Polymetallic mineralised veins in ferroan/A-type Cretaceous leucogranite, Stewart Island, New Zealand

<p>The 140 ± 1 Ma hypersolvus, ferroan, weakly peralkaline to weakly peraluminous North Red Head leucogranite in northwest Stewart Island is cut by quartz-pyrite-rich veins that contain a wide variety of Mo, Ag, Te, Bi, Au, Co, Cu, Pb, Zn, REE, Nb, Y, Th, U, Zr, Ti, Be and F-bearing minerals. Patchy hematite-pyrite alteration locally overprints leucogranite in the vicinity of the mineralised veins. Individual veins are up to 5 m thick and 200+ m long. U–Pb dating and trace-element geochemistry indicate a direct link between leucogranite crystallisation and exsolution of the vein-forming hydrothermal fluid. Mineralised veins developed along transpressional faults within the leucogranite soon after emplacement. Incipiently mineralised quartz ± pyrite veins at Waituna Bay and the northern end of West Ruggedy Beach several kilometres from North Red Head are probably part of the same hydrothermal system as the veins at North Red Head. Metal and alteration assemblages at North Red Head most closely resemble those in rare hydrothermal systems associated with oxidised fluorine-rich A-type granites.</p

Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns

During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh–Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U–Pb ages indicate the emplacement of ~3,600–3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413 million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick (~43 km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100 million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour