The Fluid Dynamic Process of Large-scale Mineralization in the Lanping Basin, Yunnan, SW China: Evidence from Fluid Inclusions and Basin Fluid Modeling

Abstract

The Lanping basin, Yunnan, SW China, is known for the giant Jinding Zn-Pb deposit and the newly-discovered Baiyangping Cu-Co-Ag super-large deposit. With a reserve of ∼ 200 Mt ore grading 6.08% Zn and 1.29% Pb (i.e., a metal reserve of ∼ 15 Mt) hosted in Cretaceous and Tertiary terrestrial rocks, the Jinding deposit is the largest Zn-Pb deposit in China, and also the youngest sediment-hosted and the only continental sediment-hosted super-large Zn-Pb deposit in the world. Differing from the known major types of sediment-hosted Zn-Pb deposits in the world, including SST, MVT and Sedex, the Jinding deposit represents a new type of sediment-hosted Zn-Pb deposits. Most previous studies assumed that the mineralizing fluids were derived from within the basin and the fluid flow was driven by topographic relief under a hydrostatic regime. However, the observations of hydraulic fractures and fluid inclusion data indicate that the mineralizing fluid system was strongly over-pressured. The study of fluid inclusions in sphalerites and associated gangue minerals (quartz, celestite, calcite, and gypsum) shows that homogenization temperatures cluster around 110–150°C, with salinities of 1.6–18.0 wt% NaCl equivalent. The fluid temperature increases with the decrease of salinities during the main ore stages, and there is a systematic westward decrease in temperature and increase in salinity in the Jinding ore district. Fluid pressures as high as (513–1364) × 10 5 Pa are indicated by CO 2-rich fluid inclusions. The basin fluid dynamic modeling results indicate that the overpressures could not have been produced by normal sediment compaction, and the overpressure related to the thrusting may not be enough to explain the high fluid pressures indicated by fluid inclusions. The injection of mantle-derived fluids is likely responsible for the building-up of the high overpressures. The mixing of two types of fluids in a structural-lithologic trap may have been the key dynamic process for the large-scale mineralization in the Lanping basin: one was a mantle-derived fluid enriched in metals and CO 2 with higher temperature and lower salinity, and the other was H 2S-rich saline formation water with lower temperature and higher salinity. The special hydrodynamic regime and potential contribution of mantle-derived fluids to the mineralizing system distinguish Jinding from other known sedimentary basin-related Pb-Zn deposits.