Co-evolution of pyrite δ⁶⁶Zn and sphalerite-pyrite-galena modal proportions in the ca. 1,828 Ma Mississippi Valley-type Zn-Pb Black Angel deposit, Greenland: insights into the chemical potential of sedimentary carbonates for Zn mineralization

Abstract

The Black Angel deposit formed 1828 ± 17 million years ago (pyrite Re-Os model age) in a foreland basin setting to the advancing Rinkian and Nagssugtoqidian orogens. Mineralizing processes were active at the time of building of the Rinkian foreland fold-and-thrust system. A source of Os, and by corollary other metals, in the Archean Rae Craton is identified in sphalerite-pyrite ± galena samples. A co-evolution of d66 Znₚᵧᵣᵢₜₑ with sulfide modal proportions is evident. The narrow and uniform δ⁶⁶ Znₚᵧᵣᵢₜₑ composition of sphalerite-pyrite ± galena samples (+0.30 to +0.33‰), which overlaps with δ⁶⁶ Zn of the upper continental crust (+0.35 ± 0.05 ‰), may be compatible with Zn saturation caused by dissolution/assimilation of carbonate which would result in near-quantitative sphalerite precipitation. Conversely, δ⁶⁶ Znₚᵧᵣᵢₜₑ in the massive pyrite body and in pyrite-galena ± sphalerite samples (+0.14 to +0.21‰) associated with more pelitic carbonate is compatible with disequilibrium conditions and rapid precipitation without sufficient carbonate dissolution/replacement to reach near-neutral pH for sphalerite precipitation. We further assume that the difference of metal endowment between Zn-Pb carbonate-hosted and Pb-Zn sandstone-hosted MVT deposits could be related to dissolution of Zn-bearing, clean, sedimentary carbonate rocks that control sphalerite saturation and precipitation.