| 1. |
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| 2. |
- Andersson, Stefan S., et al.
(författare)
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Origin of the high-temperature Olserum-Djupedal REE-phosphate mineralisation, SE Sweden : A unique contact metamorphic-hydrothermal system
- 2018
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 101, s. 740-764
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Tidskriftsartikel (refereegranskat)abstract
- The Swedish part of the Fennoscandian Shield hosts a variety of rare earth element (REE) deposits, including magmatic to magmatic-hydrothermal types. This paper focuses on the origin of the Olserum-Djupedal REEphosphate mineralisation located in the sparsely studied Vastervik region, SE Sweden. Here, mineralisation occurs in three main areas, Olserum, Djupedal and Bersummen. Primary hydrothermal REE mineralisation formed at high temperatures (about 600 degrees C), leading to precipitation of monazite-(Ce), xenotime-(Y), fluor apatite and minor (Y,REE,U,Fe)-(Nb,Ta)-oxides in veins and vein zones dominated by biotite, amphibole, magnetite and quartz. The veins are hosted primarily by metasedimentary rocks present close to, or within, the contact aureole of a local 1.8 Ga ferroan alkali feldspar granite pluton, but also occur within in the chemically most primitive granite in the outermost part of that pluton. In the Djupedal area, REE-mineralised metasedimentary bodies are extensively migmatised, with migmatisation post-dating the main stage of mineralisation. In the Olserum and Bersummen areas, the REE-bearing veins are cross-cut by abundant pegmatitic to granitic dykes. The field-relationships demonstrate a-protracted magmatic evolution of the granitic,pluton and a clear spatial and temporal relationship of the REE mineralisation to the granite. The major and trace element chemistry of ore-associated biotite and magnetite support genetic links between all mineralised areas. Biotite mineral chemistry data further demonstrate a distinct chemical trend from meta sediment-hosted ore-associated biotite distal to the major contact of the granite to the biotite in the granite hosted veins. This trend is characterised by a systematic decrease in Mg and Na and a coupled increase in Fe and Ti with proximity to the granite-hosted veins. The halogen compositions of ore-associated biotite indicate elevated contents of HCl and HF in the primary REE mineralising fluid. Calculated log(f(HF)/f(HCL)) values in the Olserum area suggest a constant ratio of about -1 at temperatures of 650-550 degrees C during the evolution of the primary hydrothermal system. In the Djupedal and Bersummen areas, the fluid locally equilibrated at lower log (f(HF)/f(HCl)) values down to -2. High Na contents in ore-associated biotite and amphibole, and the abundance of primary ore-associated biotite indicate a K- and Na-rich character of the primary REE mineralising fluid and suggest initial high-temperature K-Na metasomatism. With subsequent cooling of the system, the fluid evolved locally to more Ca-rich compositions as indicated by the presence of the Ca-rich minerals allanite-(Ce) and uvitic tourmaline and by the significant calcic alteration of monazite-(Ce). The later Ca-rich stages were probably coeval with low to medium-high temperature (200-500 degrees C) Na-Ca metasomatism variably affecting the granite and the wall rocks, producing distinct white quartz-plagioclase rocks. All observations and data lead us to discard the prevailing model that the REE mineralisation in the Olserum-Djupedal district represents assimilated and remobilised former heavy mineral-rich beds. Instead, we propose that the primary REE mineralisation formed by granite-derived fluids enriched in REE and P that were expelled early during the evolution of a local granitic pluton. The REE mineralisation developed primarily in the contact aureole of this granite and represents the product of a high temperature contact metamorphic-hydrothermal mineralising system. The REE mineralisation probably formed synchronously with K-Na and subsequent Na-Ca metasomatism affecting the granite and the wall rocks. The later Na-Ca metasomatic stage is probably related to a regional Na +/- Ca metasomatic and associated U +/- REE mineralising system operating concurrently with granitic magmatism at c. 1.8 Ga in the Vastervik region. This highlights the potential for discovering hitherto unknown REE deposits and for the reappraisal of already known deposits in this part of the Fennoscandian Shield.
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| 3. |
- Bernet, Matthias, et al.
(författare)
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Multidisciplinary petro-geo-thermochronological approach to ore deposit exploration
- 2019
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368. ; 112:Sept.
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Forskningsöversikt (refereegranskat)abstract
- The continuous search for mineral resources such as gold, copper and base metals etc. requires a refined understanding of the thermal evolution and exhumation history of potential exploration targets, as only ore deposits can be exploited that have been exhumed to the surface or near to the surface. In order to better understand the exhumation history of the ore deposits a multidisciplinary approach using single grain geo- and thermochronology double/triple dating in combination with other geochemical, fluid inclusion and petrological analyses, can significantly support prospecting for such resources, particularly in frontier areas, where little geological information exists. Geo-thermochronology provides information on the timing of initial host rock crystallization, ore deposit formation and subsequent exhumation as the thermal history can be modelled and exhumation rates can be determined. Here we present a methodological outline on which single-grain double/triple-dating techniques can be used, what sampling strategies may be applied, and how the data may be interpreted.
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| 4. |
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| 5. |
- Cave, Ben J., et al.
(författare)
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Multi-stage precipitation and redistribution of gold, and its collection by lead-bismuth and lead immiscible liquids in a reduced-intrusion related gold system (RIRGS); Dublin Gulch, western Canada
- 2019
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 106, s. 28-55
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Tidskriftsartikel (refereegranskat)abstract
- Dublin Gulch reduced intrusion-related gold system (RIRGS), located in the Selwyn Basin area of western Canada, represents one of the best examples of RIRGS mineralization globally and hence can be studied to unravel genesis and evolution of these types of deposits. Based on textural relationships, mineralogy, and trace element mineral chemistry, three auriferous vein stages were identified. The paragenetic sequence for the auriferous vein stages are: 1) Eagle Style (ES), quartz-albite, low sulfide and sulfosalt content ( < 5% vol.), As-Fe-Mo-W-Pb-Bi-Au-Ag veins; 2) Potato Hills Style-1 (PHS-1), quartz, high sulfide and sulfosalt content ( > 30% vol.), As-Fe-W-Pb-Bi-Au-Ag veins; and, 3) Potato Hills Style-2 (PHS-2), high sulfide and sulfosalt content ( > 30% vol.), Fe-Pb-Sb-Zn-Cu-Au-Ag veins. In the ES and PHS-1 veins, Au is present both as native gold (Au, Ag) and as invisible gold in arsenopyrite, whilst in the PHS-2 veins, Au is present as invisible gold in pyrite. Native gold micrograins (individual grains, 1-100's mu m in size) are observed associated with Pb minerals (in anhedral-toglobular cosalite (Pb2Bi2S5) in ES veins, or galena (PbS) in the PHS-1 veins]. Native gold is also observed as micrograins along arsenopyrite margins and in quartz fractures. We suggest a variation on the hydrothermal Bi melt collector model to explain the Au-Pb +/- Bi association. The Au, Ag, Pb, and Bi are interpreted to have been locally remobilized from arsenopyrite, which shows textures and trace element distribution patterns consistent with fluid-and-deformation assisted recrystallization. We suggest Pb and Bi were mobilized either as immiscible nanodroplets that coalesced to form larger Pb +/- Bi liquid accumulations or into the hydrothermal fluid and subsequently exsolved to form immiscible Pb +/- Bi liquids. We propose that remobilized Au and Ag were collected by these Pb +/-Bi immiscible liquids. Subsequent retrograde alteration (sulfidation) is interpreted to have converted the Au-Ag-Pb +/- Bi alloys to native gold and cosalite (ES), and native gold and galena mineral assemblages (PHS-1). The similarity of the Au/Ag ratios in native gold and arsenopyrite supports a local source for the native gold micrograins. Temperatures required to attain liquid Pb-0.Bi-5(0).(5) ( > 145.2 degrees C) and Pb (> 327.5 degrees C), are consistent with arsenopyrite geothermometry (ES 345-405 degrees C; PHS-1 approximate to 380 degrees C). These suggested new variations (Pb-Bi and Pb) on the hydrothermal melt (Bi) collector model are important, given the common association of native gold with Pb +/- Bi sulfosalts in many gold deposits, and the relatively low temperatures required to maintain these liquids and collect Au from the hydrothermal fluid.
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| 6. |
- Goodenough, K. M., et al.
(författare)
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Europe's rare earth element resource potential : An overview of REE metallogenetic provinces and their geodynamic setting
- 2016
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 72, s. 838-856
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Tidskriftsartikel (refereegranskat)abstract
- Security of supply of a number of raw materials is of concern for the European Union; foremost among these are the rare earth elements (REE), which are used in a range of modern technologies. A number of research projects, including the EURARE and ASTER projects, have been funded in Europe to investigate various steps along the REE supply chain. This paper addresses the initial part of that supply chain, namely the potential geological resources of the REE in Europe. Although the REE are not currently mined in Europe, potential resources are known to be widespread, and many are being explored. The most important European resources are associated with alkaline igneous rocks and carbonatites, although REE deposits are also known from a range of other settings. Within Europe, a number of REE metallogenetic belts can be identified on the basis of age, tectonic setting, lithological association and known REE enrichments. This paper reviews those metallogenetic belts and sets them in their geodynamic context. The most well-known of the REE belts are of Precambrian to Palaeozoic age and occur in Greenland and the Fennoscandian Shield. Of particular importance for their REE potential are the Gardar Province of SW Greenland, the Svecofennian Belt and subsequent Mesoproterozoic rifts in Sweden, and the carbonatites of the Central Iapetus Magmatic Province. However, several zones with significant potential for REE deposits are also identified in central, southern and eastern Europe, including examples in the Bohemian Massif, the Iberian Massif, and the Carpathians.
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| 7. |
- Jansson, Nils F., et al.
(författare)
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Multistage ore formation at the Ryllshyttan marble and skarn-hosted Zn-Pb-Ag-(Cu) + magnetite deposit, Bergslagen, Sweden
- 2015
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 69, s. 217-242
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Tidskriftsartikel (refereegranskat)abstract
- Numerous magnetite skarn deposits and marble- and skarn-hosted base metal sulphide deposits occur in polydeformed and metamorphosed, felsic-dominated metavolcanic inliers in the Palaeoproterozoic Bergslagen region of south-central Sweden, including the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit, approximately 2.5 km SW of the large Garpenberg Zn-Pb-Ag-(Cu-Au) deposit. The Ryllshyttan deposit, from which approximately 1 Mt of Zn-rich massive sulphide ore and 0.2 Mt of semi-massive magnetite were extracted, is located near a transition between magnesian skarn and dolomitic marble. The host unit consists of a 10-20 m-thick former calcitic limestone of likely stromatolitic origin that is commonly pervasively altered to skarn, locally hosting magnetite skarn deposits. The ore-bearing unit is one of several mineralised marble units within a more than 1 km-thick, felsic-dominated metavolcanic succession that includes a metamorphosed, large caldera-fill pyroclastic deposit, 800 m stratigraphically above the Ryllshyttan host succession. The Garpenberg stratabound Zn-Pb-Ag-(Cu)-(Au) deposit is located higher in the stratigraphy, just below the caldera fill deposits. The metavolcanic succession is bounded to the NW by a large granitoid batholith and intruded by a microgranodiorite pluton less than a 100 m from the Ryllshyttan deposit. Magnetite laminae in bedded skarns and metavolcanic rocks in the hanging wall of Ryllshyttan indicate an early (syngenetic) accumulation of Fe-rich exhalites. In contrast, the sulphide mineralisation consists of stratabound replacement-style ore associated with dolomitisation of the host and with discordant K-Mg-Fe±Si alteration of volcanic rocks and early porphyritic intrusions in the footwall and hanging wall. The microgranodiorite that intrudes the host succession crosscuts the K-Mg-Fe±Si alteration envelope and is overprinted by Na-Ca alteration (diopside and plagioclase-bearing mineral associations) that also overprints K-Mg-Fe±Si-altered rocks. The Na-Ca alteration is interpreted to be associated with the formation of calcic and magnesian iron skarn deposits semi-regionally at a similar stratigraphic position. Despite superimposed amphibolite facies regional metamorphism and substantial syn-D2-D3 remobilisation of sulphides concurrent with retrograde alteration of skarn assemblages, cross-cutting field relationships indicate that the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit results from protracted VMS-style hydrothermal activity including early seafloor mineralisation (Fe-rich exhalites), closely followed by sub-seafloor carbonate-replacement-style mineralisation (base metal-bearing massive sulphides). Both mineralisation styles were overprinted by contact metasomatism associated with the formation of abundant magnetite skarn deposits during the emplacement of granitoid intrusions. As for other deposits in the Bergslagen region, the ore-forming system at Ryllshyttan thus has similarities to both metamorphosed VMS deposits and metasomatic Fe and Zn skarn deposits. Our results suggest that the sequence of volcanic, intrusive and hydrothermal events in this region is compatible with prograde heating of a long-lived hydrothermal system, wherein a shift from a convective seawater-dominated system to a contact metamorphic and/or metasomatic environment occurred during the early stage of the 1.9-1.8 Ga Svecokarelian orogeny. This model partly resolves the controversy regarding genesis of the iron oxide and base metal sulphide deposits in Bergslagen, as we recognise that these deposits have a complex history of alteration, metamorphism, deformation and (re)mobilisation, and no unique established genetic model can account for all their features.
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| 8. |
- Jansson, Nils, et al.
(författare)
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Genesis of the Zinkgruvan stratiform Zn-Pb-Ag deposit and associated dolomite-hosted Cu ore, Bergslagen, Sweden
- 2016
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 82, s. 285-308
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Tidskriftsartikel (refereegranskat)abstract
- Zinkgruvan, a major stratiform Zn-Pb-Ag deposit in the Paleoproterozoic Bergslagen region, south-central Sweden, was overprinted by polyphase ductile deformation and high-grade metamorphism (including partial melting of the host succession) during the 1.9-1.8 Ga Svecokarelian orogeny. This complex history of post-ore modification has made classification of the deposit difficult. General consensus exists on a syngenetic-exhalative origin, yet the deposit has been variably classified as a volcanogenic massive sulfide (VMS) deposit, a sediment-hosted Zn (SEDEX) deposit, and a Broken Hill-type (BHT) deposit. Since 2010, stratabound, cobaltiferous and nickeliferous Cu ore, comprising schlieren and impregnations of Cu, Co and Ni sulfide minerals in dolomitic marble, is mined from the stratigraphic footwall to the stratiform Zn-Pb-Ag ore. This ore type has not been fully integrated into any of the existing genetic models. Based on a combination of 1) widespread hematite-staining and oxidizing conditions (Fe2O3>FeO) in the stratigraphic footwall, 2) presence of graphite and reducing conditions (Fe2O3 5 km) Zn-Pb-Ag ore was precipitated.Both ore types are characterized by significant spread in δ34S, with the sulphur in the Cu ore and associate marble-hosted Zn mineralization on average being somewhat heavier (δ34S = -4.7 to +10.5 ‰, average 3.9 ‰) than that in the stratiform Zn-Pb-Ag ore (δ34S = -6 to +17 ‰, average 2.0 ‰). The ranges in δ34S are significantly larger than those observed in syn-volcanic massive sulphide deposits in Bergslagen, for which simple magmatic/volcanic sulphur sources have been invoked. Mixing of magmatic-volcanic sulfur leached from underlying volcanic rocks and sulfur sourced from abiotic or bacterial sulfate reduction in a mixing zone at the seafloor could explain the range observed at Zinkgruvan.A distinct discontinuity in the stratigraphy, at which key stratigraphic units stop abruptly, is interpreted as a syn-sedimentary fault. Metal zonation in the stratiform ore (decreasing Zn/Pb from distal to proximal) and the spatial distribution of Cu mineralization in underlying dolomitic marble suggest that this fault was a major feeder to the mineralization. Our interpretation of ore-forming fluid composition and a dominant redox trap rather than a pH and/or temperature trap differs from most VMS models, with Selwyn-type SEDEX models, and most BHT models. Zinkgruvan has similarities to both McArthur-type SEDEX deposits and sediment-hosted Cu deposits in terms of the inferred ore fluid chemistry, yet the basinal setting has more similarities to BHT and felsic-bimodal VMS districts. We speculate that besides an oxidized footwall stratigraphy, regionally extensive banded iron formations and limestone horizons in the Bergslagen stratigraphy may have aided in buffering ore-forming brines to oxidized, near-neutral conditions. In terms of fluid chemistry, Zinkgruvan could comprise one of the oldest known manifestations of Zn and Cu ore-forming systems involving oxidized near-neutral brines following oxygenation of the Earth’s atmosphere.
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| 9. |
- Kampmann, Tobias Christoph, 1987-, et al.
(författare)
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Syn-tectonic sulphide remobilization and trace element redistribution at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden
- 2018
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Ingår i: Ore Geology Reviews. - : Elsevier. - 0169-1368 .- 1872-7360. ; 96, s. 48-71
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Tidskriftsartikel (refereegranskat)abstract
- Mineralization types at the Palaeoproterozoic Falun base metal sulphide deposit are predominantly pyritic Zn-Pb-Cu-rich massive sulphide mineralization, disseminated to semi-massive Cu-Au mineralization, auriferous quartz veins, and mineralized shear zones of talc-chlorite-dominated schist. The massive and disseminated to semi-massive sulphide mineralization types were subject to polyphase ductile deformation (D1 and D2) and metamorphism under low-P, lower-amphibolite facies conditions, which led to the development of ore textures and paragenetic relationships indicating both mechanical and chemical remobilization of sulphides. In the massive sulphide mineralization, rare inclusion-rich pyrite occurs as relic cores inside inclusion-poor metamorphosed pyrite. Imaging and spot analysis using multielement laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) reveal that inclusion-poor pyrite was depleted in trace elements, which were originally present as non-stoichiometric lattice substitutions or in mineral inclusions. The inclusion-rich pyrite was shielded from depletion and, at least partly, retained its initially higher trace element concentrations, including Au.Gold is also associated with chalcopyrite in the disseminated to semi-massive Cu-Au mineralization and in the system of auriferous quartz veins hosted therein, the latter being also affected by the D2 ductile strain. It is inferred that emplacement of the vein system took place after the peak of metamorphism, which occurred between D1 and D2, but prior to and possibly even shortly after completion of the D2 deformational event. Similarities in trace element signatures in chalcopyrite are compatible with the interpretation that the quartz veins formed by local chemical remobilization of components from the Cu-Au mineralization. Transport of liberated Au from pyrite during grain growth in the massive sulphide mineralization may have upgraded the Au endowment in the quartz veins, leading to the additional formation of native gold in the veins. A strong correspondence between elements liberated from pyrite (e.g. Pb, Bi, Se and Au) and those forming discrete and characteristic mineral phases in the quartz veins (Pb-Bi sulphosalts, native gold) supports this hypothesis.Trace element signatures for the main sulphide minerals pyrite, chalcopyrite, sphalerite and galena are similar to previously published data from other metamorphosed massive sulphide deposits. The association of the Falun mineralization with elevated Bi is reflected by its occurrence in sulphide minerals (e.g. galena) and in abundant mineral inclusions of Pb-Bi sulphosalts (e.g. weibullite), especially in the disseminated to semi-massive Cu-Au mineralization. Elevated Sn concentrations in the lattice and/or as cassiterite inclusions in chalcopyrite, sphalerite and galena are compatible with a hot, acidic and reducing fluid during formation of the syn-volcanic, base metal sulphide mineralization and associated host-rock alteration.
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| 10. |
- Konopelko, D., et al.
(författare)
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Precambrian gold mineralization at Djamgyr in the Kyrgyz Tien Shan : Tectonic and metallogenic implications
- 2017
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368. ; 86, s. 537-547
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Tidskriftsartikel (refereegranskat)abstract
- The Djamgyr gold deposit is located within the Neoproterozoic basement of the Middle Tien Shan terrane immediately west of the Talas-Fergana fault. The deposit comprises a system of auriferous quartz veins cross-cutting the Beshtor plagiogranite. The veins are surrounded by hydrothermal alteration aureoles and are oriented parallel to the Talas-Fergana fault. The Beshtor granite sampled in the vicinity of the deposit yielded a Neoproterozoic (Tonian) U-Pb zircon age of 815 ± 6 Ma, which is the first single grain zircon age of the Middle Tien Shan basement west of the Talas-Fergana fault. Ar-Ar dating of two muscovite fractions from the alteration aureoles of the auriferous quartz veins yielded ages of 804 ± 3 and 805 ± 3 Ma suggesting that the mineralization in the Djamgyr deposit occurred during the Neoproterozoic ca. 10 m.y. after emplacement of the Beshtor granite. The structural pattern of the auriferous quartz veins and the new geochronological data, combined with the results of previous structural studies, may tentatively constrain the age of pre-existing major fault possibly marking an inherited terrane boundary in the northern part of the present-day Talas-Fergana strike-slip fault. The discovery of Precambrian gold mineralization in the Middle Tien Shan suggests re-evaluation of the metallogenic potential of its Precambrian basement that occupies significant areas west and east of the Talas-Fergana fault.
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| 11. |
- Lisitsin, Vladimir A., et al.
(författare)
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Orogenic gold mineral systems of the Western Lachlan Orogen (Victoria) and the Hodgkinson Province (Queensland) : Crustal metal sources and cryptic zones of regional fluid flow
- 2016
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 76, s. 280-295
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 12. |
- Mahdy, Nasser M., et al.
(författare)
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Petrogenesis of U- and Mo-bearing A(2)-type granite of the Gattar batholith in the Arabian Nubian Shield, Northeastern Desert, Egypt : Evidence for the favorability of host rocks for the origin of associated ore deposits
- 2015
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 71, s. 57-81
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Tidskriftsartikel (refereegranskat)abstract
- Hydrothermal ore deposits associated with granitic rocks are the result of combinations and series of successive processes, events and conditions in a magma system and at post-magmatic stage. The metal content of a granitic magma as well as fractional crystallization, metal-ligand complexation, emplacement mechanisms and tectonic processes influence the metal precipitation and content of a hydrothermal ore deposit. The present study of the Gattar batholith evaluates how these processes contribute to the generation of an ore deposit. Gattar batholith in the north Eastern Desert of Egypt as part of the northern region of the Arabian-Nubian Shield (ANS) is geographically, mineralogically and geochemically divided into two distinctive areas. The southern part mainly consists of syenogranite while the northern part is dominated by highly evolved alkali-feldspar granite. Uranium and Mo mineralization occurring here is mostly limited to the margin of the highly evolved alkali-feldspar granites. New U-Pb zircon geochronology within this study indicates an age of similar to 620-600 Ma, although high common Pb and discordant age data of many zircon grains reflect alteration by F-rich fluids. Some zircons with distinct older U/Pb ages were considered to be inherited from the wall rocks representing juvenile crust of the ANS. The two different granitic rocks are comagmatic in origin and have A-type characteristics generated by partial melting of lower juvenile crust of the ANS. Geochemical data are in good agreement with a magmatic origin of the alkali-feldspar granites, suggesting fractional crystallization of a syenogranitic source as the most favorable process of their formation. Petrography, geochemistry, and geochronology studies of the Gattar granites along with field observations provide evidence for multiple intrusions as a result of successive magma pulses with different chemical composition. This led to the formation of reverse zoning within the intrusive complex. This kind of magma emplacement promotes the escape of exsolved metal-bearing volatile-rich fluids into the boundary of the Hammamat Sedimentary Rocks (HSR) and Gattar granite at the northern part of the batholith. Build-up of volatiles is a common phenomenon in granitic rocks and typically results in enhanced contents of F, alkalis and ore metals released at subsolidus stage.
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| 13. |
- Maries, Georgiana, et al.
(författare)
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Downhole physical property logging for iron-oxide exploration, rock quality, and mining : An example from central Sweden
- 2017
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Ingår i: Ore Geology Reviews. - : Elsevier. - 0169-1368 .- 1872-7360. ; 90, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Several physical properties obtained from geophysical logging and laboratory measurements were analyzed in order to characterize iron-oxide mineralization and host rocks in the Blotberget mining area of central Sweden. Seven boreholes intersecting the mineralization between 300 and 600 m depth in a volcano-sedimentary setting were downhole logged for this purpose. The downhole logging included full-waveform triple sonic, natural gamma, magnetic susceptibility, formation resistivity, fluid temperature and conductivity while laboratory measurements consisted of density, rock quality designation and magnetite content measurements. Full-waveform sonic data were used for rock quality assessments of the mineralized zones and their host rocks and proved their potential to be used for mine planning purposes. The ore-bearing rocks are primarily distinguished by increased density and dynamic elastic moduli estimated from the full-waveform sonic logging. Although seismic velocities do not follow a linear increase with the density for the mineralized rocks, as expected for igneous rocks, it is observed that a strong seismic signal from the mineralization can be expected primarily due to their high density. In addition, the full-waveform sonic data could be used as a proxy for fracture delineations and in situ rock quality assessment. For example, zones of washed-up amplitudes in these data correlate well with zones of poor quality rocks, identified by core logging. Based on the full-waveform sonic data, seismic attenuation and its reciprocal, the seismic quality factor, were calculated. A decrease in the seismic quality factor was observed at zones with low rock quality designation, but also correlated with the mineralized zones suggesting several fracture zones in the mineralization and that the ore-bearing rocks might be less competent than the surrounding host rocks. Eventual rock support and reinforcement might be required for future mining operations. Full-waveform sonic data have the potential to improve rock quality assessments for mine planning and exploration purposes.
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| 14. |
- Martinsson, Olof, et al.
(författare)
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Metallogeny of the Northern Norrbotten Ore Province, northern Fennoscandian Shield with emphasis on IOCG and apatite-iron ore deposits
- 2016
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 78, s. 447-492
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Forskningsöversikt (refereegranskat)abstract
- The Northern Norrbotten Ore Province in northernmost Sweden includes the type localities for Kiruna-type apatite iron deposits and has been the focus for intense exploration and research related to Fe oxide-Cu-Au mineralisation during the last decades. Several different types of Fe-oxide and Cu-Au +/- Fe oxide mineralisation occur in the region and include: stratiform Cu +/- Zn +/- Pb +/- Fe oxide type, iron formations (including BIF's), Kiruna-type apatite iron ore, and epigenetic Cu +/- Au +/- Fe oxide type which may be further subdivided into different styles of mineralisation, some of them with typical IOCG (Iron Oxide-Copper-Gold) characteristics. Generally, the formation of Fe oxide +/- Cu +/- Au mineralisation is directly or indirectly dated'between-2.1 and 1.75 Ga, thus spanning about 350 m.y. of geological evolution. The current paper will present in more detail the characteristics of certain key deposits, and aims to put the global concepts of Fe-oxide Cu-Au mineralisation into a regional context. The focus will be on iron deposits and various types of deposits containing Fe-oxides and Cu-sulphides in different proportions which generally have some characteristics in common with the IOCG style. In particular, ore fluid characteristics (magmatic versus non magmatic) and new geochronological data are used to link the ore-forming processes with the overall crustal evolution to generate a metallogenetic model. Rift bounded shallow marine basins developed at similar to 2.1-2.0 Ga following a long period of extensional tectonics within the Greenstone-dominated, 2.5-2.0 Ga Karelian craton. The similar to 1.9-1.8 Ga Svecofennian Orogen is characterised by subduction and accretion from the southwest. An initial emplacement of calc-alkaline magmas into similar to 1.9 Ga continental arcs led to the formation of the Haparanda Suite and the Porphyrite Group volcanic rocks. Following this early stage of magmatic activity, and separated from it by the earliest deformation and metamorphism, more alkali-rich magmas of the Perthite Monzonite Suite and the Kiirunavaara Group volcanic rocks were formed at similar to 1.88 Ga. Subsequently, partial melting of the middle crust produced large volumes of similar to 1.85 and 1.8 Ga S-type granites in conjunction with subduction related A-/I-type magmatism and associated deformation and metamorphism. In our metallogenetic model the ore formation is considered to relate to the geological evolution as follows. Iron formations and a few stratiform sulphide deposits were deposited in relation to exhalative processes in rift bounded marine basins. The iron formations may be sub-divided into BIF-(banded iron formations) and Mg rich types, and at several locations these types grade into each other. There is no direct age evidence to constrain the deposition of iron formations, but stable isotope data and stratigraphic correlations suggest a formation within the 2.1-2.0 Ga age range. The major Kiruna-type ores formed from an iron-rich magma (generally with a hydrothermal over-print) and are restricted to areas occupied by volcanic rocks of the Kiirunavaara Group. It is suggested here that 1.89-1.88 Ga tholeiitic magmas underwent magma liquid immiscibility reactions during fractionation and interaction with crustal rocks, including metaevaporites, generating more felsic magmatic rocks and Kiruna-type iron deposits. A second generation of this ore type, with a minor economic importance, appears to have been formed about 100 Ma later. The epigenetic Cu-Au +/- Fe oxide mineralisation formed during two stages of the Svecofennian evolution in association with magmatic and metamorphic events and crustal scale shear zones. During the first stage of mineralisation, from 1.89-1.88 Ga, intrusion-related (porphyry style) mineralisation and Cu-Au deposits of IOCG affinity formed from magmatic-hydrothermal systems, whereas vein-style and shear zone deposits largely formed at c. 1.78 Ga. The large range of different Fe oxide and Cu-Au +/- Fe oxide deposits in Northern Norrbotten is associated with various alteration systems, involving e.g. scapolite, albite, K feldspar, biotite, carbonates, tourmaline and sericite. However, among the apatite iron ores and the epigenetic Cu-Au +/- Fe oxide deposits the character of mineralisation, type of ore- and alteration minerals and metal associations are partly controlled by stratigraphic position (i.e. depth of emplacement). Highly saline, NaCl + CaCl2 dominated fluids, commonly also including a CO2-rich population, appear to be a common characteristic feature irrespective of type and age of deposits. Thus, fluids with similar characteristics appear to have been active during quite different stages of the geological evolution. Ore fluids related to epigenetic Cu-Au Fe oxides display a trend with decreasing salinity, which probably was caused by mixing with meteoric water. Tentatively, this can be linked to different Cu-Au ore paragenesis, including an initial (magnetite)-pyrite-chalcopyrite stage, a main chalcopyrite stage, and a late bornite stage. Based on the anion composition and the Br/Cl ratio of ore related fluids bittern brines and metaevaporites (including scapolite) seem to be important sources to the high salinity hydrothermal systems generating most of the deposits in Norrbotten. Depending on local conditions and position in the crust these fluids generated a variety of Cu-Au deposits. These include typical IOCG-deposits (Fe-oxides and Cu-Au are part of the same process), IOCG of iron stone type (pre-existing Fe-oxide deposit with later addition of Cu-Au), IOCG of reduced type (lacking Fe-oxides due to local reducing conditions) and vein-style Cu-Au deposits. From a strict genetic point of view, IOCG deposits that formed from fluids of a mainly magmatic origin should be considered to be a different type than those deposits associated with mainly non-magmatic fluids. The former tend to overlap with porphyry systems, whereas those of a mainly non-magmatic origin overlap with sediment hosted Cu-deposits with respect to their origin and character of the ore fluids.
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| 15. |
- Molnár, F., et al.
(författare)
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Boron, sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt, eastern Finland
- 2016
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 77, s. 133-162
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Tidskriftsartikel (refereegranskat)abstract
- The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73 Ga volcanic and sedimentary sequences, as well as 2.75–2.72 Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70 Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5 kbar pressures and at 500–600 °C temperatures affected the belt at around 2.70 Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe3 +–Al3 +-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ11B data for this kind of tourmaline is from − 17.2‰ to − 12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe3 +–Al3 + substitution) and δ11B values between − 19.0‰ and − 9.6‰. The uvite–foitite compositional trend and δ11B ‰ values between − 24.1% and − 13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ11B values between − 14.5‰ and − 10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ34SVCDT values measured in pyrite, chalcopyrite and pyrrhotite is from − 9.1 to + 8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ34SVCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ34SVCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ34SVCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ65CuNBS978 values from chalcopyrite is from − 1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ65CuNBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ34SVCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.
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| 16. |
- Murton, Bramley J., et al.
(författare)
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Geological fate of seafloor massive sulphides at the TAG hydrothermal field (Mid-Atlantic Ridge)
- 2019
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Ingår i: Ore Geology Reviews. - : Elsevier. - 0169-1368 .- 1872-7360. ; 107, s. 903-925
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Forskningsöversikt (refereegranskat)abstract
- Deep-sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply. Increasing demand for these metals, many of which are required to enable a low-carbon and high-technology society and to relieve pressure on land-based resources, may result in deep sea mining within the next decade. Seafloor massive sulphide (SMS) deposits, containing abundant copper, zinc, gold and silver, have been the subject of recent and ongoing commercial interest. Although many seafloor hydrothermally systems have been studied, inactive SMS deposits are likely more accessible to future mining and far more abundant, but are often obscured by pelagic sediment and hence difficult tolocate. Furthermore, SMS deposits are three dimensional. Yet, to date, very few have been explored or sampled below the seafloor. Here, we describe the most comprehensive study to date of hydrothermally extinct seafloor massive sulphide (eSMS) deposits formed at a slow spreading ridge. Our approach involved two research cruises in the summer of 2016 to the Trans-Atlantic Geotraverse (TAG) hydrothermal field at 26 N on the Mid-Atlantic Ridge. These expeditions mapped a number of hydrothermally extinct SMS deposits using an autonomous underwater vehicle and remotely operated vehicle, acquired a combination of geophysical data including sub-seafloor seismic reflection and refraction data from 25 ocean bottom instruments, and recovered core using a robotic lander-type seafloor drilling rig. Together, these results that have allowed us to construct a new generic model for extinct seafloor massive sulphide deposits indicate the presence of up to five times more massive sulphide at and below the seafloor than was previously thought.
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| 17. |
- Myint, Aung Zaw, et al.
(författare)
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Stable isotope and geochronological study of the Mawchi Sn-W deposit, Myanmar : Implications for timing of mineralization and ore genesis
- 2018
-
Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368. ; 95, s. 663-679
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Tidskriftsartikel (refereegranskat)abstract
- Myanmar is endowed with abundant Sn-W mineralization, pre-eminent amongst which is the world-class Mawchi deposit. In the Mawchi area, N-S trending vertical or steeply dipping quartz veins are hosted by both Eocene granite and Carboniferous to Early Permian metasediments. Three stages of ore formation are recognized; (i) tourmaline-cassiterite stage (ii) main ore stage and (iii) sulfide stage. Tourmaline, cassiterite and pyrite-I are early-formed minerals and are representative of the first stage. Their deposition continued together with wolframite, scheelite, molybdenite, arsenopyrite, pyrite-II, fluorite and danalite, which form the second stage. This was followed by the successive deposition of sulfides such as chalcopyrite, sphalerite, galena and Pb-Bi sulfides. A 40Ar/39Ar magmatic biotite plateau age of 41.50 ± 0.16 Ma (MSWD = 2.5) is determined for the Mawchi biotite granite which we interpret as the time the granite cooled through the biotite blocking temperature, and is consistent with a previously reported LA-ICP-MS U-Pb zircon concordia age of 42.72 ± 0.94 Ma (MSWD = 2). A molybdenite Re-Os model age of 42.4 ± 1.2 Ma indicates that Sn-W mineralization was synchronous with late Eocene granitic magmatism. Our 40Ar/39Ar hydrothermal muscovite plateau ages from the tourmaline granite (40.14 ± 0.14 Ma; MSWD = 1.48) and quartz vein selvages (40.80 ± 0.12 Ma; MSWD = 0.47) define the timing of hydrothermal alteration and simultaneous veining that accompanied the late stage of ore forming at Mawchi. Fluid inclusion microthermometry from cassiterite, scheelite, quartz, and fluorite reveals that ore fluids in the cassiterite-tourmaline stage and main ore stage are characterized by moderate homogenization temperatures (Th = 260–345 °C) and salinities (4.5–15.7 wt% NaCl equiv), while the sulfide stage is characterized by lower temperatures (Th = 175–260 °C) and moderate to low salinity (2.5–13 wt% NaCl equiv). The mean δ34S of all sulfides is 2.9 ± 2.9‰ which is suggestive that the overall system is dominated by magmatic sulfur. The similarity of δ34S values in galena and Pb-Bi sulfides (−1.3 to 2.7‰) suggest that sulfur, and inference the Pb and Bi were transported by a common fluid, probably of magmatic origin. The calculated δ18OH2O of the hydrothermal fluid associated with cassiterite and tourmaline is 7.3–8.4‰. Scheelite and quartz deposition is characterized by lighter δ18OH2O values (2.1–4.9‰) indicating that the ore fluid might be mixed with another source of water, perhaps, meteoric. Hydrogen isotopic compositions (δD = −51 to −121‰) again indicate that origin of the ore fluid is magmatic, but δD (∼−120‰) is low enough to support the δ18OH2O data that suggests, in part, a meteoric water component to the hydrothermal fluids at Mawchi.
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| 18. |
- Patten, Clifford G. C., et al.
(författare)
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Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust : Insights from the Troodos ophiolite
- 2017
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 86, s. 487-508
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Forskningsöversikt (refereegranskat)abstract
- 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.
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| 19. |
- Sosnicka, Marta, et al.
(författare)
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Fluid types and their genetic meaning for the BIF-hosted iron ores, Krivoy Rog, Ukraine
- 2015
-
Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 68, s. 171-194
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 20. |
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| 21. |
- Tindell, Thomas, et al.
(författare)
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The Kago low-sulfidation gold and silver deposit : A peripheral mineralisation to the Nansatsu high-sulfidation system, southern Kyushu, Japan
- 2018
-
Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368. ; 102, s. 951-966
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Tidskriftsartikel (refereegranskat)abstract
- The Kago deposit is a small deposit located at the southern tip of the Satsuma Peninsula of Southern Kyushu, Japan. It lies proximal to the well-known Nansatsu-type mineralisation province dominated by high-sulfidation type epithermal deposits. The deposit was heavily mined in the 18 th Century, largely for its relatively higher gold compared to that of surrounding and regional deposits. The Kago deposit is a typical low-sulfidation deposit, characterised by adularia-quartz veins, composed of electrum, Ag-tetrahedrite, polybasite, chalcopyrite and pyrite. Based on mine records, the grade ranged from 4.1 to 13.3 g/t Au and 2.6-6.6 g/t Ag. Alteration grades from low to high temperature argillic into a propylitic zone at the extremes of the vein exposures. Carbonate is absent. Fluid inclusion microthermometry reveals a typical temperature range of 220-240 °C with salinity of 0.7-2.6 NaCl eq. wt%. Electrum from high-grade ore ranges from 66 to 69 wt% Au. 40Ar/39Ar age dating of adularia bearing colloform/crustiform and brecciated veins, suggests a mineralisation event from 4.23 to 4.0 Ma. δ18O of veined and silicified quartz ranges from +4.0 to +18.4‰. δ18OH2O of fluids in equilibrium with quartz, in the dominant range of measured fluid inclusion temperatures, ranges from -6.5‰ to -0.2‰. δ34S of pyrite has a narrow range from -1.8 to 2.7‰. The deposit lies at the northern extent of the classic Nansatsu high-sulfidation epithermal area, in which a number of silicified bodies punctuate the region in a roughly semi-circular shape. The Kago deposit lies within the principle mineralisation age range of the high-sulfidation deposits, which range from 5.5 to 3.7 Ma. The structural displacement of the Kago deposit from the Nansatsu mineralisation and the differing host rocks has greatly influenced alteration, ore and rock-water interaction of the ore depositing fluids. Here we seek to establish the relationship that this extended mineral province has between the differing styles of mineralisation.
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| 22. |
- Tornos, Fernando, et al.
(författare)
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Controls on the siting and style of volcanogenic massive sulphide deposits
- 2015
-
Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 68, s. 142-163
-
Tidskriftsartikel (refereegranskat)abstract
- Volcanogenic massive sulphide (VMS) deposits form in subaqueous environments from circulating hydrothermal fluids heated by volcanic activity. These deposits form as sulphide mounds, stratiform exhalative and/or replacive bodies and commonly have stockwork/vein mineralization in their immediate footwall. These various “styles” are essentially facies of mineralization, each one being the product of a particular set of conditions that control the ore-forming processes and the consequent geometry and architecture (style) of the deposits. These controls include the physical and/or chemical nature of the host rocks, the temperature and composition of the hydrothermal fluids and the redox state of the depositional environment.The style of exhalative deposits is controlled by the salinity of the vented fluids and the redox state at the seafloor. Hydrothermal fluids with salinities less than twice that of seawater that vented into open, oxic oceanic environments, typically formed small mound and chimney complexes, unless they were rapidly covered by sediments or volcanic rocks. The massive sulphides were rapidly oxidized and partly dissolved by seawater. In contrast, stratiform sheet-like deposits are typically formed in anoxic bottom waters. Anoxic marine conditions were periodically of global extent, – particularly prior to 2.4 Ga – or of a regional nature. Local anoxic conditions can also be self-induced by the exhalation of saline and reducing hydrothermal fluids that ponded in bathymetric depressions such as second- or third-order basins to form a brine pool. These exhalative systems may have been initiated as chimney vent complexes and subsequently overlain by stratiform sulphides formed under the self-induced anoxic conditions. Deposits formed in anoxic environments can be significantly larger than those in oxic settings, and this is attributed to several factors that include longer-lived hydrothermal circulation, more efficient sulphide precipitation and reduced or inhibited oxidation thereof.Replacement of volcanic and sedimentary strata by sulphide typically occurs within the feeder zones beneath the exhalative mineralization. However, successions with abundant porous, permeable and/or reactive rocks such as glassy and/or pumiceous volcaniclastic rocks, and in some cases limestone, favoured the development of large replacive deposits, that may have had little surficial expression on the sea floor.VMS deposits at spreading centres within oceanic crust formed almost exclusively as mounds. Most of them have not been preserved, likely due to oxidation of the sulphides in the prevailing oxic environment and/or destruction of oceanic crust during subsequent subduction. Intra-continental rifts, arc rifts and back-arc rifts commonly have more complexity in their structure, and facies architecture and environments and can host all styles of VMS mineralization. In these settings, early extension favoured the formation of restricted basins with ideal conditions for the onset of hydrothermal activity and development of anoxic bottom waters, whereas in mature rifts the conditions were less conducive for the formation of regionally extensive anoxic environments. Formation of replacive deposits was permissible in all settings with porous or reactive subsea-floor strata. Replacive mineralization is the most likely to be preserved in the geological record due to the sulphides being physically shielded from oxidative weathering and mechanical erosion at the seafloor.The various styles of VMS mineralization can rarely be distinguished using a single criterion; in most cases multiple criteria are required.Mound style mineralization is distinguished by: (a) mound- or lens-shaped morphology; (b) presence of chimney fragments; (c) presence of abundant sedimented sulphide breccias; (d) location on a stratigraphic boundary (ore horizon); and, (e) association with a thin horizon or thicker stratigraphic interval of fine-grained clastic rocks (e.g., shale, mudstone) that accumulated at slow sedimentation rates.Stratiform exhalative mineralization is distinguished by: (a) sheet-like morphology prior to deformation; (b) presence of fine-grained clastic host rocks that accumulated at relatively slow rates (e.g., mudstone); (c) presence of local or extensive planar stratification.Replacive mineralization is characterized by: (a) irregular geometry and distribution of sulphide bodies; (b) gradation from massive sulphides to semi-massive sulphides and disseminated mineralization with relict textures of the host rock; and, (c) originally pumiceous, glassy or reactive host rocks emplaced at high depositional rates (mass flow deposits, lavas, carbonate-altered mass flow deposits) or limestone.One deposit or district may comprise two or more of these main styles of mineralization. In many cases the main styles of VMS mineralization present in a particular region can be predicted from examination of the facies architecture and depositional environments of the host succession. Recognition of the style(s) of mineralization that occur in a particular basin or mineral belt enables exploration models to be improved and should influence the strategy of exploration for VMS deposits.
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| 23. |
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