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Tectono-metamorphic controls on Archaean gold mineralisation in the Barberton greenstone belt, South Africa.


Team members: A. Dziggel, F.M. Meyer, A. Otto, S. Knipfer; in co-operation with A. Kisters (Stellenbosch University, SA) and M. Poujol (University of Rennes).

2int The Barberton greenstone belt, situated in the north-eastern sector of the Mpumalanga province in South Africa, is one of the oldest and best exposed Archaean greenstone belt on Earth (Fig. 1, 2). The greenstone belt comprises volcanic and sedimentary rocks formed between 3.5-3.1 billion years ago, and has long been recognized as one of the key regions for understanding the processes relating to early crustal evolution. Metamorphic grade increases from greenschist facies in the central parts of the belt to amphibolite facies at its margins, where the condensed isograds generally parallel the contact with the granitoid plutons. At least four episodes of magmatic activity have been documented from the granitoid terrain surrounding the belt, each of which being closely associated with periods of tectonism and metamorphism within the belt. Major episodes of tectono-magmatic activity have been dated at ca. 3.45, 3.23 and 3.1 Ga, the earliest of which are commonly interpreted as a result of subduction-related crustal shortening. 

Most of the deformation recorded within the belt, however, is believed to have occurred during the short-lived compressional episode at ca. 3.23 Ga, which coincides with the syndeformational deposition of the upper Fig Tree and Moodies Group sediments and the intrusion of the ca. 3227 Ma old Kaap Valley tonalite (Fig. 2). This event was responsible for the upright, tight to isoclinal folding, the formation of thrust faults, and eventually resulted in the amalgamation of the northern and southern part of the greenstone belt along the Saddleback-Inyoka fault system. The emplacement of large volumes of sheet-like potassic granites, namely the ca. 3.1 Ga Nelspruit, Piggs Peak and Mpuluzi batholiths, marked the final stabilisation of the greenstone belt and its surroundings. It has been suggested that this late-tectonic history of the belt was associated with a change from convergent to transtensional (extensional) tectonics.

Fig1hp.jpgFigure 2. General geological map of the Barberton greenstone belt and surroundings. Filled circles show location of the four major gold deposits: A = New Consort, B = Sheba, C = Fairview, D = Agnes-Princeton.

The Barberton greenstone belt hosts the oldest recognized orogenic gold ores on Earth. The majority of structurally controlled gold deposits is hosted by greenschist facies metamorphic rocks, whereas only a few record amphibolite facies grade conditions. Gold introduction occurred late in the tectonic evolution of the belt, and has been interpreted to be temporally linked with the emplacement of the late potassic granites and syenites. The close spatial association of gold deposits with the condensed isograds along the sheared margins of these batholiths has been used to postulate a direct link between granitoid emplacement and gold mineralisation, but a genetic relationship has not been established.

The present project aims to establish the metamorphic and tectonic setting of the major gold deposits in the Barberton greenstone belt. Our main interest is to study the sequence of metamorphic, magmatic and tectonic events and their influence on economic gold mineralization on a regional scale. Providing a critical link between high- and low-temperature metamorphism, tectonism, granitoid plutonism and the timing of metamorphism and mineralisation in these metamorphic domains is essential for models on gold deposit genesis, and the results should improve our present understanding considerably.

 

Publications:

Dziggel, A., Knipfer, S., Kisters, A.F.M., Meyer, F.M. (2006) P-T and structural evolution during exhumation of high-T, medium-P basement rocks in the Barberton Mountain Land, South Africa. Journal of Metamorphic Geology, 24, 535-551.

Otto, A., Dziggel, A., Kisters, A.F.M., Meyer, F.M. (2007) The New Consort gold mine, Barberton greenstone belt, South Africa: Orogenic gold mineralization in a condensed metamorphic profile. Mineralium Deposita, 42, 715-735.

Dziggel, A., Otto, A., Kisters, A.F.M., Meyer, F.M. (2007) Tectono-metamorphic controls on Archaean gold mineralization in the Barberton greenstone belt, South Africa: an example from the New Consort gold mine. In: Van Kranendonk, M., Smithies, R.H., Bennett, V. (Eds.) Earth’s oldest Rocks. Developments in Precambrian Geology, Elsevier, 15, 699-727.


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