TECTONIC CONTROLS ON THE FORMATION OF MAJOR ORE DEPOSIT TYPES, WITH FOCUS ON CONVERGENT AND COLLISIONAL MARGINS

The fundamental control of plate tectonic processes on ore formation was realized almost as soon as the plate tectonic model was established in the late 1960s-early 1970s. The formation of seafloor massive sulfide deposits at oceanic spreading centers, various types of sediment-hosted deposits in continental rifts, porphyry and epithermal deposits in volcanic arcs above subduction zones, and granite-related ore deposits in continental collision settings were quickly established. More recently, the formation of porphyry, epithermal, and some types of IOCG deposits has been recognized to occur by remobilization of lithosphere previously affected by prior episodes of subduction (or other types of mantle) metasomatism.

At root, these ore deposit types reflect the focused convection of heat and volatiles from the mantle towards the surface. Plate boundaries provide high-permeability pathways for this heat and mass flux, which is transmitted to the surface either directly as magmas or fluids (or both). At convergent margins, the flux begins with dehydration (and in some cases melting) of subducting oceanic lithosphere, which releases water, S, Cl, and other fluid-soluble components into the mantle wedge, triggering partial melting. Ascent of these partial melts into, and interaction with, the upper plate lithosphere generates hydrous intermediate-composition magmas, which rise into the upper crust where volatiles are exsolved due to decompression and crystallization. These hydrothermal fluids may go on to form porphyry and epithermal deposits if their flow is focused and sustained by a large magma supply.

Re-melting of the deep-crustal residues of arc magmatism (amphibole gabbro cumulates) during later tectonic events such as collision or back-arc extension may generate a second phase of ore deposits, indirectly related to subduction. Such remobilization events may occur soon after arc activity has ceased, or millions to billions of years later in response to unrelated tectonic events, such as continental rifting.