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Stress-transfer modelling of gold systems at the Obuasi Gold Deposit, Ghana

Odling, Angus 2022. Stress-transfer modelling of gold systems at the Obuasi Gold Deposit, Ghana. MPhil Thesis, Cardiff University.
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Coulomb stress transfer modelling has been applied to understand the dynamics of seismogenic hydrothermal mineral deposits and proposed as a tool for gold exploration. Spatial correlations between positive Coulomb stresses transferred by palaeoseismic sequences and gold resource distributions were interpreted to indicate that areas of elevated Coulomb stress promoted permeability-enhancing aftershock sequences. Despite a promising start, the technique has not been widely applied in exploration since the first studies almost two decades ago. This work further investigates the use of stress transfer modelling by applying it to a gold deposit with a complex structural and mineralisation history. The supergiant Obuasi gold deposit (34Moz current resource) in Ghana experienced two stages of gold emplacement. In the first stage (D2Ob), disseminated sulphide-hosted gold mineralization formed in large reverse fault and shear zones. The later D5Ob stage consisted of strike-slip faulting and reactivation of the D2Ob structures, emplacing predominantly free milling gold in low displacement fault-hosted quartz veins. Several stress transfer models of both stages with different levels of fault simplification were created to represent alternative approaches that a geologist might use as exploration matures. Correlation between positive Coulomb stresses induced during D5Ob and gold resources at Obuasi, which are focused on stepover structures and fault intersections, indicates that Coulomb stress-induced aftershocks could have exerted a control on mineralisation during D5Ob. No link between positive Coulomb stresses and the gold distribution was found for D2Ob. This stage may have involved the propagation of cyclical fault-valve behaviour along fault segments, and aftershocks did not control gold distribution. Post-D2Ob fault geometry modification may have made the geometry of this model not truly representative. To apply Coulomb stress transfer modelling successfully to complex multi-stage gold deposits, the mineralisation mechanism and fault geometry evolution must be constrained. For systems (typically strike-slip) where focused aftershock permeability controlled gold distribution, Coulomb modelling may be an effective tool for exploration. For mineralisation phases with different mechanisms, stress transfer modelling can provide insight into the fault dynamics but does not have a predictive capacity.

Item Type: Thesis (MPhil)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Date of First Compliant Deposit: 15 September 2022
Last Modified: 06 May 2023 01:07

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