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Journal of the Geological Society

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Discussion on extensional tectonics and magmatism as drivers of convection leading to Iberian Pyrite Belt massive sulphide deposits?

G. Digges La Touche writes: Boulter’s (1996) paper on the mechanisms leading to the formation of massive sulphide deposits in the Iberian Pyrite Belt, at Rio Tinto, interprets them as a sill sediment complex, with magmatism and extensional tectonics acting as the driving forces behind fluid convection. From the evidence presented by Boulter (1996) this interpretation appears plausible, although there do appear to be several inconsistencies in the ore-fluid convection model proposed.

Boulter (1996) invokes two mechanisms for convective fluid flow. The first of the mechanisms put forward is an adaptation of the sea water convection model of Russell (1978). It is proposed that during extensional tectonics sea water flowed down syn-sedimentary faults to be heated at depth and then flowed upwards again due to buoyancy through fracture networks or some other high permeability zone. The problem with this model is that it is essentially a low-volume system with flow down and up descrete fault and fracture zones. In order to achieve the ore-fluid volumes necessary to create the Rio Tinto massive sulphides unrealistically large long lived fluxes would be necessary. It is not suggested that this mechanism was not occurring, just that it would not have contributed significantly to the metal budget.

Additional problems with the extension driven convective cell mechanism exist in that it is hard to understand why in the model of Russell (1978) sea water should flow down extensional faults, considering that the sediments in which the syn-extensional faults are forming would already be fully saturated

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