• Two distinct generations of titanite, magmatic and hydrothermal, were distinguished in the Songshunangou deposit. • In-situ U-Pb dating of magmatic and hydrothermal titanite resolves the mineralization age controversy, confirming a short-lived Early Paleozoic ore-forming event. • Chemical signatures in magmatic titanite provide direct mineralogical evidence for the high oxidation state and elevated water content of the fertile, Au-mineralizing magma. To constrain the age of Au mineralization and to evaluate the controls on the magmatic-hydrothermal evolution of the Songshunangou porphyry-Au deposit in the North Qilian orogenic belt, titanite grains were analyzed for their chemical and U-Pb isotope compositions. Titanite from mineralized quartz dioritic porphyry (QDP) can be classified into magmatic (Ttn1) and younger hydrothermal (Ttn2) types. The former is characterized by elevated high field-strength element (HFSE) and rare earth element (REE) contents, a low LREE/HREE ratio (av. 9.65), and a significant negative Eu anomaly (av. 0.65). In contrast, hydrothermal Ttn2 has low HFSE and REE contents, a high LREE/HREE ratio (av. 28.52), and a distinct positive Eu anomaly (av. 2.02). The U-Pb ages of Ttn1 and Ttn2 are 455.1 ± 8.3 Ma (1σ, MSWD = 1.4) and 454.2 ± 8.9 Ma (1σ, MSWD = 0.84), respectively. Trace element ratios, Eu and Ce anomalies of Ttn1 indicate that the QDP magma was formed under oxidizing conditions. The distinct increase in the Eu anomaly combined with the concurrent decrease in REE + Y content during the formation of Ttn2 suggest that these elements are sensitive proxies to differentiate between magmatic and hydrothermal titanite. Titanite Sr/Y, La/Yb, Ce/Nd/Y, and Eu/Eu*/Y ratios indicate that the fertile QDP magma had a relatively high H 2 O content. Our results demonstrate that an elevated H 2 O content combined with oxidizing conditions were critical prerequisites for the formation of the porphyry-Au deposit at Songshunangou. This study illustrates that titanite can be a potential tool not only for determining the mineralization age but also for recording the magmatic-hydrothermal evolution of porphyry-Au systems.
Wu et al. (Sun,) studied this question.