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

University essay from Lunds universitet/Geologiska institutionen

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.

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