The Canadian Mineralogist
Volume 34, pages 485-502 (1996)
IMMISCIBILITY BETWEEN ARSENIDE AND SULFIDE MELTS: A MECHANISM FOR THE
CONCENTRATION OF NOBLE METALS
FERNANDO GERVILLA
Instituto Andaluz de Ciencias de la Tierra y Departamento de Mineralogia y Petrología, Facultad de Ciencias, C.S.I.C. - Universidad de Granada, 18002 Granada, Spain
MARC LEBLANC
Geofluides - Bassins - Eau, URA 1767 CNRS, Université de Montpellier II - Sciences et Techniques, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France
JOSÉ TORRES-RUIZ and PURIFICACIÓN FENOLL HACH-ALÍ
Instituto Andaluz de Ciencias de la Tierra y Departamento de Mineralogia y Petrología, Facultad de Ciencias, C.S.I.C. - Universidad de Granada, 18002 Granada, Spain
Abstract
Evidence of immiscibility between arsenide and sulfide melts can be observed locally in the magmatic ores associated with the Carratraca
(Serranía de Ronda, southern Spain) and Beni Bousera (northern Morocco) lherzolite massifs. They occur in the form of sulfide ores containing
arsenide globules. The globules are interpreted as droplets of an immiscible arsenide melt with a composition close to that
of maucherite - nickeline, suspended in the sulfide melt. Both arsenide melts depleted in As with respect to maucherite and those with
\As contents higher than nickeline tend to dissolve increasing amounts of Co, Fe and S. The above evidences support the genetic model for
the chromite - Ni arsenide ores and the sulfide - graphite ores described in the Serranía de Ronda and Beni Bousera lherzolite massifs.
The model considers that the early cyrstallization of chromite collected most Os, Ir and Ru, while gold and the
remaining PGE were partitioned into an immiscible 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 sulfide - graphite ores (0.18-0.42 ppm
PGE and 0.03-0.07 ppm Au), sulfide ores containing arsenide globules of aproximate maucherite composition display intermediate noble
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 noble metals in nature.