ABSTRACT

Considerable quantities of sedimentary analcime have been found in the rocks of the Chubutian Group (Senonian) of the anticline of the Senguerr River (Chubut, Argentina). These analcime-bearing rocks are pyroclastic, mostly tuffs that vary in grain size from claystones to conglomerates, sandstones being the most frequent type. Analcime occurs as replacement of the tuffaceous matrix, but also as crystals, fillings of cavities, veinlets, cement etc.

Analcime distribution is concentrated in the lower or Castillo Formation, part of the Chubutian Group. The mineral is authigenic and its origin is closely related to environmental and climatic factors (lacustrine or paludal conditions under arid or semiarid climate); volcanic ashes, or their alteration products, seem to be essential in analcime formation.

Sedimentary analcime may be more abundant in continental pyroclastic deposits than has hitherto been thought, since it can easily be mistaken for opal. Some partial references in the Argentine geological literature seem to point in this direction.