| The global molybdenum supply is derived almost exclusively from porphyry-type deposits. Traditional metalloginc models state that porphyry deposits mainly formed in subduction-related magmatic arc, such as the southwestern Pacific islands and Andes. The characteristics, genetic models and tectonic settings of porphyry Mo deposits, especially in the Western Cordillera and British Columbia, North America, have been extensively studied and well documented. In China, however, continental collision belts are also favorable for the development of porphyry Mo deposits, and the metallogenic models based on magmatic-arc type porphyry Mo deposits are not suitable for the deposits which formed in continental collision, and the geological differences between the two are still unknown. In this thesis, the author studied the Yaochong Mo deposit in the Dabie Shan and the Diyanqinamu Mo deposit in the Great Hinggan Range, to constrain features of porphyry Mo deposits developed in continental collision and magmatic arc settings. The author summarized the causative magma sources, the diagnostic ore-fluid and wallrock alteration characristics of the porphyry Mo deposits in continental collision and magmatic arc, and to reveal the similarities and differences between them.The Yaochong porphyry Mo deposit is located in the Xinxian UHP metamorphic unit which includes Neoproterozoic granitic gneisses and supracrustal rocks. Mo mineralization mainly occurs as numerous veinlets or stockwors in altered gneisses, with the development of potassic, phyllic, and propylitic alteration assemblages. The hydrothermal ore-forming process can be divided into four stages, from early to late: quartz + K-feldspar stage 1, quartz + K-feldspar + molybdenite stage 2, quartz + polymetal sulfide stage 3, and quartz + carbonate stage 4. Fluid inclusions(FIs) can be compositionally distinguished between pure CO2(PC-), CO2-H2O(C-), aqueous(W-) and solid-bearing(S-) types, and no halite crystals were identified. Homogenization temperatures for the fluid inclusions from stage 1 to 4 are in the ranges of 262?501 °C, 202?380 °C, 168?345 °C, and 128?286 °C, resepectively, and all the estimated salinities are ≤ 13.1 wt.% Na Cl equiv, with homogenization pressure of 62–248 MPa. The initial ore-forming fluids are characterized by relatively high temperature, CO2-rich, and Na Cl-poor, which evolved from magmatic to meteoric in sources suggested by the C-H-O isotopes study. The decrease of temperature and the escape of CO2 caused the precipitation of molybdenite. A buried porphyritic granite is spatially and genetically related to the Mo mineralization. The Yaochong porphyritic granite yields LA-ICP-MS zircon U?Pb weighted average 206Pb/238 U ages of 137.7±0.6 Ma, which is well consistent with the molybdenite Re?Os isochron age of 136.9?1.1 Ma within error, indicating that the magmatic hydrothermal activity took place at Early Cretaeous. The porphyry dike yields a LA-ICP-MS zircon U?Pb weighted average age of 130.5±1.2 Ma, younger than the metallogenic age. The porphyritic granite and porphyry dike have high contents of Si O2, K2 O and Al2O3, and low contents of Ti O2, Mg O and Ca O, showing a peraluminous to meta-aluminous high-K calc-alkaline affinity, with LREE enrichment; and Eu-depletion appears in the porphyry dike, but not in porphyritic granite. They both have high initial 87Sr/86 Sr of 0.70814-0.70854, highly negative ?Nd(t) values of-18.2 to-16.6, with TDM2(Nd) ages of 2.27?2.40 Ga. Their(206Pb/204Pb)t,(207Pb/204Pb)t and(208Pb/204Pb)t values range 16.549-16.760, 15.328-15.348, 37.440-37.576, respectively. Zircons from the porphyritic granite and porphyry dike gave negative ?Hf(t) values of-27.2 to-20.7 and-26.3 to-18.7, with TDM2(Hf) ages of 2.5?2.9 Ga and 2.4?2.8 Ga, respectively. These Sr-Nd-Pb-Hf isotopic signatures indicate that the granitic intrusions at the Yaochong deposit were mainly originated from the partial melting of the northern Dabie complex that mixed with the Precambrian rocks of the North China craton.The Diyanqinamu porphyry Mo deposit is located in the Great Hinggan Range. Mo mineralization mainly occurs as numerous veinlets in altered andesites and tuffs, with the development of potassic silicate, phyllic, propylitic alteration zones. The hydrothermal ore-forming process can be divided into four stages: quartz + K-feldspar + magnetite ± hematite stage 1, quartz + molybdenite stage 2, quartz + polymetal sulfide stage 3, and quartz + carbonate stage 4. Fluid inclusions(FIs) can be compositionally distinguished between vapor-rich aqueous(WV-), liquid-rich aqueous(WL-), and solid-bearing(S-) types with daughter minerals of halite, sylvite, anhydrite, carbonated, hematite, magnetite, molybdenite, pyrite and chalcopyrite. Homogenization temperatures for the fluid inclusions from stage 1 to 4 are in the ranges of 511?624 °C, 345?557 °C, 235?425 °C, and 115?274 °C, resepectively, with estimated salinities of 0.2–23.0 wt.% Na Cl equiv and 28.2–66.8 wt.% Na Cl equiv, and homogenization pressure of 5–90 MPa. The initial ore-forming fluids are characterized by high temperature, high salinity, high fO2, and CO2-poor. The H-O isotopic studies present low δD of-123 ‰ to-83 ‰ and large variation of δ18O from-1.1‰ to 9.3‰. Low δD could be resulted from magma degassing. The FIs studies, together with the H-O isotopes, suggest the ore-fomring fluids evolved from magmatic to meteoric in sources. The decrease of fO2 and intense fluid boiling resulted in the precipitation of molybdenite. The buried porphyritic granite and aplitic granite are spatially and genetically related to the Mo mineralization. Zircon crystals from porphyritic granite yield weighted average 206Pb/238 U ages of 158.71 ± 0.8 Ma and 157.96 ± 0.87 Ma; and zircon crystals from aplitic granite yield weighted average 206Pb/238 U ages of 156.9 ± 1.1 Ma. Eight molybdenite samples from the ores yield Re?Os isotope ages of 156.1 ± 2.2 to 158.1 ± 4.3 Ma, with the isochron age of 157.7 ± 1.3 Ma, which are consistent with the U?Pb ages for the igneous rocks. The porphyritic granite and aplitic granite have high contents of Si O2, K2 O and Al2O3, and low contents of Ti O2, Mg O and Ca O, showing a peraluminous high-K calc-alkaline affinity, with LREE enrichment, MREE and HREE depletion. These granites have a high initial 87Sr/86 Sr of 0.70669 to 0.70901, positive εNd(t) values of 2.3 to 3.8, with TDM2(Nd) ages of 0.62 to 0.76 Ga. Their(206Pb/204Pb)t,(207Pb/204Pb)t, and(208Pb/204Pb)t values range from 18.018–18.363, 15.537–15.545 and 38.002–38.142, respectively. The zircons from these granites have positive εHf(t) values of 4.2 to 9.0, with TDM2(Hf) ages of 0.63 to 0.94 Ga. This Sr-Nd-Pb and zircon Hf isotopic signatures indicate that the causative granitic intrusions for the Diyanqinamu deposit were mainly derived from juvenile lower continenetal crust, mixed with the basaltic magma generated from the hydrated and oxidized mantle wedge. The Sr-Nd-Pb isotopes of sulfides are mostly consistent with these of the causative granites, with some varying between the granites and the wallrock andesites, suggesting that metallogenic materials are major from the granites, with minor from the andesites.Comparative study of the Yaochong and the Diyanqinamu porphyry Mo deposits indicates that: the collision-type porphyry Mo deposits are commonly formed in a transition from compression to extension or subsequent extension settings, and the causative magma is derived from the thickened lower continental crust or thinned lower continental crust, with/without some mantle contribution; while, the magmatic arc-type porphyry Mo deposits generally formed in continental arc, with the magma generated from the “MASH” of the lower crust continental crust and the partial melting of hydrated and oxidized mantle wedge. The compositional differences of the causative magma sources, determine the differences of the ore-forming fluids and wallrock alteration. The ore-forming fluids of collision-type porphyry Mo deposits are characterized by high temperatue, high salinity, CO2-rich, with high ratios of K/Na and F/Cl, and the wallrock alterations are dominated by potassic alteration, with weak sericitization and propylitization. By contrast, the ore-forming fluids of magmatic arc-type porphyry Mo deposits are featured by high temperatue, high salinity, high oxygen fugacity, with little CO2 and lower ratios of K/Na and F/Cl, and the wallrock alterations are outward zoning from relatively weak potassic alteration, overprinted by phyllic alteration, to propylitic alteration. |
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