On the Geologic Characteristics and Genesis of the Shangchang Molybdenum Deposit in Pucheng County, Fujian Province

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South China Block (SCB) is the area including the Yangtze Craton and the Huanan Orogen where scattered Precambrian terranes are usually regarded as segments of Cathaysia Land. It is the third most important molybdenum metallogenic province in China, next to the Qinling-Dabie area and Northeast China, containing 29 Mo-only or Mo-dominated, 9 W-Mo(-Sn-Bi) and 8 Cu-Mo deposits. These 46 deposits are located mainly in: (1) the Lower Yangtze River Belt of the northeastern Yangtze Craton, (2) the Northern Jiangnan Orogenic Belt that is generally considered a Meso-Neoproterozoic magmatic arc complex accreted onto the southeastern margin of the Yangtze Craton, (3) the Wuyi-Yunkai Orogenic Belt characterized by local exposures of Proterozoic metamorphic terranes and the more widespread Sinian (Uppermost Proterozoic) to Triassic sedimentary sequences, and (4) the Southeast Coastal Volcanic Belt characterized by Yanshanian andesitic to felsic volcanic rocks. Their genetic types are dominated by porphyry and skarn mineral systems, with only a few quartz-vein systems. The orebodies form veins, lens, cylindrical shapes, pipes, or irregular in shape, usually controlled by faults at various scales and volcanic-subvolcanic complexes. The host-rocks are variable in lithologies, including granites, porphyries, volcanic breccias and tuffs, clastic sediments and carbonate rocks, but the high-grade orebodies are usually hosted in carbonate-shale sequences. Hydrothermal mineralization processes can be generally divided into four stages, from early to late, they are (1) K-feldspar-quartz veins or veinlets, (2) quartz-molybdenite stockworks, (3) quartz-polymetallic sulfide stockworks, and (4) quartz-carbonate-fluorite veinlets. Fluid-rock interaction as exemplified by wallrock alteration evolved from K-silicate alteration (K-feldspar-quartz-mica), through phyllic (quartz-sericite-chlorite-epidote), to propylitic or argillic alteration, with skarn alteration typically occurring in skarn-type mineral systems. Hydrothermal mineral assemblages vary between two end-members, namely the dry system formed by CO2-rich fluids and marked by quartz, K-feldspar, fluorite, carbonate and epidote, and the wet system mainly originated from CO2-poor fluids and composed of biotite, sericite and chlorite. The ore-forming fluids are magmatic in origin and show high-temperature and high-salinity. The melt-fluid systems forming Cu-Mo deposits are more oxidizing than those forming the W-Mo or Mo deposits, as suggested by accessory minerals in granitoids and daughter minerals in fluid inclusions. The Cu-Mo deposits are related to the I-type granitic rocks (adakite-like), whereas the W-Mo and Mo-only systems are related to granitic rocks of S- or A-types, although all of them show high K contents. Available isotope ages show that the Mo and Mo-bearing deposits were predominantly formed in the early Yanshanian Orogeny (170–134 Ma), followed by the late Yanshanian Orogeny (110–92 Ma) and the Caledonian Orogeny (450–410 Ma). The Caledonian Mo-mineralization has been observed only in the Wuyi-Yunkai Orogenic Belt and related to the collision between the Yangtze Craton and the pre-Devonian Huanan Orogen or terranes separated from the Cathaysia Land, linked to the assembly of the Gondwana supercontinent. The Early Yanshanian mineralization affected the entire Huanan Orogen and the eastern Yangtze Craton, and resulted from the syn- to post-collisional tectonism following the closure of eastern Paleo-Tethys. The Late Yanshanian Mo deposits mainly occur in the Southeast Coastal Volcanic Belt and the southeastern margin of the Wuyi-Yunkai Orogenic Belt, and are related to the westward subduction of the Paleo-Pacific plate. The skarn-type mineral systems generally show lower Re contents than the porphyry-type deposits in a same tectonic unit, suggesting that carbonate host-rocks have lower Re contents than the causative porphyries. The Re contents in molybdenites from porphyry or porphyry-skarn Cu-Mo systems are > 50 ppm, mainly > 100 ppm, suggesting a source significantly contributed by the mantle; whereas the Re contents in molybdenites from the Mo-only or W-Mo-dominated deposits are < 100 pm, mainly < 50 ppm, indicating a genetic relation to the crust-sourced granitic magmatism.
Mesozoic molybdenum deposits are distributed widely in Fujian, and associated with a certain amount of rhenium. This study explores Mesozoic molybdenum deposits in Fujian. It is found by analysis of characteristics of ore petrology and mineralogy of molybdenite of typical molybnum deposits (Pingdi, Shangxikeng, Luobuling, Makeng, etc.) in Fujian Province, five types of the molybdenum deposits have been determined, including the porphyry type, the magmatic hydrothermal type, the volcanic hydrothermal type, the tectonic breccia type and a few skarn type. Our systematic field investigation and petrography study finds that the ores mainly have coarse-medium fine grain, platy texture, small amount of thick tabular texture; the ore mineral(only molybdenite) is of leaden color, obvious polychroism, 2H polymorph (a small amount of 2H+3R, 3R), low contents of impurities including Re((N-N×100)×10-6, most(N-N×10)×10-6), which indicates the ores formed in moderate-high temperature condition and the ore originates from crust-mantle mixed source (most crust source); Mesozoic molybdenum mineralization is of universality in Fujian province; and as a kind of accompanying elements, Re is of some comprehensive utilization value.
Located in the southwestern Fujian Province, the Tongkeng ore deposit in Liancheng is a newly discovered molybdenum deposit. The molybdenum mineralization occurs within the contact zone between the granite porphyry stock formed in the first stage of late Yanshanian magmatic event in Tongkeng area and late Paleozoic metamorphic rocks. Ore bodies are charactered by vein, veinlet and disseminated. Re-Os dating of six molybdenite samples collected from the major ore body shows that the isochron age is (115.7±1.1) Ma and the model ages range from 115.1 Ma to 116.7 Ma, with an average value of (115.8±0.8) Ma. It can be considered that both the granite-porphyry and associated molybdenum mineralization were formed in Early Cretaceous, and that both were derived by the late Yanshanian tectonic-magmatic event in the southwestern Fujian depression. Combined with isotopic age data from other adjacent molybdenum deposits, it can be inferred that the early Yanshanian large-scale molybdenum mineralization in the southwestern Fujian depression formed during the transition stage from the Indosinian sustained platform to active continental margin, and accompanied by compressing and uplifting in early Yanshanian (about 160-145 Ma); the late Yanshanian tectonic conversion from compressing to tension (about 125-105 Ma) resulted in the magmatism which seemed to be the dominant factor for the late Yanshanian molybdenum deposit.
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