Geological and Fluid Inclusion Constraints on Gold Deposition Processes of the Dayingezhuang Gold Deposit, Jiaodong Peninsula, China

The Dayingezhuang gold deposit, hosted mainly by Late Jurassic granitoids on Jiaodong Peninsula in eastern China, contains an estimated 170 t of gold and is one of the largest deposits within the Zhaoping fracture zone. The orebodies consist of auriferous altered pyrite-sericite-quartz granites that show Jiaojia-type (i.e., disseminated and veinlet) mineralization. Mineralization and alteration are structurally controlled by the NE- to NNE-striking Linglong detachment fault. The mineralization can be divided into four stages: (K-feldspar)-pyrite-sericite-quartz, quartz-gold-pyrite, quartz-gold-polymetallic sulfide, and quartz-carbonate, with the majority of the gold being produced in the second and third stages. Based on a combination of petrography, microthermometry, and laser Raman spectroscopy, three types of fluid inclusion were identified in the vein minerals: NaCl-H2O (A-type), CO2-H2O-NaCl (AC-type), and pure CO2 (PC-type). Quartz crystals in veinlets that formed during the first stage contain mainly AC-type fluid inclusions, with rare PC-type inclusions. These fluid inclusions homogenize at temperatures of 251 degrees C-403 degrees C and have low salinities of 2.2-9.4 wt% NaCl equivalent. Quartz crystals that formed in the second and third stages contain all three types of fluid inclusions, with total homogenization temperatures of 216 degrees C-339 degrees C and salinities of 1.8-13.8 wt% NaCl equivalent for the second stage and homogenization temperatures of 195 degrees C-321 degrees C and salinities of 1.4-13.3 wt% NaCl equivalent for the third stage. In contrast, quartz crystals that formed in the fourth stage contains mainly A-type fluid inclusions, with minor occurrences of AC-type inclusions; these inclusions have homogenization temperatures of 106 degrees C-287 degrees C and salinities of 0.5-7.7 wt% NaCl equivalent. Gold in the ore-forming fluids may have changed from Au(HS)(0) as the dominant species under acidic conditions and at relatively high temperatures and fO(2) in the early stages, to Au(HS)(2-) under neutral-pH conditions at lower temperatures and fO(2) in the later stages. The precipitation of gold and other metals is inferred to be caused by a combination of fluid immiscibility and water-rock interaction.