Theoretical prediction of hydrothermal conditions and chemical equilibria during skarn formation in porphyry copper systems

A numerical model of heat and mass transfer within porphyry copper environments and equilibrium phase relations in the system CaO-FeO-MgO-Al 2 O 3 -SiO 2 -Cu 2 O-H 2 S-H 2 SO 4 -H 2 O-CO 2 are combined into a theoretical analysis of hydrothermal and chemical conditions during skarn formation in siliceous limestone.Heat and mass transfer calculations indicate temperature-pressure-fluid flux evolution within host-rock contact zones can be subdivided into three events: (1) early conductive heating (0 to [asymp]5,000 yr) when fluid fluxes remain 5 x 10 (super -7) g cm (super -2) s (super -2) are realized as temperatures decline through the H 2 O critical region to [asymp]300 degrees C, and (3) late convective cooling ([asymp]30,000 to [asymp]400,000 yr) when fluid fluxes and temperatures gradually return to ambient values. Pressure changes during this history are several tens of bars or less.Space-time variations in solution-mineral equilibria commensurate with calculated temperature-pressure evolution are described from activity diagrams that combine silicate-fluid and sulfide-fluid topologies. The diagrams incorporate explicit provision for silicate-solution compositions reported and oxidation states inferred from natural systems. Equilibrium constraints are responsible for many ore-gangue associations (e.g., chalcopyrite-andraditic garnet and bornite-wollastonite) and paragenetic features (e.g., decomposition of (garnet, clinopy-roxene)-sulfide-oxide to calcite-amphibole-sulfide-oxide at temperatures 2 concentration (in nonideal H 2 O-CO 2 fluid mixtures) during sub-400 degrees C garnet precipitation is