Textures are important features of sandstones; however, their controlling factors are not fully understood. We present a detailed textural analysis of fluvio-lacustrine sandstones and discuss the influences of provenance and depositional environments on sandstone textures. The upper Permian – lowermost Triassic Wutonggou sandstones in the Bogda Mountains, NW China, are the focus of this study. Sandstone thin-sections were studied by point counting and their textures were analysed using statistical and principal component analysis. Fluvial lithic, fluvial feldspathic, deltaic lithic, deltaic feldspathic, littoral lithic and littoral feldspathic sandstone were classified and compared. These comparisons indicate that lithic and feldspathic sandstones from the same depositional settings have significant differences in graphic mean, graphic standard deviation and roundness; in contrast, sandstones from different depositional settings but with similar compositions have limited differences in textures. Moreover, three principal components (PCs) are recognized to explain 75% of the total variance, of which the first principal component (PC1) can explain 44%. In bivariate plots of the PCs, sandstones can be distinguished by composition where lithic and feldspathic sandstones are placed in different fields of the plots along the axis of PC1. However, sandstones from different depositional settings overlap and show no clear division. These results indicate that provenance, mainly the source lithology, is the most significant controlling factor on sandstone texture, whereas the depositional environment has limited influence. This study improves our understanding of textural characteristics of fluvio-lacustrine sandstones and their controlling factors, and shows the potentiality of principal component analysis in sandstone studies.

The Duotoushan deposit is an important Fe–Cu deposit in the Aqishan–Yamansu metallogenic belt of eastern Tianshan, NW China. Magnetite occurs in two main habits which are common in many Fe–Cu deposits, i.e. platy (TD1 Mag) and granular magnetite (TD2 Mag) have been identified at Duotoushan. Platy magnetite shows two different zones (bright and dark) based on the observations by scanning electron microscopy. The bright part (TD1-L) is the main part of TD1 magnetite and lacks inclusions. The dark part (TD1-D) is very porous and has abundant tiny silicate inclusions. Granular magnetite is usually anhedral with obvious oscillatory zoning in back-scattered electron images. In general, the dark zones of magnetite are characterised by greater Si, Ca, Al and lesser Fe contents than the bright zones. In situ X-ray diffraction (XRD) analysis shows that the lattice parameter of TD1 magnetite is approximately equal to that of standard magnetite and slightly higher than that of TD2 magnetite, indicating that some cations with ionic radii smaller than those of Fe2+ or Fe3+ entered the magnetite lattice by simple or coupled substitution mechanisms in TD2 magnetite.

The results in the present study show that the effects of temperature and $f_{{\rm O}_ 2}$ on platy magnetite are very limited and the changing fluid composition might be the major controlling factor for the formation of Duotoushan platy magnetite. Although the possibility that mushketovite transformed from hematite cannot be excluded entirely, evidence from in situ XRD data, pore-volume ratio calculation and the growth habit of intergrown minerals indicates that platy magnetite (TD1) coexisting with amphibole was more likely to have been precipitated originally from hydrothermal fluid. This was then affected by changes in the fluid composition which consequently led to dissolution of primary magnetite (TD1-L) and re-precipitation of TD1-D magnetite (with abundant porosity and mineral inclusions). Meanwhile, granular magnetite (TD2) with oscillatory zoning, and coexisting with epidote and quartz, was precipitated from fluid with periodic variation in temperature. These oscillatory zones are characterised by bands enriched in Si, Al and Ca alternating with bands depleted in these elements. The present investigation revealed a complex evolutionary process for magnetite formation in the Duotoushan deposit. The importance of combined investigation of texture and compositional characterisation of magnetite for study of the ore genesis and evolution of Fe–Cu deposits is highlighted.

Sedimentary rocks from the Ediacaran–Cambrian boundary record important biological, climatic and geotectonic changes during this time. To date, only few geochemical investigations on the upper Ediacaran – upper Cambrian rocks in the Tarim Basin have been carried out. Here, we report high-resolution δ13Ccarb records from the Penglaiba, the Wushi phosphorite and the Dongergou sections from Ediacaran–Cambrian Series 3 in the Keping area of the Tarim Basin. The sections display several obvious δ13Ccarb shifts; δ13Ccarb values increased from 3‰ to 6.7‰ across the Qigebulage Formation. Moreover, a negative δ13Ccarb shift across the Ediacaran–Cambrian boundary is apparent; δ13Ccarb values decreased to a minimum of −9.8‰ in the Wushi phosphorite section (−7.7‰ in Dongergou section and −5.4‰ in Penglaiba section), followed by a positive carbonate carbon isotopic excursion across the Yuertusi Formation into the middle of the overlying Xiaoerbulak Formation. Furthermore, more or less invariable positive δ13Ccarb values characterize the middle and upper Xiaoerbulak Formation. The most negative δ13Ccarb value (−14.3‰) occurred near the base of the Shayilik Formation, which is the absolute minimum value among the studied sections of the Cambrian Series 2 to Cambrian Series 3 transition in the world. The δ13C data from Keping, Tarim Basin are in good agreement with carbon isotope profiles recorded in South China, and these events may reflect the perturbation of the carbon cycle in the Tarim Basin during the Ediacaran–Cambrian and the Cambrian Series 2 – Cambrian Series 3 transitions.