Pyrite and magnetite Re–Os isotope systematics at the Laoshankou Fe–Cu–Au deposit in the northern margin of the East Junggar terrane, NW Xinjiang, China: Constraints on the multistage mineralization and metal sources

Abstract

The northern margin of the East Junggar terrane is highly prospective for Fe–Cu–Au mineralization. However, due to the lack of precise mineralization ages, intensive debates regarding ore genesis existed. In this study, pyrite and magnetite Re–Os isotopes are first used to determine the metallogenic ages and possible genetic links with magmatism at the Laoshankou deposit, the most important Fe–Cu–Au deposit in the region.Magnetites from Fe mineralization stage yield an 187Re/188Os versus 187Os/188Os isochron age of 391 ± 55 Ma, which is in broad agreement with ages of the Beitashan Formation volcanic rocks (~390 Ma). The low initial 187Os/188Os ratio of −0.08–0.16, depleted γOs values of −161–27 and low Re/Os(common) indicated a mantle source composition, implying Fe metals are from the Beitashan Formation Fe‐rich basalts and basaltic breccias.Pyrites from sulfide mineralization stage yield a weighted average Re–Os model age of 390 ± 17 Ma with a matched 187Re versus 187Osr (radiogenic 187Os) isochron age of 391 ± 9 Ma, consistent with the symbiotic molybdenite Re–Os model age of 383.2 ± 4.5 Ma, indicating a timing of ~380 Ma for sulfide–Cu–Au mineralization. The high initial 187Os/188Os ratio of 0.53–3.6, large positive γOs values of 323–2,754 and higher Re/Os(common) indicated the increase of crustal materials, which is further limited to the Beitashan Formation S‐rich marine sedimentary rocks.In summary, based on the significant two‐stage mineralization, that is, Fe (~390 Ma) and sulfide–Cu–Au (~380 Ma), and the different nonmagmatic fluid and metal sources, we prefer considering the Laoshankou Fe–Cu–Au deposit as an IOCG‐type.