Jurassic Au–Ag mineralization of the Linjiasandaogou deposit in the Liaodong Peninsula, northeast China: Evidence from apatite U–Pb dating and the in situ geochemistry of sulfides

Abstract

The Liaodong Peninsula in northeast China hosts visible and invisible Au-dominated mineralization. Compared with visible Au, age and mechanisms of invisible Au-dominated mineralization are still unclear. The Linjiasandaogou deposit in the Liaodong Peninsula comprises disseminated and veinlet ores hosted in Paleoproterozoic schist and Triassic granite porphyry and lamprophyres. Three mineralization stages were identified in this deposit: the early pyrite (Py1), arsenian pyrite (Py2)–arsenopyrite–sphalerite (Sp1)–quartz–apatite, and freibergite–sphalerite (Sp2)–galena–quartz stages. Gold mineralization is mainly in invisible forms and was deposited during the second stage, whereas Ag was deposited during the third stage. Uranium-Pb dating of hydrothermal apatite from the second stage indicates that Au mineralization occurred at ca. 184 Ma, when the Yaojiagou granite was emplaced. In situ S isotope analysis shows that sulfides which formed during the mineralization stages have a narrow range of δ34SV-CDT values (4.80 ‰–8.53 ‰) that are typical of a magmatic–hydrothermal system. Py2 has higher As, Au, and Ag contents of 15,100–95,576, 15.4–183, and 20.8–129 ppm, as compared with Py1 (As <0.9–11,050 ppm, Au <0.03–11.1 ppm, and Ag <0.05–48.5 ppm). The negative correlations between As–S and Au–Fe in pyrite are indicative of As substitution for S and Au substitute for Fe in the pyrite. The pyrite and sphalerite trace element compositions indicate that the formation temperatures of the three stages were >250 °C, 154–252 °C, and 167–197 °C, respectively. The evolution of the hydrothermal system indicates that the deposition of Au is due to cooling and fluid-rock interaction, whereas the precipitation of Ag results from an increase in the sulfidation state. Results suggest the invisible Au-dominated mineralization in the Linjiasandaogou deposit was associated with the Early Jurassic intrusion. This study supplements the ore-forming epoch framework of the Qingchengzi ore field and improves the understanding of the Au–Ag mineralization in this area.