U-Pb and Re-Os geochronology of the Bainaimiao Cu-Mo-Au deposit, on the northern margin of the North China Craton, Central Asia Orogenic Belt: Implications for ore genesis and geodynamic setting

摘要

The Bainaimiao Cu-Mo-Au deposit, located at the northern margin of the North China Craton (NCC) in central Inner Mongolia, is a large Cu-dominant deposit. Orebodies are hosted in strongly deformed and meta morphosed granodiorite porphyry intrusions or in EW-trending shear zones in greenschist and sericite felsic schist of the Bainaimiao Group. Mineralization can be divided into early, middle and late stages, characterized by quartz-pyrite, polymetallic quartz-sulfides and carbonate-quartz, respectively. Hydrothermal activity led to pervasive silicification, K-feldspar, biotite and chlorite-epidote-carbonate alteration. Eight samples from Cu-Mo-bearing quartz veins and massive sulfide ores in greenschist were collected from the VIII orebody in the North ore belt and molybdenite separates were selected for Re-Os analyses. The con centrations of Re and ~(187)Os range from 216.6 to 1051 ppm and 2302 to 4913 ppb, respectively. All samples give Re-Os model ages between 441.2 and 445.9 Ma and yielded an isochron age of 445.0 ±3.4 Ma (MSWD = 0.26), with an initial ~(187)Os of -4±17 ppb. A granodiorite porphyry sample for SHRIMP U-Pb dating was collected from the underground of the XIII orebody of the North ore belt. Spots from 14 zircon grains have ~(206)Pb/~(238)U values between 426.2 and 462.9 Ma and form a tight cluster on Concordia yielding a weighted mean ~(206)Pb/~(238)U age of 445 ± 6 Ma. This is considered to be the best estimate of crystallization age of the granodiorite porphyry. The Re-Os isochron age of the molybdenite is consistent with the SHRIMP U-Pb age of the granodiorite porphyry, which indicates that the deposit is a porphyry system of Paleozoic age.According to geodynamic setting and geological history of the northern margin of the NCC, the deposit was probably related to magmatic activity of the Middle Paleozoic Ondor Sum subduction-accretion complex-Bainaimiao arc. During the Devonian, the Bainaimiao arc was accreted to the northern margin of the NCC. Collision between the Bainaimiao arc and NCC led to regional metamorphism of the ore-hosting terrane and ore metals were remobilized in this process. Gold mineralization was probably overprinted onto the earlier Cu-Mo mineralization.