Iron mineralization associated with a major strike–slip shear zone: Radiometric and oxygen isotope evidence from the Mengku deposit, NW China

Abstract

Some skarn-associated gold deposits display close spatial relations with shear zones in ancient orogens; however to date no skarn-associated iron mineralization has been genetically linked to a shear zone. To address this problem, we conducted a systematic study of the Mengku iron deposit in the Chinese Altai, Northwest China. All orebodies of the deposit are enclosed by skarn and stratabound by volcaniclastic rocks. LA–ICP-MS dating of zircon from a meta-rhyolitic volcaniclastic (MK19) yielded a weighted 206Pb/ 238U age of 404 ± 5 Ma. We interpret this date as the formation time of volcanic rocks in the Mengku area, which is consistent with the age of strata hosting nearby VMS deposits (400–413 Ma). Zircon from the mineralized skarn (MK13) are considered to be hydrothermal in origin and yield a weighted 206Pb/ 238U age of 250 ± 2 Ma, which we interpret to represent the time of skarn formation based on the petrographic relationship between hydrothermal zircon and skarn garnet. This age is much younger than that determined from previous studies, younger than the regional igneous and metamorphism activities (early Permian), but synchronous with the movement of the nearby regional Erqis shear zone and Abagong Fault (late Permian to Triassic). The oxygen isotope values of the hydrothermal zircon are 2.3‰ to 3.0‰, consistent with the δ 18O of garnet (1.2‰ to 3.2‰) in the skarn, but markedly different from those of zircon from igneous rocks in the mine area (MK1, 2, 3, 19; 6.3‰ to 8.0‰). The fluid source has a strong meteoric water signature as indicated by the oxygen isotope values of garnet and hydrothermal zircon, which are very different from those of the granite and volcanic rocks in the mine area. On the basis of field, chronological and geochemical evidence, we suggest that the Mengku is not a volcanogenic massive sulfide deposit, but is related to the Erqis strike–slip shear zone and Abagong Fault, which acted as fluid conduits. The upwelling of hot mantle in the Permian to Triassic supplied the heat, and thereby caused material to dehydrate and mixed with meteoric water that give rise to low-oxygen isotope fluids similar to the common genetic model for orogenic lode gold deposits.