Gold Deposits in Metamorphic Belts: Overview of Current Understanding, Outstanding Problems, Future Research, and Exploration Significance

Metamorphic belts are complex regions where accretion or collision has addedto, or thickened, continental crust. Gold-rich deposits can be formed at allstages of orogen evolution, so that evolving metamorphic belts contain diversegold deposit types that may be juxtaposed or overprint each other. This partlyexplains the high level of controversy on the origin of some deposit types,particularly those formed or overprinted/remobilized during the majorcompressional orogeny that shaped the final geometry of the hosting metamorphicbelts. These include gold-dominated orogenic and intrusion-related deposits, butalso particularly controversial gold deposits with atypical metal associations.Orogenic lode gold deposits of Middle Archean to Tertiary age are arguablythe predominant gold deposit type in metamorphic belts, and include severalgiant (>250 t Au) and numerous world-class (>100 t Au) examples. Theirdefining characteristics and spatial and temporal distributions are nowrelatively well documented, such that other gold deposit types can be comparedand contrasted against them. They form as an integral part of the evolution ofsubduction-related accretionary or collisional terranes in which the host-rocksequences were formed in arcs, back arcs, or accretionary prisms. Currentunknowns for orogenic gold deposits include the following: (1) the precisetectonic setting and age of mineralization in many provinces, particularly inPaleozoic and older metamorphic belts; (2) the source of ore fluids and metals;(3) the precise architecture of the hydrothermal systems, particularly therelationship between first- and lower-order structures; and (4) the specificdepositional mechanisms for gold, particularly for high-grade deposits.Gold-dominant intrusion-related deposits are a less coherent group ofdeposits, which are mainly Phanerozoic in age, and include a few world-class,but no unequivocal giant, examples. They have many similarities to orogenicdeposits in terms of metal associations, wall-rock alteration assemblages, orefluids, and, to a lesser extent, structural controls, and hence, some deposits,particularly those with close spatial relationships to granitoid intrusions,have been placed in both orogenic and intrusion-related categories by differentauthors. Those that are clearly intrusion-related deposits appear to be bestdistinguished by their near-craton setting, in locations more distal fromsubduction zones than most orogenic gold deposits and in provinces that alsocommonly contain Sn and/or W deposits; relatively low gold grades (