SEG Lecturer Series: Thayer Lindsley Visiting Lecturer
Forty-five years after Dick Sillitoe (1973) published his famous paper titled “Tops and Bottoms of Porphyry Copper Deposits,” the science of geology has improved our understanding based on fluid inclusion, geochronology, and petrology – geochemistry studies. Nonetheless, Sillitoe’s paper provided a framework that not changed greatly, and porphyry deposits can form at depths of ca. 2 to 10 km.
Hydrous, sulfur- and chlorine-rich intermediate to silicic magmas are likely derived via deep- to mid-crustal differentiation, and ascend to form shallow crustal magma chambers that cool and crystallize to release magmatic-hydrothermal ore fluids from granitic cupolas. Porphyry dike emplacement, veining, and hydrothermal alteration are synchronous, and make use of hydrofracture permeability.
Shallow porphyry copper deposits have a characteristic hydrothermal zones that include relatively high Cu grades, abundant sugary quartz-Cu-sulfide “A-type” veins and strong K-silicate alteration (K-feldspar-biotite) cut by later pyrite-rich veins with sericitic vein selvages. These zones are commonly overlain by shallow “lithocaps” characterized by near-surface quartz-alunite-pyrite±pyrophyllite zones that in some cases contain enargite and gold. The paired deep Cu-rich K-silicate zones and shallower quartz-alunite zones are likely produced by brine-vapor unmixing where the brine is restricted to depths > ca. 2 km and forms Cu ores, and the low-density vapor ascends and condenses into shallow zones to form lithocaps.
Deep porphyry copper deposits form at more than ca. 4 km depth and are characterized by Cu-sulfide-rich early dark micaceous veins/halos (biotite-K-feldspar-muscovite) and lower quartz vein densities. Due to greater pressures, the magmatic-hydrothermal ore fluids are principally single phase, lacking brine-vapor unmixing.