Unveiling late cretaceous Sn mineralization in the southern Baoshan Block, SW China: evidence from U–Pb dating and trace element analysis of cassiterite in the Dalong deposit

• The Dalong Sn deposit is a hydrothermal vein-type Sn deposit associated with concealed granitic rocks. • The LA-ICP-MS U-Pb ages of cassiterites (77.0 ± 1.4 Ma and 78.6 ± 4.2 Ma) from the Dalong Sn Deposit indicate that the Sn mineralization occurred in the Late Cretaceous, rather than being associated with the adjacent Triassic Muchang Granite as suggested in previous studies. • In addition to their occurrence in the northern part of the Baoshan Block, Late Cretaceous (concealed) granites and their associated Sn mineralization are also distributed in the southern part of the Block, which should be pay attention in regional ore prospecting work. The Baoshan Block is considered an integral part of the northern continuation of the world-class Southeast Asian Sn Belt. Its exceptional metallogenic geological settings make it a prime focus for Sn prospecting and exploration. In recent years, several Sn deposits have been successively discovered in the northern Baoshan Block, with formation ages clustering around 77–76 Ma, consistent with the metallogenic peak (∼80 Ma) of the Southeast Asian Sn Belt. However, despite its intermediary position, owing to the lack of outcrops of Late Cretaceous granites, Sn deposits in the southern Baoshan block remain unexplored. The scarcity of reliable geochronological data in this area severely constrains genetic models for mineralization and hinders effective exploration planning. This study presents the first high-precision cassiterite U-Pb ages (77.0 ± 1.4 Ma and 78.6 ± 4.2 Ma) for the newly discovered Dalong Sn deposit in the southern Baoshan Block, which was previously attributed to Triassic magmatism. Cassiterite from the Dalong deposit is enriched in Ti, W, and Fe, but depleted in Nb, Ta, Zr, and Mn, with high Nb/Ta ratios (3.16–545, with an average of 7.26). These geochemical signatures, combined with deposit geological characteristics, support a model in which the Dalong Sn deposit is a hydrothermal vein system sourced from Late Cretaceous concealed granites, formed during Neo-Tethyan subduction-induced back-arc extension. The results reveal that among the four Sn mineralization episodes in the Tengchong-Baoshan Block (Early Cretaceous: 124–118 Ma; Late Cretaceous: 79–69 Ma; Early Eocene: 52–47Ma; and Oligocene: 33–22 Ma), the Late Cretaceous Sn mineralization was spatially the most extensive, forming a coherent belt from the Southeast Asian Sn Belt northward through the Baoshan Block (both southern and northern parts) and into the Tengchong Block. We propose that future ore exploration in the Baoshan Block should prioritize the concealed Late Cretaceous granites and associated Sn mineralization.