Abstract Processes controlling the metal endowment of arc-related porphyry Mo deposits are not well understood. Located in northeastern China, the arc-related Luming porphyry Mo deposit has a proven reserve of 0.75 Mt Mo at an average grade of 0.092 wt % and is characterized by multiple pulses of alteration and mineralization. These features make this deposit an ideal location to investigate the role of multiple pulses of magmatism and fluid release in the evolution and formation of an arc-related porphyry Mo deposit. Molybdenum mineralization at Luming is typically observed as a series of molybdenite-bearing veins hosted within a composite intrusive complex, referred to as the Luming Intrusive Suite. Crosscutting relationships between intrusive units and offset veins indicate that the Luming Intrusive Suite is composed of five major, successive granitic intrusions: the premineralization plutonic biotite monzogranite and monzogranite units, and the synmineralization stock- and dike-like porphyritic monzogranite, granite porphyry, and syenogranite units. Each synmineralization unit is associated with similar vein sequences that comply with the general form of early EB-type biotite veins, through A-type quartz ± biotite and B-type quartz-molybdenite veins, to late D-type quartz-molybdenite ± pyrite ± chalcopyrite, molybdenite, quartz-pyrite ± calcite, and calcite ± clays veins. The intensity and volume of alteration and mineralization within a given synmineralization unit decrease from early- through inter- to late-mineralization units. Although minor Mo mineralization is associated with potassic alteration along B-type veins, the majority of the ore is associated with D-type quartz-molybdenite-pyrite and molybdenite veins rimmed by sericite-chlorite-pyrite alteration, which are primarily hosted in the two premineralization units. A combination of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb and hydrothermal biotite 40Ar/39Ar studies, together with available isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS) molybdenite Re-Os data, has resulted in a substantial reappraisal of the timing of magmatism and its association with molybdenite mineralization at Luming. The volumetrically dominant premineralization intrusive units have indistinguishable zircon U-Pb weighted mean 206Pb/238U ages ranging from 187.5 ± 2.8 to 186.5 ± 3.6 Ma (2σ), whereas the synmineralization units yield weighted mean 206Pb/238U ages from 178.6 ± 2.2 to 175.6 ± 3.0 Ma (2σ). The zircon U-Pb weighted mean 206Pb/238U ages of the synmineralization units are indistinguishable from the mean molybdenite Re-Os model (178.1 ± 2.7; 2σ) and hydrothermal biotite 40Ar/39Ar plateau (174.7 ± 1.1 Ma; 2σ) ages within uncertainty, confirming a genetic link with mineralization. Melt inclusion data show that the synmineralization intrusions were Mo poor, with Mo concentrations <4 ppm. The data presented here suggest that molybdenite mineralization at Luming was most likely accomplished through three discrete magmatic-hydrothermal events during assembly of the Mo-poor synmineralization intrusive complex. The giant Luming deposit appears to be related to multiple pulses of magmatic-hydrothermal activities, resulting in the superposition of temporally distinct mineralization events. Our results suggest that pulsed release of ore-forming magmas and fluids, which are channeled along focusing structures like small porphyry fingers within a focused area, from a large magma chamber at depth may play a major role in the formation of large to giant porphyry Mo deposits of both the arc-related and Climax types. This conclusion is in line with field observations of a number of large to giant porphyry Mo deposits, which commonly show reversals in magmatic-hydrothermal evolutionary trend and are associated with multiple pulses of small stocks and dikes that are separate in time and space.
Abstract The large Weilasituo Sn-polymetallic deposit is a recent exploration discovery in the southern Great Xing’an Range, northeast China. The ore cluster area shows horizontal mineralization zoning, from the inner granite body outward, consisting of high- T Sn–W–Li mineralization, middle- T Cu–Zn mineralization and peripheral low- T Pb–Zn–Ag mineralization. However, the intrinsic genetic relationship between Sn-W-Li mineralization and peripheral vein-type Pb–Zn–Ag–Cu mineralization, the formation mechanism and the deep geological background are still insufficiently understood. Here, we use fluid inclusions, trace elements concentrations in quartz and sphalerite, and H–O isotope studies to determine the genetic mechanism and establish a metallogenic model. Fluid inclusion microthermometry and Laser Raman spectroscopic analysis results demonstrates that the aqueous ore-forming fluids evolved from low-medium salinity, medium–high temperature to low salinity, low-medium temperature fluids. Laser Raman spectroscopic analysis shows that CH 4 is ubiquitous in fluid inclusions of all ore stages. Early ore fluids have δ 18 O H2O (v–SMOW) values from + 5.5 to + 6.2‰ and δD values of approximately − 67‰, concordant with a magmatic origin. However, the late ore fluids shifted toward lower δ 18 O H2O (v–SMOW) (as low as 0.3‰) and δD values (~ − 136‰), suggesting mixing between external fluids derived from the wall rocks and a contribution from meteoric water. Ti-in-quartz thermometry indicates a magmatic crystallization temperature of around 700 °C at a pressure of 1.5 kbar for the magmatic ore stage. Cathodoluminescence (CL) imaging and trace element analysis of quartz from a hydrothermal vug highlight at least three growth episodes that relate to different fluid pulses; each episode begins with CL-bright, Al-Li-rich quartz, and ends with CL-dark quartz with low Al and Li contents. Quartz from Episode 1 formed from early Sn-(Zn)-rich fluids which were likely derived from the quartz porphyry. Quartz from episodes 2 and 3 formed from Zn-(Sn)-Cu-rich fluid. The early magmatic fluid is characterized by low f S 2 . The SO 2 produced by magma degassing reacted with heated water to form SO 4 2− , causing the shift from low f S 2 to high f S 2 . The SO 4 2− generated was converted to S 2– by mixing with CH 4 -rich, Fe and Zn-bearing external fluid which led to late-stage alteration and dissolution of micas in vein walls, thus promoting crystallization of pyrrhotite, Fe-rich sphalerite and chalcopyrite and inhibiting the precipitation of anhydrite. This study shows that ore formation encompassed multiple episodes involving steadily evolved fluids, and that the addition of external fluids plays an important role in the formation of the later Cu–Zn and Ag–Pb–Zn mineralization in the Weilasituo ore district.
The large-scale Huanggang Skarn Fe-Sn deposit is located in the southern part of the Great Xing'an Range (SGXR), NE China. The causative granitic rocks in the Huanggang ore district are characterized by consistently high SiO2 contents, low EuN/EuN* (0.02–0.24) and Sr/Y (0.2–1.9). The pre-ore granite porphyry and syn-ore syenogranite have been dated through zircon and monazite U-Pb dating at 144.6 ± 1.4 Ma and 138.6 ± 1.3 to 135.9 ± 0.8 Ma, respectively. The U-Pb dating of garnet from ore-associated skarn reveals that the mineralization was happened at 136.5 ± 1.3 Ma, which is consistent with the emplacement age of the ore-related syenogranite. Zircon geochemistry shows that the pre-ore granite porphyry has relatively low oxygen fugacity (average zircon Ce4+/Ce3+ = 14.74), whereas the syn-ore syenogranite is more oxidized (average zircon Ce4+/Ce3+ = 178.9). We suggest that the elevation of oxygen fugacity was most likely caused by degassing. The textural features indicate that apatite crystals from the pre-ore granite porphyry are magmatic in origin. In contrast, textural and chemical characteristics of apatite grains from syn-ore syenogranite reveal that they have been modified by hydrothermal fluids in different degrees. The apatite in both pre- and syn-ore granites display relatively high F and low Cl contents, and very low SO3 concentrations (i.e., <0.032 wt%), indicate extremely low content of S in the causative magma. Apatite from syn-ore syenogranite display more strong and variable enrichment of LREE relative to HREE compared to those from pre-ore granite porphyry. The extensive present of new REE mineral inclusions (monazite and xenotime) in the altered apatite suggests that the concentrations of Ca and Na must be low in the ore-forming fluid. Apatite from syn-ore syenogranite have higher concentrations of Mo, W, Zn, Sn, and Be than those from pre-ore granite porphyry. Combined with zircon and apatite geochemistry, and whole rock components of the ore-related granites in the Huanggang deposit, we propose that the ore-forming magma has undergone long-term evolution under relatively reduced conditions with extensive fractional crystallization of plagioclase. The reduced and S-poor evolved magma could have promoted the enrichment of Sn and prevented Fe from precipitating as sulfide. This study also reveals that the compositional characteristics of ore-forming fluid, such as the enrichment of ore-forming materials, could be revealed by altered apatite geochemistry.
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