Gold vein mineralization occurs in the metamorphosed and
deformed Dalradian (Neoproterozoic) rocks of the Sperrin Mountains, Northern
Ireland. Two structures exerted a control on the location of the
mineralization; the north-south Omagh lineament and the
west-northwest-east-southeast Curraghinalt lateral ramp in the footwall of
the northeast-southwest Omagh thrust. These are Caledonian structures
resulting from the thrusting of Dalradian rocks over a possibly still active
Ordovician arc. Cathodoluminescence microscopy distinguishes four phases
of vein quartz in the Curraghinalt gold prospect. Fluid inclusion studies
and stable isotope geochemistry have defined the probable fluids responsible
for the precipitation of each quartz phase and associated sulfide and
precious metal mineralization. The initial phase (Q1) appears to have been
associated with the main Caledonian metamorphic event (ca. 470 Ma) and is
nonauriferous. The second phase (Q2) forms an extensive cement to brecciated
early quartz and is believed to have involved a fluid (~15 wt % CO 2 ,
10 wt % NaCl + KCl equiv) with a significant magmatic component of 470 to
400 Ma, which underwent phase separation and dilution with a cooler
formation water. This process resulted in precipitation of the main phase of
gold mineralization characterized by an assemblage of electrum, pyrite,
arsenopyrite, chalcopyrite, tennantite-tetrahedrite, and various tellurides.
Similar fluids are observed on a regional scale, concentrated within the
hanging wall of the Omagh thrust, indicating an extensive fluid-flow event.
The relative abundance of gold at the Curraghinalt and Cavanacaw prospects
is thought to be due to higher fluid fluxes in favorable zones of dilation
and closer proximity to the fluid source. The deposit was subsequently reactivated with the
precipitation of later quartz (Q3-Q4) from a formation water believed to be
resident in the Dalradian metasediments, which mixed with a low-temperature,
high-salinity basinal brine, probably during Carboniferous basin inversion.
Brine flow resulted in the remobilization of earlier electrum, reducing its
fineness, and also introduced base metal sulfides, carbonates, and barite.
Again, brine flow is localized by the Omagh thrust, indicating the
long-lived role of this structure in controlling regional fluid migration.
Aynak is the largest known copper deposit in Afghanistan, with indicated resources of 240 Mt grading 2.3% Cu placing it in the 'giant' category. Host rocks are Neoproterozoic metasediments comprising dolomitic marble, carbonaceous quartz schist and quartz-biotite-dolomite schist containing garnet, scapolite and apatite. Chalcopyrite and bornite dominate the hypogene ore with lesser pyrite, pyrrhotite, cobaltite and chalcocite, and rare sphalerite, molybdenite, uraninite and barite. Sulphides occur as bedding-parallel laminae, disseminations, metamorphic segregations and crosscutting veins. Sulphide δ34S ratios range –14.5 to +17.3‰ in bedded and disseminated sulphides (n = 34). This broad range favours biogenic reduction of seawater sulphate as a major source of sulphur, although thermochemical reduction processes are not precluded. The narrower δ34S range of –6 to +12.2‰ in vein and segregation sulphides (n = 21) suggests localized redistribution and partial homogenization during metamorphism. Geochemical associations suggest that Al, P, Ca, Ti and Fe were primary sedimentary constituents whereas Cu, Mg, S, Se, As, Co and Bi were introduced subsequently. We infer that Aynak originated as a shale- and carbonate-hosted stratabound replacement deposit, resembling orebodies of the Central African Copperbelt, although underlying red-beds are absent at Aynak and mafic volcanics were the probable copper source. These giant deposits formed worldwide in the Cryogenian probably due to marine enrichment in copper, magnesium and sulphate coincident with profuse basaltic volcanism and ocean oxidation.
Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Milton Keynes MK7 6AAMineral grains of the rare-earth-element-bearing minerals fergusonite-(Y) and gadolinite-(Y) are reported within alluvial sediment derived from the granitic Mourne Mountains (the Mournes) in northeast Ireland. Fergusonite (ideally YNbO
Tidal mudflats are locally enriched in heavy metals at the head of Strangford Lough in Northern Ireland, where drainage from the hinterland enters the sea lough via a tidal canal in an urban area. To characterize the metallic contaminants and investigate their provenance, heavy particles separated from stream, canal and estuarine sediments were analysed by electron microprobe and laser Raman microspectroscopic methods. Potential metal sources are mineralization in the catchment area and industrial or domestic pollution. Anthropogenic particles include metallic grains, alloys and compounds of Pb, Zn, Cu, Fe, Cr and Sn. Alteration of metallic particles includes de-zincification of brass in freshwater sediment and replacement of Cu wire by covellite in brackish to marine sediment. Mobility of Cu, Fe and S in canal and estuarine sediments is indicated by the authigenic growth of framboidal Fe sulphide on oxide substrates and of chalcopyrite rims on covellite. Intricate colloform and platy crystalline textures suggest a cyclical deposition of covellite and chalcopyrite under conditions of varying redox and salinity. Lead and Cr mobility in the contaminated estuarine sediment is shown by the authigenic formation on Pb-rich substrates of heterogeneous Pb- and Cr-rich sulphate-phosphate compounds and Pb-oxychlorides.
Abstract Compositional studies of natural gold usually have a geological focus, but are also important in archaeological provenancing. Both methodologies rely on compositional comparison of two sets of samples, one of which is geographically constrained. Here we describe how experiences in gold characterization resulting from geological studies are relevant to archaeology. Microchemical characterization of polished sections of natural gold identifies alloy compositions, alloy heterogeneity and mineral inclusions. Gold from all deposit types shows Cu and Sn values much lower than those recorded during numerous studies of artefacts. Inclusions in artefact gold include various Cu- and Sn-bearing compounds which indicate specific high temperature reactions that could ultimately illuminate the conditions of (s)melting. The use of LA-ICP-MS to generate a wide range of elemental discriminants for provenance studies may be compromised by alloy adulteration and/or unrepresentative analysis of natural/artefact alloys, which are commonly highly heterogeneous at the micron scale. Geological studies normally characterize only the earliest-formed (hypogene) alloy, whereas archaeology-focused studies should entail analyses of bulk alloy compositions and impurities that may be incorporated during (s)melting. Isotopic-based provenancing alleviates many of these problems but, to date, generates regional rather than locality-specific targets. A dual isotopic–compositional approach is recommended.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)