Abstract: It is known that the volcanic rocks of the Yingcheng Formation were altered into bentonite in the Songliao Basin, and the major bentonite deposits in the area are located in Jiutai City. These deposits are in unconformable contact with the underlying coal-bearing strata of the Shahezi Formation. Based on studying on the lithology of volcanic rocks, lithological sequence, microfeature and X-inflection, etc., bentonite in the area could be divided into the low-zeolite and high-zeolite types；pearilte, liparitic (vitreous) ash-latite with breccias and vitreous volcaniclastic rock serve as the major mineralized rocks；The fluids and their passageways were the major factors that determined if alteration happened. With increased alteration from the mineralized rocks into bentonite, their alteration of the rocks became more intensive, and hardness more weak, color more white and gradually disappearing of the lithofabric features; The thickness and continuity of the orebodies are strongly affected by volcanic facies. Generally, the best zones of bentonite are inner subfacies and middle subfacies of extrusive facies and air-fall subfacies and thermo-offal fluid subfacies of explosive facies. Besides, the erupted facies of vitreous liparite also makes up a major zone for the bentonite.

Key words: Jiutai City, volcanic rock of the Yingcheng Formation, alteration, bentonite