Metallogeny Study Of Washan Porphyry Iron Deposit In Ningwu Oredistrict

Washan iron deposit is located in the Ningwu ore district, Middle-Lower YangtzeRiver polymetallic ore Belt(MLYRB), East China. It is a typical porphyry-type irondeposit, with a multi-stage ore-forming process, and displays the iron mineralizationevolution of the Washan ore field. Based on the detailed field geological investigationand indoor experimental analysis, a detailed evolution of mineralization has beencarried out.The magnetite formation of the Washan deposit is divided into four stages,namely, disseminated magnetite, magnetite in breccia, coarse-grained vein magnetiteand pegmatitic vein magnetite. The cryptoexplosion induced large-scale ironprecipitation, and provides space for the mineralization.In situ electron microprobe and LA-ICP-MS analysis of the magnetite fromWashan iron deposit show that with the evolution of mineralization, the contents of Ti,Mn, V in magnetite are consistent; the Al and Mg contents increased; the contents ofGa, Sn and high field strength elements(Zr, Hf, Nd, Ta)change in a small range;from magnetite in breccia to pegmatitic vein magnetite, Co content graduallyincreased and Sc decreased gradually. The compositions of magnetite in variousstages indicate that the process of magnetite mineralization has a continuous evolutionwith a similar source. The iron derived from the late of magmatic evolution, and isenriched to be iron-rich fluid by the effect of immiscibility.Hydrogen and oxygen isotope characteristics of the mineralization various stagesof magnetite, apatite and quartz, and the homogenize temperature of apatite inpegmatitic vein magnetite ore indicate that during the mineralization process, the fliudcomposition was changed, combined with previous studies. The ore-forming fliud wasmainly of magmatic water, and then the meteoric water gradually added.The zircon U-Pb ages of the host rock(gabbrodiorite porphyrite)and the rockstock(granodiorite porphyry), which cut across the ore body, are 131.7±0.7Ma and126.1±0.5Ma respectly. According to the relationship between petrogenesis andmineralization, the ages of mineralization of Washan iron deposit is in the 131.7 ~126.1Ma. It belongs to the Early Cretaceous in the Dawangshan cycle. This ironmineralization is proposed to occur in an extensional tectonic regime, possiblyinduced by lithospheric thinning. The characteristics of major elements, trace elements, rare earth elements, Sr-Ndisotopic of the gabbrodiorite porphyrite and granodiorite porphyry with their zircontrace element and Hf isotope features have shown that the two type rocks have thesimilar source. The formation of magmatic rocks derived from enriched mantle andthe crust material was involved in the magma during ascent process. The granodioriteporphyry was involved more.In the late stage of volcanic activity, hydrothermal vein-type copper and/or golddeposits occurred in part of the Ningwu ore district, which are considered to beclosely related with the Niangniangshan Formation. The ages of the granodioriteporphyry is similar to that of the volcanic rocks of the Niangniangshan cycle. The Ce(Ⅳ)/Ce(Ⅲ)ratios of zircons in granodiorite porphyry are mostly less than 100,which is different(much more lower)with the one of the pluton(>300)accosiated with copper porphyry deposit, but theδEu values(>0.4). Therefore,further research is needed to study the possibility that granodioritic stocks are relatedto copper and/or gold mineralization.By the metallogenic system study, we infer that the porphyry iron mineralizationsin different typical deposits(i.e. Washan deposit, Meishan deposit, Gushan depositand Gaocun deposit)have a great similarity, experiencing disseminated magnetitemineralization - magnetite in breccia mineralization - coarse-grained vein magnetitemineralization - pegmatitic vein magnetite mineralization with the different of themainly mineralization in different stage be the effect of the different metallogenicenvironment. The relationship between iron and copper(gold)mineralizations isstill unrevealed. It needs further study and exploration to improve our understanding.