Paleomagnetism of the HYC Zn-Pb SEDEX Deposit, Australia: Evidence of An Epigenetic Origin

Abstract

The HYC Zn-Pb-Ag deposit of the McArthur River Mines Ltd. is in the late Paleoproterozoic HYC Pyritic Shale Member of the 1639 ± 2 Ma (2σ) Barney Creek Formation in the McArthur Basin of the Northern Territory, Australia. This deposit is often cited as the archtypical syngenetic-syndiagenetic McArthur-type sedimentary exhalative (SEDEX) deposit. Paleomagnetic analysis of 273 specimens from 27 sites in seven of eight ore zones (20 sites) and their host rocks (7 sites) using mostly alternating field step demagnetization isolated a well-defined stable characteristic remanent magnetization (ChRM) in all sites but one tuff site. Thermal step demagnetization, saturation remanence, and artificial mill-concentrate specimen tests show that about two-thirds of the ChRM is carried by single- and pseudosingle-domain pyrrhotite and about one-third by single-domain magnetite, which are interpreted to occur as submicroscopic inclusions in ore-stage minerals including sphalerite and galena. Paleomagnetic conglomerate and fold tests are overwhelmingly negative, showing that the ChRM postdates lithification, burial, and fold deformation. The ChRM of the host rocks and ore gives a paleopole at 77.0° S, 167.0° E ( n = 20, α95 = 3.6°). This pole is coincident with that of the 1636 ± 4 Ma (2σ) Linott Formation, providing a direct date for the ChRM. Relative dating using the apparent polar wander path for the southern McArthur Basin dates the ChRM at 1637 ± 1 Ma (1σ). Thus both dating methods show that the ChRM is ~2–3 m.y. younger than the primary age of the host rocks. Using available vitrinite reflectance values, time-temperature calculations indicate that the ore lenses have been heated to 310° ± 30°C for ~105 yr, whereas nearby and remote host rocks have been subjected to much lower temperatures. The results of this study suggest that primary sea-floor sediments, perhaps with syngenetic pyrite, were buried to a depth of ~800 m over 2–3 m.y., with attendant lithification and synclinal folding during rift fault tectonics. The preferred model discussed in this paper is that ascending 310° ± 30°C hydrothermal fluids in the Emu fault zone subsequently penetrated the pyritic zones or other metal-bearing sediments by dissolution and/or replacement, precipitating sphalerite, galena, secondary pyrite, and other ore-stage minerals with pyrrhotite and magnetite inclusions to form an epigenetic ore deposit.