Some observations on the geochemistry of gold mineralization in the weathered zone at Norseman, Western Australia

At Norseman Au mineralization is hosted by parallel easterly dipping quartz reefs in a westerly dipping sequence of Archaean basaltic flows and As is strongly correlated with this mineralization. Near-surface exploration is hampered by an extensive cover of deeply weathered soils and a veneer of silty calcareous material, probably of aeolian origin, which, because of its low Fe content of < 5%, has a very low (< 10 ppm) and often indiscernible As signature over mineralization. At a test traverse over the southern end of the Mararoa quartz reef, drill sampling to 10 m depth in highly weathered in situ material beneath the calcareous veneer produced As anomalies which are strongly correlated with the extrapolation to surface of a shear zone hosting Au mineralization which is probably blind. The As anomalies can be extended into the surface horizon by manipulating the data to compensate for the low Fe content of the calcareous layer.The Au values in the drill samples were very low and poorly correlated with the extrapolation to surface of the Mararoa shear. However, the absence of a Au halo (at a 10 ppb detection limit) may be due to the shear being barren near surface, or may be due to the samples being taken from the strongly leached zones of a laterite profile. In the area of the test traverse at the southern Mararoa area, the upper horizons of the original laterite profile have been eroded away and the remaining horizons covered by a veneer of calcareous material. There is a better correlation of the Au values of the shallow drill samples with the projection to surface of a porphyry, which hosts minor low-grade Au mineralization.The results for Au dispersion around the Mararoa shear contrast with dispersion at the nearby Hinemoa Mine where gold mineralization crops out. Channel sampling in costeans showed that an Au anomaly (threshold 40 ppb) extends 50 to 150 m into oxidized wall rocks. This well defined halo may be due to the secondary dispersion of Au from the quartz reef into the ferruginous zone of the laterite profile.Gold in the leaves and twigs of Melaleuca pauperiflora F. Muell. from the traverse at the southern end of the Mararoa shear was determined by neutron activation analysis and found to be broadly correlated with the projected Au mineralization. However, the lack of correlation with the soil Au values to 10 m may be due to the plants taking up most of their water from below the depth of sampling.