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

Abstract

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 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.

Langhofite, Pb2(OH)[WO4(OH)], a new mineral from Långban, Sweden

“…Fluid-inclusion studies of mineral assemblages with Pb oxysalts of the D period suggest formation temperatures ≤180°C and nearly atmospheric pressure conditions (Jonsson and Broman, 2002). We believe that langhofite has formed well below peak metamorphic conditions at Långban (estimated at >600°C and <3.5 kb; Grew et al, 1994;Christy and Gatedal, 2005), but still above the estimated temperature maximum for the D period, i.e. probably in the range 200−400°C.…”

New Mineral Names

“…Oxycalcioroméite (IMA 2012-022), ideally, Ca 2 Sb 2 5+ O 6 O, a member of roméite group, has been found at the Buca della Vena mine, Stazzema, Apuan Alps, Tuscany, Italy, where initially was reported as "stibiobetafite" (Orlandi and Dini, 2004). The mineral chemically corresponding to oxycalcioroméite with no crystallographic data was reported from Långban, Värmland, Sweden (Christy and Gatedal 2005). The new mineral occurs as euhedral octahedra, up to 0.1 mm, embedded in dolostone lenses at the zoned barite-pyrite-magnetite-hematite lens-shaped ore body at the contact between a metavolcanic-metasedimentary Triassic schists and dolostone and Lower Jurassic marbles.…”

“…Comment: Bindheimite, defined by Dana (1868), was related to oxyplumboroméite with the data of Christy and Gatedal (2005), and on the basis of this Atencio et al (2010) proposed it discreditation. Later on this name was restored to a questionable status by Christy and Atencio (2013), so awaits further study before the status can be changed.…”

Gatedalite, Zr(Mn2+2Mn3+4)SiO12, a new mineral species of the braunite group from Långban, Sweden

“…Flink, 1923;Magnusson, 1930;Moore, 1970;Holtstam and Langhof, 1999) and also for unusual chemical compositions of common minerals (e.g. Christy and Gatedal, 2005;Hålenius and Bosi, 2012;Hålenius et al, 2013). The Långban mine is now inaccessible and the present sample was collected from the dump near the New Shaft (Nya Schaktet) at Långban.…”

“…Peak regional metamorphic conditions are estimated to have reached~3À4 kbar and 550À600ºC (e.g. Grew et al, 1994;Christy and Gatedal, 2005). The majority of skarn (sensu lato) minerals and veins were formed during this stage.…”