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Abstract
The Pb oxychlorides comprise a group of rare minerals occurring as fissure and vein fillings in Palaeoproterozoic Långban-type Mn-Fe oxide deposits within the Bergslagen ore province, south central Sweden. Ecdemite, heliophyllite, sahlinite and nadorite occur at Harstigen; sahlinite at Jakobsberg, and all these as well as blixite, freedite, mendipite, perite and sundiusite occur at Långban. However, the presence of heliophyllite at Långban is ambiguous. Typical assemblages include ubiquitous calcite and baryte, and minor native metals, hydrocerussite, arsenites, arsenates and silicates. Electron microprobe analyses of a selection of Pb oxychloride minerals from these deposits are presented. These data indicate substitution between Sb3+ and As3+ in the ecdemite (-heliophyllite) and nadorite-type oxychloride structures. Furthermore, the composition of blixite is variable; empirically it varies between c. Pb4O3Cl2 and Pb2O(OH)Cl. The Pb oxychloride assemblages occur in two main textural positions: 1) in partly open fissures or druses, and 2) intergrown in veins or schlieren. Textural and paragenetic observations suggest that the oxychlorides formed at two main stages; firstly associated with metamorphism of the host rocks; secondly at lower P and T during post-metamorphic brittle tectonic conditions. Alteration of oxychlorides may lead to formation of native lead, litharge, hedyphane, Pb carbonates, nadorite and a finnemanite-like phase. The dominant fissure and vein host rocks comprise Fe-Mn-oxide ores or oxide±silicate-bearing carbonate rocks and Fe-Mn skarn rocks. Most of the required metals were probably sequestered from the local host rocks. It is suggested that the fissure-hosted oxychloride assemblages formed primarily from saline aqueous solutions at low pressures and temperatures.
