Base metal exchange between magnetite and a chloride-rich hydrothermal fluid

Base metal exchange reactions between magnetite and a supercritical aqueous chloride solution have been experimentally calibrated at T = 600 to 800°C, P = 2 kb, and total chloride = 2.06 M. The results are expressed as the distribution coefficients KdMFe=(XMXFe)fl(XMXFe)s, where fl and s represent the fluid and the solid solution, respectively, and XM=mM(mM + mFe total(mM + mFe total) are the mole fractions of the metal of interest and of total Fe, respectively. The experimentally determined distribution coefficients are KdMnFe = 64, 33, and 18, and KdZnFe=161, 87, and 53, at T=600, 700, and 800°C, respectively. Linear regression yields the best fit equations: ln KdZnFe = 5198/T(K) − 0.872 (r2 = 0.9999) and ln KdMnFe = 5941/T(K) − 2.63 (r2 = 0.9995). Kd values were independent of composition over the range of concentrations investigated. Clearly, both Mn and Zn are strongly fractionated into the fluid with respect to Fe. A combination of KdMnFe and KdZnFe yields KdZnMn = 2.54, 2.67, and 2.94 at 600, 700, and 800°C, where Kd = (XZnXMn)fl/(XZnXFe)s. The partitioning and temperature dependence described by KdZnMn are significantly weaker than those shown by KdMnFe and KdZnFe. Results for the CdFe and CuFe exchange reactions indicate that both Cd and Cu are extremely partitioned into the fluid with respect to Fe and very strongly partitioned into the fluid with respect to Zn and Mn. The experimental results suggest that even minor to trace concentrations of these base metals in natural magnetites indicate strong enrichment of Zn, Mn, Cu, and Cd relative to Fe in chloride-rich hydrothermal fluids. In particular, KdMnFe was applied to magnetites from skarn deposits. Calculations at 360 to 420°C suggest that skarn fluids in equilibrium with magnetites containing only minor concentrations of Mn can have Mn/Fe ratios greater than 1.