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

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Koziy, L (2007) Numerical simulation of the ore forming fluid migration in the sediment-hosted stratiform copper deposit, Zambian Copperbelt. PhD thesis, University of Tasmania.

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Abstract

A finite element numerical model was developed and applied to study densitydriven
convective flow as a potential ore-forming mechanism for sediment-hosted
stratiform copper deposits in the Zambian Copperbelt (ZCB).
The numerical model simulates free thermohaline convection in an
inhomogeneous, anisotropic, compressible porous medium completely saturated by a
compressible fluid bearing dissolved sodium chloride. The model also takes into
account possible inner salt and heat sources to simulate rock salt dissolution and heat
emission by a magmatic intrusion.
The model employs the finite element Petrov-Galerkin method with nonsymmetrical
'upwinding' weighting functions. An implicit time-stepping scheme
with 'weightings' involves an iterative procedure within each time step to handle the
nonlinearity of the problem.
The model validation was carried out by 1) comparison of the numerical results
with analytical solutions for simplified problems; 2) application of the model to three
problems of free thermal convection (anisotropy effect, convective cell aspect ratio
as a function of the Rayleigh number, and free discharge) and comparison with
published results of numerical and experimental studies; 3) simulation ofthe groundwater
flow in a complex faulted sedimentary section of the McArthur Basin
(Northern Territory, Australia) to compare the model output with the results of an
independent study (Garven et al., 2001).
The model was applied to a 43.5km x 17km numerical domain, representing a
theoretical geological section of the ZCB. The section incorporates elements of
basement, footwall succession (Mindola Clastic Formation), Ore Shale, hanging wall
Upper Roan dolomites, salt layer, Mwashia dolomitic and siliciclastic members and
overlying Kundelungu shales. As a layer-cake structure of the section shows no
significant topography, density variations due to geothermal gradient and Roan Salt
dissolution are considered the main mechanism driving pore fluid circulation.
Sixteen potential paleo-hydrologic scenarios were tested to study factors
controlling ore-forming fluid flow: fault network configuration, salt layer geometry,
rock salt physical characteristics, and piercement structures.The simulations showed notable convective flow through the basement; the
result corroborates the hypothesis of a basement source for the metals contained in
the Copperbelt orebodies (Sweeney et al., 1991). Supporting the study of fluid
inclusion data (Annels, 1989), the modeling confirms ore body formation under lowtemerature
and high salinity conditions. It also showed significant lateral flow of the
rising basement fluid through the Mindola Clastic Formation beneath the Ore Shale
seal; as a consequence, notable amounts of Cu can be precipitated in the footwall
clastics even assuming its low depositional efficiency, as supported by geochemical
and petrographic data. Low permeability shales and the salt layer form barriers to
cross-stratal fluid flow; nevertheless, horizontal flow rates in the Ore Shale are
sufficient for formation of ore bodies assuming this unit was an effective chemical
trap. Tested scenarios revealed the essential influence of Upper Roan Salt dissolution
on fluid flow and, therefore, amount of metal precipitated.
In summary, the study suggests that free convection driven by geothermal
gradient and dissolution of the massive Roan Salt sheet can be an effective oreforming
mechanism in the stratiform ZCB system. The rising basement fluid is
redirected laterally beneath the Copperbelt Orebody Member - Mindola Clastic
Formation interface. The flow rates of the metal-transporting fluid are shown to be
capable of forming ore bodies within the Ore Body Member and underlying footwall.
The modeled amount of potentially precipitated Cu agrees with the available
geochemical and petrographic data.

Item Type: Thesis (PhD)
Copyright Information:

Copyright 2007 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Date Deposited: 23 Jun 2012 13:21
Last Modified: 01 Jan 2017 16:00
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