How did Zhang Wuji die in Eternal Dragon Slayer?

At the end of the book, it is only said that Zhang Wuji put down the throne and went to the Mongolian prairie with Zhao Min, but it does not describe how Zhang Wuji died.

Zhang Wuji and Zhao Min both retired at the end of "Eternal Dragon Slayer", so he should have died of natural causes, because the place of Ice and Fire Island is hard to find, so Zhang Wuji can live safely here until the last day of his life.

Character setting

Zhang Wuji, the hero of Jin Yong's martial arts novel "Eternal Dragon Slayer", is the 34th generation leader of Zoroastrianism in Middle-earth. Zhang Cuishan, one of the seven chivalrous men in Wudang, is the son of Yin Susu, the master of the Heavenly Hawk Wei Zitang, and the adopted son of Xie Xun, the Golden Lion King of Wang Zhiyi, the four guardians of Zoroastrianism.

I've lived a primitive life in Binghuo Island since I was born. I lost my parents when I set foot in Central China, and mysterious gods were dying in cold poison. Later, I studied medicine in Butterfly Valley and sent orphans to the Western Regions, where I was saved.

Suffering from seven years of cold poison, God has combined the four great martial arts of Jiuyang, the great move of Gankun, Taiji Chuan (and Taiji Sword) and the sacred fire, making it invincible in the world. In addition, he also studied medicine and poison, and his skills were unparalleled. Kind and brave, saving countless lives, often subduing the enemy without killing people. Strength disparity, once a few to dozens of top players decisive battle.