Hydrothermal fluid evolution at the Tiegelongnan porphyry-epithermal Cu(Au) deposit, Tibet, China: Constraints from H and O stable isotope and in-situ S isotope

Abstract

The Tiegelongnan porphyry-epithermal Cu (Au) deposit is located in the Duolong porphyry district, north of the Bangong-Nujiang suture zone, Tibet, China. Mineralization is hosted by Jurassic sedimentary sandstone, and several phases of diorite and granodiorite porphyry dikes intruded between 123 and 116 Ma. The hydrothermal alteration is characterized by alunite-kaolinite-dickite overprinting quartz-muscovite-pyrite and biotite alteration zones. Porphyry chalcopyrite-pyrite ± molybdenite (Stage 1) mineralization is associated with biotite alteration. Porphyry chalcopyrite-bornite (Stage 2), and covellite (Stage 3) mineralization is associated with quartz-muscovite-pyrite alteration formed at ~121 Ma. Epithermal mineralization, consisting of pyrite-alunite (Stage 4), chalcopyrite-bornite-digenite (Stage 5), and tennantite-enargite (Stage 6), is hosted by two pulses of alunite-kaolinite breccia and veins at ~116 Ma and ~112 Ma. The fluid composition related to muscovite, with average δ18O of 8.9‰ and δD of −56‰, indicates a magmatic water origin. Fluid δ18O composition in equilibrium with quartz veins decreases from 6.7 to 2.3‰, which are likely the results of the water-rock isotopic exchange. Quartz fluid inclusions δD values between −50 to −84‰ are partly lower than that obtained from muscovite alteration fluids, which may result from H fractionation during fluid inclusions decrepitation. Epithermal stage fluid composition equilibrium with alunite yield δ18O from −1.2 to 2.7‰ and δD from −71 to −51‰, n = 11, which is comparable to the fluid composition equilibrium with Type Ⅰ kaolinite (hosting ores) with δ18O between −2.5 and 2.9‰, and δD between −72 and −51‰. It suggests that alunite and Type Ⅰ kaolinite formed with mixing between magmatic and high altitude Cretaceous meteoric water. Late Types Ⅱ and III kaolinite (filling alunite and quartz veins) fluid δ18O and δD values plot along a mixing line between magmatic and low altitude Cretaceous meteoric water, probably following the erosion and plateau subsidence. Porphyry mineralization sulfide stage 1 chalcopyrite and pyrite yield δ34S values between −5.8 and 0.9‰, with an average fluid δ34SH2S = −2.5‰ (n = 10), whereas stage 2 chalcopyrite returns δ34S values from −8.7 to −3‰ with an average δ34SH2S = −5.6‰ (n = 5). The lower fluid δ34SH2S values during sulfides stage 2 compared to that of stage 1, suggest that the chalcopyrite-bornite mineralization formed under higher oxidation conditions than that of the chalcopyrite-pyrite mineralization. Alunite yields δ34S values from 11 to 18.3‰ (n = 8), and the associated sulfide stage 4 pyrite have varying δ34S values from −32.2 to 5.4‰. Disequilibrium S isotope in alunite-pyrite pairs was likely because of rapid cooling and retrograde S isotope exchange during later sulfides emplacement. Epithermal mineralization sulfide Stage 4 S-equilibrated pyrite (−14.9 to −9.5‰), Stage 5 chalcopyrite (−11.6 to −8.2‰), and Stage 6 enargite (−5.4 to −2.6‰) display increasing δ34S values suggesting epithermal fluid compositions evolve towards more reducing conditions.