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Open File Report 126

Secondary dispersion about the Waterloo polymetallic deposit, Mt Windsor Sub-province, N.E. Queensland

K. M. Scott

Zinc-rich polymetallic mineralization occurs as steeply dipping lenses at a number of locations within the Cambro-Ordovician Trooper Creek Formation of the Mt Windsor Sub-province, south of Charters Towers. Such mineralization is, however, often obscured by regolith, especially the arenites of the flat-lying Upper Tertiary Campaspe Formation. At the Waterloo deposit a dispersion halo has been reported within the Campaspe Formation. Because of the implications for exploration, the significance of this halo has been critically investigated by studying the mineralogy and geochemistry of profiles through the Campaspe Formation into the underlying volcanics.

Along Line 4 at Waterloo a 20-25 m thick sequence of feldspar- and smectite-bearing sandstone overlies a 20 m interval where feldspar is absent and kaolinite is the dominant clay. Bands of dolomite and Fe oxides are also present immediately beneath the feldspar-bearing rocks. Below ~45 m, relatively fresh volcanics with feldspar, chlorite ± pyrite are present. The presence of a feldspar-bearing unit above an highly weathered feldspar-depleted band suggests a lithological break at that contact. Supplementary geochemical data (Ce, Cl, Zr and Ti/Zr) also suggest that the base of the plagioclase-bearing material represents a major compositional break.

These results suggest that 20-25 m of Campaspe Formation overlies a sequence of weathered volcanics with the Fe-rich horizon representing the near surface Fe enrichment of a lateritic profile. The presence of the dolomite horizon at or above the Fe-rich horizon would thus represent pedogenic or secondary dolomite formation prior to the deposition of the feldspar-bearing Campaspe Formation. It is suggested that the anomalous levels of Pb and Ba occurring immediately beneath the feldspar-bearing Campaspe Formation represent the occurrence of secondary minerals (Pb-bearing alunite type minerals and barite) mechanically transported from secondary mineralization which cropped out prior to deposition of the Campaspe Formation. It is also possible that the Pb could be at least partly residually accumulated or hydromorphically dispersed and concentrated in the Fe-rich horizon. Whatever its genesis, the presence of the Pb halo in a predictable position within the regolith profile makes its use in exploration more viable. Copper and Zn are hydromorphically dispersed within the weathered volcanics but not into the Campase Formation.


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