Gossans and Leached Cappings

This text concentrates upon field observations concerning leached cappings and gossans, occurring as oxidised surface expressions of underlying ore zones.

Although the advent of modern multielement geochemical sampling and easier mechanical excavation assist considerably in subsurface interpretation, there are still many occasions where the first observation and recognition are made by the lone field geologist. New exposures continue to be found in remote and often difficult terrains, where “on the spot” skills are of prime importance.

In general terms the text has been arranged from the broad scale to the specific, and it should be realised that all scales provide valuable input for final interpretation. The topics covered include:

• Theoretical perspectives

• Initial recognition

• General field observations

• Detailed field observations (secondary minerals, boxworks)

• Porphyry copper leached cappings