Origin of the giant Luziyuan Zn-Pb-Fe(-Cu) distal skarn deposit, Baoshan block, SE Tibet: Constraints from Pb-Sr isotopes, calcite C-O isotopes, trace elements and Sm-Nd dating

The giant Luziyuan Zn-Pb-Fe(-Cu) deposit is located in the southern Baoshan block of SE Tibet. The deposit hosts over 4.2 million tonnes (Mt) of contained Zn + Pb metal and 301 Mt Fe ore reserve. The stratiform-/vein-type orebodies are hosted along the interlayer fractures in the Upper Cambrian Shahechang Formation marble and skarn. Three alteration/mineralization stages have been identified: (I) pre-ore, (II) syn-ore (oxide-ore and sulfide-ore), and (III) post-ore. An increasing Fe(-Cu) and decreasing Zn-Pb trend is present from the shallow-level SW to deep-level NE Luziyuan.

Lead isotope compositions are similar among all Luziyuan ore sulfide types, and show positive correlations plotted between the regional Early Cretaceous granites and local wallrocks in both uranogenic and thorogenic diagrams, suggesting a mixed Pb source between intermediate-felsic magmatic fluids and sedimentary wallrocks. This is similar to many coeval northern Baoshan skarn deposits, e.g., Jinchanghe and Hetaoping. Initial ⁸⁷Sr/⁸⁶Sr ratios of the pre-ore (0.715919-0.720102), syn-ore (0.713502-0.719259) and post-ore (0.714091-0.720304) minerals lie between those of coeval granitoids and skarn deposits in the region and the average continental crust. This further supports a mixed ore-forming material source, and is suggestive of distal-skarn mineralization type. Syn-ore calcite samples yielded an Early Cretaceous Sm-Nd isochron age (130 +- 15 Ma). Regional tectonic evolution suggests that the Luziyuan mineralization was likely formed under post-collisional extension after the Lhasa-Tengchong accretion onto the Eurasian margin along Baoshan-Sibumasu. Such an extension had most likely induced crustal anatexis and the formation of the ore-causative S-type granites.