Petrography and sphalerite geochemistry of carbonate-hosted Pb-Zn deposits in meteorite impact craters: insights from Siljan (Sweden) and Kärdla (Estonia)

Low-temperature, carbonate- as well as crater-hosted Pb-Zn mineralisations located in Siljan (Boda, Sweden) and Kärdla (Hiiumaa, Estonia) consist mainly of sphalerite, galena, pyrite, and subordinate chalcopyrite. Sphalerite, pyrite, and chalcopyrite were investigated by microscopy, scanning electron microscopy (SEM), and laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Organic matter, siderite, and feldspar were identified by Raman spectroscopy. In this study, the Boda and Kärdla crater-hosted Pb-Zn mineralisations were characterised and compared, based on their sulphide mineralogy and trace element geochemistry. Although both Pb-Zn occurrences show several similarities regarding mineralogy and textures, some differences were identified. The sulphide mineralisation of Boda is characterised by the presence of gangue minerals like calcite and fluorite, whereas this is the case with accompanying calcite and dolomite in Kärdla. A two-phase genetic model for Boda and a five-phase genetic model for Kärdla is proposed. Sphalerite from Boda were divided into three major textural groups, colloform, disseminated, and vein-hosted disseminated sphalerite. Sphalerite from Kärdla were also divided into three textural groups, colloform sphalerite, euhedral sector-zoned sphalerite, and vein-hosted euhedral sector-zoned sphalerite. Sphalerite from both deposits yield nearly identical low formation temperatures about 160°C using the Ga-Ge-In-Mn-Fe sphalerite geothermometer. These temperature estimates are broadly consistent with the post-impact hydrothermal systems of the Siljan and Kärdla craters. Sphalerite from Boda show generally higher contents in trace elements than their counterparts from Kärdla. However, higher Fe contents were detected in sphalerite from Kärdla. Dark brown-coloured sphalerite correlates with Fe and Ge in both deposits. Remarkable are highly elevated Cu concentrations in the sphalerite from both deposits, especially in black-coloured colloform sphalerite sequences. The Boda deposit is characterised by elevated Ag contents in sphalerite, compared to the Kärdla Pb-Zn mineralisation. Sporadic Ag-S inclusions, presumably acanthite, were also detected in sphalerite from Boda. The Boda Pb-Zn mineralisation was targeted by mining for Pb, Zn and Ag, whereas this was not the case for Kärdla. Sphalerite is the preferred host for Ga in Boda, whereas this trend is not clearly observed in Kärdla. Redox conditions played an important role in the trace element composition of colloform sphalerite bands. Brecciation and corresponding carbonate precipitation induced trace element remobilisation in sphalerite and the formation of late-stage mineral phases such as chalcopyrite. Stratigraphically lower units (Obolus beds) were affected by the reprecipitation of sphalerite and galena derived from the main mineralisation hosted by the Boda limestone located in Boda. A later and small-scaled oxidation event of the Boda deposit is marked by the occurrence of smithsonite replacing sphalerite, whereas this was not observed in Kärdla. Arsenic prefers pyrite over sphalerite in both deposits. Outstanding is the occurrence of colloform pyrite from Kärdla, with varying trace element contents within the bands. Co/Ni ratios indicate that pyrite from Kärdla have a more pronounced hydrothermal fingerprint than their counterparts from Boda. This is in accordance with the local geology, where probable trace element-bearing shale units are missing in Kärdla, whereas they are present in Boda. The preferred incorporation of In was observed in chalcopyrite from Kärdla. In Boda, however, disseminated sphalerite is the preferred host mineral for In, and not chalcopyrite. Raman spectroscopy revealed a connection between sulphide mineralisation and organic matter in the Boda mineralisation, whereas this was not observed in Kärdla. In conclusion,