THE RISE AND FALL OF MAGMATIC NI-CU-PGE SULFIDES (Invited Presentation)

The preferential localization of magmatic Fe-Ni-Cu-(PGE) sulfide deposits within the horizontal components of dike-sill-lava complexes indicates that they were fluid dynamic traps for sulfide melts. Many authors have interpreted them to have collected fine sulfide droplets transported upwards from deeper “staging chambers”. Although fine (<1-2 cm) dilute (<10-15%) suspensions of dense (~4-5 g cm^-3) sulfide melt can be transported in ascending magmas, there are several problems with upward-transport models: 1) geological and/or S isotopic data are consistent with local crustal sources in almost all major deposits, 2) most xenoliths (if present) appear to be derived from more-or-less the same stratigraphic levels rather than deeper crustal sources, 3) lateral sheet flow/sill facies of major deposits contain few if any sulfides, 4) sulfide and/or chalcophile element enrichments rarely (if ever) occur in the volcanic components of magmatic systems – even where they overlie abundant mineralization in underlying feeder dikes/sills/chonoliths – except where there is evidence for a local S source, and 6) some overlying lavas are mildly to strongly depleted in PGE >>> Cu > Ni > Co, indicating that unerupted sulfides sequestered PGEs at depth. Two potential solutions to this paradox are that 1) natural systems contained surfactants that lowered sulfide-silicate interfacial tensions, permitting sulfide melts to coalesce and settle more efficiently than predicted from theoretical/experimental studies of artificial/analog systems, and/or 2) sulfides existed not as uniformly dispersed droplets, as normally assumed, but as fluid-dynamically coherent pseudoslugs or pseudolayers that were large and dense enough that they could not be transported upwards. Regardless of the ultimate explanation, it appears that most high-grade Ni-Cu-(PGE) sulfides formed at or above the same stratigraphic levels as they are found. Sulfides that do not appear to have formed locally were likely transported horizontally or formed at higher levels and settled backwards into the same fluid dynamic traps that have been proposed to have trapped sulfides transported from putative “staging chambers.”