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

Abstract

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).

Full text

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
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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.

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DOKLADY EARTH SCIENCES
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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.