Spectrographic studies of the minor element content of over 200 samples of pyrite in gold bearing quartz vein and replacement deposits in Quebec and Ontario have been made to determine, if possible, any distinctive trends with depth, and to acquire basic geochemical data on pyrite in such occurrences. The deposits included the rather simple quartz vein of the Powell Rouyn gold mine, Quebec, opened to the 2,450 level, the flow-type ore of the Kerr Addison mine, Ontario, extending to the 2,500-foot level, seven mines of the Kirkland Lake gold belt, Ontario, covering a length of about three miles and a depth of over 5,000 feet, and one of the major deposits in the Porcupine gold district, Ontario, to a depth of 5,375 feet.The studies included conventional direct current arc methods using different techniques, but standardized conditions for samples from individual deposits. Results are presented in log intensity ratios of minor element line vs suitable iron lines as internal standards. Quantitative methods were used for the determination of percentages of cobalt and nickel.Aside from the variable of depth of occurrence, other factors were also investigated such as size of grain, host rock or matrix, temperature readings as established electrically by Dr. F. G. Smith, University of Toronto, as well as occurrence in different ore bodies or veins of one mine, or in different mines of the one mineralized belt. In the Porcupine area, relation of composition to proximity to, or distance from quartz porphyry contacts was also studied. Trace elements studied included Au, Ag, Te, Ni, Co, Pb, Zn, Sn, Mo, Mn, Cr, Ti, V, Bi, and As. Some characteristic differences in trace elements in the different deposits are noted, as are variations which may be related to the several factors studied.Cobalt and nickel both range from 0.02 to 0.10 percent, and in most the ratio of Co: Ni is about 1:2. Kirkland Lake pyrite contains more of both than the others while pyrite from Porcupine, giving only low temperature readings, contains less.The only persistent variation in composition of pyrite with depth is an increase in cobalt, and, though locally other variations are of interest, the relative uniformity of conditions of mineralization to depths of over 5,000 feet is emphasized.Spectrographic studies of pyrite giving high and low temperature effects by methods employed by Smith, confirm differences in conditions of deposition, particularly by their content of cobalt, nickel, and arsenic. All types of pyrite of these deposits are believed to have been deposited with at least trace amounts of gold, and to have been enriched by a later introduction. Such later introductions cannot be related clearly to low-temperature readings, but are believed controlled to a greater extent by original and imposed structural factors rather than original temperature of deposition of the pyrite.