Porphyry base metal deposits A. Porphyry copper-molybdenum: San Mañuel-Kalamazoo suite

Hydrothermal Alteration Four alteration zones assoc with porphyries Potassic zone – development of secondary orthoclase-biotite-chlorite and minor sericite which replace primary orthoclase, plagioclase and mafic minerals Phyllic zone – characterized by development of quartz-sericite-pyrite assemblage with minor chlorite, illite & rutile. Sericitization affects feldspars and biotite. This rea’n releases silica resulting in silicification and quartz production Argillic zone – clay minerals dominate. Kaolin dominates near the orebody and montmorillonite further away Propylitic zone – always present! Chlorite is the predominant mineral along with pyrite, calcite and epidote. Mafic minerals are partially replaced by chlorite and carbonate, plagioclase may be unaffected. Gradually grades into surrounding rocks up to 1km.

Alteration Halos

Hypogene Mineralization Ore can be found in 3 situations Totally within the host stock (veins) Partially in stock and partially in country rock Totally within the country rock Orebodies are usually surrounded by a pyrite-rich shell, which occur in concentric zones. Usually a central barren core passing outwards firstly to Mo-rich minzn, then Cu-rich minzn as the main ore shell is encountered. Pyrite gradually increases to form a pyrite-rich halo (10-15%) but with minor chalcopy and Mo. The highest Cu values often occur at the boundary between the potassic and phyllic zones with weak minzn in the propolyitic zones

Alteration Shell

Hydrothermal Circulation