Trace Element Geochemistry of Magnetite from the Fe(-Cu) Deposits in the Hami Region, Eastern Tianshan Orogenic Belt, NW China

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to determine the trace element concentrations of magnetite from the Heifengshan, Shuangfengshan, and Shaquanzi Fe(-Cu) deposits in the Eastern Tianshan Orogenic Belt. The magnetite from these deposits typically contains detectable Mg, Al, Ti, V, Cr, Mn, Co, Ni, Zn and Ga. The trace element contents in magnetite generally vary less than one order of magnitude. The subtle variations of trace element concentrations within a magnetite grain and between the magnetite grains in the same sample probably indicate local inhomogeneity of ore-forming fluids. The variations of Co in magnetite between samples are probably due to the mineral proportion of magnetite and pyrite. Factor analysis has discriminated three types of magnetite: Ni-Mn-V-Ti (Factor 1), Mg-Ai-Zn (Factor 2), and Ga-Co (Factor 3) magnetite. Magnetite from the Heifengshan and Shuangfengshan Fe deposits has similar normalized trace element spider patterns and cannot be discriminated according to these factors. However, magnetite from the Shaquanzi Fe-Cu deposit has affinity to Factor 2 with lower Mg and Al but higher Zn concentrations, indicating that the ore-forming fluids responsible for the Fe-Cu deposit are different from those for Fe deposits. Chemical composition of magnetite indicates that magnetite from these Fe(-Cu) deposits was formed by hydrothermal processes rather than magmatic differentiation. The formation of these Fe(-Cu) deposits may be related to felsic magmatism.