Genesis of Si-rich carbonatites in Huanglongpu Mo deposit, Lesser Qinling orogen, China and significance for Mo mineralization

Abstract

The Huanglongpu carbonatite-related Mo ore field is located in the Lesser Qinling Orogenic belt in the southern margin of the North China block. The ore field is composed of six deposits, Yuantou, Wengongling, Dashigou, Shijiawan, Taoyuan and Erdaohe, all of which are genetically related to carbonatite dykes except for the Shijiawan deposit which is associated with a granitic porphyry. The Yuantou carbonatite dykes intruded into Archean gneiss and other carbonatites emplaced into Mesoproterozoic volcanic and sediment rocks. The carbonatites are mainly composed of calcite and variable amounts of quartz and K-feldspar and minor molybdenite. Re–Os dating of molybdenite from the Yuantou carbonatite yields a weighted average age of 225.0±7.6Ma, consistent with the molybdenite age (221Ma) from the Dashigou deposit. The rocks are characterized by high heavy REE (HREE) contents and consistent flat REE distribution patterns with La/Ybcn ~1. Quartz in the carbonatites from Yuantou and Dashigou deposits shows consistent O isotopes (8.1–10.2‰) similar to the associated calcite (7.2–9.5‰). The quartz and associated K-feldspar contain lower Zr, Hf and higher HREE abundances and negligible Eu anomaly relative to those from the granite porphyry in Shijiawan. Both minerals are primary products in the carbonatitic liquid, and not captured from the wall-rocks or crustal-derived silicate magmas, or a hydrothermal origin. Thus, the Huanglongpu carbonatitic liquids were enriched in Si and Mo, which may be produced by intensely fractional crystallization of non-silicate minerals.