Hydrodynamic modeling of magmatic–hydrothermal activity at submarine arc volcanoes, with implications for ore formation

Abstract

Subduction-related magmas have higher volatile contents than mid-ocean ridge basalts, which affects the dynamics of associated submarine hydrothermal systems. Interaction of saline magmatic fluids with convecting seawater may enhance ore metal deposition near the seafloor, making active submarine arcs a preferred modern analogue for understanding ancient massive sulfide deposits.We have constructed a quantitative hydrological model for sub-seafloor fluid flow based on observations at Brothers volcano, southern Kermadec arc, New Zealand. Numerical simulations of multi-phase hydrosaline fluid flow were performed on a two-dimensional cross-section cutting through the NW Caldera and the Upper Cone sites, two regions of active venting at the Brothers volcanic edifice, with the former hosting sulfide mineralization. Our aim is to explore the flow paths of saline magmatic fluids released from a crystallizing magma body at depth and their interaction with seawater circulating through the crust. The model includes a 3×2 km2 sized magma chamber emplaced at ∼2.5 km beneath the seafloor connected to the permeable cone via a ∼200 m wide feeder dike. During the simulation, a magmatic fluid was temporarily injected from the top of the cooling magma chamber into the overlying convection system, assuming hydrostatic conditions and a static permeability distribution.The simulations predict a succession of hydrologic regimes in the subsurface of Brothers volcano, which can explain some of the present-day hydrothermal observations. We find that sub-seafloor phase separation, inferred from observed vent fluid salinities, and the temperatures of venting at Brothers volcano can only be achieved by input of a saline magmatic fluid at depth, consistent with chemical and isotopic data. In general, our simulations show that the transport of heat, water, and salt from magmatic and seawater sources is partly decoupled. Expulsion of magmatic heat and volatiles occurs within the first few hundred years of magma emplacement in the form of rapidly rising low-salinity vapor-rich fluids. About 95% of the magmatically derived salt is temporarily trapped in the crust, either as dense brine or as precipitated halite. This retained salt can only be expelled by later convection of seawater during the waning period of the hydrothermal system (i.e., “brine mining”).While the abundant mineralization of the NW Caldera vent field at Brothers could not be classified as an economic ore deposit, our model has important implications for submarine metal enrichment and the origin of distinct ore types known from exposed systems on land. Sulfide-complexed metals (notably Au) will preferentially ascend during early vapor-dominated fluid expulsion, potentially forming gold ± copper rich vein and replacement deposits in near-seafloor zones of submarine volcanoes. Dense magmatic brine will initially accumulate chloride-complexed base metals (such as Cu, Fe, Pb and Zn) at depth before they are mobilized by seawater convection. The resulting mixed brines can become negatively buoyant when they reach the seafloor and may flow laterally towards depressions, potentially forming layers of base metal sulphides with distinct zonation of metals.