Late Neoproterozoic and fertile metavolcano-sedimentary sequence facilitated epigenetic Cu mineralization in the Jianglang Dome, southeastern Songpan-Ganze Orogen

Abstract

The Jianglang Dome in the southeastern Songpan-Ganze Orogen, contains a metavolcano-sedimentary sequence (Liwu Group) which hosts high-grade stratiform Cu deposits. Here we report geochemical, chronological and isotopic data of the Liwu Group, to explore the petrogenetic model, metal fertility and basement nature. This sequence is composed of metasedimentary and intercalated metabasic rocks, which contain detrital zircons and xenocrysts with U-Pb ages of 2812–540 Ma (n = 159). Among them, several zircons yield three discordia curves with lower intercepts of 547 Ma (n = 8), 543 Ma (n = 19) and 548 Ma (n = 16). Besides, the youngest concordant zircons show magmatic affinity, and have a weighted mean age of 544.5 Ma (n = 11). These age data are coeval with ca. 545 Ma arc magmatism in the Northern Gondwana, and we assign a Late Neoproterozoic age of ca. 544.5–538.8 Ma for the Liwu Group protoliths. This result, according to a chalcopyrite Re-Os isochron age of 549 ± 11 Ma (n = 5), indicates syngenetic mineralization. The metabasic rocks show depletion in HFSE, MORB-like REE patterns, whole-rock positive ε_Nd(t) values of +11.3 to +6.2, and varying zircon ε_Hf(t) values of +12.8 to −17.9. These rocks were thus derived from high-degree melting of spinel lherzolite, and suffered slightly crustal contamination. The metasedimentary rocks have moderate weathering (CIA = 82.8–54.4), moderate maturity (ICV = 1.05–0.72), whole-rock negative ε_Nd(t) values of −9.0 to −9.8, and wide zircon ε_Hf(t) values of +12.0 to −20.3; they were fed from adjacent continental blocks and volcanic arc. Our integrated data suggest that the Liwu Group protoliths were formed in an incipient back-arc basin along the Gondwana margin. Metabasic and metasedimentary rocks in this basin, were fertile in metal elements (e.g., Cu and Co) and thus facilitated epigenetic mineralization. Combined with regional comparison, our results indicate further mineralization potential in the analogous domes of the Songpan-Ganze Orogen.