Abstract:

The Yangla copper deposit is located in the middle part of the Jinshajiang belt. It was formed by the westward subduction of the Jinshajiang oceanic basin and collisional orogeny during the MidLate Triassic. Ore bodies are typically layered or bedded and controlled by interlayer fracture zones in the wall rocks. Based on the study of fluid inclusions, the present paper is to discuss characteristics and evolution of oreforming fluids and to evaluate the relationship between fluid immiscibility and mineralization. According to the petrographic study, various types of fluid inclusions including predominant vapor or liquidrich aqueous inclusions and daughter mineralsbearing multiphase inclusions, and subordinately monophase liquid and organic vapor inclusions, are present in anhydrous skarn stage (Ⅰ), hydrous skarnmagnetite stage (Ⅱ), quartzsulfide stage (Ⅲ) and calcitesulfide stage (Ⅳ). Daughter mineralsbearing multiphase inclusions are abundant in garnet (Ⅰ), epidote (Ⅱ) and especially in quartz (Ⅲ). Oreforming fluids of stage Ⅰ and Ⅱ have high homogenization temperatures (413593 ℃ and 336498 ℃,respectively) and high salinities (19.1%49.7% NaCleq and 15.7%53.3% NaCleq, respectively). Stage Ⅲ oreforming fluid has a wide homogenization temperature range (148398 ℃) and is characterized by existence of both highsalinity (35.5%65.3% NaCleq) and lowsalinity (2.1%9.6% NaCleq) end members. Stage Ⅳ oreforming fluid is characterized by low temperature (132179 ℃) and low salinity (3.4%10.4% NaCleq). Based on the characteristics of fluid inclusions together with geological features, fluid immiscibility (boiling) is suggested to be the main mechanism for metal precipitations in this deposit.

Key words: fluid inclusions, fluid immiscibility, mineralization process, Yangla copper deposit, Yunnan