Stable (Zn, Cd, Fe) and radiogenic (U, Pb) metal isotope constraints on the age, genesis and modification of the Lovisa Zn-Pb sulfide deposit, Bergslagen, Sweden

Abstract

We present isotopic data bearing on the origin of the vent-distal, Lovisa stratiform Zn-Pb sulfide deposit, Bergslagen, Sweden. The age of deposition is constrained by secondary ionisation mass spectrometry (SIMS) U–Pb zircon dating of pumiceous mass flow deposits interbedded with mineralised, fine-grained volcaniclastic strata. Two mass flow deposits in the stratigraphic footwall yield ages of 1892 ± 2 Ma and 1892 ± 5 Ma respectively, whereas a mass flow deposit in the stratigraphic hanging wall yields an age of 1891 ± 3 Ma, constraining the mineralisation to c. 1892 Ma. Positively correlated δ⁶⁶Zn (-0.299 to + 0.219‰), δ³⁴S (-0.2 to + 4.7‰) and Zn/Cd (122 to 659) in sphalerite suggest mass-dependent, kinetic fractionation during sulfide deposition. Upwards decrease in these parameters and Zn/Pb relative to stratigraphic younging suggest that the hydrothermal system matured over time, whereby later fluid batches reached the depositional site less fractionated. Thermochemical sulfate reduction and cooling of single hydrothermal fluid batches are suggested as sulfide precipitation mechanisms while bacteriogenic sulfate reduction is deemed less likely based on by absence of highly negative δ¹¹⁴Cd (-0.212 to -0.069‰), and δ³⁴S values. Sphalerite and galena Pb isotope compositions indicate leaching of c. 1.91–1.89 Ga felsic volcanic rocks as a principal metal source followed by subsequent, syn-metamorphic addition of radiogenic Pb. Limited syn-metamorphic isotopic homogenisation is suggested by retained systematic variations in different sulfide beds.