New 40Ar/39Ar ages from the Kalatag district in the Eastern Tianshan, NW China: Constraints on the timing of Cu mineralization and stratigraphy

Abstract

The Kalatag region in the Eastern Tianshan of NW China contains a number of economically important Cu deposits. Among these, the South Meiling VMS deposit is hosted in the Late Ordovician to Early Silurian dacitic tuff belonging to the Daliugou Formation. The deposit is composed of two domains: a concordant massive sulfide ore domain and a discordant vein-type ore domain located in the footwall phyllic alteration zone. Here we report new 40Ar/39Ar ages on sericite from the phyllic alteration zone to better constrain the timing and genesis of mineralization. The sericite displays markedly flat 40Ar/39Ar age spectra, yielding a 40Ar/39Ar plateau age of 438.8±3.0Ma, and an isochron age of 443.0±11.0Ma. The 40Ar/39Ar age is in good agreement with the ReOs age reported earlier, placing the timing of CuZn mineralization at 434.2±3.9Ma. The age is also consistent with the field relationships that concordant massive sulfide orebody formed later than discordant vein-type orebody. Another deposit at Hongshi is hosted in basaltic andesites of the Daliugou Formation. The orebodies cut the phyllic alteration zone indicating that the Cu mineralization formed later than the phyllic alteration. Hydrothermal sericite from the phyllic alteration zone from this deposit yielded flat 40Ar/39Ar age spectra, with 40Ar/39Ar plateau ages of 441.5±3.2Ma and 446.4±4.6Ma, and isochron ages of 445.8±5.6Ma and 448.8±7.3Ma, respectively. This age is obviously older than the ReOs age, which is also in conformity with the field relations that quartz-sulfide veins cut the sericitization zone.Our 40Ar/39Ar ages offer precise constraints on Early Paleozoic Cu mineralization in the Eastern Tianshan Orogen. These results indicate that the Lower Unit basaltic andesites of the Daliugou Formation formed earlier than 446Ma. We thus consider that the Daliugou Formation formed during Middle Ordovician to Early Silurian. We also propose several metallogenic events in the Kalatag area including: (1) an Early VMS CuZn and hydrothermal vein Cu mineralization at 440–430Ma; (2) porphyry Cu and skarn Fe mineralization at 390–380Ma; and (3) a Late CuAu mineralization at ca. 300Ma. These three metallogenic events were coeval with the distinct magmatic pulses in the Kalatag area.