SIGNIFICANT FE ISOTOPE FRACTIONATION WITHIN THE KIGLAPAIT LAYERED INTRUSION REFLECTS CRYSTAL FRACTIONATION

Layered intrusions are valued as natural laboratories of magmatic and ore-forming processes, and Fe isotopes have proven to be an increasingly useful geochemical tool to study magmatic systems. Despite recent successes in using Fe isotopes to investigate igneous processes, detailed Fe isotope studies exist for only a few layered intrusions. Published Fe isotope measurements of whole rock (WR) samples and mineral separates from the Bushveld Complex, Ilímaussaq Alkaline Complex, and Baima intrusion have provided insight into crystal fractionation, magma addition, and sub-solidus re-equilibration in layered intrusions. Iron isotopes can also provide information about fluid exsolution, redox, temperature, and alteration. The ~1.3 Ga, 8-9 km-thick Kiglapait intrusion within the Nain Plutonic Suite (Labrador, Canada), records the complete crystallization sequence of 3500 km³ of troctolitic magma. It is divided into the troctolitic Lower Zone (0-84 percent solidified [PCS]), the olivine gabbroic to ferrosyenitic Upper Zone (84-100 PCS), including the 80 cm-thick Main Ore Band, and the downward crystallized Upper Border Zone. We report the first Fe isotope measurements of Kiglapait WR samples, which show significant variation in δ⁵⁶Fe (⁵⁶Fe/⁵⁴Fe relative to IRMM-14): -0.01 ±0.06‰ to +0.18 ±0.03‰ (n=13). δ⁵⁶Fe_WR values are variable below 68 PCS, increase from 68 to 89.3 PCS into the Upper Zone, and then decrease systematically across the Main Ore Band (93.5 PCS) to 97 PCS. The decrease in δ⁵⁶Fe above 89.3 PCS is associated with the appearance of pyrrhotite and magnetite as cumulus phases. This is consistent with new measurements of magmatic pyrrhotite (n = 20) that confirm δ⁵⁶Fe_pyrrhotite <0, literature values that report δ⁵⁶Fe_magnetite >0, and simple modeling utilizing these data. The predictable shift in δ⁵⁶Fe_WR values within the Upper Zone, coupled with the variability of the Lower Zone, and a strong correlation with the radiogenic isotopic signatures of the same samples, implicates mineralogy as the primary control on the Fe isotope systematics of the Kiglapait intrusion. Using non-traditional stable isotopes like Fe constitutes a novel approach in elucidating the mechanisms at play during the assembly and evolution of layered intrusions and the concentration of ores within them.