Acta Petrologica et Mineralogica

The Hexi large strontium deposit in western Yunnan Province is located in the northern part of the Meso-Cenozoic Lanping Basin, southeast margin of the Qinghai-Tibet Plateau. The ore bodies mainly occur in the carbonate rocks of the Upper Triassic Sanhedong Formation, and are also abundant in the gypsum-bearing sand-mudstones of the Miocene Jinding Group. They are controlled by the near S-N-extending reverse fault and the near E-W-exten-ding strike-slip fault during the metallogenic stage. After the petrography observation and mineral chemical compositions analysis, the hydrothermal celestine can be divided into three generations of Cls1, Cls2 and Cls3, respectively. Cls1 mainly has an heteromorphic granular structure, while Cls2 and Cls3 exhibit an idiomorphic columnar or lamellar structure, and Cls2 is characterized by Sr-Ba solid solution oscillating rings, revealing that the ore precipitation had undergone a rapid to slow crystallization process. The Sr and S isotopic tracing results show that the celestine has an uniform ⁸⁷Sr/⁸⁶Sr value of 0.707 695~0.707 792, with an average value of 0.707 740, which is consistent with the Sr isotopic compositions of adjacent Sanhedong Formation limestones and the Late Triassic seawaters. The δ³⁴S value of celestine (17.26‰~17.49‰, with an average value of 17.36‰) is slightly higher than that of the Jinding Group gypsum (12.5‰~15.0‰). This indicates that the formation of Sr-rich fluid may be related to the high-pressure dehydration of the Sanhedong Formation limestone caused by gravity compaction and deep source heat flow, and the ore-bearing hydrothermal fluid concentrated at the favorable structural traps in the Sanhedong Formation during the early stage. Influenced by the regional tectonic stress changes and deep heat effect driven by the India-Eurasia collision process, these ore-bearing hydrothermal fluid migrated upward along the near S-N-extending reverse structure and mixed with the oxidizing lacustrine basin brine of the Jinding Group at the shallow surface. The hydrothermal sedimentary-type mineralization of the Jinding Group and hydrothermal filling metasomatism-type mineralization of the Sanhedong Formation simultaneously occurred. Hence, the Hexi strontium deposit is an epigenetic hydrothermal filling-type deposit with a continental hydrothermal sedimentary origin in nature.

celestine; mineral chemistry; Sr-S isotope; epigenetic hydrothermal filling-type deposit; continental hydrothermal sedimentary origin; Hexi strontium deposit; Lanping Basin