Controls Of Bedrocks On The Formation Of Ion-adsorption Type Ree Deposits In The Weathering Crusts

Rare earth elements(REEs)are indispensable to new energy,new materials and other high-tech technologies,especially in aerospace,national defense and military industries,and have irreplaceable applications,so they are listed as strategic resources by many countries.In the last decades,the dramatically increased demand for REEs in the global market has triggered a new wave of scientific research and exploration of REE deposits.Ion adsorption type REE(i REE)deposit is a unique REE resource mostly found in China.Its heavy rare earth elements(HREEs)resources account for more than 80% of the world's reserves,and it also contributes more than 90% of the global HREE ore products,so it has attracted worldwide attention.In recent years,great progress has been made in the prospecting of i REE deposits in China.For example,i REE deposits were found in the weathering crusts of metamorphic rocks in Ningdu County for the first time,and both light and heavy i REE deposits were found in the weathering crusts of Shangyou Caledonian granites.Therefore,new questions arise in the study of i REE deposits,such as the difference in weathering and mineralization of different types of ore-forming bedrocks(metamorphic rocks,granites)and the controls of bedrocks on i REE mineralization in the weathering crusts?What controls the different metallogenic types(light and heavy REE)in the weathering crusts? These issues are critical in the metallogenic study of the i REE deposits and the i REE deposit prospecting as well.In this study,three typical i REE deposits in the weathering crusts developed above the metamorphic rocks in Ningdu,and Shangyou granites were investigated through systematic researches on the mineralogy,petrology,geochemistry,and geochronology and carried out simulation experiments.The occurrence of REEs,weathering characteristics and bed rock controls on REE mineralization in the three profiles were identified respectively.Finally,the influences of the bed rock composition and structure on the formation of the i REE deposits are summarized.The main conclusions are as follows:1.Geochemical and mineral features of the weathering crust of metamorphic rocks in Ningdu are illustrated and the main controls on ion exchangeable REE mineralization in the weathering crusts are clarified.(1)The metamorphic bedrocks have average REE concentrations of ?198 ?g/g,which are mainly hosted in REE-bearing minerals.The mass balance calculation shows that hydrothermal rhabdophane-(Ce)and(REE-)epidote account for 62.6%REEs of the bulk.Therefore,the hydrothermal alteration of the metamorphic rocks resulted in the redistribution of the REEs,and it is likely a key factor controlling the i REE mineralization in the weathering crusts of the metamorphic rocks.(2)The low permeability of metamorphic rocks leads to the low weathering degree of weathering crust.The clay minerals in the weathering crust are primarily illite,kaolinite and hydrobiotite.The composition and evolution of the clay minerals control the pH value in the weathering crust.Under the combined action of low permeability and pH value,REEs are usually enrichment in the upper of the weathering crust.The average REEs content of ore body is ?962 ?g/g,and the leaching rate is about 75%.(3)Fractionation of REEs in the lower part of the weathering crust is controlled by the differential dissolution of minerals,while in the upper part of the weathering crust is controlled by differential adsorption of clay minerals.Ce anomalies in profile are controlled by REDOX condition.2.The controlling factors of light and heavy i REE deposits in the weathering crusts of Shangyou composite granites are demonstrated.(1)The formation of light and heavy i REE deposits in the weathering crusts of Shangyou composite granites are controlled by the REEs composition and distribution in the bedrocks.The early and late stage granites of the Shangyou batholiths are LREE-and HREE-enriched respectively,and have weathering crusts characterized by LREE-and HREE-enriched i REE deposits accordingly.Thus,the late stage granites in the Shangyou batholiths are promising prospecting targets for heavy i REE mineralization.(2)The hydrothermal alteration of the late stage granites in the Shangyou batholiths led to significant removal of REEs and formation of more weathering resistant accessory minerals,e.g.,apatite is transformed into xenotime and monazite.Therefore,unlike the conventional scenarios,hydrothermal alteration of the late stage granites in the Shangyou batholiths is unfavorable for i REE mineralization.(3)The heavy REE profile is weathered strongly,the ore body is at 4?9 m,the average content of REE is ?439 ?g/g,and the leaching rate is about 65%-80%.The light REE profile is weathered weakly,the enrichment layer is at 0?6 m,the average content of REE is ?328 ?g/g,and the leaching rate is about 67.8%?87.5%.The fractionation of REEs is mainly affected by the adsorption of clay minerals.LREE is relatively enriched in the upper part of the weathering crust and HREE is relatively enriched in the lower part of the weathering crust.3.Controls of bedrocks on the formation of i REE deposits are summarized.(1)A relatively REE-enriched bed rock is the prerequisite for ion exchangeable REE mineralization in the weathering profiles.The REE compositions and distribution of the bed rocks control the i REE mineralization types in the weathering crusts.Weathering of the heavy REE-enriched bedrocks may form the heavy i REE deposits,while weathering of the light REE-enriched bed rocks may form light i REE deposits in the weathering profiles.(2)Hydrothermal alteration directly affects the REE concentrations of the bedrocks.In fact,hydrothermal alteration may input or remove REEs from the bedrocks.Moreover,hydrothermal alteration may change the REE-bearing minerals assemblages.Both of these factors will control the i REE mineralization potential in the weathering crusts.(3)The structural of the parent rocks affect the permeability in the weathering crust,which in turn affects the weathering degree of the weathering crust,clay mineral assemblages and the enrichment layer of REEs in the weathering crust.