Review of the relationship between highly fractionated intrusions and gold mineralization

• Au mineralization is linked to oxidized, high-K calc-alkaline I-type/magnetite-series granites. • Crust-mantle boundary metasomatized by subduction fluids primes Au enrichment in magmas. • Sulfur redox shifts during magmatic evolution control Au transport and sulfide deposition. • Au deposits form in transitional tectonic settings (slab rollback, post-collisional extension). Highly fractionated intrusions plays a key role in the formation of numerous types of ore metal deposits. For example, many large-scale Au deposits are associated with highly fractionated intermediate–silicic intrusions. Since the introduction of Au deposit classification types such as intrusion-related, magmatic, and magmatic–hydrothermal Au deposits, an increasing number of studies have focused on the coupled relationships between magmatic processes (e.g., melt segregation, differentiation, and evolution) and Au mineralization. The magmatism not only provides the energy and ore-forming materials for Au mineralization, but also leads to significant Au enrichment by magmatic fractionation and evolution. Consequently, studies on magmatic fractionation-related mineralization should not be confined to ore types such as W–Sn and rare metals. This paper systematically reviews the nature of Au deposits genetically linked to highly fractionated magmas and discusses the key controls on anomalous Au enrichment during magmatic fractionation. The gold-mineralizing magmas are derived mainly from regions near the crust–mantle boundary. Metasomatic overprinting of the lithospheric mantle and enrichment of the lower crust by metallic elements, fluids, and volatiles from subducted slabs are preconditions for subsequent auriferous mineralization processes. The parental rocks of these Au deposits generally have a high O fugacity and are fractionated I-type or magnetite-series granites that have an affinity with the high-K calc-alkaline series, in which physicochemical parameters such as the O fugacity regulate the S speciation in the magmas and Au distribution. Furthermore, the timing of Au-bearing metallic sulfide saturation in the magmas and subsequent exsolution into the fluid phase is one of the critical controls on Au transportation into the shallow crust and its subsequent deposition in economically viable concentrations. Such Au deposits form mainly in transitional tectonic settings associated with slab subduction (e.g., slab rollback, break off) or during the transition from collisional to post-collisional tectonic settings. The physicochemical processes that govern melt segregation, differentiation, and Au enrichment from the magma source regions to final ore deposition have significant implications for understanding anomalous Au concentration mechanisms. The interrelationships among physicochemical parameters are the critical factor in understanding the coupling relationship between the melt-fluid evolution and Au mineralization, and therefore warrant further investigation.