Characterisation of the Metamorphic, fluid and Mineralisation History of the Zinkgruvan Zn-Pb-Ag Deposit, Sweden
Characterisation of the Metamorphic, fluid and Mineralisation History of the Zinkgruvan Zn-Pb-Ag Deposit, Sweden
The stratiform Zinkgruvan Zn-Pb(-Ag) deposit of south-central Sweden is hosted in an early Proterozoic (~1.9 - 1.84 Ga) sequence of high temperature quartzofeldspathic gneisses, calc-silicate rocks and migmatites, interpreted as original volcanics, carbonate rocks and pelitic sediments respectively, deposited in a subsiding back-arc basin or an intracontinental incipient rift environment.
Geochemical and petrological investigations show the rhyolitic-dacitic volcanic rocks of the mine have been intensely enriched in potassium and depleted in sodium as a result of sub-seafloor hydrothermal alteration associated with a mineralising system. Geochemistry, petrology and Sr isotopic analyses demonstrate that the overlying metamorphosed shallow marine carbonates contain admixed volcanic detritus, which was also selectively K-enriched. A regional, but heterogeneous dolomitisation event occurred during diagenesis. Dolomitic marbles in the mine area are enriched in Fe and Mn as well as being silicified during hydrothermal activity.
Mineral assemblages (especially the associations Qtz + Kfs + Sil + Bt, and Cal + Dol + Fo) indicate that upper amphibolite facies metamorphism related to the Svecofennian orogeny (2.0 - 1.75 Ga) reached a peak of 750 ± 50o and 5 ± 1 kbar in the mine area, decreasing to lower amphibolite facies 10 km further south. Metamorphism was 'open system' and accompanied by an aqueous (low XCO2) fluid throughout, despite significant decarbonation during calc-silicate mineral formation. Two retrogressive events have been identified at ~550oC and ~250oC, with mineral assemblages indicative of aqueous fluid infiltration.
Mineralisation is Zn-Pb rich and Fe-poor, with pyrrhotite forming the main Fe-bearing phase. The main Zn-Pb ore horizons are underlain by semi-concordant Cu-rich mineralisation in the western part of the deposit, which may represent the feeder zone for mineralising fluids. Ore fluids are interpreted to be hot (~250oC), acidic (pH = ~4), weakly reducing (aH2 S > a SO4) and saline. The precise ore-forming mechanisms remain unclear due to recrystallisation of textures in both the ore and host rocks, but expulsion of metalliferous brines via a structurally-controlled fluid conduit in a submarine setting is recognised.
Zinkgruvan is shown to have much in common with the giant Broken Hill Zn-Pb deposit in Australia. Both areas contain features common to volcanogenic massive sulphine (VMS) and sedimentary-exhalative (SEDEX) base metal deposits. This hybrid type of deposit is assigned to a separate group known as Broken Hill-type (BHT), characterised by their Proterozoic age, high metaphoric grade, distinctive stratigraphic and tectonic setting, and laterally extensive mineralisation. This fits well with local and regional features at Zinkgruvan.
University of Southampton
Gunn, Victoria Kirsten
5cbb5d6a-bea4-4b9f-a7e4-7fc4874ce7a5
2002
Gunn, Victoria Kirsten
5cbb5d6a-bea4-4b9f-a7e4-7fc4874ce7a5
Gunn, Victoria Kirsten
(2002)
Characterisation of the Metamorphic, fluid and Mineralisation History of the Zinkgruvan Zn-Pb-Ag Deposit, Sweden.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
The stratiform Zinkgruvan Zn-Pb(-Ag) deposit of south-central Sweden is hosted in an early Proterozoic (~1.9 - 1.84 Ga) sequence of high temperature quartzofeldspathic gneisses, calc-silicate rocks and migmatites, interpreted as original volcanics, carbonate rocks and pelitic sediments respectively, deposited in a subsiding back-arc basin or an intracontinental incipient rift environment.
Geochemical and petrological investigations show the rhyolitic-dacitic volcanic rocks of the mine have been intensely enriched in potassium and depleted in sodium as a result of sub-seafloor hydrothermal alteration associated with a mineralising system. Geochemistry, petrology and Sr isotopic analyses demonstrate that the overlying metamorphosed shallow marine carbonates contain admixed volcanic detritus, which was also selectively K-enriched. A regional, but heterogeneous dolomitisation event occurred during diagenesis. Dolomitic marbles in the mine area are enriched in Fe and Mn as well as being silicified during hydrothermal activity.
Mineral assemblages (especially the associations Qtz + Kfs + Sil + Bt, and Cal + Dol + Fo) indicate that upper amphibolite facies metamorphism related to the Svecofennian orogeny (2.0 - 1.75 Ga) reached a peak of 750 ± 50o and 5 ± 1 kbar in the mine area, decreasing to lower amphibolite facies 10 km further south. Metamorphism was 'open system' and accompanied by an aqueous (low XCO2) fluid throughout, despite significant decarbonation during calc-silicate mineral formation. Two retrogressive events have been identified at ~550oC and ~250oC, with mineral assemblages indicative of aqueous fluid infiltration.
Mineralisation is Zn-Pb rich and Fe-poor, with pyrrhotite forming the main Fe-bearing phase. The main Zn-Pb ore horizons are underlain by semi-concordant Cu-rich mineralisation in the western part of the deposit, which may represent the feeder zone for mineralising fluids. Ore fluids are interpreted to be hot (~250oC), acidic (pH = ~4), weakly reducing (aH2 S > a SO4) and saline. The precise ore-forming mechanisms remain unclear due to recrystallisation of textures in both the ore and host rocks, but expulsion of metalliferous brines via a structurally-controlled fluid conduit in a submarine setting is recognised.
Zinkgruvan is shown to have much in common with the giant Broken Hill Zn-Pb deposit in Australia. Both areas contain features common to volcanogenic massive sulphine (VMS) and sedimentary-exhalative (SEDEX) base metal deposits. This hybrid type of deposit is assigned to a separate group known as Broken Hill-type (BHT), characterised by their Proterozoic age, high metaphoric grade, distinctive stratigraphic and tectonic setting, and laterally extensive mineralisation. This fits well with local and regional features at Zinkgruvan.
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Published date: 2002
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Local EPrints ID: 464611
URI: http://eprints.soton.ac.uk/id/eprint/464611
PURE UUID: 47db2f63-4a20-488f-8c5d-df35afe84a48
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Date deposited: 04 Jul 2022 23:50
Last modified: 16 Mar 2024 19:38
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Author:
Victoria Kirsten Gunn
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