The Nakatatsu Mine, central Japan, is one of the largest skarn-type Pb-Zn ores in the Japanese Islands. Quartz porphyritic rocks, which is believed to relate the ore depositions, intrude into this mining district as a stock or dykes. In order to clarify the relationship between the quartz porphyry and the ore depositions, some Rb-Sr whole rock measurements have been done.
A concordant whole rock isochron has been obtained and yields an age of 64.9±4.7 m. y. and an initial ratio of Sr
87/Sr
86=0.7083±0.0003. This age agrees with a reproted K-Ar mineral age from ore deposit. It is, therefore, concluded that the emplacement of the quartz porphyry related closely to the ore depositions of the Nakatatsu Mine.
The concentrations of Rb and Sr and the initial ratio of Sr obtained in this study are identical with those of the Cretaceous igneous rocks in the inner zone of southwest Japan.
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