Oldest syenitic intrusions of the Yilgarn Craton identified at Karari gold deposit, Carosue Dam camp, Western Australia?

Abstract

Apatite was separated from four samples of syenite porphyry, taken from the Karari gold deposit, in the Kurnalpi Terrane of the Archean Kalgoorlie-Kurnalpi Rift, Eastern Goldfields Superterrane (EGST). The alkalic composition of the syenitic magmas inhibited zircon crystallisation, so apatite provided the best mineral for geochronological investigations. LA-ICP-MS analysis of U, Th and Pb isotopes in the apatite gave a relatively wide range of lower intercept ages, with large errors, ranging from 1 to 3%, using OD-306 apatite as the primary standard. Cathodoluminescent (CL)-darker cores that comprise the major volume of apatite grains are relatively homogeneous in two samples, with one having clear oscillatory zoning. These samples yielded intercept ages of 2701 ± 34 Ma and 2699 ± 25 Ma, respectively. These ages are interpreted to approximate the magmatic crystallisation age of the apatite. Younger intercept ages were generated by apatite from two other samples, which display more complex and heterogeneous patterns of CL brightness. The apatite ages from these two samples are interpreted to have been produced by integrated analysis of apatite that has been heterogeneously modified by younger events. However, the magnitude of the temporal gap between magma emplacement and closure of the U–Pb system in apatite from these two samples remains unknown. Our best estimate of the age of the magmatic apatite from at least two of the syenitic intrusions at Karari is ca 2.70 Ga, which identifies these as the oldest intrusions of the Syenitic Group of magmas yet identified in the EGST. However, if ages are corrected to offset observed in the 401 apatite secondary standard, the two oldest syenitic intrusions are dated at ca 2660 Ma.