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

摘要

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 u3c180oC 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 XCO2 = 0.15-0.16. Later, an influx of magmatic fluid or metamorphic water mixed with the early fluid. Fluid inclusion analysis shows XCO2 u3c 0.01 for the fluid mixture.