METALLOGENY OF CONVERGENT MARGIN AND COLLISIONAL MAGMATISM

Convergent margins and collision zones generate distinctive suites of broadly subduction-related calc-alkaline to mildly alkaline magma which are associated with characteristic hydrothermal mineral deposit styles, including porphyry Cu±Mo±Au and epithermal Au-Ag deposits. Intra-crustal felsic magmas are also associated with Sn-W and other granophile element deposits. Common to all these magma types is a relatively high water content, and in the case of subduction-related magmas, a relatively high oxidation state (commonly 1-2 log units above the FMQ buffer) and sulfur content (present mainly as dissolved SO₂). With these ingredients and a sufficient magma flux to shallow crustal levels, the formation of near-surface hydrothermal systems with the potential to transport and precipitate base and precious metals in sulfide deposits is almost inevitable (massive volcanic eruption could short-circuit this process). Formation of economic mineral deposits in such systems is a function of scale (mass flux and duration) and efficiency of fluid flow focusing (structure) and solute precipitation mechanisms (especially thermal gradients and/or the presence of reactive wallrocks).

Partial melting in sub-arc asthenosphere or collided subduction-modified sub-continental mantle lithosphere and/or lower crust generates similar suites of hydrous, oxidized, calc-alkaline to mildly alkaline magma, into which base and precious metals will partition due to the absence or sparsity of residual or fractionating sulfide phases (despite the high magmatic S content). The destruction of minor cumulate sulfide phases residual from arc magmatism during post-subduction re-melting of arc roots may account for the relatively high precious metal contents of some post-subduction porphyry and alkalic-type epithermal deposits. Specific enrichment of magmas (above expected mantle levels) in these elements is not a requirement for ore formation, but rather their premature loss through sulfide retention or fractionation must be avoided.