2018
The sandstone-hosted Osen lead deposit, Norway: new Pb isotope evidence for sourcing in the underlying granitoid basement
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The Osen deposit is one of several Pb–Zn deposits in Baltoscandia, hosted in Lower Cambrian sandstone, unconformably overlying a Precambrian granitoid basement and overlain by Caledonian nappes of the Lower Allochthon. In the Osen area, the Palaeoproterozoic Trysil granite (1673 ± 8 Ma) shows evidence of weathering below the unconformity. New Pb isotope data, collected by Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA–MC–ICP–MS) on K-feldspar from six samples of the Trysil granite, provide an improved internally consistent model for the age and sourcing of the Osen deposit. Data spread along the published whole-rock errorchron of the Trysil granite form two populations. The least radiogenic of these, defined by a cluster of 10 data points (average 206Pb/204Pb = 16.51 and 207Pb/204Pb = 15.37), is interpreted to represent the initial ratio of the granite. Published isotope data of Pb in galena in the Osen deposit (20.24 < 206Pb/204Pb < 20.49, 15.85 < 207Pb/204Pb < 15.89) plot on a reference line including this K-feldspar cluster and whole-rock data of the Trysil granite, as it was c. 540 Myr ago. This distribution suggests that the Osen deposit was generated shortly after deposition of the sandstone in the Early Cambrian (c. 541–511 Ma). Lead was released by weathering of the granite basement during development of the sub-Cambrian peneplain. It therefore discards alternative models involving Caledonian events in either Ordovician or Silurian time.

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