Hydrothermal iron oxide copper-gold and related deposits. Volume 1 / Медно-золотые месторождения в железистых кварцитах

Following the discovery of the giant Olympic Dam ore deposit in 1975, a realisation developed that there was an important class of mineral deposits not previously appreciated. It became apparent that this class, the Iron Oxide Copper-Gold deposits, included not only Olympic Dam, but also a number of other known deposits. It also became apparent that this was a class that could produce large, high grade prizes, of the order of 0.25 to 1 billion tonnes of around +1% Cu and 0.5 g/t Au. As a consequence this class has been one of the major targets of the exploration industry over the last decade, resulting in the discovery of further giant orebodies in Australia such as Ernest Henry, and Candelaria, Salobo, Sossego and others in South America.

This class of deposit however, does not represent a single style or a common genetic model, but rather a family of loosely related ores that share a pool of common characteristics. The principal feature they have in common is the abundance of iron oxides that accompany the ore and the intensity of the associated alteration, particularly albitisation and Fe metasomatism. The iron oxides are present as either magnetite or hematite and almost invariably precede the emplacement of the other economic minerals. These deposits are found throughout geologic time, around the globe and in settings from intra-cratonic, to continental margins above subduction zones.

There is a differences of opinion both on the processes involved in their formation, matched by the diversity in styles of mineralisation within the class, as well as which deposits should be included within the family.

The aim of this volume is to bring together a wide range of knowledge, experience and opinion from around the globe to assist in understanding this economically and geologically important family of deposits.

The discovery in 1975 of the giant Olympic Dam ore deposit on the Stuart Shelf of South Australia, and the subsequent realisation of its significance, attracted keen interest from the world's exploration industry. In the early years however, there was little knowledge of what the deposit represented, and it was not until 1983 when underground test openings were developed into the orebody that the realisation dawned it was an iron oxide rich "hydrothermal breccia complex" (Smith, 1993).

At that stage Olympic Dam was believed to represent a unique deposit, an example of a new style of mineralisation not previously recognised or sought. Consequently it sparked a rush to find a direct "look-alike". However, by the mid 1980's researchers and exploration geologists from industry had begun to realise that there were broad metallogenic similarities between Olympic Dam and other deposits around the world and that it was part of a class of deposits that included many orebodies that were already

known. This realisation culminated in the seminal paper of Hitzman, et. al. (1992) that distilled this understanding in the public domain and drew together the common threads - specifically for those deposits of Proterozoic age.

Hitzman, et. al., (1992) focussed attention on the common mineralogy of these deposits, specifically that the ores are dominated by iron oxides, usually with low Ti, present as either magnetite or hematite, with magnetite tending to prevail at depth and hematite towards the surface. In addition there is commonly an association of carbonate, Ba, P and F, with significant accompanying concentrations of light REEs. The hosts and surrounding rocks are also generally intensely altered, ranging from extensive sodic zones at depth and regionally, to potassic at shallow to intermediate levels, and sericite and silica near surface, while the hosts and surrounding rocks are intensely to variably Fe-metasomatised.

he Great Bear Magmatic zone in Canada, the Wernecke Breccias in Canada, the SE Missouri Iron Province in the US, Bayan Obo in China and the Redbank Breccias in Australia. Since then the list of orebodies that different authors assign to the family has been significantly expanded (see Haynes this volume and Pollard, this volume for lists of deposits, size, mineralogy, etc.).

Over the last decade this class of deposits has become a prime target for exploration, resulting in the discovery of two major deposits which are now in production (Candelaria in Chile and Ernest Henry in Australia) and several that are currently under development (eg. Sossego and Salobo in Brazil), from both the Proterozoic and the Phanerozoic. The attraction is obvious - the prize is both large and high grade and has not previously been the subject of concerted exploration activity, and as such is "still out there to be found".

However, despite the level of interest and exploration over the decade, and the increase in our knowledge of these deposits, there is still disagreement on how they are formed and even which deposits belong to the class. Nor has it been possible to predict barren ironstones from those that contain economic mineralisation. The aim of this volume is to bring together a diversity of knowledge, experience and opinion from around the globe to assist in understanding this important family of deposits and answering some of the questions they pose.

The iron oxide copper-gold (IOCG or FeOx-Cu-Au) class represents a family of loosely related deposits each of which shares some characteristics from a larger pool. Deposit A may overlap with B, B with C, and so on, although A and G might display significant differences.

By definition their common link is the association of low Ti iron oxides with the ore. They range from Fe-apatite ores as at Kiruna without any significant Cu-Au, to Fe-REE-F at Bayan Obo, again without any Cu or Au of economic value, to the Fe-Cu-U-Au-REE of Olympic Dam, and the Fe-Cu-Ag without Au at Mantos Blancos, etc. They also occur over an extensive depth range, from the ductile field as at Osborne in Australia to a shallow brittle regime as at Olympic Dam (Pollard, 2000). Many of the Proterozoic deposits are intracratonic, while some of the key Palaeozoic systems are found on the continental margin above a subduction zone (Hitzman, 2000). Indeed there are also deposits, such as Palabora that some say are key members the family (Vielreicher, et. al., this volume), but which others believe are un-related.

While some researchers seek to identify a common genetic link between all of the deposits attributed to the family there does not appears to be a single model that all obey, although specific models may be invoked with great success for particular groups of deposits or regions (Pollard, 2000, apply well to particular deposits, while hybrids could as readily apply in other cases).

Others believe that the family represents a number of different styles of deposit that share particular characteristics (Groves, pers. comm.). The association of elements may be no more than a compatibility of chemistry related to solubility, the common modes of both hydrothermal transport and of precipitation encountered in the Earth's crust, in much the same way that lead and zinc are associated in a wide variety of geological environments, from basinal brines, to igneous associations or in metamorphic fluids. In fact both iron oxides, copper and gold may be transported in, and deposited from all of these media under the right conditions (Yardley, et. al., 2000)

As detailed above, these deposits are characterised by an abundance of iron oxide and a relative deficiency of iron sulphides. Almost invariably gold, and copper occurring as a sulphide and often accompanied locally by substantial pyrite and/or pyrrhotite, overprints or sulphidises/replaces earlier emplaced iron oxides. The iron oxides may be an early evolutionary phase of a continuing hydrothermal event, possibly as at Olympic Dam, or alternatively be a little related pre-exiting ironstone emplaced in or along structures that continue to act as fluid conduits through geological time. Examples of the latter are in the Tennant Creek field in northern Australia (Skirrow, submitted) where only a small percentage of the 600 known early stage ironstones of the district are mineralised, or in the iron belt of Chile where there are a significant number of ironstones, many of which are barren (eg. El Romeral) and some (eg Mantos Blancos) which are mineralised.

Haynes (this volume) points out that the deposits of this class are part of a broader spectrum of ores whose mineralogy and form are the result of the surrounding country rock, its composition, oxidation state, chemistry, metal endowment, fluid and heat engine availability and structure. Similarly the iron oxide copper-gold deposits have characteristics in common with the ironstone gold deposits of the Archaean and Proterozoic.

Table of Contents

1. Hydrothermal Iron-Oxide Copper-Gold & Related Ore Deposits (T. Michael Porter)
2. Iron Oxide-Cu-Au Deposits: What, Where, When, and Why (Murray W. Hitzman)
3. Evidence of a Magmatic Fluid and Metal Source for Fe-Oxide Cu-Au Mineralisation (Peter J. Pollard)
4. Alternative Brine Sources for Fe-Oxide(-Cu-Au) Systems: Implications for Hydrothermal Alteration and Metals (MarkD. Barton and David A. Johnson)
5. The Chemistry of Crustal Brines: Tracking Their Origins (Bruce WD. Yardley, David A. Banks and Andrew C. Barnicoat)
6. Iron Oxide Copper (-Gold) Deposits: Their Position in the Ore Deposit Spectrum and Modes of Origin (Douglas W' Haynes)
7. Geology of the Olympic Dam Cu-U-Au-Ag-REE Deposit (Lachlan J Reynolds)
8. Overview of Iron Oxide-Copper-Gold Deposits in the Curnamona Province and Cloncurry District (Eastern Mount Isa Block), Australia (Patrick J. Williams and Roger G. Skirrow)
9. The Evolution of the Ernest Henry Fe-Oxide-(Cu-Au) Hydrothermal System (Geordie Mark, NickH.S. Oliver, Patrick J. Williams, Rick K. Valenta and Richard A. Crookes)
10. The Portia - North Portia Cu-Au(-Mo) Prospect, South Australia: Timing of Mineralisation, Albitisation and Origin of Ore Fluid (Graham S Teale and C Mark Fanning)
11. Gold-Copper-Bismuth Deposits of the Tennant Creek District, Australia: A Reappraisal of Diverse High-grade Systems (Roger G. Skirrow)
12. La Candelaria and the Punta del Cobre District, Chile: Early Cretaceous Iron-Oxide Cu-Au(-Zn-Ag) Mineralization (Robert Marschik, Richard A. Leveille and Walter Martin)
13. The Panulcillo and Teresa de Colmo Copper Deposits: Two Contrasting Examples of Fe-Ox Cu-Au Mineralisation from the Coastal Cordillera of Chile (David Hopper and Arturo Correa)
14. Alemao Copper-Gold (U-REE) Deposit, Carajas, Brazil (Petronilia C. Ronze, Anselmo D. V. Soares, Marcos Giovanni S. dos Santos, Celio F. Barreira)
15. The Igarape Bahia Au-Cu-(REE-U) Deposit, Carajas Mineral Province, Northern Brazil (Edison Tazava and Claudinei Gouveia de Oliveira)
16. Salobo 3 Alpha Deposit: Geology and Mineralisation (Leonardo H. Souza and Eduardo A. P. Vieira)
17. The Salobo Iron Oxide Copper-Gold Deposit, Carajas, Northern Brazil (Karin Requia and Lluis Fontbote)
18. Southeast Missouri Iron Metallogenic Province: Characteristics and General Chemistry (Cheryl M. Seeger)
19. Geology of the Proterozoic Iron Oxide-Hosted, NICO Cobalt-Gold-Bismuth, and Sue-Dianne Copper-Silver Deposits, Southern Great Bear Magmatic Zone, Northwest Territories, Canada (Robin E. Goad, A. Hamid Mumin, Norman A. Duke, Kathryn L. Neale, Derek L. Mulligan)
20. The Geology and Genesis of the Bayan Obo Fe-REE-Nb Deposit: A Review (Martin Smith and Wu Chengyu)
21. Iron Oxide Systems and Base Metal Mineralisation in Northern Sweden (Christopher J. Carlon)
22. Exploration for Iron Oxide Copper Gold Deposits in Zambia and Sweden: Comparison with the Australian Experience (Bruce Nisbet, John Cooke, Michael Richards and Craig Williams)
23. The Vergenoeg Fayalite Iron Oxide Fluorite Deposit, South Africa: Some New Aspects (Petrus J Fourie)
24. The Phalaborwa (Palabora) Deposit and its Potential Connection to Iron-Oxide Copper-Gold Deposits of Olympic Dam Type (Noreen M. Vielreicher, David I. Groves and Richard M. Vielreicher)
25. Mineralisation of the Phalaborwa Complex and the Carbonatite Connection in Iron Oxide-Cu-Au-U-REE Deposits (Robin E Harmer)