Chemical Zone of Nephrite in Alamas, Xinjiang, China

Abstract

AbstractThe Hetian deposit, located south of the Tarim Basin in Xinjiang, China, is one of the world's largest dolomite‐related nephrite deposits. In the Alamas orebody of the deposit, nephrite occurs as veins or lenses along faults or fissures of the adjacent dolomitic marble. Chemical analyses using electronic microscope probe analysis and X‐ray fluorescence spectroscope were carried out on nephrite and dolomitic marble samples collected from a cross section in Alamas to investigate zonal structure of the orebody. The nephrite in Alamas is predominately composed of tremolite with minor calcite, titanite and phlogopite, and that dolomitic marble is relatively pure with a FeO content less than 0.20 wt. %. Contents of color‐inducing elements, such as Fe, Mn, and Cr, increase gradually as color changes from white through white‐green and then to green, resulting in the formation of color‐distinctive zones. Tremolite grain size increases as color changes from white through white‐green to green. The trend may be consistent with temperature changes from dolomitic marble to granodiorite, which, in turn, suggests that both change in color from white to green and variations of grain size with increasing temperature resulting in formation of the nephrite zonal structure. Both nephrite zonal structure and minor minerals, such as calcite, titanite and phlogopite found in the contact, indicate that this dolomite‐related nephrite orebody is of a metasomatic origin under assumed pressure of 100–200 MPa and temperature <550°C.