Provenance and salt structures of gypsum formations in Pb-Zn ore-bearing Lanping basin, Southwest China

Large-scale gypsum rocks associated with world-class Pb-Zn ore formations are widely distributed in the Lanping Basin, Sowthwest China. Geochemical studies alongside field investigations were conducted in this study to determine the source and evolutionary processes of the gypsum rocks in this area. The gypsum sequences in the Lanping Basin developed in two formations: the Triassic Sanhedong Formation and the Paleogene Yunlong Formation. The gypsum hosted in the former displays a primary thick-banded structure with delta(34)S(V-CDT)values in the range of 14.5 parts per thousand-14.8 parts per thousand. Combined with the(87)Sr/Sr-86 values (0.707737-0.707783) of limestone, it can be suggested that the Sanhedong Formation is of marine origin. In contrast, the gypsum from the Paleogene Yunlong Formation is characterized by the dome, bead and diapiric salt structures, wider range of both(87)Sr/Sr-86 (0.707695-0.708629) and delta(34)S(V-CDT)values (9.6 parts per thousand-17 parts per thousand), thus indicating a marine source but with the input of continental materials. The initial layered salt formations were formed by chemical deposition in a basin and were later intensely deformed by collisional orogeny during the Himalaya period. As a result, variable salt structures were formed. We hereby propose an evolutionary model to elucidate the genesis of the gypsum formations in the Lanping Basin.