Petrography, alteration and structure of the Bronäs Zn-Pb-Ag deposits, Bergslagen, Sweden

Abstract: This report aims to characterize the alteration styles and mineralization of the Zn-Pb-Ag Bronäs deposit, in the Sala area of the Bergslagen mining district, Sweden.  It presents data collected through 237m of lithological logging and subsequent lithogeochemical data, which, coupled with thin section and SEM analysis has led to the first geological descriptions of the rocks in the deposit.  The results are presented in the report through logs and a cross-section interpreted from these logs.  The Bronäs deposit, mined between 1945 and 1962, is a satellite deposit to the well-known Sala mine, and similarly to the Sala deposit, it is hosted within an extensive marble unit interbedded with layers of felsic volcaniclastic material.  The rocks in the area have first been dolomitized and then undergone regional metamorphism to greenschist facies at 1.87 Ga.  There is a large granitic batholith to the south and east of the deposit, and a thin porphyritic intrusion with a complex geometry closely spatially associated with the mineralization.  The marble unit is the host unit for the mineralization and is the most commonly occurring rock type in the study area.  It is commonly quite impure and variably skarn altered throughout, with common gangue minerals including serpentine, chlorite, tremolite, diopside and phlogopite.  The mineralized sections of the marble are spatially related to the skarn-rich areas of marble, and common sulphides include pyrite, galena and sphalerite. The interbedded volcaniclastic beds have a rhyolitic composition, and also contain Mg-silicates such as chlorite, tremolite and phlogopite, giving them a green colour too.  The contacts between volcaniclastic and marble beds is often marked by contact skarns.  The porphyritic intrusion has a granodioritic composition.  It exhibits chilled margins, and the contact between it and the marble is often marked by increased skarn alteration in the marble.  The feldspar crystals in the unit are zoned, with a white outer rim and a green to yellow sericitic core. The cross section shows that all the units are steeply dipping to the south-west, with steeply dipping stratabound lenses of mineralization in the marble, sub-parallel to the volcaniclastic beds in the north east.  The south-western part of the section is thick uninterrupted marble.  Lithogeochemical results have been divided into two groups – a distal group within this thick marble, and a proximal group close to the mineralization and volcaniclastic beds. The proximal zone is enriched in SiO2 and Al2O3 and depleted in FeO, MnO and MgO relative to the distal zone.  The Mn and Mg values are enriched when compared to regional levels however.  The proximal zone also contains more Pb, Zn, Ag, As and Sb than the distal zone, but contains lower amounts of Au and Cu than the distal zone.  There is one hotspot in the distal zone which contains high concentrations of Pb, Zn, Ag, Au, Cu and Sb. The chemical signature is similar to the mined ore at Sala.  The common occurrence of Fe, Mg and Mn-bearing skarn minerals near the mineralization suggests these elements should be higher in the mineralized zone, but skarn minerals are common throughout the study area, and the high levels of Si and Al in the mineralization zone could be diluting the other oxides.  The lenses of mineralization are closely spatially related to the porphyritic intrusion however the intrusion is relatively minor in terms of size and is younger and unaltered.  It could have followed an existing fault which was related to the mineralization though.  Various examples of sulphide replacement and retrograde alteration can be seen, such as galena replacing amphiboles and barite.  The similarity in mineralogy between regional metamorphic calc-silicates and ore-related skarn minerals makes it hard to differentiate between the two, however the presence of high temperature clinopyroxene indicates the deposit is likely a prograde metasomatic skarn deposit.  This is backed up by a similar calc-silicate assemblage, mineralization style and replacement textures to those associated with skarn deposits.  It is also spatially related to magmatism with mineralization occurring along lithological contacts.