A newly recognised 1860–1840 Ma tectono-magmatic domain in the North Australia Craton: Insights from the Tennant Region, East Tennant area, and the Murphy Inlier

New U-Pb monazite ages from amphibolite-facies metasedimentary rocks from the Tennant Region, Murphy Inlier and intervening East Tennant area, together with existing data, reveal the presence of an 1860–1840 Ma tectono-magmatic domain over 600 km long towards the centre of the North Australian Craton. In-situ ion probe U-Pb dating of biotite-hosted monazite in amphibolite-facies schist in the Tennant Region yielded an age of 1858 ± 7 Ma, which is attributed to north–south shortening (D1) at 1860–1855 Ma. Existing data indicate that D1 was associated with east–west trending, upright folds and mostly low-grade, regional metamorphism (M1) in the Tennant Region and the Murphy Inlier. The D1 event preceded voluminous and widespread felsic magmatism between 1855 and 1845 Ma. This included the emplacement of the Tennant Creek Supersuite, as well as the Yungkulungu Formation and equivalent stratigraphy, in the Tennant Region and in East Tennant, and the Nicholson Granite and Cliffdale Volcanics in the Murphy Inlier. Newly determined monazite ages from amphibolite-facies schist from the East Tennant area and the Murphy Inlier constrain a second episode of deformation and metamorphism (D2/M2) to ~ 1845 Ma, coincident with the cessation of widespread magmatism. D2 is characterised by regional southeast to northeast trending shear zones. Phase equilibria modelling reveals that peak pressure–temperature (P–T) conditions during M2 in the East Tennant area were 2.8–3.3 kbar and 655–680 ◦C, indicating an extremely high apparent geothermal gradient (>190 ◦C/kbar) that was likely influenced by the preceding magmatism. Existing data indicate that D2 also affected the Tennant Region, where it coincided with significant Cu-Au-Bi mineralisation, albeit at significantly lower P–T conditions (sub-greenschist facies) than in the East Tennant area. The development of the 1860–1840 Ma tectono-magmatic domain, extending from west of the Tennant Region to east of the Murphy Inlier, marks an intermediate step in the migration of tectonism in the North Australia Craton, from the Arnhem Province in the north at 1880–1860 Ma to the Aileron and Tanami provinces in the south by ca. 1830 Ma.A.D. Clark, L.J. Morrissey, M.P. Doublier, N. Kositcin, A. Schofield, R.G. Skirro

Microstructural evolution and trace element mobility in Witwatersrand pyrite

Microstructural analysis of pyrite from a single sample of Witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation. Internal deformation associated with dislocation creep is heterogeneously developed within grains, shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the Central Rand conglomerates. In contrast, brittle deformation, manifest by grain fragmentation that transects dislocation-related microstructures, is spatially related to grain contacts and is interpreted to represent post-depositional deformation of the Central Rand conglomerates. Analysis of the low-angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of {100} slip systems. However, some orientation boundaries have geometrical characteristics that are not consistent with simple {100} deformation.These boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized {120} slip system. These boundaries are associated with order of magnitude enrichments in As, Ni and Co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast-diffusion pathways. The results confirm the importance of grain-scale elemental remobilization within pyrite prior to their incorporation into the Witwatersrand gold-bearing conglomerates. Since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite, the results have implications for the application of minor element geochemistry to ore deposit formation, suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing

Relationship between detrital zircon age-spectra and the tectonic evolution of the Late Archaean Witwatersrand Basin, South Africa

SHRIMP U–Pb detrital zircon age-spectra for 14 samples from the West Rand and Central Rand Groups of the Witwatersrand Basin indicate that felsic source rocks mostly formed at ~3090–3060 and ~3000–2870 Ma, with a smaller proportion formed prior to 3100 Ma. Provenance evaluation indicates that source rocks have largely vanished through erosion, such that greenstone terrains extending from the Amalia–Kraaipan Belt in the southwest to the Murchison Belt in the northeast are models of, but not the actual, source areas. Analysis of detrital zircon age-modes reveals that the West Rand and Central Rand Groups record different basins. The West Rand Group is characterised by age-subpopulations with a much-smaller range and less complex distribution than those in the Central Rand Group. Furthermore, detrital zircon age-spectra for the West Rand Group generally decrease in complexity up-section, whereas they become increasingly more-complex in the Central Rand Group. It is concluded that the source area for the West Rand Group is consistent with a passive-margin setting, with the decrease in complexity up-section of zircon age-spectra being the consequence of increasing maturity of the hinterland. Conversely, the source area of the Central Rand Group was far-more dynamic, with an increasing age-range of source rocks being exposed through time. Pulsed tectonic rejuvenation of the source area is implied. Contemporaneous emplacement, uplift and erosion of granitoids supports a magmatic fold-thrust belt as the source, and a complementary retro arc basin setting. Detrital xenotime grains record the ages of high-U granitoids emplaced in the fold-thrust belt during deposition of the Central Rand Group, and are much closer to depositional ages than are detrital zircon grains. Change from a passive margin to a retro arc basin across the West Rand-Central Rand boundary is emphasised by change from simple to increasingly complex detrital zircon age-spectra. This study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in Archaean sandstones due to severe chemical weathering and authigenic alteration, thereby enabling distinction between passive-margin and retro arc-basin stages through systematic changes in the age complexity of the source area