Trace element of epidote from the Tonglushan Cu-Fe-Au deposit, eastern China: Implications for exploration indicator for skarn mineralization

Epidote is a common alteration phase in magmatic-hydrothermal deposits. It has been widely used as an indicator for vectoring mineralization centers and fertility assessments for porphyry ore deposits. However, whether epidote from skarn deposit without significant porphyry-style mineralization can also serve as an exploration indicator remains unclear. Major and trace elements of epidote from the quartz monzodiorite (porphyry) (QMD), endoskarn, Cu-Fe ore, and exoskarn/marble at Tonglushan were analyzed by electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to constrain their variations across the deposit. The EMPA results suggest that the major compositions (e.g., Al, Fe, and Mn) of epidote are more likely to be affected by precursor minerals such as plagioclase, garnet, diopside, and features of fluid composition (e.g., fO₂, temperature, and Fe³⁺ abundance). Epidote trace elements, however, may controlled by coprecipitating minerals such as sulfides and magnetite, which may strongly enrich in certain elements (e.g., As, Sb, Co, Ti, and V) relative to the epidote, as well as precursor allanite which has significantly elevated trace elements (such as As and V) than the successor epidote replacement. The contents of Na, Mg, Mn, Co, and Sn in epidote show a general increase from QMD, through endoskarn, to Cu-Fe ore and then decreasing to exoskarn/marble whereas that of Sc, V, Y, Zr, Sb, and total REE show an opposite trend. The general patterns of these trace elements in skarn epidote may potentially indicate the direction of orebodies. The strong-moderate linear correlation (R² = 0.76 to 0.93) of As and Sb contents in epidote with the tonnage of Cu or distance from the magmatic-hydrothermal center demonstrates their potential as a quantitative evaluation tool. In general, the trace element contents of skarn epidote are expected to provide critical information for skarn deposit exploration.