Metallogenic Mechanism And Regularity Of The Shuangwang Gold Deposit, Shaanxi Province

The Shuangwang gold deposit is situated in the Feng-Tai ore-centralization area in the Qinling Orogen, owning a large gold resource of ca. 70 t. The deposit is hosted by the breccia belt in the albitite in the Upper Devonian Xinghongpu Formation. The Shuangwang gold event can be divided into sedimentary diagenesis, hydrothermal mineralization and supergenetic oxidation periods. The hydrothermal mineralization period can also be divided into albite-quartz(I), ankerite-albite-pyrite(II), calcite-ankerite-pyrite(III), calcite-pyrite-quartz(IV) and anhydrite-fluorite(V) stages, of which stages II and III are the main ore-forming stages.The Shuangwang gold deposit contains a large amount of albite. The in-situ LA-ICP-MS analysis results show that the trace and rare earth elements features of albite in sedimentary albitite are similar to those of sea water, suggesting a submarine sedimentary origin. A lot of stage I albite-quartz veins occur in albitite, and the element composition of stage I albite is almost the same as that of albite in albitite, suggesting that the materials came from albitite. The alteration of albite during the main ore-forming stages released sodium ions, which promoted the dissolution and migration of the gold contained in wall rocks. Stage II albite developed mainly in the early phase, when the physicochemical conditions were against gold sedimentation. Thus the enrichment of stage II albite normally indicates a poor gold mineralization.The breccias show the characteristics of in-site crushing. Micro fractures and stage I veins had occurred in albitite before the fragmentation of wall rocks, which significantly weakened the mechanical strength of albitite. During the main stages, albitite was preferentially broken into fragments by the hydrothermal fluids with high temperature and pressure. Fluids filled the space between fragments, forming cements.Iron and magnesium isotopes indicate that the δ57Fe value of stage II fluid is ca.-0.05‰, and the initial δ26Mg value is ca.-0.35‰. Magmatic iron and wall rock magnesium dominated the Fe and Mg isotope compositions of ore-forming fluids, respectively. The δ34S values(δ34S=2.6~14.9‰) indicate a sulfur source of both magmatic and wall rock origins. The Xinghongpu Formation has a relative high Au content(0.006-0.111ppm), suggesting a gold pre-enrichment during sedimentary diagenesis.The lamprophyre vein cutting into breccias and orebody yields a zircon U-Pb age of 214.4±2.7Ma, which limits the ceiling age of gold mineralization. Because the magmation in the Shuangwang region(219-214.9Ma) had important contribution to gold mineralization, the age of the Shuangwang gold event can be approximately limited into 219-214.4Ma.The main stages ore-forming fluids had the features of moderate-high temperature(260-340oC), high pressure(100-140MPa), alkalescence(pH = 5.18 to 6.0) and weak reduction(Eh=-0.30 to-0.12). Thermodynamic calculations indicate that Au in stage II fluid migrated mainly in the form of Au(HS) 2- with a gold solubility of more than 100 ppb. During stage III, the gold solubility had dropped to ca. 1ppb, producing a poor gold mineralization. The drop of fluid temperature and the fH2 S and fO2 decreases caused by fluid boiling and pyrite precipitation reduced the gold solubility, which resulted in the precipitation and enrichment of gold.The major indicators for gold deposit are albitite, breccias, stage II ankerite-pyrite mineral assemblage, and the enrichment of Au, Cu, Ag, Pb and Hg. There are superior metallogenic potential in the deep part of breccias in the west mine area and the peripheral breccias.