Fission-track ages were determined for the Chagai Intrusives of the Chagai District, northern Baluchistan, Pakistan. Zircon and apatite microcrystals were dated by the external detector method and yielded Oligocene and Miocene ages. On the basis of stratigraphic inference, these rocks previously had been believed to be Cretaceous to Paleocene. Although the younger fission-track ages may represent uplift ages associated with this terrane, field evidence strongly supports the interpretation that they represent ages of emplacement.
Two phases of intrusive activity have taken place in the Chagai Hills: one about 35 m.y. ago and another about 20 m.y. ago. The composition of rocks of both phases ranges from granite to tonalite, and areas of hydrothermal mineralization are associated with both phases. Fission-track ages of the rocks west of the Chagai Hills, including the Saindak porphyry copper deposit, are about 12 m.y.