Iron isotope investigation of hydrothermal and sedimentary pyrite and their aqueous dissolution products

Oxidative dissolution experiments were carried out on pyrite from multiple petrogenetic environments (hydrothermal, sedimentary, and coal-related nodules) to investigate possible variations in the iron isotopic composition of pyrite and the products of pyrite dissolution. The experimental materials were leached under carefully controlled abiotic conditions, and a subset of leachates and starting materials from these experiments was analyzed for ⁵⁶Fe/⁵⁴Fe by multicollector ICP-MS. Bulk pyrite δ⁵⁶Fe values (relative to IRMM-014) ranged from -0.1 to +1.3‰, with hydrothermal bulk pyrite values <+0.5‰ and those of coal and sedimentary nodular pyrite ≥+0.5‰, higher than most previously measured values for Phanerozoic sedimentary pyrite. We suggest that this reflects precipitation of coal pyrite from a high-δ⁵⁶Fe continental source, such as Fe derived from dissolution of Fe(III) oxides. This interpretation is consistent with pyrite rare earth element (REE) patterns. This could allow differentiation of Fe contributed from coal- and shale-related pyrite at abandoned mine drainage sites. Leachates from oxidative dissolution of the pyrite at pH = 3 yielded, with few exceptions, δ⁵⁶Fe values equal to or lower than those of the coexisting bulk pyrite, by up to about 1‰. These shifts are consistent in direction (but not magnitude) with equilibrium isotope fractionation predictions from theory, with possible second order effects from isotopic heterogeneity within individual natural pyrite samples.