A genetic link between Late Cretaceous granitic magmatism and Sn mineralization in the southwestern South China Block: A case study of the Dulong Sn-dominant polymetallic deposit

Abstract

The southwestern South China Block, composed of southwestern China and northeastern Vietnam, is one of the most important Sn-polymetallic ore districts in the world. The Dulong skarn Sn-polymetallic deposit, which is closely related to Late Cretaceous granitic magmatism, contains 5.0 mt Zn, 0.4 mt Sn, 0.2 mt Pb, and 7 kt In, and is one of the largest Sn-polymetallic deposits in the region. Sn mineralization occurs mainly as cassiterite at the contact zone between the Laojunshan granitic pluton and carbonates of the Cambrian Tianpeng Formation. The Laojunshan granitic pluton consists of three: medium- to coarse-grained two-mica monzonitic granite, medium- to fine-grained two-mica monzonitic granite, and granite porphyry, with zircon U–Pb ages of 87.5 ± 0.6 Ma, 85.3 ± 1.1 Ma, and 82.8 ± 1.6 Ma, respectively. Zircon grains from these three phases show εHf(t) values from −9.5 to −2.5, typical of a mantle origin. All samples exhibit strongly peraluminous and high-K calc-alkaline affinities, and show trace element patterns characterized by enrichment in Rb, Ta, La, Zr, and Hf, and depletion in Ba, Nb, and Ti, typical of S-type granites. Three main alteration/mineralization stages are recognized within the mineralized contact zones: (1) skarn stage; (2) sulfide mineralization stage; and (3) supergene stage. Cassiterite grains from the skarn alteration stage yield a 207Pb/206Pb–238U/206Pb concordia age of 89.4 ± 1.4 Ma, suggesting that mineralization was associated with the Laojunshan granitic pluton. Regionally, the Sn-polymetallic mineralization in the southwestern South China Block took place within a narrow time span from 97 to 79 Ma. The causative granitic magmas can be produced by partial melting of the Paleoproterozoic–Mesoproterozoic pelitic basement rocks during large-scale lithospheric thinning and asthenospheric upwelling.