Shuyin Niu

Ore-forming fluids in gold-telluride deposits in the Pingyi area, western Shandong, China

The Au-telluride district of the Pingyi area, western Shandong, mainly comprises the Guilaizhuang and Lifanggou gold deposits. The former is hosted in cryptoexplosive breccia of the Tongshi complex with a zircon SHRIMP U-Pb weighted mean age of 175.7±3.8 Ma. The latter occurs in dolomitic limestone, micrite and dolomite of the Early Cambrian Zhushadong Formation. Fluid inclusion studies indicate that the inclusions of both Au telluride deposits are both of vapor-liquid two-phase NaCl-H2O type. Homogenization temperatures of the fluid inclusions vary from 103 to 250°C, and the ice melting temperatures range from −2.5 to −13.5 °C, corresponding to a salinity range of 4.65 to 17.26 wt.% NaCl equiv. The δ34S values of pyrite associated with gold mineralization exhibit a narrow range of −0.71 to 2.99‰, implying that the sulfur was probably derived from the mantle or magma. The δ18OSMOW values of vein quartz and calcite range from 11.5 to 21.5‰, corresponding to the δ18Ofluid values of −1.13 to 10.9‰, and the δD values of the fluid inclusions between −70 and −48‰. The isotope data suggest that the ore-forming fluids of the two gold deposits were derived from the mantle, and mixed with meteoric water at shallow levels. Pressure release and boiling of the fluids played an important role in the ore-forming processes of the two deposits.

Zircon U-Pb age and Hf isotopic characteristics of the Huangtuliang monzonitic granite, North Hebei Province, China

The Huangtuliang monzonitic granite outcrops on the northern side of the Huangtuliang gold mining district, Chicheng, North Hebei Province. Our predecessors only made isotopic age determination using the K-Ar method. Through LA-MC-ICP-MS zircon U-Pb dating and zircon Hf isotopic composition determination, this study acquired the age of 244.8±2.0 Ma (MSWD=0.57) on the basis of the weighed mean 206Pb/238U ratio, indicating that the Huangtuliang monzonitic granite was formed during the Middle Triassic period, which is the product of Early Indosinian magmatic activities in the region of North Hebei. ɛHf(t) values vary relatively evenly, within the range of −10.65–−14.03, with an average of −12.14. The two-stage evolution model ages, tDM2, vary between 1943 and 2144 Ma, implying that the rock-forming materials of the Huangtiliang monzonitic granite mainly came from the Paleoproterozoic recirculated crustal materials, though a small quantity of enriched-mantle materials would have been involved.

Isotope tracers for deep-seated fluids and noble gases

The role of He and Ar isotopes in tracing the source of ore fluids has aroused great attention of the broad masses of the geological researchers. On the basis of lots of test and measurement of He and Ar isotopes in sulfides from Au, Ag polymetallic ore deposits in northern China, statistics has been made on the published He and Ar isotope data from 27 gold deposits, 13 silver polymetallic ore deposits, 8 polymetallic ore deposits, 1 rare-earth deposit, 3 oceanic incrustations, 3 volcanic springs and their wall rocks and granites. The statistical results indicate that the 3He/4Ar (×10−6) values of the Au, Ag polymetallic ore deposits are within the range of 0.24–9.39, with an average of 3.34×10−6; the He/Ar values, 0.007–6.01,with an average of 2.37; the 40Ar/36Ar values, 265.75–2361, with an average of 699.0; the 4He/40Ar values, 0.0020–643.86, with an average of 5.85, the 3He/4Ar (×10−6) values of gneiss and granite surrounding the mining area, 0.001–1.79, with an average of 1.00×10−6, reflecting great differences in source. Mantle-source He in 48 Au, Ag polymetallic ore deposits accounts for 4.55%–83.06%, averaging 29.91%. It falls near the mantle-source region which can be seen in the He isotopic concentration diagram and the 3He/4He(R/Ra)-40Ar/36Ar plot. Studies suggested that the ore-forming materials for endogenic Au, Ag polymetallic ore deposits should be derived from the deep interior of the Earth, and with the multi-stage evolution of mantle plumes the deep-seated ore fluids would be transported from the deep interior of the Earth to the shallow levels. During this process the mixing of crust/mantle-source fluids would inevitably occur, therefore, the value range always lies between the mantle and the crust.

Source of fluids in the Longquanzhan gold deposits in the Yishui area, Shandong, China

The Longquanzhan gold deposit, hosted in Archean gneiss is located along the Tanlu fault zone, on the southeastern margin of the North China craton. The orebodies occur as veins striking 15°–18° and dipping SE at 35°–62°. Wall rock alteration types include silicification, pyritization, and sericitization, and chloritization, calcitization. Metallic minerals in ores are dominated by pyrite. Gold occurs mainly in the form of electrum. All inclusions are two-phase (L+V) NaCl- H2O type. The inclusions generally range in diameter from 2 to 8 µm with a vapour/liquid ratio of 5–90% and mainly 5–10%. The homogenization temperatures of fluid inclusions in the Longquanzhan gold deposit are between 108 and 300°C. The ice-melting temperatures vary from −2.0 to −8.6°C, at a peak of −2.5 to −7.0°C. The salinities determined from the ice-melting point of the fluid inclusions range from 3.39 to 12.39 wt.% NaCl equiv. According to isotope fractionation equation and mean homogenization temperatures, the δ18O values of the mineralizing fluids are calculated, ranging from −0.28‰ to +4.07‰, showing mixing of oreformi fluids and meteoric waters. The 3He/4He ratios of fluid inclusions in pyrite are 0.14–0.24 Ra, suggesting the crustal source of ore-forming fluid. The assemblage of alteration minerals, the characteristics of fluid inclusions and stable isotopes indicate that the Longquanzhan gold deposit belongs to epithermal type deposit.

A case study of structure-controlled mineralization— the Huangtuliang gold deposit, northwestern Hebei, China

The Huangtulian gold deposit, with 10.7 tons of gold reserves at shallow levels (<150 m in depth), is situated in the Zhaojiagou-Erbaozi secondary brittle-ductile shear zone along the northern margin of the North China craton. The dominant structure in the Huangtuliang gold deposit is a steeply north-dipping, brittle-ductile shear zone south of the ore zone. Hellium and argon, sulfur and oxygen isotope studies suggest that mantle fluids were involved in the mineralization. Detailed studies of ore field structures and engineering data suggest that the Huangtuliang gold deposits has potential for a major to world-class gold resource.

The formation of a mantle-branch structure in western Shandong and its constraints on gold mineralization

On the basis of regional strata, magmatic activity, ring and radial faults, gently dipping detachments, geophysical information and isotopic age, this paper discusses the characteristics of mantlebranch structure in western Shandong. According to the characteristics of ore-control structures, gold deposits in the Luxi mantlebranch structure are divided into ductile shear zone type and detachment-slip layer type deposits. The detachment-slip layers type deposit, including the Guilaizhuang gold deposit and Mofanggou gold deposit are significant economic types. The_δ34S values of pyrite associated with gold mineralization exhibit a narrow range of-0.71 to 2.99%., implying that the sulfur was probably derived from the mantle or magma, whereas the_´18Ofluid and_´D values suggest that the ore-forming fluids of the gold deposits mixed with meteoric water in the shallow level.