Main Geological Charasteristics And Formation Mechanism Research On Bonga Carbonatite Weathering-crust In Angola,Africa

The Bonga carbonatite lies about 26km to the southwest of Quilengues,Huila Province,Angola,cropping out as an isolated circular plug in the Precambrian crystalline basement,and is one of the largest and richest carbonatites among 14 already-known Angolan carbonatites.Due to the peculiarity of primary carbonatite consisting of its main part,under long-time intense tropic weathering,there have developed extensive and thick lateritic weathing crusts,containing more niobium and phosphate resources with great economic value,in the surrounding areas,but its research is almost empty.This thesis tries to conduct a preliminary discussion on Bonga carbonatite weathering crust from the aspects of main geological characteristics formation mechanism and metallogenic mechanism respectively,combining with field geological investigation and geochemical analysis results of samples,comparing to related research materials.The results show that the wedge-shaped crusts distribute in the gently undulating slope belt between northeast and west of the carbonatite,covering an area of about 6.3km2.Its dominated colour is red,but also has variation of dark brown-brownish red-pale brown-yellowish brown from the root to the periphery of the complex with the particle size increasing.The lateritic crust profile presents vertical inhomogenous charasteristics and this characteristics mainly display in the aspects of colour,particle size and thickness,among of them,the soil colour and particle size show similar changle rules to the horizontal variation laws of surface soils,the thickness with average of 8.0m depends on topographic location,generally it is thicker closing to the carbonatite.Bonga weathering crust also has vertical zonality,according to the field drilling and actual situations,it could be divided as follows:(1)Dark brown fine grained soil layer,(2)light brown-yellowish brown coarse grained eluvial layer,(3)retaining the original rock structure,yellowish gray-yellowish white and gray syenite or granite semi-weathering layer,(4)weakly weathered or unweathered primary carbonatite or syenite or granite bedrock,but all of them may not appear.Bonga carbonatie weathering crust is a special type of laterite.it is formed under the interaction of parent rock.climate,topography.hydrology and time factors,consisting of three stages of carbonatite collapse,soil forming process and rubefaction process.Bonga weathering crust is an unique geochemical metallogenic enrichment system,it inherited the high ore-bearing potential of this primary carbonatite.especially characterizing by the enrichment of pyrochlores(Nb2O5,average 0.58%)and phosphate(P2O5,average 3.04%),and it is estimated,by the large,this crust has overall about 0.56 million tons Nb2O5 and 4 million tons P2O5.Bonga carbonatie weathering crust belongs to laterite-type weathering crust deposit,differential mobility of its components in supergene zone is a significent factor of affecting this deposit formation.Highly active rock forming components such as CaO,MgO)are subjected to large-scale migration through intense leaching,the phosphate and the inert constituents which are predominated by pyrochlores strongly concentrating in residual products is the mineralization mechanism of Bonga carbonatite weathering-crust.