LA-ICP-MS titanite U-Pb dating and mineral chemistry of the Luohe magnetite-apatite (MA)-type deposit in the Lu-Zong volcanic basin, Eastern China

Abstract

The Luohe Fe deposit is the largest magnetite-apatite (MA)-type deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB) (around 1 Gt Fe ore resource). The main orebody is hosted by volcanic rocks, and widespread titanite mineralization is developed in the footwall of the shallow ore bodies, and in the hanging wall of the deep ore bodies. Trace element chemistry of the well-crystallized hydrothermal titanite from both deep and shallow ore bodies indicates coupled substitution of (Al, Fe)3+ + (F, OH)− for Ti4+ + O2−, and the addition of elements such as Zr, Nb and REEs. Titanite Zr-thermometry yielded a mineralization temperature of ca. 700 °C, suggesting that the Luohe magnetite-apatite deposit was formed at a higher temperature than typical Fe skarn deposits in the metallogenic belt. The high ore-forming temperature may have been a key factor in Ti migration. The distinct LREE enrichment, the relatively high total REE concentrations, and the marked negative Eu anomalies all suggest that the Luohe titanite may have crystallized before apatite and epidote in a high-temperature hydrothermal environment. Variation in the negative Eu anomalies suggests the ore fluid oxygen fugacity increased from deep to shallow levels. Our study suggests that the Th/U ratio is not an efficient igneous vs. hydrothermal titanite discriminator for high-temperature titanite. LA-ICP-MS titanite U-Pb dating of the shallow and deep orebodies yielded ages of 130.0 ± 0.9 Ma and 129.1 ± 0.8 Ma to 129.7 ± 0.8 Ma, consistent with the age of the deep-seated diorite in the area. We propose that the shallow- and deep-level orebodies at Luohe were deposited from the same mineralizing system and mineralization was closely related to the deep-seated diorite.