| The Ningwu basin located in the tectonic and magmatic active belt of the lower Yangtze Region,belongs to the inheritable volcanic faulted basin.Known as porphyrite iron deposits,this basin hosts some representative Fe deposits such as Meishan,Taocun and Taipingshan.Based on systematic studies on field geological observation,lithogeochemistry and scanning electron microscope,the alteration zoning and hydrothermal evolution characteristics of the iron deposits are described.Combined with the analysis of electron microprobe,the genesis of iron deposits in the Ningwu basin is also discussed in present thesis.Meanwhile,the rock-forming and mineralization ages are set up by zircon U-Pb and pyrite Re-Os dating results.Finally,the sulfur isotopes of sulfides,Os isotopes of pyrite,zircon Hf isotope and geochemical contrast of the igneous rocks from the Ningwu and Luzong basin reveal the source of iron and the mineralization diversities of the two basins.According to the occurrences of ore bodies and the petrographic study,the Meishan iron deposit can be divided into three alteration zones from intrusion to wall rocks,i.e.sodiumisation,skarnizationand siliconizationkaolinization-carbonation-pyritization zones.It shows that the lower and middle alteration zones result from the metasomatism by magmatic fluid and the upper alteration zone by mixture of magmatic and meteoric water.Iron mineralization mainly occurred at the retrograde stage of skarn.LA-ICP-MS zircon U-Pb dating gives weighted mean ages of 129.3±1.3 Ma,129.1±1.7 Ma and 129.7±2.8 Ma for the diorite porphyrites from Meishan,Taocun and Taipingshan iron deposits,indicating that they formed in the Cretaceous.The pyrite Re-Os isochron ages of these three deposits are 127?129 Ma and represent the timing of iron mineralization,showing a close genetic relationship with the coeval diorite porphyrites.Furthermore,the Fe metallogenic epoch in the Ningwu basin coincide the second mineralization peak of the Yangtze River Metal Belt.Sulfide S isotope,pyrite Os isotope,whole rock Sr-Nd isotope and zircon Hf isotopic composition suggest that the ore materials of the Ningwu basin were derived from the enriched mantle,and the Triassic gypsum-salt layer were involved during magma ascent.In addition,geochemical comparative study of ore-bearing rocks from the Ningwu and Luzong basin suggests that the more input of crustal materials may explain the stronger Fe mineralization in the Luzong basin. |
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