Genetic Model Of The Ambatondrazaka Fe-Ti-V Deposit,Madagascar:Constraints From Petrology And Geochemistry

Fe-Ti-V ore deposit associated with mafic complex is always referred as the products of crystallization fractionation and/or liquid immiscibility,classed as typical of magmatic-tpye deposits.However,the case study of Ambatondrazaka gabbro intrusion and associated Fe-Ti-V ore deposit indicates that ore-forming process is not a certain simple magmatic process as assumed previously.This paper takes transmagmatic fluid metallogenic theory as the guiding ideology,and field geology,petrology,zirconology,geochemistry as the methods.Then systematic research is conducted on Ambatondrazaka Fe-Ti-V ore deposit,with the conclusions as follow:1.Field relation and petrographic characteristics indicate that main ore bodies are intruded into the gabbro,in spite of the fact that there are still some small-scale comformitious layed ore bodies.In addition,disseminated ores are feature of poikilitic texture and interstitial texture of magnetite.All above are similar to others in the world.2.Systematic research on zircon separated from gabbro including petrography,in-situ trace element,Ti-in-zircon thermometer,Ce-in-zircon oxygen-fugacity meter,U/Pb isopotic geochronometer indicates that the emplacement age of the host parent rock of the ore body is 784 Ma,which suggests that it is the product of the closure of Mozambique Ocean,and the gabbro experienced dual-transformation with stress and high-temperature reduced fluid(T?845?,logfo2?23.48)in 776Ma.3.The Major element and trace element of whole rocks suggest the first stage of crystallization is charactered with olivine+plagioclase,while Fe-Ti oxide+orthopyroxene?clinopyroxene ± olivine ± plagioclase in the second stage,which plays an important role in the ore forming process.However,High LILE&HFSE,low Cr&V and the above geological characteristics are in contrast to the genetic model of cumulation deduced from major element analysis.Moreover,REE tetrad effect which occurs in the massive ores is first reported about magmatic-tpye Fe-Ti-V ore deposits.All these suggest that some other processes may have a crucial role in the mineralization of Ambatondrazaka Fe-Ti-V ore deposit.4.Finally,the author proposes a genetic model that high-temperature,low-fo2,F and Cl-rich transmagmatic fluid injects and rejuvenates the near-solidus cumulated ores.This model could not only explain the first-order geological and geochemical characteristics which have been found in the ore body,but also is consistent with the event of high-temperature,low-fo2,Zr-rich fluid recorded in the zircons separated from gabbro.