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Open File Report 137

A regional overview of the Charters Towers - North Drummond Basin Region: Geomorphic Landform Provinces

S. J. Fraser

A regional-scale map (1:250,000 scale) of interpreted geomorphic provinces has been produced over the Charters Towers - North Drummond Basin area. The area has substantial mineral potential, but Tertiary sediments conceal prospective lithologies.For explorationists, the map is intended as a guide, to assist in area-selection and in formulating exploration strategies. For regolith mappers and researchers, the map provides a framework for more detailed studies. A digital version of the mapped polygons is available upon request.

The landscape evolution and weathering history of the region since the Mesozoic is complex. Erosional processes currently dominate; however, the landscape retains evidence of a complex history of at least two cycles of deposition, weathering and erosion.

The area has been subdivided on the basis of landform into seventeen geomorphic provinces, each of which contains particular regolith characteristics. These geomorphic-regolith provinces were delineated using photoform, drainage and other textural features identified on processed Landsat TM imagery. The resulting interpretation is a broadscale regional subdivision of the landscape. For planning geochemical surveys at tenement or prospect scales, a more detailed subdivision of the landscape would be required than that presented here.

Because the nature of the land's surface is critical in determining an appropriate geochemical exploration strategy, the mapped geomorphic-regolith provinces were grouped into regolith-terrain classes, which broadly convey information regarding the nature of the surface, and the state of preservation of weathering profile material. Three terrain types have been identified: Duricrust Dominated Terrain, Saprolite Dominated Terram and Alluvium and Colluvium Dominated Terrain.

In provinces belonging to the Duricrust Dominated Terrains, ferruginous weathering products could be useful geochemical sampling media because Au and other pathfinder elements such as As, Cu and Bi are generally trapped in secondary iron oxides, which are in turn, the main components of ferruginous materials. Such samples should be most informative in those areas with duricrust preservation on basement lithologies, however, extensive areas of duricrust development on basement have not been recognised. Areas of duricrust are commonly found on outcrops of Tertiary sediments, which belong to the Southern Cross (Suttor) and Campaspe Formations. Despite these sediments being predominately fluviatile, their associated secondary ferruginous weathering materials could contain trace-element concentrations indicative of underlying minetalization. Both elastic (mechanical) and hydromorphic geochemical dispersal mechanisms have possibly operated. Of particular sigmficance for mineral exploration is the development of a widespread, though not continuous layer of weakly cemented ferruginous nodules towards the top of weathering profiles associated with the Campaspe Formation (unit UFP) on the western side of the study area. The possibility that these ferruginous nodules may contain subtle geochemical anomalies indicative of basement minetalization needs to be investigated further.

In those provinces belonging to the Saprolite Dominated Terrains, secondary ferruginous materials, if available, should be sampled wherever possible. However, these will need to be supplemented with more traditional, geochemical samples, such as stream sediment, rock chip, and soil samples, in areas where saprolite has been removed.

For those provinces in the Alluvium and Colluvium Dominated Terrains, there are two geochemical strategies. In areas of colluvium, traditional geochemical sampling methods should be appropriate. Areas with a high contribution of alluvial material probably will require drilling and subsurface sampling. And if buried weathering profiles are intersected during drilling, ferruginous materials if available, should be preferentially sampled.


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