| 1. |
- Bark, Glenn, et al.
(author)
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Orogenic gold in the new Lycksele-Storuman ore province, northern Sweden : the Palaeoproterozoic Fäboliden deposit
- 2007
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In: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 32:1-2, s. 431-451
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Journal article (peer-reviewed)abstract
- Southwest of the well-known Skellefte District, northern Sweden, a new gold ore province, the so called Gold Line, is presently being explored. During the past decade a number of gold occurrences have been discovered in this area. The largest known gold occurrence is the Fäboliden deposit. Late-to post-orogenic, ca. 1.81 to 1.77 Ga, Revsund granite constitutes the main rock type in the Fäboliden area and surrounds a narrow belt of mineralized metagreywackes and metavolcanic rocks. The supracrustal rocks are strongly deformed within a roughly N-S trending subvertical shear zone. The mineralization constitutes a 30 to 50 m wide, N-S striking, steeply dipping zone. The mineralization is commonly hosted by arsenopyrite-bearing quartz-veins within the supracrustal rocks. The quartz veins parallel the main foliation in the shear zone. Gold is closely associated with arsenopyrite-löllingite and stibnite and found in fractures and as intergrowths in the arsenopyrite-löllingite. Gold is also seen as free grains in the silicate matrix of the host rock. The proximal alteration zone displays positive correlation with Ca, S, As, Ag, Sb, Sn, W, Pb, Bi, Cd, Se, and Hg, whereas K and Na show a slightly negative correlation. The hydrothermal mineral assemblage in the proximal alteration zone is diopside, calcic amphibole, biotite, and minor andalusite and tourmaline. This type of assemblage is commonly recognized in hypozonal orogenic gold deposits worldwide. Garnet-biotite geothermometry indicates amphibolite facies in the Fäboliden area. The ductile fabric that hosts the mineralization is also found in the margin of the surrounding Revsund granitoid. It is therefore suggested that at least the final stages of the gold mineralization are syn- to late-kinematic, and the minimum age for the mineralization is thus constrained at ca. 1.80 Ga (Revsund age).
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| 4. |
- Sandrin, Alessandro, et al.
(author)
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Geophysical and petrophysical study of an iron oxide copper gold deposit in northern Sweden
- 2006
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In: Ore Geology Reviews. - : Elsevier. - 0169-1368 .- 1872-7360. ; 29:1, s. 1-18
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Journal article (peer-reviewed)abstract
- A geophysical-petrophysical study has been performed in an area WSW of the city of Kiruna, northern Sweden. The sub-regional tectonic setting is dominated by two important shear zones, which define the boundary of a granitic body. Many Cu-Fe-occurrences are located in proximity of faults related to these major deformation zones. Particular attention has been given to the Tjårrojåkka iron oxide copper gold (IOCG) deposit. Here the bedrock is characterised by intermediate to mafic meta-volcanics, metamorphosed intermediate to mafic dykes, and gabbroic-dioritic intrusions of Svecofennian ages (1.96-1.75 Ga). The major Cu- and Fe-occurrences are hosted by the meta-andesites. The aim of the study is to put the deposits into a tectonic framework and test existing hypotheses for their occurrences.Glacial deposits cover almost the entire area, leading to a scarcity of outcrops and inferring that geophysical data are fundamental for geological understanding. In addition to this, petrophysical analysis is vital for the interpretation of geophysical data (gravity, airborne magnetics and radiometrics, very low frequency) and for the definition of geophysical signatures of the deposits. The anisotropy of magnetic susceptibility (AMS) was also studied for the tectonic analysis. More than 150 oriented samples were collected in a number of outcrops along a profile intersecting the major structures in the Tjårrojåkka area.From the airborne magnetic data, two major linear features are interpreted as deformation zones. The strike of these deformation zones is approximately NW-SE and E-W, respectively. The same trends have been defined from other geophysical data such as airborne VLF and ground gravity data. A third important structural trend striking SW-NE has been defined by K/Th data and ground magnetic data. Very good agreement has been found between geophysical lineaments and AMS directions. Magnetic foliations determined by AMS measurements confirm the existence of three major trends in the study area: SW-NE, E-W and NW-SE. The major Fe-orebody shows approximately a SW-NE strike direction as defined from ground magnetic data. This is parallel to the strike of magnetic foliation determined in outcrops 1 km NW of the deposit. The epigenetic nature of the Cu and Fe occurrences in Tjårrojåkka and their spatial relationship with deformation zones suggest a connection between the formation of the deposits and a tectonic event. A later tectonic episode resulted in E-W trending deformation in the central area, affecting the orebodies themselves. Other, probable, compressive deformations have been indicated from petrophysical and geophysical analyses.Thermomagnetic measurements indicate that Fe-oxides (Ti-magnetite) are common in the area, while Fe-sulphides are almost absent. Multi-domain magnetite has been identified as the most common Fe-oxide in different rock types, while an unstable magnetic mineral has been detected in metamorphosed volcanics. A good spatial correlation has been observed between Cu-deposits and high K/Th values from radiometric data, values that are expressions of potassic alteration.
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| 5. |
- Sandrin, Alessandro, et al.
(author)
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Physical properties of rocks from borehole TJ71305 and geophysical outline of the Tjårrajåkka Cu-prospect, northern Sweden
- 2007
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In: Ore Geology Reviews. - : Elsevier. - 0169-1368 .- 1872-7360. ; 30:1, s. 56-73
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Journal article (peer-reviewed)abstract
- In the last decades Fe-oxide Cu–Au (IOCG) deposits attracted the interest of exploration geologists and geophysicists. General geophysical descriptions of IOCG deposits have been published in the recent past, but there is still a lack of detailed geophysical investigations. We present a petrophysical study of rock samples from exploration borehole TJ71305, which intersects the Cu-prospect in the Tjårrojåkka IOCG mineralised area, northern Sweden. Furthermore geophysical data are compiled and analysed to tentatively define a geophysical signature, at local scale, for this type of deposit. The study area is dominated by intrusive and volcanic rocks of Middle Proterozoic age, the latter hosting the Cu and Fe occurrences. The Fe occurrences are clearly defined from both aeromagnetic and ground magnetic data, and are also indicated by gravity, geoelectric and electromagnetic data. Enrichment in ferromagnetic minerals in the area is suggested by the high values of magnetic susceptibility commonly obtained for different rock types; magnetite and Ti-magnetite are the dominant magnetic minerals. Haematite with variable contents of Ti was detected and it is probably a result of oxidation of magnetite in alteration zones. In Tjårrojåkka a clear spatial relationship is noted between Cu occurrences and high K/Th ratios. This ratio is calculated from airborne radiometric data and is an expression of enrichment in potassium, due to alteration. The Cu-prospect is also indicated by high gravity and magnetic anomalies, by clear positive anomalies in induced polarisation data and by negative anomalies for the imaginary part of ground electromagnetic (Slingram) data. However, high-density/high-susceptibility/low-resistivity (Ti)-magnetite is associated with the Cu-prospect and this may lead to misinterpretation of potential field, electromagnetic and geoelectric data.
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- Talbot, Christopher J., et al.
(author)
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Potash in a salt mushroom at Hormoz Island, Hormoz Strait, Iran
- 2009
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In: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 35:3-4, s. 317-332
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Research review (peer-reviewed)abstract
- Increasing volumes of potash are currently being discovered in a cluster of diapirs of Hormoz (formerly Hormuz) salt near Bandar Abbas, Iran. Most of the potash beds studied so far occur in complex recumbent folds in a salt mountain that would be difficult to exploit safely. However, Holocene marine erosion removed any salt mountains from a sub-group of near-shore Zagros diapirs and exposed their deeper structural levels. Even though these diapirs are still active, their potash deposits are likely more tractable to safe exploitation than in a salt mountain - as we make clear here for Hormoz Island. Geochemical surveys on Hormoz Island reveal two separate potash anomalies that are valuable pseudostratigraphic markers. Integrating field measurements of the attitudes of bedding with lineaments on air photos suggests that Hormoz Island consists of a mature bell- or plume-shaped mushroom diapir with potash beds wound around a toroidal axis of rotation near current exposure levels. 2D numerical models simulate the salt mushroom on Hormoz Island and its internal circulation. They also suggest that the diapir has a wide overhand above a narrow stem in this gas-rich region. We use the mushroom diapir model to outline a regional exploration strategy that has the potential of influencing the world potash market thereafter.
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| 7. |
- Talbot, Christopher J., et al.
(author)
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Potash in salt extruded at Sar Pohl diapir, Southern Iran
- 2009
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In: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 35:3-4, s. 352-366
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Research review (peer-reviewed)abstract
- Recent progress in the search for potassium salts in Iran is outlined. After reviewing how most potassium ores form by the evaporation of seawater +/- hydrothermal brines, we focus on how most ores are deformed within salt diapirs. We summarise the history of the 150 or so diapirs of Hormoz salt emergent in the Zagros Mountains of Iran and then consider in detail the nature of potash at Sar Pohl, 60 km west of Bandar Abbas. These deposits are unique in that they occur in salt that extruded sub-aerially and spread over the surrounding ground surface via gravity-driven collapse. Mapping and drilling of the complex structural geology of Sar Pohl found the potash beds to be dispersed in distal salt but concentrated in piles of recumbent folds with axes circumferential to the mountain over lows in the vent rim. Equivalents of the curtain folds surviving in the stems of German diapirs presumably still exist beneath Sar Pohl and would be safer to mine than the recumbent folds in this soluble mountain. However, it should be possible to continuously pump water onto the exposed salt and guide the resulting brines through evaporation ponds and then a crystallization plant on the adjacent plains. This approach would accelerate natural degradation processes but harvest potash currently draining into the gulf.
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| 9. |
- Weihed, Pär, 1959-, et al.
(author)
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Precambrian geodynamics and ore formation : the Fennoscandian Shield
- 2005
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In: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 27:1-4, s. 273-322
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Journal article (peer-reviewed)abstract
- Compared with present-day global plate tectonics, Archaean and Palaeoproterozoic plate tectonics may have involved faster moving, hotter plates that accumulated less sediment and contained a thinner section of lithospheric mantle. This scenario also fits with the complex geodynamic evolution of the Fennoscandian Shield from 2.06 to 1.78 Ga when rapid accretion of island arcs and several microcontinent–continent collisions in a complex array of orogens was manifested in short-lived but intense orogenies involving voluminous magmatism. With a few exceptions, all major ore deposits formed in specific tectonic settings between 2.06 and 1.78 Ga and thus a strong geodynamic control on ore deposit formation is suggested. All orogenic gold deposits formed syn- to post-peak metamorphism and their timing reflects the orogenic younging of the shield towards the SW and west. Most orogenic gold deposits formed during periods of crustal shortening with peaks at 2.72 to 2.67, 1.90 to 1.86 and 1.85 to 1.79 Ga. The ca. 2.5 to 2.4 Ga Ni–Cu ± PGE deposits formed both as part of layered igneous complexes and associated with mafic volcanism, in basins formed during rifting of the Archaean craton at ca. 2.5 to 2.4 Ga. Svecokarelian ca. 1.89 to 1.88 Ga Ni–Cu deposits are related to mafic–ultramafic rocks intruded along linear belts at the accretionary margins of microcratons. All major VMS deposits in the Fennoscandian Shield formed between 1.97 and 1.88 Ga, in extensional settings, prior to basin inversion and accretion. The oldest “Cyprus-type” deposits were obducted onto the Archaean continent during the onset of convergence. The Pyhäsalmi VMS deposits formed at 1.93 to 1.91 Ga in primitive, bimodal arc complexes during extension of the arc. In contrast, the Skellefte VMS deposits are 20 to 30 million years younger and formed in a strongly extensional intra-arc region that developed on continental or mature arc crust. Deposits in the Bergslagen–Uusimaa belt are similar in age to the Skellefte deposits and formed in a microcraton that collided with the Karelian craton at ca. 1.88 to 1.87 Ga. The Bergslagen–Uusimaa belt is interpreted as an intra-continental, or continental margin back-arc, extensional region developed on older continental crust. Iron oxide–copper–gold (IOCG) deposits are diverse in style. At least the oldest mineralizing stages, at ca. 1.88 Ga, are coeval with calc-alkaline to monzonitic magmatism and coeval and possibly cogenetic subaerial volcanism more akin to continental arcs or to magmatic arcs inboard of the active subduction zone. Younger mineralization of similar style took place when S-type magmatism occurred at ca. 1.80 to 1.77 Ga during cratonization distal to the active N–S-trending subduction zone in the west. Possibly, interaction of magmatic fluids with evaporitic sequences in older rift sequences was important for ore formation. Finally, the large volumes of anorthositic magmas that characterize the Sveconorwegian Orogeny formed a major concentration of Ti in the SW part of the Sveconorwegian orogenic belt under granulite facies conditions, about 40 million years after the last regional deformation of the Sveconorwegian Orogeny, between ca. 930 and 920 Ma.
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