A fluid inclusion investigation of ore formation in a tin-mineralized granite, New England, New South Wales

CSIRO Div. Mineral., North Ryde, N.S.W., Australia

Primary fluid inclusions have filling temperatures from 550 degrees to more than 620 degrees C and salt contents of 54 to 65 wt percent. Daughter crystals in multiphase inclusions were mostly halite and other chlorides. Above a temperature of 400 degrees C the hydrothermal solutions were two phase. Fluid inclusion measurements and chemical analyses indicate two stages of hydrothermal activity. In stage one, in which heat was dispersed but not dissolved salts, the temperature decreased by 200 degrees C while the salt content increased. In stage two, dissolved salts were dispersed more rapidly than heat, and fluid inclusion evidence indicates a decreasing incidence of vapor in the hydrothermal system. Formation of major fractures occurred at the transition from stage one to stage two of hydrothermal activity. Fracturing accelerated condensation of vapor and mixing of magmatic brine and ground water. Most of the tin and tungsten ores were precipitated during the decrease in salt content of the concentrated brine.--Modified journal abstract.

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