Genesis of the Jianbeigou Gold Deposit on the Southern Margin of the North China Craton: Insights from Fluid Inclusions, H-O-S Isotopes, and Pyrite in situ Trace Element Analyses

The Jianbeigou gold deposit is a typical lode gold deposit in the Qinling metallogenic belt, located on the southern margin of the North China Craton. Three stages of the hydrothermal process can be distinguished, including the quartz ± pyrite, quartz-polymetallic sulfide, and quartz-carbonate ± pyrite stages. From the early to late stages, the homogenization temperatures of primary fluid inclusions are 281–362°C, 227–331°C, and 149–261°C, respectively. The corresponding salinities estimated for these fluids are 3.9–9.9 wt%, 0.4–9.4 wt%, and 0.7–7.2 wt% Na Cl equiv. Combined with laser Raman spectroscopy data, the ore-forming fluid belongs to a H_(2)O-CO_(2)-Na Cl ± CH_4 system with medium–low temperature and salinity. The δ~(18)Ofluid and δD values for the quartz veins are-1.0‰ to 6.0‰ and-105‰ to-84‰, respectively, which indicates that the ore-forming fluid is of mixed source, mainly derived from magma, with a contribution from meteoric water. Pyrite has been identified into three generations based on mineral paragenetic sequencing, including Py1, Py2, and Py3. The pyrites have δ~(34)S sulfur isotopic compositions from three stages between 3.7‰ and 8.4‰, indicating that sulfur mainly originated from magma. Te, Bi, Sb, and Cu contents in pyrite were all high and showed a strong correlation with Au concentrations. Native gold and the Au-Ag-Bi telluride minerals were formed concurrently, and the As concentration was low and decoupled from the Au content. Therefore, Te, Bi, Sb and other low-melting point chalcophile elements play an important role for gold mineralization in arsenic-deficient ore-forming fluid. Combined with the geological setting, evolution of pyrite, and ore-fluids geochemistry, we propose that the Jianbeigou deposit can be classified as a magmatic–hydrothermal lode gold deposit. Gold mineralization on the southern margin of the North China Craton is related to Early Cretaceous magmatism and formed in an extensional setting.

Fluid Inclusion and H-O Isotope Geochemistry of the Phapon Gold Deposit, NW Laos: Implications for Fluid Source and Ore Genesis

The Phapon gold deposit, located in northern Laos, is a unique large-scale gold deposit in Luang Prabang-Loei metallogenic belt. It is hosted in the Lower Permian limestone and controlled by a NE-trending ductile-brittle fault system. There are three types of primary ore including auriferous calcite vein type, disseminated type, and breccia type, and the first two are important in the Phapon gold deposit. Based on fluid inclusion petrography and microthermometry, three types of primary fluid inclusions including type 1 liquid-rich aqueous, type 2 vapor-rich aqueous and type 3 daughter mineralbearing aqueous were identified in hydrothermal calcite grains. The ore-forming fluids are normally homogeneous, as indicated by the widespread type 1 inclusions with identical composition. The coexistence of type 1 and type 2 inclusions, showing similar final homogenization temperature but different compositions, indicate that fluid immiscibility did locally take place in both two types of ores. The results of microthermometry and H-O isotopes geochemistry indicate that there are little differences on ore-fluid geochemistry between the auriferous calcite vein-type and disseminated type ores. The oreforming fluids are characterized by medium-low temperatures(157–268 oC) and low salinity(1.6 wt.%–9.9 wt.% NaCl eq.). It is likely to have a metamorphic-dominant mixed source, which could be associated with dehydration and decarbonisation of Lower Permian limestone and Middle–Upper Triassic sandstones during the dynamic metamorphism. The fluid-wallrock interaction played a major role, and the locally occurred fluid-immiscible processes played a subordinate role in gold precipitation. Combined with the regional and ore deposit geology, and ore-fluid geochemistry, we suggest that the Phapon gold deposit is best considered to be a member of the epizonal orogenic deposit class.

Geological,Fluid Inclusion,H-O-S-Pb Isotope Constraints on the Genesis of the Erdaogou Gold Deposit,Liaoning Province

The Erdaogou gold deposit is located in the conjuncture of North China Carton and Xingmeng orogenic belt.The ore-forming process of Erdaogou gold deposit is divided into three stages(ⅠtoⅢ),which are quartz-pyrite stage(stage I),quartz-polymetallic sulfide stage(stage II)and quartz-calcite stage(stage III).Two types of fluid inclusions is distinguished in the ore-forming stage,i.e.,aqueous type(W-type)and aqueous-carbonic type(C-type)inclusions.From the stageⅠtoⅢ,the homogenization temperature of fluid inclusions are respectively 334-395,214-364 and 172-272℃,with salinities of 7.72 wt.%-11.23 wt.%NaCl equiv.,0.20 wt.%-23.18 wt.%NaCl equiv.,and 0.35 wt.%to 5.25 wt.%NaCl equiv.The ore-forming fluids of the Erdaogou deposit have the characteristics of medium-low temperatures,moderate salinities and low densities.And the fluids belong to the CO,-H2O-NaCl system.The values ofδ34SV-CDT in sulphide samples at different stages(stages I to III)are between-2.2‰ to 2.3‰,indicating sulfur source from magma volatiles or subvolcanic rock leaching.The lead isotopes suggest that ore-forming metals may be derived from a mixture of lower crust and mantle materials.Oxygen and hydrogen isotope data at Erdaogou indicate that magmatic fluid and meteoric water may both be involved in the hydrothermal system.Based on the geological characteris-tics,fluid inclusion results,stable and radiogenic isotope results of Erdaogou gold deposit,we believe that the temperature decrease,fluid boiling are the key factors leading to the ore precipitation and the genetic type of Erdaogou gold deposit is low-sulfidation epithermal deposit.

Geological and Geochemical Characteristics of the Zhulazhaga Gold Deposit in Inner Mongolia, China

Located in Alxa Zuoqi (Left Banner) of Inner Mongolia, China, the Zhulazhaga gold deposit is the first large- scale gold deposit that was found in the middle-upper Proterozoic strata along the north margin of the North China craton in recent years. It was discovered by the No. 1 Geophysical and Geochemical Exploration Party of Inner Mongolia as a result of prospecting a geochemical anomaly. By now, over 50 tonnes of gold has been defined, with an average Au grade of 4 g/ t. The ore bodies occur in the first lithological unit of the Mesoproterozoic Zhulazhagamaodao Formation (MZF), which is composed mainly of epimetamorphic sandstone and siltstone and partly of volcanic rocks. With high concentration of gold, the first lithological unit of the MZF became the source bed for the late-stage ore formation. Controlled by the interstratal fracture zones, the ore bodies mostly appear along the bedding with occurrence similar to that of the strata. The primitive ore types are predominantly the altered rock type with minor ore belonging to the quartz veins type. There are also some oxidized ore near the surface. The metallic minerals are composed mainly of pyrite, pyrrhotite and arsenopyrite with minor chalcopyrite, galena and limonite. Most gold minerals appear as native gold and electrum. Hydrothermal alterations associated with the ore formation are actinolitization, silicatization, sulfidation and carbonation. A total of 100 two-phase H 2 O-rich and 7 three-phase daughter crystal-bearing inclusions were measured in seven gold- bearing quartz samples from the Zhulazhaga gold deposit. The homogenization temperatures of the two-phase H 2 O-rich inclusions range from 155 to 401°C, with an average temperature of 284°C and bimodal distributions from 240 to 260°C and 300 to 320°C respectively. The salinities of the two-phase H 2 O-rich inclusions vary from 9.22wt% to 24.30wt% NaCl equiv, with a mode between 23 wt% and 24wt% NaCl equiv. Comparatively, the homogenization temperatures of the three- phase daughter crystal-bearing inclusions vary from 210 to 435°C and the salinities from 29.13wt% to 32.62wt% NaCl equiv. It indicates that the ore-forming fluid is meso-hypothermal and characterized by high salinity, which is apparently different from the metamorphic origin with low salinity. It suggests a magmatic origin of the gold-bearing fluid. The δ 18 O values of quartz from auriferous veins range from 11.9 to 16.3 per mil, and the calculated values in equilibrium with quartz vary from 1.06 to 9.60 per mil, which fall between the values of meteoric water and magmatic water. It reflects that the ore-forming fluid may be the product of mixing of meteoric water and magmatic water. Based on geological and geochemical studies of the Zhulazhaga gold deposit, it is supposed that the volcanism in the Mesoproterozoic might make gold pre-concentrate in the strata. The extensive and intensive Hercynian tectono-magmatic activity not only brought along a large number of ore-forming materials, but also made the gold from the strata rework. It can be concluded that the ore bodies were mainly formed in late hydrothermal reworking stage. Compared with typical gold deposits associated with epimetamorphic clastic rocks, the Zhulazhaga deposit has similar features in occurrence of ore bodies, ore-controlling structure, wall-rock alterations and mineral assemblages. Therefore, the Zhulazhaga gold deposit belongs to the epimetamorphic clastic rock type.

陕西金龙山卡林型金矿带成矿流体地球化学研究

陕西金龙山金矿的流体包裹体研究表明,成矿流体属于Na+ _Cl-型;从成矿早期到晚期,流体的氧化性逐渐增强,成矿深度逐渐变浅,大气降水混入增多,有机组分增多。石英包裹体的Na+ 、K+ 、SO2 -4、Cl-以及阴、阳离子总量都高于同期共生的方解石,而Mg2 + 和F-则相反;含铁方解石的δ13 C、δ18O和包裹体δD均低于方解石和石英。用配位化学理论将这些差异解释为同一流体系统水岩作用的结果。氢、氧、碳同位素指示了流体主要来自建造水和大气降水;