Characterisation of gold from Fiji

This is a study of the variation in chemistry and inclusion mineralogy of bedrock and placer gold from Fiji. It forms part of a large project, undertaking gold characterisation from a wide range of geological environments in Ecuador, Zimbabwe, Malaysia and Fiji. The work was carried out under the Overseas Development AdministratiodBritish Geological Survey Technology Development and Research programme (Project R5549) as part of the British Government’s provision of technical assistance to developing countries. For the Fijian component of the project, samples were collected from river gravels, primary ore, and table concentrates. In total thirty-five samples from five localities were examined. Data collected from these samples are represented by over 100 point analyses of gold, identification of associated minerals, and microgeochemical maps of gold-mineral intergrowths. A framework for identifying possible sources of alluvial gold is given. This was achieved by characterising bedrock gold mineralisation from a variety of epithermal and porphyry environments. The environments studied included alkali (the Emperor Gold Mine), high-sulphidation (the Mount Kasi prospect) and telescoped (the Tuvatu prospect) systems. A study of placer gold from the Waimanu alluvial deposit, the only alluvial gold deposit in Fiji, showed two distinct sources of gold: one, a low-silver type, associated with Cu-Fe sulphides, can be related to the nearby Namosi porphyry copper deposit. The other source, a high-silver type with abundant tellurides, indicates an alkali epithermal association, suggesting a source similar in style to the mineralisation observed at Emperor Gold Mine. The identification of two bedrock sources for the Waimanu alluvials clearly shows that there must be a, yet unknown, alkali epithermal (Emperor) source within the Waimanu catchment. This demonstrates the power of alluvial gold characterisation and its role in gold exploration. An important implication of these results is that future exploration within the area should be focussed on locating this “Emperor type” source

Quantifying the release of base metals from source rocks for volcanogenic massive sulfide deposits : effects of protolith composition and alteration mineralogy

This detailed study of the release of base metals during hydrothermal alteration from the sheeted dike complex of the Troodos ophiolite, Cyprus, aims to better understand the source of these elements in ore-forming hydrothermal fluids. The study area, ~ 10 km2 between the villages of Spilia and Kannavia, has previously been recognized as a region in which the abundance of epidote in the altered sheeted dikes is higher than average — a so-called epidosite zone. The originally basaltic to andesitic sheeted dikes have been variably altered, but the secondary mineralogy is independent of the protolith composition. Four alteration facies have been identified in the epidosite zone. With progressively increasing modal epidote, decreasing modal amphibole, and decreasing bulk-rock Mg these are: (i) diabase, which is composed of amphibole + chlorite + albitic plagioclase ± epidote ± quartz, (ii) transitional diabase–epidosite, (iii) intermediate epidosite, and (iv) rare (< 15% of the study area) end-member epidosite which consists largely of quartz + epidote. Comparing protolith base metal differentiation trends, defined by new analyses of cogenetic volcanic glass, with these altered samples indicates that the rocks originally contained 47–99 ppm Zn, 1030–1390 ppm Mn, 19–28 ppm Co, 19–57 ppm Cu and 7–50 ppm Ni. The vast majority of the altered rocks within the epidosite zone studied have low Cu (averaging 3 ppm) irrespective of alteration facies. This uniform and large depletion suggests that Cu was originally largely present in sulfides that were completely destroyed during hydrothermal alteration. With the exception of Co, the other base metals have substantially lower concentrations in the altered rocks than in their protoliths and show increasing base metal depletion with increasing modal epidote abundance. This suggests that breakdown of silicate minerals was important in controlling the release of these metals. Cobalt is enriched in the diabase and transitional diabase–epidosite alteration facies, and depleted in the end-member epidosite alteration facies, relative to protolith concentrations. This suggests that Co was redistributed within the sheeted dike complex rather than substantially leached out; the same is probably true of Mg. Mapping across the steep topography of the study area indicates that the Spilia–Kannavia epidosite zone has a volume of ~ 2 km3. Based on this estimate, hydrothermal fluids leached ~ 369 kt of Zn, ~ 52 kt of Ni, ~ 3647 kt of Mn and ~ 162 kt of Cu. These Zn and Cu losses are similar to the masses of these metals present in the largest volcanogenic massive sulfide deposits on Cyprus. Based on the differences between protolith and altered rock compositions it is predicted that alteration of primitive protoliths will tend to produce fluids with higher ratios of Cu and Ni to Zn and Mn, whereas alteration of more evolved protoliths will produce fluids with lower ratios

How much metal can you get? Quantified mass balancing of base metal release during epidosite zone alteration in ophiolite-hosted VMS systems

Understanding source-deposit relationships in VMS systems is important for mineral exploration and to increase knowledge of seafloor hydrothermal processes and ocean–crust fluxes. Although it is known that metals are stripped from oceanic crust by hydrothermal fluids and are partly redeposited in orebodies, some aspects are poorly understood. It has been proposed that metal-depleted epidosites (epidote–quartz–chlorite–Fe-oxide–titanite units within sheeted dyke complexes) were the source rocks for ophiolite-hosted VMS deposits. To test this hypothesis, the Spilia-Kannavia epidosite zone in the Troodos ophiolite, Cyprus, was investigated. This zone (≥1.9 km3), at the base of the Sheeted Dyke Complex, extends ~5 km parallel to and ~2 km across dyke strike and vertically ≥400 m. During alteration, this zone released ~0.4 Mt Zn, ~0.06 Mt Ni, ~5.2 Mt MnO, and 0.14 Mt Cu. This exceeds the Zn within any ophiolite-hosted VMS deposit, and provides enough Cu for a medium-large deposit. In comparison, published data suggest a similar volume of background diabase could mobilise less Zn, Cu and Ni and no Co or Mn. It is concluded that epidosite zone formation releases significant amounts of base metals into ophiolite-hosted VMS system

Epidosites of the Troodos Ophiolite: A direct link between alteration of dykes and release of base metals into ore-forming hydrothermal systems?

The role of source rocks in the formation of Cyprus-type Volcanogenic Massive Sulphide (VMS) deposits is not fully understood. In this paper we suggest that the formation of epidosites – episode + quartz ± chlorite ± titanite rocks commonly found at the base of ophiolitic sheeted dyke complexes – has the potential to release cobalt and zinc into active hydrothermal ore forming systems. New geochemical and mineralogical data from the sheeted dyke complex of the Troodos ophiolite indicates that progressive alteration of greenschist facies altered metabasalts to end-member epidosites results in decreasing base metal concentrations (Zn, Co) in individual dyke units. We believe this relationship provides the clearest evidence that epidosites act as source rocks for VMS deposits, and, moreover, the process of epidositisation provides a method of mobilizing metals such as Co and Ni that are considered generally immobile under typical greenschist grade alteration

Atypical gold mineralization in an orogenic setting : the Bohaun Deposit, western Irish Caledonides

The Bohaun gold deposit in County Galway, western Ireland is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks, which have only reached the late diagenetic zone of low-grade metamorphism. The mineralization consists of undeformed veins, stockworks and breccias. Quartz dominates all the veins and commonly displays multiple growth stages and a variety of classic epithermal textures, including comb, plumose and banded, indicative of low confining pressures. The veins have a simple mineralogy comprising quartz–sericite–chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite. Vein quartz is associated with two fluids compositions: Type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and Type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126 to 257oC and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175 and 245°C. The geodynamic setting of Bohaun, which comprises a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition and undeformed veins. While vein textures locally resemble those found in epithermal quartz–adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine type fluid, in a similar fashion to red-bed Au–Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic grade gold mineralization, independent of other fluids

Assessing multiple novel tracers to improve the understanding of the contribution of agricultural farm waste to diffuse water pollution

A study was undertaken on drained and undrained 1 ha grassland lysimeters to assess the effectiveness of multiple novel tracing techniques in understanding how agricultural slurry waste moves from land to water. Artificial fluorescent particles designed to mimic the size and density of organic slurry particles were found to move off the grassland via inter-flow (surface + lateral through-flow) and drain-flow. Where both pathways were present the drains carried the greater number of particles. The results of the natural fluorescence and δ13C of water samples were inconclusive. Natural fluorescence was higher from slurry-amended lysimeters than from zero-slurry lysimeters, however, a fluorescence decay experiment suggested that no slurry signal should be present given the time between slurry application and the onset of drainage. The δ13C values of >0.7 µm and 0.7 µm δ13C in water from the drain-flow pathways was higher from the lysimeter which had received naturally enriched maize slurry compared to the lysimeter which received grass slurry indicating a contribution of slurry-derived material. Values of <0.7 µm δ13C from the same pathway, however, produced counter intuitive trends and may indicate that different fractions of the slurry have different δ13C values