One-dimensional reactive transport models of alteration in the Tongchang porphyry copper deposit, Dexing district, Jiangxi Province, China

In the Tongchang porphyry copper deposit, the main hydrothermal alteration is most intense in and near the contact zone and gradually attenuates inward and outward, showing a symmetrical zoning with respect to the contact zone. Detailed field investigations of wall-rock alteration and ore-controlling fractures, combined with laboratory studies of lithology, mineral composition, and fluid inclusions, have led to an assessment of initial and boundary conditions and physical parameters such as species concentration, mineral volume fraction, temperature, porosity, reactive surface area, and flow velocity required for dynamic modeling of this system. We have developed a new program to model the coupled process of fluid flow involving advection and diffusion and chemical reactions incorporating mineral dissolution-precipitation, and have numerically simulated the spatial and temporal evolution of the systems for a duration of 100,000 yr. In the simulations, transient temperature fields are calculated for the main temperature range of ore formation from 200 degrees to 350 degrees C. Simulated mineral assemblages are generally in agreement with those observed in the real system. The important conclusions drawn from the combined researches of geology and geochemistry as well as dynamics of coupled fluid transport and reactions are as follows. Fluid flow and reactions were mainly controlled by the fractures striking around north-south and dipping to the west. In the exocontact zone, schistosity cleavages were also important fluid pathways in addition to the fractures. The formation of closely spaced fractures in the contact zone and especially in the exocontact zone by tectonic activity after potassium metasomatism and the reactive flow from the contact zone inward and outward were the principal reasons for the occurrence of the "contact zone-symmetrical alteration zone. In the porphyry body, both the dissolution and precipitation of minerals were important; the rate of dissolution or precipitation of minerals gradually decreased in time. The nearer it was to the contact zone, spatially, the earlier the dissolution or precipitation of minerals was finished. In the exocontact metamorphic wall rock the main processes were the transport and precipitation of SiO2(aq); the rate of silification gradually decreased in time and space. In the alteration of porphyry body Na+ was evidently removed from the alteration zone and Ca2+ removed from the intense alteration zone to weak alteration zone, and the fluid was weakly acidic (pH approximate to 4). In the metamorphic wall rock, SiO2(aq) was mainly brought in from fluid. The evolution of SiO2(aq), Cl-, Ca2+, and CO32- was intimately correlated with alteration-mineralization intensity.