The lower to middle Miocene Popotosa Formation records the history of sedimentation in the tectonically active closed Socorro basin before its disruption by late(?) Miocene rifting. The Ladron and Lemitar Mountains contain about 1,600 m of volcaniclastic alluvial-fan, alluvial-flat, and playa sediments intercalated with as many as 12 ash beds. Possible ash sources include the Datil-Mogollon volcanic field on the south, the Socorro caldera within the Socorro basin, and more distant calderas.

Ash samples collected from measured sections progressively farther north of Socorro caldera show increasing lithium content. Those taken 9 km north of the caldera averages 47 ppm; 17 km north, 355 ppm; and 27 km north, 2,640 ppm lithium. Still farther north, however, lithium content drops to 150 ppm.

Lithium enrichment of an individual ash within a section is generally inversely proportional to the potassium content of that ash. Yet the average potassium content of all ashes within a section remains nearly constant for all three sections, each farther north of the caldera.

Close to Socorro caldera, welded ash-flow tuffs show enrichment of potassium ascribed to postdepositional hydrothermal alteration. Such alteration could provide the mechanism for increased lithium enrichment away from the caldera.

Beds of unaltered volcanic ash have enormous surface area exposed to a variety of migrating fluids. Laboratory experiments suggest that 50% of total lithium can be leached from fresh volcanic glass by hot alkaline solutions through ionic exchange. Such (hydrothermal) solutions would increase in lithium as they travelled away from their heat source (Socorro caldera) until precipitation by cooling occurred. The same ash beds that may have been leached in the area of hydrothermal alteration have been altered farther away to dioctahedral smectites, clinoptilolite, feldspar, and silica under the less rigorous conditions of near-surface temperatures and pH's consistent with those of playa brines. Those smectites may now hold the precipitated lithium as interlayer cations.

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