Magmatic sulfide deposits fall into two major groups when considered on
the basis of the value of their contained metals, one group in which Ni,
and, to a lesser extent, Cu, are the most valuable products and a second
in which the PGE are the most important.
The first group includes komatiite- (both Archean and Paleoproterozoic),
flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a
miscellaneous group related to high Mg basalts, Sudbury, which is the only
example related to a meteorite impact melt, and a group of hitherto
uneconomic deposits related to Ural-Alaskan–type intrusions.Most Ni-rich
deposits occur in rocks ranging from the Late Archean to the Mesozoic
PGE deposits are mostly related to large intrusions comprising both an
early MgO- and SiO2-rich magma and a later Al2O3-rich, tholeiitic magma,
although several other intrusive types contain PGE in lesser, mostly
PGE deposits tend to predominate in Late Archean to Paleoproterozoic
intrusions, although the limited number of occurrences casts doubt on the
statistical validity of this observation.
A number of key events mark the development of a magmatic sulfide deposit,
partial melting of the mantle, ascent into the crust, development of
sulfide immisciblity as a result of crustal interaction, ascent of magma +
sulfides to higher crustal levels, concentration of the sulfides, their
enrichment through interaction with fresh magma (not always the case),
cooling and crystallization.
Factors governing this development include
the solubility of sulfur in silicate melts and how this varies as a
function of partial mantle melting and subsequent fractional
the partitioning of chalcophile metals between sulfide and silicate
liquids, and how the results of this vary during mantle melting and
subsequent crystallization and sulfide immiscibility (degree of
melting and crystallization, R factor and subsequent enrichment),
how effectively the sulfides become concentrated and the factors
controlling this, and
processes that occur during the cooling of the sulfide liquid that
govern aspects of exploration and mineral beneficiation.
Magmatic Base Metals Deposits
Chrome - Nickel/Copper - Platinum Group Metals (PGM)
Magmatic Deposits are so named because they are genetically linked with
the evolution of magmas emplaced into the crust (either continental or
oceanic) and are spatially found within rock types derived from the
crystallization of such magmas. The most important magmatic deposits are
restricted to mafia and ultramafic rocks which represent the
crystallization products of basaltic or ultramafic liquids. These deposit
Chromite deposits are the end product of the separation of solid
phases (Cr-rich spinets, (Fe, Mg) (Al, Cr. Fe) 2O4) from a liquid and
their accumulation into chromite-rich layers. The processes involved in
the formation of chromite layers are fractional crystallization and
gravity settling. Chromite crystallizes into mineral grains within the
silicate liquid and, because they are heavier than the liquid, they sink
to form a cummulate layer at the base of the intrusive.
There are two main types of chromite deposits:
Stratiform chromite deposits consist of laterally persistent chromite-rich
layers (a few mm to several m thick) alternating with silicate layers. The
silicate layers include ultramafic and mafic rocks such as dunite,
peridotite, pyroxenite and a variety of others, less commonly gabbroic
rocks. They are generally found within basal portions of mafic-ultramafic
layered intrusions of Archean age such as the Bushveld Complex in South
Africa. Canadian examples of stratiform chromite deposits include the Bird
River Sill deposits in Manitoba. These deposits contain substantial
reserves of poor-quality chromite (average 10.7% Cr203). Podiform
chromite deposits consist of pod to pencil-like, irregularly shaped
massive chromite bodies and they are predominantly found within dunitic
(olivine-rich) portions of ophiolite complexes. The rocks associated with
podiform chromites are generally referred to as "Alpine-type" peridotites
and they are usually found along major fault zones within mountain belts.
Ni-Cu Deposits are the end of a magmatic process known as "liquid
immiscibility''. This process involves the separation from the parental
magma of a sulphur-rich liquid containing Fe-Ni-Cu. Upon cooling, the
sulphur-rich liquid produces an immiscible sulphide phase (droplets of
sulphide liquid in silicate liquid, like oil in water) from which minerals
such as pyrrhotite (FeS), pentlandite (Fe,Ni)9S8, and chalcopyrite
(CuFeS2) crystalize. Typical magmatic Ni-Cu deposits tend to occur in
embayments at or near the base of their intrusive hosts.
They occur at the base of the intrusives because
immiscible sulphide liquids are heavier than silicate liquids and
therefore sink to the bottom of the magma chamber and
b) without the presence of sulphur, metals such as Ni become
incorporated into silicate crystal structures, such as pyroxene. Ni-Cu
deposits are found in layered intrusions, stocks and ultramafic sills
and flows. The largest deposits are of Archean and Proterozoic age.
The ore in Ni-Cu deposits can be massive, net-textured or disseminated.
Typical examples of Ni-Cu deposits include the Sudbury orebodies (layered
intrusion hosted), the Kambalda orebodies in Australia (ultramafic flow
hosted) and the orebodies in the Thompson District of Manitoba (ultramafic
3. PGM Deposits
Platinum Group Metals (Platinum, Pt; Palladium, Pd; Iridium, Ir;
Rhodium, Rh; Osmium, Os; and Ruthenium, Ru) have genetic affinities to
both Ni-Cu-sulphides and chromites. However, while the fundamental
processes involved in the formation of Ni-Cu and chromite deposits are
relatively simple, the concentration and deposition of PGM appears to be a
not too well understood, diverse sud multistage process.
Several lines of evidence indicate that PGM can
concentrate during high-temperature deposition of chromites,
be incorporated into immiscible liquids,
be remobilized and reconcentrated during metasomatic and
To date significant PGM production has come from:
The Merenski Reef of the Bushveld Complex in South Africa,
.The Ni-Cu deposits of the Noril'sk-Talnakh District in the
By-product of several Ni-Cu deposits (Sudbury, etc.),
Placers derived from zoned (Alaskan-type) uyltramafic intrusions
(Columbia, Goodnews Bay, Tulameen),
.Metasomatic dunite pipes of the Bushveld Complex. The bulk of
present world production comes from the Bushveld and Russian deposits
and most presently known reserves are within Merenski-type
environments (Bushveld and Stillwater Complexes).
The ores of the "Merenski" reef form thin (less than 1 m) but
laterally persistent, disseminated, sulphide-poor horizons within
polycyclic mafic-ultramafic cumulate sequences one-third of the way up
from the base of the Bushveld intrusion. Principal ore minerals are
pyrrhotite, chalcopyrite, pentlandite, PGM sulphides, arsenides and
tellurides. The Noril'sk-Talnakh orebodies are essentially typical Ni-Cu
deposits containing anomalously high concentrations of PGM (6 g/tonne
They occur at or near the base of complexely differentiated
gabbro-dolerite intrusions (50 to 350 m thick) emplaced during late
Permian to Triassic time during rifting of the Siberian platform. The
sills are considered to be feeders to overlying plateau basalts.
The mineralogy of the ores include pyrrhotite, chalcopyrite, pentlandite
and a great variety of PGM minerals. Placers derived fromAlaskan-type
intrusions are the results of the breakdown, transport and concentration
of Pt-Fe alloys mainly associated with Fe-rich chromite layers from the
dunitic portions of thse complexes.
The metasomatic dunite pipes of the Bushveld Complex played a significant
role as high-grade platinum producers during the early days of platinum
mining in South Africa. They consist of central zones of Fe-rich dunite
enveloped by shells of dunite and pyroxenite.
The pipes which transect at right angles the critical zone of the complex,
are 20 to 200 m in diameter and contain axially located pay zones with
surface dimensions not exceeding 20x25 m. The ores are pegmatitic and may
contain slabs of chromitite. Spot assays as high as 1,990 grams per tonne
Pt. were recorded from dunite pipes.
Before concluding this brief summary on PGM deposits, it should be
stressed that most disseminated sulphide zones carrying appreciable PGM
values are characterized by:
the presence of hydrous minerals within otherwise anhydrous layered
successions. These features,which point to high fluid activity during
magmatic segregation, are important prospecting guides.
Exploration (Prospecting) Guidelines
1.Identify well layered mafic-ultramafic intrusions;
2.Prospect below the mafic cumulate portions of the intrusions (i.e. below
the portion which is completely gabbroic).;Podiform:& 1.Carefully
prospect within all dunitic portions of Alpine-type peridotites
(Harzburgite-Dunite components of ophiolite complexes).
1.Prospect in the lowermost portion of layered and not so well layered
mefic-ultramafic intrusions (both cratonic and synorogenic), komatiitic
flows and sills;
2.Pay special attention to embayments in basal contacts.
1.Identify layered mafic-ultramafic intrusions and differentiated sills
2.Sample any sulphide-bearing material, especially if carrying visible
3.If prospecting for Merenski-type occurrences, look for very thin (1 m)
but laterally persistent disseminated sulphide-bearing horizons within
complexly interlayered peridotite-pyroxenite-troctolite-anorthosite and
4.Look for sulphide-bearing material near contact zones of
mafic-ultramafic complexes composed of several intrusive phases;
5.Look for unusual textures and mineralogy. Namely look for pegmatitic
textures and development of hydrous minerals within layers or massive
units that are normally of even grain size and anhydrous;
6.Investigate the drainage of Alaskan-type intrustions for potentially
significant Pt placers.