WHY ARE THERE NO MAJOR Ni Cu SULFIDE DEPOSITS IN LARGE LAYERED MAFIC-ULTRAMAFIC INTRUSIONS?

Abstract

Large, layered intrusions throughout the world are characterized by a remarkable paucity in economically important magmatic Ni-Cu sulfide deposits. The Sudbury intrusion in Canada is a notable exception, but sulfide ores there were formed by a unique combination of processes involving crustal melting in response to a meteorite impact. Economically important magmatic Ni-Cu sulfide deposits tend to occur in magma conduit systems. Such an environment has been documented at Noril'sk, Jinchuan, and the recently discovered Uitkomst deposit of South Africa and the Voisey's Bay intrusion of Labrador, Canada, which are associated with the Bushveld Complex and the Nain Plutonic Suite, respectively. The enhanced ore potential of conduit systems is attributed to their specific flow environment and to the exploitation of the conduits by multiple flows of magma. Sulfides that were entrained and transported by ascending silicate magma were concentrated in the widened parts of the conduits and, at Voisey's Bay, near the exit of the conduit into a larger chamber, owing to a decrease in the flow velocity of the magma. Further, the precipitated sulfides were upgraded by means of reaction with continued surges of undepleted magma using the same conduit. Thus, exploration for magmatic Ni-Cu sulfide ores should be redirected from layered intrusions to their associated conduit systems.

Palaeoproterozoic porphyry Cu–Au, intrusion-hosted Au and ultramafic Cu–Ni deposits in the Fennoscandian Shield: Temporal constraints using U–Pb geochronology

“…The Älgliden intrusion is also the only mafic-ultramafic intrusion with a dyke-like geometry in the study area. According to Maier et al (2001), ideal conditions for sulphur upgrading processes occur in the conduits to larger magma chambers, in contrast to larger intrusive bodies where precipitated sulphides would be diluted by the large quantities of silicates.…”

Géologie et indices de Ni-Cu-ÉGP de la zone Frontier dans la ceinture de Cape Smith, Nouveau-Québec /

“…Une autre option possible serait celle d'un système très dynamique où le magma, à partir duquel les sulfures ont ségrégé préalablement, fut expulsé en dehors d'un conduit magmatique laissant derrière lui les sulfures. Ces sulfures auraient ensuite été entraînés par une seconde remontée de magma non contaminée (Maier et al, 2001). Le modèle d'érosion thermique à partir de coulées invasives utilisé pour expliquer la formation des gisements de Raglan pourrait s'appliquer à la zone Frontier mais seulement en partie.…”

Platinum-group element (PGE) deposits and occurrences: Mineralization styles, genetic concepts, and exploration criteria

“…(iv) Precipitation + concentration of the sulfide melt may occur in flow-dynamic traps, e.g., widened sections of the conduits/lava channels or exits of the conduits into larger magma chambers. This model explains why massive or semi-massive sulfide ores are rare in large layered intrusions (Naldrett, 1997;Maier et al, 2001a); The latter are relatively more static systems and an effective concentration mechanism for the sulfide melt is lacking. The Platreef of the Bushveld is a case in point.…”

“…These are not only associated with large layered intrusions that are targets for PGE-reef deposits, but also with dyke-and sill-like magma conduits feeding the lava flows. The magma conduits are prime targets for massive Ni-Cu sulfide deposits (Naldrett, 1997;Maier et al, 2001a), which may contain important quantities of PGE as a byproduct (e.g., NorilÕsk).…”