Cuttings from 22 holes drilled through steeply dipping, weathered shales and volcanics at Panglo, 30 km N of Kalgoorlie, help define mineralogical and geochemical variations around flat-lying supergene Au mineralization within a stripped lateritic profile. The regolith profile contains very saline groundwaters only a few metres below the suface. Ore occurs in saprolite and saprock, below a mottled zone which is generally unconformably overlain by c. 0.5 m of fluvial/colluvial sediments. Apart from some surface enrichment associated with pedogenic carbonate, Au is strongly depleted in the saprolite above the supergene ore horizon, and only subeconomic mineralization occurs in the underlying fresh rocks.
The Au mineralization at depth is accompanied by pyrite and elevated Ag, As, Co, Cu, Sn, W and Zn contents and is associated with the destruction of albite and formation of muscovite. In the regolith, supergene Au ore at Panglo occurs in the basal portion of the saprolite and Au maintains its association with these elements because they are incorporated into secondary oxide minerals. These elements are thus useful pathfinders for Au, especially as some (e.g. As and Cu) form substantial dispersion haloes about ore. Although the ore is oxidized, it is still within its original primary hydrothermal alteration zone, characterized by muscovite and peripheral paragonite development, especially in shale sequences. Because these micas are resistant to weathering, their distributions can be readily mapped in the regolith. Alternatively, elevated K and depleted Na, Ba and Sr can be used as lithogeochemical indicators of mineralization in weathered shales. Use of Na in alteration indices is complicated by its presence in halite at Panglo, but K/Ba ratios do identify the relict zones of hydrothermal alteration within the regolith.
- © 2001 AAG/The Geological Society of London