In: Chemical Mineralogy, Smelting and Metallization ISBN 978-1-60692-853-0 Editor: E. D. McLaughlin and L. A. Breaux, pp. © 2009 Nova Science Publishers, Inc.
Chemical Mineralogy, Smelting and Metallization
Editors: Eugene D. McLaughlin and Levan A. Breaux
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Book Description: Mineralogy is an earth science focused around the chemistry, crystal structure, and physical properties of minerals. This book presents new research on mineralogy, including a review on the chemical mineralogy of ornamental rock waste and water treatment plant waste, which are produced worldwide in large scales and are a concerning issue for industry and environmentalists alike. This book also presents new research on smelting, a process that liberates the metallic element of an ore from its compound and separates it from the waste part of the charge. The environmental impact of smelting activities is discussed, as well as the combination of smelting with gasification for treatment of solid waste and the production of alternative fuel.
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Maria Economou-Eliopoulos, Athinoula L. Petrou and George Tsoupas
Chapter4
ON THE ORIGIN OF PLATINUM-GROUP ELEMENTENRICHMENT AND EXTREMELY LARGE (Os-Ir-Ru)MINERALS: EVIDENCE FROM THE ACTIVATION ENERGY VALUES ESTIMATED BY THE ARRHENIUS EQUATION Maria Economou-Eliopoulos ∗ 1, Athinoula L. Petrou2 and George Tsoupas3 1
University of Athens, Department of Geology and Geoenvironment, Athens 15784 2 University of Athens, Department of Chemistry, Athens 15784 3 University of Athens, Department of Geology and Geoenvironment, Athens 15784
ABSTRACT High refractory platinum-group elements (IPGE) contents (decades ppm) and extremely large (mm in size) Ru-Ir-Os- minerals (laurite and alloys) were found as interstitial minerals within silicate matrix of chromite ores associated with ophiolite complexes, although traditionally the abundances of PGE are low (hundreds of ppb) and IPGE-minerals occur as small (< 25 μm) inclusions in chromite grains. Also, large IPGEphases are found is placer deposits, but their origin is a subject of debate. Since chemical processes follow the minimum energy pathways, the Arrhenius equation was used as a controlling factor of the grain-size growth, and a new approach of the PGE mineralization was attempted. On the basis of literature data concerning the grain size of unaltered IPGM and temperature (in a range of temperature between 700 and 1000 oC) an Arrhenius temperature dependence allowed the use of the ln(r) versus 1/T plotting for the estimation of the Activation Energy required for the formation of IPGM. By extrapolating the line obtained, the point where it meets the axes corresponds to 1/T = 0.00065 K-1 or T = 1538 K (1265 0C), and lnr = 0 or r = 1 μm. The estimated values for grain size of the IPGM-phases and temperature applying the ln(r) versus 1/T plotting are in a very good agreement with experimental data given in literature, i.e. the stability limit of laurite at 1270 oC. Such a perfect agreement, coupled with the paucity of PGM in extruded komatiites, exceeding maximum thermal stability of the PGM in mafic magmas, provides a strong support for the reliability of the obtained slope of the straight line, and hence the estimated Activation Energy. Applying the plot of ln(r) versus 1/T on large IPGE-minerals located in placer deposits it is suggested that they have been probably formed at relatively high temperatures (700-850 °C) rather than low-temperature, during the weathering of mafic rocks and sedimentation in placers. Also, an application of the plot of ln(r) versus 1/T is ∗
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On the Origin of Platinum-Group Element-Enrichment…
3
the estimation of the formation temperature of extremely large IPGM (up to 1300 μm), located exclusively along shear zones of chromite ores hosted in the Veria ophiolites. This temperature was estimated to be approximately 730 oC, which is consistent with that suggested in previous publication based on a different approach.
INTRODUCTION The abundances of platinum-group elements (PGE) in large chromite deposits are generally low (few hundreds of ppb). However, PGE-enrichment: (a) in all PGE, (b) only in Os, Ir and Ru and (c) in Pt and/or Pd, are common features of relatively small chromite occurrences. Laurite (RuS2) occurs commonly as small inclusions within unaltered chromite grains in chromite deposits and occurrences (Cabri, 2002). Recently, Ru-Ir-Os- minerals were, however, described within silicate matrix as interstitial minerals of chromite ore from the Skyros island and Veria area (Tsoupas and Economou-Eliopoulos, 2007). The origin of both, compatible platinum-group elements (IPGE)-enrichment reaching values, up to 25 ppm, and the presence of large, up to 1.3 mm in size, PGM grains within massive chromite ores and placer deposits remains still unclear (Johan, 2000; Malitch and Thalhammer, 2000; Hattori et al., 1990, 2004; Cabri, 2002; Gervilla et al., 2005; Tolstykh et., 2005; Tsoupas and Economou-Eliopoulos, 2008). Since the chemical processes follow the minimum energy pathways (Espenson, 1981; Katakis and Gordon, 1987), Petrou and Economou-Eliopoulos (2009) indicated an Arrhenius temperature dependence and estimated the Activation Energy for the formation of IPGM (Irgroup minerals) on the basis of literature data on grain size of unaltered platinum-group minerals (PGM) and temperatures, in a range of temperature between 700 and 1000 oC. In the present study application of some new data (SEM/probe) and also literature data on the straight line obtained by a plot of ln(r) versus 1/T (Petrou and Economou-Eliopoulos, 2009), which gives the activation energy value of 425 kJ/mol, confirm the validity of our new approach and improves our understanding on the mechanism that controls the formation of large IPGM-phases and the IPGE-enrichment in ultramafic rocks and placer deposits.
ANALYTICAL METHODS Polished sections prepared from the chromite ores were examined by reflected light microscopy and scanning electron microscope. Quantitative analyses were carried out at the University of Athens, Department of Geology, using a JEOL JSM 5600 scanning electron microscope,equipped with automated OXFORD ISIS 300 energy dispersive analysis system. Analytical conditions were 20 kV a ccelerating voltage, 0.5 nA beam current, 2 μm beam diameter and 50 s count times. The following X-ray lines were used: OsMα, PtMα, IrMβ, AuMá, AgLα, AsLα, FeKα, NiKα, CoKa, CuKα, CrKα, AlKα, TiKα, CaKα, SiKα,MnKα,MgKα, ClKα. Standards used were pure metals for the elements Os, Ir, Ru, Rh, Pt, Pd, Cu,Ni, Co and Cr, indium arsenide for As and pyrite for S. Major and trace element concentrations in whole ore were determined by ICP/MS analysis after preconcentration using the nickel fire assay technique from large (30 g) samples, at Activation Laboratories,
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Maria Economou-Eliopoulos, Athinoula L. Petrou and George Tsoupas
Ltd, Canada. This method allows for complete dissolution of chromitite. Detection limits are 5 ppb for Ru and Pt, 2 ppb for Os, Ir, Pd and 1 ppb for Rh and Au.
PGE CONTENTS AND COMPOSITION OF Ru-Ir-Os-MINERALS IN CHROMITE ORES ASSOCIATED WITH OPHIOLITES Whole ore PGE analyses of massive chromite ores have shown that in certain ophiolite complexes there is a local PGE-enrichment (Table 1), although their content is commonly a few hundreds ppb. An enrichment in Pt and Pd relative to more refractory PGE (Os, Ir, Ru) has been located in some ophiolite complexes, like New Caledonia (Augé et al. 1998), the Zambales ophiolite complex in the Philippines (Bacuta et al. 1990), Thetford mines in Quebec (Corrivaux and LaFlamme 1990), Vourinos in Greece (Konstantopoulou and Economou-Eliopoulos, 1991); Bulqiza in Albania (Ohnenstetter et al. 1999), Troodos in Cyprus (Prichard and Lord 1990), Shetland in the UK (Prichard et al. 1986; Prichard and Tarkian 1988; Prichard et al. 1994), Pindos in Greece (Tarkian et al. 1996) and Veria in Greece (Tsoupas and Economou-Eliopoulos, 2008). Table 1. PGE contents in PGE-enriched chromite ores from Greece Pindos ppb
Korydallos
Os
60
60
Milia
Skyros
Veria
150
140
7400
Ir
50
50
320
480
6000
Ru
70
110
350
1200
9700
Rh
100
100
82
160
310
Pt
1120
2700
150
280
760
Pd
470
1200
7
40
750
ΣPGE
2900
4220
1059
2300
24920
Pd/Ir
9.4
24
0.22
0.1
0.12
The presence of the Pt and Pd minerals in interstitial positions to the chromite grains provides evidence for their crystallization subsequently to that of chromite. It has been suggested that the presence of only a small amount of immiscible sulfide liquid led to very high concentrations of Pt and Pd within the sulfide liquid and the association of the Pt- and Pd-bearing PGM with rare base metal sulfides (Prichard and Tarkian 1988; Ohnenstetter et al. 1999; Prichard et al., 2008). Also, the well pronounced fractionation trend (relatively high Pd/Ir ratios, Table 1; Figure 1) chromite ores enriched in Pt and Pd compared to Os, Ir and Ru, in particular those related to supra-Moho dunites has been attributed to fractionation under different fO2 and fS2 conditions (Bacuta et al., 1990; Lord and Prichard, 1997; Ohnenstetter et al., 1999; Economou et al., 1999;).
On the Origin of Platinum-Group Element-Enrichment…
5
Present study is focused on the origin of large IPGM and the IPGE-enrichment. Laurite (RuS2) occurs commonly as small inclusions within unaltered chromite grains in chromite deposits and occurrences (Figure 1a). However, Ru-Ir-Os- minerals are found within silicate matrix as interstitial minerals of chromite ore as is exemplified by the Skyros island, and Veria chromite ores (Figure 1b,c) which are rich in IPGE (Table 1). Both Ru-Ir-Os alloys and sulfides (laurite) exhibit significant Fe, Ni and Cr contents and a wide compositional variation (Table 2).
Figure 1. Chondrite-normalized PGE-patterns (sample/C1 chondrite) for the Veria chromite ores and PGE-enriched chromitites related to other ophiolite complexes. Data from Tsoupas and EconomouEliopoulos (2008) for Veria (Greece); Stockman et al (1984) for Heazlewood River, (Tasmania); Bacuta et al. (1990) for Zambales (Phillipines); Peck and Keays (1990) for Oregon (USA); Tarkian and Prichard (1987) for Shetland (Scotland); Bridges et al. (1993) for Braganca (Portugal). Normalization values (C1) after Naldrett and Duke (1980).
Table 2. Representative microprobe analyses of PGM from PGE-rich chromitites of the Skyros island and Veria (Figure 2) Skyros wt%
laurite* Figure 1b,1 Figure 1b,2 Figure
Figure 1d,1 Figure 1d,2 Figure 1d,3 Figure 1d,4
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Maria Economou-Eliopoulos, Athinoula L. Petrou and George Tsoupas 1b,3
Os
14.1
44.8
38
80.9
5.88
7.96
15.49
13.05
Ir
7.8
7.2
13.2
2.2
6.31
5.66
18.51
27.21
Ru
43.4
37.8
41.2
10.3
53.35
48.48
38.23
30.59
Pd
n.d.
n.d.
1.6
n.d.
n.d.
n.d.
n.d.
n.d.
Pt
n.d.
n.d.
n.d.
n.d.
2.74
4.37
3.61
4.59
Ni
n.d.
6.7
4.3
3.3
0.68
1.17
1.21
1.03
Fe
n.d.
1.5
1.1
0.8
0.95
0.73
0.67
0.51
Cr
n.d.
0.9
1.1
0.6
0.38
0.34
0.34
0.44
Sb
n.d.
n.d.
n.d.
n.d.
n.d.
0.52
0.63
1.14
As
n.d.
n.d.
n.d.
n.d.
2.11
2.76
5.81
10.01
S
34.8
n.d.
n.d.
n.d.
26.71
27.15
14.78
11.85
98.9
100.5
98.1
99.11
99.14
99.28
100.42
Total 100.1
Cont.
Veria
wt%
laurite
Ir-Os-Ru-alloys
Os
24.53
22.24
20.87
29.64
48.09
n.d.
6.19
n.d.
Ir
8.03
6.55
6.02
5.81
8.52
59.71
81.17
64.64
Ru
36.05
36.34
39.08
33.13
33.21
7.23
2.79
2.8
Pd
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
2.28
n.d.
Pt
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6.15
n.d.
Ni
n.d.
n.d.
n.d.
n.d.
n.d.
6.54
n.d.
1.71
Fe
0.29
n.d.
n.d.
n.d.
8.74
22.03
1.39
25.79
Cr
0.81
n.d.
n.d.
n.d.
0.77
0.96
n.d.
1.31
As
n.d.
n.d.
n.d.
n.d.
n.d.
1.24
n.d.
n.d.
S
30.14
34.77
33.59
29.92
n.d.
n.d.
n.d.
n.d.
99.9
99.56
98.5
99.33
97.71
99.97
96.25
Total 99.85
Symbol: n.d. = below detection limit.
CONTROLLING FACTORS OF THE IPGM FORMATION Phase Equilibrium Constraints on Synthetic Laurite and Ru-Os-Ir Alloy
On the Origin of Platinum-Group Element-Enrichment…
7
Experimental data have demonstrated that under appropriate conditions of T, P, fO2 and fS2, the precipitation of laurite from basaltic liquids is feasible, laurite can be formed in equilibrium with Os–Ir alloys and it is stable up to the temperature 1275°C (< 1300 oC) in mafic magmas (Brenan and Andrews 2001; Andrews and Brenan 2002). Bochrath et al (2004) showed that laurite may be the first sulfide mineral to crystallize directly out of a sulfideundersaturated silicate melt, under conditions more oxidized than ΔfO2=FMQ−1.5, which applies to most terrestrial melts. A melt that is more reduced than this value would exsolve a sulfide liquid at the fS2 in equilibrium with RuS2, and hence would fractionate all noble metals into sulfide melt, destabilizing discrete PGE sulfides. Initially, laurite is nearly stoichiometric (RuS2) with very low contents of Os and Ir. As the temperature decreases and fS2 increases Os is progressively incorporated into the laurite lattice, due to the partition coefficient of Os between laurite and silicate melt, and it significantly increases with decreasing temperature (Andrews and Brenan 2002). Ballhaus (1998) and Matveev and Ballhaus (2002) on the basis of experimental data, found that the liquidus temperature of chromite decreased with increasing H2O content (4 wt%). With lower water content (1–2 wt% H2O) higher temperatures (1430–1480 oC) at 1.5 GPa have been reported. Also, upon olivine and chromite precipitation, chromite would be collected in the hydrous liquid droplets, accompanied by PGE-rich metallic alloys, while olivine was collected in the basaltic melt. This fractionation was attributed to different interfacial energies between chromite and hydrous fluid and basaltic melt, respectively (Matveev and Ballhaus, 2002). These experimental data are consistent with the formation of chromite deposits in ophiolite complexes, only in the presence of water-rich magmas saturated in olivine and chromite, in a supra subduction zone (SSZ) environment (Bacuta et al., 1990; Zhou and Robinson, 1997; Ohnenstetter et al. 1999; Economou-Eliopoulos et al., 1999; Gervilla et al., 2005). Also, although is has been suggested that due to the high solubilities of the PGE in basaltic magma direct nucleation and crystallization of PGM from silicate melt would be precluded (Peach and Mathez, 1996) the majority of the authors agree that laurite and alloys occurring as inclusions within natural chromite crystals in ophiolite complexes and Alaskan type and layered intrusions may have formed by direct crystallization from a silicate magma under appropriate conditions of T, P, fO2 and fS2 (Merkle, 1992; Lorand et al., 1993; Tredoux et al., 1995; Matveev and Ballhaus 2002; Bockrath et al., 2004; Mungal, 2005).
IMPLICATIONS FOR THE IPGM CRYSTALLIZATION Liquidus, falling within the two-phase field defined by high-f(S2) experiments (Andrews and Brenan., 2002) suggest high- fS2 conditions during their crystallization or that the laurite formed at a lower temperature (1010-1030 °C), which is in a good agreement with conclusion on the basis of the associated silicates (pargasitic amphibole and phlogopite) resulted probably by small amounts of silicate melt that enclosed together with PGM in chromite from the Othrys ophiolite complex, Greece (Garuti et al., 1999). Such a lower temperature seems to be consistent with the association of hydrous minerals reflecting an increased H2O content in the crystallized magma (Matveew and Ballhaus, 2002). The presence of hydrous fluids and laurite in immiscible hydrous liquids, the simultaneous precipitation of PGE alloys, laurite,
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Maria Economou-Eliopoulos, Athinoula L. Petrou and George Tsoupas
olivine, and chromite are possible (Matveev and Ballhaus 2002) and consistent with a supra subduction zone (SSZ) geotectonic environment related to chromite deposits hosted in ophiolite complexes. Although the PGM crystallisation from basaltic magma may be feasible, laurite being stable up to the temperature 1275°C in mafic magmas (Brenan and Andrews 2001), at that extrusion temperature of komatiites exceeds probably the maximum thermal stability of the PGMs (Maier et al., 2003). The platinum-group elements (PGE) tend to be bonded with sulfur, oxygen or other ligands. An adequate thermodynamic description of micro-nuggets or clusters, as they may be called if they are of colloidal size (Tredoux et al. 1995), has not yet been devised and their relevance to natural systems remains an open question. Micro-nuggets, apparently in equilibrium with enclosing silicate melts or aqueous fluids are too small to be detected by optical or even scanning electron microscopy, but are detectable by micro-analytical tools like laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) (Sylvester 2001). Recently, the investigation of chromites in komatiites from the Agnew-Wiluna Belt (Western Australia) using the laser ablation ICP-MS technique demonstrated lack of PGEbearing alloys. Conversely, analyses of chromites separated from Theo’s Flow tholeiitic basalt indicate the presence of PGM, and chromites from Fred’s Flow komatiitic basalt contain Ir-rich clusters (Fiorentini et al., 2004). Despite the compositional variation of laurite in equilibrium with Os–Ir alloys, depending on the conditions of T, P, fO2 and fS2 during precipitation of chromite ore in ophiolites, they are similar in terms of (a) their precipitation directly out of a sulfide-undersaturated basaltic melt at temperatures higher than 1000 °C (lower than 1275°C), (b) their small (< 25 μm) grain size and (c) small whole PGE content in the chromite ore, and their formation is well constrained (Ohnenstetter et al. 1999; Garuti et al., 1999; Matveew and Ballhaus, 2002; Cabri, 2002; Bockrath et al., 2004). However, the growth of large IPGM as interstitial minerals in chromite ores within silicate matrix (Figure 2; Tsoupas and Economou-Eliopoulos, 2008), remains still unclear. Also, the origin of coarse-grained platinum-group minerals, reaching values up to a few mm, which have been located in many placer deposits associated with ultramafic-mafic complexes, such as Alaskan and ophiolitic type intrusions is a subject of debate, due to the paucity of coarse-grained PGM in parental mafic-ultramafic complexes. Some authors have suggested that coarse-grained PGM have formed at low-temperature, during the weathering and sedimentation (Augustithis, 1965; Ottemann and Augustithis, 1967; Cousins and Kinloch, 1976; Barker and Lamal, 1989; Bowles, 1986; Bowles et al.,2000), whilst others suggested that coarce-grained PGM may have crystallized in late magmatic pegmatitic environments in the mafic/ultramafic complexes (Cabri and Harris, 1975; Slansky et al., 1991; Cabri et al., 1996; Johan et al., 1990, Johan, 2000; Malitch and Thalhammer, 2000; Hattori et al., 2004; Tolstykh et., 2005).
On the Origin of Platinum-Group Element-Enrichment…
9
Figure 2. Back-scattered electron images of typical small laurite within unaltered chromite (Figure 1a), IPGM within silicates from the Skyros massive chromite ore (Figure 1b,c, d) and coarse-grained PGM within strongly fragmented chromite ore from the Veria area (Greece) along a shear zone (Figure 1e,f). Abbreviations: chr = chromite; Fe-chr = ferrian chromite; srp = serpentine; grt = garnet; PGE = platinum-group mineral.
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Maria Economou-Eliopoulos, Athinoula L. Petrou and George Tsoupas
Activation Energy and Minimum Energy Pathways The energy required to bring reactants to a condition necessary to form products is called Activation Energy. Under the special conditions at which chemical processes usually proceed, there is a statistical distribution in the relative orientations (pathways) of the (usually nonspherical) reacting molecules as they move and approach each other. Some of these orientations/pathways are characterized by the minimum energy pathways (Espenson, 1981; Katakis and Gordon, 1987). In an attempt to contribute to the origin of large IPGM (Ir-group minerals) occurring in chromite ores and placer deposits, Petrou and Economou-Eliopoulos (2008) based on grain size of IPGM and temperature (in a range of temperature between 700 and 1100 oC) indicated an Arrhenius temperature dependence and estimated the Activation Energy for the formation of IPGM-phases. More specifically, the rate (u) of a reaction /process is proportional to the concentration of the reactants.
u = k obs ∗ C and k obs = Ae a
On the other hand u = Ae
−
−
Eact RT
Eact RT
E
− act 1 C ⇒ n = A' C a e RT r a
Hence, combining constants: E
− act 1 RT = A ' ' e n r
( )
E E ⎛1⎞ ln⎜ n ⎟ = ln ( A' ') − act ⇒ − ln r n = const. − act ⇒ RT RT ⎝r ⎠ E E − n * ln (r ) = const. − act or n * ln(r ) = −const. + act RT RT Plotting ln(r) versus 1/T a straight line was obtained, with slope = Eact/R*n and intercept = const/n (Figure 3). From the slope of the straight line the Activation Energy was estimated: -const/n = 13.377 and Eact/n*R = slope = 20499K, hence Eact = 20499*n*8.31Jmol-1K-1*K= 170346.7*nJmol-1= n*170.3kJmol-1. Assuming that grains of IPGM can be roughly described by n = 2.5, the estimated average value is approximately 425 kJmol-1 (Petrou and EconomouEliopoulos, 2008).
On the Origin of Platinum-Group Element-Enrichment…
11
Figure 3. Plot of the ln(r) versus 1/T for the estimation of the required (minimum) activation energy of the Ru, Os, Ir-sulfide and alloy formation (after Petrou and Economou-Eliopoulos, 2008). T1 = 1270 oC corresponds to the stability limit of laurite; T2 = 1040 oC is the average temperature (ranging from 1000 to 1100 oC) corresponding to IPGM occurring as small inclusions in chromite, and T3 = 810 oC is the average temperature (ranging from 700 to 850 oC) corresponding to IPGM occurring in placer deposits.
Also, chemical diffusivities are widely reported to obey an exponential dependence on reciprocal temperature over considerable ranges in temperature, a type of relation referred to as Arrhenian.
D = D0 e
−
E act RT
The Arrhenian relation can be derived from absolute rate theory since ions move from one location in the magma to another in discrete diffusive jumps. Each jump requires an ion vibrating within a stable potential energy in the melt to acquire sufficient kinetic energy to rise over a potential energy barrier, to move to an adjacent stable site in the structure. The pre-exponential factor Do can be related to the frequency of the vibration, which allows diffusive steps to occur and has been called the frequency factor and is for most purposes a constant. However, this Arrhenian relation does not beyond the aim of the present study. Brittle deformation and fragmentation of chromite ore along thrust shear zones of Veria has been recorded in both chromite and PGM (Figure 2e). Furthemore, it provides pathways for fluid circulation and small grain dislocation have facilitated metasomatic fluids, in a low sulfidation regime. The association of PGM with Cr-rich garnets, serpentine, chlorite, ferrian chromite and magnetite like those in chromite ores from Veria and Skyros island (Figure 2) and the composition of the PGM (Table 2) has been interpreted as the products by in situ desulfurization of the PGM sulfides (Stockman and Hlava, 1984; Tarkian and Prichard, 1987; Bridges et al., 1994; Melcher et al., 1997; Garuti and Zaccarini, 1997; Tsoupas and Economou-Eliopoulos, 2008). Thus, by a diffusion process the composition of primary PGM,
12
Maria Economou-Eliopoulos, Athinoula L. Petrou and George Tsoupas
mostly laurite have substantially modified to Fe–Cr–Co–Ni-bearing Fe–Os–Ir–Ru alloys (Table 2) and oxides (Tsoupas and Economou-Eliopoulos, 2008). Nevertheless, present study is focussed on the role of the activation energy to the formation of IPGE-phases.
RELIABILITY OF THE ESTIMATED VALUE OF ACTIVATION ENERGY The timing of large IPGM formation is not well constrained, although it is an important factor regarding the behavior of IPGEs. In our attempt to constrain the activation energy for IPGE growth, the temperature of their formation was used. Literature data can be distinguished1into one group of large IPGM-phases with a grain-size ranging from 400 to1000 μm and temperature from 700 to 850 οC (average 810 οC) which differ from the small (< 25 μm) IPGM inclusions in chromite grains formed in ophiolite complexes at higher temperature, ranging from 1000 to 1100 οC (average 1040 οC ) (Johan et al., 1991; Slansky et al., 1991; Hattori et al., 1991; Tarkian et al., 1992; Melcher et al., 1997; Garuti and Zaccarini, 1997; Garuti et al., 1999a,b; Ohnenstetter et al., 1999; Johan et al., 2000; Malitch and Thalhammer, 2002; Weiser, 2002; Hattori et al., 2004; Tolstykh et al., 2005; Ahmed, 2007; Gonzalez-Jimenez et al., 2007). The available data used to construct the graph ln (r) vs 1/T (Figure 3) pose uncertainties due to multiple temperature estimates of PGM formation provided in literature, depending on the mineral associations and thermometer chosen. Also experimental data of Watson and Price (2001) showed that same spinel thickness can be achieved at different temperature, depending on the duration of the experiment. However, the large number of data with random positive and negative errors may results in eliminating the uncertainty. The estimated activation energy value, approximately 425 kJmol-1 (Petrou and Economou-Eliopoulos, 2008) resulted assuming that grains of IPGM can be roughly described by n = 2.5, where n appears in the equation relating lnr = f (1/T). If n=3 (volume of a grain), then Eact = 510. 9 kJmol-1 while when n = 2 (surface of the grain) then Eact = 340.6 kJmol-1. By extrapolating the obtained line (Figure 3) to the point where it meets the axes -1
0
corresponds a value of 1/T = 0.00065 K or T = 1538 K (1265 C) was obtained, and lnr = 0, and hence r = 1 μm. Thus, application of the ln(r) versus 1/T plotting (Figure 3, after Petrou and Economou-Eliopoulos,2008), confirms the estimation of the grain size of the IPGMphases to very small (~ 1 μm) size of PGM (“invisible PGM”) at experimental temperature of 1275 oC (Brenan and Andrews, 2001) and it is consistent with the paucity of PGM in extruded komatiites, where the high temperature exceeds the maximum thermal stability of the PGM in mafic magmas. The similarities between the estimated tempetarute applying the ln(r) versus 1/T plotting (Figure 3) and the expected negligible grain size of PGM at that temperature (Brenan and Andrews, 2001) provide a strong support for the reliability of the slope of the straight line obtained (Figure 3), and hence the estimated Activation Energy.
On the Origin of Platinum-Group Element-Enrichment…
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APPLICATIONS OF THE ACTIVATION ENERGY TO THE ORIGIN OF LARGE IPGM-PHASES (a) In Chromite Ores Besides the typical laurite that may be the first sulfide mineral to crystallize directly out of a sulfide-undersaturated silicate melt (Brenan and Andrews, 2001; Peregoedova and Ohnenstetter, 2002; Bochrath et al., 2004; Mungall, 2005) the interpretation of large IPGMphases in relatively IPGE enriched chromitites remains still unclear. Most natural IPGM are probably formed during cooling and re-crystallization of primary minerals under decreased temperature and increased H2O content (Ballhaus, 1998; Matveev and Ballhaus, 2002; Peregoedova and Ohnenstetter 2002; Peregoedova et al. 2004),). The association of PGM with Cr-rich garnets, serpentine, chlorite, ferrian chromite and magnetite like those in chromite ores from Veria and Skyros island (Figure 2) and the composition of the PGM (Table 2) has been interpreted as the products by in situ desulfurization of the PGM sulfides (Stockman and Hlava, 1984; Tarkian and Prichard, 1987; Bridges et al., 1994; Melcher et al., 1997; Garuti and Zaccarini, 1997; Tsoupas and Economou-Eliopoulos, 2008). Apart from the brittle deformation and fragmentation of chromite and PGM .along thrust shear zones (Figure 2e), providing pathways for fluid circulation and small grain dislocation have facilitated metasomatic fluids, in a low sulfidation regime and have substantially modified the composition of primary PGM, mostly laurite, generating Fe–Cr–Co–Ni-bearing Fe–Os–Ir–Ru alloys (Table 2). Besides, the presence of the laurite relicts within large PGM grains (Figure 2d, f) reveal the initial nature of laurite. The obtained temperature by application of the Plot of ln(r) versus 1/T (Figure 3) in that case of the extremely large IPGM (1300 μm), located exclusively along shear zones of the Veria chromitites hosted in ophiolites (Figure 2), is approximately 730 oC, which is comparable to that suggested in a previous publication (Tsoupas and Economou-Eliopoulos, 2008). More specifically, it has been suggested that IPGM and accompanying chromite may have re-crystallized during plastic deformation episodes at relatively lower temperatures (800 to 900 °C), along the permeable shear zones, under appropriate pressure, temperature, redox conditions and an increased H2O content (Matveev and Ballhaus 2002; Tsoupas and Economou-Eliopoulos, 2008).
(b) In Placers Placer deposit in Urals related to Alaskan type ultramafic complex was the world’s main producer of platinum and platinum-group elements (PGE) before the discovery of the Merensky reef in the Bushveld complex, South Africa. Recently, the increasing world-wide demand for platinum, has led to several Pt-bearing placers being reopened (Tolstykh et al., 2005). Large Alaskan - type complexes exposed along SE Alaska, in the Urals and southcentral British Columbia, showing a rough concentric zoning (a dunitic core is surrounded by successive shells of olivine clinopyroxenite, magnetite-rich clinopyroxenite and hornblendite) have been extensively studied. The origin of coarse-grained platinum-group minerals (PGM), reaching size up to a few mm, located in many placers associated with ultramafic-mafic complexes, such as Alaskan
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Maria Economou-Eliopoulos, Athinoula L. Petrou and George Tsoupas
and ophiolitic type intrusions is a subsect of debate, due to the paucity of coarse-grained PGM in parental rocks (Cabri and Harris, 1975; Barker and Lamal, 1989; Bowles et al., 2000; Hattori, 2004).Laurite (RuS2), from placer deposits, like the Pontyn River, Tanah Laut, Borneo, and Tambanio River, southeast Borneo, show a variety of morphologies, including euhedral grains with conchoidal fractures and pits, and spherical grains. Inclusions are rare, but one grain contains Ca-Al amphibole inclusions, and another contains an inclusion of chalcopyrite+bornite+pentlandite+heazlewoodite, with a size ranging from 700 μm to 1100 μm (average 900 μm). All grains are solid solutions of Ru and Os with Ir (2.7 -11.8 wt.%), Pd (0.3-0.7 wt.%) and Se (140 to 240 ppm). Their compositions suggest that S in laurite has not undergone redox changes and originated from by partial melting in SSZ setting (Hattori et a., 2004). Thus, the combination of literature data with temperatures estimated applying the Plot of ln(r) versus 1/T (Figure 3) indicate that they have probably formed at relatively high temperatures (700-850 °C) rather than low-temperature, during the weathering of mafic rocks and sedimentation in placers (Augustithis, 1965; Ottemann and Augustithis, 1967; Cousins and Kinloch, 1976; Barker and Lamal, 1989; Bowles, 1986; Bowles et al., 2000). More specifically, with respect to large (~1.5 mm) grains of erichmanite-laurite found in streams and residual laterite soils overlying the Freetown complex, Sierra-Leone, there is a different interpretation of their 187Os/188Os isotopic compositions. Both, their magmatic formation (Hattori and Hart, 1991; Hattori, 2002) and sedimentary origin (Bowles, 2000) have been suggested. The application of the Plot of ln(r) versus 1/T (Figure 3) confirms their formation at relatively high temperatures rather their sedimentary origin. In addition, the genetic importance of magmatic processes is reflected to the compositional trends of the IPGM in placer deposits. They provide significant criteria for discrimination between placers deposits related to Ural-Alaska type complexes and those related to ophiolite complexes reflecting differences in the composition of parent magmas and fractionation trends (Cabri and Harris, 1975; Tolstykh et al., 2005).
CONCLUSION Applying the Arrhenious equation on the basis of known grain size of unaltered platinum-group minerals (PGM) and temperature (in a range of temperature between 700 and 1000 oC) an Arrhenius temperature dependence was revealed. The application of the ln(r) versus 1/T plot for the estimation of the grain size of the IPGM-phases at the temperature of 1275 oC (the upper stability limit of the laurite) indicated a very good reliability of the estimated slope of the straight line and hence the accuracy of Activation Energy estimated by applying the Arrhenius equation. Therefore, if grain size is known, then the ln(r) versus 1/T plot can be applied to estimate roughly the temperature of IPGE formation, which is of genetic significance and improves our understanding of the mechanism that controls the formation of large IPGM-phases and the IPGE-enrichment in ultramafic rocks and placer deposits.
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