In situ U-Pb geochronology, elemental and Nd isotopic compositions of titanite from the Mesozoic porphyry Mo deposits, NE China

Abstract

Northeastern (NE) China is one of the most important regions for Mo resources in China with more than 80 porphyry/skarn Mo deposits, most of which were formed in the Mesozoic. Geology, geochemistry and geochronology of individual deposits have been studied and a large amount of data was accumulated, but the factors controlling large-scale Mo mineralization in this region is still debated. To better understand the factors controlling the Mo mineralization, three typical large to giant Mesozoic porphyry Mo deposits located in NE China, namely the Daheishan (DHS), Fu’anpu (FA) and Luming (LM) deposits have been selected. In this paper, U-Pb age, elemental and Nd isotopic compositions of titanite from these deposits have been analyzed to reveal their chemical and isotopic compositions, which provide insights into the nature of source magmas and unravel the physical and chemical conditions of Mo mineralization. In-situ titanite Nd isotopic compositions from all the ore-related granites indicate that they originate from a juvenile crustal source. Titanite occurring as euhedral grains in DHS, FA and LM ore-related granites are characterized by high Th/U and LREE/HREE ratios, as well as low Lu/Hf and 147Sm/144Nd ratios, indicate a magmatic origin. In the LM-barren granite, titanite occurs as magmatic and hydrothermally altered type. The euhedral envelope-shaped magmatic titanite has the same characteristics as the titanite in DHS, FA and LM ore-related granites, with higher Th/U and LREE/HREE ratios and lower147Sm/144Nd ratios. In contrast, hydrothermally altered titanite in the LM-barren granite has lower Th/U and LREE/HREE ratios, as well as higher Lu/Hf and 147Sm/144Nd ratios, distinct Eu anomaly and tetrad effect than the magmatic titanite. In-situ magmatic titanite U-Pb dating of DHS, FA, LM ore-related granite yields crystallization ages of ca.168 Ma, ca.166 Ma and ca.178 Ma, respectively, which are consistent with the mineralization ages within error. Hydrothermally altered titanite in the LM-barren granite yields a crystallization age of 177.9 ± 2.3 Ma (MSWD = 2.3), which is indistinguishable from the LM-ore granites and molybdenite Re-Os age, implying that the formation of the Luming porphyry Mo deposit was coeval with the wall-rock alteration. Relatively homogeneous and consistent Nd isotopic compositions of magmatic titanite from LM-ore granite and hydrothermally altered titanite from LM-barren granite suggest that the hydrothermal fluids have the same origin with the granite and could be directly exsolved from the ore-forming magma. Trace element characteristics of magmatic titanite from these porphyry Mo deposits show that the magma of ore-related granite has higher oxygen fugacity and water contents than those of the LM-barren granite. Hydrothermally altered titanite in the LM-barren granite shows higher F contents than that in the magmatic titanite. The results from this study show that high Mo content in the source region is a prerequisite for the formation of porphyry Mo deposits but the specific magma composition (e.g., with abnormally high Mo contents) may be not critical. Other factors such as the higher F, oxygen fugacity and water contents are also important for Mo mineralization, which could lead to the magma evolution and accumulation of metal elements.