The Physical Hydrology of Ore-Forming Magmatic-Hydrothermal Systems
Classifications of magmatic-hydrothermal ore deposits are largely geochemical, based on metal associations and characteristic alteration types, but the process of metal enrichment is primarily controlled by the physical hydrology of fluids flowing through rocks. Physical hydrology plays a decisive role in forming distinct ore deposit types, including volcanogenic massive sulfide deposits at mid-ocean ridges or submarine arc volcanos, porphyry-style ore deposits in continental collisional arcs, and epithermal vein and replacement deposits. Results from simulations of magmatic-hydrothermal systems using a new numerical modeling platform for thermohaline convection are used to determine the implications for ore formation in light of the different structural styles, timing, and igneous characteristics of major magma-related ore deposit types. Thermal convection, volatile expulsion, and salt water dynamics are shown to be the first-order hydrologic components, and different combinations or successions of general hydrologic patterns characterize particular oreforming systems.
Due to the nonlinear properties of fluids and rocks as a function of pressure, temperature, and composition, the physical behavior of hydrothermal systems can be counterintuitive, and understanding their self-organization requires numerically rigorous models. Thus, mid-ocean ridge hydrothermal systems do not involve broadscale lateral infiltration of seawater; instead, focused warm downflow in the immediate vicinity of hot upflow zones provides a more efficient mechanism for metal leaching and ore formation in Cyprus-type massive sulfide deposits. Phase separation in submarine magmatic-hydrothermal systems can lead to a decoupling of vapordominated venting, which is expected to favor sulfur complexing of some metals leading to the formation of Au-rich chimneys, whereas chloride-complexing metals may precipitate during the waning stages, favoring the formation of base metal-rich sulfide deposits from negatively buoyant brines. Porphyry copper mineralization is localized by a self-stabilizing hydrologic front, located at the transition from brittle to ductile rock behavior and controlled by the heat balance between an external convective cooling engine and an overpressured magmatic fluid plume. This hydrologic divide also provides a mechanism for the transition to epithermal-style deposits where magmatic and meteoric fluids mix on ascent to the surface.