'Mystery at Blind Frog Ranch' Duane Ollinger Battled Pneumonia Before Death

Duane Ollinger spent the final days of his life battling pneumonia in a Texas hospital ... TMZ has learned.

Sources with direct knowledge tell TMZ ... the star of Discovery Channel's "Mystery at Blind Frog Ranch" had been admitted to a hospital in Amarillo and was in the ICU when he died.

Sources say Duane had also been battling ALS, which was an underlying condition. However, we're told pneumonia is what ultimately led to his death.

While Duane's official cause of death has not yet been formally determined, family sources tell us doctors currently believe he died from respiratory arrest resulting from pneumonia.

We broke the story ... Duane died Tuesday at 68.

The Texas oilman became a fan favorite on the reality series, which follows Duane and his son Chad as they hunt for a legendary cache of lost Aztec gold believed to be hidden beneath their sprawling Utah property.

The show premiered in 2021 with Duane leading the charge to uncover the treasure he spent years chasing. Leadership of the hunt shifted to Josh Feldman at the start of the sixth season, which premiered last week, after Duane's health declined.