SUMMARY

A detailed investigation of the Proterozoic sediment-hosted base metal deposits in Western Gondwana is envisaged in this Project. Recent studies on the Proterozoic base metal deposits on either side of the Atlantic indicate many common sedimentological, structural, geochemical and isotopic characteristics probably arising from a common geological evolution. However, no attempt has yet been made to correlate these characteristics, and the proposed Project will carry out a detailed correlation. Such a study has important implications for mineral prospecting and also for metallogenic modeling of base metal deposits, in general, and for those in Western Gondwana, in particular. The proposal arises out of the urgent need to develop scientifically valid exploration strategies based on geotectonical, sedimentological, structural, geochemical and mineralogical characteristics of base metal deposits and also from our experience in studying many of the deposits.

The general objectives of the Projects include:

Specific aspects of the study are:

1. Structural and stratigraphical controls of the mineralization
2. Timing of the mineralization and age of host rocks
3. Sources of metal and sulfur
4. Temperatures of deposition and paleo-geothermal gradient
5. during the emplacement of the deposits
6. Nature of the ore forming fluids
7. Radiogennic and stable isotopic signatures
8. Metallogenic models
9. Clues to mineral exploration

Numerous individuals and groups with many years of experience in the study of base metal deposits will participate in the five year long Project. Active participation of graduate students is envisaged at all stages.

PROPOSITION DE PROJET IGCP 450

RESUMÉ

Ce projet propose un examen détaillé des gisements métalliques stratiformes Protérozoïques du Gondwana occidental. Des études récentes concernant les gisements Néoprotérozoïques de deux côté de l’Atlantique montrent de nombreux caractères sédimentologiques, structuraux, géochimiques et isotopiques semblables, résultant probablement d’une évolution géologique commune. Cependant, à ce jour, aucune corrélation n’a été tentée sur base de ces caractéristiques, et le projet envisagé se propose d’étudier en détail une telle corrélation. Cette étude pourra avoir d’importantes implications, tant au niveau de l’exploration des minéralisations métalliques que du modèle métallogénique général de leur mise en place, notamment pour les gisements du Gondwana occidental.

Le projet trouve sa justification dans le besoin urgent de développer des stratégies scientifiques valables d’exploration basées sur les caractéristiques géologiques, structurales, géochimiques et minéralogiques des gisements métalliques, et s’appuie sur l’expérience acquise par l’étude de nombreux gisements.

Les principaux objectifs du projet sont les suivants :

1. Contribuer au développement des modèles génétiques des gisements métalliques stratiformes Proterozoïques d’Amérique du Sud et d’Afrique.
2. Corréler et documenter l’évolution métallogénique des gisements stratiformes Protérozoïques de l’Ouest du Gondwana, en insistant particulièrement sur leurs caractéristiques sédimentologiques, minéralogiques et géochimiques.
3. Préparer une carte métallogénique des gisements métalliques stratiformes du Protérozoïque du Gondwana occidental.
4. Constituer une banque de donnees sur les gisements métalliques stratiformes Protérozoïques du Gondwana occidental.

Les aspects spécifiques du projet sont :

1. Contrôles structuraux et stratigraphiques de la minéralisation
2. Période de minéralisation et âge des sédiments hôte
3. Sources des métaux et du soufre
4. Températures de dépôt et gradient paléo-géothermique pour contraindre le rôle du cadre géotectonique dans la mise en place des minéralisations.
5. Nature des fluides porteurs de la minéralisation
6. Signatures radiogéniques et des isotopes stables
7. Modèles métallogéniques
8. Indicateurs pour l’exploration minérale

Ce projet de cinq ans bénéficiera de la participation d’individualités et de groupes possédant de nombreuses années d’expérience dans l’étude des gisements métalliques. Une participation active d’étudiants post-universitaires est envisagée à tous les stades du projet.

PROPOSTA DE PROJETO PARA O IGCP 450

RESUMO

Uma investigação detalhada a respeito dos depósitos de metais base em sedimentos proterozóicos do Gondwana Ocidental, é pretendida através deste projeto. Estudos recentes em depósitos neoproterozóicos e mesoproterozóicos situados em ambos os lados do Atlântico, indicam a existência de muitos atributos comuns, geológicos, mineralógicos, estruturais, geoquímicos e isotópicos, provavelmente relacionados a uma evolução geológica comum. Contudo, nenhuma tentativa de correlacionar esses atributos foi até agora realizada. Um trabalho dessa natureza, terá importantes implicações para a exploração e prospecção de novas áreas mineralizadas, como também para a modelagem metalogenética de depósitos de metais base de um modo geral e, em particular, para aqueles situados no Gondwana Ocidental.. A proposta foi construída, a partir da urgente necessidade de se estabelecer estratégias de exploração mineral científicamente adequadas, como também da experiência e conhecimento dos autores da proposta, no estudo de diversos depósitos. São objetivos gerais da proposta:

1. Contribuir para o desenvolvimento de modelos genéticos para os depósitos de metais base proterozóicos em sedimentos, situados na América do Sul e na África.
2. Documentar e correlacionar as informações a respeito dos depósitos de metais base proterozóicos em sedimentos do Gondwana Ocidental, dando-se ênfase aos atributos geológicos, geoquímicos, estruturais, mineralógicos e isotópicos.
3. Elaborar um mapa de depósitos minerais do Gondwana Ocidental, para os depósitos de metais base em sedimentos, com informações que servirão de base para futura elaboração de mapas metalogenéticos.
4. Construir um banco de dados para os depósitos.

São objetivos específicos deste trabalho:

1. Controles estruturais das mineralizações
2. Idade das mineralizações e das rochas encaixantes
3. Fontes dos metais e do Enxofre
4. Temperaturas de formação das mineralizações e gradiente paleo geotérmico
5. Ambiente geotctônico durante a formação das mineralizações.
6. Natureza dos fluidos mineralizantes
7. Assinaturas isotópicas (isótopos radiogênicos e isótopos estáveis)
8. Modelos metalogenéticos
9. Diretrizes para exploração mineral

O projeto, com duração de 5 anos, terá participação de pesquisadores e de grupos de pesquisa com muitos anos de experiência no estudo de depósitos de metais base proterozóicos, em diversas regiões do Gondwana Ocidental. Será estimulada a participação ativa de estudantes de pós-graduação em todos os estágios de desenvolvimento do projeto.

INTRODUCTION

The Proterozoic is a highly productive period during which huge deposits of base metals were formed. For example, the Neoproterozoic sedimentary deposits of Congo DR and Zambia contain total reserves of about 3 billion tons at a grade of 4 % Cu. In Australia, the largest examples are in the Mesoproterozoic, where McArthur, Mount Isa-Hilton and Broken Hill have world class reserves that total more than 100 million tons of metals (Pb+Zn). This suggests that the Proterozoic is an important metallogenic Eon. Thus there is a large potential for the discovery of additional base metal deposits in other extensive Proterozoic sedimentary basins of Western Gondwana.

Several studies have been carried out during the past two decades on Proterozoic base metal deposits of Africa (e.g. Binda and Mulgrew, 1974; François, 1974; Cailteux, 1976; Ryan et al., 1986; Rozendaal, 1986; Viljoen et al., 1986; Wagener and Schalkwyk, 1986; Voet and King, 1986; Unrug, 1983; 1988; Van Vuuren, 1986; Ruxton, 1986; Lombaard et. al., 1986; Innes and Chaplin, 1986; Annels, 1989; Borg, and Maiden, 1989); Mendelsohn, 1989; Lefebvre, 1989; Sweeney et al., 1991; Burnard et al., 1993; Kamona, 1993; Hanson et al, 1994; Cailteux, 1994; Cailteux et al., 1994; Sweeney and Binda, 1994; Schwartz et al., 1995; Kampunzu et al., 1998; Kamona et al., 1999) and of Brazil (e.g. Dardenne and Freitas Silva, 1999; Pedrosa Soares et al., 1992; Iyer, 1984; Iyer et al., 1992; 1995; 1999b; 1999c; Misi, 1978; Misi and Kyle, 1994; Misi et al., 1996; 1997a; 1997b; 1999a; 1999b). These studies indicate that several Proterozoic Pb-Zn deposits of the São Francisco Craton of Brazil are aligned across the Atlantic with similar deposits of Damaran and Lufilian belts.

The deposits and host rocks on either side of the Atlantic and also within Africa and South America share what appear to be common geological, structural, geochemical and isotopic characteristics. Surprisingly, there has never been any formal attempt to correlate these characteristics. The project being proposed is directed towards addressing this situation. Based on data published in the literature cited above, there is a good indication that many, if not all, of these Proterozoic deposits could have originated from the same or similar process, arising probably from a common geological evolution of the supercontinents of Rodinia and Gondwana during the Mesoproterozoic and Neoproterozoic, respectively. The geological evolution of Gondwana and the map of Gondwana published by IGCP 288 (Gondwanaland sutures and fold belts) will serve as a basis for the proposed IGCP project, which will create a database and produce a mineral deposit map of the Proterozoic base metal deposits of Western Gondwana.

Recent studies have provided new insights into the geological evolution and the process of ore formation for some individual deposits of Western Gondwana. However, an important question regarding the global occurrence and distribution of Proterozoic sediment-hosted base metal deposits still remains unanswered or only partially answered. Why is the Katangan metallogenic province in Congo DR and Zambia the only major sediment-hosted Cu-Co province in the Neoproterozoic, despite the occurrence of similar host rocks in related fold belts in other areas such as the Damaran (Namibia), Ghanzi Chobe (Botswana) and in the periphery of the São Francisco Craton (Brazil). Despite the many similarities of the deposits across the Atlantic no large base metal deposits have yet been discovered in the vast Proterozoic sedimentary basins of South America. Is this enigma due to a lack of detailed exploration or a vagary of nature?

To begin with there is an urgent need for a well-coordinated and integrated Trans-Atlantic correlation of the stratigraohy, sedimentology, age (of sediments), tectonic regimes etc of the Proterozoic sedimentary basins. This in turn will permit in correlating the large deposits of Africa with the small to medium sized deposits in the Proterozoic base metal provinces of South America. Such a correlation is possible only through a systematic, detailed and multi-disciplinary study of the various lead-zinc-copper deposits on either side of the Atlantic and the subsequent integration of the results within the context of the geological evolution of the supercontinents of Rodinia and Gondwana.

Major carbonate-hosted Pb-Zn deposits (with greater than 10 Mt of ore) include Kipushi (Congo DR), Kabwe (Zambia), and Tsumeb (Namibia) in the Pan-African Damaran-Lufilian fold belt of central Africa. Though many investigations have been carried out during the last few years on these world-class Pb-Zn deposits, the inter-relationships between these deposits are still poorly known. Recent studies (Kamona et al., 1999) indicate the need for more detailed investigations to characterize the isotopic signatures of the host rocks, the underlying basement rocks and the various ore deposits in the Damaran-Lufilian belt. In addition, a cluster of Pb-Zn deposits, possibly related to the Damaran-Lufilian ores, occurs in Angola and in Congo Brazzaville. The genetic association of these deposits with the Damaran-Lufilian deposits is poorly understood.

It is interesting to note that the Neoproterozoic Pb-Zn deposits on both side of the Atlantic show several features typical of MVT type deposits, such as platform carbonate host rocks, solution and collapse structures (some), absence of clear genetic links with igneous rocks, common occurrence of organic matter, etc. (e.g. Anderson and Macqueen, 1982; Spirakis and Heyl, 1996; Sangster, 1990). However, unlike the MVT deposits many of the Neoproterozoic Pb-Zn deposits of Western Gondwana show relatively high temperatures of formation (up to 300°C) and non-homogeneous lead isotopic signatures. This indicates the need for developing suitable metallogenic models to explain all the characteristics of the Neoproterozoic Gondwana deposits of South America and Africa. The proposed project will focus its attention in this direction.

Closely associated with the Pan-African Pb-Zn deposits in the Damaran-Lufilian fold belt are the huge stratiform Cu-Co ore deposits of the Congo DR-Zambia Copperbelt of central Africa, which are hosted by Neoproterozoic shales and quartzites. Similar but less known deposits occur in Botswana (e.g. Ngwako Pan Cu deposit: Schwartz et al., 1995) and in Namibia (e.g. Klein Aub Cu deposit: Rexton, 1986), defining the Kalahari Copperbelt. The results of IGCP 302 clearly showed that the sedimentary rocks hosting all these Cu deposits share similar geological features, but no study has yet been undertaken to correlate the geological, mineralogical and geochemical characteristics of these Cu deposits and to integrate the results into a metallogenic model.

Strange as it may seem, in Brazil only two sediment-hosted deposits containing copper have been reported in the Proterozoic, each having much lower ore grades compared to those of Africa. They are the Camaquã Cu deposits (in the State of Rio Grande de Sul) and the Pedra Verde in the state of Ceará (Brizzi and Roberto, 1988). The Camaqua deposit is hosted in a sedimentary clastic sequence of Neoproterozoic age (Remus, 1999; Remus et al., 1997; 1998; and 1999). The Pedra Verde deposit is described as a metamorphosed Precambrian supergene Cu deposit (Collins and Loureiro, 1971) and presents some similarities with the Camaquã deposit with respect to the type of wall rocks (clastic sedimentary).

Outside of Brazil but within the Bolivian portion of the Brazilian Precambrian shield, a Au-Cu deposit (Don Mario) is found within a belt of Lower Proterozoic metavolcanic and metasedimentary rock named Crystal schist, one of several NW trending supracrustal schist belts separated by tracts of orhogneisses (Thompson et al., 1997). Don Mario is comprised of two main mineralizarion zones: Upper Mineralization zone (UMZ) with oxides and sulfides of Au, Cu, Ag, Pb, and Zn in an unique calc-silicate lithology and a Lower Mineralization Zone (LMZ) with high grade of Au associated with sulfides of Cu, Bi and Ag hosted in amphibole-quartz-garnet-biotite schist. The Au-Cu mineralization appears to be spatially related to BIFs and carbonates. Few data are available about this deposit in published literature and the origin is not clearly known. There are many potential mineralization sites in the region and exploration activities are being undertaken recently by Orvana Minerals Corporation. In Uruguay the Precambrian basement is divided by first-order shear zones into three terranes: the Archean to Paleoproterozoic Piedra Alta, the Mesoproterozoic to Cambrian Nico Perez and the Neoproterozoic to Cambrian Cuchilla Dionisio (Bossi and Campal, 1992; Bossi et al., 1993; Gaucher et al., 1998; Bossi et al., 1998; Gaucher, 1999,). Campal and Schipilov (1997), Gaucher et al. (1998) and Gaucher (1999 in press) have discussed the geotectonic evolution of these terranes. In all three terranes, sedimentary and metasedimentary sequences of considerable thickness occur. The potential for the occurrence of base metal deposits in these terranes varies, but the scarcity of geological data from these units is a major problem. Some units in part have been mapped to the scale 1:100.000, but for most of them only 1:500.000 geological maps are available (Bossi et al., 1998). An investigation on these units and their correlation with similar units in the neighboring Brazil and Argentina is very important. In Argentina the Geological Survey and Universities are pursuing research projects on the Proterozoic sediments and mineral deposits present (Gamba, pers. comm., 1999). According to Cingolani (pers. comm., 1999) there are some undated sediments in the boundary of Proterozoic-Lower Paleozoic, which need to be investigated, A detailed investigations on the various South American deposits and their comparison with those of Africa is crucial due to the urgent need to supplement the falling copper reserves, especially in Brazil, whose reserves are insufficient to meet the projected internal demand for the year 2010.

It should be mentioned that the Palaeoproterozoic belts of central and southern Africa, such as those located within the Magondi belt in Zimbabwe also host Cu deposits (e.g. Master, 1991a; 1991b). In addition, there are other Palaeoproterozoic Pb-Zn deposits hosted within the Transvaal and Griqualand ore province (e.g. Martini, 1990; Duane and Kruger, 1991; Duane et al., 1991; Duane and Saggerson, 1995) at the margin of the Kaapvaal Craton. Is the repetition of the same or at least similar type of mineralization in this part of Africa at an interval of about 1000 Ma (Paleo to Neoproterozoic) fortuitous or does it indicate a consistent genetic pattern related to the same "original" source of ore metals?

Palaeoproterozoic Cu deposits assumed to be of porphyry type as well as Archaean Cu-Co- (Ni-Cr –PGE) deposits have been reported both in central and southern Africa from the same regions of Pb-Zn deposits. Are the Archean deposits representatives of the primary sources of metals eroded and later remobilized during the Paleo- and Neoproterozoic to form the sediment-hosted ore deposits? Such a hypothesis has been repeatedly put forward by many investigators without any convincing evidence. A comparative study of the Neoproterozoic/ Palaeoproterozoic base metal deposits and their potential Palaeoproterozoic and/or Archaean metal sources in the region can help verify the validity of such a hypothesis.

A similar situation is present for the sediment hosted Pb-Zn deposits in the São Francisco Craton (SFC) of Brazil (Misi et al., 1999a; 1999b). Deposits covering a time span from 2.5 Ga (Boquira) to 0.6 Ga (Morro Agudo) are present in SFC and these deposits possess similar geological, structural and isotopic characteristics, suggesting a probable repetitive nature of the ore forming process. A contribution towards understanding the probable cyclic ore formation process in Africa and South America is envisaged within the framework of this proposed IGCP Project.

OBJECTIVES AND POTENTIAL BENEFITS

The major objectives of the project may be summarized into four categories, which are elaborated later.

1. To build a database of sediment-hosted base metal deposits in Western Gondwana;
2. To prepare a mineral deposit map of Western Gondwana for Proterozoic sediment-hosted base metal deposits;
3. To correlate and document the metallogenic evolution of Proterozoic sediment-hosted base metal deposits of Western Gondwana, with particular emphasis on geological, mineralogical and geochemical characteristics;
4. To contribute towards the development of metallogenic models for Proterozoic sediment-hosted base metal deposits with a view to aid exploration programs, especially in Africa and South America;

The project is expected to bring the following benefits:

1. Raise the awareness of the mineral potential of Gondwanaland on either side of the Atlantic.
2. Produce a mineral data base enlisting much needed mineral information in one readily available form along with mineral deposit map, which should assist mineral deposit research and exploration
3. Detailed mineral deposit maps for base metal deposits where, at least, genetic links can be established.

ATTRIBUTES OF THE PROBLEM

With the exception of a few base metal deposits in Africa there is a paucity of reliable, high quality data on various aspects of the base metal deposits in Western Gondwana. Further, few studies have been undertaken to compare deposits having similar characteristics even within Africa and South America, not to speak of an inter-continental comparison. The data, wherever available for the base metal deposits of Western Gondwana are of poorer quality compared to those of North America, Europe and Australia. Many new base metal deposits have been discovered in North America over the past five years and the success is attributed to comprehensive studies (Allen, 1999). Falling investment in research activities in recent years, especially in South America is aggravating the situation (Iyer et al., 1999a). This endangers the economic well being of many nations in need of natural resources for internal consumption and for vitally needed exports.

THE ROAD AHEAD

The specific facets of the metallogenesis of Proterozoic sediment-hosted base metal deposits of Western Gondwana to be investigated are elaborated below:

1. Preparation of a mineral deposit map and database for sediment-hosted base metal deposits of Western Gondwana.

The Gondwana map of IGCP 288 and the South African Development Countries (SADC) geochronological stratigraphic table (Van Vuurtert and Johnson, 1997) will be used as a reference frame for the proposed project. The following aspects of the mineralization will be shown:

Geological and structural settings;

1. Host rock (lithology) type
2. Approximate ore grade and reserves
3. Ore and gangue minerals

(e) Age of the major deposits and their host rocks.

Figures illustrating the stratigraphic position and the depth to the basement rocks will also be prepared to accompany the map. In addition, a database, listing all the characteristics of the base metal deposits of Western Gondwana, will be prepared and updated regularly. Such a database will form the basis for ore modeling and genetic interpretations.

2. Evolution of the sedimentary basins

Large-scale crustal tectonic processes are responsible for the formation of various kinds of sedimentary basins. These processes include thermal subsidence, crustal thinning and rifting, transtensional tectonics, or tectonic loading along passive continental margins or foreland basins in compressional tectonic situations (Leach, 1997). Each basin has its own characteristic tectonics, sedimentation, and thermal history and pore fluid composition. These characteristics affect the generation and migration of ore fluids, thereby significantly controlling the mineralization. Much work on the evolution of the sedimentary basins is being carried out by IGCP 419 and we hope to supplement the data whenever necessary and prepare a database, listing the characteristics of the sedimentary basins.

3. Structural controls

For the metal bearing fluids to migrate along the sedimentary basins there must be geological features that direct the fluids into areas where geochemical processes precipitate sulfide minerals in economic proportions. A detailed regional structural investigation could reveal ancient regional fracture zones, which could have served as the fluid pathways for base metal mineralization. The relative significance of structural features, such as solution collapse breccia, faults and fractures, basement topography, unconformity features etc., for different deposits, will be investigated.

4. Timing of the mineralization

The timing of the mineralization is an important factor in developing a metallogenic model. For many of the base metal deposits (especially those of Pb-Zn) much work needs to be carried out to constrain the ages of mineralization. For these deposits the lead isotopic dating of galenas, which provide model ages for the mineralization, need to be supplemented by other age dating techniques for the host rocks and associated structures. The poorly constrained age values are major hurdles that need to be overcome in order to develop any scientifically sound metallogenic model. The following approach will be adopted to address these important problems.

In the literature there are many lead isotope evolution models, which are being used for the interpretation of model ages and source rocks of sulfide ores from base metal deposits. According to Godwin et al. (1988), the early models were based on oversimplified source evolution and ore depositional models, which invariably assumed a uniform Earth for the derivation of the lead. This assumption constituted a major weakness for these earlier models. The proposed project intends to develop craton specific models for African and South American deposits similar to that developed by Godwin and Sinclair (1982) and Godwin et al. (1988) for the Canadian Cordilliera.

Currently, Iyer and others are developing a São Francisco Craton lead isotope evolution model using whole rock lead isotope data for samples spanning a period from the Archaean to Neoproterozoic. Such growth curves are considered to be more realistic and should be used in constraining the timing of the mineralization and metal sources in specific cratons of Western Gondwana. A comparison of such evolution curves will certainly shed much light on the geological evolution of the Gondwana Supercontinent.

To constrain the timing of mineralization, other dating methods such as Nd – Sm, Ar-Ar, Re-Os, U-Pb etc. will be employed to verify the calculated model Pb-Pb ages. It is generally known that, except in a few cases, the ages obtained using the techniques mentioned above are obtained on minerals other than sulfides and therefore some of these ages can easily be questioned. The project will therefore undertake Re-Os isotopic dating of selected ore deposits. This isotopic system is unique in that it can be used for dating both the sulfides and the oxides. In addition both Re and Os are chalcophile (and siderophile) elements and therefore are incorporated directly into the sulfide structure (Stein et al., 1998). Therefore, the Re-Os dating technique has enormous potential in dating ore deposits and the project intends to apply this method in dating the base metal deposits of Western Gondwana.

5. Sources of metal and sulfur

For ore genesis modeling it is essential to know the origin of the metal and the sulfur (Sangster, 1983; 1999). They could be derived from a single or multiple sources, and radiogenic (Pb) and stable isotopic (S) signatures provide information on the type of sources involved in the ore forming process. Many studies have been carried out to ascertain the sources of Pb and S in some of the well-known base metal deposits of Africa (Sweeney et al., 1986; Kamona et al., 1991; Walraven and Chabu 1994; Kamona et al., 1999). The lead isotope results indicate that upper crustal sources were significant as metal sources in most deposits, but mantle sources seem to have played a role in some deposits (e.g. Kipushi). The sources of sulfur are not well constrained, especially for the stratiform Cu-Co deposits where bacteriogenically derived sulfur, seawater sulfate and igneous sulfur may have been involved in varying degrees. Detailed studies on mineral separate of known stratigraphic position need to be carried out to better constrain the possible metal and sulfur sources.

The important parameters to be investigated in any problem relating to ore forming fluids are:

1. fluid composition;
2. fluid-flow mechanism;
3. fluid pathways;
4. precipitation mechanism.

Though the evolutionary history of the sedimentary basin controls to a large extent the fluid composition, other factors like sediment type, pore fluid composition, presence of evaporites and organic matter, thermal and pressure regime and fluid rock interaction are also important (Hanor, 1995). The present knowledge of the chemistry and the salinity of the ore forming fluids are fragmentary, especially for the South American deposits, due to the limited data available for some deposits and total lack of such data for many other deposits. The present project proposes to bridge the gap in the knowledge by carrying out extensive fluid inclusion studies, including chemical and isotopic analyses of the entrapped phases in mineral samples. The parameters to be determined are temperature, density, pH, ionic strength, saturation pressure, ionic ratios of Na, Ca, Mg, K, Br and Cl, and concentrations of SiO₂, Fe, Zn, Pb etc. The study will include research on the regional fluid systems, and the data obtained will be used as starting points to calculate reaction pathways using the method of Plumlee et al. (1994). This should help evaluate possible depositional mechanism.

7. Temperatures of ore deposition

Information regarding the temperatures of ore deposition may be obtained from fluid inclusion studies, sulfur isotopic compositions of coexisting sulfides, and trace element geochemistry of sulfides (Bornikov et al., 1995). The temperatures of deposition for the sediment hosted base metal deposits of Western Gondwana appear to vary among deposits, but most are above ca 150⁰C. In this project thermal gradient mapping in the vicinity of the deposits will be attempted using the three approaches mentioned above.

The high temperatures obtained (in some cases above 250°C) for some Pb-Zn sulfide deposits (Morro Agudo, Novo Redenção in Brazil; Kabwe in Zambia; Tsumeb in Namibia and others) indicate the possibility of tectonic processes generate high temperature mineralizing fluids within sedimentary basins. An estimate of the temporal variation of the paleo-geothermal gradient in the different cratonic regions and basins hosting the ore deposits could probably help assist in understanding the tectonic processes. Thermal modeling using a three layer earth model and paleo-radiogenic heat production, calculated from the Pb isotope data of the whole rock samples, can be used to characterize geothermal gradients in a particular cratonic regions of Western Gondwana. The thermal modeling should also provide information on the thermal evolution of the Western Gondwana Supercontinent and will be useful in interpreting the geotectonic environment during the emplacement of the base metal deposits.

8. Possible metallogenic model and application to exploration

After the data have been collected possible metallogenic models will be developed, which may assist exploration programs. According to Leach (1997) there is a diversity of ore forming processes and geological controls on ore deposition for MVT deposits. Thus diagnostic or universally applicable geological features that can be used as a guide to exploration activities of MVT deposits are difficult to find. This applies for many other types of sediment-hosted base metal deposits. In general exploration models used by mining companies consist of some combination of empirical models (empirically observed ore controls) together with genetic models, which are essentially regional or global in nature. The scale of exploration (mine, district, regional, global) determines the degree of application of each model (Fig. 1. Leach, 1997). Leach (1997) is of the opinion that "exploration programs with strong foundations in ore-forming processes will help geologists discriminate between essential and non-essential features related to the localization of MVT deposits." The project proposes to generate geological, geochemical and isotopic data on different sediment-hosted base metal deposits and develop possible district or regional or global metallogenic models. We believe these models will help exploration geologists elaborate their own exploration models and strategies.

9. Scientific collaboration

The study of metallogenesis involves the understanding and application of many scientific disciplines and researchers need to be kept informed of the rapid advances taking place worldwide. The project will act as a source of information on sediment-hosted base metal deposits by organizing Workshops and Extension courses in Africa and South America on various aspects of base metal metallogeny. Well-known scientists from academic institutions and mining companies, familiar with the base metal deposits of Western Gondwana, will be invited to conduct the courses.

All efforts will be made to build the courses around deposit examples from Western Gondwana. Students and geologists from government agencies and private mining companies will be encouraged to participate. Whenever possible the lectures will be administered in the local language (English, French, Spanish and Portuguese). The Geological Institute of the Universidade Federal da Bahia, Brazil, is offering one such course on the metallogenesis of base metal deposits. The same Institute also organized a workshop on base metal deposits in 1998 (Iyer et al., 1999a).

Cooperation is intended with other IGCP projects involved in the research of various aspects of Rodinia and Gondwana, particularly IGCP 440 (Mesoproterozoic assembly and Neoproterozoic breakup of the Rodinia Supercontinent) and IGCP 419 (Foreland basins of the Neoproterozoic belts in central-to-Southern Africa and South America). This collaboration should provide a wealth of new information and data on the geotectonic setting and metallogeny of many of the deposits. It should be emphasized that the two African co-proposers of this proposed IGCP Project are active members of IGCP 419.

The proposed IGCP project intends to bring most geoscientists (see the list of participants) investigating similar and related problems on either side of the Atlantic in joint programs involving fieldwork, analytical investigations and exchange of data and ideas. The project aims gathering high quality data and develop geological and ore genesis models compatible with the observations. Therefore, on a more general point of view, this project should improve our present understanding of stratiform Cu and stratabound Pb-Zn deposits in general and those of western Gondwana in particular.

WORK SCHEDULE

2000: First annual meeting at 31st IGC in Rio de Janeiro, Brazil, August 7-17:

2001: Second annual meeting in Windhoek, Namibia: Field workshop and conference on Proterozoic sediment-hosted Cu-Pb-Zn deposits in Namibia;

2002: Third annual meeting in Rio de Janeiro, Brazil: Field workshop and conference on Proterozoic sediment-hosted Cu-Pb-Zn deposits in Brazil;

2003: Fourth annual meeting in Harare, Zimbabwe; Field workshop and conference on Proterozoic sediment-hosted Cu-Pb-Zn deposits in Zimbabwe;

2004: Fifth annual meeting probably in Johannesburg, South Africa: Field workshop and conference on Proterozoic sediment-hosted Cu-Pb-Zn deposits in South Africa;

REFERENCES