ALBERT

Description: Tin isotope geochemistry of cassiterite may allow for reconstructing the fluid evolution of tin ore deposits. Here, we present cathodoluminescence (CL) imaging, trace element, and in situ Sn isotope compositions of two cassiterite crystals from an early and a relatively late stage of ore formation of the Xiling vein-style Sn deposit, southeastern China, by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS). Our results show that the early-stage cassiterite from a high-temperature feldspar-stable hydrothermal environment has core, mantle, and rim zones with a systematic decrease in δ124/117Sn3161A (relative to the Sn standard NIST 3161 A) from +0.38 ± 0.06‰ in the crystal core to –0.12 ± 0.06‰ (2 SE) in the mantle zone. This isotopic evolution, also paralleled by a decrease in Ta content by two orders of magnitude, suggests a fluid batch evolving toward isotopically lighter Sn. The very rim zone of this crystal has an intermediate tin isotope composition at about +0.05‰ δ124/117Sn3161A, combined with elevated Ta, suggestive of a second fluid batch. The late-stage cassiterite crystal from a muscovite-stable hydrothermal environment has a core with an evolved Sn isotope composition at about –0.15‰ δ124/117Sn3161A combined with low Ta, and a rim with heavier Sn isotope compositions up to +0.30 ± 0.08‰ δ124/117Sn3161A and higher Ta contents. As for the early-stage crystal, two different fluid batches must be involved in the formation of this crystal. Our pilot study highlights the advantage of spatially resolved analysis compared to conventional, solution Sn-isotope analysis of bulk cassiterite crystals. The Sn isotope variations at the microscale reveal the complexity of cassiterite crystal growth by a combination of closed- and open-system fluid evolution and isotope fractionation.

Description: The Xiling Sn deposit in eastern Guangdong Province comprises the Fengdishan Sn and the Saozhoudi Sn–Pb–Zn ore blocks and has long been regarded as a volcanic–subvolcanic system related to Sn polymetallic mineralization. Here, we present fluid inclusion microthermometric data from different ore stages and H–O–S isotope data of hydrothermal minerals to constrain the genesis of the Xiling deposit. Fluid inclusions from stage I have Th values from ~ 340 to 420 °C and salinities from ~ 15 to 17 wt% NaCl equivalent, while homogenization temperatures of fluid inclusions from stages II to V range from ~ 150 to 320 °C, and salinities range between ~ 1 and 6 wt% equivalent. The oxygen and hydrogen isotopic composition of quartz and cassiterite (δDfluid − 65‰; δ¹⁸Ofluid 3.6 to 6.3‰) suggest that the ore-forming fluids from stage I have a distinct magmatic signature, whereas those from stage II through stage IV (δDfluid from − 80 to − 49‰; δ¹⁸Ofluid from − 3.7 to 2.5‰) show characteristics of mixing between meteoric and magmatic fluids. Moreover, δ³⁴S values for sulfides from the Fengdishan ore block have a narrow range of 0.6 to 2.5‰ with a mean close to 0‰, consistent with a magmatic sulfur source. By contrast, δ³⁴S values for ore minerals from the Saozhoudi ore block range from 3.4 to 11.5‰, suggesting involvement of a sedimentary sulfur source. In addition, a previous geochronological study has shown that the volcanic–subvolcanic host rocks have an age of 160–170 Ma, while the Sn polymetallic mineralization has an age of about 145 Ma. Our data support a model of mixing of magmatic brine from a hidden granitic intrusion with meteoric water. The S isotope data and the observed temperature gradient of the fluid system suggest that the Sn mineralization is developed in the central part of the ore system, while the Sn–Pb–Zn and Pb–Zn mineralization occurs in the distal part. This finding might have important implications for exploration in the region.

Description: The Yanbei tin porphyry district with a total resource of about 250,000 t Sn is hosted in the Lower Cretaceous Mikengshan volcanic basin in the southeastern part of the Nanling Range. There are several tin deposits of greisen, stockwork and vein styles associated to the intrusive center of the 8 × 8 km large Mikengshan volcano-plutonic complex. LA-ICP-MS U-Pb dating of cassiterite yielded ages of 130.4 ± 4.0 Ma, 132.0 ± 1.5 Ma and 133.5 ± 1.7 Ma for the Yanbei deposit, 135.9 ± 1.5 Ma and 136.7 ± 1.4 Ma for the Taoxiba deposit, and 135.1 ± 4.0 Ma for the Kuangbei deposit. These data overlap within error and indicate that the three deposits belong to a same tin mineral system. Combined with the previous geochronological data, we propose a scenario for the porphyry tin mineral system in the Mikengshan basin. Dacitic to rhyolitic volcanic and subvolcanic rocks formed at ca. 140 Ma, and were shallowly intruded by highly evolved magmas which released magmatic-hydrothermal fluids to form veinlet-disseminated and greisen ore in the granitic rocks, and more distal stockwork and vein tin mineralization in the volcanic and subvolcanic envelope. The porphyry tin deposits in the Mikengshan volcanic basin are part of the 145–135 Ma tin metallogenic event in South China. The Early Cretaceous volcanic basins in the eastern part of the Nanling Range are promising targets for tin exploration.

Description: Following extremely heavy rainfall, a catastrophic landslide occurred in Mazhe Village, Enshi, China, on 21 July 2020. In this study, we use C-band Sentinel-1A Interferometric Synthetic Aperture Radar (InSAR) measurements and Sentinel-2 optical imagery to retrieve the ground deformation evolution before this landslide and the aftermath. Optical imagery reveals that part of the slope body collapsed completely, while the mean normalized difference vegetation of the stable part drops from 0.703 to 0.514. Time series InSAR results show that the central and western parts of the slope body area have already undergone slow moving over the past years, which exhibited apparent seasonal variations with maximum changes when the accumulated rainfall reached its peak during the revisit period. The correlation between slope deformation and rainfall in the rainy season has been increasing, reaching over − 0.8 in 2020. The total deformation rate of the rainy season has a significantly strong correlation with the total precipitation, and the correlation coefficients at the two monitoring points reach − 0.9995 and − 0.9446, respectively, and the average value of the slope reaches − 0.8523. Multiple heavy rainfall events with short intervals are more tend to cause greater deformation. In addition, through the time-series InSAR analysis of the whole Qingjiang River in Enshi, another three creeping landslides are detected, and all show a linear trend with remarkable fluctuations in summer.