Late Jurassic magmatism and associated Au–Cu polymetallic mineralization in the North China Craton: Constraints from U–Pb–Hf–Sr–Nd isotopes of the Xiayingfang intermediate-felsic igneous rocks

Abstract

The Xiayingfang Au–Cu polymetallic deposit (22.5 Mt. @1.9 g/t Au, 0.54% Cu) in the northern margin of the North China Craton consists of the Xiajinbao and Dongliang ore blocks. It is characterized by multiple magmatic rocks, including rhyolitic tuff, granite porphyry, diorite porphyry, quartz porphyry, and monzonite porphyry. The Xiajinbao and Dongliang mineralization occurs mainly as auriferous quartz-sulfide veins in granite porphyry and diorite porphyry, respectively.Rhyolitic tuff, granite porphyry, diorite porphyry, quartz porphyry, and monzonite porphyry at Xiayingfang yielded zircon U–Pb ages of 162.3 ± 1.3 Ma (MSWD = 1.4), 161.8 ± 1.5 Ma (MSWD = 1.3), 162.7 ± 1.7 Ma (MSWD = 1.4), 162.9 ± 1.7 Ma (MSWD = 2.4), 162.7 ± 1.6 Ma (MSWD = 0.7), respectively, suggesting their Late Jurassic emplacement. The rhyolitic tuff, granite porphyry, quartz porphyry, and monzonite porphyry are characterized by high aluminum saturation index (ASI = 1.0 to 1.3), positive P2O5 vs. SiO2 correlation, negative Eu anomaly (δEu = 0.29 to 0.78), low average Nb/Th (3.05) and Nb/U (8.80) ratios, low Mg# values (14 to 42), negative zircon εHf(t) (–8.7 to –10.1) and high initial 87Sr/86Sr value (0.712 for granite porphyry), indicating a sedimentary origin. In contrast, the diorite porphyry is characterized by high Sr/Y (29.2 to 53.2), low Y (9.7 to 15.0 ppm) and Yb (1.4 to 1.8 ppm), indicating an adakite-like affinity. Moreover, its listric-shaped normalized REE patterns with no Eu anomalies, low ASI (0.8–0.9), zircon εHf(t) (–7.7 to –11.1), εNd(t) (–10 to –16) and initial 87Sr/86Sr (0.705 to 0.706), high Nb/Th (9.25), Nb/U (30.6) ratios, Mg# values (46 to 54), indicating a dominantly sub-continental lithospheric mantle (SCLM) source, which may have evolved to adakite-like signatures through amphibole fractionation. The mineral assemblages of paragenetic bornite, enargite and electrum at Xiayingfang reveals the highly oxidized condition of ore-related magma, as supported by the magnetic susceptibility (20 to 65 × 10–3SI unit) and average zircon Ce4+/Ce3+ ratio (352) of the diorite porphyry, and clearly different from those barren Xiayingfang porphyries (magnetic susceptibility below 1 × 10–3SI unit, and low average zircon Ce4+/Ce3+ ratio of 128). This reveals that the diorite porphyry may have been ore-causative at Xiayingfang, which is also supported by its clear ore-related alteration (illite, carbonate, and chlorite) and close spatial relations with the orebodies. Mineral Exploration programs in this area should therefore target the diorite porphyries instead.