Origin of paratacamite in the Kuqa Basin, Xinjiang, China: Defined by Cu isotope values

Abstract

Isotopes of Cu in some copper minerals have been used to trace the origin of metallogenic materials. However, little is known about paratacamite, a sedimentary-type Cu mineral whose formation is closely related to brines and evaporites. The Kuqa Basin is filled with a huge thickness of evaporite deposits and is located in the northern margin of the Tarim Basin, Xinjiang, northwestern China. Here, we report a study of paratacamite from the Kuqa Basin and infer its genesis from the mineral assemblage and Cu isotopes of paratacamite. Paratacamite is widely distributed in basin deposits coexisting with saline minerals in fractures and joint planes in fault/fracture zones. Cu isotopic compositions (δ65Cu) of paratacamite range from −0.49‰ to 1.02‰, with a mean of 0.41‰. These values suggest that Cu in the paratacamite originated from Cu-bearing brine that was derived from the leaching of metallic ions from surrounding rocks and migrated along and upward through faults, fractures, and joint planes and ascended to the surface while cooling. On the basis of the paragenetic association of paratacamite with saline minerals in the studied samples, the occurrence of paratacamite is inferred to have been controlled by a crystallisation sequence of saline minerals with accompanying evaporation and concentration of brines from which gypsum, secondary glauberite, paratacamite, and halite were crystallised in turn. Differences in Cu isotopic compositions among the samples from the four studied mineralisation occurrences of Kezier, Yanshuigou, Daxiagu, and Dishui may be due to variation in the processes of Cu enrichment involving brines, such as Cu extraction from different rock layers, the migration and evaporation of Cu-bearing brine, and mineral deposition.