The Scene is set in the desert in a cave in the side of a cliff, the costumes are white robes like in roman times and a man is trying to get a woman to step into a fire with him claiming it will give them either youth or immortality. She declines and he steps in alone only to perish in the fire. The male actor had white hair and the female had long blond hair. If some can help please. This is driving me nuts not remembering. Thanks

I wonder if it might be 1968's Vengeance of She, more or less inspired by the works of H. Rider Haggard.

I looked up this movie on YouTube and you are on the right path. However in the movie I remember it is the Male character who tries to persuade the Female character to enter the fire with him and she declines.
So I am still looking, thank you for trying

A reviewer of this movie states: "When Carol arrives in the hidden city, Killkrates attempts to persuade Carol to join him in the sacred flame of immortality, but she refuses and Killkrates turns to dust in the fire as he was already immortal and the flame reversed the process."