Contamination of mafic to ultramafic magmas by sulfur-bearing sediments: evaluation of the environment of deposition and tracing the unique signature of the contaminants through the magma using multiple sulfur and iron isotope data
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Date
2018
Authors
Hiebert, Russel
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Abstract
Multiple stable isotopes of sulfur (δ33S, δ34S, and Δ33S) and iron (δ56Fe) are used to identify the sources of sedimentary rock contamination at both the Voisey’s Bay (Labrador, Canada) and the Hart (Ontario, Canada) magmatic Ni-Cu-platinum-group element (PGE) deposits. At both locations, sulfide minerals were formed in sedimentary rocks during diagenesis as a result of bacterial sulfate reduction, prior to interaction with the magmas that resulted in the formation of the sulfide mineralization. In the Hart area, both exhalite and graphitic argillite were formed under predominantly anoxic conditions with localized, or transient, oxygen oases in seawater. The fluid composition was a result of mixing of seawater with hydrothermal fluids. Sulfur in the sediments in the Hart area was derived from the reduction of sulfate that had been mass-independently fractionated in the anoxic Archean atmosphere prior to delivery to the seawater.
Multiple sulfur isotopes identified the sources of contamination in both the Voisey’s Bay and Hart deposits, and determined that the Main Zone and Eastern Extension at Hart likely had different contaminants that provided sulfur to form the mineralization. Signatures of these contaminants were distinguishable up to a few hundred meters from the sulfide-rich zones, allowing this to be used as a geochemical tool to vector towards the mineralization. The iron isotopic composition of sulfides from the Voisey’s Bay deposit was too heavily influenced by the host silicate magma to recognize the signature of contamination, but could be used to identify contamination in the Hart deposit. However, this data does not uniquely identify the source of contamination in the Hart deposit, and is not able to identify the signature of contamination at distances of more than a few meters from sulfide mineralization. These data sets have different sensitivity to contamination during equilibrium isotope exchanges with the silicate magma due to the difference in the initial concentration in the magma, as indicated the difference in the distance away from sulfide mineralization at which contamination can still be recognized. In conclusion, based on this study, use of multiple isotope and elemental methods to determine the presence and extent of contamination is strongly recommended.
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Keywords
Archean, Komatiite, Sulfur, Environment, Nickel