GSA Connects 2021 in Portland, Oregon

Paper No. 101-9
Presentation Time: 3:50 PM


DILLES, John, College of Earth, Ocean & Atmospheric Sciences, Oregon State University, CEOAS Admin 104, Corvallis, OR 97331-5503 and JOHN, David, U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, 345 Middlefield Road, MS-901, Menlo Park, CA 94025

Porphyry and epithermal deposits form large economic ore bodies that are mined globally for Cu, Mo, Au, and Ag and byproduct metals (Re, Te, Se). They form in the upper crust at depths from ~10 km to 100 m. Most are related to intermediate to silicic magmas that source sulfur- and chlorine-rich hydrothermal fluids that move upward producing extensive hydrolytic and alkali wall-rock alteration, quartz veins, and sulfides.

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.