SEG Distinguished Lecturer Series
A late epithermal sulfide-rich polymetallic "Cordilleran" stage is a common feature in porphyry systems. It is frequently not economic (e.g. Chuquicamata and La Escondida, Chile), but in other cases, this stage may form large to world-class Zn-Pb-Ag-±Cu±Au deposits. Examples are the "main stage veins" in Butte, USA; Morococha, Julcani, Colquijirca and Cerro de Pasco, Peru; Bor, Serbia; Erstberg, Indonesia. Cordilleran polymetallic deposits are typically found in the upper part of porphyry systems, where they may cut earlier veins with potassic and phyllic alteration assemblages, and occur as veins (mainly in magmatic and siliciclastic host rock) and as massive replacement bodies (mainly in carbonate host rock), as well as sulfide-cemented breccia bodies.
Formation during short pulses subsequent (<1Ma) to porphyry and skarn mineralization by magmatic-dominated fluids reaching even levels close to surface and distinct paragenetic sequences (well zoned in terms of ore and alteration minerals if ore fluids are acidic and oxidizing) are key elements of the current genetic model. It is based on detailed field relationships, fluid inclusion studies, and geochemical and geochronological work on representative deposits. The occurrence of Cordilleran polymetallic mineralization can be used to explore hidden porphyry deposits.