A ductile to brittle shear zone–hosted Cu mineralization: Geological, geochronological, geochemical and fluid inclusion studies of the Lingyun Cu deposit, southern Tianshan, NW China

Abstract

The Lingyun Cu deposit is located at the northern margin of the South Tianshan Accretionary Complex, NW China. In the Lingyun mine area, the rocks are cut by a NW–trending Yanxingshan ductile to brittle shear zone. All orebodies are hosted in crystal tuff. The sulfide mineralization in the Lingyun Cu deposit can be divided into early (E) and late (L) paragenetic stages. The E-stage is characteristic of fault-controlled sulfide assemblages (chalcopyrite + bornite+chalcocite)–quartz veins in the foliation fabric of the crystal tuff with albite + sericite ± epidote ± biotite assemblages. In the L-stage, the assemblages of ankerite + chalcopyrite + bornite + chalcocite are present in veinlets that cut the E-stage assemblages. The sulfide assemblages in the Lingyun deposit are characterized by striations in the metamorphosed crystal tuff. Ribbon-like silicate minerals in the crystal tuff share common deformation features with the sulfide assemblages.The host rock of the Lingyun Cu deposit yields a concordia age of ca. 405 Ma, indicating that the Aerbishibulake Formation formed during southward subduction of the South Tianshan Ocean beneath the Tarim Block. Moreover, crystal tuff samples show well developed negative Sr anomaly and positive Rb, Th, U, Nd and P anomalies, which are common in arc-related volcanic rocks. The ca. 297 Ma Re–Os isochron age yielded by sulfides is much younger than the host rock, which implies that the Lingyun Cu deposit is a typical epigenetic deposit. Both similar REE fractionated patterns between sulfide ores and crystal tuff and the high initial 187Os/188Os ratio (18.54 ± 0.67) indicate significant crustal materials input. The 40Ar/39Ar plateau age of ca. 292 Ma from the ribbon-like sericite in crystal tuff is coeval with the mineralization occurrence in the Lingyun Cu deposit. By contrast, diorite dikes that crosscut the crystal tuff formed at ca. 221 Ma, which is much younger than the sulfide mineralization. Thus, the mineralization in the Lingyun deposit is likely to have been generated by shear zone activity rather than the intrusion of diorite dikes.Three types of fluid inclusions have been identified in the Lingyun Cu deposit: Aqueous (W-type), mixed aqueous–carbonic (M-type) and pure carbonic (C-type) inclusions. These three types of fluid inclusions are all found in the E-stage quartz whereas only the W-type fluid inclusions have been observed in L-stage ankerite. Fluid inclusions from the E-stage commonly contain CO2 and H2O, locally with N2 with low salinity, which are common in orogenic deposits. By contrast, L-stage fluid inclusions show low CO2 concentrations and salinity, indicating a significant inflow of meteoric water. Given the regional geology, ore geology, coeval ore-forming and deformation and fluid inclusion features, the Lingyun Cu deposit may be associated with the orogenic class of mineral systems.