Origin of Palaeoproterozoic, sub-seafloor Zn-Pb-Ag skarn deposits, Sala area, Bergslagen, Sweden

Abstract

Unravelling the genesis of metamorphosed mineral deposits can be complicated due to difficulties in separating between primary features and features that formed during the metamorphic overprint. Such uncertainty exists for stratabound and dolomite- and skarn-hosted Zn-Pb-Ag sulfide deposits in 1.89 Ga rocks in the Bergslagen lithotectonic unit (BLU) of Sweden, where a metasomatic vs. regional metamorphic origin for skarns has long been discussed. By integrating geological mapping with new lithogeochemical, mineralogical, and stable isotope data (C, O, S), we show that complexly zoned garnet and clinopyroxene skarns in the Sala area of the central BLU predate mineralization. Sphalerite-galena mineralization formed after the deposition of a younger, more Mn-rich ferroan diopside and andradite-grossular garnet, and is associated with phlogopite, tremolite-actinolite, chlorite, serpentine, and calcite. Mineralization in conjunction with a transition from high-T metasomatism to hydrolytic alteration is inferred. An average δ34SV-CDT of 1.6 ± 1.9‰ in sulfides is consistent with a primordial sulfur source. Trends defined by negative shifts in δ18OV-SMOW and δ13CV-PDB in dolomite and calcite are consistent with fluid infiltration at 300–500 °C. The alteration system is sharply truncated by unaltered, c. 1.89 Ga calc-alkaline granite and porphyritic intrusions, which along with F1 folding of the alteration zones and mineralization suggest that mineralization predate regional metamorphism. The Sala deposits are interpreted as Zn skarn deposits formed in conjunction with the emplacement of intrusions into penecontemporaneous marine volcanic and dolomitized limestone strata. The unusually Mg-rich mineralogy in relation to Zn skarns worldwide most likely reflects the dolomitic precursor.