SUBDUCTION-MODIFIED LITHOSPHERE AS A SOURCE FOR A SPECTRUM OF DISTAL- TO POST-SUBDUCTION MAGMATIC-HYDROTHERMAL CU-AU-MO-FE DEPOSITS: PORPHYRIES TO IOCGS

IOCG deposits share many characteristics with porphyry Cu±Au±Mo deposits, including metal inventory, fluid compositions, and association with calc alkaline to mildly alkaline, oxidized, H₂O-rich magmas. The link is strongest with post-subduction or distal-subduction porphyry deposits, which, like IOCGs, are emplaced within convergent or collisional orogenic belts.

Key to these deposits is the introduction of volatiles into the deep lithosphere during subduction, forming hydrous assemblages that preferentially melt during later tectonomagmatic events to form fertile magmas for subsequent magmatic-hydrothermal ore formation upon ascent to shallow crustal levels. The diversity of magmatic hydrothermal deposits that are formed in these settings reflects: the variety of fertile source rocks in the deep lithosphere; extent of partial melting; tectonic controls on magma ascent and emplacement; and post-emplacement system evolution. The spectrum of resultant deposit types includes: (1) calc-alkaline porphyry Cu-Mo-Au deposits formed by relatively high degree partial melting of lower crustal arc cumulates; (2) alkaline porphyry Cu-Au±Mo deposits formed by smaller degree partial melting of arc cumulates; (3) felsic porphyry Mo deposits formed by partial melting of crustal rocks ± mantle contributions in extensional settings; and (4) IOCG deposits formed by relatively low degrees of partial melting of subduction-modified lithosphere during post-orogenic stress relaxation or transtension. Compared to porphyry deposits, the prevalence of Fe-oxides, ‑silicates and ‑carbonates over Fe-sulfides, and less extensive acidic alteration associated with mineralization in IOCG systems may reflect a lower abundance of sulfur in the source magmas/fluids, resulting in the persistence of neutral pH fluids during mineral deposition. The common association of lithophile elements (such as U and REE) in IOCG deposits further suggests a low degree of partial melting and/or high degree of crustal interaction. The predominance of IOCG over porphyry systems in Proterozoic and Archean terrains and the spatial distribution of hydrothermal alteration assemblages may reflect higher ambient host rock temperatures and a greater outward-fluxing of host rock-derived metals during IOCG ore formation.