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The stratiform magnetite ore bodies occur in calcareous-volcaniclastic rocks, exhibit layered textures, and contain primary seafloor haematite ores. The sequence of mineral formation reflects the diagenetic to metamorphic evolution of the iron ores: finely dispersed haematite-1; tabular haematite-2 crystals; pseudomorphic magnetite-1 after haematite-2; zoned magnetite-2 crystals with relict haematite-2 (or magnetite-1); thin oscillatory zoned magnetite-3 crystals; and magnetite-4 porphyroblasts. A gangue assemblage of Fe-rich and Fe–Mg chlorite, illite, quartz, albite, carbonates, rutile, and apatite with rare monazite, xenotime, and zircon occurs in ore and calcareous-volcaniclastic layers. The gangue clasts are replaced by haematite and further by magnetite. Low siderite delta13C values indicate the presence of primary organic matter, siderite delta18O values are evidence of isotopic exchange between minerals and fluids during metamorphism, and negative delta34S values for pyrite probably indicate derivation of sulphur from organic matter in clastic sedimentary rocks. All element contents (except V and Ga) are significantly higher in the inner zone of magnetite (magnetite-1) than the outer zone (magnetite-2). It is proposed that the calcareous-volcaniclastic material, which was altered during submarine weathering and leaching, was a major source of iron to form oxides. The paragenetic sequence, along with mineralogical and geochemical data, all suggest the Glubochenskoe deposit can be characterised as a volcanic-sedimentary type of banded iron formation., The stratiform magnetite ore bodies occur in calcareous-volcaniclastic rocks, exhibit layered textures, and contain primary seafloor haematite ores. The sequence of mineral formation reflects the diagenetic to metamorphic evolution of the iron ores: finely dispersed haematite-1; tabular haematite-2 crystals; pseudomorphic magnetite-1 after haematite-2; zoned magnetite-2 crystals with relict haematite-2 (or magnetite-1); thin oscillatory zoned magnetite-3 crystals; and magnetite-4 porphyroblasts. A gangue assemblage of Fe-rich and Fe–Mg chlorite, illite, quartz, albite, carbonates, rutile, and apatite with rare monazite, xenotime, and zircon occurs in ore and calcareous-volcaniclastic layers. The gangue clasts are replaced by haematite and further by magnetite. Low siderite delta13C values indicate the presence of primary organic matter, siderite delta18O values are evidence of isotopic exchange between minerals and fluids during metamorphism, and negative delta34S values for pyrite probably indicate derivation of sulphur from organic matter in clastic sedimentary rocks. All element contents (except V and Ga) are significantly higher in the inner zone of magnetite (magnetite-1) than the outer zone (magnetite-2). It is proposed that the calcareous-volcaniclastic material, which was altered during submarine weathering and leaching, was a major source of iron to form oxides. The paragenetic sequence, along with mineralogical and geochemical data, all suggest the Glubochenskoe deposit can be characterised as a volcanic-sedimentary type of banded iron formation.

Recent Ar–Ar and U–Pb zircon geochronology from across the British and Irish Caledonides has revealed a prolonged period of arc-ophiolite formation (c. 514–464 Ma) and accretion (c. 490–470 Ma) to the Laurentian margin during the Grampian orogeny. The Slieve Gallion Inlier of Northern Ireland, an isolated occurrence of the Tyrone Volcanic Group, records the development of a peri-Laurentian island arc–backarc and its obduction to an outboard microcontinental block. Although a previous biostratigraphic age constraint provides a firm correlation of at least part of the volcanic succession to the Ca1 Stage of the Arenig (c. 475–474 Ma), there is uncertainty on its exact statigraphic position in the Tyrone Volcanic Group. Earliest magmatism is characterized by light rare earth element (LREE) depleted island-arc tholeiite. Overlying deposits are dominated by large ion lithophile and LREE-enriched, hornblende-phyric and feldspathic calc-alkaline basaltic andesites and andesitic tuffs with strongly negative ϵNdt values. Previously published biostratigraphic age constraints, combined with recent U–Pb zircon geochronology and new petrochemical correlations, suggest that the Slieve Gallion Inlier is equivalent to the lower Tyrone Volcanic Group. Temporal and geochemical correlations between the Slieve Gallion Inlier and Charlestown Group of Ireland suggest that they may be part of the same arc system, which was accreted at a late stage (c. 470 Ma) in the Grampian orogeny. A switch from tholeiitic volcanism to calc-alkaline dominated activity within the Lough Nafooey Group of western Ireland occurred prior to c. 490 Ma, some 15–20 Myr earlier than at Tyrone and Charlestown.

The Tyrone Plutonic Group of Northern Ireland represents the upper portions of a tectonically dissected suprasubduction-zone ophiolite accreted to the composite Laurentian margin during the Middle Ordovician. Understanding its development and relationship to the Tyrone Central Inlier, an outboard fragment of relatively high-grade, peri-Laurentian continental crust, is essential for reconstructing the closure of the Iapetus Ocean. The Tyrone Plutonic Group is composed of tectonized layered, isotropic and pegmatitic gabbros, sheeted dolerite dykes and rare pillow lavas. New U–Pb zircon thermal ionization mass spectrometry geochronology has yielded an age of 483.68 ± 0.81 Ma from pegmatitic gabbro. Geochemical characteristics, Nd and Sr isotope systematics, and zircon inheritance indicate that the Tyrone Plutonic Group formed above a north-dipping subduction zone, by the propagation of a spreading centre into a microcontinental block. Synkinematic, calc-alkaline tonalitic to granitic material preserved in the contact zone between the Tyrone Plutonic Group and the Tyrone Central Inlier has produced pressure estimates of 2.3–4.0 ± 0.6 kbar and temperatures of 525–610 °C. Coeval arc–ophiolite accretion at c. 470 Ma may explain how sillimanite-grade metamorphic conditions were reached locally in the underlying Tyrone Central Inlier. Strong temporal, geochemical and lithological similarities exist to the Annieopsquotch Ophiolite Belt of Newfoundland.

An intrusion of trachy-andesite, representative of a newly discovered suite of high-K–Ba–Sr, calc-alkaline minor intrusions (termed herein the Sperrin Mountains suite), hosted within the Grampian terrane in the north of Ireland, has been dated by U–Pb zircon at 426.69 ± 0.85 Ma (mid-Silurian; Wenlock–Ludlow boundary). Geochemistry reveals a close association with the Fanad, Ardara and Thorr plutons of the Donegal Batholith and the Argyll and Northern Highlands Suite of Scotland. The deep-seated Omagh Lineament appears to have limited eastward propagation of the Sperrin Mountains suite from beneath the main centre of granitic magmatism in Donegal. A Hf depleted mantle model age (TDMHf) of c. 800 Ma for trachy-andesite zircons indicates partial melting from a source previously separated from the mantle. Whole-rock geochemistry of the suite is consistent with a model of partial melting, triggered by slab break-off, following thrusting of Ganderia–Avalonia under the Southern Uplands–Down–Longford accretionary prism (i.e. Laurentian margin). The new age constrains the timing of this event in the north of Ireland and is consistent with the petrogenesis of Late Caledonian high-K granites, appinites and minor intrusions across the Caledonides of northern Britain and Ireland.

The Tyrone Igneous Complex is one of the largest areas of ophiolitic and arc-related rocks exposed along the northern margin of Iapetus within the British and Irish Caledonides. New U–Pb zircon data and regional geochemistry suggest that the Tyrone Plutonic Group represents the uppermost portions of a c. 480 Ma suprasubduction-zone ophiolite accreted onto an outboard segment of Laurentia prior to 470.3 ± 1.9 Ma. The overlying Tyrone Volcanic Group formed as an island arc that collided with the Laurentian margin during the Grampian phase of the Caledonidan orogeny. Early magmatism is characterized by transitional to calc-alkaline, light REE (LREE)-enriched island-arc signatures, with an increasing component of continentally derived material up sequence. Tholeiitic rhyolites with flat to U-shaped REE profiles and LREE-depleted basalts, located stratigraphically below a c. 473 Ma rhyolite of the upper Tyrone Volcanic Group, suggest initiation of intra-arc rifting at c. 475 Ma. Metamorphic cooling ages from the Tyrone Central Inlier imply arc–continent collision before 468 ± 1.4 Ma, with the emplacement of the Tyrone Volcanic Group onto the margin. A suite of 470.3 ± 1.9 Ma to 464.3 ± 1.5 Ma calc-alkaline intrusions are associated with the continued closure of Iapetus. Supplementary material: Geochemical data and petrography are available at http://www.geolsoc.org.uk/SUP18467.

The characterisation can, with appropriate treatment of data, be applied to bulk lithogeochemical data utilising principle component analysis to map dominant pyrite stages at Lisheen mine, and thus offers a new target discriminator in regional exploration. Pyrite occurs temporally and spatially throughout the stratiform carbonate-replacement deposit, overlying the sphalerite-galena ore body and forming a distal halo in the hanging-wall of fluid-controlling structures. Petrographic observations of samples from 10 vertical diamond drill-holes along a section perpendicular to a major normal fault (feeder conduit) transecting the Derryville orebody support multiple stages of dissolution and crystallisation involving pyrite±marcasite-sphalerite-galena. Early pyrite is spongy in appearance and defined as disseminated aggregated framboids; this was subsequently recrystallised into disseminated botryoidal pyrite and further recrystallised by latter hydrothermal phases. Distal to feeder structures, LA-ICP-MS data shows these two early pyrite phases are deficient in elements associated with ore-stage mineralisation (Zn-Cd-Cu-As-Tl) within the core. Both have elevated Co, Ni, Mn, Pb and Sb and a further central zone (in preserved framboids) identified by increased Sn. Distal, early-stage pyrite shows evidence of a later rim (Py4) with distinctively higher As-Tl concentrations associated with minor crystals of sphalerite. Within the ore zone, preserved early pyrite occurrs as radiating agglomerated botryoidal forms, subsequently recrystallised into euhedral crystals (Py3) with additional, fine, massive Py4 replacing the accompanying carbonate phases. As and Co were concentrated in the rims of early, hydrothermally altered diagenetic pyrite similar to the distal sample; large euhedral recrystallised pyrite concentrated Mn-Cu preferentially. Massive finely-crystalline Py4 in the proximal sample is As-rich, yet poor in many other trace elements analysed. Bladed marcasite overgrows, The characterisation can, with appropriate treatment of data, be applied to bulk lithogeochemical data utilising principle component analysis to map dominant pyrite stages at Lisheen mine, and thus offers a new target discriminator in regional exploration. Pyrite occurs temporally and spatially throughout the stratiform carbonate-replacement deposit, overlying the sphalerite-galena ore body and forming a distal halo in the hanging-wall of fluid-controlling structures. Petrographic observations of samples from 10 vertical diamond drill-holes along a section perpendicular to a major normal fault (feeder conduit) transecting the Derryville orebody support multiple stages of dissolution and crystallisation involving pyrite±marcasite-sphalerite-galena. Early pyrite is spongy in appearance and defined as disseminated aggregated framboids; this was subsequently recrystallised into disseminated botryoidal pyrite and further recrystallised by latter hydrothermal phases. Distal to feeder structures, LA-ICP-MS data shows these two early pyrite phases are deficient in elements associated with ore-stage mineralisation (Zn-Cd-Cu-As-Tl) within the core. Both have elevated Co, Ni, Mn, Pb and Sb and a further central zone (in preserved framboids) identified by increased Sn. Distal, early-stage pyrite shows evidence of a later rim (Py4) with distinctively higher As-Tl concentrations associated with minor crystals of sphalerite. Within the ore zone, preserved early pyrite occurrs as radiating agglomerated botryoidal forms, subsequently recrystallised into euhedral crystals (Py3) with additional, fine, massive Py4 replacing the accompanying carbonate phases. As and Co were concentrated in the rims of early, hydrothermally altered diagenetic pyrite similar to the distal sample; large euhedral recrystallised pyrite concentrated Mn-Cu preferentially. Massive finely-crystalline Py4 in the proximal sample is As-rich, yet poor in many other trace elements analysed. Bladed marcasite overgrows

Despite being distal to the main orebody, the 400 000 t 20% Zn, 1.6% Pb Island Pod is still very high-grade and hosts the best-quality ore at Lisheen while showing rapid lateral variation on a small scale, less than10 m, where adjacent boreholes can have zinc concentration differences of c.40%. It also displays a weaker structural control than elsewhere in the southern Irish orefield. The basic sulphide mineralogy for the Island Pod is pyrite, sphalerite and galena, with multiple generations of each observed along with several carbonate phases. Dendritic pyrite/galena, colloform sphalerite and sphalerite/dolomite intergrowth textures suggest an early, rapid mineralisation event, from a supersaturated fluid in a nonequilibrium depositional environment, marking the onset of fluid mixing. As the mineralising system came closer to equilibrium, sulphide textures changed, reflecting a slower precipitation environment. 34-S isotope data suggest a dominantly bacteriogenic source for S in the Island Pod, with minor positive 34 delta-S values recorded. This is consistent with the distal position of the orebody, away from the feeder ramp-relay fault system at Lisheen which is thought to have introduced metal-bearing hydrothermal fluids with 34 delta-S greater than 0. In addition, the orebody and its halo have very similar 34 delta-S values, signifying that a lack of hydrothermal sulphur is not what led to the halo being sub-economic; this may suggest that other factors were responsible for ore grade, such as the availability of bacteriogenic sulphur or stratigraphic controls such as the nature of host-rock permeability., Despite being distal to the main orebody, the 400 000 t 20% Zn, 1.6% Pb Island Pod is still very high-grade and hosts the best-quality ore at Lisheen while showing rapid lateral variation on a small scale, less than10 m, where adjacent boreholes can have zinc concentration differences of c.40%. It also displays a weaker structural control than elsewhere in the southern Irish orefield. The basic sulphide mineralogy for the Island Pod is pyrite, sphalerite and galena, with multiple generations of each observed along with several carbonate phases. Dendritic pyrite/galena, colloform sphalerite and sphalerite/dolomite intergrowth textures suggest an early, rapid mineralisation event, from a supersaturated fluid in a nonequilibrium depositional environment, marking the onset of fluid mixing. As the mineralising system came closer to equilibrium, sulphide textures changed, reflecting a slower precipitation environment. 34-S isotope data suggest a dominantly bacteriogenic source for S in the Island Pod, with minor positive 34 delta-S values recorded. This is consistent with the distal position of the orebody, away from the feeder ramp-relay fault system at Lisheen which is thought to have introduced metal-bearing hydrothermal fluids with 34 delta-S greater than 0. In addition, the orebody and its halo have very similar 34 delta-S values, signifying that a lack of hydrothermal sulphur is not what led to the halo being sub-economic; this may suggest that other factors were responsible for ore grade, such as the availability of bacteriogenic sulphur or stratigraphic controls such as the nature of host-rock permeability.

Preliminary Pb isotope maps of Ireland have been produced with data compiled from 11 per-reviewed sources, complemented by new data from galena occurrences across Ireland. In this preliminary dataset, variations in 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and source-rock 238U/204Pb correspond well with known terrane boundaries in most areas, with significant differences across the Iapetus Suture. Apparent 207Pb/204Pb vs 206Pb/204Pb mixing trends are deposit-specific and reflect the derivation of Pb from two sources, probably Laurentia and Ganderia. Pb isotope data from galena occurrences in Newfoundland has also been compiled and will be discussed with respect to regional terrane correlations across the Caledonian-Appalachian orogen., Preliminary Pb isotope maps of Ireland have been produced with data compiled from 11 per-reviewed sources, complemented by new data from galena occurrences across Ireland. In this preliminary dataset, variations in 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and source-rock 238U/204Pb correspond well with known terrane boundaries in most areas, with significant differences across the Iapetus Suture. Apparent 207Pb/204Pb vs 206Pb/204Pb mixing trends are deposit-specific and reflect the derivation of Pb from two sources, probably Laurentia and Ganderia. Pb isotope data from galena occurrences in Newfoundland has also been compiled and will be discussed with respect to regional terrane correlations across the Caledonian-Appalachian orogen.