GENETIC IMPLICATIONS OF SOME TRACE ELEMENTS IN CHALCOPYRITE

The trace element content of chalcopyrite is highly varied. Chalcopyrite samples representing hypogene mineralization from porphyry copper systems on islands, island arcs and cratons have been analyzed for a variety of precious and other trace elements. Variations in compositions from a suite of 40 samples representing 17 locations reveal some patterns and habits of association that can be related to geologic regions of occurrence.

Silver to gold ratios of chalcopyrite from southwestern North America reveal regional separation at a ratio of 17.5:1 related to basement, which correspond with ratios reported from production of chalcocite-dominated ores. Platinum-Palladium content in North American chalcopyrite reveals restricted regional distribution. An example in northwestern Arizona is that of elevated Pt-Pd (1635ppb) at Baghdad, U.S.A. There, platinoid-bearing chalcopyrite also contains elevated Ni and Cr and is associated with Laramide plutons that have intruded a Proterozoic terrane comprising schist, granite and a gabbro-anorthosite layered mafic body.

Gold is generally greater than 5ppm in chalcopyrite from deposits of islands and island arcs and in alkalic intrusions; high Au content (> 1ppm) from deposits of both islands and cratons is associated with high Se (>100ppm). High Ag (>75ppm) in chalcopyrite from cratonic terranes is commonly associated with elevated values of Te (50-100ppm). Bornites from three Southwestern North American deposits, Ajo, Mission, and Bisbee, contain high values of both Au (~4ppm) and Ag (>250ppm) together with elevated Tl, Se, and Te

The consistency and regional constraint of differences of precious metal composition of primary copper minerals together with constraint of some related contrasts in trace elements prompts fundamental questions. These concern the possible relationships of likely metal source regions as they relate to metallogenesis of deposits, to ore-forming processes and to effects of primary zoning. Augmentation of the sample base and its continued analysis and interpretation, based at the first level on regional settings, will continue to contribute to our knowledge and understanding of the source and process of metal concentration in porphyry systems.