Mineralogy and geochemistry of nephrite from Drosh area, district Chitral, northern Pakistan

This study evaluates the quality of nephrite by investigating its gemological, mineralogical and geochemical properties. Petrographic analysis revealed that the nephrite is primarily composed of tremolite, with minor amounts of actinolite, while the host rock, serpentinite, is predominantly composed of chrysotile and antigorite, with traces of tremolite, diopside, zoisite, talc, and plagioclase. Geochemically, the major oxides such as SiO₂, MgO, CaO and Fe₂O₃ are ranging from 54.69 to 59.1 wt. %, 20.18 to 21.89 wt. %, 10.24 to 13.98 wt. % and 2.09 to 3.01 wt. %, respectively. The trace elements, including cobalt (Co), chromium (Cr) and nickel (Ni) are ranging from 6 to 31 ppm, 401 to 1005 ppm and 789 to 987 ppm, respectively. The Fe²⁺/(Mg²⁺ + Fe²⁺) ratios for the bulk rock range from 0.10 to 0.14. Gemologically, the nephrite exhibits a color range from light-green to green, a refractive index from 1.61 to 1.62, specific gravity from 2.97 to 3.04, a hardness from 5.8 to 6.2, and Schmidt hammer values from 38.00 to 39.60 MPa. These properties are consistent with typical nephrite characteristics. The intensity of light to dark green color is controlled by variations in the contents of Fe, Cr and Ni. Additionally, the Fe²⁺/(Mg²⁺ + Fe²⁺) ratio, along with high contents of Ni and Cr, and field observations suggest that the nephrite is of serpentinite-related origin, having undergone hydrothermal metasomatic transformation through elemental exchange between the silica-rich intrusions and host ultramafic rocks. The findings of this study contribute valuable insights into the geological processes of nephrite formation, which may aid in the exploration and research of nephrite deposits in other regions.