Application of ⁸⁷Rb–⁸⁷Sr isotopic dating to single grain pyrites from a magnetite deposit

Abstract

The formation age of ore deposit is one of the most essential information in the genesis analysis of the deposits and provides clues to clarify the relationship between the deposit and the host rocks and to search for new ore deposits. Indirect dating of ore deposits using the ⁴⁰K-⁴⁰Ar and ⁸⁷Rb-⁸⁷Sr methods has been conducted for intrusive rocks and alteration minerals previously, but the inadequate sample selection of alteration minerals affects the reliability of the ages of ore deposits. In recent years, direct dating of ore minerals of the deposits has also been attempted. One example is the direct dating of pyrite, a sulfide mineral that is universally associated with various types of metal deposits, by applying the ⁸⁷Rb-⁸⁷Sr method, which is widely used to determine the age of igneous rocks and to estimate magma sources. Although the Rb and Sr contents in pyrite are extremely low, their Rb/Sr ratios show a wide range of values, and thus it is expected that highly accurate ⁸⁷Rb-⁸⁷Sr isochron ages can be obtained. In this study, with the aim of expanding the range of application of the ⁸⁷Rb-⁸⁷Sr method to pyrite, we attempted to apply the ⁸⁷Rb-⁸⁷Sr method to millimeter-sized pyrite single grains and discussed the results of quantitative analysis of major and trace elements and Sr isotopic analysis of pyrites.