Porphyry Mo Deposit Research Articles

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

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.

A geochemical study of the Sweet Home mine, Colorado Mineral Belt, USA: formation of deep hydrothermal vein–type molybdenum greisen and base metal mineralization

AbstractDeep hydrothermal Mo, W, and base metal mineralization at the Sweet Home mine (Detroit City portal) formed in response to magmatic activity during the Oligocene. Microthermometric data of fluid inclusions trapped in greisen quartz and fluorite suggest that the early-stage mineralization at the Sweet Home mine precipitated from low- to medium-salinity (1.5–11.5 wt% equiv. NaCl), CO2-bearing fluids at temperatures between 360 and 415 °C and at depths of at least 3.5 km. Stable isotope and noble gas isotope data indicate that greisen formation and base metal mineralization at the Sweet Home mine was related to fluids of different origins. Early magmatic fluids were the principal source for mantle-derived volatiles (CO2, H2S/SO2, noble gases), which subsequently mixed with significant amounts of heated meteoric water. Mixing of magmatic fluids with meteoric water is constrained by δ2Hw–δ18Ow relationships of fluid inclusions. The deep hydrothermal mineralization at the Sweet Home mine shows features similar to deep hydrothermal vein mineralization at Climax-type Mo deposits or on their periphery. This suggests that fluid migration and the deposition of ore and gangue minerals in the Sweet Home mine was triggered by a deep-seated magmatic intrusion. The findings of this study are in good agreement with the results of previous fluid inclusion studies of the mineralization of the Sweet Home mine and from Climax-type Mo porphyry deposits in the Colorado Mineral Belt.

Magmatic and Hydrothermal Evolution at Qian’echong, Central-Eastern China: Insights into Dabie-Type Porphyry Mo Mineralization

Abstract Dabie-type porphyry Mo deposits have recently been identified as a new subtype of porphyry Mo deposits, but several questions remain about the role of ore-related magmas in the formation of this type of deposit, as well as distinctions in genetic processes with the well-studied Climax-type porphyry Mo deposits. Here, mineral and melt inclusions from the giant Qian’echong deposit, Dabie orogen, central-eastern China, were studied in order to improve our understanding of the nature and the role of ore-related magmas in the genesis of Dabie-type porphyry Mo deposits. The magmatic and hydrothermal evolution of the system was reconstructed based primarily on the analysis of quartz-hosted silicate melt and mineral inclusions, in concert with field and petrographic relations as well as previously published U–Pb and Re-Os geochronology. Ore-related magmas at Qian’echong include, from early to late, quartz porphyry dikes, rhyolite porphyry dikes, granite porphyries (stock and dikes) and a newly discovered, deep-seated monzogranite porphyry stock. Based on TitaniQ thermobarometry, these lithologies were sourced from a ~16–19-km deep (500–600 MPa) magma chamber and underwent nearly isothermal decompression to ~7 km (200 MPa), at temperatures ranging from 720°C to 690°C. According to the trace element composition of melt inclusions, in combination with published whole-rock Nd isotopic compositions, the early magma evolved from quartz porphyry to rhyolite porphyry through fractionation crystallization and thus became a crystal mush. This viscous crystal mush was subsequently re-melted to produce the granite porphyries and was injected with melts from a different source to generate the monzogranite porphyry. At Qian’echong, all melt inclusions have low concentrations of Mo (2–8 ppm), >5 wt. % H2O, and little to no F (≤0.26 wt. %), arguing against the requirement for ore-related magmas to be enriched in Mo to form large porphyry Mo deposits. Rayleigh fractionation modeling shows that the concentration of Mo in the quartz and granite porphyries increased through fractionation of quartz, feldspars, biotite, magnetite, and ilmenite and that Mo was subsequently depleted in the melt through fluid exsolution, exclusively in the rhyolite and monzogranite porphyries, as a result of magma ascent and decompression. This suggests that, in addition to ore-related granite porphyry, both the rhyolite and the monzogranite porphyries also contributed to the hydrothermal mineralization. This study confirms that the formation of porphyry Mo deposits does not rely on abnormally high concentrations of Mo in ore-related magmas but instead requires efficient extraction of Mo from large volumes of magmas, with normal concentrations of Mo. Unlike the Climax-type deposits where multiple pulses of ore-forming fluids are delivered from convecting shallow magma chambers, Mo mineralization in Dabie-type deposits was achieved by the assembling of ore-forming fluids from successively emplaced, relatively deep intrusions. Although the Dabie- and Climax-type Mo deposits are respectively associated with I-type and A-type granitoids in the Dabie orogen, it is suggested that both types of magmas are derived from the partial melting of subducted Yangtze continental crust and that it is the tectonic transition from compressional to extensional settings that controlled the different styles of porphyry Mo mineralization in this orogen.

Apatite and zircon geochemistry for discriminating ore-forming intrusions in the Luming giant porphyry Mo deposit, Northeastern China

The Luming giant porphyry Mo deposit in NE China host >0.89Mt Mo, but which intrusion induced the Mo mineralization is still in debated. In this study, apatite and zircon geochemistry from three mentioned ore-forming granites, namely monzogranitic porphyry (MGP), granodiorite (GD) and monzogranite (MG) in Luming ore district are presented in order to constrain the real ore-forming granite. Apatite and zircon U-Pb dating of the MGP, GD and MG yield a crystallization age of ca. 177 Ma, ca. 187 Ma and ca. 186 Ma, respectively. Whole-rock geochemistry, in-situ apatite Sr-Nd and zircon Hf isotopic compositions suggest that these granites formed by partial melting of juvenile lower crust with different depth. Both the MGP and GD have adakitic affinities and the depth of their magma source is > 45 km with garnet as residual phase, whereas the source depths for the MG was < 33 km with plagioclase as residue. Apatite and zircon geochemical features indicated that the magma of MGP and GD have higher oxygen fugacity, water contents and lower Ti-in-zircon temperatures than these of the MG, which make the MGP and GD are more favorable for Mo mineralization, but the less volume of GD limited its metallogenic potential. Decoupled Nd-Hf isotopes of the MGP, GD and MG could be induced by recycled ocean sediment added in their source region, and the addition of recycled sediments could be account for Mo concentrate in the source and regional Mo mineralization.

Trace element and isotopic (S, Pb) constraints on the formation of the giant Chalukou porphyry Mo deposit, NE China

Abstract Porphyry-type Mo deposits have supplied most of the Mo to the world. However, the source of the Mo and the controls on its enrichment in such deposits is still a matter of great debate. In this study, we present in situ trace element and isotopic data for a giant porphyry Mo deposit (the Chalukou Mo deposit in NE China) and use these data to address these issues. Three primary paragenetic stages of mineralization were recognized at Chalukou: (Stage I) K-feldspar + quartz + minor pyrite (Py-I) + minor molybdenite (Mol-I); (Stage II) quartz + sericite + molybdenite (Mol-II) + pyrite (Py-II); (Stage III) quartz + chlorite + epidote + fluorite + pyrite (Py-III) + galena + sphalerite + minor chalcopyrite. The bulk of the molybdenite was deposited in Stage II. In situ S isotope analyses of the sulfide ores show that the δ34S values vary from –5.2 to +7.8‰ (mean = +2.9‰) and correspond to δ34SH2S values from –2.4 to +3.3‰ (mean = +1.1‰). These values are consistent with a magmatic source for the sulfur. In situ Pb isotope compositions of the sulfide ores are almost identical to those of the local Mesozoic granites and other magmatic-hydrothermal ore deposits in this region, suggesting a close genetic association between the Mo mineralization and felsic magmatism. Pyrite from the three stages of mineralization differs significantly in its trace element composition. The first generation, Py-I, has a high Cu content (8.7 ± 49.6 ppm; where the first value is the median and the second is the standard deviation) and Mo content (6.9 ± 3.8 ppm). Pyrite-II has the lowest Cu concentration (1.3 ± 2.1 ppm) and a relatively high Mo concentration (5 ± 128 ppm), and Py-III has a high Cu content (8.7 ± 37.1 ppm) but the lowest Mo content (0.05 ± 5.7 ppm). From this, we infer that pyrite recorded the chemical evolution in the Mo/Cu ratio of the ore fluid and that this ratio reached a maximum in Stage II, coinciding with the widespread saturation of the fluid in molybdenite. The evolution of the Mo/Cu ratio in pyrite implies that the fluid was undersaturated in chalcopyrite at the high temperature of Stage I, despite the Cu concentration of the fluid apparently being at its high level, and chalcopyrite only saturated later, at a lower temperature. Molybdenite, however, because of its lower solubility, saturated early (Stage I) and in the subsequent stage (Stage II) was supersaturated in the fluid. There is a significant enrichment of Mo in the syn-ore intrusions at Chalukou compared to the pre-ore monzogranite. The very low Sr/Y ratios for the Chalukou syn-ore intrusions, which are in sharp contrast to the high Sr/Y ratios of the pre-ore monzogranite and those of porphyries related to Cu deposits, suggest that fractional crystallization of plagioclase may have been a key factor in generating the syn-ore magmas. Molybdenum is a highly incompatible metal and will concentrate in the crust, and assimilation of old continental crust, therefore, may explain the Mo enrichment of the syn-ore intrusions and ultimately the formation of the giant Chalukou deposit.

Geochemical, geochronological and isotopic studies of the Taishanmiao batholith and the Zhuyuangou Mo deposit it hosted, Qinling, China

The Qinling Orogen is well known for its huge Mo resource. The porphyry Mo deposits are associated with small granite porphyries, albeit great batholiths accommodate limited Mo mineralization. In this study, detailed geochemical, isotopic, and geochronological studies were carried out to constrain the petrogenesis of the Taishanmiao batholith and time framework of magmatism and associated Mo mineralization at Zhuyuangou. Zircon geochronology reveals gradually decreasing emplacement ages, from 124.8 ± 1.0 Ma for the medium- to coarse-grained syenogranite, through 122.8 ± 1.4 Ma for fine- to medium-grained syenogranite, to 115.1 ± 0.95 Ma for porphyritic syenogranite. The last two phases hosted the Zhuyuangou Mo mineralization with a molybdenite Re-Os isochron age of 117.7 ± 1.0 Ma. Major and trace element features support highly fractionated I-type affinity for Taishanmiao. Zircon Lu-Hf isotopic studies yield εHf(t) values of −19.6 − -8, and two-stage Hf model ages (TDM2) of 1679–2408 Ma. It thus supports ancient continental crust, especially Taihua Supergroup and Xiong'er Group, as the primary source. Compared to the source rock, the low Mo concentrations of the batholith are attributed to the massive fluid flux that contributes to the formation of the Zhuyuangou and Donggou deposits. Small porphyries can effectively concentrate exsolved fluids and metal compared with great batholith. Their shallow emplacement depths result in greater temperature/pressure/composition gradient with the wall rocks, higher cooling rate, more fractures or even breccia, and fluid boiling and mixing, facilitating intensive hydrothermal alteration and mineralization.

Ore genesis of the Sadaigoumen porphyry Mo deposit, North China Craton: Constraints from pyrite trace element and lead isotope analyses

The Triassic Sadaigoumen porphyry Mo deposit is located in the northern margin of the North China Craton (NCC). To constrain the formation of this deposit, we analyzed in-situ major, trace, and Pb isotopic compositions of pyrite, as well as Pb isotopes of molybdenite and whole rocks. The formation of the deposit can be divided into five stages. Stages 1 and 2 are characterized by the massive development of magnetite with minor pyrite. Stage 3 is the main Mo mineralization stage, with much chalcopyrite deposited in sub-stage 3.1 veins, and only Mo mineralization developed in sub-stage 3.2. Stage 4 is characterized by the development of quartz–pyrite veins. Only quartz with no sulfides was deposited during stage 5. During all five stages, Mo, Cu, Pb and Bi were deposited with pyrite as mechanical inclusions; however, Zn may exist in the pyrite as isomorphic substitutions or nano scale inclusions. The pyrite from stages 1 and 2 has constant Co/Ni ratios, but the ratios were significantly more varied in the stages 3 and 4 pyrite. The variation was largely due to the mixing of meteoric water with the ore-forming system from stage 3. While magnetite and hematite were absent during sub-stage 3.1, the Ag content and Pb isotopic ratio of the pyrite rapidly increased. Geochemical signatures indicated a shift to a reduced ore-forming environment, such as an increase in the Ag content and the highly radioactive Pb isotope composition of the pyrite. The high Cu and Mo content of Py3.2 and Py4 indicated that the ore-forming elements continued to precipitate in the pyrite crystal lattice after the precipitation of chalcopyrite and molybdenite. The precipitation of Cu in the Sadaigoumen deposit is closely related to the addition of a reducing fluid from the upper crust during the evolution of the ore-forming fluid, which has important reference significance for the delicate process of Triassic porphyry Mo deposit in the northern NCC.

Geochemical and fluid evidences for fluorine-rich magmatic-hydrothermal origin of the giant Chalukou Mo deposit in the northeast China

The Chalukou deposit is a world-class porphyry Mo deposit in the Great Hingan Range, northeast China. Fluorites were analyzed by scanning electron microscope-cathodoluminescence (SEM-CL), microthermometry, and laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) to characterize the composition and temperature of mineralizing fluids. There are four types of veins in Chalukou deposit that can be categorised as A vein (quartz–fluorite–magnetite–K-feldspar vein), B vein (quartz–fluorite–molybdenite vein), D vein (quartz–fluorite–sphalerite–galena–pyrite vein) and post-mineralization vein. The CL images of fluorites are characterized by heterogeneous texture in A vein, but homogeneous texture in B and D veins. Fluorites in A vein have total rare earth elements (REE) contents of 10–4963 ppm and temperatures of 370–410 °C, which show similar geochemical features with ore-bearing porphyry. In contrast, fluorites in B and D veins exhibit total REE contents of 4.6–41.3 ppm and temperatures between 257 and 300 °C, showing the decreasing trends of total REE contents and temperatures from early to late stage. Fluid inclusions of four types of veins are composed of one-phase, two-phase and daughter mineral-bearing multiphase with decreasing trend of temperatures from 450 °C to 120 °C and salinities from 53.0% to 0.7 wt% NaCl equiv. during the evolution. It is proposed that high-fluorine fugacity has a considerable influence on metal solubility and migration in the early stage of the Chalukou deposit. Temperature decreasing and fluid mixing may be the key factors of metal precipitation during the Mo mineralization stage, but the effect of fluorine is not obvious.

Whether short-lived or prolonged duration of multistage combined magmatic and hydrothermal events in the giant Chalukou porphyry Mo deposit, China

The duration (or time scales) of multistage combined magmatic and hydrothermal events provides key information constraining the metallogenic history of porphyry deposits, but the duration (time scale) of the mineralising hydrothermal process remains uncertain. Here, we report zircon U–Pb isotopic age, fluorite Sm–Nd isochron ages, and Ti-in-quartz diffusion chronometry for the giant Chalukou porphyry Mo deposit with reserves of 2.46 Mt @ 0.087 wt% Mo in NE China. The deposit is genetically related to hydrothermal fluids associated with a Late Jurassic (148–147 Ma) porphyritic magmatic event coincident with our new fluorite Sm–Nd isochron age of 148 Ma. Porphyry Mo mineralisation is characterised by the superposition of multiple generations of crosscutting quartz-bearing veins including: quartz veins with minor magnetite and hematite associated with K-silicate alteration (A-veins), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralisation (B-veins), and late base-metal mineralisation with phyllic alteration (D-veins), as well as post-mineralisation veins with argillic alteration. Hydrothermal veins from the Chalukou porphyry Mo deposit contain multiple generations of quartz that are distinct in their Ti concentrations and scanning electron microscope-cathodoluminescence images. The A-veins preserve subhedral to euhedral characteristics with alternating CL bright and dark growth zoning. The A-veins probably formed at a near-lithostatic pressure of ∼ 1.35 kbar and contain CL-bright quartz assaying 31–71 ppm Ti with Ti-in-quartz temperatures averaging 603 ± 30 °C, and CL-dark quartz assaying 6–25 ppm Ti with Ti-in-quartz temperatures averaging 496 ± 36 °C. The mineralised B-veins also CL-dark quartz, although they display less growth zoning with lower Ti assays of 0.4–27.9 ppm. These veins have near 0.7 kbar pressure and Titanium-in-quartz temperature averaging 390 ± 79 °C. The D-veins are dominated by CL-dark quartz and systematically lower Ti concentrations of 0.4–8 ppm, being lower than the relatively latter vein generations, formed at a temperature of 348 ± 58 °C and pressure of 0.4 kbars. The Ti diffusion in quartz has become a successful method constraining time scales and duration of magmatic and hydrothermal processes. Ti diffusion modelling indicates that the A-veins were deposited and cooled in a period between 2500 and 50,000 years. This shows that the earliest magma-related hydrothermal fluids activities formed within tens of thousands of years.

Apatite fingerprints on the magmatic-hydrothermal evolution of the Daheishan giant porphyry Mo deposit, NE China

Abstract Porphyry deposits are the main source for global Cu and Mo production. The generation of hydrous silicate magmas and subsequent separation of volatile-rich magmatic fluids with hydrothermal alteration are significant processes leading to the formation of porphyry deposits. However, a specific understanding of these processes has been limited by a lack of direct mineralogical records in the evolving magmatic-hydrothermal system. In this paper, we present an integrated textural and geochemical investigation on apatite from the giant Daheishan porphyry Mo deposit in NE China, illustrating that apatite can be a potential recorder of the magmatic-hydrothermal evolution of porphyry systems. Apatite from the ore-forming porphyry displays distinctive core-rim textures, with melt inclusions in the resorption cores (Type-A1) and co-existing of melt and fluid inclusions in the euhedral rims (Type-A2), indicating a magmatic-hydrothermal origin of apatite. This is also supported by both chemical and isotopic compositions obtained by in situ analyses using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and LA-multi collector-ICP-MS. The late Type-A2 apatite is relatively enriched in incompatible elements, such as rare earth elements (REE) and Th, but slightly depleted in fluid-mobile elements such as Na and S, compared to the early Type-A1 apatite. Relatively homogeneous (87Sr/86Sr)i ratios (0.70436–0.70504) of the Type-A1 and Type-A2 apatites indicate that they were formed in a relatively closed system without detectable contamination. Meanwhile, some apatite in the wall rock (biotite granodiorite) shows characteristics of secondary altered textures, resulting from the intensive alteration by hydrothermal fluids exsolved from the porphyry system. Apatite trapped in mineral phenocrysts of the wall rock is usually unaltered (Type-B1 apatite), with clear oscillatory growth zones in cathodoluminescence (CL) images. In contrast, the intergranular apatite is commonly altered (Type-B2 apatite), with chaotic zoning in CL images, abundant micro-fractures and secondary fluid inclusions. Compositionally, the Type-B2 apatite shows notable tetrad REE patterns, relatively lower light-REE and S contents, and elevated 147Sm/144Nd ratios compared to the Type-B1 apatite. LA-ICP-MS U-Pb dating yields a lower intercept age of 171.4 ± 2.3 Ma for Type-B2, which is consistent with the age of 171.5 ± 2.4 Ma for Type-A2, but is notably younger than the Type-B1 apatite (175.5 ± 1.3 Ma). It is suggested that the Type-B2 apatite has been significantly reset by hydrothermal fluids exsolved from the porphyry system. Therefore, we conclude that the textures and geochemistry of apatite in porphyry systems can be used as a potential proxy for recording fluid exsolution and hydrothermal alteration processes.

Superimposing porphyry Mo and vein-type Cu-Pb-Zn mineralization in the Panjiaduan deposit, Great Xing’an Range (NE China): Perspective from zircon U-Pb and sphalerite Rb-Sr dating, geochemistry and S-Pb isotopes

The southern Great Xing’an Range (SGXR) is located in the eastern Central Asian Orogenic Belt (CAOB), and represents the most important Cu-Mo-Pb-Zn polymetallic mineral province in NE China. The Panjiaduan deposit in the SGXR is characterized by superimposing mineralization: (1) porphyry Mo ore hosted by syenogranite, and (2) superimposing vein-type Cu-Pb-Zn ore, hosted by fracture zones in the Manitu Formation volcanics and granodiorite. Two zircon U-Pb ages of 233.9 ± 2.8 Ma and 236.1 ± 2.8 Ma for the syenogranite (porphyry) ore host indicate that the porphyry Mo mineralization occurred in the Late Triassic, coeval with the adjacent Laojiagou porphyry Mo deposit. Sphalerite from the Cu-Pb-Zn ore veins yielded a Rb-Sr isochron age of 138.5 ± 3.5 Ma, suggesting Early Cretaceous overprinting mineralization. The syenogranite is geochemically similar to the ore-causative monzogranite at Laojiagou. They are unfractionated I-type granites, LILE- and LREE-enriched, and HFSE- and HREE-depleted. In addition, their magma source was likely derived from partial melting of the juvenile lower crust. Major sulfides (pyrite, molybdenite, and galena) from the porphyry ores have slightly lower δ34S values (−2.8 to −0.3 ‰) than the vein-type ore sulfides (pyrite, chalcopyrite, sphalerite, and galena) (1.5 to 5.3 ‰), although they are all close to the mantle δ34S values and thus suggests a magmatic sulfur source. Meanwhile, the Pb isotope compositions of the two ore types suggest that the Pb for the porphyry mineralization was likely derived from an orogenic Pb reservoir, while the latter may have crust-mantle mixed Pb source. We suggested that both mineralization events were formed in an extensional setting, induced by the post-collision tectonics after the Paleo-Asian Ocean closure and the Paleo-Pacific subduction rollback, respectively.

An ancient continental crustal source for Mo mineralisation in the eastern Central Asian Orogen: A case study of the Bilugangan Mo deposit

The Bilugangan porphyry Mo deposit is located in the southern Great Xing'an Range, and the Bilugangan area near the Hegenshan-Heihe Fault in the eastern part of the Central Asian Orogen (CAO). The deposit is hosted by a NE-trending, partly concealed, composite granite consisting of highly altered porphyritic granite (G1), porphyritic monzogranite (G2), fine- to medium-grained monzogranite (G3), medium -grained monzogranite (G4), and monzogranitic porphyry dykes (G5). The Mo mineralisation forms hydrothermal stockworks or disseminated sulphides in the G2 and G3 phases and the adjoining hornfels. Magmatic zircons from the intrusions yield 206Pb/238U weighted mean ages of 295 ± 2 Ma for G1, 240 ± 1 Ma for G2, 235 ± 2 Ma for G3, 230 ± 1 Ma for G4, and 100 ± 2 Ma for G5,indicating that multiple magmatic events have been active during the Early Permian to Early Cretaceous emplacement of the Bilugangan Granite. The age of the Mo is constrained between the Middle and Late Triassic and the magma-related hydrothermal activity proceeded in at least two pulses over a period of around 5 Ma. The first pulse has a molybdenite Re-Os weighted mean age of 238 ± 2 Ma, and the second dates at 236 ± 1 Ma. These dates are coeval with those of G2 and G3, indicating a genetic association between them and the mineralisation.The Sr-Nd-Hf isotopic systematics shows that the Bilugangan Granite was probably formed by partial melting of at least two sources. The first source is a dominant Late Mesoproterozoic to Early Carboniferous juvenile lower crust indicated by the whole rock εNd(t) values of −1.9 to 5.1 and zircon εHf(t) values of 3.5 to 14.3 with two-stage depleted mantle Nd (TDM2) and Hf (tDM2) model ages of ca. 1086 to 350 Ma, which is possibly related to the evolution of the Palaeo-Asian Ocean. The second source includes Neoarchaean to Palaeoproterozoic crustal components and a subordinate Mesoproterozoic crustal component. The Neoarchaean to Palaeoproterozoic component is indicated by the presence of ca. 2700 to 2600 Ma xenocrystic zircons, negative εHf(t) values of −16.4 to −10.1 with tDM2 ages of ca. 2300 to 1900 Ma. The Mesoproterozoic component yields εHf(t) values of −4.5 to 1.1 and tDM2 ages of ca. 1600 to 1200 Ma. These data show that pre- Mesoproterozoic substrate is present beneath the southern Great Xing'an Range. Although the post-mineralisation G4 and G5 phases have comparable positive εHf(t) values with the earlier granitic phases, the G2 and G3 phases have a more complex source with distinctly negative εHf(t) values of −16.4 to –1.3, indicating that the older crustal components in the magma’s source are also probable sources for the Mo mineralisation.

Controls on the formation of porphyry Mo deposits: Insights from porphyry (-skarn) Mo deposits in northeastern China

AbstractPorphyry Mo deposits have traditionally been classified into two major classes, arc-related and Climax-type, based on the tectonic setting and chemistry of associated intrusions. Although there is a consensus that porphyry Mo systems were formed by the optimal coincidence of geological processes operating at different scales, it is unclear what key parameter(s) render systems productive and whether the two classes of porphyry Mo deposits are unique in their mode of formation, or if they share fundamentally similar geological processes. These questions are important as a clearer understanding of the optimum conditions for the formation of porphyry Mo deposits is a prerequisite for more efficient exploration.This contribution presents a detailed assessment of the factors affecting the formation of porphyry Mo deposits through the investigation of barren and mineralized intrusions from the arc-related Songbei-Yangjiazhangzi-Lanjiagou (SYL) ore zone and the Climax-type Hashitu deposit of northeastern China. Our results show that the syn-mineralization intrusions from the SYL ore zone are quite evolved (SiO2 ~ 75 wt%; Na2O+K2O ~ 8.7 wt%) and are characterized by apparent light rare earth element (LREE) enrichments (LaN/YbN = 2.7–33.1) and moderate negative Eu anomalies (Eu/Eu* = 0.4–0.7). They show enriched zircon Hf isotopic compositions [εHf(t) = –11.9 to –4.8], indicating their parental magmas were likely derived from an ancient crustal source. Melt inclusions from the SYL syn-mineralization intrusions contain negligible F and Cl. In contrast, Hashitu syn-mineralization intrusions are characterized by weak LREE enrichments (LaN/YbN = 2.2–6.9) and strong negative Eu anomalies (Eu/Eu* = 0.02–0.10), with SiO2 and Na2O+K2O contents similar to the SYL syn-mineralization intrusions. They show depleted zircon Hf isotopic compositions [εHf(t) = 3.1–5.0], indicating their parental magmas were likely derived from a juvenile crustal source. Melt inclusions from the Hashitu syn-mineralization intrusions contain up to 0.4 wt% F and 0.03 to 0.09 wt% Cl. However, in both cases, the syn-mineralization intrusions are Mo-poor (1–7 ppm Mo), oxidized (above the quartz-fayalite-magnetite buffer), water-saturated (4.4–7.8 wt% H2O), and were emplaced at palaeodepths of 3.3 to 8.3 km. These data imply that magma source composition is not a key factor in the formation of porphyry Mo deposits. In contrast, magma oxygen fugacity, water content, and emplacement depth appear to play fundamental roles in the formation of porphyry Mo deposits of both arc-related and Climax-type.Within individual deposits, no systematic differences between pre- and syn-mineralization intrusions are observed in terms of magma source, fractionation degree, oxygen fugacity, emplacement depth, and volatile and Mo contents. Instead, a crucial apparent difference lies in the geometry of the intrusions, i.e., pre-mineralization intrusions generally occur as flat, ponded bodies, whereas syn-mineralization intrusions commonly develop as small stocks or dikes. Our results, in combination with an examination of other porphyry Mo systems, suggest that the sudden depressurization of magma chambers and subsequent venting of voluminous fluids along focusing structures (such as small stocks or dikes) most likely plays a critical role in the formation of porphyry Mo deposits of both arc-related and Climax-type. The findings of this study indicate that fluid processes in the shallow crust are pivotal for the formation of porphyry Mo deposits and that settings with ideal magmatic-hydrothermal architectures are most likely to develop into productive porphyry Mo systems.

Late Mesozoic post-orogenic granitoids and its metallogenic implications: Constraints from Nantai porphyry Mo deposit, Northern Qinling, China

Late Mesozoic Nantai porphyry Mo deposit is located in the Northern Qinling terrane, eastern China. The intrusions at Nantai include lamprophyre, quartz porphyry, and granite porphyry. Among them, molybdenum mineralization is associated with quartz porphyry and granite porphyry. The ore-bearing porphyries and ore-barren lamprophyres emplaced at ca. 155–146 Ma and ca. 156–146 Ma, respectively. The calculated zircon ΔFMQ values for Nantai felsic and mafic intrusions are –5.0 to +9.1 (an average of + 2.2) and –6.7 to +5.0 (an average of +1.2), which are similar to those of contemporaneous giant Mo deposits in the south margin of the North China Block. The quartz porphyry and granite porphyry are characterized by relatively high SiO2 (73.39–75.49%), K2O (4.24–9.61%), and Rb (291–552 ppm) contents, but low total Fe2O3 (0.13–0.83 wt%), MgO (0.1–0.3 wt%), P2O5 (0.01–0.04 wt%), Sr (22.4–141.0 ppm) and Ba (79.8–1757 ppm) concentrations. These granitic samples have TDMC ages of 2204–1592 Ma, corresponding to negative εHf(t) values of –15.8 to –6.2. Geochemical and zircon Lu-Hf signatures as well as lead isotopic analyses for Nantai granitoids suggest that they belong to the high-fractionated I-type granites and were derived from partial melting of lower continental crust with coeval mantle-derived magma contribution. The lamprophyres show low SiO2 (47.59–49.08%) and Rb (96–120 ppm) contents, whereas they have high Ba (1598–2236 ppm), and Sr (771–990 ppm) concentrations, as well as a lack of negative Eu anomalies. The concentrated εHf(t) values (–0.6 to + 3.3), high (Hf/Sm)PMN and La/Sm ratios, as well as Pb isotopic features, indicate that they were derived from an enriched lithospheric mantle that had been metasomatized by subducted components with some crustal material contribution. Combined with our datasets and previous studies, we suggest that the Nantai porphyry Mo deposit formed in a post-orogenic setting.

The Metallogeny of the Tieling Cu-Mo Porphyry Deposit in Eastern Tianshan, NW China: New Insights from Zircon U-Pb, Fluid Inclusion, and H-O-S Stable Isotope Analyses

The eastern Tianshan metallogenic belt is an important molybdenum resource base in Xinjiang and is characterized by large-scale porphyry Mo deposits formed during the Triassic. The Tieling Cu-Mo porphyry deposit, which is situated in the western part of the eastern Tianshan metallogenic belt, was recently recognized as being related to Carboniferous granite porphyry. Three stages of hydrothermal mineralization were identified, including quartz+K-feldspar+pyrite±molybdenite±magnetite (stage I), quartz+molybdenite+pyrite+chalcopyrite (stage II), and quartz+pyrite±molybdenite±epidote (stage III). Fluid inclusion petrography and microthermometry analyses indicate the presence of gas-liquid inclusions with a H2O-NaCl composition. The ore-forming fluids have a characteristic temperature ranging from 157 to 262°C under stage II and 135 to 173°C under stage III, which correspond to salinities of 7.2-17.2 wt% NaCl equiv. and 5.9 to 9.6 wt% NaCl equiv., respectively. The hydrogen and oxygen isotope data indicate that the ore-forming fluids of the Tieling deposit were originally derived from magmatic hydrothermal fluids and then mixed with meteoric water. The sulfur isotope compositions indicate that the ore-forming materials were mainly derived from the Late Carboniferous felsic magma. Furthermore, zircon U-Pb analysis of ore-bearing granite porphyry yields a concordant age of298.4±0.7 Ma, indicating that the Tieling Cu-Mo deposit formed during the Late Carboniferous and differed from that processed under pre-Early Carboniferous and Triassic mineralization in the eastern Tianshan metallogenic belt. These results also indicate that the Tieling porphyry deposit was formed in the transition condition between subduction-related accretion and postcollisional orogeny, and it should be given more attention in prospect evaluations.

Decoupling of Sr-Nd Isotopic Composition Induced by Potassic Alteration in the Shapinggou Porphyry Mo Deposit of the Qinling–Dabie Orogenic Belt, China

In our previous study on petrogenesis of quartz syenite and granite porphyry, the host rocks of the Late Mesozoic Shapinggou Mo deposit in the Qinling–Dabie orogenic belt, we found that the initial Sr isotopic composition of the host rocks is strongly affected by the degree of K-alteration. Here, we provide further isotopic evidence of the host rocks and their minerals to investigate the geochemical behaviour of trace elements and isotopes during the alteration and to explain the phenomenon of decoupling of Sr–Nd isotopic composition. The quartz syenite and granite porphyry are altered by K-alteration in varying degrees and have high K2O and Rb contents and low Na2O, CaO, Sr, and Ba contents. Rock samples of both quartz syenite and granite porphyry have variable Rb/Sr ratios and initial 87Sr/86Sr values (even &lt; 0.70) but contain quite homogeneous εNd(t) values (−12.8 to −14.8). Minerals from the rocks of moderate to intense K-alteration have very low initial 87Sr/86Sr values (even &lt; −17), while those from the weakly altered rocks have 87Sr/86Sr(t) values of 0.7044 to 0.7084. The same phenomenon of the decoupling in Sr–Nd isotopic composition can be observed from several Mo deposits within the eastern Qinling–Dabie orogenic belt. This fact suggests similar hydrothermal features and a comparable origin for both the magmatic rocks and hydrothermal fluids in this belt. A comparison between porphyry Mo and porphyry Cu deposits shows that elements and the Rb–Sr isotope system have different behaviours during the K-alteration, implying distinct material sources and igneous rocks for porphyry Mo and porphyry Cu deposits, respectively.

Multiple tectonic-magmatic Mo-enrichment events in Yuleken porphyry Cu-Mo deposit, NW China and its’ implications for the formation of giant porphyry Mo deposit

The Yuleken porphyry Cu-Mo deposit located in the Chinese Altay-East Junggar, a section of the Central Asian Orogenic Belt, the world's largest Phanerozoic subduction-accretionary orogen, witnessed the multistage tectonic-magmatic evolution and mineralization from arc to syn- and post-collision environment over a time span of about 52 Ma. Four types of molybdenite have been distinguished: (1) primary magmatic-hydrothermal, undeformed molybdenite in porphyry, (2) recrystallized molybdenite associated with ductile deformation, (3) recrystallized, intrafoliational molybdenite, and (4) fracture-related hydrothermal molybdenite, and these are characterized by decreasing Re abundances from magmatic to hydrothermal conditions. Individual molybdenite types are non-stoichiometric and exhibit the following negative correlation between the Mo and Re contents:Re (apfu) = −0.0026 Mo (apfu) + 0.00268.This trend reflects successive stages of tectonic-hydrothermal events from a magmatic island-arc to a post-collisional setting. These repetitive mineralization events produced Mo enrichment and higher overall Mo grade of the Yuleken deposit, and are recorded in progressively decreasing Re concentrations in molybdenite. Statistical evaluation of Mo and Re abundances in giant to small porphyry deposits worldwide also indicates that decreasing Re in molybdenite will lead to lower overall Re resource (in a single deposit) and to greater overall Mo resource (in the same deposit). Multiple mineralization processes related to repetitive tectonomagmatic episodes in large porphyry Mo deposits that display Re depletion in molybdenite suggest that reworking of mantle-derived porphyry systems by successive events of crustal growth may be the responsible for Mo enrichment and higher overall Mo ore grade, and eventually produce giant porphyry molybdenum deposits.

CONTROLS ON THE METAL ENDOWMENT OF PORPHYRY Mo DEPOSITS: INSIGHTS FROM THE LUMING PORPHYRY Mo DEPOSIT, NORTHEASTERN CHINA

AbstractProcesses 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 &amp;lt;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.

Molybdenum solubility and partitioning in H2O-CO2-NaCl fluids at 600 °C and 200 MPa

The Mo deposits from the Qinling-Dabie area, central China, which are characteristic for a new type of porphyry Mo deposits named the Collision-type, were formed in CO2-rich magmatic-hydrothermal environments. Yet the effects of CO2 on molybdenum transport and precipitation are still poorly known. To fill this gap and provide insight into the formation of Collision-type porphyry Mo deposit, we performed high pressure experiments to systematically quantify the role of CO2 on the solubility of molybdenum-bearing phases. Molybdenite was placed together with a single-phase H2O-CO2-NaCl fluid (8 wt% NaCl Eq.) at 600 °C and 200 MPa. The experiments were buffered by the pyrite-pyrrhotite-magnetite assemblage. At such conditions, combined microthermometric and LA-ICP-MS analysis of synthetic fluid inclusions reveals that the molybdenite solubility in the fluids coexisting with molybdenite decreases slightly (from 87 ± 17 ppm to 38 ± 13 ppm Mo) with increasing CO2 (from XCO2 = 0.10 to 0.25 M fraction). Such a Mo solubility is comparable to that determined in CO2-free fluids (61 ± 14 ppm) with the same salinity. At XCO2 = 0.33, fluid immiscibility is observed and Mo would partition preferentially into the brine phase, with a DMoliq/vap (=CMoliquid/CMovapor) value of 2.9 ± 1.0 (1σ). The evolution of molybdenite solubility with increasing CO2 in the fluid may be explained by changes of the dielectric constant of the solvent. Our results demonstrate that at the studied experimental temperature and pressure, CO2-rich fluids can transport comparable amounts of molybdenum as in H2O-dominated solutions. Combined with existing literature data over a broad range of pressure, temperature, and oxygen fugacity conditions, our data indicate that decreasing temperature and oxygen fugacity facilitate molybdenite precipitation. Albeit the nil-to-negative effect of CO2 on molybdenum solubility, the presence of CO2 would affect significantly fluid saturation of silicate melts. Notably, boiling is expected to occur at higher pressure in the presence of CO2, which may explain the deeper mineralization depth observed for the Collision-type porphyry molybdenum deposit.

Zircon Hf[sbnd]O isotope and magma oxidation state evidence for the origin of Early Cretaceous granitoids and porphyry Mo mineralization in the Tongbai-Hong'an-Dabie orogens, Eastern China

The Tongbai-Hong'an-Dabie orogens constitute the east segment of the Qinling-Dabie orogenic belt, which host several large molybdenum deposits, with a proven reserve of over 8 million tons metal Mo. In this study, we present an integrated study of in situ zircon UPb dating and HfO isotopes, in combination with magma oxidation state estimation for the Early Cretaceous granitoids in the Tongbai-Hong'an-Dabie regions. These granitoids can be divided into three groups on the basis of zircon UPb ages, including early-stage ore-related granitoids from Mushan, Xiaofan and Wangwan deposits (143–136 Ma), ore-barren granites (136–130 Ma), and late-stage ore-related granitoids from Qian'echong, Bao'anzhai, Tangjiaping and Shapinggou deposits (130–110 Ma). Early-stage ore-barren granitoids have the lowest average εHf(t) and δ18O values of −27.1 ± 1.6 and 4.25 ± 0.63‰ as well as the oldest two-stage Hf model ages (TDM2) of ~2.8 Ga, which are roughly comparable with the inferred Dabie lower crust, indicating that they were mainly derived from the partial melting of the lower crust. Ore-forming granitoids in two stages show similar two-stage Hf model ages from 2.2 to 2.3 Ga which differ from any inferred layer of crusts, while they exhibit different zircon HfO isotope and oxygen fugacity evolutionary trends. Granitoids display decreasing average εHf(t) and δ18O values from −17.1 ± 1.9 to −27.1 ± 1.6, and 5.71 ± 0.50‰ to 4.25 ± 0.63‰, and increasing average logƒO2 values from −17.4 ± 4.5 to −6.7 ± 3.8 with a decrease in age from 143 to 130 Ma. In addition, relict zircon cores with Paleoproterozic and Archean ages were observed with εHf(t) and δ18O values analogous to those of the Kongling Complex. These characteristics suggest that the early-stage granitoids were possibly produced by the remelting of the subducted lower crust mixed with the Kongling Complex, with the involvement of reducing Mo-rich black shales in the Yangtze Block that seemingly account for the lower oxygen fugacity of ore-forming granitoids with younger two-stage Hf model ages and lead to the formation of those early-stage Mo deposits. As the increasing contributions of the lower crust in the magma source, the metallogenic gap developed from 136 to 130 Ma in the Tongbai-Hong'an-Dabie orogens. Zircon HfO isotopic compositions of ore-related granitoids increase simultaneously up to −14.2 ± 4.3 and 5.82 ± 0.33‰ respectively at a lower slope after 130 Ma. Part of zircon δ18O values is slightly higher than 6.5‰, implying the involvement of supercrustal materials. Meanwhile, magma oxygen fugacity exhibits an abrupt drop at ca. 130 Ma in the Qian'echong deposit with the lowest average logƒO2 value (−19.3 ± 4.5) and Ce4+/Ce3+ ratios (1–138), then rises quickly up to a maximum average logƒO2 value of −7.3 ± 2.3. As such, these granitoids were likely generated by mixing of a blended crustal magma, derived from the lower crust and supercrustal materials, and the mantle-derived component which also contributes to the late-stage Mo mineralization. We propose that not only the highly oxidized magma but also the continuous change of oxygen fugacity in the deep magma chamber may have positive effects on the formation of porphyry Mo deposits. Given the significant geochemical change at ca. 130 Ma instead of 125 Ma, we agree that the intensive magmatism and mineralization in the Tongbai-Hong'an-Dabie orogens are closely associated with the delamination of the thickened lower crust of the Yangtze Block, which caused the asthenosphere upwelling that provided the heat source and led to partial melting of overlaying lower crust.