Hydrothermal alteration and fluid pH in alkaline-hosted epithermal systems

Epithermal gold mineralisation is found in a wide compositional range of host lithologies, but despite the diversity the alteration mineral assemblages are often similar between deposits. Notable exceptions are those gold deposits hosted in alkaline host rocks. Alkaline-hosted epithermal deposits are rare, but important, as they include some of the world’s largest known epithermal deposits by contained metal (e.g. Ladolam, Cripple Creek, Porgera). As well as the exceptional gold contents, the alkaline-hosted systems tend to exhibit different alteration mineral assemblages, with less quartz and widespread silicification than sub-alkaline-hosted equivalents, and greater enrichments in tellurium, and a scarcity of acid alteration (advanced argillic) types. In this study, geochemical modelling is used to demonstrate that 300 °C hydrothermal fluids in equilibrium with alkali, silica-undersaturated host rocks at low water/rock ratios reach significantly higher pH than equivalents in sub-alkaline lithologies. A maximum, near-neutral pH (5.5–6) is buffered by reactions involving quartz in silica-saturated alkaline and calc-alkaline lithologies. In silica-undersaturated, alkaline host rocks, quartz is exhausted by progressive water-rock interaction, and pH increases to 7 and above. Both tellurium and gold solubility are favoured by neutral to high fluid pH, and thus there is a clear mechanism within these hydrothermal systems that can lead to effective transport and concentration to produce gold telluride ore deposits in alkaline igneous hosts. This modelling demonstrates that alkaline rocks can still be altered to advanced argillic assemblages; the paucity of this alteration type in alkaline hosts instead points to NaCl ≫ HCl in magmatic volatile phases at the initiation of hydrothermal alteration

How the Neoproterozoic S-isotope record illuminates the genesis of vein gold systems: an example from the Dalradian Supergroup in Scotland

The genesis of quartz vein-hosted gold mineralization in the Neoproterozoic–early Palaeozoic Dalradian Supergroup of Scotland remains controversial. An extensive new dataset of S-isotope analyses from the Tyndrum area, together with correlation of the global Neoproterozoic sedimentary S-isotope dataset to the Dalradian stratigraphy, demonstrates a mixed sedimentary and magmatic sulphur source for the mineralization. d34S values for early molybdenite- and later gold-bearing mineralization range from 22 to +12‰, but show distinct populations related to mineralization type. Modelling of the relative input of magmatic and sedimentary sulphur into gold-bearing quartz veins with d34S values of +12‰ indicates a maximum of 68% magmatic sulphur, and that S-rich, SEDEX-bearing, Easdale Subgroup metasedimentary rocks lying stratigraphically above the host rocks represent the only viable source of sedimentary sulphur in the Dalradian Supergroup. Consequently, the immediate host rocks were not a major source of sulphur to the mineralization, consistent with their low bulk sulphur and lack of metal enrichment. Recent structural models of the Tyndrum area suggest that Easdale Subgroup metasedimentary rocks, enriched in 34S, sulphur and metals, are repeated at depth owing to folding, and it is suggested that these are the most likely source of sedimentary sulphur, and possibly metals, for the ore fluids

Stratigraphy and geochronology of the Tambien Group, Ethiopia: evidence for globally synchronous carbon isotope change in the Neoproterozoic

The Neoproterozoic Era was an interval characterized by profound environmental and biological transitions. Existing age models for Neoproterozoic nonglacial intervals largely have been based on correlation of carbonate carbon isotope values, but there are few tests of the assumed synchroneity of these records between basins. In contrast to the ash-poor successions typically targeted for Neoproterozoic chemostratigraphy, the Tonian to Cryogenian Tambien Group (Tigray region, Ethiopia) was deposited in an arc-proximal basin where volcanic tuffs suitable for U-Pb geochronology are preserved within the mixed carbonate-siliciclastic sedimentary succession. The Tambien Group culminates in a diamictite interpreted to correlate to the ca. 717–662 Ma Sturtian snowball Earth glaciation. New physical stratigraphic data and high-precision U-Pb dates from intercalated tuffs lead to a new stratigraphic framework for the Tambien Group that confirms identification of negative δ13C values from Assem Formation limestones with the ca. 800 Ma Bitter Springs carbon isotope stage. Integration with data from the Fifteenmile Group of northwestern Canada constitutes a positive test for the global synchroneity of the Bitter Spring Stage and constrains the stage to have started after 811.51 ± 0.25 Ma and to have ended before 788.72 ± 0.24 Ma. These new temporal constraints strengthen the case for interpreting Neoproterozoic carbon isotope variation as a record of large-scale changes to the carbon cycle and provide a framework for age models of paleogeographic change, geochemical cycling, and environmental evolution during the radiation of early eukaryotes

The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals

publisher: Elsevier articletitle: The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals journaltitle: Minerals Engineering articlelink: http://dx.doi.org/10.1016/j.mineng.2015.09.026 content_type: article copyright: Copyright © 2015 The Authors. Published by Elsevier Ltd.© 2015 Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Effects of magmatic volatile influx in mafic VMS hydrothermal systems: Evidence from the Troodos ophiolite, Cyprus

The Troodos ophiolite, Cyprus is the principal on- land analogue for mafic-hosted volcanogenic massive sulfide (VMS) deposits. This study, for the first time, presents sulfur isotope (δ34S) data on a regional scale from VMS deposits and other mineralised zones across the Troodos ophiolite. In combination δ34S, Se/S ratios and trace element chemistry (e.g., Se, Cu and Au) of different hydrothermal sulfides are used to assess variations in magmatic volatile influx and the source of metals and sulfur in ancient hydrothermal systems. Sulfur isotope analyses (n = 180) across 19 mineral deposits indicate a variable source of sulfur in the Troodos VMS hydrothermal system, this in turn allows a variable source of metals to be inferred. Pyrite δ34S range from −5.5‰ to +13.2‰ with an average of +4.6‰ (n = 160) for all deposits investigated. These δ34S variations cannot only be explained by variable proportions of thermochemical seawater sulfate reduction (δ34S +18 to +19‰) and leaching of primary magmatic sulfur from igneous rocks (δ34S 0-1‰). Consequently, two processes are proposed, explaining the trace metal and δ34S variation across the Troodos ore-forming systems including, i) a variable source of metals in the sheeted dyke complex and ii) the addition of a magmatic volatile-rich phase to the VMS hydrothermal systems. Two distinct lava units exist in the Troodos stratigraphy, namely the upper and lower pillow lavas (UPL and LPL). The more primitive UPL are enriched in Au, Se and Cu relative to As, Sb and Zn that are concentrated in the LPL. Some VMS deposits pre-date the formation of the UPL (e.g., Agrokipia A) suggesting Se, Cu and Au depleted source rocks. Hence, the stratigraphic position of VMS deposits and the ratio of UPL:LPL affinity elements (e.g., As + Zn + Sb vs. Cu + Se + Au) imply a systematic relationship between trace element distribution and stratigraphic depth; this relates to the relative proportion of UPL and LPL affinity lavas within the metal source region. δ34S values <0‰ recorded in some VMS deposits that are less than the Troodos magmatic mean of 0- 1‰ may be related to anhydrite buffering during fluid ascent, microbial sulfate reduction or the direct contribution of magmatically derived sulfur, to the hydrothermal system from an underlying magma chamber via volatile exsolution. We propose that negative δ34S values combined with Se/S 106 ratios >500 in pyrite suggest the contribution of a magmatic volatile component (e.g., Apliki and Skouriotissa). We demonstrate that the source of metals and sulfur in the Troodos VMS hydrothermal system is affected by regional scale processes related to i) variable source lithologies and, ii) the contribution of a magmatic volatile phase to some Troodos VMS hydrothermal systems

Stream and slope weathering effects on organic-rich mudstone geochemistry and implications for hydrocarbon source rock assessment: a Bowland Shale case study

This study contributes to the exploration and quantification of the weathering of organic-rich mudstones under temperate climatic conditions. Bowland Shales, exposed by a stream and slope, were sampled in order to develop a model for the effects of weathering on the mudstone geochemistry, including major and trace element geochemistry, Rock-Eval pyrolysis and δ13Corg. Four weathering grades (I – IV) are defined using a visual classification scheme; visually fresh and unaltered (I), chemically altered (II, III) and ‘paper shale’ that typifies weathered mudstone on slopes (IV). Bedload abrasion in the stream exposes of visually fresh and geochemically unaltered mudstone. Natural fractures are conduits for oxidising meteoric waters that promote leaching at the millimetre scale and/or precipitation of iron oxide coatings along fracture surfaces. On the slope, bedding-parallel fractures formed (and may continue to form) in response to chemical and/or physical weathering processes. These fractures develop along planes of weakness, typically along laminae comprising detrital grains, and exhibit millimetre- and centimetre-scale leached layers and iron oxide coatings. Fracture surfaces are progressively exposed to physical weathering processes towards the outcrop surface, and results in disintegration of the altered material along fracture surfaces. Grade IV, ‘paper shale’ mudstone is chemically unaltered but represents a biased record driven by initial heterogeneity in the sedimentary fabric. Chemically weathered outcrop samples exhibit lower concentrations of both ‘free’ (S1) (up to 0.6 mgHC/g rock) and ‘bound’ (S2) (up to 3.2 mgHC/g rock) hydrocarbon, reduced total organic carbon content (up to 0.34 wt%), reduced hydrogen index (up to 58 mgHC/gTOC), increased oxygen index (up to 19 mgCO + CO2/gTOC) and increased Tmax (up to 11 °C) compared with unaltered samples. If analysis of chemically weathered samples is unavoidable, back-extrapolation of Rock-Eval parameters can assist in the estimation of pre-weathering organic compositions. Combining Cs/Cu with oxygen index is a proxy for identifying the weathering progression from fresh material (I) to ‘paper shale’ (IV). This study demonstrates that outcrop samples in temperate climates can provide information for assessing hydrocarbon potential of organic-rich mudstones

A Mississippian black shale record of redox oscillation in the Craven Basin, UK

Early diagenetic redox oscillation processes have been rarely recognised in the ancient rock record but potentially exert an important control on mineral authigenesis, hydrocarbon prospectivity and supply of metals and/or reduced S as part of associated mineral systems. The upper unit of the Mississippian Bowland Shale Formation is a candidate record of diagenetic redox oscillation processes because it was deposited under a relatively high sediment accumulation rate linked to a large delta system, and under dominantly anoxic and intermittently sulphidic bottom-water conditions. In order to characterise the syngenetic and early diagenetic processes, sedimentological and geochemical data were integrated through the Upper Bowland Shale at three sites in the Craven Basin (Lancashire, UK). Organic matter (OM) comprises a mixture of Type II, II-S, II/III and III OM. ‘Redox zones’ are defined by patterns of Fe-speciation and redox-sensitive trace element enrichment and split into two groups. ‘Sulphidic’ zones (EUX, AN-III, AN-I and AN-IT) represent sediments deposited under conditions of at least intermittently active sulphate-reduction in bottom-waters. ‘Non-sulphidic’ zones (OX-RX, OX-F and OX) represent sediments deposited under non-sulphidic (oxic to ferruginous anoxic) bottom-waters. Operation of a shelf-to-basin ‘reactive Fe’ (FeHR) shuttle, moderated by sea level fluctuation and delta proximity, controlled the position and stability of redoxclines between zones of Fe and sulphate reduction, and methanogenesis. Early diagenetic redoxclines were capable of migration through the shallow sediment column relatively quickly, in response to sea level fluctuation. Preservation of syngenetic and early diagenetic geochemical signals shows redoxclines between Fe and sulphate reduction, and the upper boundary of sulphate-methane transition zone, were positioned within decimetres (i.e., 10 s cm) of seabed. Falling sea level and increasing FeHR supply is recognised as a switch from zones EUX (high sea level), AN-III and ultimately AN-I and AN-IT (low sea level). Zone AN-I defines the operation of ‘redox oscillation’, between zones of Fe and sulphate reduction in shallow porewaters, associated with enhanced degradation of OM and complete dissolution of primary carbonate. Preservation of OM and carbonate, in this system, was a function of changing bottom and pore water redox processes. Redox oscillation operated in a siliciclastic, prodeltaic environment associated with a relatively high sediment accumulation rate and high loadings of labile organic matter and metal oxides. These findings are important for understanding Late Palaeozoic black shales in the context of hydrocarbon and mineral systems

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

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