The Origin Of Corundum From Ban Huai Sai,Laos

Sapphire occurs in HTHP environment with high Al poor Si content.Most magmatic sapphires occur in alkaline basalts,which are of great value in the study of the deep earth processes.Magmatic sapphires are always founded in the continental margin of the western Pacific Ocean.Southeast Asia,located in this distribution zone,is the origin of good quality sapphire.The sapphire deposit in Huai Sai,Laos has a short history of mining time and lacks of concentration compared with the adjacent areas.Therefore,it is necessary to study the inclusions and geochemical characteristics of Hui Sai sapphire and establish a reasonable genetic model,which can also provide some reference for the future exploration of gemstone corundum.The sapphire researched occurs in deposits along the Mekong River,which has dark edges with high iron that produced by partial melting in host basalt.There are obvious two-stage crystallization process with interpenetration relationship in the interior,and phase II crystallization occurs along the crystalline direction of corundum after phase I crystallization growth ring is destroyed.Except for dark edge,the total iron content of sapphire from Hui Sai is0.4-0.9%,which is lower than other magmatic sapphires and lead to brighter color.Ti content is0.09-0.4%,which is obviously higher in stage II than in stage I,and a large number of syngenetic acicular ilmenite inclusions occur in this stage.The content of Si is 0.01-0.2%lower than that of sapphire from other areas?0.3%?and the content of alkali metal?K+Na?is 0.03-0.4%.This indicates that Huai Sai sapphire crystallizes in mafic magma.The contents of alkali metals,total Fe,Cr and Si are not significantly different in phase I and II crystals.In terms of the isotope characteristics of noble gases,the values of ⁴He vary in the range of?5.6-165.3?×10^-77 cc STP/g,the ratio of ³He/⁴He is 6.3 to 8.5 Ra,⁴⁰Ar/³⁶Ar is 488-1900,³⁸Ar/³⁶Ar is 0.152-0.197,⁴He/⁴⁰Ar^*is 6.9-16.4.The isotope compositions of the crystal edges and core are basically the same,indicating that the source of crystallization stages are all from mantle regions and with a small amount of crustal material involved.Phase II crystallization consists of titanium dioxide in the form of low-temperature anatase,a large amount of carbon dioxide two-phase inclusions and hematite crystallizable at low-temperature in alkaline magmatic water,indicating that the mother melt of phase II crystallization of Huai Sai sapphire may be carbonate melt derived from alkaline basalt.The U-Pb SIMS age of zircon mega-crystals founded in same deposit is 0.5-4.3Ma,?U-Th?/He ages is 0.9-4.2Ma,which are older than zircon inclusions in sapphire?1.2-1.3 Ma?and nearby basalts?1-2 Ma?,indicating that these zircon mega-crystals do not crystallize simultaneously in sapphire.The crystallization age of sapphire can be limited to 0.5-1.3Ma.Laos Huai Sai sapphire crystallizes in the position between upper mantle and lower crust.The basalt magma upwelling partially metasomatizes the crust and crystallizes sapphire for the phase I.The basalt magma fractionation produces new magma and phase II crystallization occurred.More alkaline basalt magma upwells rapidly,bringing sapphire to the surface and react with sapphire lead to the dark edges.