Petrogenesis And Mineralization Of The Laiziling Thick Layered Greisen,Hunan Province

Greisen is a common rock phase,which is the product of hydrothermal alteration by the reaction of the granite and F-bearing low-salinity acidic hydrothermal fluid,usually occurring on the cupola or included in the granitic body as the layer and the vein.The Laiziling granite pluton(Xianghualing district,Hunan Province)has good vertical zonation,and the greisen located above the granite is layered greisen with the thickness of?50 meters,totally different in petrological and mineralogical characteristics compared to the regular greisen formed in the hydrothermal process.Also the various types of mineralizations,implies the important scientific significance.In this study,the Laiziling greisens,the granites underneath them,and the quartz veins and pegmatites above them are selected as the samples.Combined of the observation in the field,with petrological,geochemical and mineralogical results,we define the relationship of different rock phases,confirm the detailed physicochemical condition of the Laiziling greisen during the magmatic-hydrothermal process,and discuss the formation mechanism responsible for the greisen.Moreover,the investigations on the occurrences and the chemical compositions of the ore elements-bearing minerals,help understanding the various mineralizations hosted in the different rock phases and confirm the factors causing the variation.From base to top of the Laiziling pluton,medium-grained(MG)alkali feldspar granite,fine-grained(FG)alkali feldspar granite,albite granite,greisen,quartz veins and pegmatite can be recognized according to petrography.Whole rock compositions indicate that both the granite and greisen are weakly to strongly peraluminous compositionally,and from the MG and FG alkali feldspar granite though the albite granite to the greisens,the HSFEs concentrations(such as REE,Zr,Th,Y)and parameters(e.g.Zr/Hf,Nb/Ta,and Th/U)show decreasing trends,while ore metal elements(such as Li,Rb,Nb,Ta,W)display an increasing trend.Micas from all rock phases belong to zinnwaldite,which is characterized chemically by relatively high contents of Li,Rb and F.From base to top rock phases,zinnwaldite actually presents gradually varied trends in chemical composition in both major and trace elements,as SiO2,MnO,Li2O and Ta contents increase successively,and FeO,Nb,Sn contents and Nb/Ta ratio gradually decrease.Quartz,a rock forming mineral,has Ti content decreasing from?14 ppm to?2 ppm,so the crystallization temperature drops from?560 ? to?440 ? calculated by the TitaniQ geothermometer,however,the Li content in the quartz increases from?15 ppm to?90 ppm.As a common accessory mineral,the HfO2 content of zircon increases from base?2 wt%to top?13 wt%,and the Zr/Hf ratio rapidly descending.Topaz occurs in both granite and greisen,they are all closed to the fluorine-dominated end member.Consequently,both whole-rock geochemistry and mineralogical features prove Laiziling pluton consists of highly evolved rocks and a progressive evolution from the granite in the base to the top greisen and pegmatite during the magmatism to hydrothermal process.Various W-Sn-Nb-Ta mineralizations hosted in the different rock phases from base to top of the Laiziling pluton,the occurrence and the chemical compositions of the oxide minerals are diverse greatly.In the alkali-feldspar granite,the unique oxide minerals included columbite group minerals + wolframoixiolite + qitianlingite + wolframite,suggesting the W,Nb and Ta are enriched in the original magma and fluorine enrichment in the magma reduced solidus temperature,and thus enlongate the crystallization process,resulted in the crystallization of the W-rich columbite group minerals.With the evolution of granitic magma,a W-rich fluid generated in the melt,the columbite group minerals were altered and the wolframoixiolite and qitianlingite formed around the columite-group minerals.In the magmatic to hydrothermal transition,intensive fluids and high content of volatile derived from the magma,the columite group minerals and wolframite crystallized in the albite granite and greisen and their compositions were closed to the end-member with few element substitution.Also the cassiterite contained Nb2O5+Ta2O5 content.During the late stage,the main oxide minerals occur as wolframite and cassiterite in quartz veins and pegmatite,intergrown with the sulfides.In summary,both petrogenesis and metallogenesis of the greisen in the Laiziling pluton indicate the differences from the regular greisen formed in the hydrothermal alteration.It is a kind of highly evolved rock,which characterized by the occurrence of the Li-Rb-F riched zinnwaldite and the W-Sn-Nb-Ta mineralizations obviously affected by both magmatic and hydrothermal process.It is deduced that Laiziling greisen is a specific product of peraluminous granitic magma system at magma-hydrothermal transition stage,formed between the granite and pegmatite.The Laiziling thick layered greisen may open a new window for understanding the evolution and rare-metal mineralization during the granitic magmatic-hydrothermal system.