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Open File Report 133

The significance of Campaspe-dominated terrains in exploration within the Mt Windsor Sub-province, N.E. Queensland

K. M. Scott

Weathered altered volcanics associated with mineralisation within the Mt Windsor Sub-province are characterised by very kaolinitic assemblages (sometimes with alunite-jarosite minerals and barite) derived from alteration of the feldspars. The younger Campaspe Formation is generally characterised by feldspar-bearing assemblages and is not strongly kaolinitic. Thus, the units are easily distinguished on the basis of their kaolinite and feldspar contents. However, where the volcanics are unaltered, feldspars are retained and other criteria must be used to distinguish them from the overlying Campaspe Formation sediments. If the volcanics are intermediate to basic in composition, their lower Si, Cl and higher Al and Ti/Zr relative to the Campaspe Formation can be used to discriminate. Unfortunately, if the volcanics are felsic (as at Thalanga) the geochemical parameters, e.g. Ti/Zr do not discriminate between the two units. In these cases, although the unconformity between the two units is commonly associated with Fe oxide enrichment in both units, to accurately define the boundary, the degree of rounding of the quartz grains may be the only reliable parameter for discrimination.

Chalcophile elements (Ba, Cu. Pb and Zn) may be dispersed by mechanical means into the basal 10 m of the Campaspe Formation at Waterloo and Thalanga East. The distribution of Cu. Pb and Zn within thick sequences of the Campaspe Formation at Waterloo reveals that the dispersion of Zn (and Cu) is more restricted (600 m x 300 m) than that of Pb. The usually mobile elements (viz. Zn and Cu) may be stabilised by being associated with dolomite (i.e. an alkaline environment). At Brittania, quartz -rich "sand" occurs directly over mineralised volcanics. Despite its location there is no dispersion observed in this unit, probably because there is no suitable host to immobilise chalcophile elements.


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