Porphyry to epithermal transition at the Rongna Cu-(Au) deposit, Tibet: Insights from H-O isotopes and fluid inclusion analysis

Abstract

The giant Rongna Cu-(Au) deposit is a recently discovered low-grade composite porphyry-high sulfidation epithermal system in the Duolong ore district of central Tibet. We present fluid inclusion petrography, microthermometry, stable isotope (H, O), and fluid-compositional data (LA-ICP-MS) to constrain the fluid evolution of the Rongna deposit. Hydrothermal quartz and muscovite formed in the porphyry stage yielded fluid values of 8.2 ± 1.0‰ δ18OH2O, and −75 ± 19‰ δDH2O (n = 13). Pyrophyllite, kaolinite, and dickite define the isotopic composition of the epithermal fluid (δ18OH2O = 2.2–8.3‰, δDH2O = −87 ~ −45‰). This epithermal fluid of lighter oxygen isotope and heavier hydrogen isotope composition still has a magmatic-hydrothermal signature and rules out any significant involvement of meteoric water.The very high concentration of Cu in fluid inclusions from epithermal veins and the common sulfide replacement features suggest that the epithermal mineralization may be partially related with the dissolution and re-precipitation of the porphyry-stage sulfides. Combined with fluid inclusion petrography and microthermometric data, it is indicated that a single-phase fluid exsolved from magma at depth and underwent boiling and cooling, forming the porphyry alteration and mineralization. With decrease of temperature and pressure, the ascending vapor contracted into acid aqueous fluid, leaching downward to develop the telescoped high-sulfidation epithermal alteration and mineralization.