Geology and ore-forming fluids of the Dayingezhuang gold deposit, Jiaodong Peninsula, eastern China: Implications for mineral exploration

The Dayingezhuang deposit is a representative large gold deposit within the Zhao-Ping fault zone in Jiaodong Peninsula, eastern China, and contains gold reserves of 170 t with an average grade of 3.10 g/t. The orebodies are hosted in the Late Jurassic Linglong granite intrusion and are Jiaojia-type (disseminated- or stockwork-style) ores. Mineralization and alteration are structurally controlled by the NE- to NNE-striking Linglong detachment fault located near the central section of the regional Zhao-Ping fault zone. Mineralization can be divided into primary and supergene periods. The primary period can be further divided into four hydrothermal stages: I) pyrite-(K-feldspar)-sericite-quartz; II) auriferous quartz-pyrite; III) quartz-gold-polymetallic sulfides; and IV) quartz-carbonate. Gold was mainly deposited in the second and third stages. Three types of fluid inclusion were observed (in decreasing order of abundance): AC-type (aqueous-carbonic), A-type (aqueous), and PC-type (pure carbonic). Based on petrographic, microthermometric, and laser Raman spectroscopic analyses of these fluid inclusions, three types of fluid inclusion assemblage (FIA) were identified in the quartz grains: FIA1 (AC- and PCtype), FIA2 (AC-, PC- and A-type), and FIA3 (A-type). First stage quartz grains contain mainly FIA1 -type inclusions, with trapping temperatures of 305-388 degrees C and salinities of 4.28-8.51 wt% NaCl equivalent. Quartz grains that formed in the second and third mineralization stages contain all three types of FIA. Second stage inclusions homogenize completely at temperatures of 219-317 degrees C and have salinities of 1.56-11.12 wt% NaCl equivalent, while third stage inclusions homogenize completely at temperatures of 195-319 degrees C and have salinities of 2.73-13.33 wt% NaCl equivalent. In contrast, quartz grains that formed during the last mineralization stage contain mainly FIA3-type inclusions that yield homogenization temperatures of 126-233 degrees C and salinities of 0.48-6.72 wt% NaCl equivalent. The microthermometric data indicate that the ore-forming system evolved from a CO2-rich mesothermal fluid into a CO2-poor fluid. The existence and microthermometric characteristics of AC-, PC-, and A-type inclusion assemblages (FIA2) within a single thin section of grains that formed during the second and third stages show that ore fluids underwent unmixing during the syn-ore mineralization stages, due to pressure and temperature fluctuations. The microstructural deformation of auriferous samples indicates that both ductile and brittle deformation occurred during the syn-ore mineralization. Based on the inference that fluids were immiscible in FIA2, trapping pressures during ore formation are estimated at 127-276 MPa. Assuming a fluid pressure regime controlled by fault-valve activity, these pressures are equivalent to a mineralization depth of 9.2-14.0 km. The depth to which erosion occurred in the study area has been calculated as similar to 10.3 km since the formation of the deposit at 130 Ma, which suggests good prospecting potential for gold at depth in Dayingezhuang.