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

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 NH 4 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 NH 4 + . Very high O analyses are probably caused by contamination with kaolinite. A representative structural formula for the tobelitic material is [(NH 4 ) 0.36 K 0.36 Na 0.03 ]-(Al 1.91 Mg 0.13 Fe 0.03 )(Si 3.21 Al 0.79 )O 10 (OH 1.88 F 0.12 ). 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 NH 4 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 NH 4 phyllosilicate veins in unoxidized shale and the limited occurrence of NH 4 phyllosilicates within the host shales suggests a hydrothermal source for the NH 4 .