Multiple sulfur isotopes from the Neoarchaean Serra Sul black shale, Carajás mineral province, northern Brazil

Abstract

In-situ laser ablation and multicollector–inductively coupled plasma–mass spectrometry were combined to analyse for sulfur isotopes in sulfide minerals that occur in the 2.7-billion-year-old black shale of Serra Sul, Carajás mineral province, Brazil. The black shale is part of a banded-iron-formation unit – the Carajás Formation – that hosts world-class iron-ore deposits. An obvious signal of mass-independent fractionation of sulfur isotopes (MIF-S) is not present in the black-shale sulfide minerals. Any significant MIF-S signal, above the analytical error of ∼0.5‰ Δ33S, was likely obscured by endogenic S. This explanation should also apply to the missing MIF-S signal in the clastic rocks of the overlying Águas Claras Formation. The absence of MIF-S in the latter was interpreted to indicate a depositional age younger than 2.45 Ga. However, Palaeoproterozoic hydrothermal pyrite can reconcile the non-MIF-S signature with the Neoarchaean Águas Claras Formation.