Stable isotopes and noble gases in the Xishimen gold deposit, central North China Craton: metallogeny associated with lithospheric thinning and crust–mantle interaction

Abstract

Quartz-vein type gold mineralization at Xishimen is a recently discovered gold deposit in the central North China Craton. More than 50 auriferous quartz veins occur in this region within a NNW–SSE-trending fault zone 4600 m in length and 3–10 m wide. Wall rocks are mainly Precambrian tonalite–trondhjemite–granodiorite (TTG) gneisses and associated supracrustals, modified by K-feldspathization and pyrite-phyllic hydrothermal alteration. Based on detailed field and petrographic studies, we identify five episodes of mineralization: pyrite-phyllic stage (I), coarse-grained pyrite-milky white quartz stage (II), fine-grained smoky grey quartz-pyrite stage (III), fine-grained smoky grey quartz-polymetallic sulphide stage (IV), and quartz-carbonate stage (V). We present results of δ34S analysis of sulphide minerals from the different stages which show tightly clustered values in the range of –1.0‰ to 2.1‰, close to those of mantle and meteorite sulphur. Lead isotopic ratios of pyrite from the early to main stages also show restricted ranges with 206Pb/204Pb of 16.289–17.286, 207Pb/204Pb of 15.217–15.453, 208Pb/204Pb of 37.012–38.232, implying lower crustal input. 3He/4He and 40Ar/36Ar ratios of fluid trapped in pyrite are 0.68 Ra to 1.20 Ra (where Ra is the 3He/4He ratio of air = 1.4 × 10−6) and 540.9–1065, respectively. 3He and 4Ar concentrations vary from 10.05 to 18.5 (10−7 cm3STP/g) and 6.15 to 17.4 (10−7cm3STP/g), respectively, with calculated mantle helium ranging from 8.47% to 14.96% (average 11.01%). δ18OQ and δ18DQ values of quartz range from 8.0‰ to 13.2‰ and –101.9‰ to –70.5‰, respectively, with calculated δ18OW values of the mineralizing fluid ranging from 1.11‰ to 5.72‰, suggesting the mixing of magmatic aqueous fluid with meteoric water during gold precipitation. We correlate the mixed crust–mantle signature of the ore-forming sources to magmatism and metallogeny associated with Mesozoic inhomogeneous lithosphere thinning in the central North China Craton.