Ores in ultramafic-mafic rocks B. Mishra
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Generalized Classification Scheme for ores in ultramafic- mafic rocks
Crystal Fractionation Carbonatite association Liquid fractionation i) Diamond in kimberlite REE, Nb, P, Sr, Ba i) Oxide liquid immsc. (Mt + ii) Cr ores (Layered and Zr and sometimes apatite- Kiruna type) Alpine types) Cu ii) Sulfide liquid Immiscibility iii) Fe-Ti-Oxides in gabbro- Cu + Ni ± Co ± PGE ores anorthosite association Ores in the ‘mainstream’ of ultramafic-mafic association include those formed by crystal and
liquid fractionations. Carbonatite association is included here because generation of
carbonatitic melt can be either due to (i) liquid immiscibility from phonolitic/ nephelinitic/
kimberlitic melt (Le Bas, 1987) or (ii) by direct melting fertile mantle peridotite at P > 21
kbar (Wallace and Green, 1988). Additionally, some ultramafic-mafic rocks constitute
potential protore sources for Ni- and Au- bearing laterites.
There exist several commonalties that seem to be the unifying factors for the entire
spectrum of ore deposits in ultramafic-mafic settings. These include: (i) highly selective and
specific ore-rock assoc.; (ii) a very exclusive suit of elements, whose average crustal conc. in
the barren ultramafic-mafic rocks are order-of-magnitude higher than in any other barren
rock, typically forms their ore deposits in such rocks; (iii) ore bodies with rare exceptions
typically are physically confined within the hosts; and (iv) again barring exceptions (PGEs),
pervasive wall rock alteration is strikingly absent. The consanguinity of at least the metals
with the host rocks has never been questioned, excepting diamond in kimberlite. The problem
of ore genesis thus overlaps to a large extent with the problems of petrogenesis of the
ultramafic-mafic rocks
Ores in ultramafic-mafic Association
Ores in ultramafic-mafic rocks B. Mishra
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Essential differences between the stratiform and Alpine type chromites
LAYERED ALPINE Age Pre-Camb to lower Paleozoic Paleozoic to Tertiary
Eg Bushveld Ign. Complex (SA) Urals, Philippines,
Skaergaard, Greenland Turkey, Cuba, India
Great dyke, Zimbabwe Pakistan
Muskox, NWT, Canada
Rock Comp. Peridotite at the base and Dunite to gabbro
Granite at the top Average: peridotitic
Average: gabbro
Morphology Saucer Shaped Elongated pod shaped
(inches to x100’ thick) (inches to a foot thick)
Chromite MgO/FeO = 0.6 -1.0 1.0 – 2.3 Composition Fe2O3 low (< 8 wt%) high (10 – 24 wt%) (Fig. 1) Cr/ ∑Fe = high (1.5- 4.5) low (0.75 – 1.75) Al2O3 - Cr2O3 reciprocal relation Scattered Assoc. of Ni as sulfides and arsenides Ni olivine Po- pentlandite- gersdorfite etc
Fig. 1 Schematic diagram showing solid solubility in natural spinels.
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Origin of essentially monomineralic chromite layers (Irvine, 1977)
The model is based on experimentally derived phase relations, related to contamination of a
fractionated (felsic) liquid with a more mafic liquid. The shape of the olivine-chromite
cotectic curve is such that crystallization along this leads to decrease in chromite/olivine ratio
(Fig. 2).
TWO POSSIBLE GEOLOGIC SITUATIONS:
(A) The common situation is when the intrusion contains liquids that has differentiated to
point ‘f’, where opx is on the liquidus and some new primitive (mafic) liquid (comp. a) is
introduced (Fig. 2). (1) Initially the primitive melt with high liquidus temperature crystallize
along the Ol-Chr cotectic (a→b), producing peridotite. But with progressive mixing, instead
it will follow the path b→f towards composition ‘c’. Because composition ‘c’ extends across
the edge of the chromite field, continued crystallization should for some time produce only
CHROMITE and results in driving the melt composition back to the Opx field at‘d’, leading
to formation of Opx (Fig. 2). The whole sequence is peridotite → chromite →orthopyroxenite
as seen in the Muskox intrusive, NWT, Canada.
Fig. 2 Phase diagram in part of the system (Mg, Fe)-Cr2O3-SiO2, showing the olivine-chromite cotectic and theoretical mixing line with regard to chromite crystallization.
Ores in ultramafic-mafic rocks B. Mishra
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(2) Alternatively, if the volume of the fractionated liquid (‘f’) >>> volume of new addition
(‘a’) ⇒ no peridotite will form and the sequence will be orthopyroxenite → chromite as
observed in the Bushveld Ign. Complex, SA.
(3) Alternatively, if addition of fresh liquid (‘a’) is quite large ⇒ hybrid liquid formation and
the bulk composition will return to the Ol-Chr cotectic after chromite crystallization leading
to a sequence of peridotite → chromite → peridotite →orthopyroxenite as observed in the
cyclic units of Stillwater, Montana, US.
(B) The differentiated liquid is still on the Ol-Chr cotectic at a point just above the chromite
control line (Fig. 2). The mutual solution (and mixing) of this liquid with a primitive liquid
(‘a’) ⇒ yield liquid compositions in the chromite field leading to formation of peridotite
→chromite as observed in Great dyke, Zimbabwe.
Sulfide Liquid Immiscibility: Generalized phase diagram to illustrate diverse genetic types of Ni ores
(A) Crystallization from a melt composition represented by pt.1 (92% silicate + 5%
magnetite + 3% pyrrhotite) moves the liquid composition to pt.2 and sulfide liquid of
TWO POSSIBLE CRYSTALLIZATION SEQUENCES
Fig. 3 Ternary phase relation in the system Fe-silicates (gabbro)−Fe-oxide (magnetite)−Fe-sulfide (pyrrhotite), showing the course of crystallization from melts of two compositions represented by ‘1’ and ‘8’.
Ores in ultramafic-mafic rocks B. Mishra
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composition 3 separates out with continued crystallization of silicates. Similar sequence can
be seen for pt.4 and pt.5. Continued crystallization of silicates leads to movement of residual
liquid composition moving from pt. 5 to pt.6 and leads to crystallization of magnetite and
silicate (Fig. 3). Finally, liquid composition reaches pt.7, the ternary eutectic (≈ 90%
pyrrhotite + 8% magnetite + < 5% silicate). If some trace amount (∼400 ppm) of Ni was
present in the initial melt (pt.1), then that would be greatly enriched in the final crystallization
product at pt.7, thus giving a magmatic Ni- sulfide deposit, formed due to liquid
immiscibility.
(B) On the other hand, crystallization of a liquid of composition 8, which has higher Fe3+/
Fe2+ ratio (because of relatively high fO2 compared to that at pt.1), the path will be towards
pt.9, without any liquid immiscibility. Magnetite and silicate crystallize together along the
cotectic and although crystallization stops at pt.7 and a sulfide deposit of essentially
pyrrhotite- magnetite mineralogy will form, it would be devoid of Ni, due its early strong
partitioning into crystallizing olivine and other early formed silicates along the cotectic.
These Ni-olivines serve as protore which on weathering forms Ni- laterites.
Sulfur Solubility in silicate melts
Knowledge of S-solubility in mafic and ultramafic silicate melts is important in
understanding how magmatic sulfide deposits form and also in evaluating the potential
igneous bodies as host for ore deposits of this type. At low fO2 (< 10-6 atm.) and at
temperature ∼1400−1500°C, sulfur dissolves primarily as ‘sulfide’ and the function can be
termed as “S-capacity” of the silicate melt (Cs), which is constant for melts of same
composition.
The entry of sulfur into silicate melts is governed by the simple exchange eqn.
[O]melt + 1/2S2 = [S]melt + 1/2O2
As the amount of oxygen displaced by the above reaction is very low and constant, it can be
written as Cs = Sm[fO2/fS2]1/2, where Sm is the sulfur content of the melt. Cs increases with
temperature and composition of the melt − generally increasing with increasing FeO and
MgO contents and decreasing with SiO2 and Al2O3 contents But as seen from Fig. 4, Cs is
Ores in ultramafic-mafic rocks B. Mishra
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very high for FeO-SiO2 system at any XSiO2 value compared to MgO-SiO2 and CaO-SiO2
system; thus implying that sulfur can displace the oxygens bonded to Fe2+ at ease.
FeO(melt) + 1/2S2 = FeS(melt) + 1/2O2
K = [aFes(fO2)1/2]/[aFeO(fS2)1/2]
⇒ aFes = γFeS XFeS = K x aFeO x [fS2 / fO2]1/2
⇒ logXFeS = 1/2 logfS2 + [logK + log aFeO - 1/2 log fO2 - log γFeS] (1)
If for small changes in XFeS, γFeS is assumed to remain constant and also if the amount of FeS
formed by the above reaction is sufficiently small then FeS formation has no appreciable
effect on aFeS, then it follows from eqn.(1) that XFeS vs logfS2 relation should produce a
straight line with slope = ½ (Fig. 5) , at constant fO2. This is reasonable since the amount of
The sulfur capacity is somewhat misleading. It
does not represent the overall capacity of a melt to
dissolve sulfur, but it is somewhat akin to the
equilibrium constant of the above reaction. Cs
merely relates to the amount of sulfur that will
dissolve in a given melt in response to imposed fS2
and fO2.
Maclean (1969), from his studies in the
system Fe- S- O- SiO2 found that the S- content of
a silicate melt in equilibrium with the S-rich liquid
decreases with increasing O-content. This S-
content is henceforth referred to as the ‘sulfur
content at sulfide saturation’ (SCSS). Maclean
attributed this to the fact that sulfur dissolves in
silicate melt by displacing oxygens bonded to Fe2+
and that increasing oxygen results in an increase in
Fe3+ at the expense of Fe2+ in the melt.
Shima and Naldrett (1975) studied a komatiitic
melt and obtained strong evidence supporting the
observations of Maclean. Considering the reaction
Fig. 4 The effect of melt composition on sulfur capacity of a silicate melts. Also note the variation of SiO2 content of the magma
Ores in ultramafic-mafic rocks B. Mishra
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Fe interacting with sulfur amounts to 0.2 – 0.6% in most melts containing about 8 – 12 wt%
FeO. Buchanon and Nolan (1979) performed a series of experiments with varying fO2 and
FeO (+TiO2) contents; their results are shown in Figure 6.
Effect of temperature
Buchanan et al (1983) determined the solubility of sulfur as a function of fS2 in a
basaltic melt containing 17 wt% FeO at temperature range of 1000−1400°C. At constant fO2
and fS2, the dissolved sulfur content increases by a factor of 8.5 times/ 100°C at 1000°C but
this factor falls to 3 times/ 100°C at 1400°C. Wendlandt (1982) observed that the increase in
temperature of a basaltic melt (FeO ≈ 8 wt%) at 1300−1400°C (20 kbar) and fO2 close to the
C/CO2/CO buffer (about one log unit above the FMQ buffer) caused an increase in SCSS
from 0.09 to 0.16 wt%. Possibly an increase in SCSS of 3 to 5 times from 1200 to 1450°C is
of the order expected in nature.
Effect of pressure
(1) Huang and Williams (1980) from their experiments in the Fe-Si-S-O system at 30 kbar
pointed out that the miscibility gap (between silicate and sulfide liquids) expands with
increasing pressure.
Fig. 6 Sulfur solubility as a function of FeO (+TiO2) contents of the melt and fO2.
Fig. 5 XFeS – fS2 relations in a given silicate melt at two values of fO2 (10−10.4 and 10−9.2).
Ores in ultramafic-mafic rocks B. Mishra
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(2) Wendlandt (1982) studied the SCSS- variation in two in two basalts and an andesite at P=
12.5 kbar and 30 kbar. The observation was SCSS increases with temp. and decreases
with press. This implicates that when magma rises to the surface, its ability to dissolve
sulfur increases, hence it is unlikely to approach saturation with sulfides.
(3) Wendlandt’s data have been shown as variation in FeO content (in magma) as a function
of pressure and S-solubility (Fig. 7), as modified by Naldrett (1988). The relationship
shows the variation to be expected in a basaltic melt from a depth corresponding to a
press ≈30 kbar, which looses heat and stays at its liquidus temps. (1500°C/ 30 kbar to
1330°C/ 12.5 kbar). The reduction in SCSS due to cooling would presumably to a large
extent counterbalance the effect of pressure.
Melt- Melt Equilibrium - partitioning of chalcophile elements between sulfide
and silicate melts: implication on composition of magmatic Ni- Cu sulfide ores
The partitioning coefficient of a minor or trace metal between the silicate and sulfide melts
could be expressed by the Nernst partitioning coefficient, defined as
Di(SL/SM) = wt% ‘i’ in SL / wt% ‘i’ in SM (2)
SL: sulfide liquid and SM: silicate melt
Fig. 7 Solubility of sulfur in basaltic melts of varying FeO contents estimated as function of pressure (after Wendlandt, 1982).
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If Fe, Ni, Co, Cu etc are bonded to oxygen and sulfur in SM and SL respectively then we can
consider their partitioning behavior by equilibrium (A)
NiO(SM) + 1/2 S2 = NiS(SL) + 1/2 O2 (A)
KA = (aNiS)SL / (aNiO)SM x [fO2 / fS2]1/2
⇒ = (γNiS / γNiO) x (NNiS / NNiO) x [fO2 / fS2]1/2 (3)
From eqns. (2) and (3), we see that Di = f (fO2, fS2, T, P and compositions of the two liquids).
However, reaction (A) can be combined with FeO-FeS equilibrium and we can write
NiOSM + FeSSL = NiSSL + FeOSM (B)
KB = (aNiS) / (aNiO) x (aFeO / aNiO)
⇒ = (γNiS / γNiO) x (γFeO / γFeS ) x (NNiS / NNiO) x (NFeO / NFeS) (4)
KB ≠ f(fS2, fO2), although fS2and fO2 may affect. For example, fS2 will affect γNiS and γFeS.
However,
(i) Scott et al (1974) pointed out that in liquids of same composition as mss (Fe1-xS - Ni1-
xS), γNiS and γFeS will have similar values (γNiS /γFeS =1), although both the γi terms
decrease with increasing fS2
(ii) Variation in fO2 can affect NFeO by changing the Fe3+/ Fe2+ ratio in the magma, but if
fO2 ≤ 10−8 bar, the effect in basaltic magma is small.
(iii) Doyle and Naldrett (1986) emphasized that fO2 variation also affects the O-contents
of sulfide-oxide liquids and therefore the a-X relations within them. But provided that
the fO2/fS2 ratio (and hence the O/O+S ratio in the sulfide- oxide liquids) remains
approximately constant leading to negligible effect in metal partitioning.
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Application to natural ores
Ni and Ni-Cu sulfide deposits are spatially associated with ultramafic and mafic igneous
rocks respectively. Relationship between compositions of the ores and the rocks reveal
Cu/(Cu+Ni) ratios in natural sulfide deposits show a general increasing trend with decreasing
basicity of host igneous rocks, i.e., an observed negative correlation between wt% MgO and
Cu/(Cu+Ni) ratios (Fig. 8).
Ni-Cu ores formed are formed by segregation of sulfide droplets from the host silicate
melts and their concentration by gravitational settling. The factors that govern composition of
these ores are (i) the composition of host magma at the timing of liquid immiscibility, and (ii)
partitioning behavior governing distribution coefficients of Fe, Ni, Cu and Co between the
sulfide and silicate melts. Much of the compositional changes in silicate liquid are due to
crystallization and removal of olivine, causing depletion of Ni in the residual liquid, since Ni
has a strong preference to olivine relative to silicate liquid. Cu on the other hand, tends to get
enriched in the residual magma, because it lacks any preference to early formed Fe-Mg
silicates (Fig. 9). Such contrasting behavior of Ni and Cu, results in rapid decrease in Ni/Cu
ratio during fractional crystallization of ultramafic and mafic magmas. The Nernst
distribution coefficient of an ore forming metal between sulfide liquid (SL) and silicate melt
(SM), during liquid immiscibility is given by
Fig. 8 Relationship between the Cu/(Cu + Ni) ratio of sulfide ores and the nature of its host magma or rock.
Fig. 9 Variation of Ni and Cu with wt % MgO in the successive liquids of fractionally crystallized komatiitic magma.
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SM
SL
metalwtmetalwtD⋅⋅
=%% (5)
While Di values for metals such as Pb, Zn, Sn, W, Mo < 1.0, those for metals like Cu, Ni, Co,
PGE > 1.0.
Table 1 Average experimentally determined partioning coefficients (Di) values (from Rajamani and Naldrett, 1978).
Metal Andesitic Basaltic Olivine basaltic
1255°C 1255°C 1305°C 1325°C
Ni 460 274 (±34) 257 231
Cu 243 245(±33) 180 333
Co n.d. 80 (±15) 61 n.d.
Since Fe2+ is the principal cation for the magmatic sulfide deposits, the equilibrium
relation between the SL and SM can be written as the exchange reaction
(FeS)SL + (MeO)SM = (MeS)SL + (FeO)SM (C)
a b
Fig. 10 Ni (wt %) in the sulfide liquids a function of Ni (ppm) in the coexisting basaltic melt at 1255°C.The error bars represent 1σ from the plotted mean values. The slope defines the Nernst partition coefficient (a). Cu (wt. %) in the sulfide liquid as a function of Cu (ppm) in the coexisting basaltic melt at 1255°C (b).
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The equilibrium constant (K) for the above reaction can be expressed in terms of the KD
values
D
SM
MeO
FeOSL
FeS
MeS KK ×
×
=
γγ
γγ
(7)
Where K is the equilibrium constant of the above melt-melt exchange reaction and is
independent on the melt compositions (SL and SM) at a given temperature; ‘Me’ stands
for chalcophile elements such as Ni, Cu, Co etc.
Table 2 Average distribution coefficients (KD) values.
Metal Andesitic Basaltic Olivine basaltic
1255°C 1255°C 1305°C 1325°C
Ni 59 42 (±7) 38 231
Cu 34 35(±6) 24 333
Co n.d. 15 9 n.d.
The experimentally determined KD values involving basaltic melts (Table 2) can be
theoretically extrapolated to melts of more mafic composition, for example peridotitic
melts. The procedure is briefly outlined below.
(FeS)BMS + (NiO)BM = (NiS)BMS + (FeO)BM (D)
(FeS)PMS + (NiO)PM = (NiS)PMS + (FeO)PM (E)
where BM and BMS stand for basaltic melt and the sulfide liquid segregating from the
basaltic melt respectively. Same is the case with PM and PMS. Now from reactions (D) and
(E), we can write
Ores in ultramafic-mafic rocks B. Mishra
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(6)
As mentioned before, from the theoretical study Scott et al. (1974), γNiS and γFeS will have
similar values (γNiS /γFeS =1). Hence, the first term in eqn. (6) is reduced to unity and we can
write
(7)
Because the contents of Na2O, K2O and Al2O3 generally decrease as the silicate melt
composition varies from basalt to peridotite, γFeO will decrease with the increasing basicity of
silicate magma. Using Roeder's equation (γFeO= 0.048 wt % A12O3 + 0.084 wt % Na2O +
0.12 wt % K2O + 0.19) and considering average compositions of basalt and peridotite, the
first term in eqn. (7) turns out to be 2.14. By similar theoretical considerations, the second
term is equal to 0.2. Hence, we can write
(8)
Considering similar theoretical rationale for Cu, we can obtain
(9)
Fig. 11Variation in Ni and Cu contents in the sulfide liquid that segregated from successive liquids of fractionally crystallizing komatiitic magma with wt% MgO of these liquids.
Fig.12 Calculated (open circle) relationship between the Cu/(Cu+Ni) ratios of sulfide liquids and wt% MgO of a komatiitic magma, superimposed on Figure 8.
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Eqns. (8) and (9) demonstrates theoretical predictions of kD values of Ni/Cu to a sulfide liquid
separated from a peridotitic melt, from the experimental kD values for basaltic melt. Similar
thermodynamic calculations have been performed for intermediate silicate melt compositions
and the results are shown Fig. 12, with open circles and by the curve drawn through them,
which relates the compositions of silicate liquid and magmatic sulfide ore. Conclusions
⇒ The Ni-Cu sulfide deposits in mafic-ultramafic rocks are primary and formed by
liquid immiscibility.
⇒ The ore composition depends on the silicate magma composition and timing of liquid
immiscibility.
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