PRACHI KAR

ORE DEPOSITS OF PLATINUM GROUP ELEMENTS (PGE)

● Ore forming processes that are related to evolution of magmas emplaced at crustal level have
two end members –
i. Orthomagmatic processes: Concentration of ore minerals as a direct consequence of
magmatic crystallization, with some modification by post-cumulus processes. It is
dominated by silicate melt-crystal equilibria.
ii. (Magmatic) Hydrothermal processes: Concentration of ore minerals from magmatic
hydrothermal fluids (derived from pre-existing cumulate sequence) by crystallization. It is
dominated by crystal-volatile equilibria.
● Ultramafic-mafic rocks that host orthomagmatic ore deposits are of three types.
1. Synvolcanic bodies (excluding intracratonic volcanics):
a. Komatiitic suite: (chromite, Ni-sulfide & PGE deposits)
i. Lava flows
ii. Layered sills including dunite-peridotite lenses
b. Tholeiitic suite: (Ni-sulfide, Ti-V-magnetite deposits)
i. Layered picritic intrusions
ii. Anorthosite bodies
2. Intrusions in cratonic areas:
a. Intrusions related to flood basalts (Ni-Cu-sulfide, PGE and Ti-V-magnetite deposits)
b. Small to large mafic layered intrusions (chromite, PGE, Ni-Cu-sulfide and
Ti-V-magnetite deposits)
3. Bodies related to orogenesis:
a. Synorogenic intrusions (Ni-sulfide)
b. Tectonically emplaced Alpine-type bodies including ophiolite complexes (chromite,
Cu-sulfide, PGE)
c. Alaskan type zoned complexes (chromite, PGE, Ni-Cu-sulfide)

PGE: PLATINUM GROUP ELEMENTS

● Anomalous concentrations of PGE are known from high-temperature magmatic to low
temperature hydrothermal and sedimentary environments but significant concentrations of
PGE are virtually restricted to sulfides in ultramafic rocks.
● The PGEs together with Au constitute a coherent group of siderophile and chalcophile
elements having a strong repulsion to form oxygenated compound - the so-called ‘noble’
character.
● Within the range of oxygen and sulfur fugacities prevalent in the crust and upper mantle, the
PGEs commonly exhibit chalcophile behavior.
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● The PGEs are much important because –

i. They have distinctive geochemical behaviour that helps to reconstruct some evolutionary
aspects of the Earth.
ii. They have a great economical relevance and represent an important target in ore exploration.

PLATINUM GROUP ELEMENTS: GENERAL ASPECTS

● Geochemistry of the PGEs helps to understand the chemical evolution of the Earth’s mantle.

● The PGEs can be subdivided in two groups with contrasting characters:

a. Ir-PGE, containing Os, Ir and Ru
i. very high fusion temperature
ii. More compatible in ‘refractory residue’ during mantle melting
iii. Called ‘refractory PGE’
iv. More siderophile
b. Pd-PGE, containing Pd, Pt and Rh
i. low fusion temperature
ii. Incompatible during mantle melting
iii. Called ‘fusible PGE’
iv. More chalcophile and volatile
v. Stronger affinity for sulfide phase than Ir- group
● PGE can also be described as
a. Light triad: Ru, Rh and Pd
b. Heavy triad: Os, Ir and Pt
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● The light elements have only half the density of the platinum triad; Os, Ir and Pt have
extraordinary high densities.
● Mineralogically, the PGEs form about 150 mineral species, including native metals, alloys,
arsenides, tellurides, selenides, antimonides and oxides, and they also enter into solid
solution in various base metal sulfides.
● Two types of deposit are distinguished: primary deposits in rock, and secondary deposits in
alluvium.
● Alluvial deposits include modern placers, which commonly show an association with
ultramafic/mafic complexes such as ‘Alaskan’ or ‘Alpine type’ intrusions and paleoplacers.

ECONOMIC VALUE OF PGEs

● The PGEs have tremendous economic value.

● They have many industrial uses, which are essentially based on their very high fusion
temperature, the absence of chemical reactivity and exceptional catalytic property.
● Platinum has four major market segments – jewelry (41%), autocatalyst system (28%),
industrial (25%) and investments (6%).
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PLATINUM GROUP ELEMENTS AS GEOCHEMICAL TRACER

● Geochemistry of the platinum group elements provides unique clues to the early origins of
our planet.
● The platinum group elements are particularly useful as tracers of the impacting
extra-planetary materials in the strongly PGE-depleted crust of the Earth and other
planets.
● Platinum group elements have a strong siderophile affinity.
● Laboratory experiments provide evidence that the PGEs have more than 105 times greater
preference for liquid metals than for the silicate magmas.
● Therefore, during core-mantle differentiation, the PGEs should have preferentially
partitioned into the core-forming metal, leaving the mantle depleted in these elements
relative to their original abundances.
● The measured or estimated PGE contents of planetary mantles are much higher than
predicted by these partition coefficients.
● Calculation shows that if the Earth’s mantle was in equilibrium with its core, the mantle
would contain three orders of magnitude less of the PGE than is observed, supporting a late
addition of PGE components.
● Explanations of this dispute are –
i. The last 1% of the Earth’s accretion occurred after the iron-rich core had separated from the
mantle. The Earth’s mantle underwent whole-sale bulk depletion in PGE during core
formation (at about 4.55 Gyr ago), followed by progressive re-enrichment with PGE in
response to addition of cosmic material from the Hadean to Early Archean (4.5-3.8 Gyr
ago) through heavy meteorite bombardment - the so-called ‘late veneer’.
ii. The distribution coefficients of the siderophile elements (especially Pt, Pd and Au) between
metal and silicate phases can be changed under high pressure and temperature conditions,
owing to which the lower mantle materials may contain higher amount of siderophile
elements.
iii. The PGE-rich liquid outer core material can be transported back into the mantle, as trace
elements in plumes.
● Base metal sulfide minerals are common in mantle rocks.
● Sulfide minerals in mantle peridotites control the PGE budget of mantle rocks along with
their behaviour during mantle melting.
● Hence, the sulfide minerals in mantle rocks can be used to trace the petrogenetic processes
responsible for the differentiation of the Earth.
● Because at equilibrium, the concentration of all of the PGEs is at least 10,000 times
higher in sulfide melt than in coexisting silicate melt, sulfide is an extremely potent agent
for the collection and segregation of PGE.
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TYPES OF PGE DEPOSITS

● There are two types of PGE deposits, both intimately associated with Ni-Cu sulfides,
account for about 98% of the world's identified PGE resources:
i. Stratiform or reef type (or, more correctly, strata-bound) deposits in large, layered
complexes (e.g., Bushveld, Stillwater, and Great Dyke), mined primarily for PGE; and
ii. Ni-Cu sulfide deposits mined primarily for Ni-Cu sulfides, but containing recoverable
amounts of PGE as byproducts (e.g., the Sudbury, Noril'sk-Talnakh, Jinchuan, and
Karnbalda deposits).
● The distinctive characters of stratiform deposits are –
i. Their occurrence as relatively weak disseminations of sulfides in silicate rocks, rather than
as massive concentrations, at specific horizons or "reefs" (mineralized rock layers with a
distinctive texture and/or mineralogy) within layered intrusions.
ii. Their association with chromite, either within layers of massive chromitite or within strata
that also contain small amounts of chromite, in contrast with chromite-free strata above and
below the mineralized horizons.
● The large layered intrusions contain about 90% of the world's PGE resources, with the
Bushveld Complex accounting for about 80%. About two-thirds of total world production of
PGE comes from the Bushveld Complex.
● Compared with the Bushveld and Great Dyke deposits, the Stillwater deposit is much
smaller in size, but carries a much higher PGE grade.
● PGE enrichment in massive chromitite: (example, UG-2 chromitite, Bushveld complex)
i. All chromitites, whether of a residual origin, ophiolite related, or products of crystallization
in large layered intrusions or Ural-Alaskan-type complexes, show enrichments in PGE.
ii. Most of them are not currently economic.
iii. Both sulfide-rich and sulfide-poor PGE mineralizations are present.
● For PGE in massive Ni-Cu sulfide deposits, there are two principal classes based on
petrology of host rocks and several subclasses based on the form or geologic environment of
hosting bodies.
a. Peridotite-dunite class (komatiitic association)
i. komatiitic peridotite association
ii. komatiitic dunite association
b. Gabbroid class (tholeiitic association) (> 80% of Ni resources)
i. intrusive mafic-ultramafic complex
ii. large layered complex
iii. the Sudbury Complex
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● The cumulus gabbroic and ultramafic rocks in ophiolites, particularly the units associated
with chromitite and high sulfide concentrations, may contain PGE values of economic
interest.
● Concentrations of PGE in ophiolitic chromite ores are generally low, ranging from <100 ppb
to a few hundred ppb, but samples from several ophiolite complexes have been found to
contain up to tens of ppm PGE.
● Magmatic hydrothermal type PGE deposits:
i. Found in gabbroic rocks
ii. Sulfide-rich or sulfide poor
iii. Hydrothermal alteration is more widespread

For example, Lac des Iles (Thunder Bay, Ontario); Nuasahi & Sukinda massif, India.

ORIGIN OF PGE DEPOSITS:

● There is close physical association of PGE enrichment sites with sulfide mineralizations.
● Strong correlation is present between S and Ni-Cu-PGE (PPGE) contents in all ore types.
● Magma-mixing and consequent sulfide-liquid immiscibility is the main ore-forming process.
● ‘Origin through sulfide segregation from silicate magmas’

⮚ Origin of Reef-type PGE deposits (e.g. Merensky Reef):

1. Orthomagmatic Model:
● Separation of a PGE enriched sulfide liquid from silicate magma and its accumulation on or
close to floor of magma chamber
● Settling of sulfide liquid through a column of silicate liquid
● Sulfide liquid effectively scavenges PGEs from silicate liquid, because PGEs has a high
partition coefficient in sulfides than that in silicates
● By gravitative settling PGE deposits form associated with sulfides
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Magma achieves sulfur saturation hence sulfide immiscibility by magma mixing process
mainly.

The Pt-Pd-rich sulfide horizons in layered intrusions are closely linked with the base of cyclic
units and the development of each cyclic unit is generally attributed to the influx of a new pulse
of primitive magma into the chamber.

Evidences:

● The close association of PGE reefs with specific features of magma evolution, such as cyclic
units and chromitite layers.
● The lateral persistence and relative uniformity of PGE grade of the reefs.
● Systematic stratigraphic controls on PGE tenors of sulfides.
● A strong correlation of PGE ores with Ni-Cu sulfide accumulations, which also have
orthomagmatic origin (sulfide liquid immiscibility).
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2. Hydrothermal Model:
● Platinum group elements are originally present in footwall cumulates (by the precipitation of
sulfide).
● Fluid (Cl rich, high temperature, vapor rich) exsolved from solidifying cumulates, is capable
of dissolving and transporting ore metals as it migrates upward through the largely solidified
cumulate pile and carry S and PGEs upward.
● This upward percolating fluid mixes with stratigraphically higher intercumulus silicate
liquid.
● Fluid is redissolved in the intercumulus melt, adding both S and PGE to the silicate melt.
● Due to limited solubility of S and PGE’s in silicate melt PGE-enriched sulfide precipitation
occurs.

Evidences:

● Pegmatoidal textures of the reef that are coarse grained compared with surrounding rocks.
● Abundance of hydrous and Cl-rich minerals in close association with ores.
● Intimate association with graphite
● Nature of fluid inclusion data