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Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen ore district, Fennoscandian Shield, 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.
Uppsala universitet, AGH University of Science and Technology, Mickiewicza.
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2017 (English)In: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 112, no 5, p. 1111-1152Article in journal (Refereed) Published
Abstract [en]

The Paleoproterozoic Falun Zn-Pb-Cu-(Au-Ag) pyritic sulfide deposit in the Bergslagen ore district, Sweden, is enveloped by hydrothermally altered rocks metamorphosed to the lower amphibolite facies. Immobile-element ratios suggest that the alteration precursors were volcanic rocks of mainly rhyolitic to dacitic composition. Least altered examples of these rocks plot along magmatic fractionation trends outlined by late- to post-ore feldspar-phyric metadacite dikes and post-ore granitoid plutons, consistent with a comagmatic relationship between these calc-alkaline, coeval (<10-m.y.) suites. Dolomite or calcite marble, as well as diopside-hedenbergite or tremolite skarn, form subordinate but important lithologic components in the hydrothermally altered zone. Marble occurs as fragments in the massive pyritic sulfide mineralization, suggesting that at least some mineralization formed by carbonate replacement.

Mass-change calculations suggest that the hydrothermally altered volcanic rocks gained Mg and Fe and generally lost Ca, K, and Na. Proximal, quartz-anthophyllite-rich altered rocks additionally gained Si, whereas several types of biotite-rich altered rocks lost this element. These mass changes along with mineral chemical data for anthophyllite, biotite, cordierite, and garnet, and the common occurrence of quartz indicate that chloritization, sericitization, and silicification were the dominant premetamorphic alteration styles. A zonation from distal sericitized and silicified volcanic rocks to intermediate sericitized rocks, partly overprinted by chloritization (Mg-rich chlorite), and proximal siliceous and intensely chloritized (Fe-rich chlorite) rocks has been identified. Furthermore, mass changes in more peripheral parts of the altered zone toward the southeast of the deposit suggest that the alteration weakens gradationally toward the volcanic and subvolcanic rocks surrounding the deposit. These patterns represent vectors toward mineralization.

Intensely chloritized rocks, largely represented by a single, rhyolitic precursor, envelop the central pyritic massive sulfide bodies to the east, south, and west, supporting a structural model in which the massive sulfide mineralization formed the stratigraphically highest preserved unit in the center, surrounded in a tubular manner by stratigraphic footwall rocks. The northern side represents a portion of the footwall, which was separated by a major shear zone. These spatial relationships also have implications for near-mine exploration, since quartz-rich footwall rocks locally host disseminated to semimassive stockwork Cu-Au mineralization.

Cooling of a hot (300°–400°C), acidic (pH ≤4) and reducing fluid carrying metals and sulfur is suggested for formation of stockwork Cu-Au vein mineralization and hydrothermal alteration in the stratigraphic footwall. The Zn-Pb-Cu-rich massive sulfide mineralization is inferred to have formed by fluid neutralization upon interaction with carbonates and mixing with cooler seawater upon fluid entry into porous pumice breccia in a subseafloor setting. Dissolution processes, primary porosity in the pumice breccia, and secondary porosity produced during synvolcanic faulting are all suggested to have contributed to the creation of space necessary for the formation of the massive sulfide mineralization. Falun differs from other deposits of the same type in Bergslagen mainly in the high pyrite content of the massive sulfide mineralization, the absence of related Fe oxide deposits, as well as the dominant replacement of volcaniclastic sediments compared to carbonates. The types of host rocks, the inferred premetamorphic feldspar-destructive alteration types, and the style of mineralization and alteration zonation at the deposit are reminiscent of pyritic volcanogenic massive sulfide (VMS) deposits. However, the importance of chemical trapping by fluid-limestone interaction, as well as the spatial association with subordinate skarn alteration constitute important differences to a classic VMS model.

Place, publisher, year, edition, pages
Society of Economic Geologists, 2017. Vol. 112, no 5, p. 1111-1152
National Category
Geology
Research subject
Ore Geology
Identifiers
URN: urn:nbn:se:ltu:diva-5137DOI: 10.5382/econgeo.2017.4504ISI: 000426874300005Scopus ID: 2-s2.0-85021135933Local ID: 329616f3-a1ac-49e9-8cfe-bee49ce58aa3OAI: oai:DiVA.org:ltu-5137DiVA, id: diva2:978011
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)
Note

Validerad;2017;Nivå 2;2017-06-05 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-05-04Bibliographically 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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