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Rb-Sr sphalerite data and implications for the source and timing of Pb-Zn deposits at the Caledonian margin in Sweden

Larsson, Alfred LU (2018) In Dissertations in Geology at Lund University GEOR02 20172
Department of Geology
Abstract
The eastern Caledonian erosional front hosts the world-class Pb-Zn deposit at Laisvall (64 Mt mined)
and Granberget mineralization, northern Sweden, along with several other smaller, related types of deposits consisting
of impregnation mineralization of galena and sphalerite in clastic host rocks. Previous work have discussed the
nature of the ore-forming processes and constraints for the timing of the mineralization, however, the mechanisms
are still not fully understood and previous age determinations are ambiguous. The main challenge with obtaining
reliable ages for the mineralization is the lack of minerals available for dating and the complex nature of the available
isotope data. This thesis aims to improve the understanding for... (More)
The eastern Caledonian erosional front hosts the world-class Pb-Zn deposit at Laisvall (64 Mt mined)
and Granberget mineralization, northern Sweden, along with several other smaller, related types of deposits consisting
of impregnation mineralization of galena and sphalerite in clastic host rocks. Previous work have discussed the
nature of the ore-forming processes and constraints for the timing of the mineralization, however, the mechanisms
are still not fully understood and previous age determinations are ambiguous. The main challenge with obtaining
reliable ages for the mineralization is the lack of minerals available for dating and the complex nature of the available
isotope data. This thesis aims to improve the understanding for the ore-forming processes in the Laisvall type of
deposit and, if possible, obtain an age for the mineralization by utilizing the crush-leach technique on disseminated
sphalerite from Laisvall and Granberget with subsequent ID-TIMS isotope analysis. In general, the data is complex
and our results does not provide a robust age for the mineralization. However, our data are in agreement with the
hypothesis that ore-fluids with contrasting 87Sr/86Sr were mixing during the syn-ore forming stage. These results are
opposing previous hypotheses which are favoring a post-ore disturbance causing scatter in the isotope data. Our
data provide further support for hydrothermal circulation in response to orogenic tectonics at a regional scale, forming
Pb-Zn mineralizations at multiple locations during a relatively short time-interval, either related to an event at
ca. 540 Ma (related to the Timian orogeny) or at ca. 467 Ma (related to the Caledonian orogeny). Thus, these findings
implicate that the Pb-Zn deposits in the Caledonian forefront was caused by a single, regional process. The
results provide further insights to the genetic model for the Caledonian sediment hosted Pb-Zn deposits. (Less)
Abstract (Swedish)
Längs Kaledonidernas östra erosionsfront finns den världskända bly- och zinkfyndigheten vid
Laisvall (totalt 64 miljoner ton bruten malm) och mineraliseringen vid Granberget samt flertalet mindre malmkroppar
av impregnationstyp bestående av blyglans och zinkblände som kristalliserat i klastiska bergarter. I äldre litteratur
har mineraliseringsprocesserna och tidsspannet för mineralisationen diskuterats, men processerna är fortfarande
långt ifrån klargjorda och tidigare dateringar bör beaktas med viss skepsis. Den huvudsakliga utmaningen med att
producera pålitliga dateringar av malmen är dels avsaknaden av daterbara mineral och dels det komplexa mönstret
hos den data som befintliga geokronologiska metoder producerat. Målet med denna... (More)
Längs Kaledonidernas östra erosionsfront finns den världskända bly- och zinkfyndigheten vid
Laisvall (totalt 64 miljoner ton bruten malm) och mineraliseringen vid Granberget samt flertalet mindre malmkroppar
av impregnationstyp bestående av blyglans och zinkblände som kristalliserat i klastiska bergarter. I äldre litteratur
har mineraliseringsprocesserna och tidsspannet för mineralisationen diskuterats, men processerna är fortfarande
långt ifrån klargjorda och tidigare dateringar bör beaktas med viss skepsis. Den huvudsakliga utmaningen med att
producera pålitliga dateringar av malmen är dels avsaknaden av daterbara mineral och dels det komplexa mönstret
hos den data som befintliga geokronologiska metoder producerat. Målet med denna uppsats är att klargöra och fördjupa
förståelsen för malmbildningsprocesserna vid Laisvall- och Granbergetmalmerna samt, om möjligt, datera
malmen med hjälp av ID-TIMS isotopanalys av rubidium och strontium från zinkblände. Datan är överlag komplex
och svårtolkad och våra resultat kan inte ge en precis och robust ålder för mineraliseringen. Datan styrker däremot
hypotesen om att malmfluider med kontrasterande 87Sr/86Sr-isotopskvoter blandades i samband med malmgenesen.
Dessa resultat står i motsats till tidigare hypoteser som argumenterar för att isotopsystemet skall ha blivit stört efter
mineraliseringen avslutades. Vår data stödjer hypotesen om att bergskedjebildning kan ha gett upphov till hydrotermala
fluider som cirkulerade på en regional nivå, vilket i sin tur ledde till bly- och zinkmineraliseringar på flertalet
platser under ett relativt kort tidsintervall, relaterat till antingen den Timiska orogenesen (ca. 540 miljoner år sedan)
eller ett tidigt stadium av den Kaledonska orogenesen (ca. 470 miljoner år sedan). Dessa resultat tyder på att blyoch
zinkfyndigheterna längs den Kaledoniska bergskedjans rand har skapats av en ensam, regional process. I tillägg
till detta bidrar också resultaten till en ökad kunskap och förståelse för Kaledoniska bly- och zinkmalmer i sedimentära
moderbergarter. (Less)
Please use this url to cite or link to this publication:
author
Larsson, Alfred LU
supervisor
organization
course
GEOR02 20172
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Laisvall, Granberget, geochronology, sphalerite, ore, Rb-Sr
publication/series
Dissertations in Geology at Lund University
report number
529
language
English
additional info
External supervisor: Kjell Billström, Swedish Museum of Natural History
id
8939311
date added to LUP
2018-05-04 15:42:24
date last changed
2018-05-07 21:58:52
@misc{8939311,
  abstract     = {The eastern Caledonian erosional front hosts the world-class Pb-Zn deposit at Laisvall (64 Mt mined)
and Granberget mineralization, northern Sweden, along with several other smaller, related types of deposits consisting
of impregnation mineralization of galena and sphalerite in clastic host rocks. Previous work have discussed the
nature of the ore-forming processes and constraints for the timing of the mineralization, however, the mechanisms
are still not fully understood and previous age determinations are ambiguous. The main challenge with obtaining
reliable ages for the mineralization is the lack of minerals available for dating and the complex nature of the available
isotope data. This thesis aims to improve the understanding for the ore-forming processes in the Laisvall type of
deposit and, if possible, obtain an age for the mineralization by utilizing the crush-leach technique on disseminated
sphalerite from Laisvall and Granberget with subsequent ID-TIMS isotope analysis. In general, the data is complex
and our results does not provide a robust age for the mineralization. However, our data are in agreement with the
hypothesis that ore-fluids with contrasting 87Sr/86Sr were mixing during the syn-ore forming stage. These results are
opposing previous hypotheses which are favoring a post-ore disturbance causing scatter in the isotope data. Our
data provide further support for hydrothermal circulation in response to orogenic tectonics at a regional scale, forming
Pb-Zn mineralizations at multiple locations during a relatively short time-interval, either related to an event at
ca. 540 Ma (related to the Timian orogeny) or at ca. 467 Ma (related to the Caledonian orogeny). Thus, these findings
implicate that the Pb-Zn deposits in the Caledonian forefront was caused by a single, regional process. The
results provide further insights to the genetic model for the Caledonian sediment hosted Pb-Zn deposits.},
  author       = {Larsson, Alfred},
  keyword      = {Laisvall,Granberget,geochronology,sphalerite,ore,Rb-Sr},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {Rb-Sr sphalerite data and implications for the source and timing of Pb-Zn deposits at the Caledonian margin in Sweden},
  year         = {2018},
}