Petrological and geodynamic controls on gold fertility in Au-Cu systems: Insights from the Lut block in the Alpine-Himalayan orogenic corridor

The Alpine-Himalayan orogenic belt in Iran exhibits a profound metallogenic contrast: the gold-rich late Eocene Lut block versus the copper-dominated Miocene Urumieh-Dokhtar magmatic arc (UDMA). This study investigates the geodynamic controls on this magmatic fertility dichotomy. For the Lut block, the mineralized 40−38 Ma intrusions are nonadakitic, derived from a moderately oxidized, phlogopite-bearing mantle wedge with limited crustal assimilation [87Sr/86Sr(t) = 0.70501−0.70666; εNd(t) = −2.29 to +1.91]. Eocene extensional tectonics from slab rollback induced crustal thinning and transcrustal fault networks. This architecture enabled rapid magma ascent, preventing significant staging. Critically, these magmas exhibited high sulfur activity, which efficiently scavenged and transported gold in resolving hydrothermal fluids. Delayed sulfide saturation further enriched the melt in chalcophile elements, yielding high Au/Cu ratios and deposition in structural traps. Simultaneous Eocene UDMA magmatism was deeper and less oxidized, inhibiting significant ore formation. The major UDMA porphyry Cu deposits formed later, in the early−middle Miocene, under a postcollisional compressional regime. Crustal thickening promoted partial melting, generating hydrous, oxidized, sulfur-rich adakitic magmas ideal for extensive fluid exsolution and copper transport. Subsequent Miocene magmatism in the Lut block remained barren due to metal-depleted sources and reduced fluid pathways. These divergent trajectories demonstrate that gold fertility is optimized in extensional regimes with high sulfur activity and efficient fluid pathways. In contrast, giant copper systems require compressional thickening to generate water-rich, adakitic magmas. This framework underscores the fundamental influence of tectono-magmatic conditions on metal endowment.