East Kilbride, United Kingdom
East Kilbride, United Kingdom

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Rice C.M.,University of Aberdeen | Mark D.F.,University of Aberdeen | Selby D.,Durham University | Neilson J.E.,University of Aberdeen | Davidheiser-Kroll B.,Isotope Geosciences Unit
Economic Geology | Year: 2016

The Caledonian orogenic belt of northern Britain hosts some significant quartz vein-hosted gold deposits. However, as in orogenic belts worldwide, the relationship between gold mineralization and regional tectonics, magmatism, and metamorphism is a matter of debate. This is primarily due to the absence of precise temporal constraints for the mineralization. Here we report high-precision 40Ar/39Ar and Re-Os ages for the largest known gold deposit at Curraghinalt (2.7 Moz) in Northern Ireland and use these ages to constrain the regional geologic setting of the gold mineralization and establish a genetic model. The gold resource is contained in a suite of quartz sulfide veins hosted by Neoproterozoic (Dalradian) metasediments, which have been thrust over an Ordovician island arc (Tyrone Igneous Complex). Previous studies recognized two generations of gold sulfide mineralization and we have identified a third in microshears that cut the veins. In the absence of precise geochronological data, mineralization ages from Ordovician to Carboniferous have been proposed. We have dated muscovite (40Ar/39Ar) in quartz vein-hosted clasts of Dalradian wall rock to 459.3 ± 3.4 Ma (all 40Ar/39Ar and Re-Os ages herein are reported at the 2s confidence level including all sources of uncertainty), an age that we interpret as representing the regional cooling path and which provides a maximum age constraint for all gold mineralization. This is consistent with the quartz veins postdating the end of main-stage deformation in the Grampian event of the Caledonian orogeny (ca. 465 Ma). Molybdenite (Re-Os) and sericite (40Ar/39Ar) from the newly identified gold-bearing microshears (third generation of gold mineralization) yield indistinguishable Re-Os models and 40Ar/39Ar ages, with a combined age of 455.8 ± 3.0 Ma. The radioisotope ages and field evidence temporally constrain gold mineralization at Curraghinalt to the lower Late Ordovician. Data show that the gold mineralization was emplaced during the Grampian event of the Caledonian orogeny. The ca. 10 Ma maximum possible mineralization interval (462.7-452.8 Ma) for all three episodes of gold emplacement is postpeak metamorphism and main deformation, coinciding with a period of rapid uplift and extensional tectonics following orogenic collapse. While previous studies have suggested the involvement of magmatic fluids in the deposition of the primary gold resource, the absence of magmatism throughout most of the mineralization interval and the nature of the geologic setting suggest that crustal orogenic fluids should also be considered. Overall Curraghinalt displays most of the characteristics of orogenic gold deposits but also some important differences, which may be explained by the geologic setting. The timing of mineralization at Curraghinalt broadly coincides with the shift from compressional to extensional tectonics. The extensional regime, rapid uplift, and a crustal profile comprising metasediments overlying a still hot island arc were ideal for creating large and long-lasting hydrothermal systems deriving heat, metals, and some of the fluids from the underlying arc. © 2016 by Economic Geology.


Fallick A.E.,Isotope Geosciences Unit | Melezhik V.A.,Geological Survey of Norway | Brasier A.T.,Isotope Geosciences Unit | Brasier A.T.,University of Aberdeen | Prave A.R.,University of St. Andrews
Precambrian Research | Year: 2016

A variety of carbonates of different geneses, as indicated by petrography and geochemistry, are found throughout 400 m of the volcano-sedimentary rocks of the Zaonega Formation of Palaeoproterozoic age in the Onega basin of Fennoscandia. Following intensive sampling and analysis of varied calcites from drillcore recovered during the ICDP FAR-DEEP program, we report a highly unusual depth distribution of calcite oxygen isotope values (δ18Ocal). Unprecedentedly for such rocks, the δ18Ocal values over the full depth interval of 400 m are strongly linearly correlated with depth (r2 = 0.9015, n = 178). We propose that this is the result of major oxygen isotope resetting through water–rock interaction with a fluid of relatively constant oxygen isotopic composition (δ18Ow). In this model, the observed linear δ18Ocal-depth relationship is then a consequence of the increase in temperature with depth because of the background geothermal gradient. Minor deviations from the overall linear trend are likely due to bed-scale geological factors including locally high impermeability, and oxygen isotope modification of δ18Ow by comparatively intense water–rock interaction. Were the observed δ18Ocal values to have been reset during the greenschist facies Svecofennian metamorphism which affected the rocks at c. 1800 Ma, the implied geothermal gradient of ∼560 °C km−1 is geologically unreasonable and, accordingly, this hypothesis is ruled out. Rather, the δ18Ocal variation of 5‰ over 400 m implies a near-surface depth for the rocks during fluid interaction, and this is consistent with a surface-derived origin of the infiltrating fluid (δ18Ow ∼−13.6‰ for a surface temperature of 15 °C and geothermal gradient of ∼52 °C km−1). It is speculated that the fluid accessed the carbonates from the basin edge by bed-parallel rather than cross-formational flow. There is an intriguing distribution of Na in the sedimentary rocks of the Zaonega Formation. Sodium is relatively abundant in rocks below a certain depth (the lowermost dolostone at ∼258 m), but rare in shallower sequences. It is argued that this distribution did not originate with the basin-scale fluid–rock interaction documented above, but may rather be the result of evaporite dissolution, and subsequent redistribution of soluble elements during fluid flow associated with the syndepositional emplacement of basin-wide igneous rocks. © 2016 Elsevier B.V.


Uher P.,Comenius University | Giuliani G.,French National Center for Scientific Research | Szakall S.,University of Miskolc | Fallick A.,Isotope Geosciences Unit | And 4 more authors.
Geologica Carpathica | Year: 2012

Blue, grey-pink and pink sapphires from the Cerová Highlands, Western Carpathians (southern Slovakia) have been studied using CL, LA-ICP-MS, EMPA, and oxygen isotope methods. The sapphire occurs as (1) clastic heavy mineral in the secondary sandy filling of a Pliocene alkali basaltic maar at Hajnáčka, and (2) crystals in a pyroxenebearing syenite/anorthoclasite xenolith of Pleistocene alkali basalt near Gortva. Critical evaluation of compositional diagrams (Fe, Ti, Cr, Ga, Mg contents, Fe/Ti, Cr/Ga, Ga/Mg ratios) suggests a magmatic origin for clastic blue sapphires with lower Cr and Mg, but higher Fe and Ti concentrations in comparison to the grey-pink and pink varietes, as well as similar compositional trends with blue sapphire from the Gortva magmatic xenolith. Moreover, blue sapphires show similar δ18O values: 5.1 % in the Gortva xenolith, 3.8 and 5.85 % in the Hajnáčka placer, closely comparable to mantle to lower crustal magmatic rocks. On the contrary, pink and grey-pink sapphires show higher Cr and Mg, but lower Fe and Ti contents and their composition points to a metamorphic (metasomatic) origin.


Feneyrol J.,University of Lorraine | Ohnenstetter D.,University of Lorraine | Giuliani G.,University of Lorraine | Fallick A.E.,Isotope Geosciences Unit | And 3 more authors.
Canadian Mineralogist | Year: 2012

The Namalulu tsavorite (vanadian grossular up to 0.80 wt.% V 2O 3) deposit in northeastern Tanzania occurs in metasomatized graphitic gneiss overlain by a dolomitic marble unit. These rocks belong to the Neoproterozoic metamorphic Mozambique Belt. The graphitic gneiss is composed of quartz, V-bearing kyanite (up to 0.4 wt.% V2O3), V-bearing muscovite (up to 1.0 wt.% V2O3), V-bearing rutile (up to 1.9 wt.% V2O3), and graphite. This rock is affected by calcic metasomatism precipitating calcite and mobilizing the elements for tsavorite crystallization in quartz-calcite veins. The dolomitic marble unit hosts an anhydrite-gypsum-dolomite lens (lens I) and a calcite-scapolite-diopside-sulfides-graphite lens (lens II). This last unit is characterized by the presence of F-bearing minerals (tremolite, phlogopite, tainiolite, titanite), with up to 9.4 wt.% F for tainiolite, and Ba-bearing minerals (feldspar, phlogopite), with up to 7.5 wt.% BaO for feldspar. Lithium (up to 2.0 wt.% Li 2O) and boron (up to 110 ppm) are also present in tainiolite, as well as in F-bearing tremolite. This is the first description of metamorphic tainiolite. The protoliths of the Namalulu rocks correspond to organic-rich black shales, pure and magnesian carbonates, and evaporites. They were presumably deposited in a marine coastal sabkha located at the eastern margin of the Congo-Kalahari cratons bounded by the Mozambique Ocean during early Neoproterozoic. Tsavorite formed in the Ca-metasomatized gneiss during amphibolite facies retrograde metamorphism at 5.6-6.7 kbar and 630 ± 30 °C.


Magna T.,ETH Zurich | Magna T.,University of Munster | Magna T.,Czech Geological Survey | Wiechert U.,ETH Zurich | And 5 more authors.
Geochimica et Cosmochimica Acta | Year: 2011

Lithium (Li) isotopes are thought to provide a powerful proxy for the recycling of crustal material, affected by low temperature alteration, through the mantle. We present Li isotope compositions for basaltic volcanic rocks from Hengill, Iceland, and Jan Mayen in order to examine possible links between ocean island volcanism and recycled oceanic crust and to address recent suggestions that mantle 3He/4He is also related to recycling of ancient slabs. Basaltic glasses spanning a range of chemical enrichment from the Hengill fissure system define an inverse correlation between δ7Li (3.8-6.9‰) and 3He/4He (12-20 RA). The high-3He/4He basalts have low δ18O as well as excess Eu and high Nb/U, but carry no Li isotope evidence of being the product of recycling of altered slab or wedge material. In fact, there is no clear correlation between Li or He isotopes on the one hand and any of the other fingerprints of recycled slab components. The low-3He/4He samples do have elevated Nb/U, Sr/Nd, positive Eu anomalies and high δ7Li (∼6.9‰), providing evidence of a cumulate-enriched source that could be part of an ancient altered ocean floor slab. Basalts from Jan Mayen are characterized by large degrees of enrichment in incompatible trace elements typical of EM-like basalts but have homogeneous δ7Li typical of depleted mantle (3.9-4.7‰) providing evidence for a third mantle source in the North Atlantic. It appears that oceanic basalts can display a wide range in isotope and trace element compositions associated with recycled components whilst exhibiting no sign of modern surface-altered slab or wedge material from the Li isotope composition. © 2010 Elsevier Ltd.


Herrington R.J.,Natural History Museum in London | Scotney P.M.,University of Southampton | Scotney P.M.,Waste Recycling Group Ltd | Roberts S.,University of Southampton | And 2 more authors.
Gondwana Research | Year: 2011

Whole-rock 87Sr/86Sr and δ18O analyses of volcanic rocks and 3He/4He analyses of sulphides and sulphates from mineralized rocks on Wetar, Indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted Australian craton to the south. These new data support the idea of progressive source contamination with precisely dated events showing that Wetar Island hosts the most extreme examples of crustal assimilation in the region. The increased continental contamination occurs during the Pliocene (Zanclian to Piacenzian) during distinct magmatic events between 5 and 4Ma, and at 2.4Ma when 87Sr/86Sr ratios in unaltered lavas, with whole-rock δ18O values between 5.7 and 9.6‰, increase from 0.707484 to extreme radiogenic values of 0.711656. The earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on Wetar. Samples from this yield radiogenic 3He/4He ratios between 0.5 and 1.4 R/RA, similar to the data from volcanic rocks on nearby Romang. The later magmatic event coincides with the arrival of the Australian Continental Margin at the subduction zone along the Banda arc. Progressive incorporation of continental-sourced components into the source region below the Wetar Island edifice coincides with the formation of gold-rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile. © 2010.


Petersson J.,Gothenburg University | Fallick A.E.,Isotope Geosciences Unit | Broman C.,University of Stockholm | Eliasson T.,Geological Survey of Sweden
Lithos | Year: 2014

A number of episyenite occurrences within the Neoproterozoic Bohus granite, southwestern Sweden, were investigated in order to elucidate their genesis and past associated fluid regimes. Fluid inclusion microthermometry and O-H isotope systematics integrated with information from detailed mineralogy reveal a paragenetic succession within individual episyenites, including: (1) albitization of plagioclase, accompanied by dissolution of quartz, with a vuggy feldspathic reservoir rock as a result (2) resumed infilling and replacement of igneous Fe-Ti-phases by various phyllosilicates (muscovite, chlorite and/or illite)±hematite assemblages and quartz, due to rift-related reactivation and (3) sporadic argillitic alteration and precipitation of fracture calcite. Although sparse owing to later overprinting, there are a few remnants that provide arguments for the involvement of meteoric water, shifted to a more 18O-rich composition, during the initial stage of dissolution and albitization. The main vug-filling phase is proposed to be associated with the ingress of surface-derived fluids with negative δ18O values, during the regional thermal anomaly accompanying the development of the nearby Permo-Carboniferous Oslo Rift at ~250Ma. The fluids involved during this event show consistently a marked, but variable 18O-depletion and resemble the modern-day shield brines, with modal salinities of 11-24 wt.% NaCl eq., low NaCl/(NaCl+CaCl2) weight ratios and no discernible CO2. Even though no P-T path can be defined for this phase, the fluid pressure can be restricted to 0.3-0.8kbar and temperature to 160-280°C, assuming a sedimentary overburden of ~3km. An overall variability in the δ18O and δD data can mainly be ascribed to (1) precipitation from fluids affected by isotopic exchange with the host rock and/or sedimentary overburden under highly variable water/rock ratios, and to some extent with (2) varying degrees of partial re-equilibration during subsequent hydrothermal interaction at lower temperatures, as emphasized by the presence of fluid inclusions of suspected secondary origin in quartz. © 2014 Elsevier B.V.


Giuliani G.,British Petroleum | Fallick A.E.,Isotope Geosciences Unit | Feneyrol J.,British Petroleum | Ohnenstetter D.,British Petroleum | And 2 more authors.
Mineralium Deposita | Year: 2011

The combination of oxygen isotope composition with V-Cr-Mn trace element concentrations of V-bearing garnets (tsavorites) originating from the main deposits of the Neoproterozoic Mozambique Metamorphic Belt is reported for the first time. The database enables the identification of the geological and geographical sources of the main productive areas from northern and southern Tanzania, Kenya, and Madagascar. Three consistent sets of δ18O values between 9. 5‰ and 11. 0‰, 11. 6‰ and 14. 5‰, and 15. 5‰ and 21. 1‰ have been recognized for primary deposits hosted in graphitic gneisses related to the Neoproterozic metasedimentary series. The δ18O value of tsavorite is a good tracer of the environment of its formation; the δ18O of the fluid in equilibrium with tsavorite was buffered by the host rock during metamorphism and fluid-rock interaction. This study is a first step in characterizing the geochemistry of gem tsavorite from most of the deposits and occurrences worldwide. © 2011 Springer-Verlag.


Giuliani G.,University Paul Sabatier | Giuliani G.,University of Lorraine | Pivin M.,Isotope Geosciences Unit | Fallick A.E.,Isotope Geosciences Unit | And 3 more authors.
Comptes Rendus - Geoscience | Year: 2015

Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ18O values in the range 4.3 to 5.4‰ (N=10) with a mean of 4.9±0.4‰, and rubies from 5.5 to 5.6‰ (N=3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ18O values from 4.6 to 5.2 ‰, with a mean of 4.9±0.2‰ (N=9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ18O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface. © 2015 Académie des sciences.


Unal Imer E.,Middle East Technical University | Gulec N.,Middle East Technical University | Kuscu T.,Muǧla University | Fallick A.E.,Isotope Geosciences Unit
Ore Geology Reviews | Year: 2013

Two epithermal gold deposits (Kartaldaǧ and Madendaǧ) located in NW Turkey have been characterized through the detailed examinations involving geologic, mineralogical, fluid inclusion, stable isotope, whole-rock geochemistry, and geochronology data. The Kartaldaǧ deposit (0.01-17.65ppm Au), hosted by Eocene dacite porphyry, is associated with four main alteration types with characteristic assemblage of: i) chlorite/smectite-illite±kaolinite, ii) quartz-kaolinite, iii) quartz-alunite-pyrophyllite, iv) quartz-pyrite, the last being characterized by three distinct quartz generations comprising massive/vuggy (early), fine-medium grained, vug-lining (early), and banded, colloform, comb (late) textures. Observed sulfide minerals are pyrite, covellite, and sphalerite. Oxygen and sulfur isotope analyses, performed on quartz (δ18O(quartz): 7.93 to 8.95‰ and calculated δ18O(H2O): -7.95 to 1.49‰) and pyrite (δ34S(pyrite): -4.8‰ and calculated δ34S(H2S): -6.08 to -7.20‰) separates, suggest a meteoric water source for water in the hydrothermal fluid, and an igneous source for the sulfur dissolved in ore-related fluids. Microthermometric analyses of primary fluid inclusion assemblages performed on quartz (late quartz generation) yield temperatures (Th) dominantly in the range of 245-285°C, and generally low salinity values at 0 to 1.7wt.% NaCl eq. Based on the quartz textures and the associated base metal concentrations, along with fluid inclusion petrography, the early vug-lining quartz is considered to have been associated with the mineralization possibly through a boiling and a late mixing process at >285°C. The Madendaǧ deposit (0.27-20.60ppm Au), hosted by Paleozoic mica schists, is associated with two main alteration types: sericite-illite-kaolinite, and quartz-pyrite dominated by two distinct quartz generations i) early colloform, comb and banded quartz and ii) late quartz, forming the cement in hydrothermal breccia. Whereas oxygen isotope analyses of quartz (δ18O(quartz): 9.55 to 18.19‰ and calculated δ18O(H2O): -2.97 to 5.54‰) suggest varying proportions of meteoric and magmatic sources for the ore bearing fluid, sulfur isotope ratios (δ34S(pyrite): -2.2‰ and calculated δ34S(H2S): (-3.63) to (-3.75) ‰) point to an essentially magmatic source for sulfur with or without contribution from sedimentary sources. Microthermometric analysis carried out on primary fluid inclusion populations of a brecciated sample (early quartz), give a temperature (Th) range of 235-255°C and 0.0 to 0.7wt.% NaCl eq. salinity. Based on the textural relationship, base metal and high gold contents, the ore precipitation stage is associated with late stage quartz formation via a possible boiling process. The presence of alunite, pyrophyllite and kaolinite, vuggy quartz and covellite suggest a high-sulfidation type of epithermal deposit for Kartaldaǧ. On the other hand, Madendaǧ is identified as an adularia-sericite type owing to the presence of significant sericite, neutral pH clays (mostly illite, chlorite/smectite, and kaolinite), low temperature quartz textures (e.g., colloform, comb, and banded quartz), and limited sulfide minerals. Given the geographical proximity of Kartaldaǧ and Madendaǧ deposits, the similar temperature and salinity ranges obtained from their fluid inclusions, and the similar ages of igneous rocks in both deposits (Kartaldaǧ: 40.80. ±0.36 to 42.19. ±0.45. Ma, Madendaǧ: 43.34. ±0.85. Ma) the mineralizing systems in both deposits are considered to be genetically related. © 2013 Elsevier B.V.

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