Mineralogical characteristics of tourmaline in the Jiama copper polymetallic deposit, Tibet: Insights into hydrothermal evolution

This paper is the result of mineral exploration engineering.

Objective Hornfels, as an important part of the Jima porphyry copper polymetallic system, is host rocks and a lithologic trap for ore−forming fluid. However, the origin of tourmaline in hornfels is unknown, which restricts the further understanding of the mineralization.

Methods We carried out detailed drilling logging, petrographic observations and major element analyses of tourmaline with distinctive occurrences in hornfels from the Jiama deposit to elucidate the genesis of tourmaline and evolution of magmatic hydrothermal fluids.

Results Four types of tourmalines in hornfels from the Jiama deposit have been identified in this study: 1) Tur−I, tourmaline occurring as cement in hydrothermal breccias; 2) Tur−II, quartz + tourmaline ± pyrite vein; 3) Tur−III, tourmaline ± pyrite ± chalcopyrite vein; 4) porphyritic tourmaline ± pyrite. Tourmaline with distinctive occurrences in hornfels, which belongs to alkali group and dravite−schorl solid solution series, has a wide range of Al₂O₃, Fe/(Fe+Mg) and Na/(Na+Ca). ^X□Al(NaMg)₋₁, Fe²⁺Mg₋₁ and Fe³⁺Al₋₁ exchange dominates the substitutions of Tur−I−IV.

Conclusions Tourmaline in hornfels with complicated zoning texture has a very variable compositions, indicating that the tourmaline is caused by different degrees of mixing of magmatic hydrothermal fluid and formation fluid, and the water−rock interaction between magmatic hydrothermal fluid and reduced hornfels may play an important role on the mineralization. The various textures and compositions of tourmaline with distinctive occurrences in hornfels from Jiama record some detailed information related to evolution of magmatic hydrothermal fluid, and can provides evidence for understanding the mineralization.