Metamorphic belts are complex regions where accretion or collision has added to, or thickened, continental crust. Gold-rich deposits can be formed at all stages of orogen evolution, so that evolving metamorphic belts contain diverse gold deposit types that may be juxtaposed or overprint each other. This partly explains the high level of controversy on the origin of some deposit types, particularly those formed or overprinted/remobilized during the major compressional orogeny that shaped the final geometry of the hosting metamorphic belts. These include gold-dominated orogenic and intrusion-related deposits, but also particularly controversial gold deposits with atypical metal associations. Orogenic lode gold deposits of Middle Archean to Tertiary age are arguably the predominant gold deposit type in metamorphic belts, and include several giant (>250 t Au) and numerous world-class (>100 t Au) examples. Their defining characteristics and spatial and temporal distributions are now relatively well documented, such that other gold deposit types can be compared and contrasted against them. They form as an integral part of the evolution of subduction-related accretionary or collisional terranes in which the host-rock sequences were formed in arcs, back arcs, or accretionary prisms. Current unknowns for orogenic gold deposits include the following: (1) the precise tectonic setting and age of mineralization in many provinces, particularly in Paleozoic and older metamorphic belts; (2) the source of ore fluids and metals; (3) the precise architecture of the hydrothermal systems, particularly the relationship between first-and lower-order structures; and (4) the specific depositional mechanisms for gold, particularly for high-grade deposits. Gold-dominant intrusion-related deposits are a less coherent group of deposits, which are mainly Phanerozoic in age, and include a few world-class, but no unequivocal giant, examples. They have many similarities to orogenic deposits in terms of metal associations, wall-rock alteration assemblages, ore fluids, 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 different authors. Those that are clearly intrusion-related deposits appear to be best distinguished by their near-craton setting, in locations more distal from subduction zones than most orogenic gold deposits and in provinces that also commonly contain Sn and/or W deposits; relatively low gold grades (≤1-2 g/t Au); and district-scale zoning to Ag-Pb-Zn deposits in distal zones. Outstanding problems for intrusion-related deposits include the following: (1) lack of a clear definition of this apparently diverse group of deposits, (2) lack of a definitive link for ore fluids and metals between mineralization and magmatism, (3) the diverse nature of both petrogenetic association and redox state of the granitoids invoked as the source of mineralization, and (4) mechanisms...
