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Extremely Pb-rich rock-forming silicates including a beryllian scapolite and associated minerals in a skarn from Långban, Värmland, Sweden

¹ Department of Earth and Marine Sciences, Building 47, Australian National University, Canberra, ACT 0200, Australia
² Mining Museum of Nordmark, SE-682 93 Nordmarkshyttan, Sweden

^* E-mail: andyc{at}ems.anu.edu.au

We report preliminary petrographic and mineral chemical data for a rock hosting an unusual mineral assemblage from Långban, Värmland, Sweden. The rock is a two feldspar-scapolite-spessartine-romeite skarn. The bulk composition and high degree of enrichment in Pb, Sb and As suggest that the rock was formed by reaction between a pre-existing Mn skarn containing the chalcophiles and a potassic granite, with loss of silica, alkalis and CO₂. The alkali feldspar is a Pb-rich hyalophane, averaging Or₆₃Ab₁₉Cs₁₅Pb₀₃, the plagioclase feldspar a Pb-rich labradorite, An₄₈Ab₄₈Or₀₂Pb₀₂, and the scapolite a ‘mizzonite’ (Ca/(Na+Ca) = 0.66–0.70). These minerals show their highest Pb contents recorded in nature to date: up to a maximum of 5.7 wt.% PbO in the hyalophane, 2.1% PbO in the plagioclase, and 5.3% PbO in the scapolite. Laser ablation ICP-MS of a scapolite grain detected substantial Be up to 1.7 wt.% BeO (0.6 Be per 12 tetrahedral cations), as well as Pb up to 7.05 wt.% PbO. The Be is incorporated into scapolite via the coupled exchange [Be(OH)][Al(CO₃,SO₄)]_–1. This is the first documentation of scapolite as the major repository for Be in a rock.

The romeite also contains substantial Pb, and shows extensive solid solution towards end-members containing Fe³⁺, Ti and Sb³⁺. In some analyses, the dominant end-members are Ca₂(Fe_0.5³⁺Sb_1.5⁵⁺)O₆(OH) and its Pb analogue rather than (Ca,Pb)₂Sb₂O₇. Complex exsolution textures are displayed in the hyalophane, by hancockite-epidote, romeite-bindheimite and hedyphane-johnbaumite. Ca-rich scapolite and hancockite appear to be new minerals for the Långban deposit.

The mineralogy appears consistent with the regional peak conditions of P = 3 kbar, T > 600°C. Several potential thermobarometers for Mn-rich skarns are identified in this rock.

KEYWORDS: hyalophane (plumboan), plagioclase (plumboan), scapolite (beryllian and plumboan), romeite-bindheimite, hancockite-epidote, hedyphane-johnbaumite, skarn, Långban, Sweden

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