Abstract: The Zhangjiawa skarn iron deposit is located at the Laiwu area of Luxi Block, eastern North China Craton. It is genetically associated with the Early Cretaceous highMg diorite, and occurs in the contact zones between the Ordovician Majiagou Formation limestone/dolomite and diorite. In this paper, we carried out detailed analyses of major and trace elements in magnetite using electron microprobe and laser ablation ICPMS methods, aimed to reveal the geochemical features as well as evolutional trends from the magmatic to the hydrothermal magnetites and thus provide significant constraints on the genetic processes of the Zhangjiawa Fe deposit. The results showed that igneous and hydrothermal magnetites in the Zhangjiawa deposit have distinct geochemical features. Compared with the hydrothermal magnetite, the igneous magnetite was significantly enriched in siderophile elements such as Ti, V and Cr. High field strength elements such as Nb, Ta, Zr, and Hf, and moderate compatible elements of Sn, Ga, Ge and Sc, were also relatively enriched in the igneous magnetite. However, lithophile elements such as Mg, Al, Mn, Zn and Co were remarkably enriched in hydrothermal magnetite. Ti, V, Cr, Mg, Al, Mn, and Zn typically exhibited distinct behaviors between the magmatic and hydrothermal systems. Mineralization of magmatic Ti, Cr and V in magnetite was mainly controlled by temperature, partition coefficient and fO2; whereas Mg, Al, Mn and Zn commonly enrichment in hydrothermal magnetite through isomorphic replacement was primarily controlled by waterrock interaction and later stage metasomatism of chlorite and carbonate veins. In hydrothermal magnetite, cobalt content was strongly affected by sulfide in addition to waterrock interactions and postfluid metasomatism, which would decrease drastically in the presence of sulfides. Si, Ca, Na, Sr and Ba displayed highly consistent geochemical behaviors in both the magmatic and hydrothermal magnetites systems, while Ti verus Ni/Cr ratio could be used to distinguish igneous and hydrothermal magnetites. Our analysis indicate that the Zhangjiawa hydrothermal magnetites can be divided into two stages according to the petrographical evidence. The earlystage magnetite includes early primary granular magnetite and early secondary magnetites; the latestage magnetite is composed of late primary and late secondary magnetites. The primary magnetite commonly has the typical triple point structure, while the secondary magnetite is characterized as porous and commonly shows irregular, dendritic, skeletal and metasomatic relict textures. Mg, Al, Mn, Zn and Co contents continued to increase from the early to late stage as well as from the primary to secondary magnetite, possibly due to waterrock interactions and breaking down of chlorite during later alteration. Pores are common in the late stage hydrothermal magnetite, likely resulted from elevated fluidmagnetite interaction. Therefore, the trace elements in hydrothermal magnetite not only indicate the physical and chemical conditions of the oreforming fluids, but also reflect the properties of the surrounding rocks as well as waterrock interactions.

Key words: magnetite trace elements, skarn iron deposit, laser ablation ICP-MS, metallogenic process