Change search
Refine search result
1 - 31 of 31
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Alasdair, Skelton
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fredrik, Arghe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Spatial coupling between spilitization and carbonation ofbasaltic sills in SW Scottish Highlands: evidence of amineralogical control of metamorphic fluid flow2011In: Geofluids, ISSN 1468-8115, E-ISSN 1468-8123, Vol. 11, no 3, p. 245-259Article in journal (Refereed)
    Abstract [en]

    In a geochemical and petrological analysis of overprinting episodes of fluid–rock interaction in a well-studied metabasaltic sill in the SW Scottish Highlands, we show that syn-deformational access of metamorphic fluids and consequent fluid–rock interaction is at least in part controlled by preexisting mineralogical variations. Lithological and structural channelling of metamorphic fluids along the axis of the Ardrishaig Anticline, SW Scottish Highlands, caused carbonation of metabasaltic sills hosted by metasedimentary rocks of the Argyll Group in the Dalradian Supergroup. Analysis of chemical and mineralogical variability across a metabasaltic sill at Port Cill Maluaig shows that carbonation at greenschist to epidote–amphibolites facies conditions caused by infiltration of H2O-CO2 fluids was controlled by mineralogical variations, which were present before carbonation occurred. This variability probably reflects chemical and mineralogical changes imparted on the sill during premetamorphic spilitization. Calculation of precarbonation mineral modes reveals heterogeneous spatial distributions of epidote, amphibole, chlorite and epidote. This reflects both premetamorphic spilitization and prograde greenschist facies metamorphism prior to fluid flow. Spilitization caused albitization of primary plagioclase and spatially heterogeneous growth of epidote ± calcic amphibole ± chlorite ± quartz ± calcite. Greenschist facies metamorphism caused breakdown of primary pyroxene and continued, but spatially more homogeneous, growth of amphibole + chlorite ± quartz. These processes formed diffuse epidote-rich patches or semi-continuous layers. These might represent precursors of epidote segregations, which are better developed elsewhere in the SW Scottish Highlands. Chemical and field analyses of epidote reveal the evidence of local volume fluctuations associated with these concentrations of epidote. Transient permeability enhancement associated with these changes may have permitted higher fluid fluxes and therefore more extensive carbonation. This deflected metamorphic fluid such that its flow direction became more layer parallel, limiting propagation of the reaction front into the sill interior.

  • 2.
    Boskabadi, Ahmad
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iairn K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stern, R. J.
    Azer, M. K.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mohamed, F. H.
    Majka, J.
    Carbonatite crystallization and alteration in the Tarr carbonatite-albitite complex, Sinai Peninsula, Egypt2013In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. - 239, p. 24-41Article in journal (Refereed)
    Abstract [en]

    Carbonate dykes occurring in the Arabian-Nubian Shield (ANS) are clearly intrusive in origin and carbonatites according to the IUGS classification, yet previous investigations refer to them as “intrusive carbonates”, due mainly to their low Sr, Ba, Nb, Y, Th and rare earth element (REE) contents. The Tarr carbonatite albitite complex (TCA) in SE Sinai, Egypt contains a series of small (<1.2 km2) albitite intrusions surrounded by small veins and dykes of carbonatite, which occur predominantly in a narrow zone of brecciation surrounding the intrusions. Fennitic alteration surrounding TCA has been reported but there is little consensus on the extent and origin of this alteration. Fennitic alteration surrounding the TCA carbonatites is not abundant. Alteration is dominated by precipitation of carbonates in the breccia zone surrounding the albitite intrusion with associated actinolite, chlorite, sericite and epidote. Geochemical compositions are consistent with addition of carbonates and associated secondary minerals because the altered rocks contain higher CaO, MgO, Fe2O3 and MnO and lower SiO2, Al2O3, Na2O and K2O compared to their less altered rocks. Fluid inclusion investigations show that the carbonatite magma contained a high-salinity H2O–CO2–NaCl–CaCl2 fluid, although the lack of fennitic alteration implies that this fluid was not abundant. The crystallization conditions of the carbonatite dykes and carbonatite matrix in the breccia zones have been constrained using Zr-in-rutile thermometry and fluid inclusion microthermometry. Crystallization of the carbonatite in the dykes and in the breccia zone occurred between 565 ± 38 °C and 420–480 °C, respectively and at 0.75–1.3 kbar, which corresponds to a depth of 2.8–4.9 km. Rutile hosted within the carbonatite crystallized earlier at high temperature and the carbonate matrix crystallized later after cooling. Immiscible fluid from carbonatite magma would have altered the surrounding country rocks at lower temperature (between 400 °C and 150 °C deduced from the fluid inclusion thermometry) after the intrusion of the carbonatite melt.

  • 3.
    Boskabadi, Arman
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Texas at Dallas, USA.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Boyce, Adrian
    Teagle, Damon A. H.
    Cooper, Matthew J.
    Azer, Mokhles K.
    Stern, Robert J.
    Mohamed, Fathy H.
    Majka, Jaroslaw
    Carbonate alteration of ophiolitic rocks in the Arabian-Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits2017In: International Geology Review, ISSN 0020-6814, E-ISSN 1938-2839, Vol. 59, no 4, p. 391-419Article, review/survey (Refereed)
    Abstract [en]

    Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between -8.1 parts per thousand and -6.8 parts per thousand for delta C-13, +6.4 parts per thousand and +10.5 parts per thousand for delta O-18, and Sr-87/Sr-86 of 0.7028-0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2 +/- CH4 +/- N-2) and aqueous-carbonic (H2O-NaCl-CO2 +/- CH4 +/- N-2) low salinity fluid, with trapping conditions of 270-300 degrees C and 0.7-1.1kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite-antigorite transition.

  • 4. Cave, Ben J.
    et al.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Craw, Dave
    Large, Ross R.
    Thompson, Jay M.
    Johnson, Sean C.
    A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand2017In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 52, no 4, p. 515-537Article in journal (Refereed)
    Abstract [en]

    The orogenic gold deposits of the Otago Schist, New Zealand, are enriched in a variety of trace elements including Au, As, Ag, Hg, W and Sb. We combine laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) traverses and images to show that detrital rutile is the most important host mineral for W in the subgreenschist facies rocks. Furthermore, the prograde metamorphic recrystallisation of detrital rutile to titanite releases significant amounts of W (potentially 0.41 g/tonne of rock). Scheelite development closely follows the progression of this W-liberating reaction. Scheelite micrograins form early within the fabric of the rock evolving to locally and regionally sourced scheelite-bearing veins. Scheelite from syn-metamorphic veins at Fiddlers Flat and Lake HAwea shows distinct differences in composition compared with scheelite from late-metamorphic veins at the Macraes Mine, the latter of which is enriched in REEs, Y and Sr. We suggest that the scheelite at Macraes became enriched due to the liberation of these elements during alteration of the Ca-silicate minerals epidote and titanite by the ore-forming fluid. These results are supportive of recent models for orogenic gold mineralisation in the Otago Schist, whereby prograde metamorphic recrystallisation of diagenetic or detrital metal-rich mineral phases (pyrite to pyrrhotite: Au, As, Ag, Hg and Sb; rutile to titanite: W) releases significant amounts of metals into the concurrently developing metamorphic fluids that can be subsequently focussed into regional structures and form significant tungsten-bearing orogenic gold deposits.

  • 5.
    Craw, Dave
    et al.
    Geology Department, University of Otago, Dunedin, New Zealand.
    MacKenzie, Doug
    Geology Department, University of Otago, Dunedin, New Zealand.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Teagle, Damon
    National Oceanography Centre, University of Southampton, Southampton, UK.
    Norris, Richard
    Geology Department, University of Otago, Dunedin, New Zealand.
    Geochemical signatures of mesothermal Au-mineralized late-metamorphic deformation zones, Otago Schist, New Zealand2007In: Geochemistry: Exploration, Environment, Analysis,, Vol. 7, p. 225–232-Article, review/survey (Other (popular science, discussion, etc.))
    Abstract [en]

    Hydrothermal processes along two regional-scale shear zones in the

    Otago Schist were dominated by structurally controlled fluid flow and mineralization in the host schist, with relatively minor quartz vein formation, and mineralized rocks are only subtly different from unmineralized rocks. Most Au in the shear zones is associated with sulphide minerals (pyrite and arsenopyrite) disseminated through the host schist or along microshears. Minor enrichment of Sb, Mo and Bi (ppm level) is detectable in the Hyde-Macraes Shear Zone (HMSZ). Hydrothermal muscovite is slightly more aluminous (1–2 wt%) than metamorphic muscovite in both shear zones. HMSZ muscovite averages >900 ppm N, in contrast to metamorphic muscovite that averages c. 200 ppm N. In both shear zones, rutile has replaced metamorphic titanite and epidote has altered to carbonate and phyllosilicates, but these reactions were nearly isochemical. Structurally controlled hydrothermal graphite in the HMSZ occurs in microshears (up to 3 wt%, above background <0.2 wt%). Alteration in the Rise & Shine Shear Zone (RSSZ) was accompanied by addition of abundant ankerite. The two shear zones have subtly different geochemical signatures and are not directly genetically related. However, As enrichment is a key exploration target for both shear zones.

  • 6. Craw, Dave
    et al.
    Mortensen, Jim
    Mackenzie, Doug
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Contrasting geochemistry of orogenic gold deposits in Yukon, Canada and Otago, New Zealand2015In: Geochemistry: Exploration, Environment, Analysis, ISSN 1467-7873, E-ISSN 2041-4943, Vol. 15, no 2-3, p. 150-166Article in journal (Refereed)
    Abstract [en]

    The Yukon-Tanana Terrane (YTT) of western Yukon Territory in NW Canada and Otago Schist belt (OSB) of South Island, New Zealand share similar geological evolutionary histories as convergent orogenic belts. Both belts host orogenic gold deposits of mainly Jurassic to Early Cretaceous age. Jurassic mineralization in the YTT occurred during convergent orogenesis and stacking of previously-metamorphosed (Palaeozoic) greenschist-amphibolite facies metasediments, metavolcanic rocks, and metagranitoids. Early Cretaceous OSB mineralization occurred in the latter stages of terrane accretion of un-metamorphosed turbidites with minor basaltic rocks. Metamorphism of the OSB turbidites mobilised background levels of Au (0.6-1.3 ppb), As (2-20 ppm), Sb (0.1-1 ppm), and W (< 10 ppm), primarily under greenschist to lower amphibolite facies conditions when diagenetic pyrite (Au c.0.5-2 ppm; As c.500-10000 ppm) transformed to pyrrhotite on a regional scale. In contrast, the previously-metamorphosed YTT rocks had generally low background As contents (1-2 ppm) apart from some As-rich quartzites (up to 100 ppm As). Consequently, there was less As available for orogenic mobilisation, and YTT Au deposits generally have lower concentrations of this pathfinder element compared to the OSB. YTT host rocks, especially metagranitoids, have anomalous levels of Mo (10-300 ppm), and many orogenic deposits contain elevated Mo, locally including molybdenite. OSB turbidites have elevated Mo (2-200 ppm), along with elevated Au and As, in diagenetic pyrite, but this Mo became largely dispersed through the metamorphic pile as metamorphic grade increased and pyrite transformed to pyrrhotite. OSB orogenic deposits have only marginally elevated Mo (c.1 ppm), no molybdenite, and accessory scheelite in these deposits is distinctly Mo-poor. Only minor mobilisation of base metals occurred in these orogenic belts, and orogenic Au deposits contain sparse base metal sulphides. Orogenic deposits in the YTT and OSB differ in that Au (and other associated elements) in many of the orogenic deposits in the YTT was remobilised from relatively local sources (e.g. pre-existing Cu-Mo-Au porphyry or volcanogenic sulphide mineralization) whereas Au in the OSB was mobilised from larger volumes of homogeneous rock at depth.

  • 7.
    Hamisi, Jonathan
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    MacKenzie, Doug
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Blakemore, Hamish
    Zack, Thomas
    Craw, Dave
    Hydrothermal footprint of the Birthday Reef, Reefton goldfield, New Zealand2017In: New Zealand Journal of Geology and Geophysics, ISSN 0028-8306, E-ISSN 1175-8791, Vol. 60, no 2, p. 59-72Article in journal (Refereed)
    Abstract [en]

    The Birthday Reef was the most productive gold producer during historic mining of the Reefton goldfield on the west coast of the South Island of New Zealand. Deep exploration drill holes (up to 1.6 km long) intersected the mineralised quartz vein zone and adjacent hydrothermal alteration halo beneath the historic mine workings. The Paleozoic metasedimentary host rocks contain between 0.5 and 8 ppb Au and between 4 and 30 ppm As, and metamorphic pyrite typically contains 0.1 to 1 ppm Au in solid solution. The Au and As halo, above these background values, extends < 20 m from the Birthday Reef, and other petrographic indicators of alteration are also confined to this narrow envelope. Porphyroblasts of pyrite and arsenopyrite, with minor Au enrichment, grew across the metamorphic cleavage in the alteration halo before emplacement of the Birthday Reef and associated shearing, and reflect an earlier, late metamorphic precursor to the main gold mineralisation phase.

  • 8.
    Hemmingsson, Christoffer
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fru, Ernest Chi
    Evaluation of phosphate-uptake mechanisms by Fe(III) (oxyhydr)oxides in Early Proterozoic oceanic conditions2018In: Environmental Chemistry, ISSN 1448-2517, E-ISSN 1449-8979, Vol. 15, no 1-2, p. 18-28Article in journal (Refereed)
    Abstract [en]

    Banded iron formations (BIF) are proxies of global dissolved inorganic phosphate (DIP) content in Precambrian marine waters. Estimates of Precambrian DIP rely on constraining the mechanisms by which Fe(III) (oxyhydr)oxides scavenge DIP in NaCl solutions mimicking elevated Precambrian marine Si and Fe(II) concentrations. The two DIP binding modes suggested for Early Proterozoic marine waters are (1) surface attachment on pre-formed Fe(III) (oxyhydr)oxides (adsorption), and (2) incorporation of P into actively growing Fe(III) (oxyhydr)oxides (coprecipitation) during the oxidation of Fe(II) to Fe(III) (oxyhydr)oxides in the presence of DIP. It has been suggested that elevated Si concentrations, such as those suggested for Precambrian seawater, strongly inhibit adsorption of DIP in Fe(III) (oxyhydr)oxides; however, recent coprecipitation experiments show that DIP is scavenged by Fe(III) (oxyhydr)oxides in the presence of Si, seawater cations and hydrothermal As. In the present study, we show that the DIP uptake onto Fe(III) (oxyhydr)oxides by adsorption is less than 5 % of DIP uptake by coprecipitation. Differences in surface attachment and the possibility of structural capture within the Fe(III) (oxyhydr)oxides are inferred from the robust influence Si has on DIP binding during adsorption, meanwhile the influence of Si on DIP binding is inhibited during coprecipitation when As(III) and As(V) are present. In the Early Proterozoic open oceans, Fe(III) (oxyhydr)oxides precipitated when deep anoxic Fe(II)-rich waters rose and mixed with the first permanently oxygenated ocean surface waters. Our data imply that, DIP was removed from surface waters through coprecipitation with those Fe(III) (oxyhydr)oxides, rather than adsorption. Local variations in DIP and perhaps even stratification of DIP in the oceans were likely created from the continuous removal of DIP from surface waters by Fe(III) (oxyhydr)oxides, and by the partial release of DIP into the anoxic bottom waters and buried sediments. In addition to a DIP famine, the selectivity for DIP over As(V) may have led to As enrichment in surface waters, both of which would have most likely decreased the productivity of cyanobacteria and O-2 production.

  • 9. Hofmann, Axel
    et al.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wilson, Allan
    Gold mobility during Palaeoarchaean submarine alteration2017In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 462, p. 47-54Article in journal (Refereed)
    Abstract [en]

    Seafloor alteration provides large amounts of solutes to the hydrosphere. In order to investigate gold mobility during water-rock interaction prior to 3-billion-years ago, low detection limit analysis of Au concentrations was carried out on rocks from marine alteration zones. Stratiform zones recording low temperature (<= 150 degrees C) seafloor alteration are a characteristic feature of greenstone belts older than 3.0 Ga. Hydrothermal processes were operating on, and immediately below, the seafloor, giving rise to extensive silicification of sub-seafloor volcanic rocks and silicification of seafloor sediments. In order to investigate gold mobility during silicification, unaltered and variably silicified volcanic rocks and associated cherts from Palaeoarchaean greenstone successions (c. 3.4 Ga) of South Africa were analyzed. Results show mobility of gold during silicification of mafic/ultramafic rocks and transfer to the Archaean ocean. Some gold was incorporated into carbonaceous marine sediments overlying the alteration zones. A combination of pervasive silicification, rarity of black shales, and low gold content in komatiites can explain the low mineralization potential of Palaeoarchaean greenstone belts for orogenic gold deposits.

  • 10.
    Kleine, Barbara I.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Iceland, Iceland.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mineralogical controls on metamorphic fluid flow in metabasaltic sills from Islay, Scotland2016In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 248, p. 22-39Article in journal (Refereed)
    Abstract [en]

    In this study we show that mineralogy was the primary control of metamorphic fluid flow in the well-studied metabasaltic sills in the SW Scottish Highlands. Here, basaltic sills have been partially carbonated by H2O-CO2 fluids at greenschist facies conditions. This has led to mineral zonation with carbonate-poor sill interiors separated from carbonate-rich sill margins by reaction fronts. Although deformation set the stage for metamorphic fluid flow in the SW Scottish Highlands by causing the preferred alignment of mineral grains, metamorphic fluid flow was not coupled with active deformation but occurred later utilizing the pre-existing mineral alignment as a means of accessing the sill interiors. The sills which were studied were partially carbonated with well-preserved reaction fronts. They were selected because (atypically for the SW Scottish Highlands) they are mineralogically heterogeneous making them ideal for a study of mineralogical controls of metamorphic fluid flow. Their mineralogical heterogeneity reflects chemical heterogeneity arising from magmatic flow differentiation and spilitization, which occurred before greenschist facies metamorphism. Magmatic flow differentiation resulted in parts of the sill containing large crystals with no preferred alignment. Large (up to 3 cm) plagioclase phenocrysts were concentrated in the sill interior whereas large (up to 1 cm) amphibole (after pyroxene) grains formed cumulate layers close to the sill margins. These large randomly oriented crystals were replaced by an interface-coupled dissolution-precipitation mechanism. Replacement is constant volume and with hydration and carbonation affecting the cores of these minerals while the rims are remained intact and unaltered. This finding points to intro-granular metamorphic fluid flow. In contrast inter-granular metamorphic fluid flow was facilitated by mineral alignment on different scales. Pre-metamorphic spilitization, produced layers of epidote called segregations, whereas regional deformation caused preferred alignment mainly of amphibole and chlorite. Epidote undergoes a series of volume changes during greenschist facies metamorphism. This created porosity which produced preferred pathways for metamorphic fluids affecting the advancement of fluid-driven reaction fronts. Preferred alignment of amphibole and chlorite also affected the advancement of reaction fronts. In this case, fluid flow was preferentially parallel to the foliation. In both cases, inter-granular metamorphic fluid flow utilized a pre-existing fabric albeit on different scales. These results show intra-granular metamorphic fluid flow in unfoliated rock and inter-granular metamorphic fluid flow in foliated rock. In both cases metamorphic fluid flow occurred after deformation controlled by pre-existing mineralogical heterogeneities, such as grain composition and shape anisotropy as well as preferred alignment of mineral grains.

  • 11.
    Kleine, Barbara I.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The mechanism of infiltration of metamorphic fluids recorded by hydration and carbonation of epidote-amphibolite facies metabasaltic sills in the SW Scottish Highlands2015In: American Mineralogist, ISSN 0003-004X, E-ISSN 1945-3027, Vol. 100, no 11-12, p. 2702-2717Article in journal (Refereed)
    Abstract [en]

    In this study we investigate a group of metabasaltic sills from the SW Scottish Highlands metamorphosed at epidote-amphibolite facies conditions that provide useful insight into the mechanisms and characteristics of fluid infiltration during metamorphism. The sills are amphibole and garnet bearing and exhibit a strong foliation in the sill margins that developed pre- to syn- peak metamorphism. Fluid infiltration caused hydration and carbonation in the sills, expressed as 1) replacement of garnet and amphibole by chlorite and calcite and 2) replacement of amphibole and epidote to form chlorite and calcite. Using garnet-amphibole and garnet-chlorite geothermometers we show that these reactions occurred after peak metamorphism at T = 290 to 400°C. Reaction textures show that the fluid infiltration into the sill that caused hydration and carbonation occurred in the absence of deformation. The fluid infiltration was mineralogically controlled with greater fluid access in areas of abundant fine-grained elongate minerals such as amphibole and chlorite. The replacement of garnet by chlorite most likely occurred by an interface-coupled dissolution-precipitation mechanism as evidenced by perfect pseudomorphic textures of garnet, porosity generation behind the reactive interface and fracturing ahead of this interface. Porosity generated in the product chlorite enhanced fluid access to the replacement front. The study shows that deformation was not required for extensive fluid infiltration and alteration during metamorphism. Fluid flow uses a pre-existing foliation to gain access to the rock, taking advantage of the anisotropic shape of the aligned minerals.

  • 12.
    Kleine, Barbara I.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Huet, Benjamin
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Preservation of blueschist-facies minerals along a shear zone by coupled metasomatism and fast-flowing CO2-bearing fluids2014In: Journal of Petrology, ISSN 0022-3530, E-ISSN 1460-2415, Vol. 55, no 10, p. 1905-1939Article in journal (Refereed)
    Abstract [en]

    Two types of blue halo (types I and II) composed of blueschist-facies minerals are centered around a brittle, normal shear zone in greenschist-facies rocks on the island of Syros, Aegean Sea, Greece. The shear zone is steeply dipping and cuts a near-horizontal layer of greenschist-facies rocks (albite + epidote + actinolite + chlorite + quartz). Type I and II blue haloes are 0.3 m and c. 1m wide respectively, and are seen on both sides of the shear zone. The inner type I haloes are composed of nearly pure glaucophane schist and were formed by metasomatic addition of Na2O and SiO2, and to a lesser extent of K2O and large ion lithophile elements (LILE), coupled with loss of CaO, Al2O3 and MnO. The outer type II haloes consist of a carbonated blueschist-facies assemblage (glaucophane + calcite + phengite + epidote + garnet + quartz).These experienced only slight metasomatic changes (i.e. addition of K2O and LILE), which cannot alone explain halo formation.We present  petrological, geochemical and thermodynamic evidence that this assemblage was preserved at greenschist-facies conditions because XCO2 was elevated by flow of a CO2-bearing fluid along the shear zone, which was approximately contemporaneous with greenschist-facies hydration in the surrounding rocks. We further note that the flux of CO2-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass-balance calculations reveal that the fluid flux within the shear zone was at least 100-2000 times greater than the fluid flux within the surrounding rocks. Mineral textures show greenschist-facies minerals replacing blueschist minerals in the type II haloes, supporting our interpretation that the blueschist-facies minerals were preserved during greenschist-facies retrogression. A simplified P-T vs XCO2 pseudosection confirms that preservation of carbonated blueschist can occur at greenschist-facies conditions in the presence of a CO2-bearing fluid.

  • 13.
    Kleine, Barbara Irene
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pre-metamorphic controls on the propagation of fluid-driven reaction fronts at greenschist-facies metamorphic conditionsManuscript (preprint) (Other academic)
    Abstract [en]

    In this study we show that pre-metamorphic mineralogical and chemical heterogeneities control fluid flow and fluid-driven propagation of reaction fronts during regional metamorphism. The study was undertaken at Port Ellen, Islay, in SW Scottish Highlands. Here, basaltic sills have been partially carbonated by H2O-CO2 fluids at greenschist-facies conditions. This has led to mineral zonation with carbonate poor sill interiors separated from carbonate-rich sill margins by reaction fronts. The sills which were studied are partially carbonated and reaction fronts are well-preserved. These sills are unique for the Scottish Highlands in that they show excellent evidence of 1) extensive magmatic flow differentiation and 2) spilitization having occurred before greenschist-facies metamorphism. Magmatic flow differentiation concentrated large (up to 3 cm) phenocrysts of plagioclase in the sill interior and spilitic alteration produced layers of segregated epidote and caused albitization of these plagioclase phenocrysts resulting in their preservation throughout greenschist-facies metamorphism. Coupled with this pre-metamorphic mineralogical zonation, sill margins where enriched in Ti, Fe, P, HFSEs and REEs whereas the sill interiors were enriched in Al, Na and LILEs. In this study, we show spatial correlation of metamorphic reaction fronts with pre-metamorphic mineralogical zonation produced by magmatic flow differentiation (plagioclase phenocrysts size distributions) and epidote segregations produced by spilitization. We infer a pre-metamorphic mineralogical and chemical control on the positions of fluid-driven metamorphic reaction fronts. Based on mineralogical and chemical profiles across these sills and reaction textures preserved within them, we conclude that availability of reactant minerals and mechanical factors, such as volume change in epidote and foliation development due to chlorite formation are primary controls of fluid-driven reaction front propagation during metamorphism. We further suggest that elevated oxygen fugacity in the sill margins may have further promoted carbonation.

  • 14. Lisitsin, Vladimir A.
    et al.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Orogenic gold mineral systems of the Western Lachlan Orogen (Victoria) and the Hodgkinson Province (Queensland): Crustal metal sources and cryptic zones of regional fluid flow2016In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360, Vol. 76, p. 280-295Article in journal (Refereed)
    Abstract [en]

    Orogenic gold mineral systems in the Western Lachlan Orogen (Victoria) and the Hodgkinson Province (Queensland) produced gold provinces characterised by vastly different scales of gold endowment and strongly uneven distribution of gold mineralisation within each province. The volume of hydrous pyrite-bearing rocks undergoing metamorphic devolatilisation during thermo-tectonic events driving orogenic gold mineral systems represents a fundamental first-order constraint on the total gold endowment and its broad spatial distribution, both between and within the provinces. Most of the largest gold deposits in both regions occur in linear, richly-endowed metallogenic zones, oblique to the dominant regional structures and related to deep crustal domain boundaries. These boundaries, with only subtle surface expressions, were the major regional structural controls which promoted focused near-vertical flow of mineralising metamorphic fluids above the outer margins of cratonic blocks in the lower crust. Recognised major faults represented only more local scale and often indirect controls on the focused fluid flow, particularly effective above the deep cratonic block boundaries overlain by relatively thick crustal source rocks.

  • 15. Menzies, Catriona D.
    et al.
    Wright, Sarah L.
    Craw, Dave
    James, Rachael H.
    Alt, Jeffrey C.
    Cox, Simon C.
    Pitcairn, Lain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Teagle, Damon A. H.
    Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand2018In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 481, p. 305-315Article in journal (Refereed)
    Abstract [en]

    Collisional mountain building influences the global carbon cycle through release of CO2 liberated by metamorphic reactions and promoting mechanical erosion that in turn increases chemical weathering and drawdown of atmospheric CO2. The Southern Alps is a carbonate-poor, siliciclastic mountain belt associated with the active Australian Pacific plate boundary. On-going, rapid tectonic uplift, metamorphism and hydrothermal activity are mobilising carbon. Here we use carbon isotope measurements of hot spring fluids and gases, metamorphic host rocks, and carbonate veins to establish a metamorphic carbon budget. We identify three major sources for CO2 within the Southern Alps: (1) the oxidation of graphite; (2) consumption of calcite by metamorphic reactions at the greenschist-amphibolite fades boundary, and (3) the dissolution of groundmass and vein-hosted calcite. There is only a minor component of mantle CO2 arising on the Alpine Fault. Hot springs have molar HCO3-/Ca2+ similar to 9, which is substantially higher than produced by the dissolution of calcite indicating that deeper metamorphic processes must dominate. The total CO2 flux to the near surface environment in the high uplift region of the Southern Alps is estimated to be similar to 6.4 x 10(8) mol/yr. Approximately 87% of this CO2 is sourced from coupled graphite oxidation (25%) and disseminated calcite decarbonation (62%) reactions during prograde metamorphism. Dissolution of calcite and mantle-derived CO2 contribute 10% and 3% respectively. In carbonate-rich orogens CO2 production is dominated by metamorphic decarbonation of limestones. The CO2 flux to the atmosphere from degassing of hot springs in the Southern Alps is 1.9 to 3.2 x 10(8) mol/yr, which is 30-50% of the flux to the near surface environment. By contrast, the drawdown of CO2 through surficial chemical weathering ranges between 2.7 and 20 x 10(9) mol/yr, at least an order of magnitude greater than the CO2 flux to the atmosphere from this orogenic belt. Thus, siliciclastic mountain belts like the Southern Alps are net sinks for atmospheric CO2, in contrast to orogens involving abundant carbonate rocks, such as the Himalaya, that are net CO2 sources.

  • 16.
    Patten, Clifford G. C.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Teagle, D. A. H.
    Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust: Insights from the Troodos ophiolite2017In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360, Vol. 86, p. 487-508Article, review/survey (Refereed)
    Abstract [en]

    The Troodos ophiolite is an ideal location to investigate the relationships between the mobilisation of metals by hydrothermal fluids from the lower oceanic crust and the formation of volcanogenic massive sulphide (VMS) deposits. The ophiolite hosts the classic Cyprus-type Cu-rich VMS deposits as well as abundant zones of epidosite alteration in the lower sheeted dyke section that are significantly depleted in base metals including Cu and Zn, and are considered to be the source of the metals enriched in the overlying deposits. Previous research indicates that the Troodos VMS deposits are irregularly enriched in Au and related elements As, Sb, and Se, but the behaviour of these elements during the hydrothermal alteration of the Troodos ophiolite hitherto has been poorly investigated. Low detection limit whole rock analyses of fresh glass samples reveal that the Troodos primitive crust has a similar metal content and distribution to modern-day arc-related environments such as the Manus Basin. Compared to mid oceanic ridge basalt (MORB), the Troodos primitive crust is enriched in As, Sb and Pb most likely due to addition from a subducting slab during crustal formation. During early stages of magmatic differentiation (9-3.5 wt% MgO) Au, As, Sb, Se, Cu, Zn and Pb behave as incompatible elements due to the sulphide-undersaturated nature of the melt. The onset of magnetite crystallisation, however, at similar to 3.5 wt% MgO leads to sulphide segregation and depletion of strongly chalcophile elements (Au, Cu and Se) during continued differentiation (<3.5 wt% MgO) whereas poorly chalcophile elements (As, Sb, Zn and Pb) remain incompatible. These differences in metal behaviour can account for the Cu-rich, Zn-Pb-poor of the Cyprus-type VMS deposits as the source area rocks show high Cu fertility compared to Zn and Pb. Mobilisation of metals during hydrothermal alteration of the Troodos ophiolite is more extensive than observed in hydrothermally altered MORB. Mass balance calculations show that the epidosite zones are significantly depleted in Au (-88 +/- 16%), As (-89 +/- 23%), Sb (-60 +/- 12%), Se (-91 +/- 20%), Cu (-84 +/- 18%), Zn (-63 +/- 9%) and Pb (-60 +/- 8%). Background altered diabase from outside epidosite zones shows similar metal depletions which suggests that the source areas of VMS are not restricted to epidosite zones but are extended to the lower sheeted dyke section. The masses of metals mobilised from a source area of 10.9 km(3), (composed of a 5 km(3) epidosite zone and 5.9 km(3) of background altered diabase) in the Solea graben are 47 t Au, 21 kt As, 1200 t Sb, 3100 t Se, 2.4 Mt Cu, 1.8 Mt Zn and 27 kt Pb. Comparison of metal quantities mobilised from lower sheeted dike section in the Solea graben with those hosted in VMS deposits shows trapping efficiencies ranging from 4 to 37% indicating that most of the metals is lost by other processes.

  • 17.
    Patten, Clifford G. C.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Teagle, Damon A. H.
    Sulphide mineralisation in forearc setting at ODP site 786B: evaluation of magmatic inputs into oceanic crust hydrothermal systemManuscript (preprint) (Other academic)
    Abstract [en]

    Volcanogenic massive sulphide (VMS) deposits are variably enriched in metals that are mobilised by two main processes: hydrothermal alteration of the oceanic crust’s lower sheeted dyke section and exsolution of metal-rich fluids into the hydrothermal system from differentiating magmas. The extent to which each process contributes to metal enrichment in VMS deposits varies, however, between different tectonic settings. Oceanic Drilling Program (ODP) Hole 786B recovers the volcanic section and the transitional zone of a supra-subduction zone oceanic crust including a 30 m wide mineralised zone at the base of the hole. Previous work has indicated that significant input of magmatic fluid into the hydrothermal system isresponsible for the formation of mineralisation. This study uses in-situ trace element and S-isotope analyses in sulphide minerals and whole rock data to characterise the metal endowment of the mineralised zone, the sources of the trapped metals and the signature of magmatic fluid inputs in the hydrothermal system. The mineralised zone shows strong enrichment in S, As, Se, Sb and Au, and probably Mo, Te and Bi, but little enrichment in base metals. It is subdivided in two main alteration domains: the upper alteration domain, characterised by mixing of high temperature hydrothermal fluids with sea water at relatively low temperature (150-200 °C), under reduced and near neutral pH conditions, and the central and lower alteration domain, characterised by extensive mixing of magmatic fluids with sea water at relatively high temperature (~250 °C), under oxidised and acidic conditions. Strong metal zonation occurs in the transitional zone with preferential enrichment of Zn, Cu, As, Au and Pb in the upper alteration domain and preferential enrichment of S, Se, Mo, Sb, Te and Bi in the central and lower alteration domain. This zonation is controlled by variations in fluid composition, temperature, redox, pH, and zone refining during sulphide paragenesis. The oceanic crust at Hole 786B has high As, Sb and Pb concentrations relative to mid oceanic ridge setting but similar Cu, Zn and Au, and low Se concentrations. The oceanic crust metal fertility suggests that the Cu, Zn, As, Sb and Pb enriched in the transitional zone could have been mobilised by rock buffered hydrothermal fluids but that the S, Se and Au must have been mobilised by magmatic fluids. Major and trace elements behaviour during magmatic differentiation of the oceanic crust at Hole 786B show evidences for a magnetite crisis event which is interpreted to trigger exsolution of metal-rich magmatic fluids into the hydrothermal system and which can account for the observed metal endowment in the transitional zone. The metal content in the oceanic crust at Hole 786B and the specific endowment of the mineralised zone suggests that significant fractionation between Au and base metals occur during fluid migration in supra-subduction oceanic crust and can promote the formation of Au-rich VMS deposits on the sea floor. Such process is as highlighted by the Au : base metal ratio close to unity in the mineralised zone.

  • 18.
    Patten, Clifford G. C.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Teagle, Damon A. H.
    Harris, Michelle
    Mobility of Au and related elements during the hydrothermal alteration of the oceanic crust: implications for the sources of metals in VMS deposits2016In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 51, no 2, p. 179-200Article in journal (Refereed)
    Abstract [en]

    Volcanogenic massive sulphide (VMS) deposits are commonly enriched in Cu, Zn and Pb and can also be variably enriched in Au, As, Sb, Se and Te. The behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known. Ocean Drilling Program (ODP) Hole 1256D penetrates a complete in situ section of the upper oceanic crust, providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration. A representative suite of samples was analysed for Au, As, Sb, Se and Te using low detection limit methods, and a mass balance of metal mobility has been carried out through comparison with a fresh Mid-Oceanic Ridge Basalt (MORB) glass database. The mass balance shows that Au, As, Se, Sb, S, Cu, Zn and Pb are depleted in the sheeted dyke and plutonic complexes by -46 +/- 12, -27 +/- 5, -2.5 +/- 0.5, -27 +/- 6, -8.4 +/- 0.7, -9.6 +/- 1.6, -7.9 +/- 0.5 and -44 +/- 6 %, respectively. Arsenic and Sb are enriched in the volcanic section due to seawater-derived fluid circulation. Calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as VMS mineralisation. The quantity of Au mobilised and the ratio of Au to base metals are similar to those of mafic VMS, and ten times enrichment of Au would be needed to form a Au-rich VMS. The Cu-rich affinity of mafic VMS deposits could be explained by base metal fractionation both in the upper sheeted dykes and during VMS deposit formation.

  • 19.
    Patten, Clifford G. C.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Teagle, Damon A. H.
    Harris, Michelle
    Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from IODP site 1256D2016In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 193, p. 132-159Article in journal (Refereed)
    Abstract [en]

    Fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere, supporting microbial life and the formation of sulphide ore deposits. The mechanisms responsible for metal mobilisation during the evolution of the oceanic crust are complex and are neither fully constrained nor quantified. Investigations into the mineral reactions that release metals, such as sulphide leaching, would generate better understanding of the controls on metal mobility in the oceanic crust. We investigate the sulphide and oxide mineral paragenesis and the extent to which these minerals control the metal budget in samples from International Oceanic Discovery Program (IODP) Hole 1256D. The IODP Hole 1256D drill core provides a unique sample suite representative of a complete section of a fast-spreading oceanic crust from the volcanic section down to the plutonic complex. The sulphide population at Hole 1256D is divided into five groups based on mineralogical assemblage, lithological location and texture: the magmatic, metasomatised, high temperature hydrothermal, low temperature and patchy sulphides. The initiation of hydrothermal alteration by downward flow of moderate temperature (250-350 °C) hydrothermal fluids under oxidising conditions leads to metasomatism of the magmatic sulphides in the sheeted dyke and plutonic complexes. Subsequent increase in the degree of hydrothermal alteration at temperatures >350 °C under reducing conditions then leads to the leaching of the metasomatised sulphides by rising hydrothermal fluids. Mass balance calculations show that the mobility of Cu, Se and Au occurs through sulphide leaching during high temperature hydrothermal alteration and that the mobility of Zn, As, Sb and Pb is controlled by silicate rather than sulphide alteration. Sulphide leaching is not complete at Hole 1256D and more advanced alteration would mobilise greater masses of metals. Alteration of oxide minerals does not release significant quantities of metal into the hydrothermal fluid at Hole 1256D. Mixing of rising high temperature fluids with low temperature fluids, either in the upper sheeted dyke section or in the transitional zone, triggers local high temperature hydrothermal sulphide precipitation and trapping of Co, Ni, Cu, Zn, As, Ag, Sb, Se, Te, Au, Hg and Pb. In the volcanic section, low temperature fluid circulation (<150 °C) leads to low temperature sulphide precipitation in the form of pyrite fronts that have high As concentrations due to uptake from the circulating fluids. Deep late low temperature circulation in the sheeted dyke and the plutonic complexes results in local precipitation of patchy sulphides and local metal remobilisation. Control of sulphides over Au, Se and Cu throughout fast-spreading mid-oceanic crust history implies that the generation of hydrothermal fluids enriched in these metals, which can eventually form VMS deposits, is strongly controlled by sulphide leaching.

  • 20.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Mass transfer as a function of metamorphic fluid flux2007In: American Geophysical Union Fall Meeting 2007, 2007Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Fluid-rock interaction is an integral part of metamorphism in the crust, and causes significant mass transfer most clearly demonstrated by mineral veins, ore deposits, and alteration fronts that are ubiquitous throughout metamorphic belts. Quantification of major and trace element fluxes during metamorphism and fluid-rock interaction is critical for complete understanding of global element cycles. The SW Highlands of Scotland are an ideal natural laboratory for investigation of major and trace element fluxes during metamorphism. Syn-metamorphic time integrated fluid fluxes within the 6km wide Ardrishaig anticline have been quantified and vary from $<$10m$^{2}$/m$^{3}$ on the limbs of the anticline to $>$300 m$^{2}$/m$^{3}$ in the axial zone (Skelton et al., 1995, J Petrol 36, 563). This study investigates the major and trace element fluxes that occur as a function of the variation in fluid flux within the Ardrishaig anticline. Both depletion and enrichment in chemical elements occurs and the scale of mass transfer increases with increasing fluid flux. Mobile elements include Si, C, K, Na, Sr, As, and Sb, and element mobility varies strongly with lithology. This well-constrained, localized study correlates well with larger regional scale studies (Pitcairn et al., 2006, Econ Geol 101, 1525) where many elements were shown to be locally mobile but very few were shown to have undergone large scale transport.

  • 21.
    Pitcairn, Iain
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Arndt, Nick
    LGCA, Universite de Grenoble, Grenoble, France.
    Mineralogical and chemical changes during seafloor alteration in the Archaean2008In: European Geophysical Union 2008, 2008Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Hydrothermal circulation of seawater through the oceanic crust results in alteration and recrystallisation of primary magmatic minerals and redistribution of chemical elements. Many chemical elements removed from the ocean crust during alteration are added to seawater and therefore a systematic study of the consequences of seafloor alteration in the Archaean provides insight into the composition of Archaean seawater. In addition, the alteration affects the compositions of the volcanic rocks and complicates interpretation of their magmatic histories. Finally, the types and distribution of alteration zones leaves a record of the circulation of hydrothermal fluids and thus provides information about the scale and vigour of circulation of seawater through the Archaean oceanic crust.

    We present here a mineralogical and geochemical study of alteration of basalts and komatiites from the 2.7 Ga Abitibi belt in Canada and the 3.5-3.2 Ga Barberton greenstone belt in South Africa. These belts have undergone multiple stages of alteration and a major focus of the study is to isolate the mineralogical and chemical changes that occurred during seafloor alteration from those that occurred later during the metamorphism that accompanied accretion, granite intrusion and uplift. Previous studies compared whole rock analyses of altered with unaltered rocks. This study attempts to isolate the specific mineral reactions that occurred during seafloor alteration for a more detailed account of chemical changes during seafloor alteration. Methodology used includes SEM, EMPA, and microfluorescence spectroscopy. The first stage of alteration was devitrification of glassy basaltic pillow margins which included growth of chlorite and clinozoisite, followed by replacement of primary clinopyroxene with tremolite, and plagioclase with sericite, and a later phase of carbonate veining. Preliminary results show that Abitibi belt pillow margins are depleted in Si, Fe, Mg, Ca, Na, and Mn and enriched in Al, K, and Ti. Comparison of the Barberton and Abitibi samples with modern day analogues will allow us to observe how this process has changed through time.

  • 22.
    Pitcairn, Iain
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Arndt, Nick
    LGCA, Universite de Grenoble, Grenoble, France.
    Shock brecciation around the Kidd Creek deposit, Abitibi Belt, Canada2008In: Canadian Journal of Earth Science, Vol. 45, p. 875-878Article, review/survey (Other (popular science, discussion, etc.))
    Abstract [en]

    The Kidd-Munro assemblage, Abitibi belt, Canada, is an ultramafic-mafic-felsic volcanic sequence that contains the giant Kidd Creek volcanic-hosted massive sulfide (VMS) deposit. The Kidd basin, 1.6 km NE of the deposit, contains pervasively brecciated pillowed and massive basalts. The breccia is distinctly different from most breccias in volcanic rocks, which form through volcanic processes or during later deformation or alteration. The Kidd Creek breccia occurs pervasively through otherwise undeformed pillow interiors and margins, and also in localized corridors of particularly intense brecciation. Clasts are angular, up to 4cm wide, hosted in a very fine-grained matrix, and commonly show jig-saw fit texture. The chemical compositions of the breccia fragments and matrix are generally similar although the matrix is slightly enriched in HFSE and HREE and depleted in some mobile elements such as Rb and Ba. The breccia contains altered basaltic clasts and fragments of in-filled amygdales, and is cross-cut by late-stage quartz-carbonate-sulfide veins.

    The observations imply that the breccia was formed in-situ, with minimal transport of material, and developed after solidification of the volcanic rocks. In-situ breccias such as these are known to form proximal to major fault zones but no such structure occurs in the viscinity of the Kidd Basin. We suggest the brecciation was caused by the propagation of shock waves from explosive volcanic eruption, perhaps related to the emplacement of felsic volcanic rocks observed in the Kidd Creek Mine. The breccia was subject to enhanced hydrothermal fluid flow, perhaps linked to the formation of the ore deposit.

  • 23.
    Pitcairn, Iain K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Craw, Dave
    Teagle, Damon A. H.
    Metabasalts as sources of metals in orogenic gold deposits2015In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 50, no 3, p. 373-390Article in journal (Refereed)
    Abstract [en]

    Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 +/- 0.5 ppb) and upper greenschist facies (1.0 +/- 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 +/- 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.

  • 24.
    Pitcairn, Iain K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Craw, Dave
    Teagle, Damon A. H.
    The gold conveyor belt: Large-scale gold mobility in an active orogen2014In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360, Vol. 62, p. 129-142Article in journal (Refereed)
    Abstract [en]

    The Southern Alps of New Zealand are part of an active collisional orogen where metamorphism, hydrothermal fluid flow and the formation of orogenic gold deposits are ongoing. The Southern Alps are forming due to transpressional collision between continental crust fragments on the Pacific and Australian tectonic plates. The plate tectonic rates and geometries, the sources of fluid and broad-scale fluid pathways in the hydrogeological system, and the geochemical compositions of the Torlesse Terrane rock that is being advected through the orogen are well defined so that a mass balance of metal mobility during active orogenic processing in the Southern Alps of New Zealand can be calculated. Advection of a 10 km wide x 5 km deep section of Torlesse rock through the orogen at tectonic rates (0.01 m/yr) that is then metamorphosed up to amphibolite fades causes mobilisation of over 11,27 t Au, 10.1 Mt As, 47,000 t Hg, 560,000 t Sb and 14,000 Mt H2O in 1 Myrs. The masses of elements mobilised at the same rate along the length of the Southern Alps (>200 km) for 5 Myrs would be more than 100 times greater. The metals were mobilised by the metamorphic fluid produced during the orogenic processing of the Torlesse Terrane rocks and the concentrations of Au, As, Hg and Sb in this fluid are calculated to be 0.08, 711, 3, and 40 mg/kg, respectively. The mobilised metals form the orogenic gold deposits that occur in the Southern Alps. Different styles of gold deposits form contemporaneously during the active orogenesis of the Southern Alps, including those with a fluid temperature > rock temperature that may appear to have formed after the peak of metamorphism but are instead just the product hydrothermal fluid mineralising rocks on their retrograde metamorphic path. The mass balance shows that there has been orders of magnitude more metal mobilised in the orogen than resides in the currently known deposits. There is a clear potential for large gold deposits occurring in the yet to be uplifted parts of the Southern Alps if there have been efficient enough fluid focusing and metal precipitation mechanisms occurring under the Southern Alps.

  • 25.
    Pitcairn, Iain K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arghe, Fredrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Boyce, Adrian
    Structurally focused fluid flow during orogenesis: the Islay Anticline, SW Highlands, Scotland2010In: Journal of the Geological Society, ISSN 0016-7649, E-ISSN 2041-479X, Vol. 167, no 4, p. 659-674Article in journal (Refereed)
    Abstract [en]

    Displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny. This study investigates whether hydrothermal fluid flow was focused along the Islay Anticline, Islay, SW Highlands of Scotland, as shown in the axial zone of the neighbouring Ardrishaig Anticline. Four localities from the limb to the axial plane of the Islay Anticline were investigated for isotopic homogenization of metacarbonate units to silicate values. At Mull of Oa on the limb of the anticline, metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be <1 m(3) m(-2). Within the axial zone of the Islay Anticline, metacarbonate units from Port a' Chotain and Bagh an Da Dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes. Fluid flow was enhanced along localized parasitic folds such as at Port an t-Sruthain, where metacarbonates have been isotopically reset, and there are abundant quartz-carbonate veins that precipitated during D-1-D-2 deformation. Metamorphic fluid flow was higher in the axial zone of the Islay Anticline and in localized antiformal structures. Fluid fluxes are estimated to be considerably lower than at the neighbouring Ardrishaig Anticline.

  • 26.
    Pitcairn, Iain K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlgemuth-Ueberwasser, Cora C.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mobility of gold during metamorphism of the Dalradian in Scotland2015In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 233, p. 69-88Article in journal (Refereed)
    Abstract [en]

    Mobility of Au and related metals during metamorphism has been suggested to be the source of metals enriched in orogenic Au deposits. This study investigates the mobility of Au, As, and Sb during metamorphism of the Dalradian metasedimentary rocks of Scotland. The metamorphic processes in the Dalradian of Scotland are extremely well studied, and the terrane is an ideal area to investigate mobility of these metals. Our results show that of the 25 major and trace elements analysed, only Au, As, Sb, S and volatile contents as shown by loss on ignition (LOI) values show systematic variation with the metamorphic grade of the samples. Average Au concentrations decrease from 1.1 +/- 0.55 ppb and 0.72 +/- 0.34 ppb in chlorite and biotite zone rocks down to 0.4 +/- 0.22 ppb and 034 +/- 0.13 ppb in kyanite and sillimanite zone rocks. Average As concentrations decrease from 4.8 ppm (range 0.5 to 17.8 ppm) and 1.96 +/- 1.9 ppm in chlorite and biotite zone rocks down to 0.24 +/- 0.15 ppm and 0.2 +/- 0.12 ppm in kyanite and sillimanite zone rocks. Average Sb concentrations decrease from 0.18 +/- 0.15 ppm and 0.11 +/- 0.10 ppm in chlorite and biotite zone rocks down to 0.04 +/- 0.02 ppm in both kyanite and sillimanite zone rocks. Sulphur and LOI concentrations also show significant decreases. Mass balance calculations indicate that compared to chlorite and biotite zone samples, sillimanite zone samples have an average mass loss of 62 +/- 14%, 94 +/- 4% and 74 +/- 14% for Au, As, and Sb respectively. Every 1 km(3) of chlorite-biotite zone mixed psammitic-pelitic protolith rock that is metamorphosed to sillimanite zone conditions would release 1.5 t Au, 8613 t As, 270 t Sb, and 1.02 Mt S. The mobility of these elements is strongly controlled by the paragenesis of sulphide minerals. Pyrite, sphalerite, galena and cobaltite (as well as gersdorffite) decrease in abundance with increasing metamorphic grade in the Dalradian metasedimentary rocks. A critical aspect of the sulphide paragenesis is the transition of pyrite to pyrrhotite. This transition is complete by mid greenschist facies in the Loch Lomond samples but is more gradual at Glen Esk occurring between biotite and sillimanite zones. The Au, As, and Sb content of the sulphide assemblage also decreases with increasing metamorphic grade, and we suggest that this is a controlling factor on the mobility of these metals from the Dalradian metasedimentary rocks during metamorphism. Chlorite may be an important host mineral for As in the greenschist fades rocks. Breakdown of chlorite indirectly drives the mobility of Au, As, and Sb, as this produces the bulk of metamorphic fluid that drives transition between pyrite and pyrrhotite. We suggest that there is potential for significant undiscovered mineralisation in the Central and SW Highlands of Scotland. However, as the total mass of gold mobilised is lower than observed in other metasedimentary terranes such as the Otago and Alpine Schist's, New Zealand, very efficient fluid focussing and trapping mechanisms would be required to form large deposits in the Dalradian of Scotland.

  • 27.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arghe, Fredrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Regional mapping of pre-metamorphic spilitization and associated chemical mobility in greenschist-facies metabasalts of the SW Scottish Highlands2010In: Journal of the Geological Society, ISSN 0016-7649, E-ISSN 2041-479X, Vol. 167, no 5, p. 1049-1061Article in journal (Refereed)
    Abstract [en]

    Both spilitic and non-spilitic metabasaltic sills are hosted by greenschist-facies metasediments in the SW Scottish Highlands. Spilitization is mainly characterized by enrichment in Na2O, elevated modal plagioclase and epidote segregations. Mapping of the spatial distribution of spilitic metabasalts reveals an ancient sub-sea-floor fluid cell centred on the extrusive Tayvallich Volcanics. Fluid circulation was most extensive at shallow levels where most sills were spilitized. We attribute this to pervasive flow of saline fluid, which was thermally driven by the cooling suite of lava flows and sills. Spilitization below this lithostratigraphic depth was restricted to only a few sills. Their spilitization is largely unrelated to specific properties of these sills (e. g. width, chemistry or host lithology). We conclude that fluid channelling was an intrinsic property of sub-sea-floor fluid flow either at deeper levels or earlier during fluid circulation. By profiling of the size distributions of relic phenocrysts in a partly spilitized sill, we conclude that spilitization proceeds with the symmetric propagation of a spilitization front from the sill margins towards the sill interior. Based on chemical profiling across the margin of an epidote segregation, we conclude that spilitization is associated with chemical transport on scales ranging from 0.1 to 10 m.

  • 28.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lewerentz, Alexander
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kleine, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Webster, David
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Structural Channelling of Metamorphic Fluids on Islay, Scotland: Implications for Paleoclimatic Reconstruction2015In: Journal of Petrology, ISSN 0022-3530, E-ISSN 1460-2415, Vol. 56, no 11, p. 2145-2171Article in journal (Refereed)
    Abstract [en]

    Analysis of the delta O-18 and delta C-13 values of carbonate rocks from Islay, Scotland reveals structural channelling of metamorphic fluids through the axial region of a major en echelon anticlinal fold system. Metamorphic fluid flow produced axial planar veins with higher vein density in the axial region of the fold. Fluid: rock ratios were more than 30: 1 within this axial region, at least four times greater than the regional mean ratio of 7.6 +/- 1.5:1 for carbonate rocks on Islay. This supports the interpretation that metamorphic fluids were channelled through the axial region of the Islay Anticline. Fluid: rock ratios were calculated using a model for coupled delta O-18 and delta C-13 exchange with a metamorphic fluid. The metamorphic fluid was calculated to have delta O-18 and delta C-13 values of 15.3 parts per thousand and -6.1 parts per thousand, respectively and X-CO2 of 0.2. This is in isotopic and chemical equilibrium with chlorite- and graphite-bearing metamudstones that are structurally below the folded metacarbonate rocks on Islay. Devolatilization of these metamudstones is therefore a likely source mechanism for this metamorphic fluid. Removal of the effects of metamorphic fluid flow on delta C-13 values recorded by metacarbonate rocks on Islay allows us to re-evaluate evidence used to reconstruct Neoproterozoic climate. This evidence includes a large negative delta C-13 excursion reported from the Lossit Limestone Formation. This unit underlies the Port Askaig Formation, which is dominated by diamictites that have been interpreted as glacial tillites. This 'Islay anomaly' has been correlated with other such anomalies worldwide and together with overlying tillites has been cited as evidence of major (worldwide) glaciation events. In this study, we show that the magnitude of this negative delta C-13 anomaly can partly be explained by exchange with metamorphic fluids. However, we also show that extremely negative delta C-13 values in the Bonahaven Dolomite Formation, which overlies the Port Askaig Formation and has been interpreted as a 'cap carbonate', cannot be attributed to metamorphic fluid flow.

  • 29. Sosnicka, Marta
    et al.
    Bakker, Ronald J.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Paranko, Ihor
    Burlinson, Kingsley
    Fluid types and their genetic meaning for the BIF-hosted iron ores, Krivoy Rog, Ukraine2015In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360, Vol. 68, p. 171-194Article in journal (Refereed)
    Abstract [en]

    This paper contributes to the understanding of the genesis of epigenetic, hypogene BIF-hosted iron deposits situated in the eastern part of Ukrainian Shield. It presents new data from the Krivoy Rog iron mining district (Skelevatske-Magnetitove deposit, Frunze underground mine and Balka Severnaya Krasnaya outcrop) and focuses on the investigation of ore genesis through application of fluid inclusion petrography, micro-thermometry, Raman spectroscopy and baro-acoustic decrepitation of fluid inclusions. The study investigates inclusions preserved in quartz and magnetite associated with the low-grade iron ores (31-37% Fe) and iron-rich quartzites (38-45% Fe) of the Saksaganskaya Suite, as well as magnetite from the locally named high-grade iron ores (52-56% Fe). These high-grade ores resulted from alteration of iron quartzites in the Saksaganskiy thrust footwall (Saksaganskiy tectonic block) and were a precursor to supergene martite, high-grade ores (60-70% Fe). Based on the new data two stages of iron ore formation (metamorphic and metasomatic) are proposed. The metamorphic stage, resulting in formation of quartz veins within the low-grade iron ore and iron-rich quartzites, involved fluids of four different compositions: CO2-rich, H2O, H2O-CO2(+/- N-2-CH4)-NaCl(+/- NaHCO3) and H2O-CO2(+/- N-2-CH4)-NaCl. The salinities of these fluids were relatively low (up to 7 mass% NaCl equiv.) as these fluids were derived from dehydration and decarbonation of the BIF rocks, however the origin of the nahcolite (NaHCO3) remains unresolved. The minimum P-T conditions for the formation of these veins, inferred from microthermometry are T-min = 219-246 degrees C and P-min = 130-158 MPa. The baro-acoustic decrepitation analyses of magnetite bands indicated that the low-grade iron ore from the Skelevatske-Magnetitove deposit was metamorphosed at T = similar to 530 degrees C. The metasomatic stage post-dated and partially overlapped the metamorphic stage and led to the upgrade of iron quartzites to the high-grade iron ores. The genesis of these ores, which are located in the Saksaganskiy tectonic block (Saksaganskiy ore field), and the factors controlling iron ore-forming processes are highly controversial. According to the study of quartz-hosted fluid inclusions from the thrust zone the metasomatic stage involved at least three different episodes of the fluid flow, simultaneous with thrusting and deformation. During the 1st episode three types of fluids were introduced: CO2-CH4-N-2(+/- C), CO2(+/- N-2-CH4) and low salinity H2O-N-2-CH4-NaCl (6.38-7.1 mass% NaCl equiv.). The 2nd episode included expulsion of the aqueous fluids H2O-N2CH4-NaCl(+/- CO2, +/- C) of moderate salinities (15.22-16.76 mass% NaCl equiv.), whereas the 3rd event involved high salinity fluids H2O-NaCl(+/- C) (20-35 mass% NaCl equiv.). The fluids most probably interacted with country rocks (e.g. schists) supplying them with CH4 and N-2. The high salinity fluids were most likely either magmatic-hydrothermal fluids derived from the Saksaganskiy igneous body or heated basinal brines, and they may have caused pervasive leaching of Fe from metavolcanic and/or the BIF rocks. The baro-acoustic decrepitation analyses of magnetite comprising the high-grade iron ore showed formation T = similar to 430-500 degrees C. The fluid inclusion data suggest that the upgrade to high-grade Fe ores might be a result of the Krivoy Rog BIF alteration by multiple flows of structurally controlled, metamorphic and magmatic-hydrothermal fluids or heated basinal brines.

  • 30. Webber, A. P.
    et al.
    Roberts, S.
    Taylor, R. N.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Golden plumes: Substantial gold enrichment of oceanic crust during ridge-plume interaction2013In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 41, no 1, p. 87-90Article in journal (Refereed)
    Abstract [en]

    Mantle plume events are increasingly implicated as the source of gold (Au) in regions of the Earth that show a high Au endowment. However, the process of enriching oceanic crust in Au by plume activity is poorly understood and unconstrained. We present the first systematic study of Au concentrations in oceanic basalts as a function of distance from a plume center. We show that the influence of the Iceland plume on the Mid-Atlantic Ridge progressively enriches the oceanic crust in Au along the Reykjanes Ridge by as much as 13 times normal levels, over a distance of similar to 600 km, and that the enrichment can be attributed to specific plume components. This Au enrichment by the Iceland plume implies a genetic relationship between deep mantle upwelling and major gold mineralization.

  • 31. Zoheir, Basem
    et al.
    Deshesh, Fatma
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    El-Metwally, Ahmed
    Mashaal, Shabaan
    Granitoid-associated gold mineralization in Egypt: a case study from the Atalla mine2018In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 53, no 5, p. 701-720Article in journal (Refereed)
    Abstract [en]

    Gold-bearing sulfide-quartz veins cutting mainly through the Atalla monzogranite intrusion in the Eastern Desert of Egypt are controlled by subparallel NE-trending brittle shear zones. These veins are associated with pervasive sericite-altered, silicified, and ferruginated rocks. The hosting shear zones are presumed as high-order structures of the Najd-style faults in the Central Eastern Desert (similar to 615-585 Ma). Ore minerals include an early pyrite-arsenopyrite (+/- pyrrhotite) mineralization, partly replaced by a late pyrite-galena-sphalerite-chalcopyrite (+/- gold/electrum +/- tetrahedrite +/- hessite) assemblage. Gold occurs as small inclusions in pyrite and arsenopyrite, or more commonly as intergrowths with galena and sphalerite/tetrahedrite in microfractures. Arsenopyrite geothermometry suggests formation of the early Fe-As-sulfide mineralization at 380-340 A degrees C, while conditions of deposition of the late base metal-gold assemblage are assumed to be below 300 A degrees C. Rare hessite, electrum, and Bi-galena are associated with sphalerite and gold in the late assemblage. The early and late sulfide minerals show consistently a narrow range of delta S-34 aEuro degrees (3.4-6.5) that overlaps with sulfur isotopic values in ophiolitic rocks. The Au-quartz veins are characterized by abundant CO2 and H2O +/- CO2 +/- NaCl inclusions, where three-dimensional clusters of inclusions show variable aqueous/carbonic proportions and broad range of total (bimodal) homogenization temperatures. Heterogeneous entrapment of immiscible fluids is interpreted to be caused by unmixing of an originally homogenous, low salinity (similar to 2 eq. mass % NaCl) aqueous-carbonic fluid, during transition from lithostatic to hydrostatic conditions. Gold deposition occurred generally under mesothermal conditions, i.e., 1.3 kbar and similar to 280 A degrees C, and continued during system cooling to < 200 A degrees C and pressure decrease to similar to 0.1 kbar. Based on the vein textures, sulfur isotope values, composition of ore fluids, and conditions of ore formation, we suggest that the Atalla monzogranite intrusion acted only as a competent structural host for ore deposition from shear-related, metal-rich fluids migrated up from depth. This model is also presumed for most granitoid-associated Au deposits in the region, considering the similarity in their structural control, alteration pattern and mineralogy, and chemistry of the ore fluids.

1 - 31 of 31
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf