Hydrological controls on base metal precipitation and zoning at the porphyry-epithermal transition constrained by numerical modeling

Abstract

Ore precipitation in porphyry copper systems is generally characterized by metal zoning (Cu–Mo to Zn–Pb–Ag), which is suggested to be variably related to solubility decreases during fluid cooling, fluid-rock interactions, partitioning during fluid phase separation and mixing with external fluids. Here, we present new advances of a numerical process model by considering published constraints on the temperature- and salinity-dependent solubility of Cu, Pb and Zn in the ore fluid. We quantitatively investigate the roles of vapor-brine separation, halite saturation, initial metal contents, fluid mixing and remobilization as first-order controls of the physical hydrology on ore formation. The results show that the magmatic vapor and brine phases ascend with different residence times but as miscible fluid mixtures, with salinity increases generating metal-undersaturated bulk fluids. The release rates of magmatic fluids affect the location of the thermohaline fronts, leading to contrasting mechanisms for ore precipitation: higher rates result in halite saturation without significant metal zoning, lower rates produce zoned ore shells due to mixing with meteoric water. Varying metal contents can affect the order of the final metal precipitation sequence. Redissolution of precipitated metals results in zoned ore shell patterns in more peripheral locations and also decouples halite saturation from ore precipitation.

Timing of Volatile Degassing From Hydrous Upper‐Crustal Magma Reservoirs With Implications for Porphyry Copper Deposits

“…Incremental magma growth by sill injections (Korges et al., 2020) could resolve the evolution of entire magmatic complexes with different injection frequencies, volumes and depths. Incorporating metal precipitation dynamics for copper, zinc and lead (Stoltnow et al., 2023) would resolve metal partitioning and remobilization and their effect on metal zonation. Future developments could also integrate improved formulations for bubble ascent, crystal settling, metal extraction, sulfide saturation, variable copper concentrations in the magmatic fluids and crystal lock‐up in the melt‐crystal‐volatile system, ideally resolving for compressible, multi‐phase magma flow beyond the Boussinesq approximation used in this study, which could influence convection patterns and provide further information on magma pressurization and eruption potential (Degruyter & Huber, 2014).…”