Porphyry Cu-Au-Mo deposits

Porphyry deposits provide much of the world’s copper, gold and molybdenum resources. They are typically form in convergent margin settings and have a poor preservation potential in the geological record, due to rapid uplift and exhumation. This means that most of the porphyry deposits being mined or explored today are Cretaceous or younger in age. There are older examples that have been preserved in settings where the geodynamic setting evolved from conditions favouring uplift to burial soon after the cessation of mineralisation.

Porphyry deposits are associated with oxidized hydrous intrusive complexes. Multiple episodes of magmatism are typical, with mineralisation commonly associated with early stages. Exceptions occur, whereby mineralisation is associated with late-stage intrusions. Controls on the mineralising capacity of individual intrusions remain obscure. Whole rock and mineral chemistry signatures of fertile intrusive complexes associated with porphyry mineralization are currently the subject of intensive research. These features are likely to facilitate future greenfields and brownfields exploration targeting.

Mineralisation and alteration is localised around the apex of the mineralising intrusive complexes. There is typically an evolution spatially from early proximal potassic to distal propylitic assemblages. Late-stage alteration assemblages overprint the potassic and propylitic assemblages, and are zoned from deep phyllic to shallow advanced argillic assemblages. Ore is associated with high temperature potassic assemblages and has an early timing in many deposits. In some, significant mineralisation is associated with the late-stage clay- and mica-rich alteration assemblages. Reasons for these variations remain obscure, and should stimulate further refinement to the porphyry copper model.