CRC LEME
Open File Report 126
ABSTRACT
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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