Paleomagnetism of the Mt Isa Zn–Pb–Cu–Ag and George Fisher Zn–Pb–Ag deposits, Australia

Abstract

Paleomagnetic results are reported for the Mt Isa Zn–Pb–Cu–Ag and George Fisher Zn–Pb–Ag black-shale-hosted deposits in northwestern Queensland, Australia. Both deposits are hosted in greenschist facies carbonaceous and dolomitic siltstones, mudstones and shales of the ca 1655 Ma Urquhart Shale Formation of the Mt Isa Group. Lead model ages for both deposits are ca 1655 Ma, and an Ar–Ar biotite age for Cu mineralisation at Mt Isa is ca 1523 Ma. Paleomagnetic and rock magnetic analyses of 333 specimens (28 sites) isolate, excluding modern hematite, a stable characteristic remanent magnetisation (ChRM) carried mainly by single- or pseudosingle-domain pyrrhotite for Zn–Pb–Ag ore specimens and pyrrhotite and/or titanomagnetite for Cu ore specimens. A paleomagnetic fold test shows that the ChRM postdates D3 deformation of the ca 1595 to 1500 Ma Isan orogeny, yielding a Mesoproterozoic paleopole at ca 1505 Ma on the northern Australian apparent polar wander path, thereby providing a minimum age for the ores and an age for greenschist metamorphism during the last major metamorphic peak of the Isan orogeny in the Mt Isa inlier.