In Situ Trace Element and Fe-O Isotope Studies on Magnetite of the Iron-Oxide Ores from the Takab Region, North Western Iran: Implications for Ore Genesis

The early Cambrian Takab iron ore deposit is situated in the northern part of the SanandajSirjan zone, western Iran. It consists of banded, nodular and disseminated magnetite hosted in folded micaschists. Trace element and Fe and O isotopic experiments reveal various hydrothermal precipitation environments under reduced to slightly oxidizing conditions. Disseminated magnetite has high Ti (945-1940 ppm) positively correlated with Mg + Al + Si, and heavy Fe (+0.76 to +1.86%) and O (+1.0 to +4.07%) isotopic compositions that support a magmatic/high-T hydrothermal origin. Banded magnetite has low Ti (15 200 ppm), V (<= 100 ppm), Si and Mg (mostly <= 300 ppm) and variable Al. The partial derivative Fe-56 values vary from -0.2% to +1.12% but most values also support a magmatic/high-T hydrothermal origin. However, variable partial derivative O-18 ( -2.52 to +1.22%) values provide evidence of re-equilibration with lower-T fluid at similar to 200-300 degrees C. Nodular magnetite shows high Mn (<= 1%), and mostly negative partial derivative Fe-56 values (average, 0.3%) indicative of precipitation from an isotopically light hydrothermal fluid. Re-equilibration with carbonated rocks/fluids likely results in a negative Ce anomaly and higher partial derivative O-18 (average, +6.30%). The Takab iron ore deposit has, thus, experienced a complex hydrothermal history.