Potassic alteration and veining and the age of copper emplacement at Mount Isa, Australia

Preliminary Re-187/Os-188 dating of whole rocks and sulphide separates from the Mount Isa copper orebody has generated an isochron age of 1367 +/- 80 Ma (MSWD = 49; n = 6). This age is approximately 150 myr younger than published biotite 40Ar/39Ar ages previously assumed to date the copper-forming event at ca. 1523 Ma. These older ages are from rocks in which biotite is likely to be metamorphic rather than hydrothermal in origin. Unambiguous potassic alteration related to copper formation is characterised by biotite replacement of metabasalt (brownstones) and potassium feldspar replacement of meta-tuffs. Previous 40Ar/39Ar dating of biotite from brownstone yields 1352 Ma to 1385 Ma ages, while Rb-87/Sr-86 dating of potassium feldspar altered tuffs gives 1323 Ma. Muscovite from the Buck Quartz Fault, considered a conduit for copper mineralising fluids, yields an 40Ar/39Ar age of 1324 Ma. We suggest that these ages more accurately reflect the age of copper emplacement, whereas the older 40Ar/39Ar ages more likely relate to cooling from peak metamorphism.