Uptake of Ammonium By Potassium-Bearing Silicates in Guaymas Basin Hydrothermal System, Gulf of California

Ann E. Sturdivant

Potassium (as K₂O) and fixed (lattice) and exchangeable (surface) ammonium (NH₄⁺) content of sediment samples from the Guaymas basin were measured to determine the relationship between poor fluid composition and the chemical compositions of potassium-bearing minerals from an active hydrothermal system.

In the Guaymas basin, NH₄⁺ is generated and concentrated in the pore fluid during thermal maturation and biological consumption of organic matter (up to 3.3% total organic carbon in the sediment). The NH₄⁺ is then available for ion substitution into lattice sites otherwise occupied by the potassium ion (K⁺) in clays.

The pore fluid NH₄⁺ content was found to correspond with either the fixed or exchangeable NH₄⁺ content in the sediment. Other factors such as total organic carbon content, clay mineralogy, and variable permeability may affect this relationship. Total NH₄⁺ content is as high as 1,779 ppm in the Guaymas basin sediment. No apparent relationship exists between pore fluid K^* and sediment K₂O content. No evidence has been found for a relationship between NH₄⁺ in the sediment and K⁺ in the pore fluid. This implies that a complex series of chemical reactions is occurring, and the pore fluid and the sediment may not be in equilibrium at this time.

Fluid-mineral relationships such as those in the Guaymas basin may be used to infer similar relationships between paleofluids and sediments from fossil hydrothermal systems. This information could be of economic importance because metallic mineral concentrations are often formed by hydrothermal activity, and NH₄⁺ anomalies could be used as an indicator of proximity to ore deposits.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.