The Large-scale Fluid Ore-forming Process In The Lanping Basin

The Lanping basin, Yunnan, SW-China, is famous for the giant Jinding deposit and the new-discovered Baiyangping Cu-Co-Ag super-large deposit. Differing from known major types of sediment-hosted Zn-Pb deposits in the world, including SST, MVT and Sedex, the metallogenetic mechanisms of Zn-Pb deposits in the Lanping basin are unique, although the large-scale mineralization occurred in the Mesozoic-Cenozoic strata. Because of the remarkable characteristics of epigenetic alterations and hydrothermal mineralization of the deposits in the large Pb-Zn-Cu ore concentration area, Lanping basin, the key point is about fluid for making clearly the metallogenetic mechanisms. Based on these, through the analysis of regional ore-forming geological setting, this article takes the two super-large deposits as the examples for revealing the fluid metallogenetic mechanisms.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 known major types of sediment-hosted Zn-Pb deposits in the world, the Jinding represents a new type of sediment-hosted Zn-Pb deposits. The Baiyangping Cu-Ag-Co polymetallic deposit is another super-large deposit found in the north of Jinding in the Lanping basin. So, it can improve the metallogeny study of the Lanping basin, with the comparation of metallogenetic fluids between the Jinding and Baiyangping deposits.Based on the abundant geological data collected, through the detailed study on mineragraphy for checking out the symbiotic, associated and generation relationship of the minerals and mineral assemblage in the different mineralization stages, we studied the petrography, homogeneous temperature, salinity, gas and luqid components, and rare element compositions of fluid inclusions, in order to recognizing the basic charateristics and evolution of the ore-forming fluids. Then, combing the regional geological structure setting with the H-O,He-Ne-Ar-Xe isotopes of fluis inclusions, S,Pb isotopes of sulfide ores and C-O isotopes of carbonate minerals, we have studied the metallogenic process in the Lanping basin.The major advances are as follows.â… . Rare element compositions of fluid inclusions in the various mineralization stages of the Jinding ore district shows that from the early to late mineralization stage of the fluid mineralization processes, SREE and LREE / HREE increased in the ore-forming fluid respectively, and the fluid system became more oxidizing. The rare element assemblage of the ore-forming fluid became more complex, the metallogenic element was richer in the ore-forming fluid. The fluid mineralization may be quick during the early-middle stage and slower during the middle-late stage. A mixing of a deep- and a shallow- fluid may be the essentials for the large-scale mineralization in the Jinding.â…¡. The compositions of noble gas isotopes of fluid inclusious in the hydrothermal minerals of the main mineralization stage of the Jinding and Baiyangping deposits indicates that metallogenetic fluids were the mixiture of two different fluids with different characteristics from the crust and mantle.â…¢. The mineralizing system was strongly injected by a deep overpressure fluid based on observations of hydraulic fractures, fluid inclusion data and the basin fluid modeling. The fluid pressures of (513～1364)×10⁵ Pa are revealed in CO₂-rich fluid inclusions. The basin fluid dynamic modeling results indicate that the overpressures could not have been produced by normal sediment compaction, a fluid overpressure of 170×l0⁵Pa could have been made by the thrusting nappe. An injection process of mantle-derived fluids are 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 Lanping basin.â…£. With the study on the fluid metallogenetic process of the Lanping basin, we find out that the mixing of a deep- and a shallow- fluid may be the essentials for the large-scale mineralization.â…¤. Because of the unique characteristics of the Jinding super-large Zn-Pb deposit, which is different from the MVT,SST and SEDEX type Zn-Pb deposits, it may be a new type of Zn-Pb deposit named "Jinding type".