Characterization of Hydrothermal Tobelitic Veins from Black Shale, Oquirrh Mountains, Utah

Paula N. Wilson, W. T. Parry and W. P. Nash
Department of Geology and Geophysics, 717 William Browning Building University of Utah, Salt Lake City, Utah 84112

Abstract: Hydrothermal tobelitic phyllosilicates modeled as ISII (R3) ordering with a minimum of 2–3% and a maximum of 6–8% interstratified smectite occur in veins and as replacement of fossils in hydrothermally altered black shale. These heavy metal-rich phyllosilicate veins formed during a Mesozoic-aged, regional-scale hydrothermal event that affected an area which encompasses the Mercur Au district (Wilson and Parry, 1990a, 1990b). Associated minerals include kaolinite, quartz, chlorite, Fe-oxides, I/S (R1, 45% smectite), and pyrite. N and O contents of NH4 phyllosilicates determined by microprobe analysis range from 0.19 to 1.78 and 48.6 to 52.9 elemental wt. %, respectively. Infrared absorption analysis indicates N occurs as NH4+. Very high O analyses are probably caused by contamination with kaolinite. A representative structural formula for the tobelitic material is [(NH4)0.36K0.36Na0.03]-(Al1.91Mg0.13Fe0.03)(Si3.21Al0.79)O10(OH1.88F0.12).

Correlation plots of data from microprobe analyses indicate an atypically high correlation between interlayer charge and octahedral layer charge and no correlation between (K+Na) and N. More typical correlations between N and (K+Na) and between interlayer charge and tetrahedral layer charge are obtained if 2–8% of a beidellitic smectite are factored out of the analyses. This amount of smectite is consistent with modeling of X-ray diffraction data using the computer program NEWMOD (Reynolds, 1985).

Possible sources of NH4 are from introduction by hydrothermal fluids or from thermal degradation of organic matter prevalent within the host rocks during low-grade metamorphism. The occurrence of NH4 phyllosilicate veins in unoxidized shale and the limited occurrence of NH4 phyllosilicates within the host shales suggests a hydrothermal source for the NH4.

Key Words: Ammonium illite • Hydrothermal • Microprobe • Nitrogen analysis • Tobelite

Clays and Clay Minerals; August 1992 v. 40; no. 4; p. 405-420; DOI: 10.1346/CCMN.1992.0400405
© 1992, The Clay Minerals Society
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