CRC LEME
Open File Report 140
ABSTRACT
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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