REDEFINING THE PARADIGM FOR TRONA FORMATION: A RECENTLY VALIDATED HYPOTHESIS

Trona [Na₃(CO₃)(HCO₃)·2(H₂O)] is an evaporate mineral that is used to produce soda ash (which is used in glass manufacturing, the chemical industry, and for the production of baking soda). Historically, trona was thought to form by closed-basin evaporation of waters rich in Na⁺ and HCO₃^‑ derived from silicate hydrolysis of volcanic rocks or volcaniclastic sediment.

A basin-scale hydrogeologic investigation in southeastern Arizona and northern Sonora (Mexico) revealed water chemistry distinct from the surrounding basins. Based on a suggestion by Fred Phillips to examine the mineral suites that would result from evaporation of the water from the different basins, it was determined that waters from the basin with distinct water chemistry would yield trona upon evaporation, but none of the neighboring basins would produce trona, despite the presence of similar Na⁺-rich volcanic rocks in all basins.

A new model for trona formation was developed, which involved injection of ‘excess’ CO₂; interestingly, the development of this model was made possible by an obscure piece of data collected for reasons that had nothing to do with water chemistry or evaporate mineral formation. The new model was tested by examining the potential for trona formation from waters in the Owens River (CA) tributary system. Model results suggested that trona deposition in Searles Lake resulted not from lithology alone, but also from excess CO₂ of magmatic origin. Recent work by Lowenstein et al. (2015, 2016) using dated core material and fluid inclusions supports a causal relationship between magmatic activity/CO₂ injection and the formation of trona at Searles Lake. Based on this validation, the idea that ‘excess’ CO₂ is required to drive formation of trona can be used to better understand the formation of known deposits, and to facilitate exploration for new deposits.