Agate-filled geode from the Eocene of Mexico. (Jeff Smith collection)
This agate nodule is from Mexico's famous Las Choyas Geode Deposit. At this locality, geodes occur in structurally-folded, rhyolitic volcanic tuffs (ash flow tuffs) of Middle to Late Eocene age (~35 to 44 Ma). The geodes were originally cavities in the rhyolitic rock. These cavities (lithophysae) formed before the rock completely lithified. The original ash flow deposit had some subspherical structures known as spherulites, composed of glassy to cryptocrystalline material (many felsic extrusive igneous rocks have these). Expanding gases in the spherulites destroyed the material, resulting in empty spaces. In the near-latest Eocene (~35 Ma), regional rhyolite dome intrusions resulted in hot groundwater percolating through the rocks, leaching out silica and precipitating quartz in the lithophysae/cavities.
About eighty percent of the geodes mined at this site are solid agate/quartz nodules.
Locality: Las Choyas Geode Deposit, northern Aldama County, north-central Chihuahua State, northern Mexico
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Info. synthesized from:
Keller (1977) - Quartz geodes from near the Sierra Gallego area, Chihuahua, Mexico. Mineralogical Record 10: 207-212.
Smith (2010) - The Las Choyas Geode Deposit, Chihuahua, Mexico. Rocks & Minerals 85: 112-122.
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