A Middle Ordovician Age for the Laisvall Sandstone-Hosted Pb-Zn Deposit, Sweden: A Response to Early Caledonian Orogenic Activity

Abstract

Ten sphalerite separates isolated from mineralized samples in proximal and distal positions relative to the proposed main feeder fault systems at the Laisvall deposit were used to obtain an absolute age determination of this world-class Pb-Zn deposit hosted by autochthonous Ediacaran to Lower Cambrian sandstone and located currently along the erosional front of the Scandinavian Caledonides in northern Sweden. Residue and leachate fractions of each separate were obtained using the crush-leaching technique. All samples correspond to sphalerite formed using reduced sulfur derived from thermochemical sulfate reduction, three of them from disseminated ore in the Lower Sandstone, two from the disseminated ore in the Upper Sandstone, and five from steeply dipping galena-sphalerite-calcite veinlets interpreted in previous works as remobilization of disseminated ores. The isotope dilution-thermal ionization mass spectrometry (ID-TIMS) data yield an overall complex Rb-Sr isotope pattern with two distinct trends in the 87 Sr/ 86 Sr vs. 87 Rb/ 86 Sr isochron diagram. The three sphalerite residues of disseminated mineralization from the Lower Sandstone orebody show Rb-Sr isotope systematics indicative of undisturbed primary precipitates, and yield an isochron model age of 467 ± 5 Ma (mean square weighted deviation, MSWD, 1.4). Since the isochron is based on three points, the obtained age is to be considered as preliminary. Yet, the obtained age is fully consistent with geologic evidence reported by previous authors and pointing to Middle Ordovician timing of ore formation.The ID-TIMS data were complemented by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses on the same sphalerite samples. The data support the hypothesis that the measured ID-TIMS Rb and Sr contents in these sphalerite residues are held in the sphalerite structure itself and are not related to micro-inclusions. The most viable hypothesis, in agreement with published work, is that during rapid growth, sphalerite may incorporate Rb and Sr ions from the hydrothermal fluids in its structure, most probably in octahedral voids.By contrast, the second trend in the 87 Sr/ 86 Sr vs. 87 Rb/ 86 Sr space defined by most other sphalerite residues and corresponding inclusion fluid leachates from the Upper Sandstone orebody and the veinlet samples is too steep to account for a realistic isochron age determination. This steep linear trend is interpreted to represent a postmineralization disturbance involving fluids rich in Sr. This disturbance of the Rb-Sr isotope system is consistent with the presence of the steeply dipping galena-sphalerite-calcite veinlets and the fact that the Upper Sandstone is, in places, tectonically disrupted because of its proximity to the basal Caledonian décollement. The attempt to date the Granberget deposit, located in tectonically disrupted allochthonous units inside the Caledonian orogen, failed because the Rb-Sr isotope systematics of the three analyzed sphalerite samples are also disturbed.The obtained Middle Ord...

Petroleum as source and carrier of metals in epigenetic sediment-hosted mineralization

“…1a; 64 Mt at 4% Pb, 0.6% Zn, 9 g/t Ag) in northern Sweden was mined for 40 years and was the largest Pb producer in Europe. Mineralization is found in two discrete ore horizons within a sedimentary sequence deposited on the stable, platformal margin to the continent Baltica during the Ediacaran to Early Ordovician 15,16 (Fig. 2): (1) the Pb ± Zn Lower Sandstone, and (2) the Zn ± Pb Upper Sandstone.…”

“…Epigenetic sulphide mineralization formed during the Middle Ordovician (467 ± 5 Ma) as a result of fluid migration triggered by early Caledonian orogenic activity related to an arc-continent collision 15 . Geological constraints and organic geochemistry of black shale and both mineralized and barren sandstone samples indicate that hydrocarbons migrated over several tens of kilometres from the middle Cambrian to Lower Ordovician Alum Shale Formation source rocks buried in the Caledonian foredeep into permeable Ediacaran to Cambrian sandstone in the forebulge setting 16,17 (Fig.…”

Timing and metal sources for carbonate-hosted Zn-Pb mineralization in the Franklinian Basin (North Greenland): Constraints from Rb-Sr and Pb isotopes

“…Rb-Sr isotope analysis of sphalerites appears not only useful for direct dating of Zn-Pb mineralization. In the case of disturbed isotopic systems, Rb-Sr data for sphalerite residues and corresponding inclusion fluid leachates in combination with Pb isotopes can provide important clues on post-mineralizing processes that have affected the studied ores, such as secondary fluid overprint (cf., Saintilan et al, 2015) or subrecent to recent weathering periods. 5.…”

Timing of formation of the Keketale Pb–Zn deposit, Xinjiang, Northwest China, Central Asian Orogenic Belt: Implications for the metallogeny of the South Altay Orogenic Belt

“…Along with advances in experimental techniques, dating methods have now been developed for measuring ore minerals, with the Rb–Sr isochron method on sphalerite being commonly used for dating Pb–Zn deposits (Hu et al, ; Lin, Wang, & Zhang, ; Saintilan et al, ; Yang, Wang et al, ; Zhang et al, ; Zheng et al . , ).…”

“…Along with advances in experimental techniques, dating methods have now been developed for measuring ore minerals, with the Rb-Sr isochron method on sphalerite being commonly used for dating Pb-Zn deposits (Hu et al, 2012;Lin, Wang, & Zhang, 2010;Saintilan et al, 2015;Yang, Wang et al, 2016a;Zhang et al, 2008;Zheng et al, 2014). In this paper, we first present systematic Rb-Sr isotope dates for the sphalerite from the main mineralization stage of the Keketale FIGURE 1 (a) Simplified regional geological map of the Chinese AOB (after Yang et al, 2013); (b) sketch map of the Maizi Basin (after Geological Team 706 of the Xinjiang Bureau of Nonferrous Metals, 2000) [Colour figure can be viewed at wileyonlinelibrary.com] Pb-Zn deposit, with the aim to establish more credible geochronological data.…”