3D Implicit Modelling

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.