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Jansson, N., Zetterqvist, A., Allen, R. & Malmström, L. (2018). Geochemical vectors for stratiform Zn-Pb-Ag sulfide and associated dolomite-hosted Cu mineralization at Zinkgruvan, Bergslagen, Sweden. Journal of Geochemical Exploration, 190, 207-228
Open this publication in new window or tab >>Geochemical vectors for stratiform Zn-Pb-Ag sulfide and associated dolomite-hosted Cu mineralization at Zinkgruvan, Bergslagen, Sweden
2018 (English)In: Journal of Geochemical Exploration, ISSN 0375-6742, E-ISSN 1879-1689, Vol. 190, p. 207-228Article in journal (Refereed) Published
Abstract [en]

The Zinkgruvan deposit is the largest stratiform Zn-Pb-Ag mineralization in Sweden. The most recent genetic model attributes ore formation to the discharge of oxidized, near-neutral pH, metalliferous brines into a reduced basin, forming laterally extensive, stratiform sulfide mineralization on the seafloor. It has a known strike extent of 5 km and is underlain by a regionally extensive zone of K-altered metavolcanic rock and dolomitic marble, the latter hosting Cu-(Co-Ni) replacement mineralization near the inferred hydrothermal vent to the stratiform sulfides. The deposit is stratigraphically overlain by migmatized,  pyrrhotite- and graphite-rich pelite that is in turn overlain by a banded almandine-biotite-quartz-ferrosilite-bearing unit at the base of an regionally extensive metasedimentary succession. These laterally continuous units are interpreted as metamorphosed organic-rich sulphidic mudstone and silicate-dominated Fe formation, respectively.

The favorable stratigraphic interval contains anomalously high Zn, Pb, Ag, Cu, K2O/(K2O+Na2O), Mn, Co, Tl, Ba and B relative to adjacent metatuffite. However, only Zn, Pb, Ag, K2O/(K2O+Na2O) and Mn are significantly enriched relative to adjacent strata beyond the known lateral extent of the ore. Elevated copper, Co and Tl only occur in the vent-proximal part of the deposit, whereas anomalous enrichments of Ba and B are sporadic and occur mainly in the stratigraphic footwall. Many elements such as Si, Fe, Mg, Ca and Cs are of limited use in vectoring due to low enrichment factors relative to inferred background compositions and/or strong lithological controls on their distribution.

Although ore metal (Zn, Pb and Ag) enrichments are the best quantitative and qualitative guides to ore, K, Mn and Co enrichments also provide corroborative support. The most useful elements for vectoring have been synthesized into exploration indices. The Modified Sedex Metal Index (MSMI; Zn+3Pb+100Ag) is a vector towards stratiform Zn-Pb-Ag mineralization, whereas MSMI2 [Zn+3Pb+10(Cu+Co)] alsoallows targeting of proximal Cu mineralization.

The banded iron formation and the pyrrhotite- and graphite-rich pelite of the stratigraphic hangingwall are consistently enriched in base metals (e.g. 500-1000 ppm Zn), total S and Mn throughout the entire Zinkgruvan area. However, these units are not known to grade laterally along strata into economic base metal sulfide mineralization, and they are not obviously products of the same hydrothermal system which formed the stratiform Zn-Pb-Ag deposit.

In a vent-distal setting, the somewhat spurious metal anomalies of the hangingwall units can be difficult to distinguish from those of the favorable interval. The favorable stratigraphic interval can, however, be recognized by also taking into account that positive Zn anomalies are mainly coincident with positive anomalies in both K and Mn only in the favorable interval. Furthermore, samples from the favorable interval generally have Co/Ni > 1 and displays a positive Co/Ni vs. Zn trend, whereas samples of the pyrrhotite- and graphite-rich pelite have Co/Ni < 1 and define a negative Co/Ni vs. Zn trend. Thus, the index (Co/Ni)*Zn allows easy detection of weak Zn anomalies associated with the stratiform Zn-Pb-Ag mineralization.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Geochemical vectoring; exploration index; SEDEX; VMS; Broken Hill-type; Bergslagen
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-67980 (URN)10.1016/j.gexplo.2018.03.015 (DOI)000432601600015 ()2-s2.0-85044466046 (Scopus ID)
Projects
Conceptual Modeling and Exploration Criteria for Stratiform Zn-Pb-Ag-(Cu) Deposits in Bergslagen, Sweden
Funder
VINNOVA, 2014-01792
Note

Validerad;2018;Nivå 2;2018-04-03 (rokbeg)

Available from: 2018-03-18 Created: 2018-03-18 Last updated: 2018-06-07Bibliographically approved
Kampmann, T. C., Jansson, N. F., Stephens, M. B., Olin, P. H., Gilbert, S. & Wanhainen, C. (2018). Syn-tectonic sulphide remobilization and trace element redistribution at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden. Ore Geology Reviews, 96, 48-71
Open this publication in new window or tab >>Syn-tectonic sulphide remobilization and trace element redistribution at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden
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2018 (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
Keywords
Falun deposit, Bergslagen, Fennoscandian Shield, sulphide remobilization, LA-ICP-MS, trace elements
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-61672 (URN)10.1016/j.oregeorev.2018.04.010 (DOI)000434004700004 ()2-s2.0-85045259948 (Scopus ID)
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
Jansson, N., Sädbom, S., Allen, R., Billström, K. & Spry, P. G. (2018). The Lovisa stratiform Zn-Pb deposit, Bergslagen, Sweden: Structure, stratigraphy, and ore genesis. Economic geology and the bulletin of the Society of Economic Geologists, 113(3), 699-739
Open this publication in new window or tab >>The Lovisa stratiform Zn-Pb deposit, Bergslagen, Sweden: Structure, stratigraphy, and ore genesis
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2018 (English)In: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 113, no 3, p. 699-739Article in journal (Refereed) Published
Abstract [en]

Medium- to high-grade metamorphosed, 1.9 Ga, stratiform, syngenetic Zn-Pb±Ag sulfide deposits comprise an economically important type of ore deposit in the Bergslagen lithotectonic unit of the Fennoscandian shield. The Lovisa Zn-Pb deposit occurs in a metamorphosed succession of rhyolitic ash-siltstone, rhyolitic mass flow deposits, limestone and iron formation, deposited at a stage of waning volcanism in Bergslagen.

Accessory graphite, absence of Ce anomalies in shale-normalized rare-earth element (REE) data, and absence of hematite in Mn-rich iron formations stratigraphically below the Lovisa Zn-Pb deposit indicate a suboxic-anoxic depositional environment. The uppermost Mn-rich iron formation contains disseminated, inferred syngenetic Pb-Ag mineralization with mainly negative δ34S values in sphalerite and galena (-6.1 to -1.9‰).

Deposition of this iron formation terminated during a pulse of explosive felsic volcanism. The Lovisa Zn-Pb deposit is interpreted to have formed in an alkali-rich brine pool developed immediately after this volcanic event, based on lithogeochemical and stratigraphic evidence. The first stage of mineralization deposited stratiform sphalerite mineralization with mainly positive δ34S values (-0.9 to +4.7‰). This was succeeded by deposition of more sphalerite-galena stratiform mineralization with δ34S values close to 0‰ (-2.1 to +1.5‰). The more galena-rich mineralization partitioned strain and was partly remobilized during later ductile deformation.

The stratigraphic context, sulfide mineralogy, sulfur isotopes and alteration geochemistry suggest that the metalliferous fluids and the depositional environment were H2S-deficient (S-poor or SO42--dominant). The source of sulfur is interpreted to have been a mixture of H2S derived from bacterial and thermochemical seawater sulfate reduction, and sulfur derived from leaching of volcanic rocks, with the latter becoming more important over time.

Lovisa formed in a setting where basin subsidence was periodically punctuated by the deposition of thick, syn-eruptive felsic volcaniclastic massflow deposits. Coeval volcanism was likely important for driving hydrothermal activity and supplying a reservoir of metals and sulfur. However, the high rate of deposition of volcaniclastic sediment in Bergslagen also precluded the establishment of long-lived, deep and anoxic environments favorable for accumulation of organic matter and H2S. This stratigraphic pattern is common in Bergslagen and may explain why large stratiform Zn-Pb deposits are uncommon in the region and restricted to the uppermost part of the metavolcanic succession, directly stratigraphically beneath post-volcanic pelitic rocks.

Place, publisher, year, edition, pages
Society of Economic Geologists, 2018
Keywords
Bergslagen, iron oxide, sulfide, VMS, SEDEX
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-67129 (URN)10.5382/econgeo.2018.4567 (DOI)000432889300006 ()2-s2.0-85046039155 (Scopus ID)
Projects
Conceptual modelling and exploration criteria for stratiform Zn-Pb-Ag-(Cu) Deposits in Bergslagen, Sweden
Funder
VINNOVA, 2014-01792
Note

Validerad;2018;Nivå 2;2018-05-04 (andbra)

Available from: 2017-12-28 Created: 2017-12-28 Last updated: 2018-06-08Bibliographically approved
Jansson, N., Allen, R., Sädbom, S. & Zetterqvist, A. (2017). Bergslagen & Broken Hill. Geologiskt Forum (94), 24-28
Open this publication in new window or tab >>Bergslagen & Broken Hill
2017 (Swedish)In: Geologiskt Forum, ISSN 1104-4721, no 94, p. 24-28Article in journal (Other (popular science, discussion, etc.)) Published
Place, publisher, year, edition, pages
Geologiska föreningen, 2017
Keywords
Bergslagen, Broken Hill, Aggeneys, Gamsberg
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-64322 (URN)
Funder
VINNOVA, 2014-01792
Available from: 2017-06-21 Created: 2017-06-21 Last updated: 2017-11-24Bibliographically approved
Jansson, N. (2017). Bland stromatoliter och urtidsvulkaner vid Sala silvergruva. Geologiskt Forum, 93, 4-9
Open this publication in new window or tab >>Bland stromatoliter och urtidsvulkaner vid Sala silvergruva
2017 (Swedish)In: Geologiskt Forum, ISSN 1104-4721, Vol. 93, p. 4-9Article in journal (Other (popular science, discussion, etc.)) Published
Place, publisher, year, edition, pages
Luleå: Geologiska Föreningen, 2017
Keywords
Sala, stromatoliter
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-62623 (URN)
Available from: 2017-03-22 Created: 2017-03-22 Last updated: 2017-11-24Bibliographically approved
Jansson, N. & Kampmann, T. C. (2017). Contrasting fluid types involved in the genesis of ca. 1.89 Ga, syngenetic polymetallic sulfide deposits, Falun and Zinkgruvan, Bergslagen, Sweden. In: Mineral Resources to Discover: Proceedings of the 14th SGA Biennial Meeting, Québec City, Canada. Paper presented at 14th SGA Biennial Meeting, Québec City, Canada, August 20-23, 2017 (pp. 613-616). Society for Geology Applied to Mineral Deposits, 2
Open this publication in new window or tab >>Contrasting fluid types involved in the genesis of ca. 1.89 Ga, syngenetic polymetallic sulfide deposits, Falun and Zinkgruvan, Bergslagen, Sweden
2017 (English)In: Mineral Resources to Discover: Proceedings of the 14th SGA Biennial Meeting, Québec City, Canada, Society for Geology Applied to Mineral Deposits , 2017, Vol. 2, p. 613-616Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

Metamorphosed polymetallic sulfide deposits in Bergslagen, Sweden, are currently divided into 1: Stratabound volcanic-associated limestone-skarn Zn-Pb-Ag-Cu-Au sulfide deposits (SVALS) and 2: Stratiform ash-siltstone hostedZn-Pb-Ag sulfide deposits (SAS). It has not been completely resolved if these deposit types formed from similar hydrothermal fluids. Recent investigations at the Falun SVALS deposit and the Zinkgruvan SAS deposit suggest that fluids of contrasting pH, ƒO2, salinity and Twere involved in their origin. Whereas Falun formed by cooling and neutralization of acidic (pH<4), hot (300-400ºC) and reducing fluids carrying metals and sulfur together, Zinkgruvan formed by reduction of oxidized brines at a near-neutral pH. Falun is a vent-proximal, synvolcanic carbonate-replacement deposit with similarities to VMS and skarn deposits, whereas Zinkgruvan is a post-volcanic, exhalative deposit with similarities to some SEDEX deposits. Our results suggest that the different character of SVALS and SAS deposits in part are functions of fundamental differences in fluid chemistry, controls on sulfide precipitation and relationship to volcanism.

Place, publisher, year, edition, pages
Society for Geology Applied to Mineral Deposits, 2017
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-65384 (URN)
Conference
14th SGA Biennial Meeting, Québec City, Canada, August 20-23, 2017
Available from: 2017-08-28 Created: 2017-08-28 Last updated: 2017-11-24Bibliographically approved
Jansson, N. F. & Kampmann, T. C. (2017). Contrasting fluid types involved in the genesis of ca. 1.89 Ga, syngenetic polymetallic sulfide deposits, Falun and Zinkgruvan, Bergslagen, Sweden. In: Mercier Langevin, P; Dube, B; Bardoux, M; Ross, PS; Dion, C (Ed.), Mineral Resources to Discover: . Paper presented at 14th SGA Biennial Meeting on Mineral Resources to Discover, Quebec City, Canada, AUG 20-23, 2017 (pp. 613-616). Society for Geology Applied to Mineral Deposits
Open this publication in new window or tab >>Contrasting fluid types involved in the genesis of ca. 1.89 Ga, syngenetic polymetallic sulfide deposits, Falun and Zinkgruvan, Bergslagen, Sweden
2017 (English)In: Mineral Resources to Discover / [ed] Mercier Langevin, P; Dube, B; Bardoux, M; Ross, PS; Dion, C, Society for Geology Applied to Mineral Deposits , 2017, p. 613-616Conference paper, Published paper (Refereed)
Abstract [en]

Metamorphosed polymetallic sulfide deposits in Bergslagen, Sweden, are currently divided into 1: Strata bound volcanic-associated limestone-skarn Zn-Pb-Ag-CuAu sulfide deposits (SVALS) and 2: Stratiform ash-siltstonehosted Zn-Pb-Ag sulfide deposits (SAS). It has not been completely resolved if these deposit types formed from similar hydrothermal fluids. Recent investigations at the Falun SVALS deposit and the Zinkgruvan SAS deposit suggest that fluids of contrasting pH, fO(2), salinity and T were involved in their origin. Whereas Falun formed by cooling and neutralization of acidic (pH<4), hot (300-400 C) and reducing fluids carrying metals and sulfur together, Zinkgruvan formed by reduction of oxidized brines at a near-neutral pH. Falun is a vent-proximal, synvolcanic carbonate-replacement deposit with similarities to VMS and skarn deposits, whereas Zinkgruvan is a post-volcanic, exhalative deposit with similarities to some SEDEX deposits. Our results suggest that the different character of SVALS and SAS deposits in part are functions of fundamental differences in fluid chemistry, controls on sulfide precipitation and relationship to volcanism.

Place, publisher, year, edition, pages
Society for Geology Applied to Mineral Deposits, 2017
National Category
Geology
Research subject
Ore Geology; Ore Geology
Identifiers
urn:nbn:se:ltu:diva-70284 (URN)000439764100152 ()978-2-9816898-0-1 (ISBN)
Conference
14th SGA Biennial Meeting on Mineral Resources to Discover, Quebec City, Canada, AUG 20-23, 2017
Available from: 2018-08-09 Created: 2018-08-09 Last updated: 2018-08-09Bibliographically approved
Jansson, N. F. (2017). Structural evolution of the Palaeoproterozoic Sala Stratabound Zn-Pb-Ag carbonate-replacement deposit, Bergslagen, Sweden (ed.). GFF, 139(1), 21-35
Open this publication in new window or tab >>Structural evolution of the Palaeoproterozoic Sala Stratabound Zn-Pb-Ag carbonate-replacement deposit, Bergslagen, Sweden
2017 (English)In: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 139, no 1, p. 21-35Article in journal (Refereed) Published
Abstract [en]

A structural investigation of the Sala Zn–Pb–Ag deposit in the Bergslagen mining district of southern Sweden shows that it is associated with two tectonic structures: the N–NW-trending Storgruveskölen shear zone (SSZ), which is parallel to the strike of the mined ore bodies, and the F1 Sala syncline with a fold hinge plunging c. 35° towards NNW, which is parallel to the plunge of the entire mineralised system. The Sala syncline was refolded by F2 folds, leading to flattening and local reversals in the plunge of F1 folds and the ore bodies. Field evidence suggests that the SSZ represents both a phase of D3 reverse dip-slip shearing and a later (D4) phase of dextral strike-slip reactivation. However, a high concentration of pre- to syn-D1skarn- and sulphide-bearing vein networks and breccias adjacent to the SSZ, which are gradational into the mined massive sulphide ore bodies, suggest that stages in the formation of the SSZ predated D3 and D4. It may consequently constitute a reactivated pre- to syn-D1 structure. The distribution of breccia and hydrothermal alteration together with the highly discordant nature of the deposit are consistent with a pre- to syn-D1 timing of ore formation, involving of cross-stratal fluid flow along the proto-SSZ and subordinate fluid flow parallel to volcanic interbeds in the host carbonate rock. Three δ34S determinations on sphalerite (2.1–2.4‰) and galena (1.2‰), respectively, are consistent with a magmatic source for ore sulphur, as has been suggested for many other sulphide deposits in Bergslagen.

Place, publisher, year, edition, pages
Taylor & Francis, 2017
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-9120 (URN)10.1080/11035897.2016.1196498 (DOI)000392852200004 ()2-s2.0-84988411307 (Scopus ID)7aee3838-1999-4162-aa83-f25006770f8a (Local ID)7aee3838-1999-4162-aa83-f25006770f8a (Archive number)7aee3838-1999-4162-aa83-f25006770f8a (OAI)
Note

Validerad; 2017; Nivå 2; 2017-01-11 (rokbeg)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-12-14Bibliographically approved
Kampmann, T. C., Jansson, N., Stephens, M., Majka, J. & Lasskogen, J. (2017). Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen ore district, Fennoscandian Shield, Sweden (ed.). Economic geology and the bulletin of the Society of Economic Geologists, 112(5), 1111-1152
Open this publication in new window or tab >>Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen ore district, Fennoscandian Shield, Sweden
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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
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-5137 (URN)10.5382/econgeo.2017.4504 (DOI)000426874300005 ()2-s2.0-85021135933 (Scopus ID)329616f3-a1ac-49e9-8cfe-bee49ce58aa3 (Local ID)329616f3-a1ac-49e9-8cfe-bee49ce58aa3 (Archive number)329616f3-a1ac-49e9-8cfe-bee49ce58aa3 (OAI)
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
Chmielowski, R., Jansson, N., Persson, M. F. & Fagerström, P. (2016). 3D modelling of hydrothermal alteration associated with VHMS deposits in the Kristineberg area, Skellefte district, northern Sweden (ed.). Mineralium Deposita, 51, 113-130
Open this publication in new window or tab >>3D modelling of hydrothermal alteration associated with VHMS deposits in the Kristineberg area, Skellefte district, northern Sweden
2016 (English)In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 51, p. 113-130Article in journal (Refereed) Published
Abstract [en]

This contribution presents a 3D assessment of metamorphosed and deformed, hydrothermally altered volcanic rocks, hosting the massive sulphide deposits of the Kristineberg area in the 1.9 Ga Skellefte mining district in northern Sweden, using six calculated alteration parameters: the Ishikawa alteration index, the chlorite–carbonate–pyrite index and calculated net mass changes in MgO, SiO2, Na2O and Ba. The results, which are also available as film clips in the Supplementary data, confirm inferences from geological mapping; namely that the sericite- and chlorite-rich alteration zones have complex and cross-cutting geometries and that most of these zones are semi-regional in extent and range continuously from surface to over a kilometre deep. The major known massive sulphide deposits occur proximal to zones characterised by coincidence of high values for the alteration index and chlorite–carbonate–pyrite index and large MgO gains, which corresponds to zones rich in magnesian silicates. These zones are interpreted as the original chlorite-rich, proximal parts the alteration systems, and form anomalies extending up to 400 m away from the sulphide lenses. In addition, the stratigraphically highest VHMS are hosted by rocks rich in tremolite, talc, chlorite and dolomite with lesser clinozoisite, which have high chlorite–carbonate–pyrite index and low–medium alteration index values, reflecting a greater importance of some chlorite-carbonate alteration at this stratigraphic level. Vectoring towards massive sulphide deposits in this area can be improved by combining the AI and CCPI indexes with calculated mass changes for key mobile elements. Of the ones modelled in this study, MgO and SiO2 appear to be the most useful.

Keywords
3D model, geochemistry, VHMS deposits, mass change, Earth sciences - Endogenous eartn sciences, Geovetenskap - Endogen geovetenskap
National Category
Geology
Research subject
Ore Geology
Identifiers
urn:nbn:se:ltu:diva-2722 (URN)10.1007/s00126-014-0572-x (DOI)000368078500007 ()2-s2.0-84953639332 (Scopus ID)066c7b68-5acc-4053-9723-8ec50ee62776 (Local ID)066c7b68-5acc-4053-9723-8ec50ee62776 (Archive number)066c7b68-5acc-4053-9723-8ec50ee62776 (OAI)
Note

Validerad; 2016; Nivå 2; 20140630 (reichm)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
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