Origin of the mineralised Hawke Bay conglomerate and its importance in formation of the Vassbo lead-zinc deposit, Sweden

The Vassbo deposit is part of a group of sandstone-hosted lead deposits (Laisvall, Vassbo and Osen) in lower Cambrian sandstones (series 2) close to the Precambrian basement on the Baltoscandian Shield. The mineralised sandstone at Vassbo is part of a seaward-prograding nearshore environment with aeolian dunes and lagoons at the top. In the western part of the Baltoscandian Shield there was an uplift (Hawke Bay Event) and erosion before the middle Cambrian (Miolingian) transgression with deposition of the Alum Shale Formation. The observation of clasts with ore in the Hawke Bay conglomerate may put some limits on the timing of the ore formation. The ore deposits are related to basement faults with a magnetic low signature, which probably reflects deep weathering. Galena and sphalerite in these deposits occur as cement in the sandstone, commonly together with barite, fluorite, calcite, quartz or apatite. Tidal channels in the sandstone contain clasts of dark phosphorites with sphalerite and barite indicating an early ore formation in a nearshore environment. The first cement in the aeolian sandstone is apatite that is partly replaced by a later, second-generation cement containing apatite, sulphides and quartz. The clasts are well rounded and must have been lithified before the erosion. The surface of the clasts cuts the texture of both generations of cement and the clasts are quite commonly coated by pyrite, which implies that the main mineralisation took place before the end of the Hawke Bay Event in early Cambrian time (Lower Cambrian) and the deposition of the conglomerate. The supply of phosphorous and iron adsorbed on aeolian sand gave an increased organic production, and sulphate reduction in the nearshore area. A redox front developed in the sandstone with barite and calcite forming in a sulphate zone and galena and sphalerite a sulphide zone. The sulphur isotope composition of sulphides indicates barite as a sulphur source. The lead-isotope composition of galena and barite show that lead and barium probably had a common source in the underlying basement. Orebodies in the Vassbo area show a clear spatial relationship with faults or shear-zones in the basement. The ore is epigenetic and seismic pumping in these faults/shear-zones was probably important in the ore-forming process. We propose that the hydrothermal ore solution was pumped into H2S-rich sandstones before the Hawke Bay event. The temperature of the ore-forming solution is difficult to estimate because of later metamorphism.