Leapfrog Mining

Tectonic processes and the resulting deformation of Earth's crust are known to control and influence the location as well as shape of ore bodies. Furthermore, ore body geometries and associated spatial relationships provide fundamental clues to the genesis of ore deposits. Therefore, it is important to establish consistent 3D geological models to visualise and analyse ore delineation in order to understand the controls on mineralisation processes. Implicit modelling generates objective 3D geological models directly from drill-hole data. Instead of manual linkage of hand-digitised 2D cross-sections, mathematical interpolation functions are used to generate 3D isosurfaces. These surfaces can represent ore grade shells, lithological boundaries or structural trends. The 3D implicit model can be used to define ore body geometries, grade continuity direction(s) and spatial relationships between mineralisation, lithological boundaries and major structures, helping to explain the struct...

Structurally controlled mineralisation commonly shows distinctive geometries, orientations and spatial distributions
that derive from associated structures. These structures have the ability to effectively transport, trap and focus
fluids. Moreover, structures such as faults and shear zones can offset, truncate and spatially redistribute earlier
mineralisation. We present a workflow that combines structural fieldwork with state-of-the-art 3D modelling to
assess the structural framework of an ore deposit. Traditional 3D models of ore deposits rely on manual digitisation
of cross sections and their subsequent linkage to form 3D objects. Consequently, the geological interpretation
associated with each section will be reflected in the resulting 3D models. Such models are therefore biased and
should be viewed and interpreted with caution. Conversely, 3D implicit modelling minimises the modelling bias
by using an implicit function that is fitted to spatial data such as drillhole data. This function defines a scalar
field, from which 3D isosurfaces can be extracted. Assay data can be visualised as 3D grade shells at various
threshold grade values and used to analyse and measure the shape, distribution and orientation of mineralisation.
Additionally, lithology codes from drillholes can be used to extract lithological boundaries in 3D without the need
for manual digitisation.
In our case study at the Palaeozoic Currawong Pb-Zn-Cu deposit (Victoria, Australia), orientations extracted
from ore bodies within a 3D implicit model have been compared to structural field data collected around the
deposit. The data and model suggest that Currawong’s massive sulfide lenses have been structurally modified.
Mineralisation trends are parallel to a dominant NW dipping foliation mapped in the field. This foliation
overprints earlier bedding in the host metasediments that has been deformed into upright folds. Several sets
of steep faults further increase the structural complexity of the deposit and offset mineralisation. Previously
suggested conceptual models for Currawong do not adequately explain the spatial geometry and distribution
of the mineralisation that is apparent in the 3D implicit model. We present a new structural history for the
Currawong deposit, which explains the structural evolution (folding & faulting) of the massive sulfide lenses in 3D.

In the mining industry, drillhole data is collected primarily for economic purposes such as resource estimation, quality control and mine planning. It is rarely used to gain insights on the structural framework of an ore deposit, even though abundant research has shown that many ore deposits exhibit a strong structural control. The famous and comprehensively studied orogenic gold deposits of the Victorian goldfields (Australia) are examples that emphasize the importance of structural controls on gold mineralisation. Structures such as folds, faults and shear zones allow the effective transport of hydrothermal fluids by episodically increasing permeability during seismic/aseismic events. Ascending fluids can be trapped physically within structures such as dilational jogs or extensional veins that act as low pressure sites into which fluids migrate preferentially. This process results in ore bodies with distinct geometries, sizes, orientations and spatial locations that can be outline...