Mineral association and graphite inclusions in nephrite jade from Liaoning, northeast China: Implications for metamorphic conditions and ore genesis

摘要

The Liaoning Province in the northeastern part of the North China Craton (NCC) hosts several tremolite jade (nephrite) deposits. Here we investigate the Sangpiyu tremolite jade deposit where the relationship between abundant graphite inclusions within the jade remains enigmatic. We employ petrography, electron probe microanalysis, X-ray-diffraction, and Raman spectroscopy to characterize the tremolite jade and its inclusion minerals. The Sangpiyu jade is predominately composed of tremolite with minor calcite, dolomite, serpentine, titanite, zoisite, allanite, chlorite, apatite, chromite and graphite. Raman spectroscopy of graphite inclusions shows that the D1/G intensity ratio ranges from 0.78 to 0.88 in deep green samples and from 0.05 to 0.23 in dark green samples. The ranges of D1/(D1 + G) integral area ratio for these types are from 0.0548 to 0.3037 and 0.5528 to 0.7355 respectively. The formation temperature of graphite inclusions in the dark green tremolite jade is computed as 549.8 °C, whereas that for the deep green sample is about 343.2 °C. Our results suggest that the jade formation occurred in a multi-stage process through the action of hydrothermal fluids and metamorphism possibly in a subduction-related setting at moderate to high temperatures.