The system Fe-Zn-S was studied from 850 degrees C (appearance of sphalerite-wurtzite polytypes) to 580 degrees C (where reaction rates become very slow), with control of sulfur fugacity. Results show that temperatures estimated from the 'sphalerite geothermometer,' which was based on techniques in which sulfur fugacity was not controlled, may be in serious error. Where sulfur fugacity is buffered by pyrrhotite-pyrite, Fe-content of sphalerite changes from 13 mole percent at 742 degrees C to 19 mole percent at 580 degrees , but compositions cannot be extrapolated to lower temperatures. Other sulfide assemblages that define sulfur fugacity at various temperatures form a potential basis for quantitative geothermometry using Fe-content of sphalerite.

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