Iron isotope fractionation during sulfide liquid segregation and crystallization at the Lengshuiqing Ni-Cu magmatic sulfide deposit, SW China

We report the first measurements of Fe isotope fractionation between coexisting pyrrhotite and chalcopyrite from the Lengshuiqing Ni-Cu magmatic sulfide deposit, Southwest China. The δ⁵⁶Fe values of chalcopyrite (Cp) range from 0.24‰ to 1.25‰, much higher than those in the coexisting pyrrhotite (Po, -1.25‰ to -0.13‰). The δ⁵⁶Fe values of pyroxene separates from the Lengshuiqing intrusions are in a range from -0.03‰ to 0.18‰. Pyrrhotite in sulfide-mineralized rocks with greater than ∼2 wt.% S has δ⁵⁶Fe values of -0.3 ± 0.2‰, indicating Δ⁵⁶Fe_Po-sil (pyrrhotite - silicate melt) in the range of -0.4 ± 0.2‰ with parental silicate magma δ⁵⁶Fe of 0.1‰ as calculated from the least evolved pyroxene δ⁵⁶Fe value. This fractionation is similar to that experimentally determined by Schuessler et al. (2007) for pyrrhotite and a rhyolitic melt, suggesting that Δ⁵⁶Fe_Po-sil of approximately -0.4‰ represents a near-equilibrium value for sulfide-rich mineralization in the Lengshuiqing intrusions. Pyrrhotite-dominated sulfide separates in samples with less than ∼2 wt.% S show δ⁵⁶Fe values as low as -1.25‰. The low δ⁵⁶Fe values can be modeled by a non-equilibrium process, where the fractionation between immiscible sulfide liquid and silicate magma decreases from -0.4 to -1.4‰ as the R-factor (mass ratio of silicate melt to coexisting sulfide liquid) increases from 1 to 1000. We suggest that the relative proportion of externally introduced Fe from sulfidic sedimentary country rocks may be higher in low-S samples, controlling the low δ⁵⁶Fe values of the sulfide assemblage in the mineralized rocks.

The trend of Fe isotope enrichment at Lengshuiqing is consistent with Fe-S bond strengths, but the difference between coexisting pyrrhotite and chalcopyrite is not consistent with equilibration at high temperatures. The Δ⁵⁶Fe_Cp-Po values at Lengshuiqing (1.06-1.77‰) are similar to those measured for samples from the Voisey's Bay and Eagle Ni-Cu deposits, and suggest that in a cooling sulfide liquid Fe isotope equilibration may continue to temperatures less than 200 °C.