MOLYBDENITE RE-OS AND ALBITE 40AR/39AR DATING OF CU-AU-MO AND MAGNETITE PORPHYRY SYSTEMS IN THE YANGTZE RIVER VALLEY AND METALLOGENIC IMPLICATIONS

Abstract

The area of the Middle-Lower Yangtze River valley, Eastern China, extending from Wuhan (Hubei province) to western Zhenjiang (Jiangsu province), hosts an important belt of Cu-Au-Mo and Fe deposits. There are two styles of mineralization, i.e., skarn/porphyry/stratabound Cu-Au-Mo-(Fe) deposits and magnetite porphyry deposits in several NNE-trending Cretaceous fault-bound volcanic basins. The origin of both deposit systems is much debated. We dated 11 molybdenite samples from five skarn/porphyry Cu-Au-Mo deposits and 5 molybdenite samples from the Datuanshan stratabound Cu-Au-Mo deposit by ICP-MS Re-Os isotope analysis. Nine samples from the same set were additionally analyzed by NTIMS on Re-Os. Results from the two methods are almost identical. The Re-Os model ages of 16 molybdenite samples range from 134.7 ± 2.3 to 143.7 ± 1.6 Ma (2σ). The model ages of the five samples from the Datuanshan stratabound deposit vary from 138.0 ± 3.2 to 140.8 ± 2.0 Ma, with a mean of 139.3 ± 2.6 Ma; their isochron age is 139.1 ± 2.7 Ma with an initial Os ratio of 0.7 ± 8.1 (MSWD = 0.29). These data indicate that the porphyry/skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system. Albite 40Ar/39Ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 Ma, i.e., 10-20 Ma later. Both mineralization styles characterize transitional geodynamic regimes, i.e., the period around 140 Ma when the main NS-trending compressional regime changed to an EW-trending lithospheric extensional regime, and the period of 125-115 Ma of dramatic EW-trending lithospheric extension. © 2005 Elsevier B.V. All rights reserved.