A unique Mo deposit associated with carbonatites in the Qinling orogenic belt, central China

The Qinling molybdenum belt is a prominent metallogenic structure in central China hosting several significant porphyry- and porphyry–skarn-type deposits. The Huanglongpu Mo deposit in the north-western part of the belt is unique in that it is associated with carbonatite dykes, rather than felsic magmatism. The carbonatites are composed largely of Sr–Mn-rich calcite and characterized by high concentrations of Sr and rare-earth elements (REE), and stable-isotope values indicative of a mantle source (δ13CPDB = − 6.7 ± 0.2‰ and δ18OSMOW = 8.2 ± 1.0‰). Molybdenite is associated with galena and REE minerals (parisite, bastnäsite and monazite). Both molybdenite and galena are characterized by high Re contents (up to 0.4 and 0.2 wt.%, respectively) and Re/(Mo, Pb) ratios approaching the primitive-mantle values. In contrast to the rock-forming calcite, the REE minerals are enriched in light REE, whose relative proportion increases from parisite-(Ce) [average (La/Nd)n = 2.1] to bastnäsite-(Ce) and monazite-(Ce) [average (La/Nd)n = 3.1, 4.6, respectively]. The whole-rock compositions are characterized by some of the highest Mo and heavy REE abundances reported for carbonatites to date: up to 1010 ppm Mo, 1130 ppm Y + Gd…Lu and (La/Yb)n = 1.2–2.7. The unusual trace-element geochemistry of the Huanglongpu rocks may ultimately reflect the composition of their mantle source, but their enrichment in Mo + Re was undoubtedly enhanced through preferential partitioning of these elements into a light REE–Pb–S-rich fluid derived from the carbonatitic magma modified by calcite fractionation. The present work shows that Mo can be retained, transported and deposited by carbonatitic fluids capable of generating economic Mo deposits.