The origin of brucite in hydrothermally altered limestone near Devil Peak, Nevada

Rhonda Lee Knupp, University of Nevada, Las Vegas


Open-space brucite was identified in veins crosscutting hydrothermally altered limestone near the Devil Peak rhyolite plug in southern Nevada. The brucite occurs with serpentine, calcite, chalcedony, hydromagnesite, dolomite, and clinochlore; Brucite usually forms as a replacement mineral, but textural evidence indicates that the brucite at Devil Peak precipitated in open space. The presence of chalcedony in veins indicates the temperature of the hydrothermal fluid was {dollar}<{dollar}180{dollar}\sp\circ{dollar}C during and after mineral deposition. Thermodynamic modeling shows this temperature is too low for replacement brucite to form, thus the low temperature of alteration may be a factor in this unusual occurrence of brucite; Infiltration modeling of isotopically depleted wall rocks shows that the hydrothermal fluid was initially composed of a mixture of metamorphic and meteoric water, with X{dollar}\rm\sb{CO2}{dollar} = 0.15-0.16. Later, an influx of magmatic fluid or metamorphic water mixed with the early fluid. Fluid inclusion analysis shows {dollar}\rm X\sb{CO2} < 0.01{dollar} for the fluid mixture.