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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. [ABSTRACT FROM AUTHOR]

Abstract: Paleomagnetic results are reported from the Century Zn–Pb–Ag SEDEX deposits in northwestern Queensland, Australia. The stratiform mineralization occurs in fine parallel lamellae in ∼1595Ma siltstones and black shales of the Lawn Hill Formation in the Proterozoic McNamara Group. Galena has given a Pb/Pb model age of ∼1570Ma. Paleomagnetic analysis of 333 specimens from ore zones (15 sites), hanging wall sandstones or siltstone (4 sites), and footwall siltstones (5 sites) using mostly thermal and then alternating field step demagnetization, isolates a stable characteristic remanent magnetization (ChRM) for ore sites only. Step demagnetization and rock magnetic analyses of ore, Zn and Pb concentrates and tailings show that the main remanence carriers are single- or pseudosingle-domain inclusions of titanomagnetite in sphalerite and gangue, and pyrrhotite in galena with modern goethite and/or hematite from the weathering of siderite. A paleomagnetic fold test using the ChRM directions of the ore sites is positive, showing that the ore ChRM predates the D2 deformation of the ∼1595 to 1500Ma Isan orogeny. The orogeny folded the main-stage mineralization, indicating that the ore retains a primary magnetization. The optimum 80% tilt-corrected unit mean ChRM direction for the ore yields a Mesoproterozoic paleopole at 1558±4Ma on the northern Australian apparent polar wander path. Thus this result both constrains the timing of mineralization and provides an upper age limit for D2 deformation in the orogeny. [Copyright &y& Elsevier]