Resolving temporal links between the Högberget granite and the Wigström tungsten skarn deposit in Bergslagen (Sweden) using trace elements and U-Pb LA-ICPMS on complex zircons

Detta är en Master-uppsats från Lunds universitet/Geologiska institutionen

Sammanfattning: Combined U-Pb and REE trace element LA-ICPMS analyses were performed on zircons, from the scheelite-bearing Wigström skarn, associated micro-granitic dykes and the highly evolved parental Högberget (sensu stricto) granite. Combined systematic of zircon typology and microtextures, trace elements and isotopic chemistry suggest the presence of several diverse zircon types, spanning from early- to late- magmatic stage into a hydrothermal stage. Early stage magmatic zircons (type M1 and M2-S) have high concentrations of U (3300–11500 ppm), Th (1500–7300 ppm), REEs (average ΣREEn: 11500) and Pb-C signatures and yield highly discordant data. Typological features are varying, but the majority consists of prismatic crystals with oscillatory zonations. In comparison, late stage magmatic zircons (type M2) have lower concentrations of U (200–3000 ppm), Th (100–1900 ppm), REEs (average ΣREEn: 7600) and Pb-C signatures (e.g. average 204Pb CPS: 300). Typological features include irregular, quasi-smooth forms which lack internal zonations. Geochronological analyses yield concordant data, after interpretation an 207Pb/206Pb weighted mean age of ~1.8 Ga is achieved. Hydrothermal zircon (skarn hosted, type H) associates with hydrothermal quartz, calcite, fluorite and scheelite within the skarn. Type H zircons shares similarities with type M2, yielding low concentrations of U (100–6200 ppm), Th (1–1500 ppm), and Pb-C signatures (e.g. average 204Pb CPS: 300), but slightly higher concentrations of REEs (average ΣREEn: 17800). Typological features display various forms, i.e. skeletal, irregular, oblong, and typically lacks prisms. Internal textures vary from zoned to homogenous, and spongy textures are common. Geochronological data are scattered, but display a concordant cluster around 1.8 Ga, calculated as 207Pb/206Pb weighted mean age of 1795±20 Ma (2.0 MSWD). The achieved ages further fortify a ~1.8 Ga for the tungsten metallogenesis in Bergslagen. Magmatic (early- and late types) and hydrothermal zircon can be distinguished via trace elemental discrimination diagram and in CI-chondrite normalized spidergrams. For discrimination diagram, Th versus Th/U, U versus Th, and ΣREEn versus Th diagrams clearly displays evolutionary transitional trend between the different zircon types. In spidergrams, Magmatic zircons have typical patterns for magmatic zircons (but shows LREE enrichments), whereas hydrothermal zircons display a convex pattern, which clearly distinguish the zircon populations apart.

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