In-situ LA-ICP-MS trace elements analysis of magnetite: The Fenghuangshan Cu-Fe-Au deposit, Tongling, Eastern China

- Three groups of magnetite have different chemical compositions.
- Fluid-rock interaction, T and coexisting minerals are the controlling factors.
- Magnetite from different skarns has different compositions.
- Ca is an important discriminate factor for skarn magnetite.

The Fenghuangshan deposit is a typical Cu-Fe-Au skarn deposit in the Tongling area, Anhui province, Eastern China. The deposit has a paragenetic sequence of a prograde skarn stage, followed by a retrograde skarn stage, and a final quartz-sulfide and carbonate stages. Magnetite in the Fenghuangshan deposit mainly formed in the retrograde and carbonate stages. According to the morphology of magnetite and mineral assemblage of ores, we divided magnetite-bearing ores into three groups. Group 1 (early retrograde skarn stage) is represented by a mineral assemblage of magnetite and chalcopyrite. Group 2 (late retrograde skarn stage) has a mineral assemblage of magnetite, chalcopyrite, and wollastonite with characteristic ring-like magnetite. Group 3 (carbonate stage) is characterized by large amounts of calcite veins crosscut or associated with magnetite and intensive hematization of magnetite crosscut by the veins.Laser ablation (LA)-ICP-MS was used to determine trace element concentrations of magnetite from the three mineralization stages. Positive correlations among Mg, Al, Ca, and Si in magnetite indicate that these lithophile elements have similar behavior during the skarn formation process. Calcium is an important discriminant element for magnetite in skarn deposits. Positive correlations are also evident for Pb, Sn and W in magnetite, which also indicates their similar behavior. In general, magnetite grains of different stages have similar normalized trace element patterns, indicating that they share a common source. However, some elements such as Co and Mn in magnetite decrease from early retrograde skarn stage, late retrograde skarn stage to carbonate stage, which may be attributed to the precipitation of coexisting minerals (sulfides and carbonates) or the decreasing temperature. Magnetite grains of the retrograde stage have higher Mg + Mn and Si + Al contents than those of the carbonate stage, indicating a decreasing degree of fluid-rock interaction during the skarn formation process. Trace element data of skarn magnetite indicate a more widely compositional variation than previously suggested. Magnetite from the Fenghuangshan Cu-Fe-Au deposit has similar composition to those from other Cu, Cu-Fe or Cu polymetallic skarn deposits, but different from those from Fe skarn deposits, such that magnetite composition is very powerful in establishing the origin of skarn deposits.