Numerical modelling of deformation and fluid flow in hydrothermal systems

McLellan, John George (2004) Numerical modelling of deformation and fluid flow in hydrothermal systems. PhD thesis, James Cook University.

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Geomechanical processes involved in mineralisation of several Australian deposits were tested using both continuous and discontinuous modelling techniques. Numerical modelling using both the finite difference method and discrete element method, were carried out by firstly devising conceptual models. This also allowed rigorous sensitivity testing of many input parameters throughout the modelling procedure. Fully coupled deformation and fluid flow modelling was applied to investigate and establish the influence of topography, structure and extension on porous media fluid flow. Iron ore genesis in the Hamersley Province, W.A., has been a contentious genetic issue for many years, with several ore genesis models proposed. Modelling results from this study confirm the mechanical feasibility of focusing both surface and basinal derived fluids towards sites of iron ore genesis during Proterozoic orogenic collapse, providing upward and downward migration of (reduced and oxidised)fluids during deformation, and allowing infiltration of banded iron formations and consequent silica loss during permeability increase, resulting in favourable conditions for ore genesis.

Extension and contractional models of potential scenarios of ore emplacement at the Century Zn-Pb-Ag deposit in northwest Queensland were tested by a combination of 3-D structural modelling and fully coupled fluid flow models. Three conceptual models were devised, and two of these were fully tested by numerical simulations (diagenetic and epigenetic), the other (syngenetic) by Geological Computer Aided Design (GoCad). Reconstruction of the 3-D structural model displayed thickness and grade relationships to NE trending growth faults, providing evidence that supports a syngenetic model of emplacement. Numerical models provide clear evidence that stratiform Zn-Pb mineralisation is difficult to achieve in subsurface conditions, due to the low permeability of shale units. During contractional deformation, modelling results suggest that it would most probably result in fault or vein style mineralisation at the Century deposit, due to the proximal nature of fluid focussing relative to the faults.

The Eastern Succession of the Mt Isa Inlier is a mineral rich region, with many large deposits such as Cannington Pb-Zn-Ag, Osborne Cu-Au, and Slewyn Cu-Au. This area has been the focus of mineral exploration for some years, and evaluating the prospectivity of this region is one of the current goals of the pmd*CRC. Stress partitioning in this region, during the Isan Orogeny, may have played a vital role in determining the locations of mineral deposits as a result of failure and fluid flow. Discrete element modelling of the Eastern Succession provides strong correlations with known ore deposits and prospects, particularly in the Selwyn region, and also provides several testable targets. Comparisons of these results correspond well with other current prospectivity analysis of the region. The results indicate that mineralising fluids were preferentially focussed into zones of anomalous stress in and around fault zone bends and intersections, and intrusive metasediment contacts. Furthermore the optimum far-field stress that provides the best correlation with known deposit distribution was orientated E/SE rather that E/W, suggesting a component of regional transpression late in the 1600 – 1500 Ma Isan Orogeny.

Extension related mineralisation is commonly linked to near surface fluids penetrating into deeper parts of the crust and possible fluid mixing occurring. The general mechanics of extensional faulting are fairly well understood at a range of scales, and many models have been proposed. Hydrodynamic models, however, are considerably fewer. This study applied fully coupled deformation and fluid flow numerical models to test the role of extensional deformation and its effects on fluid flow, particularly at basement-cover interfaces. Large contrasts in the permeability of the basement and cover restrict fluid exchange between these two units, unless permeable faults or shear zones are present. Downward migration of fluids is a natural consequence of the extension of dilatant materials where strain rates are high. Rapid decreases in pore pressure, as a result of dilation and failure, may provide mechanisms for mass transfer across basement cover interfaces.

Item ID: 2131
Item Type: Thesis (PhD)
Keywords: geomechanical processes, mineralisation, ore deposits, numerical modelling, deformation, fluid flow, hydrothermal systems, fully coupled deformation, fully coupled fluid flow, ore genesis, Western Australia, Hamersley Province, Queensland, Eastern Succession, Mount Isa Inlier, Mount Isa Block, Century deposit
Date Deposited: 09 Feb 2010 02:10
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 50%
04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 50%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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