In the Parkinson Pit area at Mt Magnet, 560 km NNE of Perth, mafic and felsic rocks occur within a weathering profile that has been stripped to the saprolite. Irrespective of rock type within the pit area, the weathered profile consists of fresh rock, saprock, mottled saprolite and leached saprolite below a thin transported soil cover. The depth of weathering extends to 80 m adjacent to mineralization, although weathering depths appear to be less in mafic volcanic rocks outside the pit.
Weathering has led to separation of Ag from the primary Ag-bearing gold and the redistribution of other potential pathfinders elements (As, Co, Cu, Ni, Sb and Zn ) from the original sulphides (mainly pyrite) into Fe and Mn oxides. Gold grades have not been significantly affected by weathering.
Both mafic and felsic rocks in the Parkinson Pit area, have been metamorphosed to quartz+chlorite±mica+albite assemblages. However, hydrothermal alteration has destroyed albite and formed additional mica, so that the change: minor muscovite→paragonite±muscovite→muscovite, reflects an approach to mineralization. Because micas are retained when the rocks weather, this characteristic feature is maintained in the regolith, although changes in mica compositions and abundance are more obvious in mafic than in felsic rocks because of the greater initial mica content in the latter. Thus, proximity to mineralization, reflected by mica composition and abundance in mafic volcanic rocks, could be determined by PIMA and radiometric analysis of K content, even from weathered drill cuttings.
- © 2001 AAG/The Geological Society of London