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Open File Report 140

Regolith landform relationships and geochemical dispersion around Tringadee and Brumby prospects, North Queensland

C. Phang, T.J. Munday and J.E. Wildman

The purpose of this study is to investigate the origin of a Zn anomaly (up to 1000 ppm) in Mesozoic cover at Tringadee prospect. In the Brumby prospect, ferruginous materials developed in Mesozoic cover at the surface or subsurface were studied to find any indication of concealed mineralisation. District scale regolith mapping of approximately 550 km2 was carried out to provide a framework for a geochemical dispersion study of the two prospects. Stratigraphic relationships of the main regolith units were also established to provide constraints for the interpretation of the regolith geochemistry.

Three major geomorphic environments can be delineated in the Tringadee area. These are i) the low hills and mesas developed in Mesozoic sediments in the central zone, ii) the low hills and mesas developed on the Proterozoic basement in the north and west and iii) the depositional plains with brown and well-developed 'black' clay soils over recent alluvial materials to the south and south--east. Contrasting lithologies combined with erosion, deposition, ferruginisation and silicification are important factors contributing to the variation in regolith materials and landforms observed in the area.

At the Tringadee Prospect, the Zn anomaly in the Mesozoic sediments appears to be associated with accumulated Fe and Mn oxides. The area is interpreted to have remained in a low part of the landscape before, during and after deposition of the Mesozoic sediments. Iron, Mn and Zn appear to have derived from external sources and have migrated laterally along permeable layers, precipitating at redox fronts within the sediment pile. This probably occurred after deposition, but contemporaneous accumulation of the metals with the sediments is possible. It is considered that the source of the Zn is external and that it has been scavenged by Fe-Mn oxide precipitates, now represented by ferruginous bands. There is, thus, no relationship between Zn anomalies in the sedimentary cover and potential base metal mineralisation in the basement.

The Brurnby prospect has subvertical ferruginous veins in the Mesozoic, probably associated with faulting of the underlying Proterozoic and appear to form remnants of conduits for fluids rich in Fe. The ferruginous veins give no indication of mineralisation. In contrast, the subhorizontal ferruginous bands associated with redox zones or permeability layers within the sediment pile are preferentially sampled.


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