A safety valve is configured for use at temperatures at or above 760° C. (1400° F.). The safety valve includes a closure assembly that can create a self-energizing, metal-to-metal seal. In one implementation, this closure assembly includes a disc with an arcuate finger that circumscribes an axis of fluid flow through a seat. The arcuate finger may extend inwardly toward this axis and downwardly toward the seat. This geometry permits the finger to flex in response to pressure of fluid that impinges on the downstream side of the disc, such flexure causing a first sealing surface on the disc to more forcefully contact a second sealing surface on a seat.