Garnet U-Pb dating of the Yinan Au-Cu skarn deposit, Luxi District, North China Craton: Implications for district-wide coeval Au-Cu and Fe skarn mineralization

Abstract

Several recent studies have shown that garnet U-Pb dating may potentially provide robust constraints on the ages of carbonatite, alkaline magmatism and skarn ore formation. Here we present textural, trace element, and laser ablation inductively coupled plasma mass spectrometry U-Pb isotope data of garnet from the Yinan Au-Cu skarn deposit in Luxi District, eastern North China Craton, to place tight constraints on the timing of mineralization. Three hydrothermal andradite-rich garnet samples (JL-8, THZ-38 and TJ-13) collected respectively from Jinlong, Tonghanzhuang and Gongquan ore segments of the Yinan Au-Cu skarn deposit, contain 2.31–10.36 ppm U and are characterized by flat time-resolved depth profiles for U, indicating that this element is hosted in crystallographic lattice in the garnets. The positive correlation between U and LREEs concentration suggests that the incorporation of U into the garnet is largely controlled by substitution mechanisms. The three garnet samples yield weighted mean 206Pb/238U dates of 127 ± 3 Ma, 127 ± 5 Ma and 126 ± 7 Ma (2σ), respectively. These ages are reproducible within analytical uncertainties and are consistent with a zircon U-Pb age of 129 ± 2 Ma of the ore-related diorite intrusion, and thus, are interpreted as the age of Au-Cu mineralization at Yinan. The results presented here, when combined with existing isotopic age data, demonstrate that Au-Cu skarn mineralization throughout the southeastern Luxi District occurred, coevally, in the Early Cretaceous and is genetically associated with the Tongjing and Jingchang granitic-dioritic complexes. The Au-Cu mineralization and associated magmatism coincide in time with the iron skarn mineralization and associated magmatism (134–126 Ma) across the Luxi District, strongly suggesting a causal link between the district-wide skarn mineralization and pervasive intermediate to felsic magmatism; both are products of destruction of lithospheric mantle beneath the North China Craton. This study confirms that garnet U-Pb dating can be a robust geochronometer for garnet-bearing hydrothermal deposits and can elucidate associated geological processes.