The base metal sulfide deposits of northern New Brunswick, [Canada], were studied with special emphasis on temperature determinations made with the sphalerite geothermometer. The Fe content of the sphalerite was determined by X-ray fluorescence analysis of 0.25 mg. samples drilled from the surface of the polished sections. Appreciable differences in the Fe content of adjacent areas of sphalerite are attributed to varying amounts of Fe picked up by the sphalerite from the replaced minerals. Exsolution of pyrrhotite was found to be the only reliable evidence that sphalerite formed in the presence of excess Fe. The temperature of formation of the Heath Steele and Brunswick mines is given as 610 degrees C. at 2,000 bars, equivalent to burial under 15,000 ft. of younger rocks.