Implicit 3D geological modelling in structurally complex environments

Abstract

Geological structures are closely related to the genesis of ore deposits due to their ability to transport and trap hydrothermal fluids. Structurally-controlled mineralisation commonly shows distinctive geometries, orientations and spatial distributions that derive from associated structures. Moreover, structures such as faults and shear zones can offset, truncate and redistribute earlier mineralisation. Consequently, the analysis of these structures provides fundamental insights on the genesis of ore deposits and their evolution in a regional context. Accurate three-dimensional (3D) geological models are therefore required to visualise and analyse ore bodies and their host rock geometries in order to understand the underlying controls on mineralisation or remobilisation processes. The aim of this thesis is to understand the relationship between ductile structures (i.e., folds), brittle structures (i.e., fractures) and mineralisation. 3D implicit modelling of dense lithogeochemical drillhole datasets is employed to assess mineralisation in structurally complex environments. Such models are used to determine the geometry, size and orientation of ore bodies and first-order structures, and to understand their spatial relationships to host rocks. Working hypotheses based on such geometrical analyses can then be tested and compared to field measurements and to other independent datasets (e.g., geophysical maps). This approach is applied here to both the Navachab lode gold (Namibia) and the Currawong base metal (Victoria, Australia) deposits. In both cases, links are established between modelled ore bodies and local to regional structural patterns. The modelling also constrains the structural frameworks of the deposits and explains their temporal evolution. First order (regional) structural controls are also identified for future exploration in these areas and similar geological environments worldwide. In a third case study at Cape Liptrap and Cape Paterson (Victoria, Australia), the complex structural framework of basement folds and faults and their influence on brittle deformation in younger cover sediments are assessed by a combination of 3D implicit modelling and unmanned aerial vehicle (UAV) photogrammetry. The latter results in the computation of dense 3D point clouds and corresponding orthorectified photographs at sub-centimetre resolution. Bedding orientation measurements from Cape Liptrap are extracted from the point cloud and used to generate a 3D implicit model, which facilitates the analysis of fold geometries and the estimation of bulk shortening strain. Orthorectified photographs are used to measure and analyse fracture orientations in order to understand their relationship to the development of folds and fractures in the overlying cover rocks. This approach is applied to constrain the structural history of the southern part of the Lachlan Orogen and to elucidate the role of basement structure inheritance on subsequent basin development and deformation. The understanding of deformation-related structures and their influence on the emplacement and redistribution of mineralisation can be significantly enhanced by combining of modern modelling and mapping techniques with well-established geological methods and principles.