Abstract

Biotite from quartz monzonite to granodiorite stocks in the Basin and Range province of the western United States has been analyzed for Fe, Mg, Ti, Ba, Cl, and F by electron microprobe methods. Analyses are of biotite from stocks which host the porphyry copper deposits at Santa Rita, Bingham, and Ely; from the Hanover-Fierro, Last Chance, and Weary Flat stocks which are in close proximity to the stocks hosting the porphyry copper mineralization at Santa Rita, Bingham, and Ely, respectively; from stocks spatially related to the mineralization at Park City-Little Cottonwood, Gold Hill and Tintic, Utah, and at Texas Canyon, Arizona; and from stocks which have little or no known mineralization in the Mineral Range, Utah, and the Quitman Mountains and Marble Canyon, Texas. Three distinct types of biotite have been analyzed: magmatic biotite, replacement biotite (that which has replaced hornblende or resulted from the recrystallization of magmatic biotite), and hydrothermal biotite which occurs in veins or is pervasively disseminated.Hydrothermal biotite from the Santa Rita porphyry copper deposit is more magnesian than magmatic or replacement biotite from the same stock and contains less Ti and Ba and more F. Hydrothermal biotite from Bingham and Ely is also magnesium rich. Magmatic biotite from the Santa Rita stock cannot be distinguished chemically from the magmatic biotite of the nearby Hanover-Fierro stock or from magmatic biotite of stocks in other categories. The systematic variation in biotite composition from magmatic to replacement to hydrothermal types documented at Santa Rita might be characteristic of the environment of porphyry copper deposits.Temperatures of biotite crystallization determined from ferrous iron, ferric iron, and magnesium using the method of Beane (1974) are 380 degrees C for hydrothermal biotite at Santa Rita and 600 degrees C for hydrothermal biotite from Bingham. Distribution of fluorine between hydrothermal biotite and hydrothermal apatite from Santa Rita gives temperatures of 350 degrees to 400 degrees C using the method of Stormer and Carmichael (1971).

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