Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit,Eastern Tianshan Mountains,NW China: Constraints on the metallogenesis |
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| Institution: | 1. State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing, 100083, China;2. Division of Interdisciplinary Science, Kochi University, Kochi 780-8520, Japan;3. Department of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, MO 63108, USA;1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;2. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;3. College of Geological Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China;4. Department of Earth Sciences, The University of Hong Kong, Pokfulam, Road, Hong Kong, China;5. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. Graduate University of Chinese Academy of Sciences, Beijing 100049, China;3. Guangdong Provincial Key Laboratory of Mineral Physics and Materials, 511 Kehua Street, Guangzhou 510640, China;4. School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China;5. Wuhan Institute of Geology and Mineral Resources, China Geological Survey, Wuhan 434205, China |
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| Abstract: | The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4 ± 0.94 and 323.47 ± 0.95 Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO2 (42.90–46.61 wt.%), P2O5 (0.08–0.12 wt.%), MnO (0.35–0.97 wt.%) and TiO2 (0.74–0.82 wt.%), and relatively large variations in CaO (6.93–15.13 wt.%), Al2O3 (14.71–19.93 wt.%), total Fe2O3 (8.14–12.66 wt.%) and MgO (4.96–8.52 wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type. |
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