ARNE, D.C., BROOME, J.M.N. and RAMSAY, W.R.H., 1998. Metatholeiites and interflow sediments from the Cambrian Heathcote greenstone belt, Australia: Sources for gold mineralisation in Victoria. Economic Geology, 93, pp. 84-101.

BURROWS, D.R. and SPOONER, E.T.C., 1989. Relationships between Archean gold vein-shear zone mineralisation and igneous in the Val d'Or and Timmins areas, Abitibi Subprovince, Canada. Economic Geology Monograph, 6, pp. 424-444.

FYFE, W.S. and KERRICH, R., 1984. Gold: Natural concentration processes. In: R.P. FOSTER, ed, Gold '82: The geology, geochemistry, and genesis of gold deposits. Rotterdam: Balkema, pp. 99-127.

GOLDFARB, R.J., BAKER, T., DUBE, B., GROVES, D.I., HART, C.J.R. and GOSSELIN, P., 2005. Distribution, character, and genesis of gold deposits in metamorphic terranes. Economic Geology 100th Anniversary volume, pp. 407-450.

ROCCI, G., BRONNER, G. and DESCHAMPS, M., 1991. Crystalline basement of the West African Craton. In: R.D. DALLMEYER and J.P. LECORCHE, eds, The West African Orogens and circum-Atalantic correlatives. Berlin Hiedelberg: Springer-Verlag, pp. 31-64.

PITCAIRN, I.K., TEAGLE, D.A., CRAW, G.R., OLIVO, G.R., KERRICH, R. and BREWER, T.S., 2006. Sources of metals and fluids in orogenic gold deposits: Insights from the Otago and Alpine Schists, New Zealand. Economic Geology, 101, pp. 1525-1546.

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