In-situ major and trace element chemistry of melanite from Tieshan Fe–Cu skarn deposit, Hubei Province, Eastern China: Implications for hydrothermal fluid evolution

Abstract

The Tieshan Fe–Cu skarn deposit is located in the Edong ore district, the largest ore district of the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in Eastern China. The Fe–Cu mineralization is spatially associated with the Tieshan pluton, which intruded the carbonates of the Lower Triassic Daye Formation. The orebodies are predominantly hosted along the intrusive contact zone between the (quartz)-diorite and (dolomitic)-marble. EMPA analysis shows that the Tieshan garnets belong to high-Ti andradite, with TiO2 contents ranging from 0.03 to 2.59 wt%, and is designated as melanite. The melanite contains low Na content, suggesting the absence of Na+-REE3+ coupled substitution. LA–ICP–MS analysis shows high total LREE (∑LREE), low ∑HREE and positive Eu anomalies, suggesting high fO2 and rapid growth of the melanite. Compared with other Ti-rich garnets worldwide, the Tieshan melanite grains are more Al-rich and Si-rich, and shows negative SiO2 vs. TiO2 correlation, which suggests predominant Ti-substitution in the tetrahedral site. The more positive Eu anomalies and higher percentage of andradite are present in the rim (than in the core), indicating lower salinities and higher fO2 of the late-stage ore-forming fluid. The magmatic-hydrothermal fluid source of these garnets supports the metasomatic ore genesis at Tieshan.