The Role Of Carbonate/CO₂ In Rare Metal Mineralization In Peraluminous Granitic Systems

Rare metals（Li,Be,Nb,Ta,Zr,Hf,Sr,Rb,Cs,and W,Sn）have been regarded as crucial and strategic resources nowadays.A better understanding of the enrichment mechanism of rare metals are of great significance to better resource exploration and evaluation.In view of this,the role of carbonate/CO₂ in rare metal mineralization was determined by the means of fluid inclusion study and real-time high pressure（P）and high temperature（T）experiments using a hydrothermal diamond-anvil cell（HDAC）and Raman spectroscopy.The main results of this study are as follows:（1）Research on fluid inclusions indicates that the ore-forming fluids of rare metal deposits such as tungsten,tin,and lithium are often rich in CO₂/alkali carbonate,implying a close relationship of CO₂/alkali carbonate and rare metal mineralization.A comparative study of fluid inclusions from“basement”and“five floors”of Taoxikeng tungsten deposit revealed that CO₂ effervescence,as a result of fluid immiscibility,may account for the deposition of wolframite.Raman spectroscopic analyses of fluid inclusions in pegmatite minerals indicate carbon dioxide is very abundant in the aqueous liquid and vapor phases.In the aqueous liquid,HCO₃^-was detected in fluid inclusions in tantalite-（Mn）from the Morrua Mine and in late-stage quartz from the Naipa Mine,and Muiane and Nuaparra pegmatites,all in the Alto Ligonha District,Mozambique.Moreover,a carbonate（calcite group）was observed in fluid inclusions in garnet from the Naipa Mine and in beryl from the Morrua Mine,both in the Alto Ligonha District,Mozambique.A calcite-group carbonate and whewellite（Ca C₂O₄?H₂O）were identified in fluid inclusions in topaz from Khoroshiv,Ukraine.The occurrence of whewellite has not previously been reported from a peraluminous granitic pegmatite,and is interpreted to be due to a reaction of some form of carbon（possibly CO）with a peralkaline fluid.The evidence from fluid inclusions in pegmatites minerals indicates that hydrogen carbonate-rich peralkaline fluids may be involved in the evolution of Li-rich and other peraluminous granitic pegmatites.（2）The results of high P–T experiments show that dissolution of wolframite resulted in formation of WO₄^2-in the CO₂-and alkali carbonate（e.g.,Li₂CO₃,Na₂CO₃）-bearing aqueous solutions.Hydrolysis of carbonate produced alkaline conditions,which cause the increase of wolframite solubility in aqueous solution and are favorable for the crystallization of wolframite during cooling.The experimental results show significant control of P and T on wolframite crystallization and existing of lower limits of P–T conditions suitable for wolframite crystallization（e.g.,P not lower than～200 MPa）.The crystal shapes of wolframite from different experiments vary from coarse prism,acicular shape,to hair shape along with the decreasing of P–T conditions.The results of this study indicate that CO₂-and alkali carbonate-rich fluids are favorable medium for the transport of tungsten and wolframite precipitation.Cooling and Consumption of alkali carbonate via such as CO₂ effervescence or carbonate precipitation may be key factors accounting for the precipitation of wolframite from hydrothermal fluids.（3）The experimental results show that heating of alkali carbonate（e.g.Li₂CO₃,Na₂CO₃）aqueous solutions leads to the dissolution of tin dioxide in the form of Sn（OH）₆^2-,and crystallization of long prismatic cassiterite during subsequent cooling.The average growth rates of cassiterite obtained from in situ measurements range from 0.6×10^-6–8.22×10^-6 cm/s in length,and 3.40–19.07μm³/s in volume.Cassiterite grew mainly at 400–850°C and 300–850 MPa,and ended in rare metal pegmatite-forming conditions.The experimental results of cassiterite crystallization are in good agreement with geological observations of cassiterite in granitic pegmatites,and therefore reach a conclusion that alkali carbonate can play a significant role in the formation of tin-mineralized pegmatites.The results of this study agree with the argument that Sn（IV）can be efficient tin-transporting species at least in alkaline fluids.（4）The behavior of spodumene-bearing assemblages in aqueous carbonic solution and conditions at which carbonate can be generated in granitic pegmatites were studies using a HDAC and Raman spectroscopy,to provide experimental constraint on the role of carbonate in the formation of rare metal granitic pegmatites.No obvious reaction of spodumene was observed in the runs in which oxalic acid was used as the source of CO₂（up to 600–800°C）.Heating oxalate or hydrogen carbonate solutions produced CO₂ and HCO₃^--rich peralkaline fluids,which resulted in strong corrosion of spodumene（and“lepidolite”）and,in one run,formation of zabuyelite（Li₂CO₃）crystals at low temperatures.In addition,spodumene can crystallize upon heating pegmatite-forming minerals quartz and muscovite,and aqueous lithium carbonate solutions.The results of this study indicate that an alkaline fluid was required for the formation of the crystal-rich inclusions in spodumene,and therefore underscore the important role of hydrogen carbonate-rich peralkaline fluids in the evolution of Li-rich and other peraluminous granitic pegmatites.This study demonstrates that carbon dioxide-and alkali carbonate-rich fluids are favorable media for transportation of tungsten and tin and precipitation of wolframite and cassiterite,and are involved in the evolution of Li-rich and other peraluminous pegmatites.The results of this study support the conclusion that alkali carbonate is a type of significant but easily overlooked fluxing agent and can play a crucial role in the formation of W,Sn,Li and other rare metal deposits.