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

摘要

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 delta Fe-56 values of chalcopyrite (Cp) range from 0.24 parts per thousand to 1.25 parts per thousand, much higher than those in the coexisting pyrrhotite (Po, -1.25 parts per thousand to -0.13 parts per thousand). The delta Fe-56 values of pyroxene separates from the Lengshuiqing intrusions are in a range from -0.03 parts per thousand to 0.18 parts per thousand. Pyrrhotite in sulfide-mineralized rocks with greater than similar to 2 wt.% S has delta Fe-56 values of -0.3 +/- 0.2 parts per thousand, indicating Delta Fe-56(po-sil) (pyrrhotite - silicate melt) in the range of -0.4 +/- 0.2 parts per thousand with parental silicate magma delta Fe-56 of 0.1 parts per thousand as calculated from the least evolved pyroxene delta Fe-56 value. This fractionation is similar to that experimentally determined by Schuessler et al. (2007) for pyrrhotite and a rhyolitic melt, suggesting that Delta Fe-56(po-sil) of approximately -0.4 parts per thousand represents a near-equilibrium value for sulfide-rich mineralization in the Lengshuiqing intrusions. Pyrrhotite-dominated sulfide separates in samples with less than similar to 2 wt.% S show delta Fe-56 values as low as -1.25 parts per thousand. The low delta Fe-56 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 parts per thousand 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 delta Fe-56 values of the sulfide assemblage in the mineralized rocks.