Zhang Zhaochong

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.

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.