The Lovisa Stratiform Zn-Pb Deposit, Bergslagen, Sweden: Structure, Stratigraphy, and Ore Genesis

Medium- to high-grade metamorphosed, 1.9 Ga, stratiform, syngenetic Zn-Pb +/- Ag sulfide deposits constitute an economically important type of ore deposit in the Bergslagen lithotectonic unit of the Fennoscandian Shield. The Lovisa Zn-Pb deposit occurs in a metamorphosed succession of rhyolitic ash-siltstone, rhyolitic mass flow deposits, limestone, and Fe formation, deposited at a stage of waning volcanism in Bergslagen. Accessory graphite, absence of Ce anomalies in shale-normalized rare earth element (REE) data, and absence of hematite in Mn-rich Fe formations stratigraphically below the Lovisa Zn-Pb deposit indicate a suboxic-anoxic depositional environment. The uppermost Mn-rich Fe formation contains disseminated, inferred syngenetic Pb-Ag mineralization with mainly negative delta S-34 values in sphalerite and galena (-6.1 to -1.9 parts per thousand). Deposition of this Fe formation terminated during a pulse of explosive felsic volcanism. The Lovisa Zn-Pb deposit is interpreted to have formed in an alkali-rich brine pool developed immediately after this volcanic event, based on lithogeochemical and stratigraphic evidence. The first stage of mineralization deposited stratiform sphalerite mineralization with mainly positive delta S-34 values (-0.9 to 4.7 parts per thousand). This was succeeded by deposition of more sphalerite-galena stratiform mineralization with delta S-34 values close to 0%0 (-2.1 to 1.5 parts per thousand). The more galena-rich mineralization partitioned strain and was partly remobilized during later ductile deformation. The stratigraphic context, sulfide mineralogy, S isotopes, and alteration geochemistry suggest that the metal-liferous fluids and the depositional environment were H2S deficient (S poor or SO42- dominant). The source of S is interpreted to have In a mixture of H2S derived from bacterial and thermochemical seawater sulfate reduction and S derived from leaching of volcanic rocks, with the latter becoming more important over time. Lovisa formed in a setting where basin subsidence was periodically punctuated by the deposition of thick, syneruptive felsic volcaniclastic mass flow deposits. Coeval volcanism was likely important for driving hydrothermal activity and supplying a reservoir of metals and S. However, the high rate of deposition of volcaniclastic sediment in Bergslagen also precluded the establishment of long-lived, deep, and anoxic environments favorable for accumulation of organic matter and H2S. This stratigraphic pattern is common in Bergslagen and may explain why large stratiform Zn-Pb deposits are uncommon in the region and restricted to the uppermost part of the metavolcanic succession, directly stratigraphically beneath postvolcanic politic rocks.