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

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 Linjia-sandaogou 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 de-posit: the early pyrite (Py1), arsenian pyrite (Py2)-arsenopyrite-sphalerite (Sp1)-quartz-apatite, and frei-bergite-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 delta 34SV-CDT values (4.80 parts per thousand-8.53 parts per thousand) that are typical of a magma-tic-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 degrees C, 154-252 degrees C, and 167-197 degrees 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 mineral-ization in this area.