WALL-ROCK ALTERATION IN PORPHYRY COPPER DEPOSITS: PREDICTION OF HYDROTHERMAL FLUID COMPOSITIONS, FLOW PATHS, AND SULFIDE ORE DEPOSITION

Porphyry Cu(Mo-Au) deposits contain ore sulfides in narrow veins and disseminations in hydrothermally altered wall-rocks associated with granitoid porphyries emplaced at ~1-10 km depth. Hydrothermal fluids produce characteristic mineral assemblages that are zoned spatially and temporally and may be used for exploration.

Magmatic-hydrothermal fluids supply ore metals, sulfur, acids, and alkalis, and hydrofracture overlying rock. Pressure gradients and permeability cause flow to be dominantly vertical but also lateral at shallow depth and in D veins. Similar fluids may produce potassic alteration at high temperature, but hydrolytic alteration via cooling and acidification. Acids may be produced via dissociation of HCl and disproportionation of SO₂ into water to produce H₂SO₄ and H₂S. Early and shallowly derived fluids remain hot on ascent and depressurization, and commonly unmix to brine and vapor phases. Such fluids may produce potassic alteration, A and B veins, and early Cu±Mo sulfides at depth, and overlying advanced argillic (AA) alteration associated with low density vapor. Younger fluids are progressively more deeply derived and may remain single phase. They cut previously altered rocks at lower temperature to produce D veins with sericitic and local AA alteration. These zones may contain shallow Cu±Au ores. As the geothermal gradient diminishes, end-stage clays may form.

Non-magmatic fluids ranging from dilute meteoric water to saline sedimentary brine are driven by magmatic heat and advect through fractured rock. Meteoric waters may advect to <300°C but dense brines may advect to ~450°C as governed by permeability reduction caused by the precipitation of quartz on heating. Non-magmatic saline fluids are chiefly responsible for large zones of relatively barren and common propylitic alteration and locally present sodic-calcic alteration that flank the central ore zone. Sodic-calcic alteration results as heated neutral brines add Na to and remove K from rock.

Ore Cu-Fe sulfide precipitation is promoted by cooling, supply of H₂S, and wall-rock alteration that consumes acid and adds Fe to the fluid. SO₂ disproportionation to produce H₂S increases with pressure so that at high temperature deep porphyries form sulfide (Cp + Py) whereas shallow porphyries are H₂S-deficient and may form magnetite (+Bn ± Cp).