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Source and character of syntaxial hydrothermal calcite veins in Paleoproterozoic crystalline rocks revealed by fine-scale investigations
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science. (Geochemistry research group)
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.ORCID iD: 0000-0001-7230-6509
Stockholm University.
University of Gothenburg.
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2014 (English)In: Geofluids, ISSN 1468-8115, E-ISSN 1468-8123, Vol. 14, no 4, 495-511 p.Article in journal (Refereed) Published
Abstract [en]

Calcite veins in Paleoproterozoic granitoids on the Baltic Shield are the focus of this study. These veins are distinguished by their monomineralic character, unusual thickness and closeness to Neoproterozoic dolerite dykes and therefore have drawn attention. The aim of this study was to define the source of these veins and to unravel their isotopic and chemical nature by carrying out fine-scale studies. Seven calcite veins covering a depth interval of 50–420 m below the ground surface and composed of breccias or crack-sealed fillings typically expressing syntaxial growth were sampled and analysed for a variety of physicochemical variables: homogenization temperature (Th) and salinity of fluid inclusions, and stable isotopes (87Sr/86Sr, 13C/12C, 18O/16O), trace-element concentrations (Fe, Mn, Mg, Sr, rare earth elements) and cathodoluminescence (CL) of the solid phase. The fluid-inclusion data show that the calcites were precipitated mainly from relatively low-temperature (Th = 73–106°C) brines (13.4–24.5 wt.% CaCl2), and the 87Sr/86Sr is more radiogenic than expected for Rb-poor minerals precipitated from Neoproterozoic fluids. These features, together with the distribution of δ13C and δ18O values, provide evidence that the calcite veins are not genetic with the nearby Neoproterozoic dolerite dykes, but are of Paleozoic age and were precipitated from warm brines expressing a rather large variability in salinity. Whereas the isotopic and chemical variables express rather constant average values among the individual veins, they vary considerably on fine-scale across individual veins. This has implications for understanding processes causing calcite-rich veins to form and capture trace metals in crystalline bedrock settings.

Place, publisher, year, edition, pages
2014. Vol. 14, no 4, 495-511 p.
National Category
Geochemistry
Research subject
Natural Science
Identifiers
URN: urn:nbn:se:lnu:diva-37702DOI: 10.1111/gfl.12092ISI: 000344334600008OAI: oai:DiVA.org:lnu-37702DiVA: diva2:756158
Available from: 2014-10-16 Created: 2014-10-16 Last updated: 2017-02-07Bibliographically approved
In thesis
1. Abundance and fractionation of rare earth elements in calcite and other secondary minerals in fractures in the upper kilometre of crystalline bedrock, SE Sweden
Open this publication in new window or tab >>Abundance and fractionation of rare earth elements in calcite and other secondary minerals in fractures in the upper kilometre of crystalline bedrock, SE Sweden
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the geochemistry of secondary minerals, mainly calcite but also others such as fluorite and Ca/Al silicates, precipitated throughout the last 1.5 billion years in fractures of crystalline rock, SE Sweden. The work was based on previous reconnaissance studies and has been possible thanks to access to high-quality drill cores and associated mapping data provided by the Swedish Nuclear Fuel and Waste Management Co (SKB). Concentrations of rare earth elements (REEs) and occasionally other metals were determined in a variety of secondary minerals from fractures (mainly open systems) and veins (mainly sealed systems) and in primary minerals from the bedrock. Stable-isotope composition was measured in the secondary minerals. The overall aim was to define the sources, uptake and fractionation of REEs in calcite, and a few other co-genetic minerals, precipitated throughout the geological history under conditions ranging from hydrothermal to low temperatures.

Collectively, the findings of the individual studies show that there is no easy and straightforward control of REE abundance and fractionation in calcite and other minerals in fractures and veins in crystalline bedrock settings. For example, the REE features in calcite vary extensively within sub-generations of single vein-precipitating events, on micro scale in transects across individual veins, and unsystematically over the geological history characterised by successively decreasing temperatures of mineral formation. Although the REE content in, and release from, the crystalline bedrock can have an influence on REE distribution in calcite and other minerals, it is of overall minor importance within a given bedrock domain. The main advantage of determining REEs in secondary minerals in fractures and veins in crystalline rock is therefore, as revealed in this work, to assess the character and evolution of the conditions (including features of the paleofluids) during confined mineral-precipitating events.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2014
Series
Linnaeus University Dissertations, 197/2014
Keyword
secondary minerals, rare earth elements, isotopes, crystalline bedrock
National Category
Geochemistry
Identifiers
urn:nbn:se:lnu:diva-37547 (URN)978-91-87925-25-2 (ISBN)
Public defence
2014-11-14, Fullriggaren (B135), Landgången 4, Kalmar, 09:30 (English)
Opponent
Supervisors
Available from: 2014-11-03 Created: 2014-10-09 Last updated: 2017-02-07Bibliographically approved

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