Geochronology and isotope geochemistry studies of an epithermal gold deposit in the northern Lesser Khingan Range, NE China: The Gaosongshan example

Abstract

The Gaosongshan epithermal gold (Au) deposit is located in the northern margin of the Lesser Khingan Range polymetallic ore belt, northeast (NE) China, and contains Au resources of more than 23 tons at an average grade of 6.3 g/t. Gold orebodies are mainly hosted in intensely altered volcanic rocks of the Lower Cretaceous Banzifang Formation, and as exploration has continued, gold has also been discovered in extensively altered porphyritic intrusions with the highest Au grade of 70 g/t. However, the ages of metallogenesis and related magmatism have not been well constrained yet, which have hindered the summary of metallogenic regularities. In this study, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating from the granite porphyry yields a weighted mean 206Pb/238U age of 101.6 ± 1.2 Ma, slightly older than the Rb–Sr isotope age of 98 ± 1.6 Ma for the auriferous quartz samples, indicating that the Gaosongshan Au mineralization is genetically related to the granite porphyry. The fluid δD and δ18OH2O values vary from −136% to −111% and from −9.3% to −2.5%, respectively, and the 4He and 40Ar isotope compositions range from 15.7 × 10−9 to 35.5 × 10−9 cm3 STP/g and 13.4 × 10−7 cm3 to 34.1 × 10−7 cm3 STP/g, respectively. The obtained isotopic data show that the ore-forming fluids were mixtures of magmatic and meteoric water but were dominated by meteoric water. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of ores in the Gaosongshan deposit are 17.752–18.856, 15.514–15.674, and 38.005–39.921, respectively, which are similar to those of the Gaosongshan granite porphyry, implying that Pb and possibly Au originated primarily from late Early Cretaceous magmatic intrusions. Combining these results with data from previous studies, we propose that the Gaosongshan Au deposit formed in an extensional tectonic setting during the rollback of the Paleo-Pacific oceanic plate in NE China.