Both gold-rich sulphides and ultra-high grade native gold oreshoots are common but poorly understood phenomenon in orogenic-type mineral systems, partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations. The world-class Obuasi gold deposit, Ghana, has gold-rich arsenopyrite spatially associated with quartz veins, which have extremely high, localised concentrations of native gold, contained in microcrack networks within the quartz veins where they are folded. Here, we examine selected samples from Obuasi using a novel combination of quantitative electron backscatter diffraction analysis, ion microprobe imaging, synchrotron XFM mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations. The auriferous arsenopyrites are shown to have undergone partial replacement (∼15%) by Au-poor, nickeliferous arsenopyrite, during localised crystal-plastic deformation, intragranular microfracture and metamorphism (340–460°C, 2kbars). Our results show the dominant replacement mechanism was pseudomorphic dissolution-reprecipitation, driven by small volumes of an infiltrating fluid that had relatively low ƒS2 and carried aqueous NiCl2. We find that arsenopyrite replacement produced strong chemical gradients at crystal-fluid interfaces due to an increase in ƒS2 during reaction, which enabled efficient removal of gold to the fluid phase and development of anomalously gold-rich fluid (potentially 10ppm or more depending on sulphur concentration). This process was facilitated by precipitation of ankerite, which removed CO2 from the fluid, increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation. Gold re-precipitation occurred over distances of 10μm to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides. We suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic/ultramafic rocks, which act as a source of Ni and Co scavenged by chloride-bearing fluids. Both the preference of the arsenopyrite crystal structure for Ni and Co, rather than gold, and the release of sulphur during reaction, can drive gold remobilisation in many deposits across broad regions.

Oxygen isotope ratios have been determined using laser fluorination techniques on olivine and plagioclase phenocrysts and bulk glasses from the Reykjanes Ridge and Iceland. δ18O in Reykjanes Ridge olivines shows hyperbolic correlations with Sr–Nd–Pb isotope ratios that terminate at δ18O=+4.5‰ at compositions almost identical to those of moderately enriched lavas on the Reykjanes Peninsula, Iceland. Samples with low δ18O show no indication of contamination by oceanic crust such as elevated Cl/K, and are too deep to have been influenced by meteoric water hydrothermal systems. They cannot represent Icelandic melts contaminated in the crust and transferred laterally along the ridge since fissure systems are strongly oblique to the ridge axis. It follows that Icelandic mantle advected along the ridge has low δ18O. The hyperbolic 143Nd/144Nd–δ18O correlation appears to be more strongly curved than magma mixing trajectories and suggests that melt fractions are ∼4.5× greater and source Nd contents ∼9× greater in the mantle at 63°N compared with that at 60°N. Primitive lavas from the Reykjanes Peninsula show linear correlations between olivine δ18O and 143Nd/144Nd or 206Pb/204Pb, extending to δ18O of +4.3‰ at 143Nd/144Nd close to the lowest ratios observed in Icelandic magmas. These correlations cannot be produced by melt mixing or crustal contamination because these would yield strongly hyperbolic trajectories. Lower δ18O seen in more evolved samples from the Eastern Rift Zone may reflect crustal contamination, though there is some evidence of a mantle source with lower δ18O in eastern Iceland. It is very difficult to explain the low δ18O of enriched Icelandic mantle sources on current understanding of mantle and crustal oxygen isotopes. There is no obvious reason why such low-δ18O sources should not contribute to other ocean islands. No oceanic crustal lithologies exist that could produce the low-δ18O enriched sources by recycling into the mantle, and there is no evidence for changes in δ18O of ophiolite suites with time, nor of changes during high-P metamorphism. Low δ18O appears to be associated with high 3He/4He, and we speculate that this signature may be characteristic of the host mantle into which ocean crust was recycled.

High precision Sr-Nd isotope ratios together with Pb isotope ratios corrected for mass fractionation using a double spike are reported for an extensive suite of late Quaternary to Recent lavas of Iceland, the Kolbeinsey and Reykjanes Ridges, and a small number of basalts from further south on the Mid-Atlantic Ridge. Compared with global MORB, the Icelandic region is distinguished by having low Pb-207/Pb-204 for any given Pb-206/Pb-204, expressed by negative Delta(207)Pb (-0.8 to -3.5) in all but four Icelandic samples. Most samples also have elevated Pb-208/Pb-204 (strongly positive Delta(208)Pb), which combined with their negative Delta(207)Pb is very unusual in MORB worldwide. The negative Delta(207)Pb is interpreted as a consequence of evolution in high-g mantle sources for the last few hundred Ma. The region of negative Delta(207)Pb appears to correspond with the region of elevated He-3/He-4, suggesting that both lithophile and volatile elements in melts from the whole region between 56 and 70degreesN are dominantly sourced in a plume that has incorporated recycled Palaeozoic ocean crust and unradiogenic He, probably from the deep mantle. At least four mantle components are recognized on Iceland, two with an enriched character, one depleted and one that shows some isotopic affinities to EM1 but is only sampled by highly incompatible-element-depleted lavas in this study. Within restricted areas of Iceland, these components contribute to local intermediate enriched and depleted components that display near binary mixing systematics. The major depleted Icelandic component is clearly distinct in Pb isotopes from worldwide MORB, but resembles the depleted mantle source supplying the bulk of the melt to the Kolbeinsey and southern Reykjanes Ridges. However, an additional depleted mantle source is tapped by the northern Reykjanes Ridge, which with very negative Delta(207)Pb and less positive Delta(208)Pb is distinct from all Icelandic compositions. These components must mostly mix at mantle depths because a uniform mixture of three Icelandic components is advected southward along the Reykjanes Ridge.Despite strong covariation with isotope ratios, incompatible trace element ratios of Icelandic magmas cannot be representative of old mantle sources. The observed parent-daughter ratios in depleted and enriched Icelandic lavas would yield homogeneous Sr, Nd, Hf and Pb-206 isotope signatures similar to170 Ma ago if present in their sources. The heterogeneity in Pb-207/Pb-204 is not however significantly reduced at 170 Ma, and the negative present day Delta(207)Pb cannot be supported by the low mu observed in depleted lavas from Iceland or the adjacent ridges. Since A is higher in melts than in their sources, it follows that all the depleted sources must be residues from <170 Ma partial melting events. These are thought to have strongly affected most incompatible trace element ratios. Copyright (C) 2004 Elsevier Ltd.

A linear, axis-parallel, array of glacial and postglacial basalt samples from the elevated midocean ridge in southwest Iceland, the Reykjanes Peninsula, shows three MgO lows and two MgO highs at similar to 40 km intervals. Similar patterns are observed with other fractionation indices, e,g,, Cr. These along-axis variations in elements affected by fractional crystallization are interpreted as evidence for segment-scale variation in crustal residence times arising from the focusing of magmatic activity at regular intervals along this elevated mid-ocean ridge. In the majority of the samples, Nb/Zr, generally considered to be unaffected by crystal fractionation, does not show a systematic variation with MgO. Lavas with unusually low Nb/Zr, erupted at the end of the last glaciation, are the only exception. These low-Nb/Zr lavas are generally restricted to the MgO highs, resulting in a wider range of lava Nb/Zr in these areas than in the MgO lows,It is proposed that low-Nb/Zr melts are available along the entire ridge section at all times, but are modified before eruption by mixing,vith melts that are more enriched in incompatible elements. Crustal processes at this ridge axis are governing the distribution of chemistry associated with the mantle.

Three hundred stratigraphically constrained samples from the Reykjanes Peninsula, SW Iceland, provide the basis for this study. This area is an elevated section of mid-ocean ridge influenced by the Iceland Plume. Selected chemical, Sr, Nd and laser-assisted fluorination oxygen isotope data are presented. The dataset is subdivided into groups based on criteria which are independent of degree of fractionation and petrography. Two of these groups, Depleted and Stapafell, include high-MgO aphyric samples with Δ18Oolivine values in equilibrium with normal peridotite mantle. Depleted group samples have high 143Nd/144Nd, low Nb/Zr and low incompatible element abundances compared with the dataset as a whole, the reverse of the Stapafell group. The majority of the remaining samples have radiogenic isotope ratios, and incompatible element concentrations and ratios intermediate between the Depleted and Stapafell groups. Some samples, however, define a range in 87Sr/86Sr and δ18Oolivine at constant 143Nd/144Nd, and others possess positive Sr anomalies when normalized to primitive mantle values. We explore the possibility that these and other chemical characteristics have been produced by shallow crustal processes, including assimilation of xenocrysts, cumulates and hydrothermally modified crust. We conclude that although these processes are important, the major crustal process acting to modify characteristics indicative of mantle heterogeneity is magma mixing. Chemical variation previously thought to be a consequence of dynamic melting is more readily explained by magma mixing.

We present chemical and isotopic data from Reykjanes Peninsula rift zone tholeiites which show dramatic changes during the past 20 kyr, effectively defining the 9–13 kyr BP period of fluctuations in glacioisostacy associated with the end of the Weichselian glaciation and the Younger Dryas readvance. During this period primitive lavas were erupted with `depleted' characteristics; low Nb/Zr, high 143Nd/144Nd, low 87Sr/86Sr, and low 206Pb/204Pb. We propose that the clustered eruption of these chemically and isotopically anomalous lavas is primarily a reflection of reduced crustal residence times for the parental magmas. These `depleted' magmas largely escaped crustal processes such as mixing, assimilation and fractional crystallisation which would overprint their distinctive chemical and isotopic characteristics. There is some evidence that the northern rift zone may also have been similarly affected during this period of rapid glacioisostatic readjustment. The parental melts to these depleted glacioisostatic lavas may only be produced during fluctuations in glaciation, or could be a normal component in all Iceland magmas.

Thermally buoyant mantle, in the form of a plume, rises beneath Iceland creating a major topographic anomaly along the Mid-Atlantic Ridge and in the surrounding ocean basin. However, the influence of the Iceland plume on the composition of lavas erupted on adjacent ridges remains a contentious issue. Trace element systematics and radiogenic isotope ratios of Sr, Nd and Pb suggest that the plume influences a region 1200 km in length. In contrast, the3He anomaly associated with Iceland closely corresponds to the 2400 km ridge section affected by thermal uplift. We present evidence that the Sr, Nd and Pb isotope signature of the Iceland plume is in fact as widespread as its thermal and3He anomalies. Results imply that much of the source of North Atlantic ridge basalts has been contaminated by lateral outflow of asthenosphere from the Icelandic plume. Consequently, estimates of the average composition of mid-ocean ridge basalt (MORB) sources are likely to be biased by including data from plume-contaminated regions. True MORB values, and perhaps upper mantle geochemistry, can be constrained only by considering data untainted by plume asthenosphere.