Abstract:

Stable (C, H, O and S) and radiogenic (Pb) isotopes of hydrothermal solutions from the Aolunhua Mo-Cu deposit in southern Da Hinggan Mountains were analyzed with the purpose of determining the source of the ore-forming fluids and tracing the ore-forming process. New isotopic data indicate that the hydrothermal solutions were not from a single source. C, H, O and S isotopic compositions of the ore-forming fluids show that the metallogenic complexing agent had a hybrid origin, derived mainly from the mantle and subordinately from the crust. The Pb isotopic compositions of the molybdenite suggest that molybdenum came mainly from the juvenile orogenic belt, with a small amount derived from the mantle. Distinct metallogenic mechanism was identified based on stable isotope compositions and previous fluid inclusion studies. The mineralization at the early stage was caused by a mixing between magmatic fluid and meteoric water. However, mineralization at the prevailing metallogenic stage resulted from the fluid boiling. Distinct from the porphyry deposits formed in arc settings and collisional orogenic settings, the Aolunhua porphyry Mo-Cu deposit was formed in a unique geological setting related to reactivation of a juvenile orogen under an extensional backarc regime during the Late Jurassic and Early Cretaceous period. Interaction between magmas and hydrothermal fluids from the mantle and the crust formed a hybrid system, which created essential conditions for subsequent mineralization. It is thus held that more importance should be attached to this ore-forming process so as to recognize potential fertile intrusions.

Key words: isotope determination, fluids evolution, ore-forming mechanism, Aolunhua ore deposit, southern Da Hinggan Mountains