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The composition of inclusions in osmium minerals as an indicator of the formation conditions of the Guli ultramafic Massif

Authors:
  • Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia

Abstract and Figures

The morphology and chemical composition of solid inclusions in the heavy concentrate-derived osmium-rich alloy grains associated with the Guli massif (Maimecha-Kotui Province, Russia) were studied using a scanning electron microscope and a CAMECA SX microprobe. In addition to various PGM inclusions reported previously (Malitch et al., 1995), a spectrum of the oxide and silicate inclusions entrapped by osmium minerals was discovered for the first time. Distinct types of inclusions comprise: (1) monophase silicate (forsterite and serpentine) and oxide (chromite and Cr-magnetite) inclusions; (2) polyphase silicate and oxide inclusions; (3) inclusions of base metal sulfides (pentlandite and djerfisherite); (4) polyphase inclusions of sulfides and PGM (isoferroplatinum, moncheite, and telluropalladinite); (5) monophase PGM inclusions (laurite, erlichmanite and rutheniridosmine). The silicate and oxide inclusion series corresponds to two mantle assemblages. One assemblage, which controls the PGE mineralization, includes the ultramafic rock minerals (chromite and forsterite). Another assemblage is related to the alkaline metasomatism and comprises the mafic and alkaline rock minerals (hortonolite, diopside, magnetite, pargasite, titanite, and other K–Na-rich silicate phases).
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888
Doklady Earth Sciences, Vol. 361A, No. 6, 1998, pp. 888–890.
Translated from Doklady Akademii Nauk, Vol. 361, No. 6, 1998, pp. 812–814.
Original Russian Text Copyright © 1998 by Malich, Auge.
English Translation Copyright © 1998 by
åÄàä ç‡Û͇
/Interperiodica Publishing (Russia).
The geological survey (scale 1:20000), prospecting
and thematic works carried out in 1985–1995 allowed
us to obtain new original data on the geology, mineral-
ogy, and geochemistry of the ultramafics of the world’s
largest clinopyroxenite–dunite massif and related PGM
placers [1, 2, etc.]. Analysis of the obtained data allows
us to reestimate the metallogenic potential of the Mai-
mecha–Kotui province [3] located at the northern Sibe-
rian Platform. Osmium is a major PGM-forming ele-
ment in the placers of the Ingarinda, Gule, and Sabyda
rivers within the Gule massif. In terms of the significant
prognostic resources of a PGE (about 15 t), this massif
is comparable with the unique gold and uranium placer
deposits of Witwatersrand (Republic of South Africa),
where, from 1921 to the present day, about 10 t Os and
Ir was mined.
Low
187
Os/
186
Os
values [1, 2] indicate the mantle ori-
gin of the PGM in the Gule massif and suggest that the
primary mantle melt was generated at a depth of 400–
500 km. New data on inclusions in osmium minerals,
which are the subject of this paper, were used both for
an applied purpose and genetic interpretation. In partic-
ular, the compositional features of the inclusions,
together with their abundance, may be indicative of the
parental source of the PGM.
The morphology and chemical composition of the
inclusions in the heavy concentrate-derived PGM were
studied using a scanning electron microscope and a
CAMECA SX microprobe (about 200 quantitative
multicomponent determinations). In addition to various
PGM inclusions [1], a spectrum of the oxide and sili-
cate inclusions entrapped by osmium minerals was dis-
covered for the first time. We distinguished five types of
inclusions: (1) monophase silicate (forsterite and ser-
pentine) and oxide (chromite and Cr-magnetite) inclu-
sions; (2) polyphase silicate and oxide inclusions;
(3) inclusions of base metal sulfides (pentlandite and
djerfisherite; (4) polyphase inclusions of sulfides and
PGM (isoferroplatinum, moncheite, and telluropalla-
dinite); (5) monophase PGM inclusions (laurite,
erlichmanite and rutheniridosmine).
The silicate and oxide inclusions, 5–30
µ
m across,
are represented by both individual minerals and struc-
turally complex mineral assemblages (Fig. 1). Many
inclusions occur as “negative” crystals. Chromite, for-
sterite, and serpentine are the most widespread. Cli-
nopyroxene, hortonolite, chrome-magnetite, fer-
rispinellide, amphibole, phlogopite, biotite, ilmenite,
and titanite are less common. In addition, a consider-
able number of mineral phases are likely composed of
silicate glass, because their chemical compositions do
not correspond to known mineral analogs (Table 1).
The Composition of Inclusions in Osmium Minerals
as an Indicator of the Formation Conditions
of the Gule Ultrabasic Massif
K. N. Malich and T. Auge
Presented by Academician D.V. Rundquist May 30, 1996
Received June 4, 1996
All-Russia Institute of Geology and Mineral Resources
of World Ocean, Angliiskii pr., 1, St. Petersburg,
190121 Russia
Bureau of the Geological and Mining Investigations,
45060 Orlean cedex, B.P. 6009, France
GEOCHEMISTRY
10
µ
m
11
12
10
Fig. 1.
Features of the internal structure of the polyphase
oxide–silicate inclusion in native osmium (white) in A4
flake. Numbers denote the areas analyzed with the micro-
probe and correspond to those in Table 1. The white spot
within the three-phase inclusion of the silicate phase is
ilmenite. Back-scattered electron image.
DOKLADY EARTH SCIENCES
Vol. 361A
No. 6
1998
THE COMPOSITION OF INCLUSIONS IN OSMIUM MINERALS 889
Depleted sums of the analyzed phases suggest the pres-
ence of H
2
O. Two compositional groups of silicate
glasses were distinguished (Table 1): high-Na (Na
2
O up
to 6.1 wt %) and high-K (K
2
O up to 6.7 wt %). Detailed
study of the Na- and K-rich silicate phases supports the
existence of discrete compositional fields without tran-
sitional values (Fig. 2).
Chromite (or Cr-spinellide) occurs as monophase
inclusions both within the osmium mineral matrix and
between the euhedral crystal of the latter. Compositions
of the studied Cr-spinellide inclusions (Table 2) are
similar to those of dunites rather than chromitites [4,
etc.]. This is indicative of the high prospect of the dis-
covery of PGE mineralization in these rocks.
Olivine occurs as individual euhedral inclusions as
well as in polyphase inclusions. Olivine from the indi-
vidual inclusions contain considerably more of the for-
sterite component (
Fo
83.7–91.9
) than olivine from
polyphase inclusions (Fo
65.0–68.4
) (Table 1, Fig. 1). All
chemical compositions (Table 1) correspond to diop-
side which is relatively rich in Na
2
O (0.85–1.22 wt %)
and TiO
2
(0.70–1.09 wt %).
The composition of one inclusion differs from the
aforesaid Cr-spinellide (Table 2) and is highly similar
to ishkulite [5] assigned to the isomorphous series of
solid solutions
FeCr
2
O
4
–FeFe
2
O
4
. The polyphase
inclusions contain the following assemblages: magne-
tite with sulfides and isoferroplatinum (Table 2),
ilmenite and pargasite with diopside and highly alka-
line silicate phases, and the phlogopite–biotite series
Fe–Mg-micas with diopside and high-Na silicate glass
(Fig. 1). Silicate inclusions, which are not completely
embedded in the osmium mineral matrix, are usually
replaced by various hydroxides dominated by serpen-
tine minerals.
The first discovered wide spectrum of inclusions
entrapped by the osmium minerals is typical of heavy
concentrate-derived PGM in the ultramafic zonal com-
Al
2
O
3
Na
2
OK
2
O
Fig. 2.
Composition of the silicate glass inclusions in the
diagram
Al
2
O
3
–Na
2
O–K
2
O
(wt % adjusted to 100%).
Table 1.
Chemical composition of silicate phase inclusions in osmium minerals, wt %
Ordi-
nal no. Sample
no. SiO
2
TiO
2
Al
2
O
3
Cr
2
O
3
FeO MnO MgO CaO Na
2
OK
2
O NiO Total
1 A3 38.96 0.00 0.00 0.04 15.04 0.40 43.34 0.18 0.00 0.00 0.44 98.40
2 A4 40.70 0.03 0.05 0.00 7.79 0.29 49.01 0.21 0.00 0.00 0.20 98.28
3 A6 40.45 0.00 0.01 0.00 8.96 0.22 47.70 0.43 0.00 0.00 0.39 98.16
4 A3/1 45.79 2.53 6.53 0.21 5.06 0.21 17.96 8.73 4.17 1.03 0.09 92.31
5 A3/1 40.40 1.27 12.04 0.08 6.05 0.05 24.22 0.09 1.46 7.94 0.18 93.78
6 A4/1 36.57 0.05 0.31 0.02 29.62 0.72 30.81 0.08 0.11 0.00 0.05 98.34
7 A4/1 52.19 0.94 0.86 0.09 4.96 0.08 14.28 22.90 1.09 0.00 0.00 97.39
8 A4/1 39.88 1.70 12.14 0.08 7.88 0.03 21.73 0.13 2.33 6.49 0.14 92.53
9 A4/1 40.86 0.90 7.90 2.99 6.94 0.03 19.14 6.72 2.20 4.33 0.00 92.01
10 A4/2 48.22 1.80 4.56 0.03 10.46 0.15 15.62 7.48 6.12 0.19 0.09 94.72
11 A4/2 36.14 3.10 12.03 0.20 13.55 0.14 15.82 0.08 1.03 8.24 0.16 90.49
12 A4/2 52.34 0.70 0.86 0.02 5.64 0.25 13.92 23.19 1.22 0.00 0.00 98.14
13 B15 36.50 0.11 0.00 0.00 27.62 0.52 33.52 0.10 0.00 0.00 0.15 98.52
14 B15 52.42 1.09 0.91 0.07 4.90 0.18 14.46 22.90 0.85 0.00 0.00 97.78
15 B15 43.99 3.99 8.07 0.24 7.42 0.02 15.76 10.71 3.78 0.37 0.09 94.44
16 B15 31.87 3.32 10.25 1.08 6.45 0.07 12.07 0.29 0.99 6.70 0.10 73.19
17 B33 49.27 0.23 10.16 0.09 6.84 0.11 8.84 14.68 6.02 1.12 0.25 97.61
18 B33 52.61 0.27 1.79 0.04 5.40 0.17 22.04 9.79 2.13 0.00 0.71 94.95
Note: (1–3) Forsterite; (4, 8–10, 15–18) silicate glasses; (5, 11) Mg–Fe-mica of the phlogopite–biotite series; (6, 13) hortonolite; (7, 12,
14) diopside.
890
DOKLADY EARTH SCIENCES
Vol. 361A
No. 6
1998
MALICH, AUGE
plex of the Uralian–Alaskian and Konder types, which
are distinguished within dunite–clinopyroxenite–gabbro
and clinopyroxenite–dunite formations, respectively.
However, similar inclusions previously described [6–9,
etc.] associate with Fe–Pt solid solutions. The Pt–Pd-
bearing PGM inclusions [1] discovered in the osmium
minerals emphasize the presence of these PGE in the
mineral-forming environment.
The silicate and oxide inclusion series corresponds
to two mantle assemblages. One assemblage, which
controls the PGE mineralization, includes the ultrama-
fic rock minerals (chromite and forsterite). Another
assemblage is less deep-seated and comprises the mafic
and alkaline rock minerals (hortonolite, diopside, mag-
netite, pargasite, titanite, and other K–Na-rich silicate
phases). The formation of the minerals of the latter
assemblage is related to the alkaline metasomatism of
the ultramafic rock minerals under the action of the
emanation of the alkaline melts that produced the ijo-
lite–carbonatite rock association. Fluids also facilitated
mobilization and accumulation of PGE in the apical
parts of the ultramafic substrate [4, 10, etc.].
The results obtained supplement the geological,
mineralogical, geochemical, and isotopic data [1, 2,
11–14], which suggest the existence of two autono-
mous magmatic complexes within the heterogeneous
Gule massif: the Gule clinopyroxenite–dunite complex
and the Maimecha–Kotui ijolite–carbonatite complex,
which are characterized by different rock assemblages
and metallogenic specifics.
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Table 2.
Chemical composition of inclusions of Fe- and Cr-spinellides in osmium minerals, wt %
Component 12345678910
TiO
2
1.87 1.92 1.88 1.93 3.57 4.26 1.47 1.35 0.00 0.11
Al
2
O
3
12.16 11.95 11.91 12.36 6.34 6.25 0.14 0.15 0.03 0.00
Cr
2
O
3
41.45 40.33 39.84 41.56 45.49 46.17 11.84 11.48 0.00 0.00
FeO 21.82 21.71 21.62 22.40 21.52 22.29 27.65 27.46 28.28 27.17
Fe
2
O
3
12.73 12.33 13.93 12.06 13.29 12.49 52.58 52.36 67.12 66.77
MnO 0.56 0.48 0.47 0.47 0.27 0.29 0.16 0.39 0.96 1.58
MgO 8.35 8.02 8.29 8.07 9.30 9.49 2.35 2.06 0.54 0.83
NiO 0.30 0.22 0.16 0.16 0.27 0.44 0.11 0.01 0.00 0.00
Total 99.24 99.26 98.10 99.01 100.05 101.68 96.30 95.26 96.63 96.46
Note: Chromite: (1, 2) sample A6, (3, 4) sample A10, (5, 6) sample A25; (7, 8) Cr-magnetite, sample B11, (9, 10) magnetite, sample A5.
... However, X-ray powder-diffraction data do not always prove the mineral identity unequivocally because ordering reactions may occur during mechanical treatment of alloys (i.e., preparation of sample for X-ray analysis). Furthermore, Pt-Fe alloy is commonly designated isoferroplatinum both in the western and particularly in the eastern literature, although X-ray data are lacking (e.g., Toma & Murphy 1977, Johan et al. 1991, Mochalov et al. 1991, Slansky et al. 1991, Augé & Legendre 1992, Rudashevsky et al. 1992a, b, Nekrasov et al. 1994, Palandzhian et al. 1994, Tolstykh & Krivenko 1997, Mochalov & Khoroshilova 1998, Augé et al. 1998, Ohnenstetter et al. 1999. The latter comment applies, in our opinion, to Pt-Fe alloy compositions close to Pt:Fe = 1, which are commonly referred to as tetraferroplatinum. ...
... terize chromitites and PGM mineral concentrates from zoned ultramafic massifs, clearly distinct from those of podiform and stratiform chromitites in ophiolites and layered complexes (Page et al. 1983, Barnes et al. 1985, Naldrett & Von Gruenewaldt 1989, Leblanc 1991, Yang et al. 1995, Augé et al. 1998, Melcher et al. 1999, Malitch et al. 2001). This finding is consistent with the observations of Fleet & Stone (1991), that PGE fractionate between alloy and sulfide liquid according to their atomic weight rather than melting point. ...
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Combined structural, compositional and osmium-isotope data on selected Pt–Fe nuggets from economically important placer deposits closely linked to clinopyroxenite–dunite massifs of the Siberian Platform (Kondyor, Inagli, Guli) and the Middle Urals (Nizhny Tagil), Russia, are presented for the first time. Pt–Fe alloys investigated are ferroan platinum (space group Fm3m) with a composition close to Pt 3 Fe. This emphasizes the necessity of an X-ray study in identifying the particular Pt–Fe alloy species. Less common are compositions such as Pt2Fe and an intimate intergrowth of Pt3Fe2 and PtFe. Other platinum-group minerals (PGM) observed in ferroan platinum include a diversity of Os–Ir–Ru alloys, PGE sulfides [laurite, malanite, cuproiridsite, cooperite, and an unnamed base metal – (Ir,Pt) monosulfide], PGE sulfarsenides (hollingworthite, irarsite), Pt–Pd tellurides (moncheite, telluropalladinite) and stibiopalladinite. This suite of PGM is consistent with those from other zoned or Uralian- Alaskan-type massifs. However, unusually Ru-rich alloys included in ferroan platinum of Guli are characteristic of PGM derived from an ophiolite source and underline the transitional signature of the Guli massif between zoned-and ophiolite-type complexes. Pd-rich ferroan platinum nuggets indicate a derivation from clinopyroxenite source-rock, whereas Ir-rich Pt–Fe alloys suggest a chromitite source. The presence of numerous Os–Ir–Ru exsolution lamellae in ferroan platinum are indicative of a high-temperature origin of the PGM. The first Os-isotope data from Os-rich minerals from chromitites and placers closely associated to the Kondyor and Inagli massifs reveal low 187Os/188Os values with a very narrow range, indicative of a common mantle source of the PGE, implying that the PGM are of primary origin. Disintegration of parent ultramafic source-rocks and short-range mechanical transport of liberated PGM formed the placers. Os-isotope model ages in the range of 340 to 355 Ma constrain the formation age of the Kondyor and Inagli massifs of the Aldan Province at the southeastern part of the Siberian Craton, and closely match those from the Guli massif (370 Ma) of the Maimecha–Kotui Province at the northern part of the Siberian Craton.
... 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) 4 Sb 3 (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 Pt 3 Fe and (Ru, Os, Ir) that could have formed by incomplete exsolution from a complex primary solid solution. ...
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¶Geological, mineralogical and Os isotopic data for detrital PGE-mineralization derived from the Guli and Bor-Uryah ultramafic massifs, within the Maimecha-Kotui Province (the northern part of the Siberian Platform, Russia), are presented for the first time. The detrital platinum-group minerals (PGM) are dominated by Os–Ir–(Ru) species, which is typical for ophiolites or Alpine-type complexes. However, the PGM assemblage in the placers investigated is similar to that derived from zoned platiniferous clinopyroxenite–dunite massifs (also known as Uralian-, Alaskan-type and Aldan-type massifs). The unique features of the Au-PGE placers at Guli are (1) the dominance of Os-rich alloys over other PGM and Au, and (2) the considerable predicted resources of noble metals, particularly osmium. Dominant chromite, olivine and clinopyroxene inclusions recorded in Os–Ir–(Ru) alloys imply that they were derived from ultramafic sources (e.g., chromitite, dunite and clinopyroxenite). The first in situ osmium-isotope measurements by laser ablation - multiple collector - inductively coupled plasma mass spectrometry of different, intimately intergrown, PGM (e.g., laurite and Os-rich alloys) in various nuggets from Guli have revealed low 187Os/188Os and γOs values. They yield a very narrow range of 187Os/188Os (0.12432 to 0.12472) and γOs (− 2.39 to − 2.07). These values are indicative of a common chondritic or subchondritic mantle source of PGE. 187Os/188Os and γOs values of Os-rich alloys, derived from the Bor-Uryah massif, are different (i.e., γOs ranges from − 2.67 to − 1.30). The mineral-isotopic data obtained are consistent with the conclusion that the PGM were derived from parent ultramafic source rocks. Os-isotope model ages in the range of 495 to 240 Ma constrain the age of ultramafic protoliths in the northern part of the Siberian Craton. The variation in 187Os/188Os values for detrital PGM, where the provenance source is unknown, is considered to be a useful technique for distinguishing parent bedrock sources.
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The Maimecha-Kotui economical region, traditionally known as a source of apatite-magnetite and phlogopite ores related to ijolite-carbonatite massifs, is a new Russian platinum Province.
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Mineralogical and geochemical criteria for assessing platinum potential of zoned-type clinopyroxenite-dunite massifs within the Siberian Platform (Kondyor, Chad, Inagli, and Guli) and adjacent territories (Feklistov, etc.), are briefly outlined.
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Textures and compositions of platinum-group minerals (PGM) and coexisting silicate, oxide and base-metal minerals have been examined in platinum-group-element-enriched chromities of the Alaskan-type Tulameen complex and spatially associated placers. Major objectives of this study were to compare the PGM in bedrock and placer occurrences and to evaluate their origin: The common Pt-Fe-Ni-Cu alloys in chromitities are tentatively identified as 'tetraferroplatinum' [Pt(Fe,Ni,Cu)] and 'isoferroplatinum' [Pt2.5(Fe,Ni,Cu)1.5], whereas placers commonly contain isoferroplatinum [Pt3Fe to Pt2.6(Fe,Cu,Ni)1.4], native and ferroan platinum. Most alloys (with the notable exception of tulameenite and platinian copper), laurite and erlichmanite are considered to represent a primary high-temperature paragenesis. These data indicate quite conclusively that the PGE mineralization in the placers was derived from chromitites in the dunite core of the Tulameen complex. Other silicate inclusions in the nuggets comprise primary clinopyroxene (Mg-to Fe-rich diopside), magnesian phlogopite to Fe-rich biotite, ferroan pargasitic hornblende and plagioclase, and secondary sericitic mica, chromian chlorite and Cr-bearing epidote. The primary mineral assemblage evidently crystallized from inclusions of silicate melt trapped within platinum alloy at the time of chromitite formation; the secondary minerals formed during subsequent greenschist-facies metamorphism. The origin of PGE-enriched chromitites in the Tulameen and other Alaskan-type intrusions is related to segregation of predominantly Pt-Fe alloys directly from the melt during conditions that enhanced the precipitation of chromite [e.g., increase in f(O2)]. There is no evidence for high-temperature subsolidus concentration of PGE by either exsolution from chromite or desulfurization of primary magmatic sulfides.
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Isoferroplatinum nuggets from Milverton (N.S.W., Australia) contain inclusions of: Os-Ir-Pt alloys, "cuprorhodsite', bowieite-kashinite, Pd3(Te, As) and Rh5Pd5As4. Additionally, silicate inclusions filling negative crystals in isoferroplatinum are composed of diopside + muscovite + glass + apatite + sphene + CaSO4. Iridosmine and osmiridium exsolutions indicate a high temperature of formation of the nuggets. The study shows Pt3Fe co-existing with an aluminosilicate melt rich in H2O, K, Mg, Ca and containing P and S. -Authors
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