LA-ICP-MS titanite U-Pb dating and its signification in the Luohe iron deposit in the Lu-Zong volcanic basin

The Luohe Fe deposit is the largest iron deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (around 1 Gt Fe ore resource). Widespread hydrothermal titanite is developed in deposit. Trace element chemistry of the well-crystallized hydrothermal titanite from both deep and shallow orebodies indicates the presence of (Al, Fe)³⁺ + (F, OH) ^- = Ti⁴⁺ + O^2- replacement reaction 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 porphyritic type iron deposit was formed at a higher temperature than typical Fe skarn deposits in the metallogenic belt. The high ore-forming temperature may be the key factor of Ti migration. The distinct LREE enrichment, the relatively high total REE concentrations and marked negative Eu anomalies all suggest that the Luohe titanite crystallized before apatite and epidote under a high temperature hydrothermal environment. Variation in the negative Eu anomalies suggests increasing ore fluid oxygen fugacity from deep to shallow level. In-situ LA-ICP-MS U-Pd dating for the titanite from the shallow and deep orebodies yielded 130.0 ± 0.9 Ma and 129.1 ± 0.8 Ma to 129.7 ± 0.8 Ma, consistent with the age of the buried diorite in the area. Combined with ore deposit geology, we proposed that the Luohe porphyritic type iron deposit mineralization was closely related to the deep-seated diorite, and was a product of the Late Yanshanian (Early Cretaceous) crustal thinning-related tectono-thermal events.