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- Weihed, Jeanette Bergman, et al.
(författare)
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Geology, tectonic setting, and origin of the Paleoproterozoic Boliden Au-Cu-As deposit, Skellefte District, northern Sweden
- 1996
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - : Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 91:6, s. 1073-1097
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte district in northern Sweden comprises more than 85 pyritic volcanic-hosted massive sulfide deposits which mainly occur within, and at the top of, a felsic-dominated volcanic unit overlain by a sedimentary sequence. The Boliden Au-Cu-As deposit was one of the first discovered in the district, and it has attracted a continuous interest since then due to its significant size and high gold grade (avg 15 ppm). The Boliden ore can be divided into massive ore, with arsenopyrite- and pyrite-dominated lenses, and vein ore which comprises a quartz-chalcopyrite-sulfosalt-dominated assemblage, occurring in brecciated parts of the arsenopyrite bodies, and quartz-tourmaline veins mainly in host rocks below the massive ore. As a rule, the gold is found in deformational structures in vein ore. Most gold is present as an Au-Ag-Hg alloy with variable compositions, from Au (sub 0.17) Ag (sub 0.68) Hg (sub 0.16) to Au (sub 0.93) Ag (sub 0.07) (in atomic proportions).For the last two decades, the approximately 1.88 Ga massive sulfide ores in the Skellefte district have collectively been interpreted as volcanic exhalative formations resembling the Miocene kuroko ores of Japan. However, this view has recently been challenged and a subsurface replacement origin has been proposed for some of the ores in the district.The Boliden ore is not bound to one particular host rock but occurs in feldspar porphyritic dacite, quartz porphyry, and basalt-andesite. Textural observations suggest that these rocks represent intrusions or lavas. Geochemically, they are typical calc-alkaline volcanic rocks, enriched in large ion lithophile elements, depleted in heavy rare earth elements, and with troughs for Th, Nb, Hf, and Ti. The ore zone, in its present setting, is in a more or less vertical position and oblique to lithological contacts. Ore-related hydrothermal and regional metamorphic processes (lower amphibolite facies) have created a complex alteration system around the ore. This forms a symmetric pattern with an inner sericite-rich zone, locally containing abundant andalusite, and an outer chlorite-dominated zone. The nature of the alteration is consistent with leaching of elements and a silica-alumina-rich residue--features which are often found in epithermal environments.Structural observations suggest that three ductile foliation-forming events have affected the rocks near the ore. These include a regional S (sub 1) foliation, formed during isoclinal folding, which was subsequently sheared causing formation of a strong cleavage S (sub s) and extensive deformation of the ore itself. A late S (sub 2) cleavage crenulated earlier fabrics.The available data and observations are not consistent with a volcanic exhalative model for the ore and the following scenario is favored. Shallow intrusions of dacite and andesite into unlithified sediments occurred around 1.87 Ga. At this time, the earlier marine environment had been lifted up to a shallow-marine or possibly subaerial position. Shortly thereafter, fluids which generated the massive ore at Boliden were focused along a fault, and arsenopyrite and pyrite lenses were precipitated in more than one host rock discordantly to lithological contacts. Regional deformation with folding and shearing, possibly at around 1.85 Ga, led to brecciation of previously formed ores and stretching of orebodies. In relation to this shearing event, Au was introduced and/or remobilized and concentrated in brecciated portions of the ore zone. Thereafter, ores and host rocks recrystallized during peak metamorphism at around 1.82 Ga, and a second deformation at around 1.80 Ga caused crenulation of early fabrics.The crosscutting nature of the ore with respect to the host rocks, the hydrothermal alteration pattern with strongly leached host rocks, and the ore association with early massive sulfides followed by gold, chalcopyrite, and sulfosalts in brittle structures all indicate that a modern analogue for ore formation may be a high-sulfidation epithermal environment. The epigenetic nature of the Boliden deposit has significant implications for exploration of gold deposits elsewhere in the region.
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| 3. |
- Weihed, Pär, 1959-, et al.
(författare)
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Post-deformation, sulphide-quartz vein hosted gold ore in the footwall alteration zone of the Palaeoproterozoic Långdal VHMS deposit, Skellefte District, northern Sweden
- 2002
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 124:4, s. 201-210
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Tidskriftsartikel (refereegranskat)abstract
- The Palaeoproterozoic, c. 1.88 Ga old Långdal VHMS deposit is situated in the eastern part of the Skellefte District, northern Sweden. In the stratigraphic footwall to the VHMS ore a sulphidequartz vein system with high gold grades was mined in the second half of the 1990’s. The Långdal VHMS ore is hosted by the uppermost part of the Skellefte Group volcanic rocks, close to the contact with an overlying fine-grained sedimentary unit. Regional structural studies indicate that bedding surfaces in volcanic rocks are parallel to the contact between the volcanic and the sedimentary rocks. Based on the differences in structural style on each side, the contact is interpreted as a major structural break. The Långdal ore is situated close to this break that may have focussed fluid flow during metamorphism and deformation. The orientation of the contact indicates that it either is a D2 structure or that it was at least active during D2. The structural development in the altered footwall rocks to the Långdal VHMS ore indicates that gold-bearing sulphide and sulphide-quartz veins both pre- and post-date the first deformation. Gold associated with the vein system can thus not only be attributed to syngenetic exhalative or replacement processes. The close spatial relationship with the massive sulphide deposits suggests, however, that the gold was remobilized from these syngenetic systems. It is concluded that sulphides were introduced at several stages during the geological evolution of the area as: a) syngenetic disseminations of sulphide and folded, pre-S1 stringer sulphide±quartz veins in the footwall related to the syngenetic VHMS deposit, b) syn-S1 sulphide veins in the footwall gold ore, c) main, post-S1, sulphide-quartz veins associated with the gold ore in the footwall rocks to the Långdal VHMS deposit, and d) post-S1 to pre-S2 galena and sphalerite rich veins post-dating the main, post-S1, sulphide-quartz vein system in the footwall to the Långdal ore. From these relationships it is suggested that gold was re-mobilized from the sulphide rich parts of the VHMS system into post-D1 structures during or slightly after the peak metamorphism. The timing of this event is poorly constrained to post-date the syngenetic ore emplacement by 20–80 m.y.
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| 4. |
- Weihed, Pär, 1959-, et al.
(författare)
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Relationship between 1.90–1.85 Ga accretionary processes and 1.82–1.80 Ga oblique subduction at the Karelian craton margin, Fennoscandian Shield
- 2002
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 124:3, s. 163-180
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Tidskriftsartikel (refereegranskat)abstract
- The three main intrusive suites: early calc-alkaline, late I/Atype, and late S-type intrusive rocks in relation to the Svecokarelian orogeny (1.9–1.8 Ga) have been dated at the Archaean craton margin in the Palaeoproterozoic Skellefte district and surrounding areas in northern Sweden. In addition, new SIMS data have been obtained on a calcalkaline intrusion for which unusually young TIMS ages existed, compared to similar calc-alkaline intrusions elsewhere in the region. Titanite and zircon from a subvolcanic intrusion affected by a major N–S trending shear zone have also been dated to constrain the last ductile deformation in the area. The 1895+14–12 Ma zircon age for a calc-alkaline intrusion is interpreted as the crystallisation age and is significantly older than the existing 1825 Ma age on titanite from a pyroxene skarn in a marble horizon close to the contact. The latter age is instead interpreted as the age of peak metamorphism in this area. The 1798±4 Ma age for the S-type granite confirms that the S-type magmatism is largely coeval with the I/A type magmatism previously dated at 1803±6 Ma. At a larger scale, a zoned belt over 2000 km long with A/I-type magmatism in the west and S-type magmatism in the east can be inferred. Either mafic underplating or Cordilleran type settings can explain the magmatic belt, which trends oblique to the roughly NE-directed subduction that led to the accretion of volcanic arcs onto the older craton between 1.95 and 1.87 Ga. An intimate temporal relationship between the extensive 1.80 Ga magmatism and regional N–S-trending shear zones in the area is confirmed by the titanite age of c. 1.80 Ga from one such shear zone. Kinematics on this shear zone suggest E–W shortening. SIMS data from a calc-alkaline intrusion at Sikträsk indicate that the previously obtained conventional zircon ages of 1.85–1.86 Ga are actually mixed ages of 1.88 Ga magmatic zircons, and c. 1.80 to 1.82 Ga metamorphic overgrowths. This shows that the 1.80 Ga event was not only constrained to shear zones. It is argued that both the 1.80 to 1.82 Ga deformation and metamorphism discussed here is related to E-W shortening and the voluminous magmatism at 1.82–1.80 Ga. This is in contrast to the older c. 1.88 Ga deformation identified to the north and east within the Karelian craton that was related to Svecokarelian accretionary processes.
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- Weihed, Pär, 1959-, et al.
(författare)
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Structural Evolution of the Björkdal Gold Deposit, Skellefte District, Northern Sweden: Implications for Early Proterozoic Mesothermal Gold in the Late Stage of the Svecokarelian Orogen
- 2003
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 98:7, s. 1291-1309
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Tidskriftsartikel (refereegranskat)abstract
- The Björkdal gold deposit is situated in the eastern part of the Paleoproterozoic Skellefte district in northern Sweden. The Skellefte district constitutes a 1.89 to 1.88 Ga volcanic arc with numerous volcanic massive sulfide deposits and lode gold deposits of which the Björkdal deposit is the largest, at ca. 20 Mt with 2.5 g/t Au. The gold at Björkdal is associated with centimeter- to meter-wide, subvertical quartz veins at the northwestern contact between a quartz-monzodioritic to tonalitic intrusion and the surrounding supracrustal rocks. The main quartz veins strike north-northeast, and a minor set of veins strike east-northeast. The quartz veins terminate against a major thrust duplex at the contact between the intrusion and the structurally overlying supracrustal rocks. The mylonitic thrust zone has a 20° to 40° dip toward north and trends approximately east-west. A few kinematic observations indicate reverse to obliquely reverse slip on the thrust. Deformed quartz veins exist in lithons between thrusts within the duplex. In the mine, the quartz veins in the footwall to the thrust are spatially and temporally associated with moderately to steeply west dipping reverse shear zones with a northeast strike. It is suggested here that the quartz veins and the steep reverse shear zones are related to the thrust duplex and formed more or less simultaneously. Fluid inclusion and isotopic results from previous studies indicate that juvenile magmatic fluids were responsible for the precipitation of quartz and sulfides at moderate temperatures and pressures. Furthermore, titanites from the quartz veins give ages of ca. 1.78 to 1.79 Ga, whereas the host pluton is dated at ca. 1.90 Ga, indicating a time gap of over 100 m.y. between the emplacement of the host rock and titanite growth in quartz veins. The regional deformation and metamorphism are poorly constrained in the area to some time between 1.87 and 1.80 Ga. As the quartz veins are virtually undeformed and do not exhibit metamorphic fluid inclusions or other evidence of premetamorphic origin, we interpret the titanite ages in the quartz veins as the age of emplacement of the veins. The ca. 1.78 to 1.79 Ga age is also constrained for the crustal-scale, north-south–striking shear zones in the area, and it is suggested here that the thrust duplex and steep reverse shear zones in the mine are third-order structures related to east-west shortening at ca. 1.80 Ga. Gravity data from the Björkdal area indicate the presence of a less dense body at depth beneath the Björkdal pluton. The geophysical signature is best explained by the presence of a 1.80 Ga Skellefte-type intrusion at depth. Magmatic fluids from this S-type granite may have interacted with the host pluton and precipitated gold in the more competent pluton during the east-west shortening. The common occurrence of scheelite in the quartz veins is further evidence for magmatic fluids derived from a younger pluton at depth.
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| 7. |
- Abdelnasser, Amr, et al.
(författare)
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REE geochemical characteristics and satellite-based mapping of hydrothermal alteration in Atud gold deposit, Egypt
- 2018
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Ingår i: Journal of African Earth Sciences. - : Elsevier. - 0899-5362 .- 1464-343X. ; 145, s. 317-330
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Tidskriftsartikel (refereegranskat)abstract
- New geochemical data of the hydrothermal alteration zones associated with gold-bearing quartz veins at Atud mine are used for better understanding the ore evolution and exploration vectoring. ASTER and Landsat 8 OLI data are used to elucidate the distribution of gold-associated alteration zones. Three alteration zones are defined; zone 1 (sericite-kaolinite-quartz-pyrite), zone 2 (quartz-sericite-albite-pyrite), and zone 3 (chlorite-carbonate-epidote ± pyrite). Sericite and hydrothermal quartz are confined to the mineralized quartz veins. Fe-OH and OH-bearing minerals are observed along NW- and NE-trending shear zones in the Main Atud mine. The association of gold-bearing quartz veins and sericite alteration is constrained by processing ASTER- and OLI-imagery data. The geochemical data of the ore-enveloping hydrothermally altered rocks are used to assess the behavior of the REEs during the mineralization process. Mild enrichment in LREE and significant enrichment in the HREE are associated with sericite in zones (1) and (2) alterations. Carbonate alteration (zone 3) is enriched in LREE and in immobile HREE. Moreover, LREE and Eu anomalies have negative correlated with the Alteration Index (A.I.) and K2O index (K.I.) in zones 1 and 2, suggesting high mobility of LREE in K-rich hydrothermal fluids. On the other hand, HREE anomalies with increasing MgO index (M.I.) in alteration zone 3 may imply low solubility of these elements in alkaline solutions. Au anomalies linked to sericite/silica alteration is a rather meaningful vector for further exploration in the area.
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