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


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Duffett, ML, Roach, M and Leaman, DE 2007 , 'Inferring central McArthur Basin shape at HYC time: Integration of geophysical interpretation and geology using GIS', in T. Munson and G. Ambrose (eds.), Proceedings of the Central Australian Basins Symposium, Alice Springs, 16-18 August, 2005. , Northern Territory Geological Survey Special Publication, vol. 2 , Northern Territory Geological Survey, Darwin, NT.

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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.

Item Type: Book Section
Authors/Creators:Duffett, ML and Roach, M and Leaman, DE
Keywords: Northern Territory, McArthur Basin, geographic information systems (GIS), sedimentary basins, reconstruction, mineral deposits, metallogenesis, Sedex, exhalative processes, base metals, geophysical interpretation, geological interpretation, regional mineral exploration
Publisher: Northern Territory Geological Survey
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