BORATE DEPOSITS OF NORTHERN SONORA, MEXICO - STRATIGRAPHY, TECTONICS,STABLE ISOTOPES, AND FLUID INCLUSIONS

Mid-Tertiary tectonic extension produced basins with lacustrine sedime nts bearing berates, zeolites, gypsum, and detrital gold in northern S onora. Colemanite deposits associated with howlite, gypsum, and celest ite are present in the Magdalena and Tubutama basins. In the Magdalena basin, the La Tinaja del Oso deposit contains at least two generation s of colemanite, the first replacing stratiform and probably syngeneti c ulexite, and the second, more voluminous, of clear epigenetic origin . Fluid inclusions in epigenetic colemanite indicate deposition from v ery low-salinity hydrothermal fluid at temperatures up to 180 degrees C, mainly 100 degrees to 140 degrees C. Water of crystallization from epigenetic colemanite has SD values of -35 to -20 per mil, indicating that water in the hydrothermal fluid was of meteoric origin but was no t lacustrine brine because of the low salinity. The delta(34)S values of gypsum associated with colemanite, 4.1 to 10.6 per mil, are typical of sulfate in extensional basins of the region. A realgar specimen ha s a delta(34)S value of -32.9, consistent with formation of sulfide by bacterial reduction of sulfate. Berate was initially deposited as ule xite, then replaced by colemanite after burial. Epigenetic colemanite formed after lacustrine brine had disappeared, probably by circulating meteoric water heated as a result of high thermal gradients in the up per plates of metamorphic core complexes. We formally propose the name s Magdalena, Cuesta, El Torreon, and Tubutama Formations for mid-Terti ary sedimentary and volcanic rocks of the upper plates in the Magdalen a and Tubutama basins. The name Baucarit Formation should be restricte d to the younger sedimentary rocks overlying these formations.