Microstructures, Formation Mechanisms, and Depth-Zoning of Phyllosilicates in Geothermally Altered Shales, Salton Sea, California1

Yau Yu-Chyi2, Donald R. Peacor2, Richard E. Beane2, 4, Eric J. Essene2 and S. Douglas McDowell3
2 Department of Geological Sciences, University of Michigan Ann Arbor, Michigan 48109
3 Department of Geology and Geological Engineering Michigan Technological University Houghton, Michigan 49931
1 Contribution No. 433 from the Mineralogical Laboratory, Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109.
4 Present address: St. Joe American Corporation, 2002 N. Forbes Blvd., Tucson, Arizona 85745.

Abstract: Scanning, transmission, and analytical electron microscopy studies of shales from the Salton Sea geothermal field revealed that phyllosilicates progress through zones of illite-muscovite (115°–220°C), chlorite (220°–310°C), and biotite (310°C) These phyllosilicates occur principally as discrete, euhedral to subhedral crystals which partly fill pore space. The structural and chemical heterogeneity, which is typical of phyllosilicates in shales subject to diagenesis, is generally absent. Textures and microstructures indicate that the mineral progression involves dissolution of detrital phases, mass transport through interconnecting pore space, and direct crystallization of phyllosilicates from solution.

Phyllosilicate stability relations indicate that either increase in temperature or changing ion concentrations in solutions with depth are capable of explaining the observed mineral zoning. Textural and compositional data suggest that the observed mineral assemblages and the interstitial fluids approach equilibrium relative to the original detrital suites. The alteration process may have occurred in a single, short-lived, episodic hydrothermal event in which the original detrital phases (smectite, etc.) reacted directly to precipitate illite, chlorite, or biotite at different temperatures (depths) without producing intermediate phases.

Key Words: Chlorite • Electron microscopy • Fluid-mineral equilibria • Geothermal field • Illite • Salton Sea • Smectite • Texture

Clays and Clay Minerals; February 1988 v. 36; no. 1; p. 1-10; DOI: 10.1346/CCMN.1988.0360101
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)