IMMISCIBILITY BETWEEN ARSENIDE AND SULFIDE MELTS - A MECHANISM FOR THE CONCENTRATION OF NOBLE-METALS

Evidence of immiscibility between arsenide and sulfide melts can be ob served locally in the magmatic ores associated with the Carratraca (Se rrania de Ronda, southern Spain) and Beni Bousera (northern Morocco) l herzolite massifs. They occur in the form of Fe-Ni-Cu sulfide ores con taining arsenide globules. The globules are interpreted as droplets of an immiscible arsenide melt with a composition close to that of mauch erite - nickeline, suspended in the sulfide melt. Both arsenide melts depleted in As with respect to maucherite and those with As contents h igher than nickeline tend to dissolve increasing amounts of Co, Fe and S. The above lines of evidence support the genetic model for the chro mite - Ni arsenide ores and the sulfide - graphite ores described in t he Serrania de Ronda and Beni Bousera lherzolite massifs. The model co nsiders that the early cyrstallization of chromite collected most Os, Ir and Ru, while gold and the remaining PGE were partitioned into an i mmiscible arsenide melt leaving a sulfide melt depleted in PGE and Au. Thus, although chromite - Ni arsenide ores show higher PGE (0.9-5 ppm ) and Au (3-18 ppm) contents than Fe-Ni-Cu sulfide - graphite ores (0. 18-0.42 ppm PGE and 0.03-0.07 ppm Au), sulfide ores containing arsenid e globules of aproximate maucherite composition display intermediate n oble metals abundances (0.6-1.07 ppm PGE and 0.07-0.45 ppm Au). These data suggest that PGE and Au have a higher affinity for arsenide melts than for sulfide melts, remarking their importance as collectors of n oble metals in nature.