| The Datangpo sedimentary manganese ore deposits forming during Nanhua period located in eastern of Guizhou and adjacent areas was one of most important manganese resources in our country,and showed better prospecting potential,but the genesis of manganese ore deposits still was controversial and held a long-standing argument.Based on Geochemical characteristics of ore deposits,the thesis discussed the forming environment and process of manganese ores.And then it discussed the metallogenic mechanism and source of metallogenic material to proposed the genetic model of Datangpo-type manganese deposits by taking into account comprehensively including Geochemical properties of manganese,sedimentary environment evolution,Neoproterozoic glaciation and regional geological evolution.Based on geochemical data in several manganese ore deposits including Yanglizhang,Minle,Rongxi,Daotuo,Xixibao and Baishixi,compared to black shales,the Mn ores showed relative high content of P2O5?MnO?CaO?MgO,higher degree fractionation between Mn and Fe,and no clear enrichment or depletion of redox sensitive trace elememt U,Mo and V,cap-shaped PAAS-normalized rare earth element distribution patterns and evident positive Ce anomalies and strong positive Eu anomalies.The comprehensive analysis suggested that manganese was precipitated as insoluble Mn-oxyhydroxides particle in oxic environment,and then transferred to rhodochrosite in reducing alkaline pore-water by early diagenetic organic carbon-mediated changes.Besides,according geochemical comparison with ferromanganese oxyhydroxide precipitates in modern marine,the precipitation of Datangpo-type manganese ores is similar to hydrogenetic crusts and nodules,and the analysis of differences indicated some information:higher degree of fractionation between Fe and Mn prior to deposition,more quickly precipitation during deposition and strong reducing condition and early diagenetic condition during post-deposition.Geochemical investigation of sedimentary section indicated that the early of Datangpo period witnessed a gradual evolution form anoxic reduction to oxidation environment,which led a question that there was a incoordination between environment forming-rock and forming-ore.Depth analysis based on geological and geochemical characteristics state clearly that the Mn ores and black shales formed in different sedimentary environments.The fromer represented rapid deposition under condition of more stronger hydrodynamic force,and the latter stand for slow deposition under stagnant water conditions.These differences were great likely result from periodical changes of sedimentary environments led by marine transgression:redox stratified water was appeared in restricted basin during intermittent period of transgression resulting in dissolved oxygen and soluble manganese enriched in upper and lower water column,respectively;while circulation flow and mass exchange happened between upper and lower water column leading to oxidation precipitation of manganese finally.More geochemical evidence indicated that contemporaneous submarine hydrothermal activity were not the main source ore-forming materials.Howerer,submarine hydrothermal activity occurred in the more earlier period,such as Nahhua rift development stage during early Neoproterozoic,probably were the main source ore-forming materials.In Changan glacial period,a large number of Fe and Mn derived from rift-related hydrothermal sources were pre-concentrated in dissolved state in stagnant,highly saline,O2-deficient seawater beneath the ice cover.During Fulu interglacial,along with progressively increasing of O2-content in surface seawater,Fe and Mn were oxygenated and precipitated successively resulting in the iron-bearing deposits appear in lower Fulu Formation,while manganese-bearing deposits appear in upper Datangpo Formation finally.Based on the above analysis,the genetic model of Datangpo-type manganese deposits was proposed. |
PDF Full Text Request