A HYDROTHERMAL LIQUID LINE OF DESCENT FROM MAGMA TO EPITHERMAL FLUID

Porphyry copper deposit fluids equilibrate with feldspars and biotite in magma, then react with wall rock when they emerge at 700̊C to form veins bordered by feldspars, biotite, andalusite, anhydrite and quartz –the potassic alteration assemblage. Cross-cutting relations in Butte (Montana) show that potassic veins are followed by quartz-molybdenite veins, then pyrite veins with intense sericite-pyrite alteration, and finally fissure veins of covellite-enargite-pyrite bordered by advanced argillic alteration. The same sequence occurs worldwide, reflecting a fundamental process of hydrothermal fluid evolution that has been credited to magma evolution controlling fluid character at its source. However, matching NaCl and CO₂ concentrations in fluid inclusions from all vein stages, and computations of reaction of the fluids with the host granite show that a single fluid composition may yield veins of every variety. Differences among vein types result from differences in temperature and extent of reaction between fluid and rock. Apparently the cause of variations in veins and alteration lies not in the magma chamber but in the hydrothermal regime.

The fundamental reaction that enables a feldspar-equilibrated fluid to produce feldspar-biotite alteration at 700̊C then quartz-sericite-pyrite at 500̊C, then advanced argillic at 350̊C is SO₂(aq) + H₂O(aq) = 1½ H⁺(aq) + ¾ SO₄^2-(aq) + ¼ H₂S(aq). Between 600̊C and 100̊C, the equilibrium constant for this SO₂ disproportionation reaction increases by 17 orders of magnitude, yielding hydrogen ion, sulfate and H2S at lower temperature. This acidic fluid, which forms the covellite-enargite assemblages of the late Butte veins and of Nansatsu-Summitville type epithermal deposits, is neutralized by wall rock reaction so it yields distal assemblages of Cu-Pb-Zn sulfides and rhodochrosite. Feldspar-buffered meteoric water dilution of the fluid increases bisulfide concentration, driving base metal sulfides to precipitate while increasing Au solubility in Au(HS)₂^-, then transporting Au to distal epithermal deposits. Individual natural systems experience the evolution outlined above with variations in rates of temperature change and extent of wall rock reaction, yielding a large diversity of porphyry and epithermal ore styles–from a single magmatic fluid.