Lode Gold Deposits in Time and Space

Different types of primary or “lode” gold deposits possess unique patterns in space and time. Most such lode gold deposits formed from hydrothermal fluids in association with magmatic or metamorphic thermal events during times of continental growth. Granite-related gold deposits (porphyry, skarn, epithermal) formed in the upper 5 km of the crust from mainly magmatic-hydrothermal fluids exsolved from oxidized magmas above subducting slabs in oceanic and continental arcs; in the case of low-sulfidation epithermal ores, convection of meteoric waters around a causative intrusion is also important. Although there are uncommon examples of granite-related gold deposits older than the Mesozoic Era, most gold endowment in such deposit types is Jurassic and younger. This reflects the relatively shallow depth of gold ore formation and, particularly for oceanic arcs, rapid rate of exhumation and erosion of the lodes over a few tens of millions of years. Because of the limited preservation, most granite-related gold deposits are located in the young arcs of the circum-Pacific rim and in the Tethyan belt. Orogenic gold deposits form from metamorphic-hydrothermal fluids at depths of anywhere from 3 to 20 km and along deep crustal structures in metamorphic belts of all ages. Their relatively deeper formation levels have led to better preservation over geological time, such that they are particularly well recognized in Neoarchean and Paleoproterozoic cratons and in most late Neoproterozoic-Phanerozoic accretionary orogens. They are notably not preserved in terranes with ages between 1.7 and 0.7 Ga, which commonly are characterized by widespread rock units exhumed from depths > 20 km. Other types of gold deposits, such as Carlin-type and iron oxide-gold-copper occurrences, are less well understood and are described from a limited number of locations, such that their spatial-temporal distribution cannot be defined.