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Syn-tectonic sulphide remobilization and trace element redistribution at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0003-1867-2342
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0002-2634-6953
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
CODES ARC Centre of Excellence and TMVC ARC Research Hub, University of Tasmania.
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Number of Authors: 62018 (English)In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360, Vol. 96, p. 48-71Article in journal (Refereed) Published
Abstract [en]

Mineralization types at the Palaeoproterozoic Falun base metal sulphide deposit are predominantly pyritic Zn-Pb-Cu-rich massive sulphide mineralization, disseminated to semi-massive Cu-Au mineralization, auriferous quartz veins, and mineralized shear zones of talc-chlorite-dominated schist. The massive and disseminated to semi-massive sulphide mineralization types were subject to polyphase ductile deformation (D1 and D2) and metamorphism under low-P, lower-amphibolite facies conditions, which led to the development of ore textures and paragenetic relationships indicating both mechanical and chemical remobilization of sulphides. In the massive sulphide mineralization, rare inclusion-rich pyrite occurs as relic cores inside inclusion-poor metamorphosed pyrite. Imaging and spot analysis using multielement laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) reveal that inclusion-poor pyrite was depleted in trace elements, which were originally present as non-stoichiometric lattice substitutions or in mineral inclusions. The inclusion-rich pyrite was shielded from depletion and, at least partly, retained its initially higher trace element concentrations, including Au.

Gold is also associated with chalcopyrite in the disseminated to semi-massive Cu-Au mineralization and in the system of auriferous quartz veins hosted therein, the latter being also affected by the D2 ductile strain. It is inferred that emplacement of the vein system took place after the peak of metamorphism, which occurred between D1 and D2, but prior to and possibly even shortly after completion of the D2 deformational event. Similarities in trace element signatures in chalcopyrite are compatible with the interpretation that the quartz veins formed by local chemical remobilization of components from the Cu-Au mineralization. Transport of liberated Au from pyrite during grain growth in the massive sulphide mineralization may have upgraded the Au endowment in the quartz veins, leading to the additional formation of native gold in the veins. A strong correspondence between elements liberated from pyrite (e.g. Pb, Bi, Se and Au) and those forming discrete and characteristic mineral phases in the quartz veins (Pb-Bi sulphosalts, native gold) supports this hypothesis.

Trace element signatures for the main sulphide minerals pyrite, chalcopyrite, sphalerite and galena are similar to previously published data from other metamorphosed massive sulphide deposits. The association of the Falun mineralization with elevated Bi is reflected by its occurrence in sulphide minerals (e.g. galena) and in abundant mineral inclusions of Pb-Bi sulphosalts (e.g. weibullite), especially in the disseminated to semi-massive Cu-Au mineralization. Elevated Sn concentrations in the lattice and/or as cassiterite inclusions in chalcopyrite, sphalerite and galena are compatible with a hot, acidic and reducing fluid during formation of the syn-volcanic, base metal sulphide mineralization and associated host-rock alteration.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 96, p. 48-71
Keywords [en]
Falun deposit, Bergslagen, Fennoscandian Shield, sulphide remobilization, LA-ICP-MS, trace elements
National Category
Geology
Research subject
Ore Geology
Identifiers
URN: urn:nbn:se:ltu:diva-61672DOI: 10.1016/j.oregeorev.2018.04.010ISI: 000434004700004Scopus ID: 2-s2.0-85045259948OAI: oai:DiVA.org:ltu-61672DiVA, id: diva2:1069245
Projects
Structural evolution, hydrothermal alteration and tectonic setting of the Falun base metal and gold deposit, Bergslagen region, Sweden
Funder
The Geological Survey of Sweden (SGU), 61-1441/2011
Note

Validerad;2018;Nivå 2;2018-04-16 (andbra)

Available from: 2017-01-27 Created: 2017-01-27 Last updated: 2018-06-28Bibliographically approved
In thesis
1. Age, origin and tectonothermal modification of the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden
Open this publication in new window or tab >>Age, origin and tectonothermal modification of the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, situated in the Palaeoproterozoic (1.9–1.8 Ga) Bergslagen lithotectonic unit in the south-western part of the Fennoscandian Shield, is one of the major base and precious metal sulphide deposits in Sweden. Felsic volcanic rocks and limestone hosting the deposit, as well as their hydrothermally altered equivalents and the mineralization, were affected by heterogeneous ductile strain and metamorphism under low-pressure, lower amphibolite-facies conditions during the Svecokarelian orogeny (2.0–1.8 Ga). These processes reworked the mineral assemblages of the original hydrothermal system and the mineralization, and also reshaped the structural geometry of the deposit.

A three-dimensional modelling approach has been used in order to evaluate geometric relationships between lithologies at the deposit. The polyphase character (D1 and D2) of the strong ductile deformation at Falun is apparent. The main rock-forming minerals in the altered silicate-rich rocks are quartz, biotite and anthophyllite with porphyroblasts of cordierite and garnet, as well as retrogressive chlorite. Major static grain growth occurred between D1 and D2, inferred to represent the peak of metamorphism, as well as after D2 with growth (or recrystallization) of anthophyllite. A major shear zone with chlorite, talc and disseminated sulphides bounds the pyritic Zn-Pb-Cu-rich massive sulphide mineralization to the north, the latter being surrounded elsewhere by disseminated to semi-massive Cu-Au mineralization. F2 sheath folding along axes plunging steeply to the south-south-east is suggested as a key deformation mechanism, accounting for the cone-shaped mineralized bodies, which pinch out at depth, and explaining the similar character of intensely altered rocks on all sides of the massive sulphide mineralization. Immobile-element lithogeochemistry suggests that they share a common volcanic precursor. These relationships are consistent with a model in which the pyritic massive sulphide mineralization is located in the core of a sheath fold structure, surrounded by the same altered stratigraphic footwall rocks with Cu-Au mineralization.

The geological evolution in the metavolcanic inlier that hosts the Falun deposit, constrained by secondary ion mass spectrometry (SIMS) U–Pb (zircon) geochronology, involved emplacement of a felsic volcanic and sub-volcanic rock suite at 1894±3 Ma, followed by hydrothermal alteration and mineralization. Subsequent burial and intrusion of late- to post-mineralization dykes occurred between 1896±3 Ma and 1891±3 Ma, followed by further burial and emplacement of plutons with variable composition during the time span 1894±3 Ma to 1893±3 Ma. The age determinations for all these magmatic suites overlap within their uncertainties, indicating a rapid sequence of continuous burial and different magmatic pulses. A metamorphic event, herein dated at 1831±8 Ma and 1822±5 Ma (SIMS U–Pb monazite), falls in the age range of a younger Svecokarelian metamorphic episode (M2). U-Th-Pb isotope systematics in monazite was completely reset during this event.

During hydrothermal alteration and mineralization, a hot, reducing and acidic fluid carrying metals and sulphur together flowed upward along syn-volcanic faults, leading to intense chloritization, sericitization and silicification of calc-alkaline volcanic rocks in the stratigraphic footwall to the deposit. This resulted in proximal siliceous associations including Fe-rich chlorite, and dominant Mg-rich chlorite and sericite in more peripheral parts. Cu-Au stockwork mineralization formed in the siliceous core of the hydrothermal system as result of fluid cooling. Neutralization of the metal-bearing fluids upon carbonate interaction stratigraphically higher in the sub-seafloor regime led to formation of Zn-Pb-Cu-rich massive sulphide mineralization, the space for which was created by a combination of carbonate dissolution, primary porosity in the overlying volcanic rocks and secondary porosity produced during syn-volcanic faulting. A hybrid model for mineralization is suggested by alteration styles, metal zoning and textures indicating replacement of carbonate rock or highly porous pumice breccia by pyritic massive sulphide. Aspects of a sub-seafloor volcanogenic massive sulphide (VMS) system and carbonate replacement are both present. Partly Zn-Pb-(Ag) mineralized skarns comprise a separate and subordinate type of mineralization, probably formed after burial of the hydrothermal system to the contact-metasomatic regime.

Textures and microstructures in the massive sulphide mineralization indicate that the ductile deformation and metamorphism resulted in internal mechanical and chemical remobilization of sulphide minerals. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analysis of the main sulphide minerals suggests, for example, that trace elements (including Au) were liberated from pyrite during metamorphism. A system of auriferous quartz veins, affected by D2 ductile strain, occurs in intensely altered and mineralized rocks on the eastern side of the deposit. It is suggested that they formed after the peak of metamorphism and prior to the completion of the D2 tectonic event, as a result of fluid-assisted remobilization of sulphides and Au in the disseminated to semi-massive Cu-Au mineralization and possibly also the massive sulphide mineralization.

Place, publisher, year, edition, pages
Luleå University of Technology, 2017
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-61620 (URN)978-91-7583-796-3 (ISBN)978-91-7583-797-0 (ISBN)
Supervisors
Projects
Structural evolution, hydrothermal alteration and tectonic setting of the Falun base metal and gold deposit, Bergslagen region, Sweden
Available from: 2017-01-27 Created: 2017-01-27 Last updated: 2017-11-24Bibliographically approved

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Kampmann, Tobias ChristophJansson, Nils F.Stephens, Michael B.Wanhainen, Christina

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