Mid-Tertiary tectonic extension produced basins with lacustrine sediments bearing borates, zeolites, gypsum, and detrital gold in northern Sonora. Colemanite deposits associated with howlite, gypsum, and celestite are present in the Magdalena and Tubutama basins. In the Magdalena basin, the La Tinaja del Oso deposit contains at least two generations of colemanite, the first replacing stratiform and probably syngenetie ulexite, and the second, more voluminous, of clear epigenetic origin. Fluid inclusions in epigenetic colemanite indicate deposition from very 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 delta D values of -35 to -20 per mil, indicating that water in the hydrothermal fluid was of meteoric origin but was not 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 has a delta 34 S value of -32.9, consistent with formation of sulfide by bacterial reduction of sulfate. Borate was initially deposited as ulexite, then replaced by colemanitc 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 upper plates of metamorphic core complexes. We formally propose the names Magdalena, Cuesta, El Torre?n, and Tubutama Formations for mid-Tertiary sedimentary and volcanic rocks of the upper plates in the Magdalena and Tubutama basins. The name Baucarit Formation should be restricted to the younger sedimentary rocks overlying these formations.

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