SHRIMP U–Pb zircon ages, mineral compositions and geochemistry of placer nephrite in the Yurungkash and Karakash River deposits, West Kunlun, Xinjiang, northwest China: Implication for a Magnesium Skarn

Abstract

The most highly valued placer nephrite deposits in the world occur in the Yurungkash and Karakash Rivers (also known as the White and Black Jade Rivers) in Hetian, Xinjiang, Northwest China. However, the ages of these deposits and the geological environment in which they formed are not known, and their genesis, mineral compositions, and geochemical and petrographic characteristics are poorly constrained. More than 50 samples of white, green, brown, and black nephrites were examined from the placers with the aim of understanding the timing and processes of formation of the nephrite and its archeological significance. The samples collected comprise fine-grained tremolite with highly variable Fe2O3 (total) contents (green nephrite=0.77–3.97wt.%; black nephrite=0.56–16.23wt.%) and FeO concentrations (green=0.67–3.18wt.%; black=0.48–9.55wt.%). XRD and petrographic studies indicate that placer nephrite is more mineralogically complex than primary nephrite and contains tremolite (most samples >80.1wt.%), diopside (≤21.3wt.%), chlorite (≤10.5wt.%), zircon, titanite, apatite, spinel, garnet, epidote, barite, actinolite, sphalerite, pyrrhotite, graphite, calcite, and iron-hydroxide. The black nephrite contains significant amounts of graphite and Fe(OH)3 as mineral inclusions. The presence of strong negative Eu anomalies (0.04–0.26), flat REE patterns, and δ18O=1.3‰–8.4‰ (330°C) and δD=−17.7‰ to −87.1‰ (350–650°C) for the ore-forming fluids suggests that the deposit is a Mg-skarn formed by the metasomatism of dolomite by fluids derived from local granite/granodiorite intrusions with quite few samples showing geochemical characteristics of serpentinite-related nephrite.Zircons inclusions in the placer nephrite are oval in shape, <200μm long and <100μm wide. Their geochemical characteristics are typical of magmatic zircons, with high Th/U ratios (>0.1), positive Ce anomalies (Ce/Ce*) of ≥1.31. Zircons from both Alamas and two placer rivers have similar geochemical signatures, suggesting that the zircon grains in the placer nephrite were derived from the magmatic rocks that host the nephrite deposits. Zircon grains from two samples of black nephrite yield concordant SHRIMP U–Pb ages of 389±4Ma, 397.1±3.5Ma, and 440.7±4.4Ma. Zircon grains from one sample of green nephrite yield concordant SHRIMP U–Pb ages of 377.8±6.2Ma. These ages are roughly close to those of the emplacement ages of the Alamas and Buya granodiorites and probably represent the recrystallization of zircons or formation ages of the nephrite within the Hetian Nephrite Belt. Some igneous zircon grains yield ages of ~600Ma and were probably sourced from the wall rocks and entrained in the ~400Ma magma.Petrographic studies indicate that the Mg-skarns that host the primary nephrite have been thermally metamorphosed. The nephrite formed during early retrogression from diopside skarn→tremolite skarn (coarse-grained), epidote skarn→tremolite skarn and coarse-grained tremolite→fined-grained tremolite. The first step in the formation of nephrite was alteration of the Mg-dolomite to form an assemblage of diopside, epidote, and minor grossular. This was followed by metasomatism of the skarn assemblage by pneumatolitic/hydrothermal fluids derived from local granitoid intrusions, which, combined with later brittle deformation, formed the fine-grained tremolite (nephrite). Zircon inclusions in the primary and placer nephrite at Hetian have similar geochemical characteristics, implying that the nephrites are genetically related. However, the placer nephrite contains more complex phases or associated minerals than does the primary nephrite. Primary nephrite is generally regarded as the main source of the downstream placer nephrite deposits. The fine grain-size of the tremolite and small quantities of accessory minerals are the main characteristics of high-quality placer nephrite. A combination of transport by glacial melt-water and the relatively high altitude (the primary source region and the placer deposits differ in elevation by 3km) led to the formation of the two extensive placer nephrite deposits at Hetian.