This course covers the principles and practices required for the visual field-scale assessment of leached outcrops for a range of deposit types. Special emphasis is placed on the surface expression of porphyry copper styles and their related oxide zones. The class component runs for 2 days, and is generally supplemented by field visits to gossanous leached outcrops of current interest on the third day.

Gossan/leached caps – general.

Basic principles of oxidizing profiles above ore zones

Coping with limonites: styles, compositional effects of neutralizing wall rocks, indigenous versus transported

Secondary mineral recognition and factors governing their formation. Emphasis on ‘copper oxide’ grouping. Interpretation of assemblages (Cu, Pb, Zn, Fe, Ni, Ag)

Boxworks – recognition, mode(s) of development, overprinting effects. Rock versus sulphide styles

Porphyry copper – leached cappings and their links to supergene enrichment and the development of ‘oxide’ zones.

Leached capping – development, textural features, limonite styles – methods of assessment. Examples from arid (Escondida) and tropical (Grasberg) environments.

Oxide zones – factors governing development, different types of oxide zones, mineral recognition.

Supergene zones – factors controlling development, relationships to leached caps, influence of overprinting high-sulphidation systems.