In situ U–Pb geochronology and geochemistry of garnets in the Hongniu Cu-polymetallic distal skarn deposit, SW China: Implication for ore-forming process

• Genetic link between Hongniu deposit and Indosinian diorite porphyrite is excluded by garnet U–Pb dating. • Mg 2+ –REE 3+ coupled substitution is the dominant mechanism for REE incorporation into garnet. • The skarn system evolved from diffusion to infiltration alteration, with higher f O 2 and alkalinity. The Hongniu deposit is a typical distal skarn-type Cu-polymetallic deposit situated in the central Geza arc of the Sanjiang Tethyan region. This area records multiple magmatic events, represented by partially overlapping NNW-trending Late Indosinian and approximately NS-trending Late Yanshanian magmatic rock belts. Although both intrusive suites occur in the Hongniu district, the skarn shows no direct contact with them. Consequently, the genesis and geochronology of the deposit remain controversial, primarily due to the absence of spatiotemporally related intrusions. In this study, we performed in situ U–Pb dating and major and trace element analyses on garnets from Hongniu using LA-ICP-MS and EMPA. The garnets in the skarn were divided into two generations (Grt I and Grt II). Grt I is grossular-rich (Gro 59.71 And 37.71 to Gro 82 And 14 ) and exhibits LREE-enrichment with a pronounced positive Eu anomaly. In contrast, Grt II belongs to the grossular-andradite solid solution series (Gro 11.87-69.27 and And 29.49-86.93 ) and is characterized by HREE-enrichment with a negative Eu anomaly. The lower intercept U–Pb age of Grt I is 82.53 ± 1.10 Ma (MSWD = 1.3, n = 56), constraining the skarn formation to the Late Yanshanian period, consistent with the emplacement age of the local quartz monzonite porphyry. Geochemical data suggest that REE 3+ incorporation into garnet is governed mainly by a Mg 2+ –REE 3+ coupled substitution mechanism, with a minor contribution from an Al 3+ –REE 3+ (YAG-type) substitution. From early to late stages, decreasing U concentrations reflect a continuous increase in oxygen fugacity of the ore-forming system, while declining ΣREE and δEu values indicate a gradual rise in the pH of the ore-forming fluids. Concurrently, the dominant fluid–rock interaction mechanism shifted from diffusion to infiltration metasomatism. This study establishes a clear temporal link between the Hongniu deposit and Yanshanian magmatism, confirming that mineralization is unrelated to Indosinian diorite porphyrite emplacement.