LLHR-type chalcopyrite Re-Os geochronology and geochemistry of the Xiaotongchang Cu deposit in south Ailaoshan and its geological significances

The Xiaotongchang Cu deposit is located in the south Ailaoshan and at the contact between Yunnan province of China and Vietnam. Four Cu ore bodies are distributed parallel to each other in an interlayer fracture zone in the Permian Emeishan basalt. The major ore mineral in the deposit is chalcopyrite, and the main gangue minerals are quartz and calcite. New Re-Os data for chalcopyrite from the Xiaotongchang Cu deposit show tens of ppb Re abundances (1–86 ppb) and contain essentially no common Os. All the samples have extremely high ¹⁸⁷ Re/ ¹⁸⁸ Os (up to 2.3 × 10 ⁴ ), and the dominance of radiogenic Os and high ¹⁸⁷ Re/ ¹⁸⁸ Os are diagnostic features of “LLHR” (low-level, highly radiogenic) sulfides. The Re-Os isochrone age for the No. 1 and No. 3 Cu bodies show that the phase of Cu mineralization predominantly occurred in the Middle Triassic, with an age of 230.6 ± 1.1 Ma. Thus, the mineralization occurred much later than the periods of major eruption of the Emeishan large igneous provinces, indicating a closer relationship with the late collisional processes in the Ailaoshan orogenic belt. Combined with REEs patterns and consistently positive δ ³⁴ S values (7.4–9.2‰), it can be concluded that the ore-forming materials of the Cu deposit are derived from a mixture of mantle and crust components.