Paleomagnetism of the world-class Century Zn–Pb–Ag deposits, Australia

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 ∼ 1595 Ma siltstones and black shales of the Lawn Hill Formation in the Proterozoic McNamara Group. Galena has given a Pb/Pb model age of ∼ 1570 Ma. 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 D 2 deformation of the ∼ 1595 to 1500 Ma 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 ± 4 Ma on the northern Australian apparent polar wander path. Thus this result both constrains the timing of mineralization and provides an upper age limit for D 2 deformation in the orogeny.