Peter Schaubs

Factors controlling the location of gold mineralisation around basalt domes in the stawell corridor: insights from coupled 3D deformation – fluid-flow numerical models

We present coupled 3D deformation – fluid-flow models which place constraints on the importance of basalt dome shape and interpreted synmineralising shortening direction in localising gold mineralisation around basalt domes in the Stawell corridor, western Victoria. Gold mineralisation in the Magdala orebody at the Stawell mine occurs predominantly within a thin metasomatised unit named the Stawell Facies which blankets the basalt domes and also occurs close to parasitic fold-like basalt lobes on the basalt domes. In dome-scale models that do not contain basalt lobes, areas with the maximum fluid-flow rates occur on the tops of the flanks of the domes where there is a dramatic change in dip of the basalt, and a change from contraction to dilation which creates a significant pore-pressure gradient. In models that contain basalt lobes, the location of high fluid-flow rates is strongly controlled by the presence of these lobes. High fluid-pressure gradients are created between the contracting Stawell Facies in the area between the lobe and the main domes and those areas dilating above. Areas of significant dilation occur on the shallow-dipping portion at the top of the dome and cause fluid to flow towards them. Areas that have significant dilation are also areas of tensile failure in some cases and are coincident with areas of known quartz vein-associated mineralisation. In the Magdala Dome models, only the east-northeast – west-southwest- and east – west-shortened models record high fluid-flow rates in areas of known mineralisation, which is consistent with the interpreted synmineralisation-shortening directions. Therefore in this situation, fluid-flow rates during east-northeast – west-southwest- and east – west-shortening can be used to indicate the potential location of gold mineralisation. In numerical models of the Kewell Dome (a prospect to the north), the position of areas of high fluid-flow rate when shortened in the east-northeast – west-southwest and east – west direction, combined with information from limited drilling, indicated the potential for gold mineralisation at the southwest end of the dome. Diamond drillholes in this area yielded significant gold values.

Application of 3D models and numerical simulations as a predictive exploration tool in western Victoria

3D models and computer-based numerical simulations have been used in the exploration industry for some time to visualise the geometry and mechanisms resulting in the formation of orebodies. However, due in part to computational limitations, few numerical simulations have been run on complex (real) geometries in order to predict the location of new ore systems. Presented here are the results of an exploration program developed by the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) and MPI (now Leviathan Resources) in the orogenic-gold system of western Victoria that utilised 3D modelling and numerical finite-element simulations to successfully target several new orebodies and predict their geometries and extent. Existing drillcore databases were utilised to constrain the geometries of known deposits and associated mafic domes, the effects of known post-mineralisation faulting was systematically removed and syndeformation fluid flow was then modelled within the system. The results of these simulations were compared with the known geometry of the mineralised systems about these deposits in order to test the simulation parameters and accuracy. 3D models were also developed of poorly constrained target domes in regions with no outcrop utilising potential-field datasets and limited drilling data. Simulations were then run on these model geometries using the tested parameters in order to predict the likelihood of mineralisation in these systems, its geometry and (most importantly) its location. These targets were then drilled resulting in the discovery of previously unknown gold deposits associated with the Kewell Dome northwest of Stawell.

Predictive targeting in Australian orogenic-gold systems at the deposit to district scale using numerical modelling

3D numerical models of coupled deformation and fluid flow provide a useful tool for exploration in orogenic-gold systems. Numerical modelling of ore-forming processes can lead to a reduction in targeting and detection risk, thus improving the value proposition of mineral exploration. Hydrothermal mineralisation arises from a complex interplay of deformation, fluid flow, conductive and advective heat transport, solute transport and chemical reactions. Coupled simulation of all of these processes represents a significant computational challenge that cannot be solved within the time-scale of a mineral exploration program. However, the problem can be simplified by identifying a subset of processes representing the first-order controls on mineralisation at the scale of interest. For most orogenic-gold systems, it is argued that the first-order controls on mineralisation at the camp to deposit scale are deformation-induced dilation, fluid flow and fluid focusing. Hence, numerical models of coupled deformation and fluid flow can provide a quantitative insight into the localisation of ore-forming fluids in this type of system. In two case studies, known deposits were modelled in order to determine the critical deformation and fluid-flow-related factors controlling the localisation of mineralisation in these systems. The quantitative results from the forward models were then used as a basis for constructing predictive models that were applied to regional targeting, prospect ranking and selecting the choice of detection methods. Both case studies show that numerical modelling is capable of reproducing the distribution of known anomalism, and that it can predict anomalies that were not expected or accounted for by purely empirical analysis.

A numerical modelling approach to fluid flow in extensionalenvironments: implications for genesis of large microplaty hematite ores

Abstract Numerical modelling of deformation and fluid flow on a regional scale was utilised to test recent models for deep (≥ 5 km)penetration of surface fluids involved in genesis of Whaleback style microplaty hematite ores in the Pilbara, Western Australia. Current models suggest they formed in the waning stages of the ca. 2300 Ma Ophthalmian Orogeny. This study uses a finite difference continuum modelling code, Fast Lagrangian Analysis of Continua (FLAG), to test whether deep penetration of surface fluids is mechanically feasible in relation to ore formation. Several “conceptual models” were tested and model boundary conditions were conducive to an extensional collapse of the mountain range. During extensional deformation, downward fluid flow is evident along a subvertical fault for reasonable strain rates and topographic elevation. Throughout deformation, fluids move progressively deeper into the model. Deeper seated fluid is forced upwards from the base of the model due to perturbations in hydraulic head and pore pressure. Both fluids mix at intermediate levels and this mixing process becomes progressively deeper within the model as extension takes place. Fluid mixing is apparent at the banded iron formation (BIF) and fault boundaries, as is lateral fluid migration along the BIF layers. This study supports the hypothesis that downward flow of meteoric fluid may have played a role in the evolution of the microplaty hematite ores. However, unlike the model of Morris [Morris. R.C., 1985. Genesis of iron ore in banded iron formation by supergene and supergene-metamorphic processes—a conceptual model. In: Wolff, K.H. (Ed.), Handbook of Strata-Bound and Stratiform Ore Deposits, vol. 13, Elsevier, Amsterdam, pp. 73–235] in which downward penetration is essentially superficial, the association of fluid flow with extension may have allowed deep fluid penetration (≥ 5 km) and potential fluid mixing, as proposed by Powell et al. [Geology 27 (1999) 175] and Taylor et al. [Econ. Geol. 96 (2001) 837].