Gold remobilisation and formation of high grade ore shoots driven by dissolution-reprecipitation replacement and Ni substitution into auriferous arsenopyrite

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, 2 kbars). Our results show the dominant replacement mechanism was pseudomorphic dissolution-reprecipitation, driven by small volumes of an infiltrating fluid that had relatively low fS2 and carried aqueous NiCl2. We find that arsenopyrite replacement produced strong chemical gradients at crystal-fluid interfaces due to an increase in fS2 during reaction, which enabled efficient removal of gold to the fluid phase and development of anomalously gold-rich fluid (potentially 10 ppm 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 10 µm 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

The potential for reconstructing primary ocean chemistry from hypogene and supergene altered banded iron formations: An example from Weld Range, Western Australia

Pristine banded iron formations (BIF) are established paleo-environmental proxies for reconstructing the elemental and isotopic signatures of the ancient seawater that they precipitated from. Negligible changes in shale-normalised rare earth element patterns in BIF throughout Earth’s history, including features such as low La/Yb ratios, and positive La, Eu, Gd, and Y anomalies, and near- to super-chondritic Y/Ho ratios support the preservation of ancient seawater signatures. Nevertheless, limiting paleo-environmental reconstructions to pristine BIF imparts a significant sampling bias and restricts understanding of the temporal evolution of the oceans. However, altered BIF samples are problematic for paleo-environmental reconstructions due to the risk of disturbance of their primary signatures. Instead, mineral-/fraction-specific analysis potentially provides robust paleo-environmental reconstructions where primary mineral phases are preserved, with the three main mineral fractions in pristine and altered BIF including carbonates (e.g., siderite and ankerite), Fe oxides (e.g., magnetite, hematite, and goethite), and silicates (e.g., quartz and Fe-silicates). This study investigates samples from the ca. 2.7 Ga Weld Range BIF, located in the Youanmi Terrane, Yilgarn Craton, Western Australia. The lower-greenschist facies BIF varies from least-altered to progressively hypogene- and/or supergene-altered. Whole-rock analysis of these rocks revealed the preservation of seawater-like signatures despite significant alteration, such as positive La, Eu and Y anomalies. Additionally, sequential extraction techniques were performed on the least-altered and altered BIF samples to separately analyse the carbonate, Fe oxide, and silicate mineral fractions. In both the least- and hypogene-altered samples all fractions preserved evidence for seawater-like chemistry despite extensive precipitation of secondary hypogene carbonate and Fe oxide minerals in the latter. The seawater-like characteristics preserved in the hypogene carbonate and Fe oxide-fractions are the result of the seawater-magmatic fluid mixture that precipitated hypogene replacement minerals. Therefore, we interpret the silicate-fraction to be the most indicative of the primary seawater that precipitated the Weld Range BIF, where the quartz/chert reflects amorphous silica signatures that are unaffected by low-grade metamorphism and hypogene alteration. The preservation of primary mineral phases (i.e., silicates) and characteristic seawater signatures in the extensively altered Weld Range BIF, suggests that altered BIF should be more widely investigated to improve the breadth and representativeness of global paleo-environmental reconstructions