Mao Jingwen

RE-OS ISOTOPIC DATING OF MOLYBDENITES IN THE XIAOLIUGOU W (MO) DEPOSIT IN THE NORTHERN QILIAN MOUNTAINS AND ITS GEOLOGICAL SIGNIFICANCE

Abstract The Xiaoliugou is a large-scale W (Mo) deposit newly explored in the Northern Qilian Mountains Caledonian fold belt, Northwestern China. It is a vein and skarn type deposit genetically associated with the Xiaoliugou Caledonian granodiorite stock. Re-Os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 ± 13 Ma (2σ) and model ages of 436 to 496 Ma. These results suggest that W-Mo mineralization occurred before the collision of plates, and mineralized substances mainly originated from the crust.

Geology and Metallogeny of the Shizhuyuan Skarn-Greisen Deposit, Hunan Province, China

The Shizhuyuan deposit is the largest among the economically important polymetallic tungsten deposits in China. The deposit occurs within the thermal aureole of Yanshanian felsic intrusions that were emplaced into Devonian carbonates and marls. The mineralization can be divided into three phases that are genetically associated with three episodes of granitic emplacement-pseudoporphyritic biotite granite, equigranular biotite granite, and granite porphyry. During the emplacement of pseudoporphyritic biotite granite, thermal metamorphism and subsequent skarnization developed around the stock. The pure limestone was transformed to marble, whereas marls and argillite interlayers were changed to a series of metamorphic rocks such as grossular-diopside hornfels, wollastonite hornfels, diopside hornfels, wollastonite-vesuvianite hornfels, muscovite-K-feldspar-anorthite hornfels, and prehnitevermiculite hornfels. Because of the subsequent strong skarn development, most hornfelses later were transformed into skarn...

Nitrogen isotope and content record of Mesozoic orogenic gold deposits surrounding the North China craton

As an effective tracer, nitrogen isotopes have been used to determine the source of ore materials in recent years. In this study, the nitrogen isotopes and contents were measured on K-feldspar and sericite of gold deposits and some related granitic intrusions in Jiaodong, Xiao-qinling-Xiong’ershan, west Qinling, the west part of North Qilian and the Zhangjiakou-Xuanhua district around the North China craton (NCC). Although the gold deposits around the NCC are hosted in Precambrian metamorphic rocks, Phanerozoic sedimentary rocks, mafic volcanic rocks or granite, comparison of which with the nitrogen contents and isotope data of previous studies on mantle-derived rocks, granites, metamorphic rocks and gold deposits indicates that those deposits are closely related to granitic rocks. In addition, mantle-derived materials may have been involved in the ore-forming processes to a certain degree. This conclusion is consistent with the result of previous hydrogen, oxygen and carbon isotopic studies of those gold deposits.

The Hanshan gold deposit in the Caledonian North Qilian orogenic belt, NW China

Abstract The recently discovered Hanshan gold deposit in northern Gansu Province, northwestern China, is hosted by a WNW-striking shear zone in Ordovician andesite and basalt. Mineralization consists of surface to near-surface oxidized ore (the yellow sandy gossan type) and three types of primary ore, i.e. early-stage quartz-sericite-pyrite ores in stockworks, early-stage disseminated ore, and the most important late-stage quartz ± calcite-sulfide veins. The ore system is characterized by variable degrees of potassic and silicic alteration. Late-stage gold-related fluid inclusions have homogenization temperatures between 170 to 310 °C, with a peak around 260 °C and low salinities. The ore fluids had high contents of CO2, CH4, and N2. Sulfur isotope measurements of −1.9 to +1.7 per mil for hydrothermal pyrites could be consistent with a hydrothermal fluid source from the mantle, but the oxygen and carbon isotope data from calcite and quartz suggest mixing between mantle and crustal fluid sources. K-Ar ages for hydrothermal sericite from ore zones are 213.9 ± 3.1 and 224.4 ± 3.2 Ma. Due to the arid Cenozoic climate, a yellow gold-bearing gossan developed, which consists of jarosite, gypsum, and relict quartz. It could be a widespread and useful prospecting guide for gold in northwestern China.

Tourmalinite from northern Guangxi, China

The extensive development of tourmalinite is a feature that distinguishes the northern Guangxi polymetallic tin province of China from similar metallogenic provinces elsewhere. Two types of tourmalinite occur in the province. The first type, in the lower part of the Early Proterozoic Sibao Group, is bedded, stratiform or lenticular tourmalinite that shows well-developed laminated, gel, and degelatinized structures. Its mineral assemblage is very simple and the grain size ranges from 2 to 8 μm. This tourmaline is relatively rich in Mg, with an Fe/(Fe + Mg) ratio of 0.25–0.50.The second type of tourmalinite occurs as lodes distributed in the exocontact zone of Late Proterozoic biotite-granite intrusions. Its mineral assemblage is relatively complex; the tourmaline is present as euhedral or subhedral crystals ranging from 0.1 to 3.5 mm, mostly from 0.5 to 1 mm. This tourmaline commonly exhibits a radiating, zoned structure with Fe/(Fe + Mg) ratios of 0.64–0.79. It is suggested that the bedded tourmalinite formed by exhalation in an Early Proterozoic spreading-ridge environment, whereas the vein tourmalinite formed in a plate-convergence setting genetically associated with emplacement of Late Proterozoic biotite granite. As the tourmalinites themselves are related to mineralized rocks and orebodies, their origin and the related boron cycle of the region reflect to some extent the formation and evolution of the associated polymetallic tin deposits of the region.

Timing of the formation of the Tianhuashan Basin in northern Wuyi as constrained by geochronology of volcanic and plutonic rocks

The Tianhuashan Basin is one of the most important volcanic basins in the northern Wuyi, southeastern China, comprising two successive volcanic units, the Daguding Formation and the overlying Ehuling Formation, along with several small associated igneous intrusions. The Lengshuikeng super-large-scale Ag-Pb-Zn deposit, which is closely related to these volcanic-intrusive rocks, is located in the northwestern part of the basin. In order to understand the basin evolution and magmatism, we determined LA-ICP-MS U-Pb zircon ages for the volcanic successions and associated intrusive rocks. U-Pb zircon dating of volcanic units yielded precise ages of 144±1 Ma for crystal tuff in the lower member of the Daguding Formation, 142±1 Ma for andesite within the upper member of the Daguding Formation, 140±1 Ma for tuffite of the first (i.e., lowermost) member of the Ehuling Formation, and 137±1 Ma for rhyolitic ignimbrite within the third volcano-stratigraphic member of the Ehuling Formation. Three types of intrusive igneous rocks (quartz syenite porphyry, K-feldspar granite porphyry, and rhyolite porphyry) yielded precise weighted mean 206Pb/238U ages of 144±1, 140±1, and 140±1 Ma, respectively, suggesting that these intrusions along with the aforementioned volcanics were all emplaced during the Early Cretaceous. In addition, the weighted mean 206Pb/238U ages determined on zircon from two samples of a granite porphyry intrusion, which hosts ore mineralization of the Lengshuikeng Ag-Pb-Zn deposit, are 158±1 and 157±1 Ma, indicating emplacement in the Late Jurassic. These new geochronological results for igneous rocks of the Tianhuashan Basin constrain the timing of volcanic and plutonic activity in the basin, and have important implications for our understanding the tectonic history of the region, and for identifying metallogenic types and the timing of ore deposition of the Lengshuikeng deposit.

The Aoyougou Mafic-Ultramafic Complex in the North Qilian Mountains, Northwest China: A Possible Middle Proterozoic Ophiolite along the Southern Margin of the North China Craton

The Aoyougou mafic-ultramafic complex lies in the Paleozoic orogenic belt of the western part of the North Qilian Mountains near Aoyougou valley in Gansu Province, northwestern China. It consists of serpentinite, a cumulate sequence of gabbro and diorite, pillowed and massive lavas, diabase dikes, and chert, an assemblage tentatively interpreted as an ophiolite. SHRIMP dates on zircons from the diabase dikes indicate a crystallization age of 1777 ± 28 Ma. The basalts show light rareearth element enrichment and have relatively high TiO2 and low Al2O3 contents, characteristic of present-day E-MORB or ocean-island lavas. All of the lavas have relatively low MgO contents and Mg numbers [100Mg/(Mg+Fe)], indicating a somewhat evolved character. The diabase dikes have flat chondrite-normalized REE patterns and are significantly more primitive than the lavas. Based on the lava geochemistry, the nature of the serpentinite, and the regional geology, the Aoyougou complex is interpreted as a fragment of oceanic lithosphere that may have formed at an oceanic spreading axis. Later subduction of oceanic lithosphere in the Middle Proterozoic produced a trench-arc-basin system, which is preserved in the North Qilian Mountains. The tectonically dismembered Aoyougou complex is similar in many respects to Phanerozoic ophiolites and suggests that modern-style plate tectonic processes in China may have begun as early as the Middle Proterozoic.