THE
LATE PERMIAN CAO BẰNG PGE-CU-NI-BEARING COMPLEX OF THE SÔNG HIẾN STRUCTURE,
A. I. GLOTOV1,
G. V. POLYAKOV2, TRẦN TRỌNG H̉A2,
NGÔ THỊ PHƯỢNG2, A. E. IZOKH1, S. V. KOVYAZIN1,
P. A. BALYKIN1,
HOÀNG HỮU THÀNH2, BÙI ẤN NIÊN2, AND PHẠM
THỊ DUNG2
1Institute of Geology, SB of RAS, Koptyug Str., Novosibirsk,
630090, Russia,
2Institute of Geological
Sciences, NCNST of Việt Nam, Hoàng Quốc Việt, Cầu Giấy,
Hà Nội.
Abstract: The peridotites
of the Cao Bằng lherzolite-gabbronorite-diabase complex host PGE-Cu-Ni sulfide
mineralization formed at early stages of the complex history. The specific composition
of the mineralization and the petrology of the host rocks are discussed on the example
of the Suối Củn intrusion. With the composition of primary melt inclusions
taken into account, it is shown that the mineralization is related to the early
sulfide-silicate liquid immiscibility of a picritoid
(25-26% MgO) magma. The sulfide melt was crystallized under high-temperature low-sulfur
conditions, with Pd and Pt accumulating together with Ni in the residual melt of
the sulfide system. There is evidences that the mantle-derived
Cu- and Ni-bearing magmas are contaminated by crustal material.
I. INTRODUCTION
Small lherzolite-gabbronorite-diabase and diabase-granophyre
intrusions are widespread in
II. SPECIFIC FEATURES OF COMPOSITION OF PGE-CU-NI
SULFIDE MINERALIZATION IN PLAGIOPERIDOTITE OF THE CAO BẰNG COMPLEX
1. Composition and location of sulfide mineralization
A more comprehensive study was given to sulfide mineralization
in the Suối Củn massif, where it is irregularly spread in plagiolherzolite,
being most concentrated in the southwestern part of the eastern block of the intrusion
(Fig. 1). The mineralization is represented by interstitial impregnation, which
is present throughout the section of a peridotite body making up
Worthy of note is the higher content of PGE in the
association expressed in all of its facies varieties. The available analytical data
suggest the Pd specialization of noble metallization at a higher level of Pt and
rather ordinary contents of Au and Ag (Table 1). Largest content of Pt and Pd occurs
in plagiolherzolites of the southwestern part of the Suối Củn massif
(sp. H1500-H1505, H1607), in gabbro from a dike (H1506) cutting plagiolherzolite,
and also in quenching picrites of the southeastern contact (sp. M006, M009), with
very lean sulfide mineralization.
2. Features of sulfide composition
Pyrrhotite and pentlandite are predominant in the
sulfide association, chalcopyrite gravitates toward the
periphery of sulfide particles and is less abundant. Sulfide phenocrysts in plagiolherzolite
of the complex typically contain subparallel separates of pentlandite and chalcopyrite
in pyrrhotite, less frequently, pentlandite occurs in the form of thin flame-shaped
exsolution structures. In some cases, sulfides are zoned in lherzolites: the core
is pyrrhotite, surrounded by a double rim, with pentlandite inside and chalcopyrite
outside. The impregnation in picrites displays a different arrangement of main sulfides:
chalcopyrite often lies inside pyrrhotite grains, occasionally rimmed with pentlandite.
Among columnar crystals of pyroxene "spinifex" groundmass of rocks, homogeneous
elongate separates of chalcopyrite and pentlandite occur in places.
Iron sulfides are represented chiefly by hexagonal
pyrrhotite with Me/S = 0.88-0.92, with rare troilite (Me/S
= 1-1.01). Pyrrhotite in lherzolite contains 0.08-0.23% Ni and insignificant impurities
of Co and Cu (0.0n%). Pyrrhotite from picrite is richer
in Ni: 0.42-0.66% (Table 2). Pentlandite in plagiolherzolite of the complex is represented
by an iron variety, and in the quenching picrites and lherzolite of the endocontact
of the Suối Củn massif, its composition corresponds to pentlandite proper
(Ni/Fe = 0.95 -1.11). In all studied specimens pentlandite is poor in Co, though
the pentlandite from picrites contains 0.83-1.32 wt.% Co,
which is twice as much as Co content in the pentlandite from lherzolite. Microprobe
analysis also shows no considerable impurities of Pd and Ag in pentlandite. Compositionally,
chalcopyrite is close to sulfide phenocrysts in the central part of the studied
section, represented by a typomorphous triad of major sulfides in plagiolherzolite.
It contains less copper (35.8-36.7 wt.% Cu), becomes Fe-low
and S-low in an association with cubanite from sulfide occurrences in the southwestern
part of the Suối Củn massif, thus reflecting a shift of equilibrium
in the sulfide system toward the increasing copper potential. The highest content
of Pd, Pt, and Au are recorded there as well (see Table 1). The content of Ni in
chalcopyrite from an association with cubanite is also higher (0.47-0.57 wt.%), because it isomorphically substitutes for some parts of
iron. Cubanite in this association usually occurs on the periphery of sulfide separates,
whose core is made up chiefly of pyrrhotite and less frequently of pentlandite.
Chalcopyrite appears in cubanite-dominated phenocrysts. The composition of cubanite
is stoichiometric, with an insignificant impurity of Ni: 0.02-0.12 wt.% (see Table 2).
3. Regularities in distribution of major ore-forming and noble metals
The main petro-geochemical feature of the rocks in
peridotite part of the sections of massifs is weak variability of composition of
rocks expressed, first of all, in complementary variations of Mg and Al contents,
connected with the distribution of deep-seated subliquidus olivine, which usually
contains Cr-spinel. Quantitative distribution of early olivine in the Suối
Củn massif is governed by its weak concentration in the direction from east
to west in the section of plagiolherzolite as expressed in increasing content of
Mg (Fig. 1). The absence of regular variability in rock composition connected with
fractional crystallization of the intruded magma in situ accords with the irregular
distribution of compatible Cr relative to incompatible LREE (Fig. 1) [2, 3].
Distribution of major ore metals (MOM), PGE, and Au
in lherzolite of the complex is linked chiefly to the variability of composition
of the sulfide component of the ore-magmatic system and its amount in the rocks.
To analyze the evolution of the sulfide-phase composition, the data on chalcophile
elements were converted to 100% sulfide (Table 1) and given in Fig. 2. Figure 2a
shows that the PGE-Cu-Ni mineralization of the complex is characterized by the Fe-Ni
trend and is not accompanied by any regular relation of copper amount to the rest
of MOM and PGE. The noble metal Pd-specialized mineralization is concentrated in
the sulfide system together with Ni at the advanced stages of sulfide crystallization,
which is also accompanied by Pt- and Rh-rich mineralization. Distribution of Au
and Ag is weakly linked to PGE distribution and only the sulfide phase in the picrite
specimen (M009) is abnormally rich in all noble metals. The sulfide mineralization
in the dike of amphibole gabbro (sp. H1506) among plagiolherzolite corresponds in
distribution of MOM, PGE, and Au to the mineralization of the main peridotite phase
of the complex.
Nickel behaviour during the crystallization of silicate
and sulfide-silicate systems can be analyzed with the help of the Ni-Ol-Fa diagram
for the Cao Bằng Complex given in Fig. 3. Here, olivine of all studied massifs
corresponds virtually to a single field, whose orientation follows the trend of
Ni depletion of the silicate system, while olivine crystallizes and during this
process becomes richer in iron. In most cases, the silicate melt seems to be in
equilibrium with a small amount of early sulfide phase not forming considerable
excessive concentration. Composition points somewhat lie apart for quenching picrites
to form a steeper trend, which suggests that Ni in the form of early sulfide phase
was transferred from periphery to core of the body by redistribution of phases at
the expense of differentiation on melt migration. For the complex as a whole this
diagram shows that olivine composition is a poor criterion for Ni potential of intrusive
bodies.
Table 2. Representative
of glass in primary inclusions of Cao Bằng picrites (wt%)
|
1 |
Sample |
SiO2 |
TiO2 |
Al2O3 |
FeO |
MgO |
CaO |
Na2O |
K2O |
Cr2O3 |
NiO |
Total |
|
1 |
M006/7 |
43.33 |
0.99 |
10.30 |
4.34 |
25.99 |
1.10 |
0.38 |
4.90 |
0.01 |
0.08 |
91.42 |
|
2 |
M009/5 |
62.32 |
0.95 |
20.84 |
0.11 |
0.10 |
9.05 |
2.59 |
1.48 |
0.00 |
0.01 |
97.45 |
|
3 |
M009/2 |
66.46 |
0.77 |
19.30 |
0.82 |
0.56 |
4.98 |
2.70 |
2.05 |
0.22 |
0.02 |
97.88 |
Note: 1-2: Inclusion in olivine; 3: Inclusion in chrome spinel,
embedded in olivine.
Analyzed by Camebax electron probe microanalyzer at the
Center for Analysis - United Institute
of Geology-Geophysics-Mineralogy- Siberian Branch – RAS by E.H. Nigmatulina.
III. PHYSICOCHEMICAL CONDITIONS OF SULFIDE MINERALIZATION IN THE ORE-MAGMATIC
SYSTEM OF THE CAO BẰNG COMPLEX
The reported material shows that the PGE-Cu-Ni complex
in question characterizes the stage of development of the sulfide-silicate OMS as
weakly differentiated in composition in the silicate part and having high-temperature
characteristics of the sulfide component, such as generally higher content of Fe
and a stable Fe-Ni trend in distribution of ore composition. High-temperature conditions
of sulfide ore formation typical for the deposits related to komatiite and picritoid
magmas well conform to the experimental data on sulfide systems obtained in the
last decade [5], demonstrating that the coefficient of Ni distribution among the
monosulfide solid solution and residual melt, Dmss/LNi = 0.07-1.90 is
inversely dependent on temperature. At T > 1000oC, Ni is predominantly
concentrated in the residual melt, which should be reflected in the distribution
of ore composition in the form of Fe-Ni trends, which is observed for the mineralization
of the Cao Bằng complex. At the mineralogical level, this conforms to the
presence of troilite in the sulfide association and predominantly Fe-rich pentlandite.
During the complex formation, the ore-magmatic system
in its sulfide part was characterized by a relatively S - low potential. Its estimation
by pentlandite composition, made on the basis of data by Kosyakov et al. [6], shows
that lgf (S2) for the sulfide system of mineralization in the Suối
Củn massif does not exceed -11.1 – -12.4, and only on the margin of the intrusion
and in quenching picrite its value grows to -9.4 – -8.7. In principle, this may
reflect the higher concentration of S along the contacts with host rocks as well
as the pentlandite crystallization under lower temperature conditions on the background
of higher sulphur potential, which conforms to the distribution of temperature field
during the intrusion emplacement and physicochemical regularities of fractional
crystallization of sulfide systems.
Since the sulfide mineralization in the Suối
Củn is of complex nature, with noble metals playing an important role along
with Ni and Cu, we analyze the ore composition using a special technique [7]. Analysis
of partitioning of native metals among high-Fe and high-Cu kinds of ores in a certain
deposit is based on their average content in these varieties normalized by high-Fe
kind of ores, as shown for the sulfide mineralization in the Suối Củn
massif (Fig. 4). Small deviations of the normalized values of all noble metals from
unity are indicative of a weak degree of fractionation of the sulfide melt. In most
occurrences, Pt and Pd are concordantly concentrated in the later fractions of the
sulfide system and corresponding types of mineralization, enriched in Ni in the
Suối Củn massif. The relative Pt enrichment of the high-Fe kind of ores,
displayed in Fig. 4a in the form of the normalized value below unity, may reflect
the appearance of ferroplatinum alloys co-existing with monosulfide solid solution.
For all sulfide occurrences in peridotite, the ore formation proceeded under low-S
conditions, which is inferred from the Rh-normalized values higher than unity, in
accordance with experimental data in PGE-bearing sulfide systems varying in sulphur
saturation [8]. According to this parameter, the sulfide melt seems to become richer
in sulphur, which was expressed in the composition of sulfide phase from gabbro
dike (Fig. 4d).
Table 3.
Representative of Cu-Ni sulphides and PGE of Suối Củn
massif, wt%.
|
No |
Sample |
Fe |
Ni |
Co |
Cu |
Ag |
Pd |
S |
TOTAL |
|
Troilite, pyrrhotite |
|||||||||
|
1 |
1604-1 |
63.11 |
0.06 |
0.03 |
0.02 |
|
|
36.30 |
99.52 |
|
2 |
1605-1 |
62.80 |
0.08 |
0.04 |
0.02 |
|
|
35.81 |
98.75 |
|
3 |
1500-1 |
63.01 |
0.04 |
0.04 |
|
|
0.02 |
35.81 |
98.91 |
|
4 |
1603-5 |
60.70 |
0.29 |
0.04 |
0.05 |
|
0.01 |
38.57 |
99.65 |
|
5 |
006-2 |
60.32 |
0.66 |
0.03 |
0.05 |
|
|
39.02 |
100.08 |
|
6 |
006-8 |
59.94 |
0.57 |
0.04 |
|
0.01 |
0.01 |
38.99 |
99.56 |
|
7 |
009-1 |
60.44 |
0.42 |
0.04 |
0.09 |
0.03 |
|
38.80 |
99.81 |
|
Pentlandite |
|||||||||
|
8 |
1603-3 |
32.05 |
33.70 |
0.96 |
0.06 |
0.02 |
0.01 |
32.75 |
99.55 |
|
9 |
1603-6 |
32.83 |
32.75 |
0.69 |
0.16 |
|
0.02 |
32.25 |
98.70 |
|
10 |
1604-2 |
37.86 |
28.20 |
0.58 |
0.08 |
|
|
32.85 |
99.57 |
|
11 |
1604-4 |
37.84 |
28.07 |
0.52 |
0.15 |
|
0.01 |
32.86 |
99.44 |
|
12 |
1605-2 |
38.82 |
27.22 |
0.62 |
0.06 |
|
0.01 |
32.26 |
98.98 |
|
13 |
1500-2 |
39.21 |
26.46 |
0.61 |
0.14 |
|
0.01 |
32.10 |
98.53 |
|
14 |
1500/2-3 |
39.74 |
25.81 |
0.60 |
0.50 |
|
0.02 |
32.69 |
99.35 |
|
15 |
1505-1 |
37.03 |
28.44 |
0.58 |
0.06 |
|
|
32.31 |
98.42 |
|
16 |
006-4 |
30.04 |
34.68 |
1.32 |
0.06 |
|
|
32.56 |
98.66 |
|
17 |
006-9 |
30.29 |
35.07 |
1.22 |
0.03 |
|
0.02 |
31.40 |
98.03 |
|
18 |
009-3 |
31.47 |
34.03 |
0.83 |
0.08 |
|
0.01 |
32.19 |
98.61 |
|
Chalcopyrite |
|||||||||
|
19 |
1603-4 |
30.78 |
0.01 |
0.03 |
34.43 |
|
|
34.44 |
99.69 |
|
20 |
1603-7 |
30.63 |
0.11 |
0.02 |
33.93 |
0.04 |
0.01 |
33.88 |
98.61 |
|
21 |
1604-3 |
30.67 |
0.05 |
0.01 |
33.87 |
|
0.01 |
34.62 |
99.23 |
|
22 |
1605-3 |
30.48 |
0.08 |
0.02 |
34.12 |
|
|
34.24 |
98.94 |
|
23 |
1500-3 |
30.64 |
0.15 |
0.02 |
33.47 |
|
|
33.61 |
97.89 |
|
24 |
1500/2-2 |
30.26 |
0.49 |
0.01 |
35.84 |
|
|
32.66 |
99.26 |
|
25 |
1500/2-4 |
29.95 |
0.57 |
0.01 |
36.56 |
|
|
33.29 |
100.38 |
|
26 |
1500/2-6 |
29.73 |
0.50 |
0.02 |
36.49 |
|
|
32.74 |
99.48 |
|
27 |
006-1 |
31.38 |
0.13 |
0.02 |
33.47 |
|
|
34.07 |
99.08 |
|
28 |
009-2 |
30.48 |
0.14 |
0.02 |
34.02 |
0.01 |
|
34.07 |
98.74 |
|
Cubanite |
|||||||||
|
29 |
1500/2-1 |
41.27 |
0.01 |
0.03 |
23.48 |
|
0.03 |
35.36 |
100.18 |
|
30 |
1500/2-5 |
40.71 |
0.02 |
0.02 |
23.73 |
|
|
35.54 |
100.02 |
|
31 |
1500/2-7 |
40.78 |
0.02 |
0.03 |
23.65 |
|
0.01 |
35.23 |
99.71 |
|
32 |
1505-2 |
41.16 |
0.07 |
0.03 |
23.00 |
|
|
35.12 |
99.37 |
|
33 |
1505-3 |
41.17 |
0.12 |
0.04 |
23.11 |
|
0.02 |
34.54 |
99.00 |
Note: Samples belong
to the Project: "Intraplate magmatism and related mineralization of Việt
The composition of the parental magma for Pt-bearing
ultramafic-mafic intrusions and the melts that appear in the process of its intrusion,
produced by its interaction with the crustal matter, may be inferred from the composition
of primary melt inclusions in olivine and Cr-spinel from quenching picrites (Table
3). These inclusions found and tentatively examined by A.I. Glotov and S.V. Kovyazin
are glass, often with blebs of gas-liquid phase, idiomorphic crystals of Cr-spinel
(in inclusions in olivine), and, in one case, pyroxene microspinifex. Some inclusions,
with Mg ones among them, contain microglobules of sulfide, whose size does not permit
a correct examination of their composition by an ordinary microprobe. The first
glass composition of those reported in Table 3 corresponds to Al-high, K-high picrite
and best of all matches the composition of the parental magma intruded to the level
of complex emplacement. However, in P.A. Balykin's opinion, this composition does
not correspond to the composition of the parental magma because of abnormally low
content of Ca, Na, and abnormally high content of K. Balykin also believes that
the content of Mg in the glass of primary inclusion (25.99%) is too high for the
parental melt. Two other compositions typical of the majority of the explored inclusions
are close to Al-high dacite and, most likely, characterize the hybrid melts, whose
origin is related to the assimilation of the crustal material by the primary picritoid
magma. This is in accordance high value of 87Sr/86Sr =
0.709 in the rocks of the massif. Worthy of note is the closeness of glass composition
in these inclusions entrapped by different subliquidus minerals, which suggest early,
probably, precrystallization contamination of the primordial mantle
magma with crustal material
and the appearance of a hybrid silicate melt, immiscible with the primordial magma,
which is present in the form of thin emulsion. This melt was fluid-saturated, as
inferred from the presence of gas-liquid inclusions in glasses and a deficit in
sums in microprobe analyses of theses glasses, where volatiles were not identified.
CONCLUSIONS
The formation of the volcanoplutonic association of
the Cao Bằng district and generation of PGE-Cu-Ni-enriched peridotite of the
Cao Bằng Complex is linked, first of all, with the supply, to the emplacement
level, of a Mg-high mantle magma bearing subliquidus varieties
of olivine and Cr-spinel, and, probably, a segregated sulfide phase. The PGE-Cu-Ni
specialization of the peridotite of the Cao Bằng Complex is governed by the
early sulfide-silicate liquation of a picritoid melt with the formation of sulfide
mineralization under high-temperature and S-low conditions of sulfide crystallization.
In the process, the sulfide liquid was considerably enriched in Pt metals, having
the highest coefficient of distribution in favour of sulfides as compared with other
ore elements. The further fractional crystallization of the sulfide melt proceeded
with enrichment in fusible PGE, together with Ni, residual sulfide melt, with a
weak tendency toward accumulation of Au and Ag, predominantly with Cu.
This work is supported by Project "Intraplate
magmatism and related mineralization of Việt Nam" from Ministry for Sciences
and Technology of Việt Nam and grant 02-05-65087, 03-05-65088 from Russian
Foundation for Basic Research and "Leading Scientific Schools" (grant
NSH-1573.2003.5).
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