Abstract: Permeable subterranean underground formations and fractures can be sealed using lost circulating material fluids having borate and metallic crosslinked polysaccharide gels that are crosslink in the well bore. Compositions and methods are illustrated whereby lost circulation material solids can be mixed in a mixing tank on the surface with a crosslinkable polysaccharide and a crosslinking agent under conditions whereby the lost circulating material fluid can be pumped into the well bore before undergoing gelation. Once the lost circulation material fluid equilibrates to the reservoir temperature the polysaccharide component of the lost circulation material fluid will crosslink.