Geothermal waters in the Shiquanhe-Yarlun Zangbo geothermal field of southern Tibet are highly enriched in rare alkali metals (RAMs), which form a globally unique geyserite Cs-deposit. Boron isotopes, together with the geochemistry of a local leucogranite, geothermal springs and geyserite were investigated to constrain the migration and accumulation of RAMs in the geyserite cesium deposit. The delta B-11 values of -9.7 parts per thousand to -8.4 parts per thousand in tourmalines from the Himalayan leucogranite have shown typical S-type granites. A mass balance calculation indicates that the magma of the S-type granite contained < 10 % dehydrated fluid, which resulted in an enrichment of RAMs by 4-5 times during the partial melting of crustal materials, compared to that in the UCC. The linear relationships of Sigma RAMs vs. B and Na + K vs. soluble Si, indicate that further enrichment of RAMs in the geothermal fluids was promoted by intense water-rock interaction at temperatures of 210 to 250 degrees C in the geothermal system reservoir. A strong adsorption process controlled the hyper enrichment of RAMs (especially Cs) during silica precipitation from the upwelling geothermal water, with the sharp decreases in temperature and pressure leading to the enrichment of Cs in fresh geyserites. Despite loss of RAMs during dehydration of the geyserite during phase transformation from opal-A, opal-CT, and chalcedony to quartz, freshly deposited geyserites have great potential for exploration of RAMs resources in this area.