Three-Dimensional Oxygen Isotope Imaging of Convective Fluid Flow around the Big Bonanza, Comstock Lode Mining District, Nevada

摘要

Oxygen isotope analyses of propylitized andesitesfrom the Con Virginia and California mines allow constructionof a detailed, three-dimensional image of the isotopic surfacesproduced by the convective fluid flows that deposited thefamous Big Bonanza orebody. On a set of intersecting maps andsections, the #delta#~(18)O isopleths clearly show the intricateand conformable relationship of the orebody to a deep, ~500 mgyre of meteoric-hydrothermal fluid that circulated along andabove the Comstock fault, near the contact of the DavidsonGranodiorite. The core of this gyre (#delta#~(18)O valence 0 to3.8 per thousand) encompasses the bonanza and is almost totallysurrounded by rocks having much lower #delta#~(18)O values (-1.0 to 4.4 per thousand). This deep gyre may represent aconvective longitudinal roll superimposed on a large unicellularmeteoric-hydrothermal system, producing a complex flow fieldwith both radial and longitudinal components that is consistentwith experimentally observed patterns of fluid convection in permeable media.