Hydrothermal Mineralogy of Calcareous Sandstones from the Colorado River Delta in the Cerro Prieto Geothermal System, Baja California, Mexico

Peter Schiffman1, 2, Dennis K. Bird3 and Wilfred A. Elders1
1 Institute of Geophysics and Planetary Physics, University of California, Riverside, California 92521 USA.
2 Department of Geology, University of California, Davis, California 95616 USA.
3 Department of Geology, Stanford University, Stanford, California 94305 USA.

Abstract: The Cerro Prieto geothermal system provides a unique opportunity for the detailed study of calc-silicate mineral transitions between the diagenetic clay-carbonate and greenschist facies within the terrigenous sediments of the Colorado River delta. In this system, progressive devolatization reactions within carbonate-cemented, quartzofeldspathic sediment have produced a distinct hydrothermal mineral zonation at temperatures between 200–370°C and fluid pressures below 0.3 kbar. Descriptive and compositional data are presented for these minerals which include wairakite, epidote, prehnite, actinolite, clinopyroxene, garnet, sphene, biotite, microcline, and calcite. Partitioning of octahedral Fe, Mg, and Al between coexisting authigenic silicates is comparable with data from higher temperature metamorphic rocks and demonstrates an approach to local equilibrium within this system. Calculated fugacities of oxygen at temperatures above 300°C are (with rare exception) more reducing than that defined by the quartz-fayalite-magnetite buffer, a result consistent with the scarcity of hematite and grandite and the ubiquitous presence of organic material in Cerro Prieto sandstones.

Keywords: sandstones • geothermal system • wairakite • epidote • prehnite • actinolite • clinopyroxene • biotite • sphene • garnet • microcline • Cerro Prieto • Mexico

Mineralogical Magazine; June 1985 v. 49; no. 352; p. 435-449; DOI: 10.1180/minmag.1985.049.352.14
© 1985, The Mineralogical Society
Mineralogical Society (www.minersoc.org)