Abstract

The Hitachi volcanogenic massive sulfide (VMS) deposit in the Hitachi belt is one of the largest Cu-Zn sulfide deposits in Japan. However, its primary formation age has not been precisely determined because regional and contact metamorphism have disturbed some radiometric isotope systems, e.g., K-Ar ages on wall rocks. Further metamorphism has greatly reduced the fossil record, which has limited the use of biostratigraphy. To establish the ore deposit age, we applied Re-Os isotope dating to samples from two representative sulfide deposits of the greater Hitachi VMS deposit: the Fudotaki and Fujimi deposits. Re-Os isotope data from the Fudotaki deposit yields a model 3 age of 533 ± 13 Ma, with an initial 187Os/188Os ratio of 0.35 ± 0.20. In contrast, Re-Os isotope data from the Fujimi sulfide ore did not exhibit a linear correlation, suggesting that the Re-Os isotope system there has been disturbed by regional metamorphism of higher grade than the Fudotaki deposit as well as contact metamorphism during intrusion of Cretaceous granitic rocks. Although the Fudotaki sulfide ores have been metamorphosed to lower epidote-amphibolite facies, the well-defined Re-Os isochron allowed us to interpret the timing of primary sulfide deposition on the paleoseafloor. Some orebodies of the greater Hitachi VMS deposit have been estimated to be Early Carboniferous based on the scarce fossil records and considered to be distributed in the Daioin Formation, but our results revealed that the Fudotaki deposit were formed during the Cambrian and occurs in the Upper Akazawa Formation, which strongly supports the idea that the Cambrian beds are widely distributed in the Hitachi belt and that a significant unconformity exists between the Cambrian and Early Carboniferous above the Fudotaki deposit. The Cambrian Re-Os date classifies the Hitachi VMS deposit as the oldest dated ore deposit in Japan, and in turn provides an insight to understand the geologic framework and genesis of the proto-Japanese islands and the East Asian region.

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