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Alluvial platinum-group minerals as indicators of primary PGE mineralization (placers of southern Siberia)
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ScienceDirect Russian Geology and Geophysics 57 (2016) 1437–1464 www.elsevier.com/locate/rgg
Alluvial platinum-group minerals as indicators of primary PGE mineralization (placers of southern Siberia) S.M. Zhmodik a,b,*, G.V. Nesterenko a, E.V. Airiyants a, D.K. Belyanin a,b, V.V. Kolpakov a, M.Yu. Podlipsky a, N.S. Karmanov a a
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 21 September 2015; accepted 24 September 2015
Abstract The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. Different characteristics of minerals can be determined by a set of conventional and modern in situ analytical techniques (scanning electron microscopy (SEM) and electron probe microanalysis (EPMA)). A study of PGM from placers of southern Siberia (Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge) shows that the morphology and composition of PGM grains, the texture, morphology, and composition of silicate, oxide, and intermetallic microinclusions, and the type of mineral alteration can serve as efficient indicators of the primary sources of PGM. The widespread rock associations in the Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge, the compositions of PGM and microinclusions in them, and the dominant mineral assemblages testify to several possible primary sources of PGE mineralization: (1) Uralian–Alaskan-type intrusions; (2) ophiolite associations, including those formed in a subduction zone; (3) ultramafic alkaline massifs; and, probably, (4) rocks of the picrite–basalt association. The preservation of poorly rounded and unrounded PGM grains in many of the studied placers of the Altai–Sayan Folded Area (ASFA) suggests a short transport from their primary source. © 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: platinum-group minerals (PGM); gold; placers; Altai–Sayan folded area; Kuznetsk Alatau; Salair; Gornaya Shoria
Introduction Gold and platinum-bearing placer deposits have been worked in South Siberia since they were first discovered in the 1830s–1840s. Despite an extensive exploration and mining history, substantial production of gold and platinum-group minerals from placers of Kuznetsk Alatau, Gornaya Shoria, and Salair (Agafonov et al., 1996, 2000; Chernykh and Uvarov, 2003; Polyakov and Bognibov, 1995; Podlipsky and Krivenko, 2001; Sazonov et al., 2000; Tolstykh, 2004; Vysotsky, 1933; and others), and the recognition of the Altai–Sayan platinum-bearing province (Dodin et al., 1999; Izokh et al., 2004), virtually none of these PGE occurrences have been traced to their bedrock source. The aims of the study were to determine (1) the assemblages and types of platinum-group minerals (PGM) in alluvial placers of West Siberia; (2) their properties, compositions, and textural features pointing to the
* Corresponding author. E-mail address: [email protected] (S.M. Zhmodik)
primary sources of the placer PGM, based on local analysis of individual PGM grains. The study provides detailed mineralogical description of the placer PGM from southern West Siberia along with the data on grain morphologies, textures, compositions, and types of inclusions that bear information on potential primary sources for the majority of PGM placer grains. All PGM grains were found in the gold placers in the study area. Most of the PGM grains were recovered from gold-bearing alluvial sediments. The history of gold placer exploration and mining in the region was discussed in earlier works (Butvilovskii et al., 2011; Nesterenko, 1991; Vysotsky, 1933; and others), and information on the first PGM discoveries was summarized by Vernadsky (1955) in 1908. According to Vysotsky (1933), the PGM were found in many gold-bearing placer deposits of southern Siberia. Kyuz (1935) noticed that PGM were present in virtually all rivers in Kuznetsk Alatau that were mined for gold. New data obtained over the next 50 years are scarce. The PGE potential of this region was evaluated by Syrovatskii in 1990–1991 and the composition of alluvial PGM grains was systematically studied
1068-7971/$ - see front matter D 201 6, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 6.09.002
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within the same period (Krivenko et al., 1994; Podlipsky, 1999; Podlipsky et al., 2007; Tolstykh, 2004; Tolstykh et al., 1999; Zhmodik et al., 2004). Although these papers provide the first assumptions on the types of original sources of PGM from gold-bearing placers of southern Siberia, the possible source rocks of placer PGM in the Kuznetsk Alatau, Gornaya Shoria, and Salair remain unknown. Following traditions in earlier works on other PGE occurrences worldwide (Cabri et al., 1996; Gornostayev et al., 1999; McClenaghan and Cabri, 2011; Nakagawa and Franco, 1997; Nixon et al., 1990; Okrugin, 1999; Weiser and Bachmann, 1999; and others), the available samples were analyzed in detail using modern analytical methods. The results of this study show that the compositions of new PGE compounds, mineral and melt inclusions, as well as exsolution textures can account for the existence of gold-rich PGE and composite polyphase melts formed by liquation or hydrothermal-metasomatic replacement.
Geological setting and PGE potential The study area is in the Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge, which represent the western part of the Altai–Sayan folded area (ASFA) (Fig. 1) formed during the Caledonian and Hercynian orogenies. Variations in the structural architecture and lithologies were controlled by different geodynamic processes that led to the formation of oceanic and island-arc complexes, subsequent collision during accretion of the Siberian craton and protracted plume-related magmatism during the Neoproterozoic–Mesozoic (Buslov et al., 2013; Dobretsov, 2003; Dobretsov et al., 2013; Kuzmin and Yarmolyuk, 2014; Rudnev et al., 2013a). Little published information on the primary PGE mineralization in the region is available and its potential bedrock source is still unknown. The geology of the region is dominated by rock complexes that are considered as having potential for PGE-bearing mineralization, e.g., Neoproterozoic–Lower Cambrian ophiolite complexes that have variably originated in mid-ocean ridge, back-arc, oceanic island, and island arc settings, or Lower–Middle Paleozoic bimodal volcanic complexes (Alabin and Kalinin, 1999; Buslov, 2011; Explanatory Note…, 2007; Izokh, 1999; Kurenkov et al., 2002; Pinus et al., 1958; Plotnikov et al., 2000). The compositions of the placer PMG grains from this region were used to recognize a series of Uralian–Alaskan-type mafic-ultramafic complexes (Tolstykh, 2004). For example, the Kaigadat massif in the northwestern part of Kuznetsk Alatau was attributed to Uralian–Alaskan-type zoned mafic-ultramafic intrusions, based on its bulk rock composition and the widespread occurrence of ferroan platinum mineral assemblage as the dominant PGM in the alluvium of nearby rivers and streams (Podlipsky and Krivenko, 2001). For example, although the ophiolitic nature of the Srednyaya Ters’ massif has long been recognized (Gertner and Krasnova, 2000; Krasnova and Gertner, 2000; Kurenkov et al., 2002), this has been disputed by some workers who argue that this massif
represents a layered intrusion. The data of A.E. Izokh (Polyakov and Bognibov, 1995) show that dunites of the Srednyaya Ters’ massif have high Pd (up to 1 ppm) and Pt (up to 0.6 ppm) contents (atomic absorption spectrometry). The dunites are relatively enriched in disseminated sulfides and PGM, represented by a wide variety of Pt and Pd compounds with Sb, As, and Te. Low-grade PGE mineralization (Ru–Ir– Os alloys) was found in serpentinites from the Seglebir massif of Gornaya Shoria (Gusev et al., 2004) and rodingites from the Togul-Sungai massif of the Central Salair Ridge (Agafonov et al., 1996, 2000). Ordovician clinopyroxenite-gabbro massifs that have significant PGE anomalies are known in southern Salair (Shokal’skii et al., 2000). In addition, high Pt and Pd values were identified in early Cambrian chromite-rich ultramafic rocks, layered peridotite-gabbro massifs, and carbonaceous schists of some Late Riphean, Late Vendian and Early Cambrian complexes of Kuznetsk Alatau, Gornaya Shoria, and Salair (Explanatory Note…, 2007). Small mafic intrusions and dikes were also regarded by some workers as the most probable source of the PGE in placer deposits. Gold mineralization contributing to the many gold and PGM-bearing placers of the ASFA has long been considered to be genetically related to the dikes of the Middle–Upper Cambrian gabbro-diorite-diabase complex (Bulynnikov, 1948). Most alluvial gold-PGM placer occurrences are related to Quaternary sediments (Butvilovskii et al., 2011; Nesterenko, 1991; and others). Some of these occurrences were re-explored and revived for exploitation. The irregular distribution of placers within the study area is largely controlled by bedrock sources and geomorphology. Most placers are typically found at medium altitudes in river valleys formed by erosional and depositional processes. A few placers occur at lower elevations, and are virtually absent in rugged, high-mountain areas. No published information on the PGM content of most placer occurrences is available, but PGM is generally present at a low grade, ranging from 0.03–0.05% to a few percent of native gold (from 0.5–10.0 mg/m3 to 500–800 mg/m3 by rock). At some localities, the proportion of PGM make up as much as 10–30% of native gold (Kyuz, 1935; Syrovatskii, 1991; Vysotsky, 1933; our data).
Samples and methods All samples were taken using gold dredgers and hydraulic dredgers. The material was washed at the gravity concentration plant using sluice boxes and wash pans to obtain concentrate samples. PGM were extracted from concentrates after they were panned to recover gold grains. The final volume of the concentrate was 5–10 L. The concentrate comprises a heavy black sand. Substantial quantities of native gold and PGM (the degree of enrichment ranging from 5 to 100,000 times) are present in the concentrate sample. Large-scale bulk sampling was employed to obtain an initial sand volume ranging from hundreds to a few thousands of m3. Heavy mineral concentrate sampling was conducted at several used placer deposits. The initial sand volume was 50–400 L. Final treatment of all
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Fig. 1. Geological map showing the location of PGM and gold placers in the western Altai–Sayan folded area. 1, sedimentary rocks (MZ–P); 2, granite (MZ–P). − 3–O); 5, volcanosedimentary rocks (D2–C1); 6, 7, rocks of ophiolite association(V2–C − 2); 8, terrigenous-volcanosedi3, coal-bearing deposits (C1–P2); 4, gabbro (C − 3); 9, mafic rocks (R3–C − 1); 10, ultramafic massifs: K, Kaigadat, ST, Srednyaya Ters’, S, Seglebir, A, Atalyk, TS, Togul-Sungai. 11, major mentary sequence (V2–C faults; 12, Au-placer with PGM grains: with Pt-dominant minerals (a); with Ru–Ir–Os-dominant minerals (b). 13, studied placers: 1, Kaurchak River, dredge 138; 2, Andoba River, dredge 315, 3, Koura River, dredge 317; 4, Taenza River; 5, Bol. Orton River; 6, Bol. Tuluyul River; 7, Poludennyi Kundat River; 8, Pryamoi Kundat River; 9, Shaltyr’-Kozhukh River; 10, Srednyaya Ters’ River; 11, Kel’bes River; 12, Sella River; 13, Simonovskii Creek; 14, Mostovaya River. The KuznetskAlatau–Altai platinum-bearing belt is indicated by black color in the inset (modified from Izokh et al., 2004): 1, Kaigadat massif; 2, Srednyaya Ters’ massif; 3, placers of Gornaya Shoria; 4, Ureg-Nuur area (Mongolia).
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Table 1. Comparison of selected WDS (microprobe) and EDS (SEM) analyses of PGM from pacers of southern Siberia (wt.%) Placer
Grain
Analysis
Os
Ir
Pt
Ru
Rh
Pd
Fe
Ni
Cu
Total
Koura R.
82-44
WDS
45.09
16.61
11.48
25.42
0.75
B.d.l.
0.48
B.d.l.
0.13
99.97
82-44
EDS
43.8
19.2
10.3
25.6
1.0
B.d.l.
0.5
B.d.l.
B.d.l.
100.4
82-44
EDS
45.2
18.2
9.1
26.2
1.3
B.d.l.
0.3
B.d.l.
B.d.l.
100.3
Taenza R.
Kaurchak R.
66-68
WDS
31.68
54.73
7.10
6.28
0.34
0.16
0.25
0.03
0.02
100.57
66-68
EDS
29.5
53.9
6.1
6.3
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
96.1
66-12
WDS
0.91
3.92
87.01
0.43
1.00
0.44
6.29
0.16
0.32
100.39
66-12
EDS
1.2
3.8
85.8
B.d.l.
0.8
B.d.l.
6.3
B.d.l.
0.3
98.2
66-12
EDS
0.0
3.9
88.3
0.5
1.0
0.5
6.2
B.d.l.
0.3
100.7
WDS
9.09
75.5
0.38
10.54
1.70
B.d.l.
1.04
0.22
0.04
98.5
82-146
EDS
10.6
77.0
B.d.l.
10.4
1.8
B.d.l.
1.1
B.d.l.
B.d.l.
100.9
82-146
EDS
10.8
77.7
B.d.l.
10.7
1.6
B.d.l.
1.1
B.d.l.
B.d.l.
101.9
82-176
WDS
19.92
49.34
14.51
13.51
0.70
B.d.l.
0.58
0.29
0.04
98.93
82-176
EDS
21.8
48.3
13.1
13.0
1.4
B.d.l.
0.6
0.4
B.d.l.
98.6
82-162
WDS
43.53
42.84
0.91
10.57
0.22
B.d.l.
0.34
0.08
0.13
98.63
82-162
EDS
44.1
42.4
1.5
10.1
B.d.l.
B.d.l.
0.4
B.d.l.
B.d.l.
98.5
82-162
EDS
45.3
42.9
B.d.l.
10.7
0.7
B.d.l.
0.6
B.d.l.
B.d.l.
100.2
82-166
WDS
26.35
67.3
3.73
0.29
0.32
B.d.l.
0.25
0.08
0.047
98.36
82-166
EDS
26.4
69.9
2.5
B.d.l.
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
99.1
82-166
EDS
27.0
67.2
3.3
0.7
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
98.5
Poludennyi Kundat R. 82-146
Pryamoi Kundat R.
Note. WDS, Results of electron microprobe analysis; EDS, Results of scanning electron microscopy. B.d.l., Below detection limit.
samples comprised hand-panning using a stepwise procedure (Boitsov et al., 2005) to minimize loss of precious metals. PGM grains were handpicked from the final concentrates under a binocular microscope and then examined for grain size, morphology, and surface texture. Selected PGM grains were mounted in epoxy blocks and polished with diamond paste for further analysis using a set of analytical methods. The composition and morphology of the nanoscale PGM grains were investigated using a MIRA 3 LMU (Tescan) scanning electron microscope with an attached INCA Energy 450 XMax 80 (Oxford Instruments—NanoAnalysis) energy dispersion system, which allowed detection of nanoparticles. The analytical conditions were accelerating voltage 20 kV, beam current 1600 pA (1.6 nA), with 30 s counting time, energy resolution (MIRA) 126–127 eV at the Mn Kα line, and spot size 12 nm (MIRA). Spatial resolution of the analysis was a function of the spot size of the X-ray beam (3–5 µm and more), depending on the average atomic number of element and the wavelength of any particular line. The minimum detection limit for most elements was 0.2–0.3% (3-σ criterion) and may reach 0.5–0.8% and more in the case of element interferences or when using the L-series X-rays for “heavy” elements (Z > 72). Analytical accuracy was often as low as 1 rel.% for major elements (C > 10–15%) and varied from 2–6 to 10 rel.% for minor elements present in excess of 1–10%, reaching 20–30% rel. for elements present near the detection limits. Low detection limit and high accuracy for trace elements were obtained by increasing the counting time. At the same time, the detection limit and accuracy decreased
by half as the counting time was increased by a factor of four. The counting times ranged from 30 s for the main elements in the PGM to 150 s for the trace elements. The analyses were conducted at the X-ray Laboratory of the Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (analysts N.S. Karmanov, M.V. Khlestov, D.K. Belyanin). The standards used were SiO2 (O and Si), NaAlSi3O8 (Na), MgCaSi2O6 (Mg and Ca), Al2O3 (Al), FeS2 (S), NaCl (Cl), KAlSi3O8 (K), Cr2O3 (Cr), PtAs2 (As), SnO2 (Sn), HgTe (Hg), PbTe (Pb and Te), Bi2Se3 (Bi, Se), and pure metals (Ti, V, Mn, Pt, Ir, Os, Pd, Rh, Ru, Fe, Cu, Ni, Co, Au, and Ag). Detection limits of the elements (wt.%) were 0.48 for Ru, 0.36 for Os, 0.38 for Ir, and 0.13–0.17 for Rh, Pt, and Pd. Additional control was performed by analyzing all PGM grains on a Camebax-micro electron microprobe at the Analytical Center of the Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, in wavelength dispersive X-ray spectrometry mode (WDS) with a finely focused beam (<2 µm). The analytical conditions were accelerating voltage 20 kV, 20–30 nA beam current, and 10 s counting time. The following X-ray lines and standards were used: PtLα, IrLα, OsMα, PdLα, RhLα, RuLα, AgLα, AuLα (pure metals), AsLα (synthetic InAs), SbLα (synthetic CuSbS2), SKα, FeKα, CuKα (synthetic CuFeS2), NiKα, CoKα (synthetic FeNiCo), BiMα (synthetic Bi2Se3). Element interference was corrected using experimentally measured coefficients (Lavrent’ev and Usova, 1994). Detection limits of the elements (wt.%) were 0.17 for Pt, 0.15 for Ir; 0.04 for Os, 0.04 for Pd, 0.04 for Rh, 0.04 for Ru, 0.03 for Fe, 0.06 for
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Cu, 0.07 for Ni, 0.05 for Co, 0.02 for S, 0.05 for As, and 0.06 for Sb. A comparison of the WDS (wavelength dispersive X-ray spectrometry, microprobe) and EDS (energy dispersive X-ray spectrometry, SEM) data on PGM (Table 1) with the results obtained on olivine, garnet, pyroxene, ilmenite, and Cr-spinel (Lavrent’ev et al., 2015) has demonstrated good reproducibility, which enables the use of the EDS data in mineralogical and geochemical studies. It should be noted that identification of sample heterogeneity and high spot resolution of the EDS analysis make it more preferable over WDS for analysis of the mineral chemistry of the assemblages and aggregates (e.g., PGM exsolutions) in the nanometer-size range. In these cases, the microprobe data can be used to characterize bulk compositions of these nanoscale polymetallic aggregates. Microtextural observations of PGM were performed by means of reflected light microscopy with a Zeiss AxioScope. A1 microscope.
Results PGM morphology. Overall, the morphology of the PGM grains from different placers is remarkably similar but may exhibit slight variation. Grain sizes range from 0.1 to 1.0– 1.5 mm, being up to 2 mm in individual samples. Most grains (over 70%) are smaller than 0.25 mm. Grains in the size range of –0.25 mm are less abundant (7–35 wt.%), whereas PGM grains 0.25–0.5 and 0.5–1.0 mm in size are more common. Grains of Pt–Fe and Os–Ir–Ru alloys are characterized by different surface morphologies. The Pt–Fe alloy grains appear smoother and more rounded, whereas most Os–Ir–Ru alloy grains are oblate. The PGM grains with well-rounded shapes are most common in the fine size range (smaller than 0.2 mm) (Fig. 2). The PGM grains display a wide range of shapes. Knobby, subrounded, spherical, thin-tabular, deformed octahedral and cubic grains with smooth edges or druse-like, irregularly shaped, acicular grains are present. Less common are grains with an embayed, rough surface. All the above morphologies can be found only locally and in various proportions. The surfaces of Pt–Fe alloy grains often bear negative imprints of host crystals. Many grains are mantled by dark layers, which may either partially in-fill cavities on the surfaces or cover the entire surface of individual grains. Os–Ir–Ru alloys occurs as oblate, tabular, poorly rounded grains, often with flat, smooth surfaces and a prominent pinacoidal cleavage. Well-formed cubic and hexagonal crystals are common. Some grains have shagreen surfaces and growth steps. The pinacoidal surfaces may exhibit negative growth pyramids. Rounded grains with “rock candy” reflective surfaces are present in one sample from the Taenza River placer. Sperrylite occurs as isolated angular and rarer rounded grains. Rare angular grains of laurite and irarsite are present in the Bolshoi Orton River placer. Two sets of fractures are detected in PGM grains. The curved fractures with contorted edges filled with supergene minerals (kaolinite, goethite, illite, etc.) are erratically distrib-
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Fig. 2. PGM concentrate (dredge 138), Kaurchak River, Gornaya Shoria. The concentrate composition (%): PtFe—88, RuIrOs—8, PtAs2—4. Photomicrograph shows small grains of oval and rounded shapes.
uted on the surfaces of the PGM grains (Fig. 3a, b). Fractures of this type contain no alteration products. Rectilinear fractures commonly form en-echelon or parallel arrays within the entire grain volume. They have relatively smooth surfaces and are characterized by the development of endogenic minerals (rutile, amphibole, chlorite) and alteration products at their contacts (Fig. 3c). Composition of PGM. The minerals are dominated by Pt–Fe(±Cu) and Os–Ir–Ru(±Pt) solid solutions, and subordinately by sperrylite (up to 3–4%, rarely to 17%) and other PGE-bearing minerals (sulfides, arsenites, sulfarsenides) (Table 2). The proportions of Fe–Pt and Os–Ir–Ru alloy grains vary considerably in placers of Kuznetsk Alatau and Salair. The principal PGM in placers of Gornaya Shoria is ferroan platinum, whereas Os–Ir–Ru alloys predominate in the Taenza and Bolshoi Orton Rivers placer located to north, at the junction between Gornaya Shoria and Kuznetsk Alatau. Pt–Fe(±Cu) solid solutions exhibit some similarities but also important differences. The majority of grains are isoferroplatinum (Pt3Fe) with Fe + Ni = 20–30 at.%, and subordinate grains are native platinum Pt(Fe) with Fe + Ni = 10– 20 at.% (Fig. 4). Cu (<1.0–1.5 to 3–4 at.%) is invariably present in Pt–Fe alloys (Table 3). The rims of some grains from different placers of Gornaya Shoria (Fig. 4b) are extremely enriched in Cu (up to 15 at.% and more) due to the presence of tulameenite, Cu-rich isoferroplatinum, Pt3(Fe0.6⋅ Cu0.4)–Pt3(Fe0.4Cu0.6), and hongshiite, Pt1.1Cu0.9–Pt1.2Cu0.8. Four groups of Pt(Fe-Cu) solid solutions can be divided based on their Cu content: (1) Pt–Fe(–Cu)—Cu < 1 at.%; (2) Pt– Fe–Cu—Cu 1–5 at.%; (3) Pt–Fe–Cu—Cu 7–15 at.%; (4) Pt– Cu(–Fe) (hongshiite)—Cu 40–50 at.%. The Cu-rich varieties of Pt–Fe are the principal PGM in placers of Gornaya Shoria. The contents of other trace elements vary from 2 to 6 wt.%, while the maximum concentrations of individual elements may be as high as 2–5 wt.%. In decreasing order of abundance, the following elements are present in PGM grains: Rh → Ir → Os → Pd, Ru → Ni. Based on various combinations of these trace elements, all Pt–Fe alloys can be divided into the following groups: Cu–Rh-dominant alloys, Os-dominant alloys
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Fig. 3. Fractures in the deformed placer PGM grains from southern Siberia. a, Isoferroplatinum (Kaurchak River) displaying a network of curve fractures; b, rectilinear parallel fractures in rutheniridosmine (Kaurchak River); c, thin parallel fractures in rutheniridosmine with rutile, tourmaline, amphibole (Poludennyi Kundat RIver). a, b, Photomicrographs, a Zeiss AXIO Scope A1 microscope; c, Back-scattered electron image (SEM BSE, Teskan Orsay Holding).
enriched in Cu, Rh, and Ru, Ir-dominant alloys enriched in Cu, Rh, and Ru. Os–Ir–Ru (±Pt) alloys range in composition from the most common Os-dominant alloys and subordinate Ir-dominant alloys to minor Ru-dominant alloys (Fig. 5, Table 3). In the Os–Ru–Ir diagram, these compositions form two distinct Osand Ru-enrichment trends. The compositions defining an Os trend are typical of the placer PGM from Gornaya Shoria. The grains of Os–Ir–Ru(±Pt) alloys forming a Ru trend are reported from all placer PGM analyzed. However, the individual Os–Ir–Ru(±Pt) grains are mostly homogeneous in composition, except for Os–Ir–Ru alloys in the Taenza River placer, which exhibit variations in the concentration of Ir from rim to core. Compositional zoning may be manifested in the development of parallel rims of Ru-rich iridosmine, Os47.6Ir35.9Ru16.4, rutheniridosmine rims with elevated contents of Pt (Ru52.7Os20.6Ir20.6Pt8.2Fe0.9), or in the growth of boxy grains (Fig. 6). Figure 6b shows photomicrographs of a
PGM grain with a hexagonal crystal of native Os (Os89.9Ir8.6Ru1.6) occurring as a broken, partly splintered core mantled by osmiride (Ir61.6Os33.8Ru4.6), which is replaced by a more Ru-rich phase toward a rim. The splintered portion of the osmium crystal is filled with kaolinite. Microtextures of PGM. Based on the timing and conditions of crystallization, the observed textures of the placer PGM grains are interpreted to have either a magmatic or postmagmatic origin. Massive microtextures (homogeneous), exsolution-induced textures and inclusions are typical of heterogeneous mineral grains formed at the magmatic stage. Texturally homogeneous grains are uncommon. Inclusions are commonly observed in combination with the massive textures in the matrix. Inclusions are interpreted to have formed during sequential crystallization of PGE-rich phases from the magma or during entrapment of primary magmatic material (aluminosilicate inclusions). Osmium (with traces of Ru and Ir) is commonly present as euhedral inclusions in isoferroplatinum.
Table 2. Types of assemblages of PGM in placers of southern Siberia Region
Placer
PGM, % Pt–Fe alloys
I
II
III
Os–Ir–Ru alloys
Sperrylite PtAs2
As-, S-rich PGE
Kaurchak R., dredge
88
8
4
–
Andoba R., dredge
88
9
3
–
Koura R., dredge*
93
2
4
1
Taenza R.
2
98
–
–
Bol. Orton R.
21
75
–
4
Bol. Tuluyul R.
65
24
11
–
Poludennyi Kundat R.
22
78
–
–
Pryamoi Kundat R.
17
58
17
8
Shaltyr’-Kozhukh R.
67
33
–
–
Srednyaya Ters’ R.
67
33
–
–
Kel’bes R.
72
28
–
–
Sella R.
60
37
3
–
Simonovskii Creek
100
–
–
–
Mostovaya R.*
8
92
–
–
Note. Hereafter in the tables: I, Gornaya Shoria; II, Kuznetsk Alatau; III, Salair. * Data from this study and from Krivenko et al. (1994) and Tolstykh et al. (1996).
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
1443
Fig. 4. Composition of Pt, Fe (±Cu) solid solutions. a, Matrix (grain); b, inclusions and rims. 1, Gornaya Shoria; 2, Kuznetsk Alatau; 3, Salair.
The Cu-rich Pt–Fe alloy grains containing lamellae of native gold are found in PGM from the Andoba River placer. Such textures formed by exsolution from PGE-bearing solid solutions have been discussed in the literature over a period of more than 50 years (Betekhtin et al., 1958; Cabri et al., 1996). Lattice-like intergrowth (exsolution fabric) of complex solid solutions of (Pt, Fe, Ir, Os, Rh, Ru), Pt–Ir (Rh), and Pt-Os, which may also contain gold, are uncommon. One placer grain from the Koura River alluvium, Gornaya Shoria, displays evidence of exsolution of a composite compound Pt63Fe29Ru3Rh2Ir2PdCu (Fig. 7a) and formation of Ru–Osand Ru–Ir–Os-bearing exsolution phases. This alloy appears to reflect the primary composition of PGE-bearing melt. Some platinum grains from the Polydennyi Kundat River placer (Kuznetsk Alatau) contain exsolution phases consisting
of platiniridium and iridioplatinum (or Ir–Pt phases) in association with (Rh,Pt,Ir)4Sb3 (an Rh-dominant analogue of genkinite) (Fig. 7b). Some grains display complex intergrowth relationships between isoferroplatinum with a lattice of nanoscale Os lamellae and PGE-bearing sulfides and sulfarsenides, replacing this isoferroplatinum, but not Os lamellae, which remain unaltered (Fig. 7c). One grain of isoferroplatinum exhibits a columnar (subgraphic) integrowth of cooperite and xingzhongite in the rim zone (Fig. 7d). This texture was probably a late-stage product that formed as a result of interaction of Cu-, S-, As-rich melt with the exsolution lamellae of Pt-, Ir-, and Rh-dominant alloy. The most common microtextures (patchy, banded, cellular, zoned) formed during the late stages as a result of metasomatic replacement of primary textures (Fig. 8) and are locally
Fig. 5. Composition of Os, Ir, Ru solid solutions. Symbols are the same as in Fig. 4.
1444
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Fig. 6. Rims and zoned boxy PGM grains, Taenza River. Electron images. a, Zoned grains of Os62Ir23Ru15 alloy (2) surrounded by a rim of Ru-dominant alloy (Ru53Os21Ir14Pt8Rh4) (1), inclusions of sanidine (3) and epidote (4); b, a composite boxy grain: native osmium (1), kaolinite (2), osmiride (3), Ru-rich phase (4).
observed in combination with exsolution textures. These textures are composed of PGE-bearing sulfides, arsenides, and sulfarsenides (sperrylite, irarsite, rhodarsenite, etc.). In the Cu-bearing Pt–Fe alloy grains, such textures represented by Cu-rich varieties of platinum (up to hongshiite) are observed in the rim zones. Rims on PGM grains. The presence of rims is typical of many placer PGM grains from Gornaya Shoria and other regions. There are two main types of rims, as indicated by the relationship between a rim and a primary matrix mineral: replacement-induced rims (Fig. 8a, d; Fig. 9a, c) and rims formed by overgrowth and precipitation of minerals (Fig. 9b, f). These rims around PGM grains are formed by PGE-bearing sulfides, sulfarsenides, and arsenides present in variable proportions (Tables 4 and 5): cooperite, sperrylite, irarsite, cuprorhodsite, malanite, hollingworthite, rhodarsenite, platarsite, Pt–Cu–Fe alloys, hongshiite, kharaelakhite, erlichmanite, native gold, etc. The sequence of crystallization of these minerals is not similar in all rims but it shows the tendency of early formation of cooperite and native gold-I (Fig. 9) and late formation of sperrylite. Cooperite replaces platinum leaving inclusions of native Os intact (Fig. 10a). Cooperite tends to be fully replaced by arsenides and sulfarsenides at the margins. Early native gold-I having fineness of 700–800‰ occurs as inclusions along boundaries of PGE-bearing grains and/or within replacement-induced rims. As noted above, in some grains of Cu–Pt–Fe alloys from the Andoba River placer, native gold is present as emulsion–liquation inclusions (Fig. 10b) or branching dendritic segregations (Fig. 10c). A characteristic exsolution texture is locally preserved in the rim zones of one grain of isoferroplatinum (with 0.47 wt.% Cu, 1.31 wt.% Pd, 0.72 wt.% Ru, and 1.93 wt.% Rh) from the Kaurchak River placer. This texture is represented by a thin alternation of bent, dendtritic, elongated nanoscale lamellae of gold (Au84Ag16) and an arsenide phase (Pt and Rh), which formed as a result of late-stage metasomatic-hydrothermal alteration. As a result
of replacement of such zones by cooperite and sperrylite (along the grain margins), gold-I is concentrated and distributed parallel to the isoferroplatinum–sperrylite interface or forms nanoscale inclusions in sperrylite. High-fineness native gold-II may also be present as late-stage smallest (5–10 µm) overgrowths on the surface of native platinum or preexisting rims (Figs. 9b, 10d). Gold-II is characterized by very low Ag (1.4–1.6 wt.%) and the invariable presence of Hg (3.5– 12.5 wt.%, up to 18.3 wt.%). The genesis of this supergene gold remains poorly understood. Microinclusions in PGM grains. Most PGM grains contain submicroscopic and microscopic inclusions (Figs. 8, 11, and 12): (1) formed by exsolution from a primary solid solution; (2) melt and fluid-melt inclusions showing different degrees of alteration and crystallization; (3) relic mineral inclusions (silicates, oxides, etc.); (4) emulsion–liquation inclusions. The shape of the inclusions varies considerably. They appear as tabular, crystallomorphic, lath-shaped or irregular, smoothed, oval and roundish (drops, balls) crystals. They can be monomineralic or polymineralic and are composed of aluminosilicates, oxides, sulfides, and PGM. Some PGM grains exhibit a complex internal structure formed by exsolution, replacement, and recrystallization textures in combination with inclusions of oxides, silicates, and newly-formed PGM. The role of inclusions as an indicator requires further investigation. Common microinclusions in PGM comprise sulfides, sulfarsenides, and arsenides, whereas intermetallic compounds (tellurides and Sb compounds) are rare. Inclusions are more abundant in Pt–Fe solid solutions than in Ru–Ir–Os alloys. PGM grains in Gornaya Shoria placers contain the most abundant and diverse inclusions, which show a far more varied array of minerals (Tables 4–6). More than 20 minerals were identified, including sulfides (cooperite, laurite, braggite, erlichmanite, xingzhongite, bowieite) and rarer minerals. For example, the analyzed PGE-rich thiospinels represent an extensive series, which includes a Co-rich variety, dayingite,
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464 Table 3. Chemical composition of selected PGM grains from placers of southern Siberia (wt.%) Region Placer
Analysis
Solid Sol.
Os
I
EDS
Pt–Fe
EDS
Pt–Fe
EDS
Pt–Fe
Kaurchak R.
Andoba R.
Koura R.
Taenza R.
II
Bol. Tuluyul R.
Poludennyi Kundat R.
Pryamoi Kundat R.
Pt
Ru
Rh
Pd
Fe
Ni
Cu
Total
B.d.l.
B.d.l.
89.7
0.7
2.3
0.9
5.7
1.3
B.d.l.
87.4
0.6
2.1
0.8
6.4
B.d.l.
B.d.l.
99.3
B.d.l.
0.6
B.d.l.
3.9
88.2
0.5
1.0
0.5
6.2
B.d.l.
99.2
0.3
100.6
EDS
Pt–Fe
1.4
3.1
84.6
0.6
B.d.l.
0.5
6.0
B.d.l.
B.d.l.
96.2
EDS
Pt–Fe
B.d.l.
2.0
89.7
B.d.l.
0.7
0.5
6.7
B.d.l.
B.d.l.
99.6
EDS
Pt–Fe
B.d.l.
B.d.l.
90.5
B.d.l.
1.0
B.d.l.
5.6
B.d.l.
0.9
98.0
EDS
Ir–Os
20.5
71.8
B.d.l.
1.0
B.d.l.
B.d.l.
0.5
0.3
B.d.l.
94.1
EDS
Pt–Fe
B.d.l.
1.9
89.7
0.5
0.6
0.5
8.1
B.d.l.
0.7
102.0
EDS
Pt–Fe
B.d.l.
B.d.l.
88.0
B.d.l.
0.5
B.d.l.
8.1
B.d.l.
0.5
97.1
EDS
Pt–Fe
B.d.l.
B.d.l.
90.8
B.d.l.
B.d.l.
B.d.l.
8.3
B.d.l.
0.4
99.5
EDS
Os–Ir–Ru
33.9
31.4
4.9
30.4
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
100.6
EDS
Os–Ir–Ru
36.6
30.2
2.3
28.6
B.d.l.
B.d.l.
0.5
B.d.l.
B.d.l.
98.2
EDS
Os–Ir
74.9
21.9
B.d.l.
3.2
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
100.0
EDS
Os–Ir
67.0
22.6
2.3
5.5
0.7
B.d.l.
0.2
B.d.l.
B.d.l.
98.3
EDS
Ir–Os
34
60.7
B.d.l.
1.0
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
95.7
EDS
Ir–Os
31.5
61.0
B.d.l.
3.1
B.d.l.
B.d.l.
0.2
B.d.l.
B.d.l.
95.8
EDS
Os–Ir–Ru
51.3
39.1
B.d.l.
9.4
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
99.8
EDS
Pt–Fe
1.4
1.5
86.5
0.4
1.8
1.0
5.5
B.d.l.
0.6
98.7
EDS
Pt–Fe
B.d.l.
2.2
87.6
B.d.l.
B.d.l.
B.d.l.
7.8
B.d.l.
1.2
98.8
EDS
Os–Ir–Ru
48.0
41.9
B.d.l.
9.4
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
99.6
EDS
Os–Ir–Ru
45.6
38.3
B.d.l.
12.0
0.7
B.d.l.
B.d.l.
B.d.l.
B.d.l.
96.6
EDS
Ru–Os–Ir
36.4
14.7
B.d.l.
48.1
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
99.2
EDS
Pt–Fe
B.d.l.
B.d.l.
86.9
0.5
0.8
B.d.l.
10.1
0.6
1.0
99.9
EDS
Ir–Os–Pt–Ru
21.8
48.3
13.1
13.0
1.4
B.d.l.
0.6
0.4
B.d.l.
98.6
EDS
Os–Ir–Ru
44.4
37.4
1.8
15.9
0.7
B.d.l.
B.d.l.
B.d.l.
B.d.l.
100.2
EDS
Ir–Pt
B.d.l.
64.9
20.9
6.3
7.2
B.d.l.
B.d.l.
B.d.l.
B.d.l.
99.3
EDS
Os–Ir–Ru
45.3
42.9
B.d.l.
10.7
0.7
B.d.l.
0.6
B.d.l.
B.d.l.
100.2
EDS
Pt–Fe
B.d.l.
B.d.l.
88.2
B.d.l.
B.d.l.
B.d.l.
8.4
B.d.l.
B.d.l.
96.6
EDS
Os–Ir–Ru
36.5
26.4
B.d.l.
32.3
1.9
B.d.l.
B.d.l.
B.d.l.
B.d.l.
97.1
EDS
Ir–Os
26.4
69.9
2.5
B.d.l.
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
99.1
Shaltyr’-Kozhukh R.
EDS
Pt–Fe
B.d.l.
B.d.l.
88.8
B.d.l.
0.5
0.6
9.0
0.8
B.d.l.
99.7
EDS
Pt–Fe
B.d.l.
1.9
87.8
B.d.l.
B.d.l.
B.d.l.
8.5
B.d.l.
B.d.l.
98.2
Srednyaya Ters’ R.
WDS
Pt–Fe
0.09
0.20
89.49
B.d.l.
1.24
0.45
8.09
0.07
1.02
100.65
WDS
Pt–Fe
0.07
0.22
89.6
0.16
1.58
0.52
6.46
0.09
0.71
99.41
WDS
Os–Ir–Ru
44.22
36.03
2.55
16.94
0.54
B.d.l.
0.25
0.03
0.08
100.64
WDS
Os–Ir–Ru
30.47
29.53
3.24
35.14
1.08
B.d.l.
0.34
0.02
0.08
99.91
WDS
Os–Ir
89.05
8.20
1.44
0.74
0.50
B.d.l.
0.07
0.07
B.d.l.
100.07
Kel’bes R
Sella R.
III
Ir
Simonovskii Creek* Mostovaya R.**
WDS
Pt–Fe
B.d.l.
B.d.l.
89.00
0.08
0.14
1.11
8.00
0.73
B.d.l.
99.06
WDS
Pt–Fe
B.d.l.
0.29
89.60
0.06
0.77
B.d.l.
7.69
0.40
0.04
98.84
WDS
Pt–Fe
B.d.l.
0.54
90.38
0.03
0.10
B.d.l.
7.47
1.32
0.04
99.88
WDS
Pt–Fe
B.d.l.
0.15
89.89
0.02
0.91
0.48
7.73
0.93
0.01
100.12
WDS
Os–Ir–Ru
38.80
37.00
3.61
20.05
0.56
B.d.l.
0.20
B.d.l.
0.06
100.28
WDS
Os–Ir–Ru
13.95
67.47
5.98
9.31
2.29
B.d.l.
1.06
B.d.l.
0.12
100.18
WDS
Pt–Fe
B.d.l.
1.14
89.73
B.d.l.
0.23
B.d.l.
7.29
B.d.l.
0.76
99.16
WDS
Pt–Fe
B.d.l.
B.d.l.
89.94
B.d.l.
0.62
0.29
7.75
B.d.l.
0.33
98.92
WDS
Pt–Os–Ir
6.38
8.19
76.04
0.77
0.41
B.d.l.
7.59
B.d.l.
0.11
99.49
WDS
Pt–Ru–Ir–Os
12.93
26.23
43.94
13.78
1.95
B.d.l.
0.08
B.d.l.
B.d.l.
98.81
WDS
Pt–Ru–Ir–Os
7.77
12.73
49.54
21.89
6.5
B.d.l.
0.12
B.d.l.
B.d.l.
98.55
WDS
Pt–Fe
0.35
B.d.l.
87.2
B.d.l.
0.28
0.22
10.08
B.d.l.
0.49
98.62
Note. B.d.l., Below detection limit. * Data from Podlipsky et al. (2007). ** Data from Tolstykh et al. (2002).
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Fig. 7. Exsolution textures. Backscattered electron images (SEM) of PGM. a, Lattice-like texture: 1, platinum Pt63Fe29Ru3Rh2Ir2Pd1Cu1, 2, Ru55Ir16Os15Pt10Rh2Fe2, 3, Ru39Os39Ir15Pt7, 4, isoferroplatinum (Koura River); b, lattice fabric of platinum: 1, Ir56Pt19Rh12Ru10Fe3, 2, Pt46Ir32Fe12Rh6Ru4, 3, Rh-genkinite (Rh,Pt,Ir)4Sb3 (Poludennyi Kundat River); c, isoferroplatinum grain with a lattice of osmium lamellae replaced by PGE sulfides and sulfarsenides: 1, isoferroplatinum, 2, hollingworthite, 3, cooperite, 4, sperrylite, 5, platarsite (Koura River); d, columnar, subgraphic texture in the rim around isoferroplatinum grain: 1, 2, cooperite, 3, xingzhongite (Koura River).
Cu(Co, Pt)2S4 (Fig. 10a), a rare derivative of the malanite– carrollite CuPt2S4–CuCo2S4 isomorphic series (up to 39 mol.% Co). Inclusions of dayingite occur in association with malanite at the grain margins or in arsenides occurring as rims around the grains of isoferroplatinum in the placers of Gornaya Shoria (Andoba and Koura Rivers). Simple sulfides like laurite and erlichmanite often form medium and large isolated idiomorphic inclusions (up to 25 µm) (Fig. 10a), whereas more complex sulfides constitute crystalline polymineralic inclusions where they are found in association with Fe–Cu–Ni sulfides. These complex, polyphase sulfide inclusions in the grain of ruthernosmiride from the Polydennyi Kundat River placer contain an unnamed sulfide
phase, a possible Cu-dominant analogue of vysotskite (Pd,Ni)S (Fig. 11, Table 4). Arsenides and sulfarsenides of Ru, Rh, Ir, Pt, and Pd occur as polyphase inclusions or rims (sperrylite), which often reach a thickness of up to 50 µm. Such rims often surround grains of Pt–Fe alloys and are uncommon in Os–Ir–Ru alloys. The mineral composition of these rims directly correlates with that of their host mineral, i.e., arsenide phases tend to be composed of trace elements from a matrix mineral. For example, rhodarsenide (Rh,Pd)2As is commonly found as Rh- and Pd-bearing inclusions in the grains of Pt–Fe alloy. The most common arsenide phases in Os–Ir–Ru alloys are iridsite, iridarsenite.
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
1447
Fig. 8. Secondary microtextures in PGM grains formed by PGE sulfides and sulfarsenides (Koura River). a, Lattice-like formed by sperrylite; b, patchy: sperrylite, hongshiite, platarsite, rhodarsenite occuring as inclusions in Pt–Pd alloy; c, banded; d, zoned. Backscattered electron images (SEM) of PGM.
The microinclusions in placer samples from the Andoba River (Gornaya Shoria) contain PGE-bearing tellurides: telluropalladinite, Pd9Te4, and maslovite, Pt(BiTe). They fill microfractures or occur as inclusions within rim sulfarsenides (Fig. 11). Of special interest are rare microinclusions of Sb intermetallic compounds, like genkinite (Pt,Pd)4Sb3, and their derivatives. They are present as inclusions in isoferroplatinum (Andoba River) or as exsolution textures within a Pt grain represented by alternation of oriented zones made up of platiniridium and iridioplatinum (Poludennyi Kundat River) (Figs. 10b, 11f, and 7b). In the latter case, the exsolution textures are formed by iridioplatinum and coexisting phases of the Rh-dominant analogue of genkinite (Pt,Pd)4Sb3, which incorporates Rh (up to 12 wt.%) instead of Pd.
Besides various PGM, inclusions of silicates, sulfides, oxides and, more rarely, phosphates were identified in some PGE-bearing minerals. Sulfides of Fe, Ni, and Cu are commonly present in some crystalline inclusions: chalcopyrite and pentlandite coexisting with laurite, ferrorhodsite, and irarsite; chalcopyrite and bornite in association with braggite, bowieite, and telluropalladinite; cubanite in association with braggite, ferrorhodsite, and malanite (Fig. 11). Some grains of the Pt–Fe alloys contain Fe and Cr oxides as inclusions. Inclusions of magnetite were detected in placers from the Polydennyi Kundat River area. Large zoned inclusions with variable Cr content are present in Pt–Fe alloy grains from the Koura River placer (Fig. 12). Such inclusions record changes in redox or P–T conditions during melt crystallization.
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Fig. 9. Inclusions and rims in PGM. Electron images. a–c, Kaurchak River; d, Andoba River; e, f, Kaurchak River. a, Sperrylite occurring as a replacement rim (1) and inclusions of early native gold-I with fineness of 700–800‰ (2) in the isoferroplatinum grain; b, overgrowths of late native gold-II with fineness of 863–985‰ on isoferroplatinum; c, overgrowths of late native gold-II (1) on isoferroplatinum containing inclusions of gold-I and rim sperrylite (1); d, overgrowths of late gold-II on isoferroplatinum containing inclusions of gold-I (1) and rim sperrylite (2); e, rim sperrylite (1) surrounding a rounded Pt grain (2), PGM inclusions occuring subparallel to the grain boundary, the grain varies in composition from Pt63Fe20Ir13Os3Ru1 (2) to Ir44Os25Pt23Ru3Rh3Fe2 (3); f, sperrylite (1) and cooperite (2) rim around isoferroplatinum grains (3).
In addition, some PGM grains may contain inclusions of oxide minerals. Various PGE oxides have been documented so far in chromitites from ophiolite complexes of the Urals, Finland, and Oman (Ahmed and Arai, 2003; Garuti et al., 1997; Gornostayev et al., 2000), alluvial occurrences in the Chukotka Peninsula (Mochalov et al., 1992; Gornostayev et al., 1999; and others). The formation of PGE oxides is believed to occur as a result of low-temperature transformation of rocks (Ahmed and Arai, 2003) or even grains during burial diagenesis or low-temperature metamorphism (Gornostayev et al., 1999). No structural data could be obtained for these minerals, because of their small grain-size. Two complex PGE oxides, Ir- and Hg-rich (Table 7, Fig. 13) and Ru-, Fe-, and Ir-rich (Table 7) were identified in two grains of Pt–Fe alloy in the Koura River placer (Gornaya Shoria). These minerals occur at the grain margins, in zones of microfracturing, in intimate association with lamellae of osmium (Fig. 13) and Ru-bearing osmium, probably replacing them. Like the oxides from the Southern Urals (Garuti et al., 1997), the PGE oxides reported in the Koura River placer have stoichiometries between XO2 and X2O3 and an analytical total less than 100 wt.% (up to 93.5 wt.% in the first case and no more than 96.8 wt.% in the second case, Table 7). This was ascribed to the possible presence of OH-groups (Gornostayev et al., 1999; Mochalov et al., 1992) and/or H2O molecules in their composition. It cannot be ruled out that the presence of fractures in the known oxides may lead to a deficit in the analytical total (Ahmed and Arai, 2003). The formulas calculated for these minerals in this study are presented in Table 7. In addition to major
elements, the chemical analysis of the PGE oxides demonstrates the presence of a considerable amount (>1 wt.%) of Pt, Rh, Ca, Mn, and Ti, and traces (≤1 wt.%) of Al, Si, V, As, and S. Not all constituents detected in these compounds are minor elements, but their presence can be explained by a close association with other minerals, e.g., kaolinite. Silicates occur as crystallized melt inclusions in PGM and are commonly represented by amphibole and pyroxene (Fig. 12). Similar aluminosilicate melt inclusions were reported in ferroplatinum grains from placer deposits of the northeastern Siberian platform (Airiyants et al., 2014; Okrugin et al., 2012). In addition, these grains were found to contain olivine, plagioclase, feldspar, biotite, muscovite, quartz, chlorite, garnet (grossular, andradite, schorlomite), epidote, titanite, rutile, and phosphates such as monazite and fluorapatite (3.5 wt.% F). Supergene minerals observed within fractures and on grain surfaces are represented mostly by goethite and kaolinite.
Discussion A detailed analysis of the placer PGM from southern Siberia revealed considerable variation in the degree of abrasions, from well-rounded to unrounded, reflecting longer transportation (Gornostayev et al., 1999; Hattori et al., 2010; McClenaghan and Cabri, 2011; Podlipsky et al., 2007; Tolstykh et al., 2002). Usually, the PGM from the studied placers are poorly rounded, which indicates relatively little transport from their bedrock sources. At the same time, smooth,
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464 Table 4. Composition of PGE sulfides in rims and inclusions from PGM grains Region
Placer
Matrix
Location Os
Ir
Pt
Ru
Rh
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 85.9
B.d.l. 0.9
Pd
Fe
Co
Ni
Cu
Pb
S
Total
Cooperite PtS I
Koura R.
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.5
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 82.9
2.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 85.3
B.d.l. B.d.l. B.d.l. 0.6
I
Kaurchak R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 81.7
1.3
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 16.1
99.1
I
Andoba R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 84.3
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.4
98.7
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 85.2
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.3
99.5
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 85.9
B.d.l. B.d.l. 0.9
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 1.2
0.6
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
II
Pryamoi Kundat R.
(Fe, Cu)Pt
II
Shaltyr’-Kozhukh R.
(Fe, Cu)Pt
81.2
B.d.l. B.d.l. 0.2
0.7
B.d.l. B.d.l. 0.7
101.3
B.d.l. 12.6
98.8
B.d.l. B.d.l. B.d.l. B.d.l. 13.9
99.8
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.5
B.d.l. 0.5
101.3
B.d.l. B.d.l. B.d.l. B.d.l. 14.9
99.1
B.d.l. B.d.l. 84.5
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.1
98.6
r
B.d.l. B.d.l. 86.0
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 15.9
101.9
r
B.d.l. B.d.l. 83.4
B.d.l. B.d.l. 1.03
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.2
98.6
Braggite (Pt, Pd)S I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 59.0
B.d.l. B.d.l. 19.4
0.3
B.d.l. 3.6
B.d.l. B.d.l. 18.1
100.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 62.3
B.d.l. B.d.l. 12.7
0.5
B.d.l. 4.9
B.d.l. B.d.l. 17.6
98.0
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 56.1
B.d.l. B.d.l. 17.7
2.1
B.d.l. 3.2
1.9
B.d.l. 19.9
100.9
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 60.2
B.d.l. B.d.l. 17.5
1.1
B.d.l. 3.6
0.4
B.d.l. 20.6
103.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 59.1
B.d.l. B.d.l. 17.5
0.3
B.d.l. 3.3
0.7
B.d.l. 17.5
98.3
incl.
B.d.l. 31.3
B.d.l. 18.8
14.3
98.2
Xingzhongite (Pb, Cu, Fe)(Ir, Pt, Rh)2S4 I
Andoba R.
(Fe, Cu)Pt
10.1
I
Andoba R.
(Fe, Cu)Pt
incl.
B.d.l. 27.6
11.7
B.d.l. 20.9
B.d.l. 0.5
B.d.l. B.d.l. 5.1
15.4
19.2
100.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 43.1
11.0
B.d.l. 15.2
B.d.l. 1.0
B.d.l. 0.8
6.6
3.0
18.5
99.2
I
Kaurchak R.
(Fe, Cu)Pt
r
B.d.l. 35.8
21.9
1.2
B.d.l. 1.2
B.d.l. B.d.l. 3.6
8.1
17.5
97.4
I
Andoba R.
(Fe, Cu)Pt
r
1.7
28.2
24.8
B.d.l. 9.9
B.d.l. B.d.l. B.d.l. B.d.l. 5.0
10.0
18.5
98.0
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 18.7
28.1
0.3
B.d.l. B.d.l. B.d.l. B.d.l. 5.1
17.8
21.5
98.7
8.2
7.2
B.d.l. 1.0
B.d.l. B.d.l. 4.8
17.9
Kingstonite (Rh, Ir, Pt)3S4 I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 10.1
12.1
B.d.l. 51.4
B.d.l. 0.3
B.d.l. B.d.l. B.d.l. B.d.l. 25.7
99.6
II
Shaltyr’-Kozhukh R.
(Fe, Cu)Pt
incl.
B.d.l. 17.3
14.1
B.d.l. 42.2
B.d.l. 2.6
B.d.l. B.d.l. B.d.l. B.d.l. 24.0
100.2
(Fe, Cu)Pt
incl.
7.6
Laurite RuS2 I
Koura R.
B.d.l. B.d.l. 57.0
2.9
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 32.3 B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 34.9
99.8
I
Taenza R.
Os–Ir–Ru
incl.
11.7
5.9
B.d.l. 46.6
0.7
I
Taenza R.
Os–Ir–Ru
incl.
9.8
5.4
B.d.l. 47.9
B.d.l. 0.5
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 35.0
98.6
99.7
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
6.8
5.2
B.d.l. 52.1
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 35.8
99.9
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
6.9
5.0
B.d.l. 50.7
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 36.4
99.0
Os–Ir–Ru
incl.
39.2
10.5
B.d.l. 15.8
2.2
B.d.l. 30.5
99.4
Erlichmanite OsS2 I
Taenza R.
I
Taenza R.
Os–Ir–Ru
incl.
39.9
11.7
B.d.l. 16.5
1.9
1.3
0.3
B.d.l. B.d.l. B.d.l. B.d.l. 28.3
99.9
I
Koura R.
(Fe, Cu)Pt
r
56.7
7.0
4.6
1.9
B.d.l. 0.6
B.d.l. B.d.l. B.d.l. B.d.l. 25.5
98.7
1.3
B.d.l. B.d.l. 2.0
B.d.l. 11.2
98.0
1.4
B.d.l. 2.1
15.6
B.d.l. 17.0
96.1
B.d.l. B.d.l. 12.1
B.d.l. 31.3
98.9
B.d.l. 29.3
102
2.4
B.d.l. 0.6
B.d.l. 0.4
0.2
Vasilite Pd16S7 I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 1.7
B.d.l. B.d.l. B.d.l. 81.8
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
B.d.l. B.d.l. B.d.l. 0.5
B.d.l. 59.5
Cuprorhodsite CuRh2S4 I
Kaurchak R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 3.7
B.d.l. 50.3
B.d.l. 1.5
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 20.1
B.d.l. 40.1
B.d.l. B.d.l. 0.6
B.d.l. 11.9
(continued on next page)
1450
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Table 4 (continued) Region
Placer
Matrix
Location Os
(Fe, Cu)Pt
incl.
Ir
Pt
Ru
Rh
38.4
B.d.l. 19.1
Pd
Fe
Co
Ni
Cu
Pb
S
Total
Malanite Cu(Pt, Ir)2S4 I
Koura R.
B.d.l. 3.7
I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 38.6
5.8
I
Koura R.
(Fe, Cu)Pt
r
3.8
30.0
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. 6.2
36.7
II
Shaltyr’-Kozhukh R.
(Fe, Cu)Pt
incl.
B.d.l. 39.3
22.4
B.d.l. 0.5
B.d.l. B.d.l. 11.8
B.d.l. 18.3
0.1
B.d.l. B.d.l. 9.6
B.d.l. 26.4
100.4
B.d.l. 11.3
B.d.l. B.d.l. B.d.l. B.d.l. 8.4
B.d.l. 24.5
100.4
B.d.l. 19.3
B.d.l. 1.1
B.d.l. B.d.l. 11.2
B.d.l. 25.2
99.7
14.4
B.d.l. 8.6
B.d.l. 1.2
B.d.l. B.d.l. 11.8
B.d.l. 23.9
99.2
1.6
B.d.l. 26.4
99.9
Ferrorhodsite (Fe, Cu)(Rh, Ir, Pt)2S4 I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 3.2
10.9
B.d.l. 32.2
B.d.l. 14.4
B.d.l. 1.2
7.0
B.d.l. 29.5
98.4
II
Tuluyul R.
Os–Ir–Ru
incl.
B.d.l. 17.6
1.4
B.d.l. 33.7
B.d.l. 13.6
B.d.l. 1.9
5.8
B.d.l. 26.9
100.9
Dayingite Cu(Co,Pt)2S4 I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 43.7
B.d.l. 2.2
B.d.l. B.d.l. 11.5
B.d.l. 14.0
B.d.l. 28.4
99.8
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 43.1
B.d.l. 1.5
B.d.l. B.d.l. 11.5
B.d.l. 14.0
B.d.l. 29.0
99.0
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 43.1
B.d.l. 1.5
B.d.l. B.d.l. 11.1
B.d.l. 13.5
B.d.l. 29.8
98.9
Os–Ir–Ru
incl.
B.d.l. B.d.l. B.d.l. 0.5
B.d.l. 15.0
96.1
(Pd, Cu)S phase II
Poludennyi Kundat R.
B.d.l. 58.5
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
B.d.l. 1.3
1.2
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
B.d.l. 1.3
B.d.l. B.d.l. B.d.l. 72.0
B.d.l. B.d.l. 66.5
1.4
B.d.l. 5.1
15.6
0.6
B.d.l. 2.1
15.3
B.d.l. 13.7
100.7
B.d.l. B.d.l. 1.5
11.4
B.d.l. 13.4
99.6
Note. Here and in Table 5: B.d.l., below detection limit; incl., in inclusions; r, in grain rims.
well-rounded to ball-shaped grains (Fig. 2) are common in the finest size fraction (less than 0.2 mm), which, at first glance, appear inconsistent with the assumption about a short transportation distance. In this case, a high degree of roundness and smooth surfaces of grains in the fine size fraction does not reflect the degree of abrasion of PGM grains during fluvial transport, but instead has a primary, endogenic nature. Since the fine particles can be carried as suspended load by stream water, they will become rounded slowly (Nesterenko, 1991; Nesterenko et al., 2013). In some cases, discrimination between PGM grains subjected to mechanical wear and well-rounded PGM grains of primary endogenic origin can be problematic because of a lack of the respective criteria. Here, though, we may notice a certain analogy with native gold. For example, the gold microinclusions in pyrite often have a roundish, drop-like shape, which can be preserved in alluvium (Nesterenko and Kolpakov, 2010). We suggest that the following distinctive features may be indicative of primary endogenic origin of smooth well-rounded PGM grains: (1) the preservation of rims on PGM grains of endogenic origin which were not mechanically eroded (Fig. 9f, e); (2) the presence of textures at the grain margin that are conformable to the outline of the grain, e.g., see Figs. 9e and 14a. The rim of the grain of native osmium (Os70–74Ir20–21 Ru5–11) contains a veinlet of irarsite, which is contorted subparallel to the grain edge (surface); (3) concentric zoning in rounded grains; (4) the presence of remnants of primary minerals and rocks on the surface of PGM grains. The rounded grain of sperrylite (Fig. 14b) displays a pronounced chemical and textural concentric zoning, with a core composed of
chalcopyrite, irarsite, and platarsite, and titanite occurring as inclusions in a common microzone; (5) smooth dendritic segregations within PGM grains with relatively deep embayments and negative crystal imprints (Fig. 14c). Such outlines can be produced at the endogenic stage of PGM formation, but they are unlikely to have been derived from erosion and abrasion processes. The analysis of the grain structure and the presence of rims, first of all, can be of crucial importance in this respect. As noted above, two types of rims were identified within Pt-bearing mineral grains: replacement rims and overgrowth (precipitation) rims. The latter have sharp, smooth boundaries with subtle signs of interaction between the rim and matrix minerals. They are internally homogeneous and monotonic, whereas replacement-induced rims have an irregular, patchy texture. These rims are products of late-magmatic and postmagmatic processes. In contrast to overgrowth (precipitation) rims, metasomatic replacement rims have been widely discussed in the literature. In our opinion, the overgrowth (precipitation) rims may record important genetic information. They may be indicative of the transportation of Pt in magmatic fluids and are commonly developed around PGM grains in placers of Gornaya Shoria. This testifies to the widespread development of fluid-saturated ore-forming systems in this region, e.g., along the Kaurchak, Koura, and Andoba Rivers (e.g., Seglebir massif). At the same time, the presence of relic rims of Pt-rich sulfides in some alluvially worn PGM grains from the Shaltyr’–Kozhukh River area (Kuznetsk Alatau) may indicate negligible alluvial reworking of the material in Gornaya Shoria compared to the central part of the Kuznetsk
1451
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464 Table 5. Composition of PGE sulfarsenides and arsenides in inclusions and rims from PGM grains Region
Placer
Matrix
Location Os
Ir
Pt
Ru
Rh
Pd
Fe
Co
Cu
S
As
Sb
Te
Total
1.2
98.5
Sperrylite PtAs2 I
Koura R.
Os–Ir–Ru
incl.
2.5
7.5
45.8
1.0
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 39.6
0.9
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 2.2
52.6
0.5
2.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 1.7
53.9
I
Kaurchak R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 53.6
I
Kaurchak R.
(Fe, Cu)Pt
r
I
Kaurchak R.
(Fe, Cu)Pt
r
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. B.d.l. B.d.l. 1.1
40.5
B.d.l. B.d.l. 99.2
B.d.l. 0.7
B.d.l. 0.3
B.d.l. B.d.l. 0.9
39.8
B.d.l. B.d.l. 97.3
B.d.l. 2.4
B.d.l. 0.3
B.d.l. B.d.l. 1.8
40.0
B.d.l. B.d.l. 98.0
B.d.l. B.d.l. 49.5
B.d.l. 2.3
0.6
0.7
0.8
B.d.l. 1.4
42.3
B.d.l. B.d.l. 97.6
B.d.l. B.d.l. 55.5
B.d.l. B.d.l. 1.0
0.4
0.9
0.4
2.1
38.5
B.d.l. 1.3
B.d.l. 4.5
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 1.1
40.0
B.d.l. B.d.l. 98.8
53.2
100.1
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 2.3
52.5
B.d.l. 1.1
B.d.l. B.d.l. B.d.l. B.d.l. 1.0
40.4
B.d.l. B.d.l. 97.3
II
Pryamoi Kundat R.
(Fe, Cu)Pt
incl
B.d.l. 2.8
59.1
B.d.l. 1.7
B.d.l. 2.0
32.8
B.d.l. B.d.l. 99.6
Os–Ir–Ru
r
0.4
B.d.l. 7.9
B.d.l. B.d.l. 1.2
Iridarsenite IrAs2 I
Taenza R.
47.0
1.0
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 44.0
B.d.l. B.d.l. 100.3
Cherepanovite RhAs I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 9.4
B.d.l. 49.2
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 42.3
B.d.l. B.d.l. 100.9
I
Taenza R.
Os–Ir–Ru
incl.
0.8
1.7
1.4
7.8
47.9
B.d.l. 0.3
B.d.l. B.d.l. B.d.l. 40.0
B.d.l. B.d.l. 99.9
I
Taenza R.
Os–Ir–Ru
incl.
1.5
1.1
2.7
7.1
44.1
B.d.l. 0.4
B.d.l. B.d.l. B.d.l. 39.9
B.d.l. B.d.l. 96.8
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 1.9
0.8
49.6
18.2
B.d.l. B.d.l. B.d.l. 26.7
B.d.l. B.d.l. 98.1
Rhodarsenide (Rh, Pd)2As I
Kaurchak R.
0.9
I
Andoba R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 2.5
B.d.l. 49.3
18.7
2.5
B.d.l. B.d.l. B.d.l. 24.2
B.d.l. B.d.l. 97.1
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 0.4
B.d.l. 46.7
25.4
2.1
B.d.l. 0.5
B.d.l. B.d.l. 100.2
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 5.0
B.d.l. 48.6
19.1
0.2
B.d.l. B.d.l. B.d.l. 25.4
B.d.l. B.d.l. 98.2
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 5.5
1.7
49.0
20.7
0.7
B.d.l. B.d.l. B.d.l. 23.0
B.d.l. B.d.l. 100.5
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 7.6
B.d.l. 39.1
28.0
B.d.l. B.d.l. B.d.l. B.d.l. 26.2
B.d.l. B.d.l. 100.8
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 5.6
B.d.l. 41.9
22.1
3.3
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 11.3
B.d.l. 46.6
18.4
B.d.l. B.d.l. B.d.l. B.d.l. 24.0
B.d.l. B.d.l. 100.3
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 10.4
B.d.l. 45.1
20.9
B.d.l. B.d.l. B.d.l. B.d.l. 23.1
B.d.l. B.d.l. 99.5
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 11.2
B.d.l. 43.0
19.0
B.d.l. B.d.l. B.d.l. B.d.l. 25.9
B.d.l. B.d.l. 99.1
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 54.8
B.d.l. 10.1
B.d.l. 2.7
B.d.l. B.d.l. 7.3
25.9
B.d.l. B.d.l. 100.8
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 0.9
50.5
B.d.l. 8.5
B.d.l. 1.2
B.d.l. B.d.l. 7.4
29.4
B.d.l. B.d.l. 97.9
I
Kaurchak R.
(Fe, Cu)Pt
r
2.1
65.9
0.9
B.d.l. B.d.l. B.d.l. B.d.l. 12.8
8.8
B.d.l. B.d.l. 96.3
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 58.9
B.d.l. 3.8
B.d.l. 0.9
0.4
B.d.l. 7.4
24.3
B.d.l. B.d.l. 95.7
I
Koura R.
(Fe, Cu)Pt
r
1.6
6.5
34.5
2.5
13.0
B.d.l. 0.2
B.d.l. B.d.l. 7.4
34.1
B.d.l. B.d.l. 99.8
I
Koura R.
(Fe, Cu)Pt
r
2.2
4.6
38.0
2.0
9.6
B.d.l. 0.5
B.d.l. B.d.l. 5.7
34.6
B.d.l. B.d.l. 97.2
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. 1.4
50.5
2.8
10.4
B.d.l. 0.4
B.d.l. B.d.l. 6.5
29.2
B.d.l. B.d.l. 101.2
II
Pryamoi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. 1.9
49.5
0.5
8.9
B.d.l. 1.8
(Fe, Cu)Pt
incl.
B.d.l. 0.6
49.8
7.6
B.d.l. B.d.l. 5.8
(Fe, Cu)Pt
incl.
0.0
35.8
2.2
13.3
7.7
B.d.l. 25.2
B.d.l. B.d.l. B.d.l. 25.3
1.3
B.d.l. 99.5
Platarsite PtAsS I
II
1.4
4.4
B.d.l. 0.3
10.7
B.d.l. B.d.l. 9.0
28.0
B.d.l. B.d.l. 101.6
25.6
B.d.l. B.d.l. 98.4
28.4
B.d.l. B.d.l. 100.1
Irarsite (Ir, Ru, Rh, Pt)AsS I
Koura R.
B.d.l. B.d.l. B.d.l. B.d.l. 12.7
I
Koura R.
(Fe, Cu)Pt
incl.
1.7
34.6
2.1
13.9
9.9
B.d.l. 0.4
0.5
B.d.l. 11.2
25.6
B.d.l. B.d.l. 99.9
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 34.4
3.6
12.4
8.7
B.d.l. 1.4
B.d.l. B.d.l. 13.2
25.3
B.d.l. B.d.l. 99.0
I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 49.1
B.d.l. 5.1
7.6
B.d.l. B.d.l. B.d.l. B.d.l. 12.6
25.7
B.d.l. B.d.l. 100.0
I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 43.6
B.d.l. 6.0
12.2
B.d.l. 0.3
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 32.5
2.3
28.2
B.d.l. B.d.l. B.d.l. B.d.l. 8.2
I
Koura R.
(Fe, Cu)Pt
r
0.2
33.6
2.4
1.7
30.2
B.d.l. 0.7
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 35.0
4.4
2.0
21.2
B.d.l. B.d.l. B.d.l. B.d.l. 10.1
1.2
B.d.l. B.d.l. 12.2
B.d.l. B.d.l. 5.5
23.8
B.d.l. B.d.l. 98.1
26.5
B.d.l. B.d.l. 98.9
26.4
B.d.l. B.d.l. 100.7
27.6
B.d.l. B.d.l. 100.3
(continued on next page)
1452
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Table 5 (continued) Region
Placer
Matrix
Location Os
I
Koura R.
(Fe, Cu)Pt
r
I
Taenza R.
Os–Ir–Ru
Ir
Pt
Ru
Rh
Pd
Fe
Co
Cu
S
As
Sb
B.d.l. 33.7
3.2
1.5
21.5
B.d.l. B.d.l. B.d.l. B.d.l. 9.1
r
2.8
51.4
B.d.l. 9.0
Te
Total
30.2
B.d.l. B.d.l. 99.1
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 12.5
24.7
B.d.l. B.d.l. 100.4
Hollingworthite (Rh, Pt, Pd)AsS I
Kaurchak R.
(Fe, Cu)Pt
incl.
0.1
6.6
29.4
10.1
22.5
B.d.l. 0.3
B.d.l. B.d.l. 12.3
19.5
B.d.l. B.d.l. 100.8
I
Kaurchak R.
(Fe, Cu)Pt
incl.
0.2
5.0
12.4
6.2
27.2
B.d.l. 0.3
B.d.l. B.d.l. 15.2
31.0
B.d.l. B.d.l. 97.5
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 6.5
23.2
3.4
23.7
B.d.l. 0.3
B.d.l. B.d.l. 12.9
29.6
B.d.l. B.d.l. 99.6
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 9.9
22.0
3.3
22.1
B.d.l. B.d.l. B.d.l. B.d.l. 10.6
30.8
B.d.l. B.d.l. 98.7
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 10.1
29.0
4.6
15.4
B.d.l. 0.3
32.1
B.d.l. B.d.l. 100.4
B.d.l. B.d.l. 9.1
I
Koura R.
(Fe, Cu)Pt
r
0.1
8.2
28.2
3.2
22.6
B.d.l. 1.2
B.d.l. B.d.l. 8.6
25.1
B.d.l. B.d.l. 97.2
II
Tuluyul R.
(Fe, Cu)Pt
incl.
1.4
4.9
14.2
7.4
37.2
B.d.l. 0.2
B.d.l. B.d.l. 7.7
27.4
B.d.l. B.d.l. 100.4
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 29.2
7.6
22.9
B.d.l. 0.6
B.d.l. 0.4
13.3
25.5
B.d.l. B.d.l. 99.5
II
Pryamoi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. 5.7
0.7
27.8
B.d.l. 0.4
B.d.l. B.d.l. 10.8
29.9
B.d.l. B.d.l. 95.7
20.4
Table 6. Composition of PGE-bearing intermetallic compounds occurring as inclusions and fracture-fill in PGM grains Region
Placer
Matrix
Os
Ir
Pt
Ru
Rh
Pd
Fe
Ni
Cu
Bi
S
As
Sb
Te
Total
Pt3(Rh,Pd)Sb3 phase I
Andoba R.
(Fe, Cu)Pt
N.a.
3.1
45.1
N.a.
6.8
N.a.
3.4
0.4
N.a.
N.a.
N.a.
N.a.
39.6
N.a.
98.4
I
Koura R.
Os–Ir–Ru
1.7
3.1
43.4
4.7
7.7
1.5
N.a.
0.5
N.a.
N.a.
N.a.
0.7
37.2
N.a.
100.5
I
Koura R.
Os–Ir–Ru
1.7
1.2
47.5
2.8
9.5
B.d.l. N.a.
I
Koura R.
Os–Ir–Ru
2.0
2.4
50.6
B.d.l. 8.1
1.5
0.1
N.a.
N.a.
N.a.
B.d.l. 38.0
N.a.
100.8
B.d.l. 0.8
N.a.
N.a.
N.a.
B.d.l. 35.9
N.a.
101.3
II
Poludennyi Kundat R.
Os–Ir–Ru
N.a.
5.5
42.5
0.8
12.4
4.5
0.2
N.a.
N.a.
N.a.
N.a.
N.a.
31.3
N.a.
97.2
II
Poludennyi Kundat R.
Os–Ir–Ru
N.a.
3.9
40.9
B.d.l. 17.9
3.6
B.d.l. N.a.
N.a.
N.a.
N.a.
N.a.
33.5
N.a.
99.8
II
Poludennyi Kundat R.
Os-Ir-Ru
N.a.
4.9
46.2
0.4
11.6
5.4
0.2
N.a.
B.d.l. N.a.
N.a.
N.a.
33.0
N.a.
101.7
II
Poludennyi Kundat R.
Os-Ir-Ru
N.a.
4.5
50.1
0.5
10.8
4.5
1.0
N.a.
0.4
N.a.
N.a.
N.a.
30.2
N.a.
102.0
(Fe, Cu)Pt
N.a.
2.2
27.9
N.a.
5.7
N.a.
1.8
N.a.
0.4
39.4
N.a.
N.a.
2.2
22.5
102.1
Maslovite PtBiTe I
Andoba R.
I
Andoba R.
(Fe, Cu)Pt
N.a.
2.8
20.1
N.a.
9.1
N.a.
1.4
N.a.
0.4
51.1
N.a.
N.a.
1.4
14.2
100.5
I
Andoba R.
(Fe, Cu)Pt
N.a.
2.4
26.6
N.a.
6.5
N.a.
1.1
N.a.
B.d.l. 37.9
N.a.
N.a.
1.6
22.0
98.1
(Fe, Cu)Pt
N.a.
N.a.
4.6
B.d.l. 1.8
58.9
1.1
N.a.
0.4
1.9
N.a.
N.a.
30.0
98.6
Telluropalladinite Pd9Te4 I
Kaurchak R.
N.a.
Note. B.d.l., Below detection limit; N.a., not analyzed.
Alatau area and less intense erosion in Gornaya Shoria during the Cenozoic. The preservation of rims around placer PGM grains can serve as additional evidence to support a short distance from their bedrock sources. Surface fracturing is another important morphological feature of PGM grains. We also note, without going into details, that the studied PGM grains display two types of microfractures, one of which is probably associated with mechanical deformation of grains during alluvial transport. The more interesting type II includes linear, en-echelon, parallel fractures filled with amphibole, rutile, and chlorite. Obviously, such fractures could have formed by deformation processes during crystallization of PGM or as a result of formation of ophiolite complexes in a stress regime (Coleman, 1977). It is not surprising that type II fractures are typical of
most of the examined Os–Ir–Ru alloy grains, possibly because they are more brittle than Pt–Fe alloys. Analysis of microfractures in bedrock minerals is important because it provides clues as to the directions and magnitudes of principal stresses, effective confining pressure, differential stress, brittle and ductile behavior, and multiphase deformations. The evaluation of the potential bedrock sources of PGE mineralization using the compositions of PGM in placer occurrences has been discussed in many previous papers (Cabri et al., 1996; Fedortchuk et al., 2010; Gornostayev et al., 1999; Harris and Cabri, 1991; Krivenko et al., 1994; McClenaghan and Cabri, 2011; Nixon et al., 1990; Polyakov and Bognibov, 1995; Slansky et al., 1991; Tolstykh, 2004; Vysotsky, 1933; and others). PGE mineralization represented by Pt–Fe and Pt–Pd alloy species of PGM are traditionally
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
1453
Fig. 10. Rims and exsolution textures in PGM. a, a grain of isoferroplatinum with Os lamellae (1) and inclusions of native gold-I (2) is overgrown by a rim consisting of cooperite (3), malanite, dayingite (4), cuprorhodsite (5) and erlichmanite (6) (Koura River); b, emulsion inclusions of gold (1), geversite (2) (Andoba River); c, isoferroplatinum with inclusions of gold-I (1) is corroded by rims consisting of cooperite (2) and sperrylite (3). The inset shows the exsolution texture: lamellae of gold Au84Ag16 and an arsenide phase (Kaurchak River); d, Pt grain with Os lamellae (1), rim cooperite (2) with tiny inclusions of gold (3), overgrown by native gold-II (4) with fineness of 900‰ (Kaurchak River).
related to concentrically zoned complexes and layered mafic– ultramafic intrusions (O’Driscoll and Gonzáles-Jiménez, 2016), whereas rutheniridosmine is thought to have originated from ophiolitic associations. Considering the above, there is the working hypothesis that two types of PGM placer occurrences prevail in southern Siberia, the Pt–Fe alloys associated with Uralian–Alaskan-type concentrically zoned complexes and rutheniridosmine associated with ophiolite ultramafic rocks (Krivenko et al., 1994; Podlipsky et al., 2007; Polyakov and Bognibov, 1995; Tolstykh et al., 1996, 1999), with the exception of PGM in the Taenza River placer, which are represented mostly by Os–Ir–Ru minerals (Fig. 1). The
latter are interpreted to have originated from Fe-rich ultramafic rocks, mainly small elongated bodies of serpentinized lherzolites and, more rarely, dunites (Tolstykh, 2004), which were previously described as “dike-like bodies of serpenitinized porphyrites” (Kyuz, 1935). In the ternary diagram, the compositions of Os–Ir–Ru alloys from these rocks form an osmium trend, as opposed to ophiolitic compositions defining a ruthenium-enrichment trend. At the same time, the PGM assemblage found in the Taenza and Bolshoi Orton River placers (dominantly Os–Ir–Ru alloy species) is distinctly different from the PGM of many other placers of Gornaya Shoria (dominantly Pt–Fe alloy species). Despite the absence
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Fig. 11. Complex heterogeneous internal exsolution textures with inclusions of newly-formed PGM. a, b, h, Taenza River, c, Poludennyi Kundat River, d, Kaurchak River, e, Bol. Tuluyul River, f, g, Koura River, i, Andoba River. Backscattered electron images (SEM) of PGM. a, Polyphase sulfide inclusion: xingzhongite (1), braggite (2), vasilite (3) with laurite occurring as a idiomorphic grain in the core (4) and a thin rim of erlichmanite (5); b, tiny inclusions of hongshiite (1), rhodarsenite (2) in the Pt–Fe-Cu alloy; c, crystalline sulfide inclusion of pentlandite (1), bornite (2), laurite (3) and (Pd, Cu)S phase; d, polyphase inclusion of chalcopyrite (1), bornite (2), bowieite (3), telluropalladinite (4), braggite (5); e, idiomorphic grain of laurite (1) in a sulfide inclusion of pentlandite (2), irarsite (3), chalcopyrite (4); f, polyphase rounded crystalline inclusions of Rh-rich geversite (1) and isoferroplatinum (2), inclusions of irarsite (3), laurite (4); g, Os lamellae (1), idiomorphic inclusions of laurite (2), irarsite (3), rim cooperite (4); h, dendritic textures formed by cherepanovite (1), rim irarsite (2), sperrylite (3); i, inclusions of xingzhongite (1) and maslovite (2) in isoferroplatinum containing emulsion inclusions of gold.
of the ruthenium-enrichment trend in the compositions of PGM, these placers are spatially associated with ophiolite − 1) along the northern margin of the Mrassu uplift. rocks (V–C The PGM may be derived from small serpentinite massifs. In its upper course, the Bolshoi Orton River drains a lens-like body (1.5 km in length) of fully serpentinized wehrlite-clinopyroxenite of the Usa complex. According to Kyuz (1935), outcrops of serpentinite from which the samples analyzed in
this study were collected are also located in the estuary of the Taenza River (near the Nikolka-Talovaya Creek). In these placers, isolated grains or inclusions of laurite, irarsite, and pentlandite were identified in addition to rutheniridosmine and subordinate Pt–Fe alloys. However, previous data (Polyakov and Bognibov, 1995) show that laurite, erlichmanite, PGE-, Cu-, Fe-, and Ni-rich sulfides, Pd telluride, and palladian gold were also reported from the Taenza River placer. Besides the
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
1455
Fig. 12. Polymineralic inclusions in isoferroplatinum and Ru–Ir–Os alloy (d). Backscattered electron images of PGM. a, Rounded, poorly crystalline silicate inclusion, tiny inclusions of sperrylite (1) (Koura River); b, c, rounded crystalline inclusions of silicates (Koura River): 1, plagioclase, 2, titanite, 3, biotite, 4, pargasite, 5, pyroxene, 6, Fe-rich hornblende; d, olivine inclusion (Taenza River); e, f, relic inclusions of magnetite, chromite, Cr-magnetite in the Pt3Fe grain with Os lamellae (1) and rim cooperite (2) (Koura River); g, rounded poorly crystalline silicate inclusion (Andoba River); h, crystalline inclusions of silicates: pyroxene (1), Cr-magnetite (2), biotite (3), chlorite (4); hongshiite (5) and rhodarsenite (6) developed at the inclusion edges (Koura River); i, a grain of ferroan platinum with Os lamellae (1) inclusions of monazite (2) and irarsite (3) (Koura River).
above two localities, laurite also occurs in placers along the Koura, Poludennyi and Pryamoi Kundat, Bol. Tuluyul Rivers and more commonly in placers of Kuznetsk Alatau (in association with pentlandite), yet rarely found in placers of Gornaya Shoria. For example, similar PGM assemblages (laurite, erlichmanite, irarsite, and sperrylite) and ophiolitetype sources were reported for Au-PGM placer occurrences of the Gar ore cluster (Amur River region) (Molchanov et al., 2003). The presence and amount of PGE-rich sulfides depend on the temperature regime and the activity of S in the ore-forming system of ophiolite complexes (Polyakov and Bognibov, 1995).
The observed variations in the proportions of Pt–Fe and Os–Ir–Ru alloys in placer PGM from southern Siberia indicate the existence of different sources (Table 2). Zaccarini et al. (2004) and Murzin et al. (2015) provided evidence for the existence of both PGM assemblages associated with Uralian– Alaskan-type and ophiolite-type sources, the first assemblage dominated by Pt–Fe alloys and the second dominated by Os–Ir–Ru alloys. Assuming a similar bedrock source for these placer PGM assemblages, the proportions of Pt–Fe and Os–Ir–Ru minerals should be almost identical. However, the situation is different. The proportion of Pt–Fe alloys varies from 93 to 2% (by number of PGM grains) in placers of
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Table 7. Composition of Ir oxide Element
82-28-11*
82-28-12
82-28-13
82-29-8
82-29-9
82-29-13
Pt, wt.%
6.5
4.7
5.2
5.7
6.6
5.3
Ir
49.3
42.1
38.6
16.8
22.2
17.4
Ru
1.4
1.3
1.7
17.4
11.0
16.6
Rh
2.3
2.0
1.6
0.8
1.5
0.9
Al
0.7
0.6
0.7
0.7
0.6
0.6
Ca
2.9
2.2
3.0
0.7
0.9
0.8
Si
N.a.
N.a.
0.3
0.2
0.4
0.2
Ti
N.a.
N.a.
0.5
1.4
0.8
1.4
V
0.1
0.8
0.6
0.6
0.5
0.6
Mn
1.6
2.1
3.1
0.3
0.4
0.3
Fe
0.6
0.7
1.8
13.6
13.4
12.9
Hg
7.0
20.0
16.5
11.0
11.4
10.8
As
0.1
0.2
0.1
0.5
0.8
0.6
S
0.1
0.3
2.1
1.9
2.1
1.8
O
17.0
15.5
17.6
25.2
22.7
23.2
Total
89.6
92.5
93.4
96.8
95.3
93.3
O = 2.3
O=2
O=2
O = 2.3
O = 2.3
O = 2.3
Pt, apfu
0.07
0.05
0.05
0.04
0.06
0.04
Ir
0.56
0.45
0.37
0.13
0.19
0.14
Ru
0.03
0.03
0.03
0.25
0.18
0.26
Rh
0.05
0.04
0.03
0.01
0.02
0.01
Al
0.06
0.04
0.05
0.04
0.04
0.04
Ca
0.16
0.11
0.14
0.03
0.04
0.03
Si
–
–
0.02
0.01
0.02
0.01
Ti
–
–
0.02
0.04
0.03
0.05
V
0.00
0.03
0.02
0.02
0.01
0.02
Mn
0.06
0.08
0.10
0.01
0.01
0.01
Fe
0.02
0.03
0.06
0.36
0.39
0.37
Hg
0.08
0.21
0.15
0.08
0.09
0.09
As
0.00
0.00
0.00
0.01
0.02
0.01
S
0.01
0.02
0.12
0.09
0.11
0.09
O
2.3
2
2
2.3
2.3
2.3
ΣPt, Ir, Ru, Rh, Ca, V, Mn, Fe, Hg
1.03
1.03
0.95
0.93
0.99
0.97
Note. N.a., Not available. * Grain no.
Kuznetsk Alatau, from 72 to 17% in placers of Gornaya Shoria and from 100 to 8% in placers of Salair. Considerable variations in the proportions of Os–Ir–Ru alloys, sperrylite and sulfarsenides in the same placer occurrences are also observed. In diagrams, the compositions of all studied grains show Os- and Ru-enrichment trends, typical of the Uralian–Alaskanand ophiolite-type sources (Bird and Bassert, 1980; Fedortchuk et al., 2010; Polyakov and Bognibov, 1995; Tolstykh et al., 1996). In some cases, the presence of melt inclusions (lamellae) of native gold enclosed within the Cu-rich Pt–Fe alloy grains (Andoba River) and the occurrence of electrum—native gold as exsolutions (Kaurchak River) may testify to the existence
of primary composite Au-Pt–Fe alloys (Fig. 10). Textural data indicate the following sequence of crystallization: (1) crystallization of isoferroplatinum at temperatures below 1350 ºC from a composite melt containing PGE, Au, Ag, Fe, As, S, and Te; (2) formation of dendritic textures with a complex eutectic between gold-silver and PGE sulfarsenides; (3) crystallization of sperrylite at 912–1016 ºC from a gas–vapor phase (because of the high vapor pressure of As) by reaction with the early isoferroplatinum and eutectic gold-silver–PGE sulfarsenides to form zones composed of bowieite (Rh,Ir, Pt)2S3 and platarsite, sperrylite. Lager grains of gold begin to precipitate at the same interface. This crystallization sequence
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Fig. 13. A grain of ferroan platinum with Os lamellae replaced by an Ir–(Hg)-bearing oxide phase. a, Backscattered electron images (SEM) of PGM; b–f, element map for Os, Ir, Pt, Fe, and O using EDS on SEM.
may point to an emplacement of layered intrusions or concentrically zoned massifs and is uncommon for ophiolites. The presence of Cu-rich varieties, including hongshiite, as overgrowths on Pt–Fe alloys has been reported by many workers (Barkov et al., 2002, 2005; Betekhtin et al., 1958; Nekrasov et al., 1994; and others). Such overgrowths are interpreted to have formed by metasomatic replacement of magmagtic Pt–Fe alloys by postmagmatic hydrothermal solutions. The evidence for overgrowth of Cu-rich platinum was first reported in the rim-like zone around Pt–Fe alloy grains from the Nizhny Tagil complex in 1930 and linked by A.G. Betekhtin to serpentinization of dunites and mobilization of Cu. In some cases (Fig. 11b), our data may point to the existence of the original Cu–Pt–Fe fluid, which crystallizes during cooling to form a patchy exsolution texture, with alternating Fe-rich zones and more Cu-rich zones consisting of almost pure hongshiite. This heterogeneity is not limited to the surface of grains and the network of fractures or deformations. A more complex situation involving both mechanisms cannot be ruled out. The theoretical predictions that Pt may be transported by hydrothermal fluids have been discussed in many previous papers (Boudreau et al., 1986; Distler et al., 2000; Nixon et al., 1990) and the strongest evidence for this is the presence of PGM in quartz veins (Distler et al., 2000; Ramdohr, 1962; Vysotsky, 1933). These results confirm that a variety of micro- and nanoscale precious mineral grains may have crystallized from the melt enriched in precious metals
(Pt, Rh, Pd, Ru, Os, Ir, Au, and Ag), as well as S, Te, and As. As noted above, isoferroplatinum may contain exsolution phases consisting of platiniridium and iridioplatinum (or Ir–Pt phases), which are replaced by a compound (Rh,Pt,Ir)4Sb3. Similar compounds were reported from placers in the Witwatersrand (Feather, 1976) and Guli massif (Manur and Ozue, 1998). Feather (1976) suggested that such compounds represent the end-members Pt3Fe and (Ru, Os, Ir) that could have formed by incomplete exsolution from a complex primary solid solution. It is known that Pt and Ir form a continuous solid solution series at temperatures above 845 ºC (Andryushchenko et al., 1984). The solid solutions break down at lower temperatures forming the end-members enriched in the accompanying element. The observed exsolution textures of Pt–Ir indicate a high temperature origin for these alloys. Typical exsolution textures of PGE solid solutions have been widely discussed in the literature (Betekhtin et al., 1958; Cabri et al., 1996). The grains of Pt–Fe alloy with exsolution textures are most common in placers of Gornaya Shoria. It was assumed (Betekhtin et al., 1958; and others) that such textures could have been developed during slow cooling and solidification of a melt or solid solution of hydrothermal origin. Such conditions may exist within relatively large massifs originated at great depths. The absence of exsolution textures can be indicative of special conditions of formation, e.g., in the mafic dikes, which are widespread in the study area, or in the picrtite-basalt association commonly developed
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Fig. 14. Pseudorounded PGM grains of primary endogenic origin. Backscattered electron images (SEM) of PGM. a, A veinlet of irarsite (1) parallel to the margin of the grain of native osmium Os73Ir21Ru6, inclusion of amphibole (2) (Taenza River); b, rounded zoned sperrylite grain with inclusions of native gold (1), hollingworthite (2), (Fe,Cu)2(Rh,Pt)S3 (3), titanite (4), chalcopyrite (5) (Kaurchak River); c, pseudorounded endogenic grains of Pt3(Fe,Cu), arrows indicate smooth dendrites, which could not be preserved during abrasion (Koura River).
along extensions of the studied structures to Western Mongolia (Izokh et al., 2001; Oyunchimeg et al., 2009). Some of the analyzed grains of Pt–Fe alloys from the Polydennyi Kundat River placer exhibit exsolution textures formed by platiniridium and iridioplatinum (or Ir–Pt phases), which are replaced by a compound (Rh, Pt, Ir)4Sb3 (Rh-dominant analogue of genkinite) (Fig. 9e). Similar compounds were reported from placers in the Witwatersrand (Feather, 1976) and Guli massif (Malitch and Auge, 1998), as well as from Pt–Fe–Ni–Cu ore deposits hosted by layered mafic-ultramafic intrusions. Feather (1976) suggested that such compounds represent the end-members Pt3Fe and (Ru, Os, Ir) that could have formed by incomplete exsolution from a complex primary solid solution. It is known that Pt and Ir form a continuous solid solution series at temperatures above 845 ºC (Andryushchenko et al., 1984). The solid solutions break down at lower temperatures forming the end-members enriched in the accompanying element. The observed exsolution textures of Pt–Ir indicate high temperatures of formation. Of particular importance is the discovery of Pt-based intermetallic alloys exhibiting large compositional variations: Pt40Ir35Os13Fe11Ru–Pt63Fe20Ir13Os3Ru; Ir55Pt20Ru10Os2Fe– Pt66Fe16Rh12Ir5Sb5Cu3Rh2Pd2; and others. Such alloys were reported in the PGM placer grains from the Evander Goldfield in the eastern part of the Witwatersrand basin (Badanina et al., 2015). The existence of composite solid solutions is related to the systems, which are slightly differentiated with respect to PGE and reflect high P–T conditions. The chemical analysis of the inclusions in chromian spinel and magnetite revealed that they have relatively high TiO2 contents and cannot be classified as Cr-spinel from ophiolitic complexes, which are characterized by TiO2 < 0.3 wt.% (Fig. 15). In the Fe–Al–Cr diagram (Garuti et al., 2012), the compositions of the studied Cr-rich magnetites plot close to the epigenetic trend of Uralian–Alaskan-type concentrically zoned intrusions. Katophorite and edenite are members of the sodic amphibole group (richterite–katophorite–edenite), which is a common constituent of alkaline rocks and carbonatites, the
products of intraplate (plume) magmatism. Experimental results for a variety of alkaline silicate melts show that alkali amphiboles of the richterite–katophorite–pargasite–edenite series formed at ~650 ºC (Koval’skii et al., 2006). Hastingsite is a common constituent of alkaline rock complexes. Numerous alkaline plutons are widely developed within the Kuznetsk Alatau and Gornaya Shoria areas. These plutons are associated with the Goryachegorsk and Karadat complexes represented by the alkaline-gabbro association (Goryachegorsk, Kiya-Shaltyr, Petropavlovka and other nepheline syenite–urtite–melteigite plutons) and alkaline syenites with minor nepheline syenites (Tuim–Karysh plutons), alkaline syenites and nordmarkites (Beresh and other small plutons). The rocks of the Beresh pluton have a relatively homogeneous composition, displaying minor variations in the proportions of the main mineral constituents, K-feldspar and plagioclase. The alkali amphiboles are mainly riebeckite–osannite and ferrorichterite, while potassic amphiboles are edenite and ferro-edenite (Explanatory Note…, 2007). The common PGM assemblage in the rocks of the Goryachegorsk and KiyaShaltyr plutons comprises sperrylite, isoferroplatinum, ferroan platinum and, rarely, rutheniridosmine (Sazonov et al., 2000). The analysis of sources and geodynamic conditions revealed two stages in the formation of gabbroic and feldspathic rocks (theralite, basic foidolite, nepheline syenite) and carbonatites of the Kuznetsk Alatau area: Middle Cambrian–Early Ordovician (~510–480 Ma) and Early–Middle Devonian (~410– 385 Ma). It was suggested that the highly alkaline rocks and carbonatites in the western Central Asian Fold Belt have a mantle plume origin (Vrublevskii, 2015; Vrublevskii et al., 2014). The presence of amphibole in association with biotite, pyroxene, and plagioclase in the Pt–Fe alloy grains may reflect a picritic source for the placer PGM in southern Siberia. The compositions of amphiboles and pressure estimates (P) (Table 8, Fig. 16) indicate that there are several distinct groups of amphiboles found as inclusions in PGM: (1) pargasite (P = 10.1–11.1 kbar); (2) hastingsite (P = 8.2–8.8 kbar); (3) alkali ferromagnesian hornblende (P = 4.5–6.6 kbar); (4) ede-
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Fig. 15. Diagrams showing the compositions of: a, Cr-spinel from inclusions in PGM grains from placers of the Koura and Poludennyi Kundat Rivers, arrows indicate the epigenetic trend; b, Cr-magnetite from inclusions in PGM grains. Samples: K, Koura River; PolK, Poludennyi Kundat River; T, Taenza River.
nite (P = 3.0–4.5 kbar); (5) aluminoedenite (P = 5.1–6.5 to 8 kbar); (6) ferromagnesian hornblende (P = 0.6–1.1 to 2.5 kbar). The presence of pargasite in the PGM grains from the Koura River place suggests that the rocks may have formed in a subduction-zone setting at high pressure (P = 10.1–11.1 kbar) and relatively low temperature conditions. This is also supported by the presence of subalkaline and alkaline magnesian hastingsite that formed at pressures of up to 8.8 kbar. This assumption is consistent with the previous results (Anikina et al., 2001) reported for the Uralian–Alaskan-type dunite–clinopyroxenite–gabbro complexes. Thus, it was assumed that dunite–clinopyroxenite–gabbro complexes of the Ural platinum-bearing belt formed in a suprasubduction setting. The same is true for the Alaskan-type Akarem amphibole-rich mafic-ultramafic complex (Eastern Desert, Egypt) (Abd El-Rahman et al., 2012). A study of plagiogranitoid magmatism of Gornaya Shoria and Western Sayan revealed two stages of magmatic evolution in the island-arc settings. The early stage (550–540 Ma) is marked by formation of high-alumina plagiogranites spatially associated with adakites of the Kshta (545 ± 8 Ma) and Taraskyr massifs (545 ± 7 Ma). These plagiogranites are considered to be derived by melting of a subducting oceanic metabasic protolith with N-MORB-like composition at P ≥ 15 kbar, in equilibrium with a garnet-bearing restite. The rocks of the Taraskyr massif are low-alumina plagiogranites that could be derived by melting of metabasic source regions at the base of an island arc at P = 3–8 kbar, in equilibrium with a plagioclase-bearing restite. The late stage (525–520 Ma) is marked by formation of plagiogranitoids of the Maina complex of the Yenisei (524 ± 2 Ma) and Tabat plutons (Rudnev et al., 2013a,b). Pyroxene is also present as part of composite crystalline and melt inclusions in the PGM. The range of temperatures and fO2 was calculated for clinopyroxene from microinclusions using the original version of monomineralic oxythermometry (Ashchepkov et al., 2010). Figure 17 shows the T–fO2 relationship for clinopyroxene from melt and crystalline inclusions
in PGM. These results indicate, at least, four groups of clinopyroxenes that have formed at (1) T = 1200–1400 °C and fO2 (FMQ) = ca. –4.5; (2) T = 900–1100 °C and fO2 (FMQ) between ca. –1.5 and –0.5; (3) T = 1100–1300 °C and fO2 (FMQ) between –0.1 and +0.5; (4) T = 600–800 °C and fO2 (FMQ) between ca. –1.1 and + 0.1. One crystalline inclusion of iridosmine (Taenza River, Gornaya Shoria) contains olivine with f = Fe/(Fe + Mg) equal to 0.1, which can be indicative of a dunite-harzburgite association. The presence of PGE oxide–hydroxide mineral species may be indicative of hydrothermal alteration. Similar Ru–Os–Ir oxides in association with other PGM have been previously documented in chromitites of the Nurali complex, southern Urals (Garuti et al., 1997), and Oman ophiolite complexes (Ahmed and Arai, 2003). These authors concluded that Ru–Os–Ir oxides were formed by desulfurization of Os and Ru sulfides during hydrothermal alteration and serpentinization. It was also suggested that the PGE oxides may result from the alteration of magmatic PGM alloys and from primary crystallization of oxides in lateritic conditions. Their presence indicates the existence of mechanisms of transport and recrystallization of PGE (as oxide–hydroxide) in surface conditions, mechanisms contributing to the redistribution and enrichment of PGE in laterite (Auge and Legendre, 1994).
Conclusions The use of a set of modern techniques for the in situ quantitative analysis (microprobe and field emission scanning electron microscopy) allows effective determination of different characteristics of minerals. The chemical data for minerals obtained with energy dispersive spectrometry (SEM-EDS) show good agreement with the microprobe results and can be used for mineralogical and geochemical studies.
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Table 8. Amphiboles from inclusions in PGM grains, with P estimates (kbar) Placer
Grain no.
M*
A**
Amphibole
[1]
[2]
[3]
[4]
Koura R.
1-2
PtFe3
Px+Bt+Ttn+Mgh
Alkali-rich ferropargasite
10.1
10.8
8.2
10.1
Gr+Px+Ab+Mgh
Alkali-rich ferropargasite
10.4
11.1
8.4
10.4
11-3
PtFe3, PtAs
Bt
Fe-Mg hornblende
0.4
0.7
0.4
0.4
23-1
PtFe3
Ab+Ap+Bt+Ep+Kfs
Subalkaline hastingsite
8.4
8.8
6.7
8.4
1-3
23-3
Alkaline high-Al edenite
23-2
Alkaline high-Fe hornblende
6.6
6.6
5.1
6.6
27-2
Pt(Fe, Cu)3, PtCu
Pl+Px+Bt+Ttn
Alkaline edenite
4.8
4.5
3.5
4.8
Magnesian hornblende
4.5
4.2
3.3
4.5
27-3
Iron-rich hornblende
5.0
4.7
3.6
5.0
27-4
Magnesian-alkaline hastingsite
8.2
8.5
6.5
8.2
27-9
Edenite
3.5
3.0
2.3
3.5
27-8
Subalkaline hastingsite
4.1
3.7
2.9
4.1
Edenite-hornblende
6.4
6.4
4.9
6.4
27-1
35-15 35-27
PtFe3, RhAs, PtCu, IrAs
35-18
Pl+Bt+Px
Alkaline aluminous katophorite Ol+Px+Chl
Mg-cummingtonite
35-17
Subalkaline anthophyllite
35-19
Tremolite-hornblende
35-14
Fe-Mg hornblende
0.6
0.4
0.2
0.6
35-16
Subalkaline hornblende
2.5
1.8
1.4
2.5
35-20
Subalkaline Fe-rich hornblende
1.1
0.1
0.1
1.1
35-7
Subalkaline hastingsite
4.1
3.7
2..9
4.1
6.5
6.5
5.0
6.5
35-13
Ol+Chl+Grt
35-22
Tremolite Subalkaline hastingsite
Andoba R.
115-8
PtFe3
An
Al-edenite
8.2
8.5
6.5
8.2
Pol. Kundat R.
135-18
Pt(Fe, Cu)3
Pl+Px+Mgh
Subalkaline hastingsite
6.2
6.2
4.8
6.2
Al-edenite
6.6
6.7
5.1
6.6
Mg-hornblende
4.8
4.5
3.5
4.8
2.2
1.4
1.2
2.2
135-19 134-5
OsIrRu
134-7 Taenza R.
73-15
Actinolite-hornblende OsIrRu
Mgh
Cr-Fe-Mg-hornblende
Note. Mgh, Magnetite; Px, pyroxene; Bt, biotite; Ab, albite; An, anorthite; Pl, plagioclase; Ap, apatite; Ep, epidote; Grt, garnet; Ttn, titanite; Kfs, K-feldspar; Ol, olivine; Chl, chlorite. Geobarometers: [1], Hammarstrom and Zen (1986); [2], Hollister et al. (1987); [3], Johnson and Rutherford (1989); [4], Schmidt (1992). Pressure estimates are based on the empirical and experimental calibrations using aluminum content of hornblende. No data are provided where calculations of pressure are not impossible. * Composition of host mineral. ** Association of coexisting mineral phases in inclusions.
The platinum-group minerals identified in 14 placer deposits of Kuznetsk Alatau, Gornaya Shoria, and Salair comprise two distinct mineral assemblages consisting mainly of Pt–Fe alloy species (55%) and subordinately of rutheniridosmine (41%). In two placers, this proportion was close to 2:1. Of all analyzed PGM, about 3–4% are represented by sperrylite and 1% by PGE compounds enriched in As and S. Considerable variations in the proportions of Pt–Fe and Os–Ir–Ru alloys in placers of southern Siberia point to the existence of different bedrock sources. The textural observations and compositions of PGM from alluvial placers (as well as gold placers) provide important evidence for the possible sources of PGE mineralization. The morphology and compositions of the PGM grains and microin-
clusions of silicates, oxides, intermetallic compounds and the type of alteration can be used as indicators for distinguishing parent bedrock sources of PGE. The rock suites of the Kuznetsk Alatau, Gornaya Shoria, and Salair areas and compositions of the host PGM and microinclusions indicate several potential source areas for PGE-bearing mineralization: (1) Uralian–Alaskan-type intrusions (northwestern Kuznetsk Alatau, Simonovskii Creek of the Salair Ridge); (2) ophiolite suites, including those formed in a subduction zones (central Kuznetsk Alatau, Mostovaya River of the Salair Ridge); (3) ultramafic alkaline and carbonatite massifs (northwestern and central Kuznetsk Alatau), alkaline gabbroic suites and, probably, (4) rocks of the picrite-basalt association and small mafic rock bodies and dikes. The presence of composite
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Fig. 16. A plot of Fe3/(Fe3+AlVI) vs. P, kbar for amphiboles from melt and crystalline inclusions in PGM.
crystalline inclusions in PGM.
PGE-bearing compounds of variable composition, Pt40Ir35⋅ Os13Fe11Ru; Pt63Fe20Ir13Os3Ru; Ir55Pt20Ru10Os2Fe; Pt66Fe16⋅ Rh12Ir5Sb5Cu3Rh2Pd2; Os–Ir–Ru–Pt(±Fe), may be indicative of the systems with weak differentiation with respect to PGE and high P–T conditions. In addition, the discovery of emulsion—liquation inclusions and lamellae of native golds within the Cu-rich Pt–Fe alloy grains, and exsolution textures of electrum–native gold provides the first supportive evidence for the existence of Au—Pt–Fe alloys. These data could point to the existence of Uralian–Alaskan-type complexes in some areas of Gornaya Shoria (Kaurchak, Koura, and Andoba Rivers). We suggest that all placer PGM grains with well-rounded smooth shapes may be and often are of primary endogenic origin. Such grains cannot be considered as indicators of greater transportation distance from their bedrock sources. We assume that the following distinctive features may be indicative of primary endogenic origin of smooth well-rounded PGM grains: (1) the preservation of rims on PGM grains of endogenic origin which were not mechanically eroded (Fig. 9f, e); (2) the presence of textures at the grain margin that are conformable to the outline of the grain, e.g., see Figs. 9e and 14a. The rim of the grain of native osmium (Os70–74Ir20– 21Ru5-11) contains a veinlet of irarsite, which is contorted subparallel to the grain edge (surface); (3) concentric zoning in rounded grains; (4) the presence of remnants of primary minerals and rocks on the surface of PGM grains, i.e., chemical and textural concentric zoning; (5) smooth dendritic segregations within PGM grains with relatively deep embayments and negative crystal imprints (Fig. 14c). The preservation of poorly rounded and unrounded PGM grains in the studied placers within the ASFA indicates relatively little transport from their primary sources. The presence of replacement and overgrowth-precipitation rims around the grains of Pt–Fe alloys suggests different mechanisms of their formation. The formation of replacementinduced rims is related to metasomatic alteration at the final stages of PGM crystallization, as indicated by the development of a reaction zone between rims and their host mineral and
data on the same their 187Os/188Os ages (Malitch et al., 2014). The presence of overgrowth-precipitation, represented mainly by sperrylite, provides the additional evidence for hydrothermal mobilization of Pt at the postmagmatic stage. The widespread development of endogenic rims around PGM grains appear to have been related to postmagmatic hydrothermal processes during ore formation, while their good preservation may reflect a short distance from bedrock sources. It should be remembered when interpreting analytical results obtained for placer PGM that the physicochemical processes operating within the weathering profiles on slopes and along river valleys in the exogenic evironments lead to the various transformations of the PGM assemblages. As a result, the assemblages become successively enriched in the more stable components, such as Pt–Fe and Os–Ir–Ru solid solutions, and depleted in unstable components, i.e., PGE sulfides and sulfarsenides, as well as compound containing Pd, Cu, etc. Sperrylite appears to be intermediate in terms of its stability. During formation of alluvial autochthonous placers, PGM undergo flow separation in fluvial environments, which is manifested as changes in the particle sizes, with an increase in the small-size fractions and a decrease in the fine-size fractions and flatness index in the same particle size. It is impossible to discuss in one paper all the large amounts of data that have been accumulated over a long period of investigations. In this study, we would like to emphasize the opportunity for collecting data (on both endogenic and exogenic history of mineral formation) on placer PGM using a combination of modern in situ analytical techniques. Further studies will provide greater insights into the characteristic features of PGM in placer deposits. Acknowledgments. The authors are grateful to L.P. Boboshko and E.V. Lazareva for her assistance in data preparation. This study was supported by the Earth Science Division, Russian Academy of Sciences (project no. 5.1), the Russian Foundation for Basic Research (project nos. 12-05-01164, 13-05-12056, 15-05-06950, and 13-05-12056 ofi-m), and the Ministry of Education and Science.
Fig. 17. A plot of T, °C vs. ∆log fO2(FMQ) for clinopyroxenes from melt and
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Alluvial platinum-group minerals as indicators of primary PGE mineralization (placers of southern Siberia) S.M. Zhmodik a,b,*, G.V. Nesterenko a, E.V. Airiyants a, D.K. Belyanin a,b, V.V. Kolpakov a, M.Yu. Podlipsky a, N.S. Karmanov a a
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 21 September 2015; accepted 24 September 2015
Abstract The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. Different characteristics of minerals can be determined by a set of conventional and modern in situ analytical techniques (scanning electron microscopy (SEM) and electron probe microanalysis (EPMA)). A study of PGM from placers of southern Siberia (Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge) shows that the morphology and composition of PGM grains, the texture, morphology, and composition of silicate, oxide, and intermetallic microinclusions, and the type of mineral alteration can serve as efficient indicators of the primary sources of PGM. The widespread rock associations in the Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge, the compositions of PGM and microinclusions in them, and the dominant mineral assemblages testify to several possible primary sources of PGE mineralization: (1) Uralian–Alaskan-type intrusions; (2) ophiolite associations, including those formed in a subduction zone; (3) ultramafic alkaline massifs; and, probably, (4) rocks of the picrite–basalt association. The preservation of poorly rounded and unrounded PGM grains in many of the studied placers of the Altai–Sayan Folded Area (ASFA) suggests a short transport from their primary source. © 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: platinum-group minerals (PGM); gold; placers; Altai–Sayan folded area; Kuznetsk Alatau; Salair; Gornaya Shoria
Introduction Gold and platinum-bearing placer deposits have been worked in South Siberia since they were first discovered in the 1830s–1840s. Despite an extensive exploration and mining history, substantial production of gold and platinum-group minerals from placers of Kuznetsk Alatau, Gornaya Shoria, and Salair (Agafonov et al., 1996, 2000; Chernykh and Uvarov, 2003; Polyakov and Bognibov, 1995; Podlipsky and Krivenko, 2001; Sazonov et al., 2000; Tolstykh, 2004; Vysotsky, 1933; and others), and the recognition of the Altai–Sayan platinum-bearing province (Dodin et al., 1999; Izokh et al., 2004), virtually none of these PGE occurrences have been traced to their bedrock source. The aims of the study were to determine (1) the assemblages and types of platinum-group minerals (PGM) in alluvial placers of West Siberia; (2) their properties, compositions, and textural features pointing to the
* Corresponding author. E-mail address: [email protected] (S.M. Zhmodik)
primary sources of the placer PGM, based on local analysis of individual PGM grains. The study provides detailed mineralogical description of the placer PGM from southern West Siberia along with the data on grain morphologies, textures, compositions, and types of inclusions that bear information on potential primary sources for the majority of PGM placer grains. All PGM grains were found in the gold placers in the study area. Most of the PGM grains were recovered from gold-bearing alluvial sediments. The history of gold placer exploration and mining in the region was discussed in earlier works (Butvilovskii et al., 2011; Nesterenko, 1991; Vysotsky, 1933; and others), and information on the first PGM discoveries was summarized by Vernadsky (1955) in 1908. According to Vysotsky (1933), the PGM were found in many gold-bearing placer deposits of southern Siberia. Kyuz (1935) noticed that PGM were present in virtually all rivers in Kuznetsk Alatau that were mined for gold. New data obtained over the next 50 years are scarce. The PGE potential of this region was evaluated by Syrovatskii in 1990–1991 and the composition of alluvial PGM grains was systematically studied
1068-7971/$ - see front matter D 201 6, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 6.09.002
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within the same period (Krivenko et al., 1994; Podlipsky, 1999; Podlipsky et al., 2007; Tolstykh, 2004; Tolstykh et al., 1999; Zhmodik et al., 2004). Although these papers provide the first assumptions on the types of original sources of PGM from gold-bearing placers of southern Siberia, the possible source rocks of placer PGM in the Kuznetsk Alatau, Gornaya Shoria, and Salair remain unknown. Following traditions in earlier works on other PGE occurrences worldwide (Cabri et al., 1996; Gornostayev et al., 1999; McClenaghan and Cabri, 2011; Nakagawa and Franco, 1997; Nixon et al., 1990; Okrugin, 1999; Weiser and Bachmann, 1999; and others), the available samples were analyzed in detail using modern analytical methods. The results of this study show that the compositions of new PGE compounds, mineral and melt inclusions, as well as exsolution textures can account for the existence of gold-rich PGE and composite polyphase melts formed by liquation or hydrothermal-metasomatic replacement.
Geological setting and PGE potential The study area is in the Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge, which represent the western part of the Altai–Sayan folded area (ASFA) (Fig. 1) formed during the Caledonian and Hercynian orogenies. Variations in the structural architecture and lithologies were controlled by different geodynamic processes that led to the formation of oceanic and island-arc complexes, subsequent collision during accretion of the Siberian craton and protracted plume-related magmatism during the Neoproterozoic–Mesozoic (Buslov et al., 2013; Dobretsov, 2003; Dobretsov et al., 2013; Kuzmin and Yarmolyuk, 2014; Rudnev et al., 2013a). Little published information on the primary PGE mineralization in the region is available and its potential bedrock source is still unknown. The geology of the region is dominated by rock complexes that are considered as having potential for PGE-bearing mineralization, e.g., Neoproterozoic–Lower Cambrian ophiolite complexes that have variably originated in mid-ocean ridge, back-arc, oceanic island, and island arc settings, or Lower–Middle Paleozoic bimodal volcanic complexes (Alabin and Kalinin, 1999; Buslov, 2011; Explanatory Note…, 2007; Izokh, 1999; Kurenkov et al., 2002; Pinus et al., 1958; Plotnikov et al., 2000). The compositions of the placer PMG grains from this region were used to recognize a series of Uralian–Alaskan-type mafic-ultramafic complexes (Tolstykh, 2004). For example, the Kaigadat massif in the northwestern part of Kuznetsk Alatau was attributed to Uralian–Alaskan-type zoned mafic-ultramafic intrusions, based on its bulk rock composition and the widespread occurrence of ferroan platinum mineral assemblage as the dominant PGM in the alluvium of nearby rivers and streams (Podlipsky and Krivenko, 2001). For example, although the ophiolitic nature of the Srednyaya Ters’ massif has long been recognized (Gertner and Krasnova, 2000; Krasnova and Gertner, 2000; Kurenkov et al., 2002), this has been disputed by some workers who argue that this massif
represents a layered intrusion. The data of A.E. Izokh (Polyakov and Bognibov, 1995) show that dunites of the Srednyaya Ters’ massif have high Pd (up to 1 ppm) and Pt (up to 0.6 ppm) contents (atomic absorption spectrometry). The dunites are relatively enriched in disseminated sulfides and PGM, represented by a wide variety of Pt and Pd compounds with Sb, As, and Te. Low-grade PGE mineralization (Ru–Ir– Os alloys) was found in serpentinites from the Seglebir massif of Gornaya Shoria (Gusev et al., 2004) and rodingites from the Togul-Sungai massif of the Central Salair Ridge (Agafonov et al., 1996, 2000). Ordovician clinopyroxenite-gabbro massifs that have significant PGE anomalies are known in southern Salair (Shokal’skii et al., 2000). In addition, high Pt and Pd values were identified in early Cambrian chromite-rich ultramafic rocks, layered peridotite-gabbro massifs, and carbonaceous schists of some Late Riphean, Late Vendian and Early Cambrian complexes of Kuznetsk Alatau, Gornaya Shoria, and Salair (Explanatory Note…, 2007). Small mafic intrusions and dikes were also regarded by some workers as the most probable source of the PGE in placer deposits. Gold mineralization contributing to the many gold and PGM-bearing placers of the ASFA has long been considered to be genetically related to the dikes of the Middle–Upper Cambrian gabbro-diorite-diabase complex (Bulynnikov, 1948). Most alluvial gold-PGM placer occurrences are related to Quaternary sediments (Butvilovskii et al., 2011; Nesterenko, 1991; and others). Some of these occurrences were re-explored and revived for exploitation. The irregular distribution of placers within the study area is largely controlled by bedrock sources and geomorphology. Most placers are typically found at medium altitudes in river valleys formed by erosional and depositional processes. A few placers occur at lower elevations, and are virtually absent in rugged, high-mountain areas. No published information on the PGM content of most placer occurrences is available, but PGM is generally present at a low grade, ranging from 0.03–0.05% to a few percent of native gold (from 0.5–10.0 mg/m3 to 500–800 mg/m3 by rock). At some localities, the proportion of PGM make up as much as 10–30% of native gold (Kyuz, 1935; Syrovatskii, 1991; Vysotsky, 1933; our data).
Samples and methods All samples were taken using gold dredgers and hydraulic dredgers. The material was washed at the gravity concentration plant using sluice boxes and wash pans to obtain concentrate samples. PGM were extracted from concentrates after they were panned to recover gold grains. The final volume of the concentrate was 5–10 L. The concentrate comprises a heavy black sand. Substantial quantities of native gold and PGM (the degree of enrichment ranging from 5 to 100,000 times) are present in the concentrate sample. Large-scale bulk sampling was employed to obtain an initial sand volume ranging from hundreds to a few thousands of m3. Heavy mineral concentrate sampling was conducted at several used placer deposits. The initial sand volume was 50–400 L. Final treatment of all
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Fig. 1. Geological map showing the location of PGM and gold placers in the western Altai–Sayan folded area. 1, sedimentary rocks (MZ–P); 2, granite (MZ–P). − 3–O); 5, volcanosedimentary rocks (D2–C1); 6, 7, rocks of ophiolite association(V2–C − 2); 8, terrigenous-volcanosedi3, coal-bearing deposits (C1–P2); 4, gabbro (C − 3); 9, mafic rocks (R3–C − 1); 10, ultramafic massifs: K, Kaigadat, ST, Srednyaya Ters’, S, Seglebir, A, Atalyk, TS, Togul-Sungai. 11, major mentary sequence (V2–C faults; 12, Au-placer with PGM grains: with Pt-dominant minerals (a); with Ru–Ir–Os-dominant minerals (b). 13, studied placers: 1, Kaurchak River, dredge 138; 2, Andoba River, dredge 315, 3, Koura River, dredge 317; 4, Taenza River; 5, Bol. Orton River; 6, Bol. Tuluyul River; 7, Poludennyi Kundat River; 8, Pryamoi Kundat River; 9, Shaltyr’-Kozhukh River; 10, Srednyaya Ters’ River; 11, Kel’bes River; 12, Sella River; 13, Simonovskii Creek; 14, Mostovaya River. The KuznetskAlatau–Altai platinum-bearing belt is indicated by black color in the inset (modified from Izokh et al., 2004): 1, Kaigadat massif; 2, Srednyaya Ters’ massif; 3, placers of Gornaya Shoria; 4, Ureg-Nuur area (Mongolia).
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Table 1. Comparison of selected WDS (microprobe) and EDS (SEM) analyses of PGM from pacers of southern Siberia (wt.%) Placer
Grain
Analysis
Os
Ir
Pt
Ru
Rh
Pd
Fe
Ni
Cu
Total
Koura R.
82-44
WDS
45.09
16.61
11.48
25.42
0.75
B.d.l.
0.48
B.d.l.
0.13
99.97
82-44
EDS
43.8
19.2
10.3
25.6
1.0
B.d.l.
0.5
B.d.l.
B.d.l.
100.4
82-44
EDS
45.2
18.2
9.1
26.2
1.3
B.d.l.
0.3
B.d.l.
B.d.l.
100.3
Taenza R.
Kaurchak R.
66-68
WDS
31.68
54.73
7.10
6.28
0.34
0.16
0.25
0.03
0.02
100.57
66-68
EDS
29.5
53.9
6.1
6.3
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
96.1
66-12
WDS
0.91
3.92
87.01
0.43
1.00
0.44
6.29
0.16
0.32
100.39
66-12
EDS
1.2
3.8
85.8
B.d.l.
0.8
B.d.l.
6.3
B.d.l.
0.3
98.2
66-12
EDS
0.0
3.9
88.3
0.5
1.0
0.5
6.2
B.d.l.
0.3
100.7
WDS
9.09
75.5
0.38
10.54
1.70
B.d.l.
1.04
0.22
0.04
98.5
82-146
EDS
10.6
77.0
B.d.l.
10.4
1.8
B.d.l.
1.1
B.d.l.
B.d.l.
100.9
82-146
EDS
10.8
77.7
B.d.l.
10.7
1.6
B.d.l.
1.1
B.d.l.
B.d.l.
101.9
82-176
WDS
19.92
49.34
14.51
13.51
0.70
B.d.l.
0.58
0.29
0.04
98.93
82-176
EDS
21.8
48.3
13.1
13.0
1.4
B.d.l.
0.6
0.4
B.d.l.
98.6
82-162
WDS
43.53
42.84
0.91
10.57
0.22
B.d.l.
0.34
0.08
0.13
98.63
82-162
EDS
44.1
42.4
1.5
10.1
B.d.l.
B.d.l.
0.4
B.d.l.
B.d.l.
98.5
82-162
EDS
45.3
42.9
B.d.l.
10.7
0.7
B.d.l.
0.6
B.d.l.
B.d.l.
100.2
82-166
WDS
26.35
67.3
3.73
0.29
0.32
B.d.l.
0.25
0.08
0.047
98.36
82-166
EDS
26.4
69.9
2.5
B.d.l.
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
99.1
82-166
EDS
27.0
67.2
3.3
0.7
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
98.5
Poludennyi Kundat R. 82-146
Pryamoi Kundat R.
Note. WDS, Results of electron microprobe analysis; EDS, Results of scanning electron microscopy. B.d.l., Below detection limit.
samples comprised hand-panning using a stepwise procedure (Boitsov et al., 2005) to minimize loss of precious metals. PGM grains were handpicked from the final concentrates under a binocular microscope and then examined for grain size, morphology, and surface texture. Selected PGM grains were mounted in epoxy blocks and polished with diamond paste for further analysis using a set of analytical methods. The composition and morphology of the nanoscale PGM grains were investigated using a MIRA 3 LMU (Tescan) scanning electron microscope with an attached INCA Energy 450 XMax 80 (Oxford Instruments—NanoAnalysis) energy dispersion system, which allowed detection of nanoparticles. The analytical conditions were accelerating voltage 20 kV, beam current 1600 pA (1.6 nA), with 30 s counting time, energy resolution (MIRA) 126–127 eV at the Mn Kα line, and spot size 12 nm (MIRA). Spatial resolution of the analysis was a function of the spot size of the X-ray beam (3–5 µm and more), depending on the average atomic number of element and the wavelength of any particular line. The minimum detection limit for most elements was 0.2–0.3% (3-σ criterion) and may reach 0.5–0.8% and more in the case of element interferences or when using the L-series X-rays for “heavy” elements (Z > 72). Analytical accuracy was often as low as 1 rel.% for major elements (C > 10–15%) and varied from 2–6 to 10 rel.% for minor elements present in excess of 1–10%, reaching 20–30% rel. for elements present near the detection limits. Low detection limit and high accuracy for trace elements were obtained by increasing the counting time. At the same time, the detection limit and accuracy decreased
by half as the counting time was increased by a factor of four. The counting times ranged from 30 s for the main elements in the PGM to 150 s for the trace elements. The analyses were conducted at the X-ray Laboratory of the Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (analysts N.S. Karmanov, M.V. Khlestov, D.K. Belyanin). The standards used were SiO2 (O and Si), NaAlSi3O8 (Na), MgCaSi2O6 (Mg and Ca), Al2O3 (Al), FeS2 (S), NaCl (Cl), KAlSi3O8 (K), Cr2O3 (Cr), PtAs2 (As), SnO2 (Sn), HgTe (Hg), PbTe (Pb and Te), Bi2Se3 (Bi, Se), and pure metals (Ti, V, Mn, Pt, Ir, Os, Pd, Rh, Ru, Fe, Cu, Ni, Co, Au, and Ag). Detection limits of the elements (wt.%) were 0.48 for Ru, 0.36 for Os, 0.38 for Ir, and 0.13–0.17 for Rh, Pt, and Pd. Additional control was performed by analyzing all PGM grains on a Camebax-micro electron microprobe at the Analytical Center of the Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, in wavelength dispersive X-ray spectrometry mode (WDS) with a finely focused beam (<2 µm). The analytical conditions were accelerating voltage 20 kV, 20–30 nA beam current, and 10 s counting time. The following X-ray lines and standards were used: PtLα, IrLα, OsMα, PdLα, RhLα, RuLα, AgLα, AuLα (pure metals), AsLα (synthetic InAs), SbLα (synthetic CuSbS2), SKα, FeKα, CuKα (synthetic CuFeS2), NiKα, CoKα (synthetic FeNiCo), BiMα (synthetic Bi2Se3). Element interference was corrected using experimentally measured coefficients (Lavrent’ev and Usova, 1994). Detection limits of the elements (wt.%) were 0.17 for Pt, 0.15 for Ir; 0.04 for Os, 0.04 for Pd, 0.04 for Rh, 0.04 for Ru, 0.03 for Fe, 0.06 for
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Cu, 0.07 for Ni, 0.05 for Co, 0.02 for S, 0.05 for As, and 0.06 for Sb. A comparison of the WDS (wavelength dispersive X-ray spectrometry, microprobe) and EDS (energy dispersive X-ray spectrometry, SEM) data on PGM (Table 1) with the results obtained on olivine, garnet, pyroxene, ilmenite, and Cr-spinel (Lavrent’ev et al., 2015) has demonstrated good reproducibility, which enables the use of the EDS data in mineralogical and geochemical studies. It should be noted that identification of sample heterogeneity and high spot resolution of the EDS analysis make it more preferable over WDS for analysis of the mineral chemistry of the assemblages and aggregates (e.g., PGM exsolutions) in the nanometer-size range. In these cases, the microprobe data can be used to characterize bulk compositions of these nanoscale polymetallic aggregates. Microtextural observations of PGM were performed by means of reflected light microscopy with a Zeiss AxioScope. A1 microscope.
Results PGM morphology. Overall, the morphology of the PGM grains from different placers is remarkably similar but may exhibit slight variation. Grain sizes range from 0.1 to 1.0– 1.5 mm, being up to 2 mm in individual samples. Most grains (over 70%) are smaller than 0.25 mm. Grains in the size range of –0.25 mm are less abundant (7–35 wt.%), whereas PGM grains 0.25–0.5 and 0.5–1.0 mm in size are more common. Grains of Pt–Fe and Os–Ir–Ru alloys are characterized by different surface morphologies. The Pt–Fe alloy grains appear smoother and more rounded, whereas most Os–Ir–Ru alloy grains are oblate. The PGM grains with well-rounded shapes are most common in the fine size range (smaller than 0.2 mm) (Fig. 2). The PGM grains display a wide range of shapes. Knobby, subrounded, spherical, thin-tabular, deformed octahedral and cubic grains with smooth edges or druse-like, irregularly shaped, acicular grains are present. Less common are grains with an embayed, rough surface. All the above morphologies can be found only locally and in various proportions. The surfaces of Pt–Fe alloy grains often bear negative imprints of host crystals. Many grains are mantled by dark layers, which may either partially in-fill cavities on the surfaces or cover the entire surface of individual grains. Os–Ir–Ru alloys occurs as oblate, tabular, poorly rounded grains, often with flat, smooth surfaces and a prominent pinacoidal cleavage. Well-formed cubic and hexagonal crystals are common. Some grains have shagreen surfaces and growth steps. The pinacoidal surfaces may exhibit negative growth pyramids. Rounded grains with “rock candy” reflective surfaces are present in one sample from the Taenza River placer. Sperrylite occurs as isolated angular and rarer rounded grains. Rare angular grains of laurite and irarsite are present in the Bolshoi Orton River placer. Two sets of fractures are detected in PGM grains. The curved fractures with contorted edges filled with supergene minerals (kaolinite, goethite, illite, etc.) are erratically distrib-
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Fig. 2. PGM concentrate (dredge 138), Kaurchak River, Gornaya Shoria. The concentrate composition (%): PtFe—88, RuIrOs—8, PtAs2—4. Photomicrograph shows small grains of oval and rounded shapes.
uted on the surfaces of the PGM grains (Fig. 3a, b). Fractures of this type contain no alteration products. Rectilinear fractures commonly form en-echelon or parallel arrays within the entire grain volume. They have relatively smooth surfaces and are characterized by the development of endogenic minerals (rutile, amphibole, chlorite) and alteration products at their contacts (Fig. 3c). Composition of PGM. The minerals are dominated by Pt–Fe(±Cu) and Os–Ir–Ru(±Pt) solid solutions, and subordinately by sperrylite (up to 3–4%, rarely to 17%) and other PGE-bearing minerals (sulfides, arsenites, sulfarsenides) (Table 2). The proportions of Fe–Pt and Os–Ir–Ru alloy grains vary considerably in placers of Kuznetsk Alatau and Salair. The principal PGM in placers of Gornaya Shoria is ferroan platinum, whereas Os–Ir–Ru alloys predominate in the Taenza and Bolshoi Orton Rivers placer located to north, at the junction between Gornaya Shoria and Kuznetsk Alatau. Pt–Fe(±Cu) solid solutions exhibit some similarities but also important differences. The majority of grains are isoferroplatinum (Pt3Fe) with Fe + Ni = 20–30 at.%, and subordinate grains are native platinum Pt(Fe) with Fe + Ni = 10– 20 at.% (Fig. 4). Cu (<1.0–1.5 to 3–4 at.%) is invariably present in Pt–Fe alloys (Table 3). The rims of some grains from different placers of Gornaya Shoria (Fig. 4b) are extremely enriched in Cu (up to 15 at.% and more) due to the presence of tulameenite, Cu-rich isoferroplatinum, Pt3(Fe0.6⋅ Cu0.4)–Pt3(Fe0.4Cu0.6), and hongshiite, Pt1.1Cu0.9–Pt1.2Cu0.8. Four groups of Pt(Fe-Cu) solid solutions can be divided based on their Cu content: (1) Pt–Fe(–Cu)—Cu < 1 at.%; (2) Pt– Fe–Cu—Cu 1–5 at.%; (3) Pt–Fe–Cu—Cu 7–15 at.%; (4) Pt– Cu(–Fe) (hongshiite)—Cu 40–50 at.%. The Cu-rich varieties of Pt–Fe are the principal PGM in placers of Gornaya Shoria. The contents of other trace elements vary from 2 to 6 wt.%, while the maximum concentrations of individual elements may be as high as 2–5 wt.%. In decreasing order of abundance, the following elements are present in PGM grains: Rh → Ir → Os → Pd, Ru → Ni. Based on various combinations of these trace elements, all Pt–Fe alloys can be divided into the following groups: Cu–Rh-dominant alloys, Os-dominant alloys
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Fig. 3. Fractures in the deformed placer PGM grains from southern Siberia. a, Isoferroplatinum (Kaurchak River) displaying a network of curve fractures; b, rectilinear parallel fractures in rutheniridosmine (Kaurchak River); c, thin parallel fractures in rutheniridosmine with rutile, tourmaline, amphibole (Poludennyi Kundat RIver). a, b, Photomicrographs, a Zeiss AXIO Scope A1 microscope; c, Back-scattered electron image (SEM BSE, Teskan Orsay Holding).
enriched in Cu, Rh, and Ru, Ir-dominant alloys enriched in Cu, Rh, and Ru. Os–Ir–Ru (±Pt) alloys range in composition from the most common Os-dominant alloys and subordinate Ir-dominant alloys to minor Ru-dominant alloys (Fig. 5, Table 3). In the Os–Ru–Ir diagram, these compositions form two distinct Osand Ru-enrichment trends. The compositions defining an Os trend are typical of the placer PGM from Gornaya Shoria. The grains of Os–Ir–Ru(±Pt) alloys forming a Ru trend are reported from all placer PGM analyzed. However, the individual Os–Ir–Ru(±Pt) grains are mostly homogeneous in composition, except for Os–Ir–Ru alloys in the Taenza River placer, which exhibit variations in the concentration of Ir from rim to core. Compositional zoning may be manifested in the development of parallel rims of Ru-rich iridosmine, Os47.6Ir35.9Ru16.4, rutheniridosmine rims with elevated contents of Pt (Ru52.7Os20.6Ir20.6Pt8.2Fe0.9), or in the growth of boxy grains (Fig. 6). Figure 6b shows photomicrographs of a
PGM grain with a hexagonal crystal of native Os (Os89.9Ir8.6Ru1.6) occurring as a broken, partly splintered core mantled by osmiride (Ir61.6Os33.8Ru4.6), which is replaced by a more Ru-rich phase toward a rim. The splintered portion of the osmium crystal is filled with kaolinite. Microtextures of PGM. Based on the timing and conditions of crystallization, the observed textures of the placer PGM grains are interpreted to have either a magmatic or postmagmatic origin. Massive microtextures (homogeneous), exsolution-induced textures and inclusions are typical of heterogeneous mineral grains formed at the magmatic stage. Texturally homogeneous grains are uncommon. Inclusions are commonly observed in combination with the massive textures in the matrix. Inclusions are interpreted to have formed during sequential crystallization of PGE-rich phases from the magma or during entrapment of primary magmatic material (aluminosilicate inclusions). Osmium (with traces of Ru and Ir) is commonly present as euhedral inclusions in isoferroplatinum.
Table 2. Types of assemblages of PGM in placers of southern Siberia Region
Placer
PGM, % Pt–Fe alloys
I
II
III
Os–Ir–Ru alloys
Sperrylite PtAs2
As-, S-rich PGE
Kaurchak R., dredge
88
8
4
–
Andoba R., dredge
88
9
3
–
Koura R., dredge*
93
2
4
1
Taenza R.
2
98
–
–
Bol. Orton R.
21
75
–
4
Bol. Tuluyul R.
65
24
11
–
Poludennyi Kundat R.
22
78
–
–
Pryamoi Kundat R.
17
58
17
8
Shaltyr’-Kozhukh R.
67
33
–
–
Srednyaya Ters’ R.
67
33
–
–
Kel’bes R.
72
28
–
–
Sella R.
60
37
3
–
Simonovskii Creek
100
–
–
–
Mostovaya R.*
8
92
–
–
Note. Hereafter in the tables: I, Gornaya Shoria; II, Kuznetsk Alatau; III, Salair. * Data from this study and from Krivenko et al. (1994) and Tolstykh et al. (1996).
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Fig. 4. Composition of Pt, Fe (±Cu) solid solutions. a, Matrix (grain); b, inclusions and rims. 1, Gornaya Shoria; 2, Kuznetsk Alatau; 3, Salair.
The Cu-rich Pt–Fe alloy grains containing lamellae of native gold are found in PGM from the Andoba River placer. Such textures formed by exsolution from PGE-bearing solid solutions have been discussed in the literature over a period of more than 50 years (Betekhtin et al., 1958; Cabri et al., 1996). Lattice-like intergrowth (exsolution fabric) of complex solid solutions of (Pt, Fe, Ir, Os, Rh, Ru), Pt–Ir (Rh), and Pt-Os, which may also contain gold, are uncommon. One placer grain from the Koura River alluvium, Gornaya Shoria, displays evidence of exsolution of a composite compound Pt63Fe29Ru3Rh2Ir2PdCu (Fig. 7a) and formation of Ru–Osand Ru–Ir–Os-bearing exsolution phases. This alloy appears to reflect the primary composition of PGE-bearing melt. Some platinum grains from the Polydennyi Kundat River placer (Kuznetsk Alatau) contain exsolution phases consisting
of platiniridium and iridioplatinum (or Ir–Pt phases) in association with (Rh,Pt,Ir)4Sb3 (an Rh-dominant analogue of genkinite) (Fig. 7b). Some grains display complex intergrowth relationships between isoferroplatinum with a lattice of nanoscale Os lamellae and PGE-bearing sulfides and sulfarsenides, replacing this isoferroplatinum, but not Os lamellae, which remain unaltered (Fig. 7c). One grain of isoferroplatinum exhibits a columnar (subgraphic) integrowth of cooperite and xingzhongite in the rim zone (Fig. 7d). This texture was probably a late-stage product that formed as a result of interaction of Cu-, S-, As-rich melt with the exsolution lamellae of Pt-, Ir-, and Rh-dominant alloy. The most common microtextures (patchy, banded, cellular, zoned) formed during the late stages as a result of metasomatic replacement of primary textures (Fig. 8) and are locally
Fig. 5. Composition of Os, Ir, Ru solid solutions. Symbols are the same as in Fig. 4.
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Fig. 6. Rims and zoned boxy PGM grains, Taenza River. Electron images. a, Zoned grains of Os62Ir23Ru15 alloy (2) surrounded by a rim of Ru-dominant alloy (Ru53Os21Ir14Pt8Rh4) (1), inclusions of sanidine (3) and epidote (4); b, a composite boxy grain: native osmium (1), kaolinite (2), osmiride (3), Ru-rich phase (4).
observed in combination with exsolution textures. These textures are composed of PGE-bearing sulfides, arsenides, and sulfarsenides (sperrylite, irarsite, rhodarsenite, etc.). In the Cu-bearing Pt–Fe alloy grains, such textures represented by Cu-rich varieties of platinum (up to hongshiite) are observed in the rim zones. Rims on PGM grains. The presence of rims is typical of many placer PGM grains from Gornaya Shoria and other regions. There are two main types of rims, as indicated by the relationship between a rim and a primary matrix mineral: replacement-induced rims (Fig. 8a, d; Fig. 9a, c) and rims formed by overgrowth and precipitation of minerals (Fig. 9b, f). These rims around PGM grains are formed by PGE-bearing sulfides, sulfarsenides, and arsenides present in variable proportions (Tables 4 and 5): cooperite, sperrylite, irarsite, cuprorhodsite, malanite, hollingworthite, rhodarsenite, platarsite, Pt–Cu–Fe alloys, hongshiite, kharaelakhite, erlichmanite, native gold, etc. The sequence of crystallization of these minerals is not similar in all rims but it shows the tendency of early formation of cooperite and native gold-I (Fig. 9) and late formation of sperrylite. Cooperite replaces platinum leaving inclusions of native Os intact (Fig. 10a). Cooperite tends to be fully replaced by arsenides and sulfarsenides at the margins. Early native gold-I having fineness of 700–800‰ occurs as inclusions along boundaries of PGE-bearing grains and/or within replacement-induced rims. As noted above, in some grains of Cu–Pt–Fe alloys from the Andoba River placer, native gold is present as emulsion–liquation inclusions (Fig. 10b) or branching dendritic segregations (Fig. 10c). A characteristic exsolution texture is locally preserved in the rim zones of one grain of isoferroplatinum (with 0.47 wt.% Cu, 1.31 wt.% Pd, 0.72 wt.% Ru, and 1.93 wt.% Rh) from the Kaurchak River placer. This texture is represented by a thin alternation of bent, dendtritic, elongated nanoscale lamellae of gold (Au84Ag16) and an arsenide phase (Pt and Rh), which formed as a result of late-stage metasomatic-hydrothermal alteration. As a result
of replacement of such zones by cooperite and sperrylite (along the grain margins), gold-I is concentrated and distributed parallel to the isoferroplatinum–sperrylite interface or forms nanoscale inclusions in sperrylite. High-fineness native gold-II may also be present as late-stage smallest (5–10 µm) overgrowths on the surface of native platinum or preexisting rims (Figs. 9b, 10d). Gold-II is characterized by very low Ag (1.4–1.6 wt.%) and the invariable presence of Hg (3.5– 12.5 wt.%, up to 18.3 wt.%). The genesis of this supergene gold remains poorly understood. Microinclusions in PGM grains. Most PGM grains contain submicroscopic and microscopic inclusions (Figs. 8, 11, and 12): (1) formed by exsolution from a primary solid solution; (2) melt and fluid-melt inclusions showing different degrees of alteration and crystallization; (3) relic mineral inclusions (silicates, oxides, etc.); (4) emulsion–liquation inclusions. The shape of the inclusions varies considerably. They appear as tabular, crystallomorphic, lath-shaped or irregular, smoothed, oval and roundish (drops, balls) crystals. They can be monomineralic or polymineralic and are composed of aluminosilicates, oxides, sulfides, and PGM. Some PGM grains exhibit a complex internal structure formed by exsolution, replacement, and recrystallization textures in combination with inclusions of oxides, silicates, and newly-formed PGM. The role of inclusions as an indicator requires further investigation. Common microinclusions in PGM comprise sulfides, sulfarsenides, and arsenides, whereas intermetallic compounds (tellurides and Sb compounds) are rare. Inclusions are more abundant in Pt–Fe solid solutions than in Ru–Ir–Os alloys. PGM grains in Gornaya Shoria placers contain the most abundant and diverse inclusions, which show a far more varied array of minerals (Tables 4–6). More than 20 minerals were identified, including sulfides (cooperite, laurite, braggite, erlichmanite, xingzhongite, bowieite) and rarer minerals. For example, the analyzed PGE-rich thiospinels represent an extensive series, which includes a Co-rich variety, dayingite,
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464 Table 3. Chemical composition of selected PGM grains from placers of southern Siberia (wt.%) Region Placer
Analysis
Solid Sol.
Os
I
EDS
Pt–Fe
EDS
Pt–Fe
EDS
Pt–Fe
Kaurchak R.
Andoba R.
Koura R.
Taenza R.
II
Bol. Tuluyul R.
Poludennyi Kundat R.
Pryamoi Kundat R.
Pt
Ru
Rh
Pd
Fe
Ni
Cu
Total
B.d.l.
B.d.l.
89.7
0.7
2.3
0.9
5.7
1.3
B.d.l.
87.4
0.6
2.1
0.8
6.4
B.d.l.
B.d.l.
99.3
B.d.l.
0.6
B.d.l.
3.9
88.2
0.5
1.0
0.5
6.2
B.d.l.
99.2
0.3
100.6
EDS
Pt–Fe
1.4
3.1
84.6
0.6
B.d.l.
0.5
6.0
B.d.l.
B.d.l.
96.2
EDS
Pt–Fe
B.d.l.
2.0
89.7
B.d.l.
0.7
0.5
6.7
B.d.l.
B.d.l.
99.6
EDS
Pt–Fe
B.d.l.
B.d.l.
90.5
B.d.l.
1.0
B.d.l.
5.6
B.d.l.
0.9
98.0
EDS
Ir–Os
20.5
71.8
B.d.l.
1.0
B.d.l.
B.d.l.
0.5
0.3
B.d.l.
94.1
EDS
Pt–Fe
B.d.l.
1.9
89.7
0.5
0.6
0.5
8.1
B.d.l.
0.7
102.0
EDS
Pt–Fe
B.d.l.
B.d.l.
88.0
B.d.l.
0.5
B.d.l.
8.1
B.d.l.
0.5
97.1
EDS
Pt–Fe
B.d.l.
B.d.l.
90.8
B.d.l.
B.d.l.
B.d.l.
8.3
B.d.l.
0.4
99.5
EDS
Os–Ir–Ru
33.9
31.4
4.9
30.4
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
100.6
EDS
Os–Ir–Ru
36.6
30.2
2.3
28.6
B.d.l.
B.d.l.
0.5
B.d.l.
B.d.l.
98.2
EDS
Os–Ir
74.9
21.9
B.d.l.
3.2
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
100.0
EDS
Os–Ir
67.0
22.6
2.3
5.5
0.7
B.d.l.
0.2
B.d.l.
B.d.l.
98.3
EDS
Ir–Os
34
60.7
B.d.l.
1.0
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
95.7
EDS
Ir–Os
31.5
61.0
B.d.l.
3.1
B.d.l.
B.d.l.
0.2
B.d.l.
B.d.l.
95.8
EDS
Os–Ir–Ru
51.3
39.1
B.d.l.
9.4
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
99.8
EDS
Pt–Fe
1.4
1.5
86.5
0.4
1.8
1.0
5.5
B.d.l.
0.6
98.7
EDS
Pt–Fe
B.d.l.
2.2
87.6
B.d.l.
B.d.l.
B.d.l.
7.8
B.d.l.
1.2
98.8
EDS
Os–Ir–Ru
48.0
41.9
B.d.l.
9.4
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
99.6
EDS
Os–Ir–Ru
45.6
38.3
B.d.l.
12.0
0.7
B.d.l.
B.d.l.
B.d.l.
B.d.l.
96.6
EDS
Ru–Os–Ir
36.4
14.7
B.d.l.
48.1
B.d.l.
B.d.l.
B.d.l.
B.d.l.
B.d.l.
99.2
EDS
Pt–Fe
B.d.l.
B.d.l.
86.9
0.5
0.8
B.d.l.
10.1
0.6
1.0
99.9
EDS
Ir–Os–Pt–Ru
21.8
48.3
13.1
13.0
1.4
B.d.l.
0.6
0.4
B.d.l.
98.6
EDS
Os–Ir–Ru
44.4
37.4
1.8
15.9
0.7
B.d.l.
B.d.l.
B.d.l.
B.d.l.
100.2
EDS
Ir–Pt
B.d.l.
64.9
20.9
6.3
7.2
B.d.l.
B.d.l.
B.d.l.
B.d.l.
99.3
EDS
Os–Ir–Ru
45.3
42.9
B.d.l.
10.7
0.7
B.d.l.
0.6
B.d.l.
B.d.l.
100.2
EDS
Pt–Fe
B.d.l.
B.d.l.
88.2
B.d.l.
B.d.l.
B.d.l.
8.4
B.d.l.
B.d.l.
96.6
EDS
Os–Ir–Ru
36.5
26.4
B.d.l.
32.3
1.9
B.d.l.
B.d.l.
B.d.l.
B.d.l.
97.1
EDS
Ir–Os
26.4
69.9
2.5
B.d.l.
B.d.l.
B.d.l.
0.3
B.d.l.
B.d.l.
99.1
Shaltyr’-Kozhukh R.
EDS
Pt–Fe
B.d.l.
B.d.l.
88.8
B.d.l.
0.5
0.6
9.0
0.8
B.d.l.
99.7
EDS
Pt–Fe
B.d.l.
1.9
87.8
B.d.l.
B.d.l.
B.d.l.
8.5
B.d.l.
B.d.l.
98.2
Srednyaya Ters’ R.
WDS
Pt–Fe
0.09
0.20
89.49
B.d.l.
1.24
0.45
8.09
0.07
1.02
100.65
WDS
Pt–Fe
0.07
0.22
89.6
0.16
1.58
0.52
6.46
0.09
0.71
99.41
WDS
Os–Ir–Ru
44.22
36.03
2.55
16.94
0.54
B.d.l.
0.25
0.03
0.08
100.64
WDS
Os–Ir–Ru
30.47
29.53
3.24
35.14
1.08
B.d.l.
0.34
0.02
0.08
99.91
WDS
Os–Ir
89.05
8.20
1.44
0.74
0.50
B.d.l.
0.07
0.07
B.d.l.
100.07
Kel’bes R
Sella R.
III
Ir
Simonovskii Creek* Mostovaya R.**
WDS
Pt–Fe
B.d.l.
B.d.l.
89.00
0.08
0.14
1.11
8.00
0.73
B.d.l.
99.06
WDS
Pt–Fe
B.d.l.
0.29
89.60
0.06
0.77
B.d.l.
7.69
0.40
0.04
98.84
WDS
Pt–Fe
B.d.l.
0.54
90.38
0.03
0.10
B.d.l.
7.47
1.32
0.04
99.88
WDS
Pt–Fe
B.d.l.
0.15
89.89
0.02
0.91
0.48
7.73
0.93
0.01
100.12
WDS
Os–Ir–Ru
38.80
37.00
3.61
20.05
0.56
B.d.l.
0.20
B.d.l.
0.06
100.28
WDS
Os–Ir–Ru
13.95
67.47
5.98
9.31
2.29
B.d.l.
1.06
B.d.l.
0.12
100.18
WDS
Pt–Fe
B.d.l.
1.14
89.73
B.d.l.
0.23
B.d.l.
7.29
B.d.l.
0.76
99.16
WDS
Pt–Fe
B.d.l.
B.d.l.
89.94
B.d.l.
0.62
0.29
7.75
B.d.l.
0.33
98.92
WDS
Pt–Os–Ir
6.38
8.19
76.04
0.77
0.41
B.d.l.
7.59
B.d.l.
0.11
99.49
WDS
Pt–Ru–Ir–Os
12.93
26.23
43.94
13.78
1.95
B.d.l.
0.08
B.d.l.
B.d.l.
98.81
WDS
Pt–Ru–Ir–Os
7.77
12.73
49.54
21.89
6.5
B.d.l.
0.12
B.d.l.
B.d.l.
98.55
WDS
Pt–Fe
0.35
B.d.l.
87.2
B.d.l.
0.28
0.22
10.08
B.d.l.
0.49
98.62
Note. B.d.l., Below detection limit. * Data from Podlipsky et al. (2007). ** Data from Tolstykh et al. (2002).
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Fig. 7. Exsolution textures. Backscattered electron images (SEM) of PGM. a, Lattice-like texture: 1, platinum Pt63Fe29Ru3Rh2Ir2Pd1Cu1, 2, Ru55Ir16Os15Pt10Rh2Fe2, 3, Ru39Os39Ir15Pt7, 4, isoferroplatinum (Koura River); b, lattice fabric of platinum: 1, Ir56Pt19Rh12Ru10Fe3, 2, Pt46Ir32Fe12Rh6Ru4, 3, Rh-genkinite (Rh,Pt,Ir)4Sb3 (Poludennyi Kundat River); c, isoferroplatinum grain with a lattice of osmium lamellae replaced by PGE sulfides and sulfarsenides: 1, isoferroplatinum, 2, hollingworthite, 3, cooperite, 4, sperrylite, 5, platarsite (Koura River); d, columnar, subgraphic texture in the rim around isoferroplatinum grain: 1, 2, cooperite, 3, xingzhongite (Koura River).
Cu(Co, Pt)2S4 (Fig. 10a), a rare derivative of the malanite– carrollite CuPt2S4–CuCo2S4 isomorphic series (up to 39 mol.% Co). Inclusions of dayingite occur in association with malanite at the grain margins or in arsenides occurring as rims around the grains of isoferroplatinum in the placers of Gornaya Shoria (Andoba and Koura Rivers). Simple sulfides like laurite and erlichmanite often form medium and large isolated idiomorphic inclusions (up to 25 µm) (Fig. 10a), whereas more complex sulfides constitute crystalline polymineralic inclusions where they are found in association with Fe–Cu–Ni sulfides. These complex, polyphase sulfide inclusions in the grain of ruthernosmiride from the Polydennyi Kundat River placer contain an unnamed sulfide
phase, a possible Cu-dominant analogue of vysotskite (Pd,Ni)S (Fig. 11, Table 4). Arsenides and sulfarsenides of Ru, Rh, Ir, Pt, and Pd occur as polyphase inclusions or rims (sperrylite), which often reach a thickness of up to 50 µm. Such rims often surround grains of Pt–Fe alloys and are uncommon in Os–Ir–Ru alloys. The mineral composition of these rims directly correlates with that of their host mineral, i.e., arsenide phases tend to be composed of trace elements from a matrix mineral. For example, rhodarsenide (Rh,Pd)2As is commonly found as Rh- and Pd-bearing inclusions in the grains of Pt–Fe alloy. The most common arsenide phases in Os–Ir–Ru alloys are iridsite, iridarsenite.
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
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Fig. 8. Secondary microtextures in PGM grains formed by PGE sulfides and sulfarsenides (Koura River). a, Lattice-like formed by sperrylite; b, patchy: sperrylite, hongshiite, platarsite, rhodarsenite occuring as inclusions in Pt–Pd alloy; c, banded; d, zoned. Backscattered electron images (SEM) of PGM.
The microinclusions in placer samples from the Andoba River (Gornaya Shoria) contain PGE-bearing tellurides: telluropalladinite, Pd9Te4, and maslovite, Pt(BiTe). They fill microfractures or occur as inclusions within rim sulfarsenides (Fig. 11). Of special interest are rare microinclusions of Sb intermetallic compounds, like genkinite (Pt,Pd)4Sb3, and their derivatives. They are present as inclusions in isoferroplatinum (Andoba River) or as exsolution textures within a Pt grain represented by alternation of oriented zones made up of platiniridium and iridioplatinum (Poludennyi Kundat River) (Figs. 10b, 11f, and 7b). In the latter case, the exsolution textures are formed by iridioplatinum and coexisting phases of the Rh-dominant analogue of genkinite (Pt,Pd)4Sb3, which incorporates Rh (up to 12 wt.%) instead of Pd.
Besides various PGM, inclusions of silicates, sulfides, oxides and, more rarely, phosphates were identified in some PGE-bearing minerals. Sulfides of Fe, Ni, and Cu are commonly present in some crystalline inclusions: chalcopyrite and pentlandite coexisting with laurite, ferrorhodsite, and irarsite; chalcopyrite and bornite in association with braggite, bowieite, and telluropalladinite; cubanite in association with braggite, ferrorhodsite, and malanite (Fig. 11). Some grains of the Pt–Fe alloys contain Fe and Cr oxides as inclusions. Inclusions of magnetite were detected in placers from the Polydennyi Kundat River area. Large zoned inclusions with variable Cr content are present in Pt–Fe alloy grains from the Koura River placer (Fig. 12). Such inclusions record changes in redox or P–T conditions during melt crystallization.
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Fig. 9. Inclusions and rims in PGM. Electron images. a–c, Kaurchak River; d, Andoba River; e, f, Kaurchak River. a, Sperrylite occurring as a replacement rim (1) and inclusions of early native gold-I with fineness of 700–800‰ (2) in the isoferroplatinum grain; b, overgrowths of late native gold-II with fineness of 863–985‰ on isoferroplatinum; c, overgrowths of late native gold-II (1) on isoferroplatinum containing inclusions of gold-I and rim sperrylite (1); d, overgrowths of late gold-II on isoferroplatinum containing inclusions of gold-I (1) and rim sperrylite (2); e, rim sperrylite (1) surrounding a rounded Pt grain (2), PGM inclusions occuring subparallel to the grain boundary, the grain varies in composition from Pt63Fe20Ir13Os3Ru1 (2) to Ir44Os25Pt23Ru3Rh3Fe2 (3); f, sperrylite (1) and cooperite (2) rim around isoferroplatinum grains (3).
In addition, some PGM grains may contain inclusions of oxide minerals. Various PGE oxides have been documented so far in chromitites from ophiolite complexes of the Urals, Finland, and Oman (Ahmed and Arai, 2003; Garuti et al., 1997; Gornostayev et al., 2000), alluvial occurrences in the Chukotka Peninsula (Mochalov et al., 1992; Gornostayev et al., 1999; and others). The formation of PGE oxides is believed to occur as a result of low-temperature transformation of rocks (Ahmed and Arai, 2003) or even grains during burial diagenesis or low-temperature metamorphism (Gornostayev et al., 1999). No structural data could be obtained for these minerals, because of their small grain-size. Two complex PGE oxides, Ir- and Hg-rich (Table 7, Fig. 13) and Ru-, Fe-, and Ir-rich (Table 7) were identified in two grains of Pt–Fe alloy in the Koura River placer (Gornaya Shoria). These minerals occur at the grain margins, in zones of microfracturing, in intimate association with lamellae of osmium (Fig. 13) and Ru-bearing osmium, probably replacing them. Like the oxides from the Southern Urals (Garuti et al., 1997), the PGE oxides reported in the Koura River placer have stoichiometries between XO2 and X2O3 and an analytical total less than 100 wt.% (up to 93.5 wt.% in the first case and no more than 96.8 wt.% in the second case, Table 7). This was ascribed to the possible presence of OH-groups (Gornostayev et al., 1999; Mochalov et al., 1992) and/or H2O molecules in their composition. It cannot be ruled out that the presence of fractures in the known oxides may lead to a deficit in the analytical total (Ahmed and Arai, 2003). The formulas calculated for these minerals in this study are presented in Table 7. In addition to major
elements, the chemical analysis of the PGE oxides demonstrates the presence of a considerable amount (>1 wt.%) of Pt, Rh, Ca, Mn, and Ti, and traces (≤1 wt.%) of Al, Si, V, As, and S. Not all constituents detected in these compounds are minor elements, but their presence can be explained by a close association with other minerals, e.g., kaolinite. Silicates occur as crystallized melt inclusions in PGM and are commonly represented by amphibole and pyroxene (Fig. 12). Similar aluminosilicate melt inclusions were reported in ferroplatinum grains from placer deposits of the northeastern Siberian platform (Airiyants et al., 2014; Okrugin et al., 2012). In addition, these grains were found to contain olivine, plagioclase, feldspar, biotite, muscovite, quartz, chlorite, garnet (grossular, andradite, schorlomite), epidote, titanite, rutile, and phosphates such as monazite and fluorapatite (3.5 wt.% F). Supergene minerals observed within fractures and on grain surfaces are represented mostly by goethite and kaolinite.
Discussion A detailed analysis of the placer PGM from southern Siberia revealed considerable variation in the degree of abrasions, from well-rounded to unrounded, reflecting longer transportation (Gornostayev et al., 1999; Hattori et al., 2010; McClenaghan and Cabri, 2011; Podlipsky et al., 2007; Tolstykh et al., 2002). Usually, the PGM from the studied placers are poorly rounded, which indicates relatively little transport from their bedrock sources. At the same time, smooth,
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464 Table 4. Composition of PGE sulfides in rims and inclusions from PGM grains Region
Placer
Matrix
Location Os
Ir
Pt
Ru
Rh
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 85.9
B.d.l. 0.9
Pd
Fe
Co
Ni
Cu
Pb
S
Total
Cooperite PtS I
Koura R.
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.5
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 82.9
2.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 85.3
B.d.l. B.d.l. B.d.l. 0.6
I
Kaurchak R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 81.7
1.3
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 16.1
99.1
I
Andoba R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 84.3
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.4
98.7
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 85.2
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.3
99.5
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 85.9
B.d.l. B.d.l. 0.9
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 1.2
0.6
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
II
Pryamoi Kundat R.
(Fe, Cu)Pt
II
Shaltyr’-Kozhukh R.
(Fe, Cu)Pt
81.2
B.d.l. B.d.l. 0.2
0.7
B.d.l. B.d.l. 0.7
101.3
B.d.l. 12.6
98.8
B.d.l. B.d.l. B.d.l. B.d.l. 13.9
99.8
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.5
B.d.l. 0.5
101.3
B.d.l. B.d.l. B.d.l. B.d.l. 14.9
99.1
B.d.l. B.d.l. 84.5
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.1
98.6
r
B.d.l. B.d.l. 86.0
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 15.9
101.9
r
B.d.l. B.d.l. 83.4
B.d.l. B.d.l. 1.03
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 14.2
98.6
Braggite (Pt, Pd)S I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 59.0
B.d.l. B.d.l. 19.4
0.3
B.d.l. 3.6
B.d.l. B.d.l. 18.1
100.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 62.3
B.d.l. B.d.l. 12.7
0.5
B.d.l. 4.9
B.d.l. B.d.l. 17.6
98.0
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 56.1
B.d.l. B.d.l. 17.7
2.1
B.d.l. 3.2
1.9
B.d.l. 19.9
100.9
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 60.2
B.d.l. B.d.l. 17.5
1.1
B.d.l. 3.6
0.4
B.d.l. 20.6
103.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 59.1
B.d.l. B.d.l. 17.5
0.3
B.d.l. 3.3
0.7
B.d.l. 17.5
98.3
incl.
B.d.l. 31.3
B.d.l. 18.8
14.3
98.2
Xingzhongite (Pb, Cu, Fe)(Ir, Pt, Rh)2S4 I
Andoba R.
(Fe, Cu)Pt
10.1
I
Andoba R.
(Fe, Cu)Pt
incl.
B.d.l. 27.6
11.7
B.d.l. 20.9
B.d.l. 0.5
B.d.l. B.d.l. 5.1
15.4
19.2
100.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 43.1
11.0
B.d.l. 15.2
B.d.l. 1.0
B.d.l. 0.8
6.6
3.0
18.5
99.2
I
Kaurchak R.
(Fe, Cu)Pt
r
B.d.l. 35.8
21.9
1.2
B.d.l. 1.2
B.d.l. B.d.l. 3.6
8.1
17.5
97.4
I
Andoba R.
(Fe, Cu)Pt
r
1.7
28.2
24.8
B.d.l. 9.9
B.d.l. B.d.l. B.d.l. B.d.l. 5.0
10.0
18.5
98.0
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 18.7
28.1
0.3
B.d.l. B.d.l. B.d.l. B.d.l. 5.1
17.8
21.5
98.7
8.2
7.2
B.d.l. 1.0
B.d.l. B.d.l. 4.8
17.9
Kingstonite (Rh, Ir, Pt)3S4 I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 10.1
12.1
B.d.l. 51.4
B.d.l. 0.3
B.d.l. B.d.l. B.d.l. B.d.l. 25.7
99.6
II
Shaltyr’-Kozhukh R.
(Fe, Cu)Pt
incl.
B.d.l. 17.3
14.1
B.d.l. 42.2
B.d.l. 2.6
B.d.l. B.d.l. B.d.l. B.d.l. 24.0
100.2
(Fe, Cu)Pt
incl.
7.6
Laurite RuS2 I
Koura R.
B.d.l. B.d.l. 57.0
2.9
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 32.3 B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 34.9
99.8
I
Taenza R.
Os–Ir–Ru
incl.
11.7
5.9
B.d.l. 46.6
0.7
I
Taenza R.
Os–Ir–Ru
incl.
9.8
5.4
B.d.l. 47.9
B.d.l. 0.5
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 35.0
98.6
99.7
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
6.8
5.2
B.d.l. 52.1
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 35.8
99.9
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
6.9
5.0
B.d.l. 50.7
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 36.4
99.0
Os–Ir–Ru
incl.
39.2
10.5
B.d.l. 15.8
2.2
B.d.l. 30.5
99.4
Erlichmanite OsS2 I
Taenza R.
I
Taenza R.
Os–Ir–Ru
incl.
39.9
11.7
B.d.l. 16.5
1.9
1.3
0.3
B.d.l. B.d.l. B.d.l. B.d.l. 28.3
99.9
I
Koura R.
(Fe, Cu)Pt
r
56.7
7.0
4.6
1.9
B.d.l. 0.6
B.d.l. B.d.l. B.d.l. B.d.l. 25.5
98.7
1.3
B.d.l. B.d.l. 2.0
B.d.l. 11.2
98.0
1.4
B.d.l. 2.1
15.6
B.d.l. 17.0
96.1
B.d.l. B.d.l. 12.1
B.d.l. 31.3
98.9
B.d.l. 29.3
102
2.4
B.d.l. 0.6
B.d.l. 0.4
0.2
Vasilite Pd16S7 I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 1.7
B.d.l. B.d.l. B.d.l. 81.8
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
B.d.l. B.d.l. B.d.l. 0.5
B.d.l. 59.5
Cuprorhodsite CuRh2S4 I
Kaurchak R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 3.7
B.d.l. 50.3
B.d.l. 1.5
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 20.1
B.d.l. 40.1
B.d.l. B.d.l. 0.6
B.d.l. 11.9
(continued on next page)
1450
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Table 4 (continued) Region
Placer
Matrix
Location Os
(Fe, Cu)Pt
incl.
Ir
Pt
Ru
Rh
38.4
B.d.l. 19.1
Pd
Fe
Co
Ni
Cu
Pb
S
Total
Malanite Cu(Pt, Ir)2S4 I
Koura R.
B.d.l. 3.7
I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 38.6
5.8
I
Koura R.
(Fe, Cu)Pt
r
3.8
30.0
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. 6.2
36.7
II
Shaltyr’-Kozhukh R.
(Fe, Cu)Pt
incl.
B.d.l. 39.3
22.4
B.d.l. 0.5
B.d.l. B.d.l. 11.8
B.d.l. 18.3
0.1
B.d.l. B.d.l. 9.6
B.d.l. 26.4
100.4
B.d.l. 11.3
B.d.l. B.d.l. B.d.l. B.d.l. 8.4
B.d.l. 24.5
100.4
B.d.l. 19.3
B.d.l. 1.1
B.d.l. B.d.l. 11.2
B.d.l. 25.2
99.7
14.4
B.d.l. 8.6
B.d.l. 1.2
B.d.l. B.d.l. 11.8
B.d.l. 23.9
99.2
1.6
B.d.l. 26.4
99.9
Ferrorhodsite (Fe, Cu)(Rh, Ir, Pt)2S4 I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 3.2
10.9
B.d.l. 32.2
B.d.l. 14.4
B.d.l. 1.2
7.0
B.d.l. 29.5
98.4
II
Tuluyul R.
Os–Ir–Ru
incl.
B.d.l. 17.6
1.4
B.d.l. 33.7
B.d.l. 13.6
B.d.l. 1.9
5.8
B.d.l. 26.9
100.9
Dayingite Cu(Co,Pt)2S4 I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 43.7
B.d.l. 2.2
B.d.l. B.d.l. 11.5
B.d.l. 14.0
B.d.l. 28.4
99.8
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 43.1
B.d.l. 1.5
B.d.l. B.d.l. 11.5
B.d.l. 14.0
B.d.l. 29.0
99.0
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 43.1
B.d.l. 1.5
B.d.l. B.d.l. 11.1
B.d.l. 13.5
B.d.l. 29.8
98.9
Os–Ir–Ru
incl.
B.d.l. B.d.l. B.d.l. 0.5
B.d.l. 15.0
96.1
(Pd, Cu)S phase II
Poludennyi Kundat R.
B.d.l. 58.5
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
B.d.l. 1.3
1.2
II
Poludennyi Kundat R.
Os–Ir–Ru
incl.
B.d.l. 1.3
B.d.l. B.d.l. B.d.l. 72.0
B.d.l. B.d.l. 66.5
1.4
B.d.l. 5.1
15.6
0.6
B.d.l. 2.1
15.3
B.d.l. 13.7
100.7
B.d.l. B.d.l. 1.5
11.4
B.d.l. 13.4
99.6
Note. Here and in Table 5: B.d.l., below detection limit; incl., in inclusions; r, in grain rims.
well-rounded to ball-shaped grains (Fig. 2) are common in the finest size fraction (less than 0.2 mm), which, at first glance, appear inconsistent with the assumption about a short transportation distance. In this case, a high degree of roundness and smooth surfaces of grains in the fine size fraction does not reflect the degree of abrasion of PGM grains during fluvial transport, but instead has a primary, endogenic nature. Since the fine particles can be carried as suspended load by stream water, they will become rounded slowly (Nesterenko, 1991; Nesterenko et al., 2013). In some cases, discrimination between PGM grains subjected to mechanical wear and well-rounded PGM grains of primary endogenic origin can be problematic because of a lack of the respective criteria. Here, though, we may notice a certain analogy with native gold. For example, the gold microinclusions in pyrite often have a roundish, drop-like shape, which can be preserved in alluvium (Nesterenko and Kolpakov, 2010). We suggest that the following distinctive features may be indicative of primary endogenic origin of smooth well-rounded PGM grains: (1) the preservation of rims on PGM grains of endogenic origin which were not mechanically eroded (Fig. 9f, e); (2) the presence of textures at the grain margin that are conformable to the outline of the grain, e.g., see Figs. 9e and 14a. The rim of the grain of native osmium (Os70–74Ir20–21 Ru5–11) contains a veinlet of irarsite, which is contorted subparallel to the grain edge (surface); (3) concentric zoning in rounded grains; (4) the presence of remnants of primary minerals and rocks on the surface of PGM grains. The rounded grain of sperrylite (Fig. 14b) displays a pronounced chemical and textural concentric zoning, with a core composed of
chalcopyrite, irarsite, and platarsite, and titanite occurring as inclusions in a common microzone; (5) smooth dendritic segregations within PGM grains with relatively deep embayments and negative crystal imprints (Fig. 14c). Such outlines can be produced at the endogenic stage of PGM formation, but they are unlikely to have been derived from erosion and abrasion processes. The analysis of the grain structure and the presence of rims, first of all, can be of crucial importance in this respect. As noted above, two types of rims were identified within Pt-bearing mineral grains: replacement rims and overgrowth (precipitation) rims. The latter have sharp, smooth boundaries with subtle signs of interaction between the rim and matrix minerals. They are internally homogeneous and monotonic, whereas replacement-induced rims have an irregular, patchy texture. These rims are products of late-magmatic and postmagmatic processes. In contrast to overgrowth (precipitation) rims, metasomatic replacement rims have been widely discussed in the literature. In our opinion, the overgrowth (precipitation) rims may record important genetic information. They may be indicative of the transportation of Pt in magmatic fluids and are commonly developed around PGM grains in placers of Gornaya Shoria. This testifies to the widespread development of fluid-saturated ore-forming systems in this region, e.g., along the Kaurchak, Koura, and Andoba Rivers (e.g., Seglebir massif). At the same time, the presence of relic rims of Pt-rich sulfides in some alluvially worn PGM grains from the Shaltyr’–Kozhukh River area (Kuznetsk Alatau) may indicate negligible alluvial reworking of the material in Gornaya Shoria compared to the central part of the Kuznetsk
1451
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464 Table 5. Composition of PGE sulfarsenides and arsenides in inclusions and rims from PGM grains Region
Placer
Matrix
Location Os
Ir
Pt
Ru
Rh
Pd
Fe
Co
Cu
S
As
Sb
Te
Total
1.2
98.5
Sperrylite PtAs2 I
Koura R.
Os–Ir–Ru
incl.
2.5
7.5
45.8
1.0
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 39.6
0.9
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 2.2
52.6
0.5
2.4
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 1.7
53.9
I
Kaurchak R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 53.6
I
Kaurchak R.
(Fe, Cu)Pt
r
I
Kaurchak R.
(Fe, Cu)Pt
r
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. B.d.l. B.d.l. 1.1
40.5
B.d.l. B.d.l. 99.2
B.d.l. 0.7
B.d.l. 0.3
B.d.l. B.d.l. 0.9
39.8
B.d.l. B.d.l. 97.3
B.d.l. 2.4
B.d.l. 0.3
B.d.l. B.d.l. 1.8
40.0
B.d.l. B.d.l. 98.0
B.d.l. B.d.l. 49.5
B.d.l. 2.3
0.6
0.7
0.8
B.d.l. 1.4
42.3
B.d.l. B.d.l. 97.6
B.d.l. B.d.l. 55.5
B.d.l. B.d.l. 1.0
0.4
0.9
0.4
2.1
38.5
B.d.l. 1.3
B.d.l. 4.5
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 1.1
40.0
B.d.l. B.d.l. 98.8
53.2
100.1
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 2.3
52.5
B.d.l. 1.1
B.d.l. B.d.l. B.d.l. B.d.l. 1.0
40.4
B.d.l. B.d.l. 97.3
II
Pryamoi Kundat R.
(Fe, Cu)Pt
incl
B.d.l. 2.8
59.1
B.d.l. 1.7
B.d.l. 2.0
32.8
B.d.l. B.d.l. 99.6
Os–Ir–Ru
r
0.4
B.d.l. 7.9
B.d.l. B.d.l. 1.2
Iridarsenite IrAs2 I
Taenza R.
47.0
1.0
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 44.0
B.d.l. B.d.l. 100.3
Cherepanovite RhAs I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 9.4
B.d.l. 49.2
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 42.3
B.d.l. B.d.l. 100.9
I
Taenza R.
Os–Ir–Ru
incl.
0.8
1.7
1.4
7.8
47.9
B.d.l. 0.3
B.d.l. B.d.l. B.d.l. 40.0
B.d.l. B.d.l. 99.9
I
Taenza R.
Os–Ir–Ru
incl.
1.5
1.1
2.7
7.1
44.1
B.d.l. 0.4
B.d.l. B.d.l. B.d.l. 39.9
B.d.l. B.d.l. 96.8
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 1.9
0.8
49.6
18.2
B.d.l. B.d.l. B.d.l. 26.7
B.d.l. B.d.l. 98.1
Rhodarsenide (Rh, Pd)2As I
Kaurchak R.
0.9
I
Andoba R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 2.5
B.d.l. 49.3
18.7
2.5
B.d.l. B.d.l. B.d.l. 24.2
B.d.l. B.d.l. 97.1
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 0.4
B.d.l. 46.7
25.4
2.1
B.d.l. 0.5
B.d.l. B.d.l. 100.2
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 5.0
B.d.l. 48.6
19.1
0.2
B.d.l. B.d.l. B.d.l. 25.4
B.d.l. B.d.l. 98.2
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 5.5
1.7
49.0
20.7
0.7
B.d.l. B.d.l. B.d.l. 23.0
B.d.l. B.d.l. 100.5
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 7.6
B.d.l. 39.1
28.0
B.d.l. B.d.l. B.d.l. B.d.l. 26.2
B.d.l. B.d.l. 100.8
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 5.6
B.d.l. 41.9
22.1
3.3
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 11.3
B.d.l. 46.6
18.4
B.d.l. B.d.l. B.d.l. B.d.l. 24.0
B.d.l. B.d.l. 100.3
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 10.4
B.d.l. 45.1
20.9
B.d.l. B.d.l. B.d.l. B.d.l. 23.1
B.d.l. B.d.l. 99.5
II
Poludennyi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 11.2
B.d.l. 43.0
19.0
B.d.l. B.d.l. B.d.l. B.d.l. 25.9
B.d.l. B.d.l. 99.1
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 54.8
B.d.l. 10.1
B.d.l. 2.7
B.d.l. B.d.l. 7.3
25.9
B.d.l. B.d.l. 100.8
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 0.9
50.5
B.d.l. 8.5
B.d.l. 1.2
B.d.l. B.d.l. 7.4
29.4
B.d.l. B.d.l. 97.9
I
Kaurchak R.
(Fe, Cu)Pt
r
2.1
65.9
0.9
B.d.l. B.d.l. B.d.l. B.d.l. 12.8
8.8
B.d.l. B.d.l. 96.3
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. B.d.l. 58.9
B.d.l. 3.8
B.d.l. 0.9
0.4
B.d.l. 7.4
24.3
B.d.l. B.d.l. 95.7
I
Koura R.
(Fe, Cu)Pt
r
1.6
6.5
34.5
2.5
13.0
B.d.l. 0.2
B.d.l. B.d.l. 7.4
34.1
B.d.l. B.d.l. 99.8
I
Koura R.
(Fe, Cu)Pt
r
2.2
4.6
38.0
2.0
9.6
B.d.l. 0.5
B.d.l. B.d.l. 5.7
34.6
B.d.l. B.d.l. 97.2
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. 1.4
50.5
2.8
10.4
B.d.l. 0.4
B.d.l. B.d.l. 6.5
29.2
B.d.l. B.d.l. 101.2
II
Pryamoi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. 1.9
49.5
0.5
8.9
B.d.l. 1.8
(Fe, Cu)Pt
incl.
B.d.l. 0.6
49.8
7.6
B.d.l. B.d.l. 5.8
(Fe, Cu)Pt
incl.
0.0
35.8
2.2
13.3
7.7
B.d.l. 25.2
B.d.l. B.d.l. B.d.l. 25.3
1.3
B.d.l. 99.5
Platarsite PtAsS I
II
1.4
4.4
B.d.l. 0.3
10.7
B.d.l. B.d.l. 9.0
28.0
B.d.l. B.d.l. 101.6
25.6
B.d.l. B.d.l. 98.4
28.4
B.d.l. B.d.l. 100.1
Irarsite (Ir, Ru, Rh, Pt)AsS I
Koura R.
B.d.l. B.d.l. B.d.l. B.d.l. 12.7
I
Koura R.
(Fe, Cu)Pt
incl.
1.7
34.6
2.1
13.9
9.9
B.d.l. 0.4
0.5
B.d.l. 11.2
25.6
B.d.l. B.d.l. 99.9
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 34.4
3.6
12.4
8.7
B.d.l. 1.4
B.d.l. B.d.l. 13.2
25.3
B.d.l. B.d.l. 99.0
I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 49.1
B.d.l. 5.1
7.6
B.d.l. B.d.l. B.d.l. B.d.l. 12.6
25.7
B.d.l. B.d.l. 100.0
I
Taenza R.
Os–Ir–Ru
incl.
B.d.l. 43.6
B.d.l. 6.0
12.2
B.d.l. 0.3
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 32.5
2.3
28.2
B.d.l. B.d.l. B.d.l. B.d.l. 8.2
I
Koura R.
(Fe, Cu)Pt
r
0.2
33.6
2.4
1.7
30.2
B.d.l. 0.7
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 35.0
4.4
2.0
21.2
B.d.l. B.d.l. B.d.l. B.d.l. 10.1
1.2
B.d.l. B.d.l. 12.2
B.d.l. B.d.l. 5.5
23.8
B.d.l. B.d.l. 98.1
26.5
B.d.l. B.d.l. 98.9
26.4
B.d.l. B.d.l. 100.7
27.6
B.d.l. B.d.l. 100.3
(continued on next page)
1452
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Table 5 (continued) Region
Placer
Matrix
Location Os
I
Koura R.
(Fe, Cu)Pt
r
I
Taenza R.
Os–Ir–Ru
Ir
Pt
Ru
Rh
Pd
Fe
Co
Cu
S
As
Sb
B.d.l. 33.7
3.2
1.5
21.5
B.d.l. B.d.l. B.d.l. B.d.l. 9.1
r
2.8
51.4
B.d.l. 9.0
Te
Total
30.2
B.d.l. B.d.l. 99.1
B.d.l. B.d.l. B.d.l. B.d.l. B.d.l. 12.5
24.7
B.d.l. B.d.l. 100.4
Hollingworthite (Rh, Pt, Pd)AsS I
Kaurchak R.
(Fe, Cu)Pt
incl.
0.1
6.6
29.4
10.1
22.5
B.d.l. 0.3
B.d.l. B.d.l. 12.3
19.5
B.d.l. B.d.l. 100.8
I
Kaurchak R.
(Fe, Cu)Pt
incl.
0.2
5.0
12.4
6.2
27.2
B.d.l. 0.3
B.d.l. B.d.l. 15.2
31.0
B.d.l. B.d.l. 97.5
I
Koura R.
(Fe, Cu)Pt
incl.
B.d.l. 6.5
23.2
3.4
23.7
B.d.l. 0.3
B.d.l. B.d.l. 12.9
29.6
B.d.l. B.d.l. 99.6
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 9.9
22.0
3.3
22.1
B.d.l. B.d.l. B.d.l. B.d.l. 10.6
30.8
B.d.l. B.d.l. 98.7
I
Koura R.
(Fe, Cu)Pt
r
B.d.l. 10.1
29.0
4.6
15.4
B.d.l. 0.3
32.1
B.d.l. B.d.l. 100.4
B.d.l. B.d.l. 9.1
I
Koura R.
(Fe, Cu)Pt
r
0.1
8.2
28.2
3.2
22.6
B.d.l. 1.2
B.d.l. B.d.l. 8.6
25.1
B.d.l. B.d.l. 97.2
II
Tuluyul R.
(Fe, Cu)Pt
incl.
1.4
4.9
14.2
7.4
37.2
B.d.l. 0.2
B.d.l. B.d.l. 7.7
27.4
B.d.l. B.d.l. 100.4
II
Tuluyul R.
(Fe, Cu)Pt
incl.
B.d.l. B.d.l. 29.2
7.6
22.9
B.d.l. 0.6
B.d.l. 0.4
13.3
25.5
B.d.l. B.d.l. 99.5
II
Pryamoi Kundat R.
(Fe, Cu)Pt
incl.
B.d.l. 5.7
0.7
27.8
B.d.l. 0.4
B.d.l. B.d.l. 10.8
29.9
B.d.l. B.d.l. 95.7
20.4
Table 6. Composition of PGE-bearing intermetallic compounds occurring as inclusions and fracture-fill in PGM grains Region
Placer
Matrix
Os
Ir
Pt
Ru
Rh
Pd
Fe
Ni
Cu
Bi
S
As
Sb
Te
Total
Pt3(Rh,Pd)Sb3 phase I
Andoba R.
(Fe, Cu)Pt
N.a.
3.1
45.1
N.a.
6.8
N.a.
3.4
0.4
N.a.
N.a.
N.a.
N.a.
39.6
N.a.
98.4
I
Koura R.
Os–Ir–Ru
1.7
3.1
43.4
4.7
7.7
1.5
N.a.
0.5
N.a.
N.a.
N.a.
0.7
37.2
N.a.
100.5
I
Koura R.
Os–Ir–Ru
1.7
1.2
47.5
2.8
9.5
B.d.l. N.a.
I
Koura R.
Os–Ir–Ru
2.0
2.4
50.6
B.d.l. 8.1
1.5
0.1
N.a.
N.a.
N.a.
B.d.l. 38.0
N.a.
100.8
B.d.l. 0.8
N.a.
N.a.
N.a.
B.d.l. 35.9
N.a.
101.3
II
Poludennyi Kundat R.
Os–Ir–Ru
N.a.
5.5
42.5
0.8
12.4
4.5
0.2
N.a.
N.a.
N.a.
N.a.
N.a.
31.3
N.a.
97.2
II
Poludennyi Kundat R.
Os–Ir–Ru
N.a.
3.9
40.9
B.d.l. 17.9
3.6
B.d.l. N.a.
N.a.
N.a.
N.a.
N.a.
33.5
N.a.
99.8
II
Poludennyi Kundat R.
Os-Ir-Ru
N.a.
4.9
46.2
0.4
11.6
5.4
0.2
N.a.
B.d.l. N.a.
N.a.
N.a.
33.0
N.a.
101.7
II
Poludennyi Kundat R.
Os-Ir-Ru
N.a.
4.5
50.1
0.5
10.8
4.5
1.0
N.a.
0.4
N.a.
N.a.
N.a.
30.2
N.a.
102.0
(Fe, Cu)Pt
N.a.
2.2
27.9
N.a.
5.7
N.a.
1.8
N.a.
0.4
39.4
N.a.
N.a.
2.2
22.5
102.1
Maslovite PtBiTe I
Andoba R.
I
Andoba R.
(Fe, Cu)Pt
N.a.
2.8
20.1
N.a.
9.1
N.a.
1.4
N.a.
0.4
51.1
N.a.
N.a.
1.4
14.2
100.5
I
Andoba R.
(Fe, Cu)Pt
N.a.
2.4
26.6
N.a.
6.5
N.a.
1.1
N.a.
B.d.l. 37.9
N.a.
N.a.
1.6
22.0
98.1
(Fe, Cu)Pt
N.a.
N.a.
4.6
B.d.l. 1.8
58.9
1.1
N.a.
0.4
1.9
N.a.
N.a.
30.0
98.6
Telluropalladinite Pd9Te4 I
Kaurchak R.
N.a.
Note. B.d.l., Below detection limit; N.a., not analyzed.
Alatau area and less intense erosion in Gornaya Shoria during the Cenozoic. The preservation of rims around placer PGM grains can serve as additional evidence to support a short distance from their bedrock sources. Surface fracturing is another important morphological feature of PGM grains. We also note, without going into details, that the studied PGM grains display two types of microfractures, one of which is probably associated with mechanical deformation of grains during alluvial transport. The more interesting type II includes linear, en-echelon, parallel fractures filled with amphibole, rutile, and chlorite. Obviously, such fractures could have formed by deformation processes during crystallization of PGM or as a result of formation of ophiolite complexes in a stress regime (Coleman, 1977). It is not surprising that type II fractures are typical of
most of the examined Os–Ir–Ru alloy grains, possibly because they are more brittle than Pt–Fe alloys. Analysis of microfractures in bedrock minerals is important because it provides clues as to the directions and magnitudes of principal stresses, effective confining pressure, differential stress, brittle and ductile behavior, and multiphase deformations. The evaluation of the potential bedrock sources of PGE mineralization using the compositions of PGM in placer occurrences has been discussed in many previous papers (Cabri et al., 1996; Fedortchuk et al., 2010; Gornostayev et al., 1999; Harris and Cabri, 1991; Krivenko et al., 1994; McClenaghan and Cabri, 2011; Nixon et al., 1990; Polyakov and Bognibov, 1995; Slansky et al., 1991; Tolstykh, 2004; Vysotsky, 1933; and others). PGE mineralization represented by Pt–Fe and Pt–Pd alloy species of PGM are traditionally
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
1453
Fig. 10. Rims and exsolution textures in PGM. a, a grain of isoferroplatinum with Os lamellae (1) and inclusions of native gold-I (2) is overgrown by a rim consisting of cooperite (3), malanite, dayingite (4), cuprorhodsite (5) and erlichmanite (6) (Koura River); b, emulsion inclusions of gold (1), geversite (2) (Andoba River); c, isoferroplatinum with inclusions of gold-I (1) is corroded by rims consisting of cooperite (2) and sperrylite (3). The inset shows the exsolution texture: lamellae of gold Au84Ag16 and an arsenide phase (Kaurchak River); d, Pt grain with Os lamellae (1), rim cooperite (2) with tiny inclusions of gold (3), overgrown by native gold-II (4) with fineness of 900‰ (Kaurchak River).
related to concentrically zoned complexes and layered mafic– ultramafic intrusions (O’Driscoll and Gonzáles-Jiménez, 2016), whereas rutheniridosmine is thought to have originated from ophiolitic associations. Considering the above, there is the working hypothesis that two types of PGM placer occurrences prevail in southern Siberia, the Pt–Fe alloys associated with Uralian–Alaskan-type concentrically zoned complexes and rutheniridosmine associated with ophiolite ultramafic rocks (Krivenko et al., 1994; Podlipsky et al., 2007; Polyakov and Bognibov, 1995; Tolstykh et al., 1996, 1999), with the exception of PGM in the Taenza River placer, which are represented mostly by Os–Ir–Ru minerals (Fig. 1). The
latter are interpreted to have originated from Fe-rich ultramafic rocks, mainly small elongated bodies of serpentinized lherzolites and, more rarely, dunites (Tolstykh, 2004), which were previously described as “dike-like bodies of serpenitinized porphyrites” (Kyuz, 1935). In the ternary diagram, the compositions of Os–Ir–Ru alloys from these rocks form an osmium trend, as opposed to ophiolitic compositions defining a ruthenium-enrichment trend. At the same time, the PGM assemblage found in the Taenza and Bolshoi Orton River placers (dominantly Os–Ir–Ru alloy species) is distinctly different from the PGM of many other placers of Gornaya Shoria (dominantly Pt–Fe alloy species). Despite the absence
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S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
Fig. 11. Complex heterogeneous internal exsolution textures with inclusions of newly-formed PGM. a, b, h, Taenza River, c, Poludennyi Kundat River, d, Kaurchak River, e, Bol. Tuluyul River, f, g, Koura River, i, Andoba River. Backscattered electron images (SEM) of PGM. a, Polyphase sulfide inclusion: xingzhongite (1), braggite (2), vasilite (3) with laurite occurring as a idiomorphic grain in the core (4) and a thin rim of erlichmanite (5); b, tiny inclusions of hongshiite (1), rhodarsenite (2) in the Pt–Fe-Cu alloy; c, crystalline sulfide inclusion of pentlandite (1), bornite (2), laurite (3) and (Pd, Cu)S phase; d, polyphase inclusion of chalcopyrite (1), bornite (2), bowieite (3), telluropalladinite (4), braggite (5); e, idiomorphic grain of laurite (1) in a sulfide inclusion of pentlandite (2), irarsite (3), chalcopyrite (4); f, polyphase rounded crystalline inclusions of Rh-rich geversite (1) and isoferroplatinum (2), inclusions of irarsite (3), laurite (4); g, Os lamellae (1), idiomorphic inclusions of laurite (2), irarsite (3), rim cooperite (4); h, dendritic textures formed by cherepanovite (1), rim irarsite (2), sperrylite (3); i, inclusions of xingzhongite (1) and maslovite (2) in isoferroplatinum containing emulsion inclusions of gold.
of the ruthenium-enrichment trend in the compositions of PGM, these placers are spatially associated with ophiolite − 1) along the northern margin of the Mrassu uplift. rocks (V–C The PGM may be derived from small serpentinite massifs. In its upper course, the Bolshoi Orton River drains a lens-like body (1.5 km in length) of fully serpentinized wehrlite-clinopyroxenite of the Usa complex. According to Kyuz (1935), outcrops of serpentinite from which the samples analyzed in
this study were collected are also located in the estuary of the Taenza River (near the Nikolka-Talovaya Creek). In these placers, isolated grains or inclusions of laurite, irarsite, and pentlandite were identified in addition to rutheniridosmine and subordinate Pt–Fe alloys. However, previous data (Polyakov and Bognibov, 1995) show that laurite, erlichmanite, PGE-, Cu-, Fe-, and Ni-rich sulfides, Pd telluride, and palladian gold were also reported from the Taenza River placer. Besides the
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
1455
Fig. 12. Polymineralic inclusions in isoferroplatinum and Ru–Ir–Os alloy (d). Backscattered electron images of PGM. a, Rounded, poorly crystalline silicate inclusion, tiny inclusions of sperrylite (1) (Koura River); b, c, rounded crystalline inclusions of silicates (Koura River): 1, plagioclase, 2, titanite, 3, biotite, 4, pargasite, 5, pyroxene, 6, Fe-rich hornblende; d, olivine inclusion (Taenza River); e, f, relic inclusions of magnetite, chromite, Cr-magnetite in the Pt3Fe grain with Os lamellae (1) and rim cooperite (2) (Koura River); g, rounded poorly crystalline silicate inclusion (Andoba River); h, crystalline inclusions of silicates: pyroxene (1), Cr-magnetite (2), biotite (3), chlorite (4); hongshiite (5) and rhodarsenite (6) developed at the inclusion edges (Koura River); i, a grain of ferroan platinum with Os lamellae (1) inclusions of monazite (2) and irarsite (3) (Koura River).
above two localities, laurite also occurs in placers along the Koura, Poludennyi and Pryamoi Kundat, Bol. Tuluyul Rivers and more commonly in placers of Kuznetsk Alatau (in association with pentlandite), yet rarely found in placers of Gornaya Shoria. For example, similar PGM assemblages (laurite, erlichmanite, irarsite, and sperrylite) and ophiolitetype sources were reported for Au-PGM placer occurrences of the Gar ore cluster (Amur River region) (Molchanov et al., 2003). The presence and amount of PGE-rich sulfides depend on the temperature regime and the activity of S in the ore-forming system of ophiolite complexes (Polyakov and Bognibov, 1995).
The observed variations in the proportions of Pt–Fe and Os–Ir–Ru alloys in placer PGM from southern Siberia indicate the existence of different sources (Table 2). Zaccarini et al. (2004) and Murzin et al. (2015) provided evidence for the existence of both PGM assemblages associated with Uralian– Alaskan-type and ophiolite-type sources, the first assemblage dominated by Pt–Fe alloys and the second dominated by Os–Ir–Ru alloys. Assuming a similar bedrock source for these placer PGM assemblages, the proportions of Pt–Fe and Os–Ir–Ru minerals should be almost identical. However, the situation is different. The proportion of Pt–Fe alloys varies from 93 to 2% (by number of PGM grains) in placers of
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Table 7. Composition of Ir oxide Element
82-28-11*
82-28-12
82-28-13
82-29-8
82-29-9
82-29-13
Pt, wt.%
6.5
4.7
5.2
5.7
6.6
5.3
Ir
49.3
42.1
38.6
16.8
22.2
17.4
Ru
1.4
1.3
1.7
17.4
11.0
16.6
Rh
2.3
2.0
1.6
0.8
1.5
0.9
Al
0.7
0.6
0.7
0.7
0.6
0.6
Ca
2.9
2.2
3.0
0.7
0.9
0.8
Si
N.a.
N.a.
0.3
0.2
0.4
0.2
Ti
N.a.
N.a.
0.5
1.4
0.8
1.4
V
0.1
0.8
0.6
0.6
0.5
0.6
Mn
1.6
2.1
3.1
0.3
0.4
0.3
Fe
0.6
0.7
1.8
13.6
13.4
12.9
Hg
7.0
20.0
16.5
11.0
11.4
10.8
As
0.1
0.2
0.1
0.5
0.8
0.6
S
0.1
0.3
2.1
1.9
2.1
1.8
O
17.0
15.5
17.6
25.2
22.7
23.2
Total
89.6
92.5
93.4
96.8
95.3
93.3
O = 2.3
O=2
O=2
O = 2.3
O = 2.3
O = 2.3
Pt, apfu
0.07
0.05
0.05
0.04
0.06
0.04
Ir
0.56
0.45
0.37
0.13
0.19
0.14
Ru
0.03
0.03
0.03
0.25
0.18
0.26
Rh
0.05
0.04
0.03
0.01
0.02
0.01
Al
0.06
0.04
0.05
0.04
0.04
0.04
Ca
0.16
0.11
0.14
0.03
0.04
0.03
Si
–
–
0.02
0.01
0.02
0.01
Ti
–
–
0.02
0.04
0.03
0.05
V
0.00
0.03
0.02
0.02
0.01
0.02
Mn
0.06
0.08
0.10
0.01
0.01
0.01
Fe
0.02
0.03
0.06
0.36
0.39
0.37
Hg
0.08
0.21
0.15
0.08
0.09
0.09
As
0.00
0.00
0.00
0.01
0.02
0.01
S
0.01
0.02
0.12
0.09
0.11
0.09
O
2.3
2
2
2.3
2.3
2.3
ΣPt, Ir, Ru, Rh, Ca, V, Mn, Fe, Hg
1.03
1.03
0.95
0.93
0.99
0.97
Note. N.a., Not available. * Grain no.
Kuznetsk Alatau, from 72 to 17% in placers of Gornaya Shoria and from 100 to 8% in placers of Salair. Considerable variations in the proportions of Os–Ir–Ru alloys, sperrylite and sulfarsenides in the same placer occurrences are also observed. In diagrams, the compositions of all studied grains show Os- and Ru-enrichment trends, typical of the Uralian–Alaskanand ophiolite-type sources (Bird and Bassert, 1980; Fedortchuk et al., 2010; Polyakov and Bognibov, 1995; Tolstykh et al., 1996). In some cases, the presence of melt inclusions (lamellae) of native gold enclosed within the Cu-rich Pt–Fe alloy grains (Andoba River) and the occurrence of electrum—native gold as exsolutions (Kaurchak River) may testify to the existence
of primary composite Au-Pt–Fe alloys (Fig. 10). Textural data indicate the following sequence of crystallization: (1) crystallization of isoferroplatinum at temperatures below 1350 ºC from a composite melt containing PGE, Au, Ag, Fe, As, S, and Te; (2) formation of dendritic textures with a complex eutectic between gold-silver and PGE sulfarsenides; (3) crystallization of sperrylite at 912–1016 ºC from a gas–vapor phase (because of the high vapor pressure of As) by reaction with the early isoferroplatinum and eutectic gold-silver–PGE sulfarsenides to form zones composed of bowieite (Rh,Ir, Pt)2S3 and platarsite, sperrylite. Lager grains of gold begin to precipitate at the same interface. This crystallization sequence
S.M. Zhmodik et al. / Russian Geology and Geophysics 57 (2016) 1437–1464
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Fig. 13. A grain of ferroan platinum with Os lamellae replaced by an Ir–(Hg)-bearing oxide phase. a, Backscattered electron images (SEM) of PGM; b–f, element map for Os, Ir, Pt, Fe, and O using EDS on SEM.
may point to an emplacement of layered intrusions or concentrically zoned massifs and is uncommon for ophiolites. The presence of Cu-rich varieties, including hongshiite, as overgrowths on Pt–Fe alloys has been reported by many workers (Barkov et al., 2002, 2005; Betekhtin et al., 1958; Nekrasov et al., 1994; and others). Such overgrowths are interpreted to have formed by metasomatic replacement of magmagtic Pt–Fe alloys by postmagmatic hydrothermal solutions. The evidence for overgrowth of Cu-rich platinum was first reported in the rim-like zone around Pt–Fe alloy grains from the Nizhny Tagil complex in 1930 and linked by A.G. Betekhtin to serpentinization of dunites and mobilization of Cu. In some cases (Fig. 11b), our data may point to the existence of the original Cu–Pt–Fe fluid, which crystallizes during cooling to form a patchy exsolution texture, with alternating Fe-rich zones and more Cu-rich zones consisting of almost pure hongshiite. This heterogeneity is not limited to the surface of grains and the network of fractures or deformations. A more complex situation involving both mechanisms cannot be ruled out. The theoretical predictions that Pt may be transported by hydrothermal fluids have been discussed in many previous papers (Boudreau et al., 1986; Distler et al., 2000; Nixon et al., 1990) and the strongest evidence for this is the presence of PGM in quartz veins (Distler et al., 2000; Ramdohr, 1962; Vysotsky, 1933). These results confirm that a variety of micro- and nanoscale precious mineral grains may have crystallized from the melt enriched in precious metals
(Pt, Rh, Pd, Ru, Os, Ir, Au, and Ag), as well as S, Te, and As. As noted above, isoferroplatinum may contain exsolution phases consisting of platiniridium and iridioplatinum (or Ir–Pt phases), which are replaced by a compound (Rh,Pt,Ir)4Sb3. Similar compounds were reported from placers in the Witwatersrand (Feather, 1976) and Guli massif (Manur and Ozue, 1998). Feather (1976) suggested that such compounds represent the end-members Pt3Fe and (Ru, Os, Ir) that could have formed by incomplete exsolution from a complex primary solid solution. It is known that Pt and Ir form a continuous solid solution series at temperatures above 845 ºC (Andryushchenko et al., 1984). The solid solutions break down at lower temperatures forming the end-members enriched in the accompanying element. The observed exsolution textures of Pt–Ir indicate a high temperature origin for these alloys. Typical exsolution textures of PGE solid solutions have been widely discussed in the literature (Betekhtin et al., 1958; Cabri et al., 1996). The grains of Pt–Fe alloy with exsolution textures are most common in placers of Gornaya Shoria. It was assumed (Betekhtin et al., 1958; and others) that such textures could have been developed during slow cooling and solidification of a melt or solid solution of hydrothermal origin. Such conditions may exist within relatively large massifs originated at great depths. The absence of exsolution textures can be indicative of special conditions of formation, e.g., in the mafic dikes, which are widespread in the study area, or in the picrtite-basalt association commonly developed
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Fig. 14. Pseudorounded PGM grains of primary endogenic origin. Backscattered electron images (SEM) of PGM. a, A veinlet of irarsite (1) parallel to the margin of the grain of native osmium Os73Ir21Ru6, inclusion of amphibole (2) (Taenza River); b, rounded zoned sperrylite grain with inclusions of native gold (1), hollingworthite (2), (Fe,Cu)2(Rh,Pt)S3 (3), titanite (4), chalcopyrite (5) (Kaurchak River); c, pseudorounded endogenic grains of Pt3(Fe,Cu), arrows indicate smooth dendrites, which could not be preserved during abrasion (Koura River).
along extensions of the studied structures to Western Mongolia (Izokh et al., 2001; Oyunchimeg et al., 2009). Some of the analyzed grains of Pt–Fe alloys from the Polydennyi Kundat River placer exhibit exsolution textures formed by platiniridium and iridioplatinum (or Ir–Pt phases), which are replaced by a compound (Rh, Pt, Ir)4Sb3 (Rh-dominant analogue of genkinite) (Fig. 9e). Similar compounds were reported from placers in the Witwatersrand (Feather, 1976) and Guli massif (Malitch and Auge, 1998), as well as from Pt–Fe–Ni–Cu ore deposits hosted by layered mafic-ultramafic intrusions. Feather (1976) suggested that such compounds represent the end-members Pt3Fe and (Ru, Os, Ir) that could have formed by incomplete exsolution from a complex primary solid solution. It is known that Pt and Ir form a continuous solid solution series at temperatures above 845 ºC (Andryushchenko et al., 1984). The solid solutions break down at lower temperatures forming the end-members enriched in the accompanying element. The observed exsolution textures of Pt–Ir indicate high temperatures of formation. Of particular importance is the discovery of Pt-based intermetallic alloys exhibiting large compositional variations: Pt40Ir35Os13Fe11Ru–Pt63Fe20Ir13Os3Ru; Ir55Pt20Ru10Os2Fe– Pt66Fe16Rh12Ir5Sb5Cu3Rh2Pd2; and others. Such alloys were reported in the PGM placer grains from the Evander Goldfield in the eastern part of the Witwatersrand basin (Badanina et al., 2015). The existence of composite solid solutions is related to the systems, which are slightly differentiated with respect to PGE and reflect high P–T conditions. The chemical analysis of the inclusions in chromian spinel and magnetite revealed that they have relatively high TiO2 contents and cannot be classified as Cr-spinel from ophiolitic complexes, which are characterized by TiO2 < 0.3 wt.% (Fig. 15). In the Fe–Al–Cr diagram (Garuti et al., 2012), the compositions of the studied Cr-rich magnetites plot close to the epigenetic trend of Uralian–Alaskan-type concentrically zoned intrusions. Katophorite and edenite are members of the sodic amphibole group (richterite–katophorite–edenite), which is a common constituent of alkaline rocks and carbonatites, the
products of intraplate (plume) magmatism. Experimental results for a variety of alkaline silicate melts show that alkali amphiboles of the richterite–katophorite–pargasite–edenite series formed at ~650 ºC (Koval’skii et al., 2006). Hastingsite is a common constituent of alkaline rock complexes. Numerous alkaline plutons are widely developed within the Kuznetsk Alatau and Gornaya Shoria areas. These plutons are associated with the Goryachegorsk and Karadat complexes represented by the alkaline-gabbro association (Goryachegorsk, Kiya-Shaltyr, Petropavlovka and other nepheline syenite–urtite–melteigite plutons) and alkaline syenites with minor nepheline syenites (Tuim–Karysh plutons), alkaline syenites and nordmarkites (Beresh and other small plutons). The rocks of the Beresh pluton have a relatively homogeneous composition, displaying minor variations in the proportions of the main mineral constituents, K-feldspar and plagioclase. The alkali amphiboles are mainly riebeckite–osannite and ferrorichterite, while potassic amphiboles are edenite and ferro-edenite (Explanatory Note…, 2007). The common PGM assemblage in the rocks of the Goryachegorsk and KiyaShaltyr plutons comprises sperrylite, isoferroplatinum, ferroan platinum and, rarely, rutheniridosmine (Sazonov et al., 2000). The analysis of sources and geodynamic conditions revealed two stages in the formation of gabbroic and feldspathic rocks (theralite, basic foidolite, nepheline syenite) and carbonatites of the Kuznetsk Alatau area: Middle Cambrian–Early Ordovician (~510–480 Ma) and Early–Middle Devonian (~410– 385 Ma). It was suggested that the highly alkaline rocks and carbonatites in the western Central Asian Fold Belt have a mantle plume origin (Vrublevskii, 2015; Vrublevskii et al., 2014). The presence of amphibole in association with biotite, pyroxene, and plagioclase in the Pt–Fe alloy grains may reflect a picritic source for the placer PGM in southern Siberia. The compositions of amphiboles and pressure estimates (P) (Table 8, Fig. 16) indicate that there are several distinct groups of amphiboles found as inclusions in PGM: (1) pargasite (P = 10.1–11.1 kbar); (2) hastingsite (P = 8.2–8.8 kbar); (3) alkali ferromagnesian hornblende (P = 4.5–6.6 kbar); (4) ede-
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Fig. 15. Diagrams showing the compositions of: a, Cr-spinel from inclusions in PGM grains from placers of the Koura and Poludennyi Kundat Rivers, arrows indicate the epigenetic trend; b, Cr-magnetite from inclusions in PGM grains. Samples: K, Koura River; PolK, Poludennyi Kundat River; T, Taenza River.
nite (P = 3.0–4.5 kbar); (5) aluminoedenite (P = 5.1–6.5 to 8 kbar); (6) ferromagnesian hornblende (P = 0.6–1.1 to 2.5 kbar). The presence of pargasite in the PGM grains from the Koura River place suggests that the rocks may have formed in a subduction-zone setting at high pressure (P = 10.1–11.1 kbar) and relatively low temperature conditions. This is also supported by the presence of subalkaline and alkaline magnesian hastingsite that formed at pressures of up to 8.8 kbar. This assumption is consistent with the previous results (Anikina et al., 2001) reported for the Uralian–Alaskan-type dunite–clinopyroxenite–gabbro complexes. Thus, it was assumed that dunite–clinopyroxenite–gabbro complexes of the Ural platinum-bearing belt formed in a suprasubduction setting. The same is true for the Alaskan-type Akarem amphibole-rich mafic-ultramafic complex (Eastern Desert, Egypt) (Abd El-Rahman et al., 2012). A study of plagiogranitoid magmatism of Gornaya Shoria and Western Sayan revealed two stages of magmatic evolution in the island-arc settings. The early stage (550–540 Ma) is marked by formation of high-alumina plagiogranites spatially associated with adakites of the Kshta (545 ± 8 Ma) and Taraskyr massifs (545 ± 7 Ma). These plagiogranites are considered to be derived by melting of a subducting oceanic metabasic protolith with N-MORB-like composition at P ≥ 15 kbar, in equilibrium with a garnet-bearing restite. The rocks of the Taraskyr massif are low-alumina plagiogranites that could be derived by melting of metabasic source regions at the base of an island arc at P = 3–8 kbar, in equilibrium with a plagioclase-bearing restite. The late stage (525–520 Ma) is marked by formation of plagiogranitoids of the Maina complex of the Yenisei (524 ± 2 Ma) and Tabat plutons (Rudnev et al., 2013a,b). Pyroxene is also present as part of composite crystalline and melt inclusions in the PGM. The range of temperatures and fO2 was calculated for clinopyroxene from microinclusions using the original version of monomineralic oxythermometry (Ashchepkov et al., 2010). Figure 17 shows the T–fO2 relationship for clinopyroxene from melt and crystalline inclusions
in PGM. These results indicate, at least, four groups of clinopyroxenes that have formed at (1) T = 1200–1400 °C and fO2 (FMQ) = ca. –4.5; (2) T = 900–1100 °C and fO2 (FMQ) between ca. –1.5 and –0.5; (3) T = 1100–1300 °C and fO2 (FMQ) between –0.1 and +0.5; (4) T = 600–800 °C and fO2 (FMQ) between ca. –1.1 and + 0.1. One crystalline inclusion of iridosmine (Taenza River, Gornaya Shoria) contains olivine with f = Fe/(Fe + Mg) equal to 0.1, which can be indicative of a dunite-harzburgite association. The presence of PGE oxide–hydroxide mineral species may be indicative of hydrothermal alteration. Similar Ru–Os–Ir oxides in association with other PGM have been previously documented in chromitites of the Nurali complex, southern Urals (Garuti et al., 1997), and Oman ophiolite complexes (Ahmed and Arai, 2003). These authors concluded that Ru–Os–Ir oxides were formed by desulfurization of Os and Ru sulfides during hydrothermal alteration and serpentinization. It was also suggested that the PGE oxides may result from the alteration of magmatic PGM alloys and from primary crystallization of oxides in lateritic conditions. Their presence indicates the existence of mechanisms of transport and recrystallization of PGE (as oxide–hydroxide) in surface conditions, mechanisms contributing to the redistribution and enrichment of PGE in laterite (Auge and Legendre, 1994).
Conclusions The use of a set of modern techniques for the in situ quantitative analysis (microprobe and field emission scanning electron microscopy) allows effective determination of different characteristics of minerals. The chemical data for minerals obtained with energy dispersive spectrometry (SEM-EDS) show good agreement with the microprobe results and can be used for mineralogical and geochemical studies.
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Table 8. Amphiboles from inclusions in PGM grains, with P estimates (kbar) Placer
Grain no.
M*
A**
Amphibole
[1]
[2]
[3]
[4]
Koura R.
1-2
PtFe3
Px+Bt+Ttn+Mgh
Alkali-rich ferropargasite
10.1
10.8
8.2
10.1
Gr+Px+Ab+Mgh
Alkali-rich ferropargasite
10.4
11.1
8.4
10.4
11-3
PtFe3, PtAs
Bt
Fe-Mg hornblende
0.4
0.7
0.4
0.4
23-1
PtFe3
Ab+Ap+Bt+Ep+Kfs
Subalkaline hastingsite
8.4
8.8
6.7
8.4
1-3
23-3
Alkaline high-Al edenite
23-2
Alkaline high-Fe hornblende
6.6
6.6
5.1
6.6
27-2
Pt(Fe, Cu)3, PtCu
Pl+Px+Bt+Ttn
Alkaline edenite
4.8
4.5
3.5
4.8
Magnesian hornblende
4.5
4.2
3.3
4.5
27-3
Iron-rich hornblende
5.0
4.7
3.6
5.0
27-4
Magnesian-alkaline hastingsite
8.2
8.5
6.5
8.2
27-9
Edenite
3.5
3.0
2.3
3.5
27-8
Subalkaline hastingsite
4.1
3.7
2.9
4.1
Edenite-hornblende
6.4
6.4
4.9
6.4
27-1
35-15 35-27
PtFe3, RhAs, PtCu, IrAs
35-18
Pl+Bt+Px
Alkaline aluminous katophorite Ol+Px+Chl
Mg-cummingtonite
35-17
Subalkaline anthophyllite
35-19
Tremolite-hornblende
35-14
Fe-Mg hornblende
0.6
0.4
0.2
0.6
35-16
Subalkaline hornblende
2.5
1.8
1.4
2.5
35-20
Subalkaline Fe-rich hornblende
1.1
0.1
0.1
1.1
35-7
Subalkaline hastingsite
4.1
3.7
2..9
4.1
6.5
6.5
5.0
6.5
35-13
Ol+Chl+Grt
35-22
Tremolite Subalkaline hastingsite
Andoba R.
115-8
PtFe3
An
Al-edenite
8.2
8.5
6.5
8.2
Pol. Kundat R.
135-18
Pt(Fe, Cu)3
Pl+Px+Mgh
Subalkaline hastingsite
6.2
6.2
4.8
6.2
Al-edenite
6.6
6.7
5.1
6.6
Mg-hornblende
4.8
4.5
3.5
4.8
2.2
1.4
1.2
2.2
135-19 134-5
OsIrRu
134-7 Taenza R.
73-15
Actinolite-hornblende OsIrRu
Mgh
Cr-Fe-Mg-hornblende
Note. Mgh, Magnetite; Px, pyroxene; Bt, biotite; Ab, albite; An, anorthite; Pl, plagioclase; Ap, apatite; Ep, epidote; Grt, garnet; Ttn, titanite; Kfs, K-feldspar; Ol, olivine; Chl, chlorite. Geobarometers: [1], Hammarstrom and Zen (1986); [2], Hollister et al. (1987); [3], Johnson and Rutherford (1989); [4], Schmidt (1992). Pressure estimates are based on the empirical and experimental calibrations using aluminum content of hornblende. No data are provided where calculations of pressure are not impossible. * Composition of host mineral. ** Association of coexisting mineral phases in inclusions.
The platinum-group minerals identified in 14 placer deposits of Kuznetsk Alatau, Gornaya Shoria, and Salair comprise two distinct mineral assemblages consisting mainly of Pt–Fe alloy species (55%) and subordinately of rutheniridosmine (41%). In two placers, this proportion was close to 2:1. Of all analyzed PGM, about 3–4% are represented by sperrylite and 1% by PGE compounds enriched in As and S. Considerable variations in the proportions of Pt–Fe and Os–Ir–Ru alloys in placers of southern Siberia point to the existence of different bedrock sources. The textural observations and compositions of PGM from alluvial placers (as well as gold placers) provide important evidence for the possible sources of PGE mineralization. The morphology and compositions of the PGM grains and microin-
clusions of silicates, oxides, intermetallic compounds and the type of alteration can be used as indicators for distinguishing parent bedrock sources of PGE. The rock suites of the Kuznetsk Alatau, Gornaya Shoria, and Salair areas and compositions of the host PGM and microinclusions indicate several potential source areas for PGE-bearing mineralization: (1) Uralian–Alaskan-type intrusions (northwestern Kuznetsk Alatau, Simonovskii Creek of the Salair Ridge); (2) ophiolite suites, including those formed in a subduction zones (central Kuznetsk Alatau, Mostovaya River of the Salair Ridge); (3) ultramafic alkaline and carbonatite massifs (northwestern and central Kuznetsk Alatau), alkaline gabbroic suites and, probably, (4) rocks of the picrite-basalt association and small mafic rock bodies and dikes. The presence of composite
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Fig. 16. A plot of Fe3/(Fe3+AlVI) vs. P, kbar for amphiboles from melt and crystalline inclusions in PGM.
crystalline inclusions in PGM.
PGE-bearing compounds of variable composition, Pt40Ir35⋅ Os13Fe11Ru; Pt63Fe20Ir13Os3Ru; Ir55Pt20Ru10Os2Fe; Pt66Fe16⋅ Rh12Ir5Sb5Cu3Rh2Pd2; Os–Ir–Ru–Pt(±Fe), may be indicative of the systems with weak differentiation with respect to PGE and high P–T conditions. In addition, the discovery of emulsion—liquation inclusions and lamellae of native golds within the Cu-rich Pt–Fe alloy grains, and exsolution textures of electrum–native gold provides the first supportive evidence for the existence of Au—Pt–Fe alloys. These data could point to the existence of Uralian–Alaskan-type complexes in some areas of Gornaya Shoria (Kaurchak, Koura, and Andoba Rivers). We suggest that all placer PGM grains with well-rounded smooth shapes may be and often are of primary endogenic origin. Such grains cannot be considered as indicators of greater transportation distance from their bedrock sources. We assume that the following distinctive features may be indicative of primary endogenic origin of smooth well-rounded PGM grains: (1) the preservation of rims on PGM grains of endogenic origin which were not mechanically eroded (Fig. 9f, e); (2) the presence of textures at the grain margin that are conformable to the outline of the grain, e.g., see Figs. 9e and 14a. The rim of the grain of native osmium (Os70–74Ir20– 21Ru5-11) contains a veinlet of irarsite, which is contorted subparallel to the grain edge (surface); (3) concentric zoning in rounded grains; (4) the presence of remnants of primary minerals and rocks on the surface of PGM grains, i.e., chemical and textural concentric zoning; (5) smooth dendritic segregations within PGM grains with relatively deep embayments and negative crystal imprints (Fig. 14c). The preservation of poorly rounded and unrounded PGM grains in the studied placers within the ASFA indicates relatively little transport from their primary sources. The presence of replacement and overgrowth-precipitation rims around the grains of Pt–Fe alloys suggests different mechanisms of their formation. The formation of replacementinduced rims is related to metasomatic alteration at the final stages of PGM crystallization, as indicated by the development of a reaction zone between rims and their host mineral and
data on the same their 187Os/188Os ages (Malitch et al., 2014). The presence of overgrowth-precipitation, represented mainly by sperrylite, provides the additional evidence for hydrothermal mobilization of Pt at the postmagmatic stage. The widespread development of endogenic rims around PGM grains appear to have been related to postmagmatic hydrothermal processes during ore formation, while their good preservation may reflect a short distance from bedrock sources. It should be remembered when interpreting analytical results obtained for placer PGM that the physicochemical processes operating within the weathering profiles on slopes and along river valleys in the exogenic evironments lead to the various transformations of the PGM assemblages. As a result, the assemblages become successively enriched in the more stable components, such as Pt–Fe and Os–Ir–Ru solid solutions, and depleted in unstable components, i.e., PGE sulfides and sulfarsenides, as well as compound containing Pd, Cu, etc. Sperrylite appears to be intermediate in terms of its stability. During formation of alluvial autochthonous placers, PGM undergo flow separation in fluvial environments, which is manifested as changes in the particle sizes, with an increase in the small-size fractions and a decrease in the fine-size fractions and flatness index in the same particle size. It is impossible to discuss in one paper all the large amounts of data that have been accumulated over a long period of investigations. In this study, we would like to emphasize the opportunity for collecting data (on both endogenic and exogenic history of mineral formation) on placer PGM using a combination of modern in situ analytical techniques. Further studies will provide greater insights into the characteristic features of PGM in placer deposits. Acknowledgments. The authors are grateful to L.P. Boboshko and E.V. Lazareva for her assistance in data preparation. This study was supported by the Earth Science Division, Russian Academy of Sciences (project no. 5.1), the Russian Foundation for Basic Research (project nos. 12-05-01164, 13-05-12056, 15-05-06950, and 13-05-12056 ofi-m), and the Ministry of Education and Science.
Fig. 17. A plot of T, °C vs. ∆log fO2(FMQ) for clinopyroxenes from melt and
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Editorial responsibility: N.L. Dobretsov