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- Frank, Katherine S., et al.
(författare)
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Magnetite as a provenance and exploration tool to metamorphosed base-metal sulfide deposits in the Stollberg ore field, Bergslagen, Sweden
- 2022
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Ingår i: Mineralogical magazine. - : Cambridge University Press. - 0026-461X .- 1471-8022. ; 86:3, s. 373-396
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Tidskriftsartikel (refereegranskat)abstract
- Magnetite is a common mineral in the Paleoproterozoic Stollberg Zn–Pb–Ag plus magnetite ore field (~6.6 Mt of production), which occurs in 1.9 Ga metamorphosed felsic and mafic rocks. Mineralisation at Stollberg consists of magnetite bodies and massive to semi-massive sphalerite–galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite and magnetite) hosted by metavolcanic rocks and skarn. Magnetite occurs in sulfides, skarn, amphibolite and altered metamorphosed rhyolitic ash–siltstone that consists of garnet–biotite, quartz–garnet–pyroxene, gedrite–albite, and sericitic rocks. Magnetite probably formed from hydrothermal ore-bearing fluids (~250–400°C) that replaced limestone and rhyolitic ash–siltstone, and subsequently recrystallised during metamorphism. The composition of magnetite from these rock types was measured using electron microprobe analysis and LA–ICP–MS. Utilisation of discrimination plots (Ca+Al+Mn vs. Ti+V, Ni/(Cr+Mn) vs. Ti+V, and trace-element variation diagrams (median concentration of Mg, Al, Ti, V, Co, Mn, Zn and Ga) suggest that the composition of magnetite in sulfides from the Stollberg ore field more closely resembles that from skarns found elsewhere rather than previously published compositions of magnetite in metamorphosed volcanogenic massive sulfide deposits. Although the variation diagrams show that magnetite compositions from various rock types have similar patterns, principal component analyses and element–element variation diagrams indicate that its composition from the same rock type in different sulfide deposits can be distinguished. This suggests that bulk-rock composition also has a strong influence on magnetite composition. Principal component analyses also show that magnetite in sulfides has a distinctive compositional signature which allows it to be a prospective pathfinder mineral for sulfide deposits in the Stollberg ore field.
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| 2. |
- Frank, Katherine S, et al.
(författare)
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Whole Rock-Rare Earth Element and Magnetite Chemistry as Guides to Exploration for Metamorphosed Base Metal Sulfide Deposits in the Stollberg Ore Field, Bergslagen, Sweden
- 2014
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Stollberg ore field (~12 Mt), 50 km W of the giant Garpenberg Zn-Pb-Ag-(Cu-Au) district (>100 Mt) occurs in the regional Stollberg F2 syncline within 1.9 Ga bimodal felsic and mafic rocks metamorphosed to the amphibolite facies. Sulfide mineralization is hosted by volcanic rocks and skarn and consists of massive to semi-massive sphalerite-galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite, and magnetite). The trace element composition of magnetite, which locally forms ore-grade masses and occurs as a common accessory in most rocks types at Stollberg, has previously proven to be a pathfinder in the exploration for ore deposits elsewhere and is evaluated here along with the rare earth element (REE) chemistry of altered rocks. At Stollberg, the dominant country rocks are metamorphosed rhyolitic pumice breccia and rhyolitic ash-silt-sandstone with minor amphibolite sills. On the eastern side of the Stollberg syncline, mineralization at Stollberg and Dammberget occurs as stratabound replacement of limestone/skarn that grades into iron formation spatially related to garnet-biotite and gedrite-albite alteration. At Gränsgruvan on the western side of the syncline, sulfides occur in a silicified zone along with garnet-biotite and quartz-garnet-pyroxene alteration. Although the Tvistbo and Norrgruvan deposits along the north end of the syncline are small, they show geological characteristics that are transitional to deposits found on the western and eastern side of the syncline in that the ore is hosted by skarn rock and associated with quartz-garnet-pyroxene alteration. The Gränsgruvan deposit more closely resembles deposits found at Garpenberg than those located on the eastern limb of the Stollberg syncline. Whole-rock analyses of altered and unaltered host rocks suggest that most components were derived from a felsic volcaniclastic component and that elements were immobile during alteration. These rocks (including altered rocks in the stratigraphic footwall) are light REE enriched, heavy REE depleted, and show negative Eu anomalies, whereas mineralized rocks (Fe- and base metal-rich) and altered rocks in the ore zone show the same REE pattern but with positive Eu anomalies. Trace element compositions (using LA-ICP-MS techniques) of magnetite in high-grade ore, limestone/skarn, massive magnetite, and garnet-biotite, gedrite-albite, garnet-pyroxene alteration show a range of compositions. Such ranges in composition are inconsistent with previous studies in other ore fields that suggest the composition of magnetite can be used to define compositional fields characteristic of ore deposit type (e.g., Al+Mn vs. Ti+V wt. %) or approximate temperature of the ore-forming fluid. Magnetite in garnet-biotite and gedrite-albite alteration spatially associated with Dammberget typically contains > 200 ppm Ga, > 10 ppm Sn, and Ti/V ratios of >10 whereas magnetite in garnet-biotite alteration associated the smaller Cederkreutz deposit contains < 25 ppm Ga, < 2 ppm Sn, and Ti/V ratios < 0.1. Magnetite in garnet-biotite alteration associated with the Gränsgruvan deposit contains > 10 ppm Sn, 20 to 180 ppm Ga, and Ti/V ratios of 0.1 to 2. These and other trace element compositions of magnetite as well as REE patterns of altered host rocks show potential as exploration guides to ore in the Stollberg district.
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| 3. |
- O'Brien, Joshua J, et al.
(författare)
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The major-trace element chemistry of garnet in metamorphosed hydrothermal alteration zones, Proterozoic Stollberg Zn-Pb-Ag-(Cu-Au) ore field, Bergslagen district, Sweden : implications for exploration
- 2014
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Altered and exhalative rocks are used as exploration guides to ore deposits since they are generally more extensive than the massive sulfide target. Major and trace element compositions of silicates (e.g., garnet) and oxides (e.g., gahnite and magnetite) in meta-exhalites have recently been used as a vectoring tool in the search for metamorphosed massive sulfide deposits. Here, we evaluate the major-trace element chemistry of garnet in altered (i.e., gedrite-albite, garnet-biotite, and garnet-pyroxene-carbonate alteration) and unaltered (i.e. rhyolitic ash-siltstone) rocks spatially associated with volcanogenic massive sulfide Zn-Pb-Ag-(Cu-Au) and magnetite deposits in the Stollberg ore field (metamorphosed to the amphibolite facies), to determine the spatial distribution of major/trace element compositions of garnet and the potential of garnet chemistry as a guide to ore. Garnet in garnet-biotite alteration (extends intermittently for ~8 km along strike) and high-grade sulfides is Fe-rich (almandine) whereas garnet in skarn and garnet-pyroxene alteration contains significantly higher amounts of Ca (grossular), and Mn (spessartine). Concentrations (425 analyses) of trace elements in garnet were obtained from 38 samples in the Dammberget (n = 14), Gränsgruvan (n = 17), and Tvistbo (n = 7) deposits. Garnet contains elevated concentrations of Sc, Ti, V, Cr, Co, Zn, Ga, Ge, Y, and rare earth elements (REEs). Chondrite-normalized rare earth element patterns of garnet are depleted in light REEs (LREEs) and enriched in heavy REEs (HREEs). Garnet in sulfide-bearing altered rocks (i.e., garnet-biotite and garnet-pyroxene alteration) show a strong positive Eu anomaly, regardless of its major element composition, and contains elevated Zn (> 100 ppm) and Ga (> 15 ppm) contents, and low concentrations of Ti (<200 ppm). Garnet-biotite alteration adjacent to unaltered rhyolitic ash-siltstone contains garnet which is LREE depleted, HREE enriched, and typically shows no Eu anomaly, or in some cases, minor negative Eu anomalies. In sulfide-free quartz-garnet-pyroxene rocks, garnet possesses no Eu anomaly and contains elevated concentrations of Ga (> 10 ppm), Sc (> 5 ppm), and Ti (> 100 ppm), but low concentrations of Co (< 1 ppm), Cr (< 5 ppm), and V (< 20 ppm). Garnet in gedrite albite alteration exhibits a relatively flat chondrite-normalized REE profile, and contains elevated (> 10 ppm) Sc content, and low concentrations of V (< 2 ppm), Cr (< 3 ppm), and Zn (< 30 ppm). Garnet in mafic dikes and marbles contain the highest Cr (> 10 ppm), Co (> 5 pm), V (25-250 ppm) and Ti contents, whereas garnet in rhyolitic ash-siltstone typically shows no Eu anomaly, and low concentrations of Zn (< 100 ppm), Ga (< 15 ppm), Cr (< 5 ppm), and V (< 3 ppm). Garnet in massive sulfides and sulfide-bearing alteration assemblages can be distinguished from sulfidepoor or sulfide-free rocks of the same alteration type on the basis of their positive Eu anomaly, and Zn, Ga, and Ti content, which suggests garnet chemistry may be used as a vectoring tool to ore in the Stollberg ore field, and elsewhere in the Bergslagen district.
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