Trace Element Geochemistry of Magnetite: Implications for Ore Genesis of the Huanggangliang Sn-Fe Deposit, Inner Mongolia, Northeastern China

摘要

The Huanggangliang deposit is a super-large Sn-Fe deposit in the Huanggangliang–Ganzhuermiao metallogenic belt in the southern section of the Great Hinggan Range. The Sn-Fe deposits mainly occur in the skarn contact zone and were formed via the interaction of biotite-bearing alkali feldspar granite with limestone strata of the Permian Dashizhai and Zhesi Formations. Based on the intersecting relations among the ore-bearing veins and the different types of mineral assemblages within these veins, the Sn-Fe mineralization could be divided into two periods and four stages: the skarn period, which includes the garnet–diopside–magnetite (T1) stage (stage 1) and epidote–idocrase–cassiterite–magnetite (T2) stage (stage 2); and the quartz–magnetite period, which can be divided into the quartz–cassiterite–magnetite (T3) stage (stage 3) and quartz–magnetite (T4) stage (stage 4). In this paper, we discuss the genesis of magnetite, controlling factors for magnetite compositions, and type of ore genesis based on petrographic studies and LA-ICP-MS analyses of trace elements in these four types of magnetite from the Huanggangliang Sn-Fe deposit. The results demonstrate that the four types of magnetite are generally depleted in Ti (0.002–3.030 wt %), Al (0.008–1.731 wt %), and Zr (<1.610 ppm). In addition, the low Ni and Cr contents and relatively high and stable Fe contents in the four types of magnetite are indicative of hydrothermal genetic features. Compositions of the ore fluids and host rocks, formation of coexisting minerals, and other physical and chemical parameters (such as fO2) may have influenced the variable magnetite geochemistry in the different Huanggangliang ore types, with fluid compositions and fO2 probably playing the most important roles. The geological, petrographic, and geochemical characteristics of magnetite of the Huanggangliang Sn-Fe deposit lead us to conclude that the deposit is a skarn-type Sn-Fe deposit associated with Yanshanian medium-acidic magmatic activities.