Timing of formation and cause of coloration of brown nephrite from the Tiantai Deposit, South Altyn Tagh, northwestern China

Abstract

The world’s largest and economically valuable nephrite deposits are distributed along the Western Kunlun Orogen in northwestern China. During the past 20 years, extensive white and brown nephrite deposits have been discovered in both China and Russia, and several of these deposits have been found in South Altyn Tagh in the eastern part of the Western Kunlun Orogen. However, the timing of formation, genesis, petrographic characteristics, mineralogical composition, and reasons for the white–brown color of the nephrite remain unclear. We analyzed 50 samples of white, brown, and white–brown nephrite from the Tiantai nephrite deposit in Qiemo County with the aim of understanding the processes of formation, age, and diversity of these nephrites. Three main types of nephrite can be distinguished: white, brown, and dark brown. X-ray diffraction and petrographic studies indicate that the brown and dark-brown nephrite samples have fewer mineral inclusions compared with the white nephrites. The nephrite formed during early retrogression through the sequence of diopside skarn → coarse-grained tremolite skarn and coarse-grained tremolite → fine-grained tremolite. The higher bulk Fe content of the white nephrite (0.76 wt% FeO) relative to the brown nephrite (0.39 wt%), and the uniform, very low Fe2O3 contents of both of these color varieties (<0.15 wt%), suggest that the brown coloration is unrelated to Fe content and oxidation state. The white nephrite is characterized by a wide range of SiO2 contents (49.6–56.7 wt%), in contrast to the brown nephrites, which are characterized by nearly constant SiO2 contents (~58 wt%), close to the ideal content for tremolite. On average, the white nephrite is enriched in Al2O3 (3.22 wt%) and water (3.96 wt% loss on ignition) and depleted in CaO (12.35 wt%) relative to the brown nephrite (0.80 wt% Al2O3, 2.23 wt% loss on ignition, 12.95 wt% CaO). These characteristics, combined with greater transparency and higher Al and water contents in the chlorite formula relative to that of tremolite, as well as a near-absence of Ca in the chlorite formula, can explain the lower transparency and associated more intense coloration (i.e., yellow to brown) of the brown nephrite in terms of the lower modal abundance of chlorite. The spatial distribution of selected elements agrees with this hypothesis, whereby the brown-colored zones are dominated by Ca- and Sr-rich (substituting Ca) domains reflecting the highest modal volume of the tremolite. In contrast, the white zones are characterized by a substantially higher volumetric ratio of Al-rich domains, reflecting the higher chlorite content. Thus, the different coloration is not caused by specific elements, but by modal proportions of tremolite and chlorite. The slightly higher average Cr (10.1 ppm) and Ni (6.4 ppm) contents of brown nephrite, relative to white nephrite (8.7 ppm Cr and 5.5 ppm Ni), might also contribute to the color intensity and brown coloration. Ore-forming fluids involved in the formation of the studied nephrite have isotopic compositions of δ18O = 1.5‰ to 9.4‰ (330–430 °C) and δD = −87‰ to −50‰ (330–450 °C), suggesting that the formation of the Mg-skarn deposit was related to metasomatism of dolomite by fluids derived from local granite/granodiorite intrusions. Zircons from three brown–white samples yield concordant SHRIMP ages of 438 ± 14 Ma (2σ, MSWD=10.4), 916 ± 10 (2σ, MSWD = 1.5), 438 ± 9 Ma (2σ, MSWD = 5.3), and 431.1 ± 2.5 Ma (2σ, MSWD = 2.4). The younger ages (ca. 430 Ma) are close to those for placer nephrite from the Yurungkash and Karakash rivers, as well as those for Yecheng, Alamas, and Buya granodiorites, and probably represent the primary formation ages of nephrite within the Hetian Nephrite Belt. Overall, these ages indicate that the primary deposits formed during pre- or post-orogenic stages in the Western Kunlun Orogen. 40Ar/39Ar dating of hydrothermal muscovite and phlogopite intergrown with tremolite in the nephrite yielded ages of 418.8 ± 3.4 Ma (2σ, MSWD = 0.65), 379.5 ± 3.0 Ma (2σ, MSWD = 0.81), and 354.5 ± 3.6 Ma (2σ, MSWD = 0.71), which suggest multiple metasomatic activity. The similar formation ages of nephrites from Altyn Tagh (433 Ma) and from the previously studied areas of West Kunlun (441–378 Ma) and East Kunlun (416 Ma) indicate that these nephrites formed during the closure of the Proto-Tethys and in the accompanying post-collisional extensional environment.