Zircon constraints on gold enrichment in a high-silica porphyry, Yixingzhai region, north of the Trans-North China Orogen

Abstract

The Hewan granite porphyry is a newly discovered, highly evolved Au deposit in north China. Two types magmatic zircon are found in the porphyry. Type I is CL-dark and only occurs in the core of some zircons. Type II is CL-bright and mantles Type I zircon or occurs as individual grains. The two Types are close in age, i.e. 138.0 ± 2.2 Ma for Type I and 135.9 ± 2.1 Ma for Type II. Type I zircon is enriched in Y, U, Th, Nb, Ta, P, REE, while Type II zircon is relatively depleted in these elements, which reflects a reverse in the magma evolution. Whole-rock geochemistry and the presence of xenocrystic zircons (2522 ± 18 Ma), combined with εHf(t) (−20.2 to −22.3) and δ18O (5.3 to 6.6 ‰) for Type I zircons, suggests that the Hewan granite porphyry was mainly derived from the melting of the surrounding older, country rock gneiss. The higher εHf(t) (−16.5 to −19.4) and lower δ18O (5.0 to 6.0‰) of Type II zircon indicates that Type II zircon formed during infiltration of a later fluid. This later fluid also had a higher oxygen fugacity (Ce/Ce* Z = 248 to 3663) and a higher H2O activity. We propose that the Hewan granite porphyry originated from the shallow melting of regional gneisses due to heat from underplating mafic magmas related to a subduction event. Type I zircons, which formed during this event where later mantled or totally replaced by Type II zircon due to an influx of Cl-, S-, and Au-bearing fluids from these crystallizing underplating mafic magmas. The high F and pH in this shallow magma promoted decoupling of AuCl− in the upwelling fluids, which allowed for the deposition of elemental Au.