AsianScientist (Sep. 23, 2014) – Scientists have determined that the Hitachi deposit dates back to the Cambrian era, and not the Early Carboniferous as previously thought. These results have been published in Economic Geology.
The Hitachi volcanogenic massive sulfide (VMS) deposit in the Ibaraki prefecture is one of the largest copper/zinc sulfide deposits in Japan. So far, efforts to determine when it was first formed have been hampered by geologic changes that have disturbed the radiometric isotope systems traditionally used for dating.
In the present study, Professor Yasuhiro Kato from The University of Tokyo, and Drs. Tatsuo Nozaki and Katsuhiko Suzuki from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) have successfully determined the primary formation age of the Hitachi deposit using the Rhenium-Osmium (Re-Os) isotope dating method.
Re has two isotopes, 185Re and 187Re, one of which (187Re) radioactive, producing 187Os through β-decay with a half-life of 41.6 billion years. From the ratio of 187Re/188Os and 187Os/188Os obtained by isotope analysis, the age of the sample can be determined.
Using this method, the authors were able to show that the Hitachi deposit dates to the Cambrian era some 485.4 to 541 million years ago, rather than the Early Carboniferous period (323.2 to 358.9 million years ago) as estimated previously. This dating makes the Hitachi deposit the oldest dated ore deposit in Japan to date.
These results support previous studies that indicated the existence of a large age gap (hiatus) in the geologic record of 150 million years and the wide distribution of Cambrian beds in the Hitachi area. It is expected to provide important age constraints for use in reconstructing the initial tectonic history of the Japanese Islands.
The article can be found at: Nozaki et al. (2014) Re-Os Geochronology of the Hitachi Volcanogenic Massive Sulfide Deposit: The Oldest Ore Deposit in Japan.
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Source: Japan Agency for Marine-Earth Science and Technology.
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