Inferring central McArthur Basin shape at HYC time: Integration of geophysical interpretation and geology using GIS

Abstract

Sediment-hosted metallogeny results from sedimentary basin fluid flow, which in turn, is controlled by the evolving architecture of the basin. Understanding and predicting the location of ore deposits therefore depends on knowledge of the three-dimensional geometry of the target basin through time (4-D basin architecture). However, quantitative basin analysis is severely handicapped in the absence of extensive seismic reflection data from the target terranes, such as the Proterozoic of northern Australia, host to a world-class base metal endowment. Geological mapping and regional potential field geophysical data, on the other hand, are widely available, but their interpretation in terms of 4-D basin architecture is not straightforward. GIS and geophysical modelling were deployed to assist. A GIS with 1:250 000-scale geological map and geochemical data was designed and implemented for a region in the McArthur Basin encompassing the giant HYC Zn-Pb-Ag deposit. The GIS incorporates geological attributes that encode depth information implicit in the stratigraphic column. This data structure, in conjunction with topological attributes, allows queries based on the stratigraphic relationships of spatial elements. An initial 3-D picture of the basin, relying solely on surface geological data and measured stratigraphic thicknesses, was developed by generation of layers comprising 'predicted' structure contour values for any given stratigraphic unit. This prediction is analogous to calculation of the theoretical Bouguer gravity value during reduction of gravity data. The predicted value (for example, of basement depth) does not necessarily indicate the true elevation of the surface being considered at a given location; rather, it is a baseline for comparison. Lateral variations from this baseline indicate departures of basin shape from 'layer-cake' geometry. By this mechanism, elements of the basin fill, lost due to deformation and erosion following terminal deposition, may be restored for comparative purposes. The development of stratigraphic topology enables automatic identification of the location and magnitude of unconformities on geological maps. These indicate areas and periods of uplift through the sedimentation history of the basin, from which fluid flow may have been topographically driven. Conversely, the distribution of unconformities circumscribes regions of more continuous sedimentation, where accommodation space was developed more consistently. Both gravity and magnetic data were forward modelled in an extensive interlocking array of cross-sectional 2-D profiles. Several basin units are resolvable from regional data using these methods. In particular, the HYC-hosting upper McArthur Group is distinguishable due to its carbonate-dominant composition, resulting in a positive density contrast. These interpretations, initially expressed as structure contours and isopachs (Leaman 1998), were interpolated into 3-D models of the present disposition of basin units. These may be compared directly with the basin unit depths and thicknesses 'predicted' from outcrop-derived data. Residuals, after removal of 'predicted' or 'layer-cake' McArthur Group thickness from the 'actual' (geophysically interpreted) present thickness, directly map the location and size of active sub-basins at the time of the formation of HYC mineralisation. The subbasins thus defined are congruent with indications from unconformity distribution. HYC's situation at the northeastern edge of one of these sub-basins is consistent with topographic and bounding growth fault control on the palaeohydrogeological regime that focused mineralising fluids in the vicinity of the deposit. Other sub-basin edges are indicated as sites of potential base metal mineralisation.