Magnetite as a provenance and exploration tool to metamorphosed base metal sulfide deposits in the Stollberg ore field, Bergslagen, Sweden

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2022-Spry-MagnetiteProvenanceManuscript.pdf (2.46 MB)

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2022-04-25
Authors
Frank, Katherine
Spry, Paul
O'Brien, Joshua J.
Koenig, Alan
Allen, Rodney L.
Jansson, Nils
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Copyright © The Author(s), 2022
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Spry, Paul
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Geological and Atmospheric Sciences
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 likely 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 were 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 spider 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 from previously published compositions of magnetite in metamorphosed volcanogenic massive sulfide deposits. Although spider 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 chemistry. Principal component analyses also show that magnetite in sulfides has a distinctive chemical signature that allows it to be a prospective pathfinder mineral in the Stollberg ore field.
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This is a manuscript of an article published as Frank, Katherine S., Paul G. Spry, Joshua J. O'Brien, Alan Koenig, Rodney L. Allen, and Nils Jansson. "Magnetite as a provenance and exploration tool to metamorphosed base metal sulfide deposits in the Stollberg ore field, Bergslagen, Sweden." Mineralogical Magazine (2022). doi:https://doi.org/10.1180/mgm.2022.39. Posted with permission.
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