Abstract
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 <10 (super -7) g cm (super -2) s (super -1) as temperatures increase from <150 degrees to [asymp]550 degrees C, (2) main-stage convective cooling ([asymp]5,000 to [asymp]30,000 yr) when fluid flux maxima >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 <300 degrees C) observed in porphyry-related calcic skarns, and indicate that aqueous CO 2 concentration (in nonideal H 2 O-CO 2 fluid mixtures) during sub-400 degrees C garnet precipitation is < 0.05 mole fraction.Equilibrium phase relations and preliminary mass transfer calculations suggest that the following chemical evolution coincides with predicted hydrothermal events: (1) early metasomatism--calcite recrystallization, wollastonite formation (around siliceous impurities), and minor development of zoned calc-silicate assemblages (in the vicinity of endogenous pore fluids); (2) main-stage metasomatism--metasomatic growth of silicate-sulfide-oxide as a consequence of mass transfer between marble-wollastonite rock and exogenous fluid; and (3) late metasomatism--development of hydrous silicate-calcite-sulfide-oxide assemblages as a function of continued fluid-rock mass transfer and/or metastability of main-stage anhydrous silicates.Scale-recursive compositional zoning within skarn is controlled by reaction progress during fluid-rock mass transfer that proceeds in the context of space-time variations in hydrothermal conditions and the state of chemical equilibrium. Observed features such as gross outward zoning of garnet-chalcopyrite to wollastonite-bornite, amphibole selvages on fractures that cut anhydrous skarn, and core to rim iron enrichment of andradite-grossular garnet are consistent with the reaction paths, equilibrium constraints, and hydrothermal evolution presented in this study.
