Mineralization Characteristic And Genesis Of The Fancha And Yisishan Gold Deposit In The Eastern Xiaoqinling District

The Xiaoqinling gold district is situated in the southern margin of North China Craton and to the north of Qinling terrain, and has been one of the most important gold metallogenic belts and the second largest gold district with proven gold reserves of500t. This district is dominated by the EW-trending Taiyao and Xiaohe Faults and the Xiaoqinling metamorphic core complex. Gold deposits in the Xiaoqinling district consist mainly of auriferous quartz veins, which cluster in three mineralization belts, including the Dayueping-Laoyacha-Yangzhaiyu belt (southern belt), the Qishuping-Leijiapo belt (central belt), and the Wulicun-linhu belt (northern belt). Gold mineralization is hosted in amphibolites facies metamorphic rocks of the Neoarchean-Paleoproterozoic (2.3-2.7Ga) Taihua Group and strictly structurally controlled, with major ore bodies in brittle-ductile to brittle faults.The Fancha and Yisishan gold deposits are located in the eastern part of the southern and northern belts, respectively. Gold mineralization in the Fancha deposit is composed of auriferous quartz veins, whereas it consists of gold-quartz veins and alteration assemblages in ductile shear zones in the Yisishan deposit. In the Fancha gold deposit, ore-bodies are preferentially hosted in the amphibolite of the Taihua Group and Paleoproterozoic granite, and structurally confined by a series of NE-NNE trending brittle fractures. The most important ore minerals are pyrite, chalcopyrite, galena, sphalerite, native gold and electrum, with minor amounts of tellurides. Hydrothermal alteration is moderately developed, consisting of silification, sericitization, sulfidation, and carbonatization. Four stages of mineral precipitation are recognized based on field and microscopic observations, including the quartz-coarse grained pyrite (stage Ⅰ), quartz-pyrite-chalcopyrite (stage Ⅱ), quartz-polymetallic sulfides (stage Ⅲ) and carbonate-quartz (stage Ⅳ).The Yisishan gold deposit is hosted in the biotite plagioclase gneiss of the Yangzhaiyu Formation of Taihua Group. Ore-bodies are jointly controlled by the Wulicun anticline and a series of ductile-brittle faults of NE-, EW-, and NW-orientations. Gold mineralization can be classified as gold-bearing alteration assemblages in ductile shear zones and auriferous quartz veins in brittle structures. The predominant ore minerals include pyrite, chalcopyrite, galena, sphalerite, bismuthinite, and native gold. Ore-related hydrothermal alteration has produced various mixtures of quartz, biotite, plagioclase, microcline, chlorite, hornblende, and carbonate. Four hydrothermal stages can be recognized based on field and petrographic relations: quartz-massive pyrite (stage I), quartz-disseminated pyrite (stage II), quartz-polymetallic sulfides (stage III), and carbonate-quartz (stage IV).Scanning electron microscopy combined with energy dispersive spectrum (SEM-EDS) and electron probe microprobe (EMP) reveal that both Fancha and Yisishan deposits contain abundant telluride minerals that formed mostly in the gold-polymetallic stage and commonly present as complex with sulfides and native gold, or as fillings within micro-fracture of pyrite and quartz. Textural relationship suggests that telluride minerals precipitated in an order of altaite, sylvanite, hessite, and petzite. LA-ICP-MS in situ trace element analysis of pyrite suggests a prominent positive correlation between Te and Au-Ag. Additionally, at the beginning of the mineralization stage, the ore-bearing fluids were enriched in Te-Au-Ag elements. However, the precipitation of tellurides was suppressed by the high temperature and high fS2. Hence, the Au-Ag-Te elements could be transported by telluride melts or tellurium complex. Tellurides were precipitated due to a change of the chemical and physical parameters of the ore-bearing fluids and consequently over saturation of Te and tellurides. Presence of abundant tellurides indicates a close genetic association between gold mineralization and magmatism.Fluids inclusions are widely distribuyed in auriferous quartz veins both from the Fancha and Yisishan deposits. In the Fancha gold deposit, three types of inclusions are recognized on the basis of phases at room and subzero temperatures and textural relationships:three-phase CO2-rich inclusions, two-phase aqueous inclusions and one-phase CO2inclusions. Microthermometric analysis for these fluid inclusions reveals final homogenization temperatures ranging mainly from280℃to330℃and calculated salinities of ore fluids of12.5to15.75wt.%NaCl equiv. In the Yisishan gold deposit, three-phase CO2-rich inclusions, two-phase aqueous inclusions and two-phase CH4-rich inclusions are recognized. Microthermometric measurements suggest that the gold and associated minerals were precipitated at115-275℃and275-350℃from ore fluids with salinities of2-5wt.%NaCl equiv and14.5-19wt.%NaCl equiv, respectively. Combined with Laser Raman spectroscopic results and existing chemical data of fluid inclusions, it is inferred that the ore fluids responsible for the Fancha and Yisishan deposits were of CO2-H2O-NaCl-(±CH4) composition with middle-high temperature and intermediate salinities. The Th vs. salinity diagram demonstrates phase separation and fluid-mixing have been the major mechanisms leading to gold deposition.The834S values of sulfides in the Fancha gold deposit can be divided into two groups. The first is close to zero (0.9～1.8‰), indicating a deep-seated, most likely magmatic source for the sulfur. The second has negative834S values ranging from-3.8to-1.4‰, reflecting minor isotopic fractionation in the evolution of the mineralizing fluids due to phase separation. In contrast, pyrite separates of the Yisishan gold deposit have834S values ranging from7.5to10.6‰, consistent with the values of local granitic dikes, indicating a magmatic source of the ore-bearing fluids.40Ar/39Ar dating of hydrothermal sericites yields well-defined plateau ages of130.5±1.3Ma and120.2±2.2Ma, indicating two separate hydrothermal events. LA-ICP-MS zircon U-Pb dating of mafic dike coexisting with auriferous quartz veins reveals that the mafic dike and gold mineralization were controlled by extensional tectonic setting, which favored large-scaled magmatism and well development of brittle structures. Extensive magmatism could have provided sufficient heat energy, fluids, components, whereas extensional structures have been the pathways of ore fluids and loci of gold ore bodies.