Abstract Hangi stones, used to retain heat in traditional Maori earth ovens (hangi), may carry records of Earth's magnetic field when they were last used. Sixteen archaeological features, including 12 hangi, from eight sites were sampled and their palaeomagnetic data used to construct the first archaeointensity record for New Zealand, covering the past 700 years. A combination of radiocarbon dating of associated charcoal and archaeomagnetic dating of palaeomagnetic directions was used to obtain a ‘preferred’ date of each palaeointensity. A plot of virtual axial dipole moment (VADM) for the SW Pacific region outlines steady VADM values of about 8 × 10 22 A m 2 from 1000 to 1300 AD and 9.5 × 10 22 A m 2 from 1500 AD to the present, with a sharp peak in the early fifteenth century when the VADM reached about 13 × 10 22 A m 2 . This peak bears many similarities to archaeomagnetic ‘jerks’ and ‘spikes’ in Northern Hemisphere records from the first millennia BC and AD. However, it is the first such feature to be found in the Southern Hemisphere at this date, suggesting, in accordance with recent modelling, that it may be a feature of the non-dipole field, associated with rapid growth and decay of an intense flux patch on the core–mantle boundary.

Abstract Epithermal, Carlin, and orogenic Au deposits form in diverse geologic settings and over a wide range of depths, where Au precipitates from hydrothermal fluids in response to various physical and chemical processes. The compositions of Au-bearing sulfidic hydrothermal solutions across all three deposit types, however, are broadly similar. In most cases, they comprise low-salinity waters, which are reduced, have a near-neutral pH, and CO 2 concentrations that range from <4 to >10 wt %. Experimental studies show that the main factor controlling the concentration of Au in hydrothermal solutions is the concentration of reduced S, and in the absence of Fe-bearing minerals, Au solubility is insensitive to temperature. In a solution containing ~300 ppm H 2 S, the maximum concentration of Au is ~1 ppm, representing a reasonable upper limit for many ore-forming solutions. Where Fe-bearing minerals are being converted to pyrite, Au solubility decreases as temperature cools due to the decreasing concentration of reduced S. High Au concentrations (~500 ppb) can also be achieved in strongly oxidizing and strongly acidic chloride solutions, reflecting chemical conditions that only develop during intense hydrolytic leaching in magmatic-hydrothermal high-sulfidation epithermal environments. Gold is also soluble at low to moderate levels (10–100 ppb) over a relatively wide range of pH values and redox states. The chemical mechanisms which induce Au deposition are divided into two broad groups. One involves achieving states of Au supersaturation through perturbations in solution equilibria caused by physical and chemical processes, involving phase separation (boiling), fluid mixing, and pyrite deposition via sulfidation of Fe-bearing minerals. The second involves the sorption of ionic Au on to the surfaces of growing sulfide crystals, mainly arsenian pyrite. Both groups of mechanisms have capability to produce ore, with distinct mineralogical and geochemical characteristics. Gold transport and deposition processes in the Taupo Volcanic Zone, New Zealand, show how ore-grade concentrations of Au can accumulate by two different mechanisms of precipitation, phase separation and sorption, in three separate hydrothermal environments. Phase separation caused by flashing, induced by depressurization and associated with energetic fluid flow in geothermal wells, produces sulfide precipitates containing up to 6 wt.% Au from a hydrothermal solution containing a few ppb Au. Sorption on to As-Sb-S colloids produces precipitates containing tens to hundreds of ppm Au in the Champagne Pool hot spring. Sorption on to As-rich pyrite also leads to anomalous endowments of Au of up to 1 ppm in hydrothermally altered volcanic rocks occurring in the subsurface. In all of these environments, Au-undersaturated solutions produce anomalous concentrations of Au that match and surpass typical ore-grade concentrations, indicating that near-saturated concentrations of dissolved metal are not a prerequisite for generating economic deposits of Au. The causes of Au deposition in epithermal deposits are related to sharp temperature-pressure gradients that induce phase separation (boiling) and mixing. In Carlin deposits, Au deposition is controlled by surface chemistry and sorption processes on to rims of As-rich pyrite. In orogenic deposits, at least two Au-depositing mechanisms appear to produce ore; one involves phase separation and the other involves sulfidation reactions during water-rock interaction that produces pyrite; a third mechanism involving codeposition of Au-As in sulfides might also be important. Differences in the regimes of hydrothermal fluid flow combined with mechanisms of Au precipitation play an important role in shaping the dimensions and geometries of ore zones. There is also a strong link between Au-depositing mechanisms and metallurgical characteristics of ores.