TEMPERATURE – DEPTH RANGES OF CU-AU-MO-(AG-PB-ZN) ARC-RELATED MAGMATIC-HYDROTHERMAL SYSTEMS: PORPHYRY – EPITHERMAL ORE FORMATION FROM 10 KM TO THE SHALLOW SUBSURFACE
Phase petrology of hydrothermal mineral assemblages and fluid inclusions provide evidence for pressure (depth) and temperature conditions. Porphyry-type deposits form where magmatic-hydrothermal fluids hydrofracture rock at 700 to 350°C and pressures ranging from supra-lithostatic to supra-hydrostatic at ~10 to 2 km depth. In deep deposits (>4 km), “early halo” vein selvages, containing muscovite, K-feldspar, ± biotite and andalusite with Cu-Fe sulfides, are associated with sparse quartz veins formed at ~500-650°C, whereas in shallow deposits (2-4 km), Cu-Fe sulfides ± magnetite are found in abundant (1->10 vol%) A-style sugary quartz veins (Proffett, 2009). In shallow deposits, ore formation extends to lower temperatures (~400°C) but not to depths <2 km where metal transport is limited by the halite+vapor field. In both cases, alkali (potassic) alteration is widespread and contains added biotite ± K-feldspar. Younger D-style pyrite-quartz veins with sericitic (muscovite; hydrolytic) selvages cross-cut potassic veins and form at lower temperatures (~450-350°C).
Where present, Cordilleran base metal lodes of Cu, Zn, Pb, Ag ± Au, are intermediate in position between porphyry and epithermal environments. Lodes have selvages with combinations of muscovite or illite, pyrophyllite, and alunite. They form between 200-350°C and at hydrostatic pressures (1-6 km depth) from mixed meteoric and magmatic water.
Epithermal Au-Ag deposits form at depths <~1.5 km, under hydrostatic conditions, and at temperatures of ~100-300°C from low- to moderate-salinity, commonly boiling fluids of mixed meteoric and magmatic origin. High and intermediate sulfidation varieties are transitional to Cordilleran lodes. The stabilities of pyrophyllite, dickite, kaolinite, illite, illite-smectite, zeolites, and alunite and the hydrostatic boiling curve provide useful temperature and depth constraints.
Therefore, porphyry-epithermal ores form in specific pressure (depth) - temperature environments in the upper crust that can be recognized by the type of hydrothermal alteration mineralogy, veins, and ore minerals.