Collision-related tin metallogeny: Insight from a new discovery of the volcanic-subvolcanic-hosted Sn deposit in the western Gangdese, Xizang

The Gangdese metallogenic belt in Xizang, a world-class copper polymetallic province, has a poorly understood western segment due to extensive volcanic cover and limited historical exploration. The recent discovery of the Sangmoladong (SMLD) deposit, the first undocumented volcanic- to subvolcanic-hosted, tin-dominant polymetallic system in western Gangdese, provides a unique opportunity to investigate collisional metallogeny. Through integrated LA-ICP-MS U-Pb geochronology of zircon and cassiterite, comprehensive whole-rock geochemistry, and Nd-Hf isotopes, this research establishes a genetic link between Paleocene magmatism and Sn-polymetallic mineralization. The mineralization is hosted within granite porphyry stocks and associated rhyolitic tuff breccias of a volcanic dome complex. It comprises two main stages: an early, disseminated cassiterite-sulfide stage with chloritic alteration, followed by later fluorite-cassiterite-tourmaline veins and veinlets. New LA-ICP-MS U-Pb dating constrains the timing of granite porphyry emplacement to 61.3 ±0.2 Ma and rhyolitic tuff deposition to 61.8 ±0.5 Ma. Cassiterite mineralization occurred between 61.0 ±2.2 Ma and 59.3 ±3.5 Ma, confirming that Sn metallogenesis was coeval with this Paleocene magmatic pulse during the Indo-Asian collision. The ore-forming granitic porphyries are highly evolved, A-type granites, characterized by high SiO₂ (77.50 - 80.40 wt.%), elevated zircon saturation temperatures (831 - 870 ℃), and high Ga/Al ratios (10,000×Ga/Al = 5.75 - 6.51). Their Nd-Hf isotopic signatures (ε_Nd(t) = -6.5 to -6.3; zircon ε_Hf(t) = -5.2 to +2.1) indicate an origin from anatexis of ancient Lhasa terrane metapelites, likely triggered by lithospheric extension during Neo-Tethyan slab rollback. This generated a reduced, fluorine- and boron-rich magmatic-hydrothermal system highly efficient at mobilizing and concentrating tin. On a regional scale, a metallogenic framework is proposed where Fe-Cu mineralization is sourced from hybridized mantle-crust magmas, whereas Pb-Zn and Sn systems derive from similar crustal-dominated sources. This metallogenic divergence of Pb-Zn and Sn reflects contrasting thermal regimes and magma crystallization pathways during a transtensional setting with local extension in pull-apart and uplift structures. The formation of the SMLD tin deposit is attributed to prolonged fractional crystallization of a high-temperature, low fO₂ magma within a subvolcanic dome complex. These conditions suppressed early cassiterite saturation and promoted extreme tin enrichment in the residual melt. This study makes two key contributions: (1) it identifies the first volcanic-hosted tin system in the western Gangdese, challenging traditional exploration models focused on porphyry skarn Cu-Pb-Zn deposits; and (2) it establishes a new tectono-metallogenic model that elucidates the spatiotemporal evolution of Paleocene mineralization (Fe-Cu → Pb-Zn → Sn) during orogenesis. These findings provide crucial insights into metallogeny in continental collision zones and pave the way for new exploration targets for tin resources throughout the central-western Tethyan metallogenic domain, especially in underexplored volcanic terrains with analogous geodynamic histories.