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Petrography and geochemistry of a section through the Blötbergetapatite-iron oxide deposit, Bergslagen, south central Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
2011 (English)Independent thesis Advanced level (degree of Master (One Year)), 40 credits / 60 HE creditsStudent thesis
Abstract [en]

The Blötberget apatite-iron oxide ores are situated in the Grängesberg mining district (GMD). This is located in the famous Bergslagen ore province, within the southern part of the Svecokarelian domain in Sweden. Apatite-iron oxide ores of comparable type are generally known as „Kiruna-type‟ deposits; they are economically important, geologically interesting and genetically controversial.This study utilizes petrography and geochemistry to characterize and in part investigate the genesis of apatite-iron oxide ores and their host rocks in Blötberget. The ores are dominated by massive and banded magnetite and hematite with variable contents of phosphorous. Disseminated iron oxides are also present in the rock units close to the ores. The Palaeoproterozoic host rocks are mainly intermediate to felsic metavolcanic to meta-subvolcanic rocks, geochemically ranging between dacitic and rhyolitic in composition, accompanied by localized phyllosilicate-rich and amphibole-bearing units. The host rocks also include large amounts of cross-cutting older granitoids as well as younger granite-aplite-pegmatite.Fluorapatite, being one of the most important non-oxide mineral in the ores, is responsible for the high contents of REE in apatite-rich ores. The studied apatite-rich ore samples have highest ΣREE contents, and the ΣREE contents for all ores analyzed range between 195 ppm and 3009 ppm, while the host rocks range between 58 ppm and 366 ppm. Adding to the dominant role of apatite influencing the ΣREE contents, monazite-(Ce), xenotime-(Y) and allanite-(Ce) are locally important REE carriers. The REE patterns in the ores, and the phyllosilicate-rich and amphibole-bearing assemblages exhibit clear similarities, indicating a similar genetic origin. Furthermore, similarities in REE patterns and ferride geochemistry between Blötberget and other apatite-iron oxide ores, including Grängesberg and Kiruna, lend support to the fact that apatite-iron oxide ores in Blötberget belong to the Kiruna-type deposit class.The metavolcanic to metasubvolcanic rocks in the studied ores at Blötberget show regional K-(Na) alteration, similar to many volcanic and subvolcanic rocks in central and southern Sweden. In contrast, the phyllosilicate-rich (mainly biotite) and amphibole-bearing units spatially close to the ores show enrichment in Mg-(K), which is suggested to be a result of localized and comparatively small-scale hydrothermal alteration. They also share systematic similarities in major and trace elements geochemistry with the ores, thus indicating that they are genetically related. Furthermore, disseminated iron oxides occur within the localized alteration assemblages, highlighting the fact that ores and alteration units are closely related. Nevertheless, the presence of hydrothermal alteration does not exclude a main process of orthomagmatic ore formation. Not least, the presences of structures such as magnetite ore breccias are indications of an orthomagmatic origin.

Place, publisher, year, edition, pages
2011. , 72 p.
Keyword [en]
Blötberget, Grängesberg mining district, Bergslagen, apatite-iron oxide ore, REE, „Kiruna type‟ deposit, geochemistry.
National Category
URN: urn:nbn:se:uu:diva-166183OAI: oai:DiVA.org:uu-166183DiVA: diva2:475539
Educational program
Master Programme in Earth Science
Physics, Chemistry, Mathematics
Available from: 2012-02-21 Created: 2012-01-11 Last updated: 2012-02-21Bibliographically approved

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