With many new discoveries of giant Mo deposits in the past decade, northeastern China has become the largest molybdenum mineralization region in China. The Xilamulun district is in the southern part of northeastern China and contains 25 Mo deposits and prospects, dominantly of porphyry type. These deposits are mostly along the E-striking Xilamulun fault. Re-Os dating of hydrothermal molybdenite from four deposits in the northern Xilamulun district revealed mineralization ages of 137 to 129 Ma (129.4 ± 3.4 Ma for Aolunhua, 135.3± 2.6 Ma for Shabutai, 136.4 ± 0.8 Ma for Haisugou, and 136.1 ± 6.6 Ma for Banlashan), in general agreement with the crystallization ages of their host granitoids.\ud \ud The compilation of existing data on Mo-bearing deposits in northeastern China, including the new data of this study, shows that Mesozoic Mo deposits in this region have a wide range of ages, from ~250 to 90 Ma. We propose that they are linked to three tectonic-magmatic events: (1) Triassic Mo deposits (250–220 Ma) are mainly distributed along the E-trending Xilamulun fault and are related to postcollisional crustal extension following the final closure of the Paleo-Asian ocean; (2) Jurassic Mo mineralization (200–140 Ma) displays a clear younging trend from southeast to northwest, coincides with the regional magmatism trend, and is interpreted to be related to the northwestward flat-slab subduction of the Paleo-Pacific plate beneath the Eurasian continent that started in the Early Jurassic (ca. 200 Ma); (3) Cretaceous Mo mineralization (140–90 Ma) shows a reversed migration trend from northwest to southeast, and can be explained by the coastward migration of slab rollback related to lower crust delamination, asthenospheric upwelling, and lithospheric thinning in eastern China. The spatial-temporal distribution of Mesozoic Mo mineralization is important for regional metallogeny and exploration. Recently numerous epithermal Au (Cu, Mo) deposits have been recognized in the southeast margin of northeastern China. According to this study, we predict that hidden porphyry/skarn systems could be located beneath these epithermal deposits

The Baiyinnuo’er skarn deposit is one of the largest Zn-Pb deposits in northeastern China, with 32.74 million metric tons (Mt) resources averaging 5.44% Zn, 2.02% Pb, and 31.36 g/t Ag. The deposit formed in three stages: the preore stage (prograde skarn minerals with minor magnetite), the synore stage (sulfides and retrograde skarn minerals including calcite and minor quartz), and the postore stage (late veins composed of calcite, quartz, fluorite, and chlorite; cutting the above mineral assemblages). In this study we analyzed the composition of single fluid inclusions using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to (1) determine the composition of the fluids and the evolution through the stages, (2) infer the fluid and metal sources, and (3) explore the metal deposition mechanisms. The preore fluids trapped in pyroxene have higher homogenization temperatures (432°–504°C), higher salinity (36.5–46.1 wt % NaCl equiv), and higher concentrations of Zn (~0.9 wt %), Pb (~1.4 wt %), and other elements (e.g., Na, K, Li, As, Rb, Sr, Cs, Ba, Cl, and Br) than synore mineralizing fluids (<370°C, <10 wt % NaCl equiv, ~450 ppm Zn, and ~290 ppm Pb). The postore fluids show lower temperatures (<250°C) and a rather dilute composition (<4 wt % NaCl equiv, ~33 ppm Zn, and ~24 ppm Pb). Geochemically, the fluids of all paragenetic stages in Baiyinnuo’er have magmatic signatures based on the element mass ratios, including elevated K/Na, Zn/Na, and Rb/Na ratios, lower Ca/K ratios, and combined Cl/Br-Na/K ratios, which are distinctively different from basinal brines. Inclusion fluids in preore stage show little variation in composition between ~510° and ~430°C, indicative of a closed cooling system. In contrast, the major components of the syn- and postore fluids, including Cl, Na, and K, decrease and correlate with a drop of homogenization temperatures from ~370° to ~200°C, indicating a dilution by mixing with groundwater. The Baiyinnuo’er mineralizing fluids (trapped in sphalerite) have higher Ca/K mass ratios (avg ~0.78) than other proximal magmatic hydrothermal systems (0.06–0.52) but lower than that of the distal El Mochito skarn (avg ~6.4), probably reflecting a gradually weakened magmatic signal away from the causative intrusions. The metal contents in preore fluids are significantly higher than those in synore fluids, but no mineralization occurred. This confirms that the early fluids were, although enriched in metals, not responsible for ore precipitation, most likely due to their high temperature and high salinities. One important factor controlling Zn-Pb mineralization was mixing with groundwater, which resulted in temperature decrease and dilution that significantly reduced the metal solubility, thereby promoting metal deposition. Another main driving force was the interaction with carbonate wall rock that buffered the acidity generated during the breakdown of Zn and (Pb)-Cl complexes and the precipitation of sulfides. Phase separation occurred in both the preore and the early part of the synore stages, but no evidence indicated that it caused metal deposition. The prograde minerals and retrograde minerals (including ore minerals) coexisting in the same samples could have been caused by two (or more) successive pulses of hydrothermal fluids released from residual melts of a progressively downward crystallizing magma. Each fluid produced a series of proximal high-temperature prograde to distal low-temperature assemblages, with the lower temperature assemblages of later fluids overprinting the higher temperature assemblages at most locations.

There are more than 80 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1·09 Mt Mo with an average Mo grade of 0·07 %. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar ε_Nd(t) values from –1·1 to 1·4, corresponding to T_DM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of +1·8 to +4·1 (+2·7 on average). This is also consistent with the high whole-rock Fe₂O₃/FeO ratios (1·3–26·4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0·7–1·1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g. >5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from those for other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km³ with ∼10 ppm Mo was required to form the Mo ores containing 1·09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit such as Daheishan, whereas a specific magma composition (e.g. with unusually high Mo and/or S concentrations) might be less critical.

Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ²⁶Mg values (−1.89 to −1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo.