Numerical modelling of the ore forming fluid migration in the sediment-hosted stratiform copper deposit, Zambian Copperbelt

Abstract

As an aid to better understanding fluid flow processes and potential source rocks for the Zambian Copper Belt, preliminary numerical fluid flow modelling has been undertaken, coupled to salinity and heat transfer. The computer code used for the modelling is from Yang and Large (2001), and has been previously used to successfully model fluid flow-salinity-heat transfer in the McArthur Basin related to Stratiform Zn-Pb-Ag ore genesis (Large, et al., 2002). A 43.5km ´ 17km theoretical geological section was constructed, based on our current understanding of the structure, stratigraphy and basin architecture in the ZCB. The geological model section incorporates elements of basement, footwall succession, Ore Shale, hangingwall lower Roan siltstones and carbonates, upper Roan siltstones, carbonates and a layer of salt, Mwashia and overlying Kundelungu shales. Porosity and permeability parameters have been assigned based on our understanding of the sedimentary facies, alteration zones, petrographic evidence and local to regional structures. The most permeable elements assigned in the model are; the Mindola Clastics in the immediate footwall to the Ore Shale; crosscutting breccias in the hangingwall (below salt layer) and fault zones. The least permeable elements assigned are the Ore Shale, hangingwall carbonates and siltstones, the Mwashia shales and the basement. A one km thick salt layer in the Upper Roan has been assigned a salinity of 30 wt% (the maximum allowable by the computer code). Other sedimentary units have been given lesser initial salinities depending on the interpreted sedimentary environment (evaporitic, marine or lacustrine). A normal geothermal gradient through the basin of 30°C per km has been assumed. The model has been designed to test fluid flow and temperature gradients associated with the downward flow and convective circulation of saline fluid, from the hanging wall evaporitic salt layer into the Lower Roan stratigraphy and basement. A number of scenarios have been tested by varying the permeability of certain basin elements and fault structures. The results indicate that the most likely scenario to account for the stratiform Cu deposits involves penetration of high salinity brines from the Upper Roan salt layer down into the basement along a series on normal master faults, with brine circulation and leaching of Cu from the basement terrain. The oxidised metalliferous fluids more upwards along second order faults and are channelled into the Mindola Clastics below the organic-bearing shale cap rocks where potassic alteration and Cu-mineralisation occurs.