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Citation: Sun, X.; Ren, Y.; Li, J.;
Huang, M.; Sun, Z.; Li, Z. Magma
Evolution and Constraints on the
Graphite Mineralization Hosted by
the Huangyangshan Alkaline Granite
Suite in the East Junggar of Xinjiang
Province: Evidence from In Situ
Analyses of Silicate Minerals.
Minerals 2022,12, 1458. https://
doi.org/10.3390/min12111458
Academic Editor: John A. Jaszczak
Received: 23 October 2022
Accepted: 17 November 2022
Published: 18 November 2022
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minerals
Article
Magma Evolution and Constraints on the Graphite
Mineralization Hosted by the Huangyangshan Alkaline
Granite Suite in the East Junggar of Xinjiang Province:
Evidence from In Situ Analyses of Silicate Minerals
Xinhao Sun 1, Yunsheng Ren 2,3,*, Jingmou Li 3, Mengjia Huang 4, Zhenjun Sun 2and Zuowu Li 5
1Chengdu Center of China Geological Survey, Chengdu 610081, China
2Hebei Key Laboratory of Earthquake Dynamics, Institute of Disaster Prevention, Sanhe 065201, China
3School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
4Sichuan Institute of Land Science and Technology, Chengdu 610045, China
5Xinjiang Branch of China National Geological Exploration Center of Building Materials Industry,
Urumqi 830092, China
*Correspondence: rys@cidp.edu.cn
Abstract:
The Huangyangshan super-large graphite deposit, located in the East Junggar area of the
Xinjiang Province, is hosted in and has closely temporal, spatial, and genetic relationships with the
Huangyangshan alkaline granites. There are such silicate minerals as amphibole, biotite, pyroxene,
and plagioclase occurring in the graphite-bearing granites. The integration of the electron microprobe
analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)
enabled us to reveal the physicochemical conditions and evolution process, as well as the relationship
of alkaline magmatism with graphite mineralization. The results show that the amphiboles generally
have low Al and high Ti, K, Si, and Fe contents, as well as similar rare-earth elements (REEs) patterns
and trace element distribution patterns to granites with significantly negative Eu anomalies. In the
analyzed samples, primary biotite belongs to Fe-biotite and has characteristics of high Si and Fe and
low Al and Mg contents. In the graphite orbicules, the pyroxene phenocrysts develop multiple zonal
structures and are characterized by high Si and low Ca and Fe contents. The dominant plagioclase
phenocrysts in the graphite orbicules are oligoclase and andesine, with normal and occasionally
oscillatory zoning. The calculated crystallization temperature of the pyroxene, amphibole, and
primary biotite in graphite orbicules are 840–1012
◦
C, 681–761
◦
C, and 658–720
◦
C, respectively,
corresponding with their crystallization order. The pressure and depth calculation results of the
amphibole, representing those of the magmatism, are 157–220 Mpa and 5.95–8.32 km, respectively.
Both amphibole and biotite crystallized in a reducing environment with extremely low oxygen
fugacity. The elemental compositions of these silicates indicate that the Huangyangshan pluton
experienced significant mixing of mafic mantle-derived magma and felsic crust-derived magma. The
cores of graphite orbicules were formed in a relatively earlier magmatic stage, while the granites
and their dioritic enclaves were formed in a later magmatic stage. During magmatism, the mixing of
mantle-derived basic magma had an important influence on the evolution and differentiation of the
melts. According to the coexisting sulfides with graphite and compositional difference of amphibole
and biotite in the granites and graphite ores, the graphite mineralization might be triggered by a
magma mixing process.
Keywords:
mineral elemental composition; EPMA; crystallization conditions; magma mixing;
Huangyangshan graphite deposit; NW China
1. Introduction
Generally, there are three main mineralization processes that graphite crystallizes,
namely, organic matter transforming by high-grade metamorphism in regional metamor-
Minerals 2022,12, 1458. https://doi.org/10.3390/min12111458 https://www.mdpi.com/journal/minerals
Minerals 2022,12, 1458 2 of 23
phic graphite deposit [
1
–
3
], conversion of organic matter in contact with pyrometamor-
phism between igneous intrusions (mostly felsic) and coal-bearing sedimentary rocks [
4
–
6
],
and carbon transforming by certain reactions in carbonaceous fluids or melts in fluid-
deposited graphite [
7
–
10
]. The fluid-deposited graphite is the most complicated type due
to the variability of ore-hosting rocks, melts/fluids properties, and ore-forming mecha-
nisms [8,11–13].
In China, the first fluid-deposited graphite deposit, the Sujiquan occurring in the Kala-
maili area in Xinjiang Province, was distinguished in the 1960s. In the last ten years, more
crystalline graphite deposits have been discovered in this region such as the Huangyang-
shan super-large deposit. Then, the graphite deposits in the Eastern Junggar have attracted
more attention. Some scholars have reported the ages and properties of the Huangyangshan
pluton and concluded that belongs to the A-type granites formed in the Late Carbonifer-
ous [
14
–
16
]. Besides, the ore genesis of the Huangyangshan deposit has been involved
in some references on the basis of fluid inclusion analysis laser Raman and graphite C
isotopic analyses, as well as S and Pb isotopic analyses of associating sulfides in the graphite
ores [
16
,
17
]. The C isotopic composition of the Huangyangshan graphite showed organic
characteristics [
16
], whereas the S and Pb isotopic composition of associating sulfides
showed a magmatic origin [
17
]. As a result, there are two major genetic viewpoints on the
ore-forming mechanism of the Huangyangshan and the Sujiquan graphite deposits. One
suggests that graphite is directly crystallized from magma [
17
–
19
]. The other holds that
the graphite deposited from the hydrothermal process in the late stage of magma evolu-
tion [
10
,
16
]. The critical factors for the two controversial points of view are the accurate
understanding on the magma evolution process and the relationship between graphite
mineralization and the magmatic evolution of the ore-hosting granites.
Some magmatic minerals such as amphibole, biotite, pyroxene, and feldspar are gen-
erally used to reflect the magma composition and physicochemical condition when they
precipitated according to their geochemical behavior [
20
–
26
]. As the main minerals in
granitic rocks, they all have broad stability field to precipitate during the whole magmatic
process and are capable of recording the temperature, pressure, and oxygen fugacity [
27
,
28
].
Our present study focuses mainly on the amphibole, biotite, pyroxene and plagioclase
textures and chemical compositions in the graphite-hosting granites, the pluton microgran-
ular dioritic enclaves, and orbicular graphite ores from the Huangyangshan deposit. We
present in situ major and trace element analyses in order to reveal the physicochemical
conditions of different magmatic stages and their implication on the mineralization of
the investigated graphite. These studies are presumably helpful for better understanding
the magma evolution process and unravel the genetic relationship between graphite and
magmatism, which would have an impact on the mineral exploration for graphite deposits
in China and elsewhere in the world.
2. Geological Setting and Graphite Mineralization
2.1. Regional Geology
The northern Xinjiang area, tectonically located in the southern part of the Central
Asian Orogenic Belt (CAOB; Figure 1a), is split into the Altay, Junggar, and Tianshan
blocks from north to south (Figure 1b). The East Junggar orogenic belt (EJOB) is situated in
the northeast of the Junggar terrane and comprises the Durat–Baytag arc, the Aermantai
ophiolite belt, the Yemaquan arc, the Kalamaili ophiolite belt, and the Harlik–Dananhu
arc from north to south (Figure 1b); [
29
–
32
]. Based on the available geochronological
data for this area, the Junggar ocean separating the Junggar and Altai block was formed
at least from early Ordovician, and gradually closed during the late Paleozoic [
33
–
38
].
The Cambrain Aermantai ophiolite and early Devonian Kalamaili ophiolite witnessed the
tectonic evolution process in the Paleozoic [
39
–
42
]. After its closure, the EJOB transferred
into the post-collisional orogenic stage when the Huangyangshan alkaline pluton were
emplaced, which is the main concern of the present study [19,43,44].
Minerals 2022,12, 1458 3 of 23
The Kalamaili area is located in the central part of the EJOB and is controlled by the
NWW-trending Kalamaili fault system, which hosts typical ophiolite and several metallic
deposits (Figure 1c). The exposed strata in the area mainly comprise small-scale Silurian
sediments, Permian volcanogenic sediments, and a thick pile of Devonian and Carbonifer-
ous volcano-sedimentary rocks. Some Mesozoic and Cenozoic strata are distributed in the
eastern and southern parts of this area. All these sediments are controlled by the Beitashan,
Kubusu, Qingshui–Sujiquan, and Kalamaili faults and are deposited in a forearc basin-type
sedimentary system dominated by active continental margin sediments [
45
–
47
]. The mag-
matic activities were characterized by late Paleozoic post-collisional granitic magmatism
according to the geochronology and geochemical data from previous studies [
14
,
32
,
48
,
49
].
They produced many alkaline granitic plutons including the Laoyaquan, Beilekuduke,
Yemaquan, Huangyangshan, and Yebushan plutons, as well as multiple Au, Cu, Sn, and
graphite mineralization (Figure 1c); [44,50–53].
Minerals 2022, 12, x FOR PEER REVIEW 3 of 24
evolution process in the Paleozoic [39–42]. After its closure, the EJOB transferred into the
post-collisional orogenic stage when the Huangyangshan alkaline pluton were emplaced,
which is the main concern of the present study [19,43,44].
The Kalamaili area is located in the central part of the EJOB and is controlled by the
NWW-trending Kalamaili fault system, which hosts typical ophiolite and several metallic
deposits (Figure 1c). The exposed strata in the area mainly comprise small-scale Silurian
sediments, Permian volcanogenic sediments, and a thick pile of Devonian and Carbonif-
erous volcano-sedimentary rocks. Some Mesozoic and Cenozoic strata are distributed in
the eastern and southern parts of this area. All these sediments are controlled by the Beit-
ashan, Kubusu, Qingshui–Sujiquan, and Kalamaili faults and are deposited in a forearc
basin-type sedimentary system dominated by active continental margin sediments [45–
47]. The magmatic activities were characterized by late Paleozoic post-collisional granitic
magmatism according to the geochronology and geochemical data from previous studies
[14,32,48,49]. They produced many alkaline granitic plutons including the Laoyaquan,
Beilekuduke, Yemaquan, Huangyangshan, and Yebushan plutons, as well as multiple Au,
Cu, Sn, and graphite mineralization (Figure 1c); [44,50–53].
Figure 1. (a) Sketch map of the Central Asian Orogenic Belt (after [54]); (b) Geological map of the
northern Xinjiang Province (after [38]); (c) Simplified geological map of the Kalamaili area, Eastern
Junggar (modified after [19]).
Figure 1.
(
a
) Sketch map of the Central Asian Orogenic Belt (after [
54
]); (
b
) Geological map of the
northern Xinjiang Province (after [
38
]); (
c
) Simplified geological map of the Kalamaili area, Eastern
Junggar (modified after [19]).
Minerals 2022,12, 1458 4 of 23
2.2. Huangyangshan Pluton
The Huangyangshan pluton, located in the northeastern Kalamaili fault, is a typical
A-type alkaline granitic pluton within the Kalamaili felsic intrusions of alkaline affinity. It
roughly shows a NW–SE trend and crops out as a nearly elliptical shape on the surface,
generating a concentrically zoned domain as a result of multiple phases of magmatism
(Figure 2a). It intruded into the Devonian strata and shows sharp contact combined
with small-scale contact metamorphism, indicating shallow depth for emplacement [
14
].
According to the grain sizes and main types of dark minerals, this pluton is divided
into five lithofacies: medium-grained arfvedsonite granite, from medium- to fine-grained
arfvedsonite granite, from medium- to fine-grained amphibole granite, medium-grained
biotite granite, and fine-grained biotite granite. Their crystallizing ages show a slightly
younger trend from north to south (~320–300 Ma) [
16
,
19
]. In our study, we focus on the
arfvedsonite and amphibole granites, which occur near orebodies, consisting of K-feldspar
(~60%), quartz (~25%), arfvedsonite (~3%–5%), hornblende (~5%–8%), plagioclase (<3%),
and some accessory minerals such as zircon, titanite, ilmenite, and apatite.
Minerals 2022, 12, x FOR PEER REVIEW 4 of 24
2.2. Huangyangshan Pluton
The Huangyangshan pluton, located in the northeastern Kalamaili fault, is a typical
A-type alkaline granitic pluton within the Kalamaili felsic intrusions of alkaline affinity.
It roughly shows a NW‒SE trend and crops out as a nearly elliptical shape on the surface,
generating a concentrically zoned domain as a result of multiple phases of magmatism
(Figure 2a). It intruded into the Devonian strata and shows sharp contact combined with
small-scale contact metamorphism, indicating shallow depth for emplacement [14]. Ac-
cording to the grain sizes and main types of dark minerals, this pluton is divided into five
lithofacies: medium-grained arfvedsonite granite, from medium- to fine-grained arfved-
sonite granite, from medium- to fine-grained amphibole granite, medium-grained biotite
granite, and fine-grained biotite granite. Their crystallizing ages show a slightly younger
trend from north to south (~320–300 Ma) [16,19]. In our study, we focus on the arfved-
sonite and amphibole granites, which occur near orebodies, consisting of K-feldspar
(~60%), quartz (~25%), arfvedsonite (~3%–5%), hornblende (~5%–8%), plagioclase (<3%),
and some accessory minerals such as zircon, titanite, ilmenite, and apatite.
Figure 2. (a) Geological map of the Huangyangshan pluton (after [15]). (b) Simplified map of the
No.1 and 2 ore bodies of the Huangyangshan graphite deposit. (c) Cross-section along line 7–7′ in
Figure 2b (modified after [17]).
Figure 2.
(
a
) Geological map of the Huangyangshan pluton (after [
15
]). (
b
) Simplified map of the
No.1 and 2 ore bodies of the Huangyangshan graphite deposit. (
c
) Cross-section along line 7–7
0
in
Figure 2b (modified after [17]).
Minerals 2022,12, 1458 5 of 23
One of the most remarkable features of the Huangyangshan granites is a great abun-
dance of microgranular dioritic enclaves. They are arranged randomly in different sizes and
develop black chilled margins (Figure 3a,b). These enclaves are mostly diorite in composi-
tion, consisting of plagioclase (~45%), amphibole (mainly richterite, ~30%), quartz (~10%),
biotite (<5%), and K-feldspar (~5%). Their grain sizes (~0.2–0.5 mm) are significantly
smaller than those of the host granites (~5 mm). Some gabbro and diorite porphyrite dykes
are occurred inside the Huangyangshan granites and their strikes are nearly east–west,
which have clear boundaries with the granites on the surface. The diorite porphyrite dykes
are mainly distributed in from medium- to fine-grained amphibole granite in the north,
while the gabbro dykes are mainly distributed in medium-grained biotite granite in the
south of the pluton (Figure 2a).
Minerals 2022, 12, x FOR PEER REVIEW 5 of 24
One of the most remarkable features of the Huangyangshan granites is a great abun-
dance of microgranular dioritic enclaves. They are arranged randomly in different sizes
and develop black chilled margins (Figure 3a,b). These enclaves are mostly diorite in com-
position, consisting of plagioclase (~45%), amphibole (mainly richterite, ~30%), quartz
(~10%), biotite (<5%), and K-feldspar (~5%). Their grain sizes (~0.2–0.5 mm) are signifi-
cantly smaller than those of the host granites (~5 mm). Some gabbro and diorite porphyrite
dykes are occurred inside the Huangyangshan granites and their strikes are nearly east–
west, which have clear boundaries with the granites on the surface. The diorite porphyrite
dykes are mainly distributed in from medium- to fine-grained amphibole granite in the
north, while the gabbro dykes are mainly distributed in medium-grained biotite granite
in the south of the pluton (Figure 2a).
Figure 3. Photographs (a–c) and photomicrographs (d–o) of representative granite and graphite ore
samples of the Huangyangshan pluton. The pink points represent some measuring locations. (a) A
Figure 3.
Photographs (
a
–
c
) and photomicrographs (
d
–
o
) of representative granite and graphite ore
samples of the Huangyangshan pluton. The pink points represent some measuring locations. (
a
) A
dark-colored ME in arfvedsonite granite. (
b
) Enclaves of different sizes in amphibole granite. (
c
) Zoning
Minerals 2022,12, 1458 6 of 23
graphite orbicules consisting of graphite shell, light-colored core, and fine-grained dark inner core.
(
d
) Subhedral arfvedsonite phenocryst with ilmenite and titanite inclusions in arfvedsonite granite
(
−
). (
e
) Contact zone between granite and ME containing subhedral richterite phenocryst and fine-
grained richterite (
−
). (
f
) Altered amphibole phenocryst and biotite at the edge in enclaves (
−
). (
g
)
Amphibole and its chloritization in amphibole granite. (
h
) Euhedral–anhedral hornblendes in the
light-colored core of graphite orbicules (
−
). (
i
) Fine-grained hornblendes and distributed carbon
materials in the dark inner core of graphite orbicules (
−
). (
j
) Altered amphibole, biotite, and chlorite
in the granite collected on the surface (
−
). (
k
) Fine-grained biotite coexisting with richterite in ME
(
−
). (
l
) Euhedral biotite and hornblendes in the light-colored core of graphite orbicules (
−
). (
m
)
A pyroxene phenocryst besides cluster graphite aggregate (
−
). (
n
) Corona texture of pyroxene,
amphibole, and biotite (BSE). (
o
) A plagioclase phenocryst without any alteration and inclusions
in light-colored cores (+). Gr = graphite, Arf = arfvedsonite, Ilm = ilmenite, Ttn = titanite, Kfs = K-
feldspar, Rct = richterite, Qz = quartz, Pl = plagioclase, Bt = biotite, Amp = amphibole, CM = carbon
materials, Chl = chlorite, Mag = magnetite, Po = pyrrhotite, and Px = pyroxene.
2.3. Graphite Mineralization
The graphite deposit hosted in the Huangyangshan pluton is a large-scale deposit with
over 78 million tons predicted graphite reserves [
55
]. All the orebodies occur as lenticular,
saddle, or irregular shapes in the middle and southern part of the pluton (Figure 2a).
Among them, the No. 1 lenticular and No. 2 saddle orebodies are the largest in scale, with
a predicted resource of more than 70 million tons (Figure 2b) [
15
,
55
]. The graphite ore
is often characterized by the unique spherical structure and coexist with metal sulfides
such as pyrrhotite and chalcopyrite (Figure 3c). The graphite orbicules are abundant and
randomly oriented in the orebodies. Most of them are composed of a dark graphite shell
and light color core forming typical “zonal structures”. Some of them include two cores
in a dark and a light color, respectively (Figure 3c). The cores, as well as the host granites,
have obvious different mineral compositions. The light-colored cores contain euhedral
to anhedral amphiboles in brown or green color (Figure 3h,k). The dark-colored cores
consist of fine-grained amphiboles, plagioclases, graphite, and some pyroxene phenocrysts
(Figure 3i,m).
3. Sampling and Analytical Methods
Twelve samples were collected either from the surface outcrops or the drill cores,
including the granites, enclaves, and graphite ore in the Huangyangshan graphite deposit
(Figure 2). The HYI and HYS are the slightly weathered samples collected on surface from
the arfvedsonite and amphibole granites, respectively. The HYB are the dark enclaves in
arfvedsonite and amphibole granites. From ZK12 to ZK43 represents the fresh ore samples
collected from the drill cores in No. 1 and No. 2 orebodies. All the samples were polished
into thin sections for petrographic observation, electron microprobe analysis (EMPA), and
LA-ICP-MS analysis.
Major elements compositions of amphibole, biotite, and plagioclase were analyzed
using a JEOL JXA-8230 electron probe microanalyzer hosted at the State Key Laboratory of
geological process and mineral resources, China University of Geosciences (Wuhan). The
operating conditions were 15 kV accelerating voltage, 20 nA probe current, and 2–5
µ
m
beam diameter. The natural minerals and oxides standards were used for calibrating the
elemental X-ray intensity. A modified ZAF matrix-correction was applied during the data
reduction. The analytical precision for SiO
2
, MgO, FeO
T
, Al
2
O
3
, TiO
2
, MnO, Na
2
O, K
2
O,
and Cr2O3are all below 0.01 wt%. The analytical errors are generally less than 2%.
The trace element analysis of the amphiboles was performed using the LA-ICP-MS
method at the State Key Laboratory of Ore Deposit Geochemistry, IGCAS. The laser sam-
pling was performed using an ASI RESOLution-LR-S155 laser microprobe equipped with a
Coherent Compex-Pro 193 nm ArF excimer laser. An Agilent 7700x ICP-MS instrument was
used to acquire the ion-signal intensities. Helium (350 mL/min) was applied as a carrier
gas. The ablated aerosol was mixed with Ar (900 mL/min) as a transport gas, before exiting
Minerals 2022,12, 1458 7 of 23
the cell. Each analysis incorporated a background acquisition of approximately 30 s (gas
blank) followed by 60 s of data acquisition from the sample. The calibration line for each
element was constructed by analyzing three USGS reference glasses (BHVO-2G, BIR-1G and
BCR-2G) with Si as an internal standard element. The spot size and frequency of the laser
were set to 40
µ
m and 7 Hz, respectively. An Excel-based software, ICPMSDataCal, (10.2,
Wuhan, China) was used to perform the offline selection and integration of background and
analyzed signals, time–drift correction, and quantitative calibration for the trace element
analysis [56].
4. Results
4.1. Mineral Petrography
4.1.1. Amphibole
The amphibole in the Huangyangshan pluton can be divided into five types: Type-I
is typical arfvedsonite in the arfvedsonite granite and occurs pervasively as phenocrysts
(size up to 3 mm). It is euhedral and deep green in color (Figure 3d). There always occur
some inclusions inside such as ilmenite and titanite. Type-II occurs as phenocryst at the
contact zone between the arfvedsonite granite and enclaves (Figure 3e). It is from deep
to light green in color, from euhedral to subhedral, has a relatively smaller size (size in
1–2 mm) than Type-I, and has no inclusions inside. Type-III amphibole is fine-grained
(size in 0.2–0.5 mm) occurring in enclaves (Figure 3f). It is green-colored and mostly in a
columnar or acicular shape. The common alterations such as chloritization, epidotization,
and kaolinization are mainly due to the weathering on the surface. Type-IV is the subhedral-
anhedral and yellow-green amphibole developing in the amphibole granite. It is mainly
short columnar or granular in shape and develops chloritization at the edge (Figure 3g).
Type-V amphibole developing in the cores of the graphite orbicules is enclosed by graphite.
It is from brown to green in color, mostly from euhedral to subhedral, and almost fresh
(Figure 3h). The interior of the graphite orbicule is diorite, whereas the exterior is granitic.
The dark-colored cores in graphite orbicules contain sparse graphite grains, whereas the
graphite shells are packet aggregates (Figure 3i), possibly showing the growing process
of graphite.
4.1.2. Biotite
The biotite contents in both granites and enclaves are significantly lower than the
amphibole. They can be divided into two types according to the alteration degree. Altered
biotite mostly occurs in enclaves and granites metasomatizing arfvedsonite and/or de-
veloping chloritization (Figure 3f,j). It mainly has a rough surface, reabsorbed structure,
and symbiosis with secondary minerals. Some relatively fresh biotites in enclaves are
fine-grained (<0.1 mm) in light brown color and coexist with amphibole (Figure 3k). Fresh
biotite all occurs in the graphite orbicule core. It is commonly euhedral in flake, dark-brown,
and coexists with Type-IV amphibole, plagioclase, apatite, and some pyroxene (Figure 3l).
4.1.3. Pyroxene
The pyroxene occurs only in the graphite orbicules and is mostly colorless and in a
euhedral granular shape. Most of them are phenocrysts and do not coexist with or sur-
rounded by graphite, indicating their formation was earlier. A small amount of pyrrhotite
and graphite can be found in the cracks inside some pyroxene (Figure 3m). Some other py-
roxene phenocrysts are wrapped by amphibole and biotite in a corona structure (
Figure 3n
).
4.1.4. Plagioclase
The Huangyangshan granites barely contains plagioclase resulted from the high
alkalinity. The plagioclase content of the dioritic enclaves is more than 40% but the grains
are too small (<0.5 mm) to observe oscillatory zones. Therefore, we mainly focused on the
relatively coarse-grained plagioclase in graphite orbicules and their host granites collected
in the drill cores. The plagioclase in granites are commonly 0.5–3 mm in size and distributed
Minerals 2022,12, 1458 8 of 23
in clusters, with some accessory mineral inclusions developed inside such as apatite and
zircon. The plagioclase content in the graphite orbicules is significantly higher than that
in the granites. They are mostly euhedral and hardly contain fine-grained amphibole and
biotite as inclusions (Figure 3o). Most plagioclases in the granites are coarse-grained and
develop normal zoning, while those in the graphite orbicules are medium- to fine-grained
and develop inapparent normal and oscillatory zoning (Figure 3o).
4.2. Mineral Chemistry
4.2.1. Amphibole
The amphiboles in the Huangyangshan pluton and graphite ores are characterized by
low Al
2
O
3
(commonly < 6%, Figure 4), extremely high SiO
2
(mostly > 45%, Figure 4a), FeO
T
(>20%, Figure 4d), and relatively high TiO
2
(mostly > 0.8%), K
2
O (0.7%–1.9%, Figure 4f).
These results are shown in Table S1. The MgO, CaO, and Na
2
O contents of the five types
of amphibole are significantly different (Figure 4b,c,e). According to the major element
compositions of the amphiboles, their structural formulae on a 23-cations anhydrous basis
after adjusting the ferrous iron charge balance were calculated following the procedure of
Leake et al. [
57
] The results show that Type-I amphibole has a high FeO
T
(
32.54%–34.03%
),
Na
2
O (6.37%–7.03%) contents, and low MgO (0.1%–0.24%), CaO (1.5%–2.5%) contents,
belonging to sodic amphiboles [Na
B≥
1.5, (Na + K)
A≥
0.5)]. It is classified as typical
arfvedsonite in Figure 5a. Type-II and -III amphiboles belong to sodic–calcic amphiboles
[Ca
B
< 1.5; (Na + K)
A≥
0.5] and have a relatively high Na
2
O (3.57%–4.99%) and low CaO
(4.73%–7.93%) content. They are significantly different in MgO (<1% and >8%, respectively)
and FeO (32.82%–33.56% and 18.87%–26.65%, respectively) contents, being consistent with
the characteristics that the enclaves are neutral (rich in magnesium) and the granites are
acidic (rich in iron). They are both plotted in the field of ferrorichterite in Figure 5b. Type-IV
and -V amphiboles both have high CaO (9.37%–10.72%), medium MgO (
4.97%–8.44%
), and
low Na
2
O (1.61%–2.07%) content, showing calcic-amphiboles characteristics [
CaB≥1.5
;
(Na + K)
A
< 0.5]. They both are classified as ferrohornblende in Figure 5c. The main
difference between them is that Type-IV has a low Mg/(Mg + Fe
2+
) value and most
measuring points do not contain Fe
3+
(Table S1). There is no obvious difference between
the amphiboles in light-colored and dark-colored cores.
The trace element compositions of 17 amphibole points in this study collected from
enclaves and graphite orbicules and 7 arfvedsonite points reported by Ye et al. [
58
] are
shown in Table S2. They have relatively consistent REE patterns that show significantly
negative Eu anomalies and inapparent fractionation of the REEs (Figure 6a). Type-I ar-
fvedsonite in granite and Type-II and -III richterites in enclaves have almost the same
patterns with a slight HREE enrichment of Er–Tm–Yb–Lu compared with whole-rock REE
patterns [
19
], except for one of the HYB-2 points (
Σ
REE = 52.67 ppm). Type-IV and -V
amphiboles in amphibole granite and graphite orbicules have similar REE patterns, show-
ing a significant enrichment in the REE content (
Σ
REE = 515.3–569.29 and 887.64–2126.49
ppm, respectively). The REE content of the Type-V amphibole is obviously higher than
that from the Type-I to -IV amphiboles, indicating that the cores of graphite have not
experienced the crystallization process of REE-rich minerals similar to the granites. The
primitive mantle-normalized multi-element variation diagram patterns (Figure 6b) suggest
all types of amphiboles have similar distribution characteristics to the granite with stronger
Ba and Sr negative anomalies. Compared to the other four types, Type-V amphibole shows
a slightly stronger La–Ce–Sm–Nd positive and Pb–Zr negative anomalies.
Note that one point in the enclaves (HYB-2) shows altered amphibole characteristics
with low Nb (0.48 ppm) and REE (
Σ
REE = 52.67) contents and significantly different patterns
in Figure 6b, hence the data were eliminated in the statistics. From calcic to sodic amphiboles,
the Li content has an upward trend (average of 36, 485 and 1627 ppm, respectively), whereas
the Sc–Y contents show a downward trend. The V–Cr contents also have a decreasing trend
from calcic to sodic–calcic amphiboles due to the lack of sodic amphiboles data.
Minerals 2022,12, 1458 9 of 23
Minerals 2022, 12, x FOR PEER REVIEW 9 of 24
Figure 4. Diagrams of SiO2 (a), MgO (b), CaO (c), FeOT (d), Na2O (e), and K2O (f) vs. Al2O3 of am-
phibole in the Huangyangshan deposit in wt%.
Figure 5. Classification diagrams of (a) sodic, (b) sodic‒calcic, and (c) calcic amphiboles (after [57]).
Symbols are as in Figure 4.
Figure 4.
Diagrams of SiO
2
(
a
), MgO (
b
), CaO (
c
), FeO
T
(
d
), Na
2
O (
e
), and K
2
O (
f
) vs. Al
2
O
3
of
amphibole in the Huangyangshan deposit in wt%.
Minerals 2022, 12, x FOR PEER REVIEW 9 of 24
Figure 4. Diagrams of SiO2 (a), MgO (b), CaO (c), FeOT (d), Na2O (e), and K2O (f) vs. Al2O3 of am-
phibole in the Huangyangshan deposit in wt%.
Figure 5. Classification diagrams of (a) sodic, (b) sodic‒calcic, and (c) calcic amphiboles (after [57]).
Symbols are as in Figure 4.
Figure 5.
Classification diagrams of (
a
) sodic, (
b
) sodic–calcic, and (
c
) calcic amphiboles (after [
57
]).
Symbols are as in Figure 4.
Minerals 2022,12, 1458 10 of 23
Minerals 2022, 12, x FOR PEER REVIEW 10 of 24
Figure 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element
variation patterns (b) of amphiboles and granites in the Huangyangshan deposit. The normalizing
values for the REEs and trace elements are from Sun and McDonough [59]. The yellow, red, blue,
green lines and the shadow parts represent the trace element data of Type-I, Type-II and -III, Type-
IV, Type-V and granites, respectively. The granites data are from Sun et al. [19].
Note that one point in the enclaves (HYB-2) shows altered amphibole characteristics
with low Nb (0.48 ppm) and REE (ΣREE = 52.67) contents and significantly different pat-
terns in Figure 6b, hence the data were eliminated in the statistics. From calcic to sodic
amphiboles, the Li content has an upward trend (average of 36, 485 and 1627 ppm, respec-
tively), whereas the Sc–Y contents show a downward trend. The V–Cr contents also have
a decreasing trend from calcic to sodic‒calcic amphiboles due to the lack of sodic amphi-
boles data.
4.2.2. Biotite
The major element compositions of the biotite are shown in Table S3. The cation num-
ber and structural formulae of biotite were calculated by 22 oxygen atoms as a unit. We
obtained only three available biotite EPMA points in dioritic enclaves, two of which are
altered and one is relatively fresh. They have a relatively high SiO2 (40.53%–42.32%) and
MgO (9.02%–11.48%) and low Al2O3 (8.75%–9.51%) and FeO (19.6%–21.87%) contents; 3
and 12 valid data were obtained from amphibole granite and graphite orbicules, respec-
tively, which have similar compositional characteristics. They have relatively low SiO2
(35.57%–38.51%) and MgO (4.62%–10.12%), and high Al2O3 (11.12%–12.95%) and FeO
(23.47%–30.17%) contents. The altered and fresh biotite samples are mainly different in
TiO2 content (0.99%–1.59% and 2.09%–4.46%, respectively) and are plotted in primary bi-
otites and re-equilibrated biotites fields, respectively (Figure 7a). In the biotite classifica-
tion plot (Figure 7b), the fresh biotite is classified as an Fe-biotite, whereas the altered
biotite is classified as a Mg-biotite, indicating the alteration led to the loss of Fe or Mg
enrichment.
Figure 6.
Chondrite-normalized REE patterns (
a
) and primitive mantle-normalized trace element
variation patterns (
b
) of amphiboles and granites in the Huangyangshan deposit. The normalizing
values for the REEs and trace elements are from Sun and McDonough [
59
]. The yellow, red, blue,
green lines and the shadow parts represent the trace element data of Type-I, Type-II and -III, Type-IV,
Type-V and granites, respectively. The granites data are from Sun et al. [19].
4.2.2. Biotite
The major element compositions of the biotite are shown in Table S3. The cation
number and structural formulae of biotite were calculated by 22 oxygen atoms as a unit.
We obtained only three available biotite EPMA points in dioritic enclaves, two of which
are altered and one is relatively fresh. They have a relatively high SiO
2
(40.53%–42.32%)
and MgO (9.02%–11.48%) and low Al
2
O
3
(8.75%–9.51%) and FeO (19.6%–21.87%) con-
tents; 3 and 12 valid data were obtained from amphibole granite and graphite orbicules,
respectively, which have similar compositional characteristics. They have relatively low
SiO
2
(35.57%–38.51%) and MgO (4.62%–10.12%), and high Al
2
O
3
(11.12%–12.95%) and FeO
(23.47%–30.17%) contents. The altered and fresh biotite samples are mainly different in TiO
2
content (0.99%–1.59% and 2.09%–4.46%, respectively) and are plotted in primary biotites
and re-equilibrated biotites fields, respectively (Figure 7a). In the biotite classification plot
(Figure 7b), the fresh biotite is classified as an Fe-biotite, whereas the altered biotite is
classified as a Mg-biotite, indicating the alteration led to the loss of Fe or Mg enrichment.
Minerals 2022, 12, x FOR PEER REVIEW 11 of 24
Figure 7. (a) Ternary 10 × TiO2–(FeO + MnO)–MgO discrimination diagram of primary, re-equili-
brated, and epigenetic biotite (after [60]). (b) Ternary Mg–(Fe2+ + Mn)–(AlVI + Fe3+ + Ti) classification
diagram of biotite (after [61]).
4.2.3. Pyroxene
Due to the low content of pyroxene in the Huangyangshan pluton, 16 measuring
points of the 6 pyroxene phenocrysts in the graphite orbicules were analyzed in this study.
According to their major element compositions (Table S4), 6 pyroxene phenocrysts were
classified into 4 clinopyroxene phenocrysts with high calcic pyroxene endmembers (Wo)
and 2 orthopyroxene phenocrysts with low Wo (Figure 8). The analyzed clinopyroxenes
are mainly subhedral and fine-grained, which were found in granites, graphite orbicule
shells, and deep-colored cores. Their enstatite endmembers (En) are similar (18.26–27.64)
and have low Al2O3 (0.32%–1.12%), MgO (5.98%–9.26%), FeO (17.24%–24.26%), and high
CaO (12.09%–20.43%) contents. The orthopyroxenes only occurred in graphite orbicules
in the shape of coarse phenocrysts. They have extremely lower CaO (0.36%–1.69%), higher
FeO (25.49%–36.33%), MgO (7.36%–19.96%), and similar Al2O3 (0.33%–1.01%) contents
compared to the clinopyroxenes.
Figure 8. Ternary Calcic pyroxene (Wo)—Enstatite (En)—Orthoferrosilite (Fs) classification diagram
of pyroxene (after [62]).
Figure 7.
(
a
) Ternary 10
×
TiO
2
–(FeO + MnO)–MgO discrimination diagram of primary, re-
equilibrated, and epigenetic biotite (after [
60
]). (
b
) Ternary Mg–(Fe
2+
+ Mn)–(Al
VI
+ Fe
3+
+ Ti)
classification diagram of biotite (after [61]).
Minerals 2022,12, 1458 11 of 23
4.2.3. Pyroxene
Due to the low content of pyroxene in the Huangyangshan pluton, 16 measuring
points of the 6 pyroxene phenocrysts in the graphite orbicules were analyzed in this study.
According to their major element compositions (Table S4), 6 pyroxene phenocrysts were
classified into 4 clinopyroxene phenocrysts with high calcic pyroxene endmembers (Wo)
and 2 orthopyroxene phenocrysts with low Wo (Figure 8). The analyzed clinopyroxenes
are mainly subhedral and fine-grained, which were found in granites, graphite orbicule
shells, and deep-colored cores. Their enstatite endmembers (En) are similar (18.26–27.64)
and have low Al
2
O
3
(0.32%–1.12%), MgO (5.98%–9.26%), FeO (17.24%–24.26%), and high
CaO (12.09%–20.43%) contents. The orthopyroxenes only occurred in graphite orbicules in
the shape of coarse phenocrysts. They have extremely lower CaO (0.36%–1.69%), higher
FeO (25.49%–36.33%), MgO (7.36%–19.96%), and similar Al
2
O
3
(0.33%–1.01%) contents
compared to the clinopyroxenes.
Minerals 2022, 12, x FOR PEER REVIEW 11 of 24
Figure 7. (a) Ternary 10 × TiO2–(FeO + MnO)–MgO discrimination diagram of primary, re-equili-
brated, and epigenetic biotite (after [60]). (b) Ternary Mg–(Fe2+ + Mn)–(AlVI + Fe3+ + Ti) classification
diagram of biotite (after [61]).
4.2.3. Pyroxene
Due to the low content of pyroxene in the Huangyangshan pluton, 16 measuring
points of the 6 pyroxene phenocrysts in the graphite orbicules were analyzed in this study.
According to their major element compositions (Table S4), 6 pyroxene phenocrysts were
classified into 4 clinopyroxene phenocrysts with high calcic pyroxene endmembers (Wo)
and 2 orthopyroxene phenocrysts with low Wo (Figure 8). The analyzed clinopyroxenes
are mainly subhedral and fine-grained, which were found in granites, graphite orbicule
shells, and deep-colored cores. Their enstatite endmembers (En) are similar (18.26–27.64)
and have low Al2O3 (0.32%–1.12%), MgO (5.98%–9.26%), FeO (17.24%–24.26%), and high
CaO (12.09%–20.43%) contents. The orthopyroxenes only occurred in graphite orbicules
in the shape of coarse phenocrysts. They have extremely lower CaO (0.36%–1.69%), higher
FeO (25.49%–36.33%), MgO (7.36%–19.96%), and similar Al2O3 (0.33%–1.01%) contents
compared to the clinopyroxenes.
Figure 8. Ternary Calcic pyroxene (Wo)—Enstatite (En)—Orthoferrosilite (Fs) classification diagram
of pyroxene (after [62]).
Figure 8.
Ternary Calcic pyroxene (Wo)—Enstatite (En)—Orthoferrosilite (Fs) classification diagram
of pyroxene (after [62]).
Some orthopyroxene phenocrysts develop typical zonal structures, including orthopy-
roxene, amphibole, and biotite from the core to the edge (Figure 3n). In the backscattered
electron photograph (Figure 9a), an orthopyroxene has high protrusion and developed
cleavage at the core, but low protrusion and undeveloped cleavage at the edge, indicating
the orthopyroxene may have undergone metasomatic replacement and belongs to transi-
tional phase minerals. From the core to the edge, the Fe content decreases, whereas the Mg
content increases, showing the characteristics of reversely zoned structure. However, if it
continues outward, the Fe content increases and the Mg content decreases (Figure 9b), sug-
gesting that magma have experienced multiple mixing during the formation of pyroxene.
4.2.4. Plagioclase
The plagioclase grains in both granites and graphite orbicules have similar Al
2
O
3
con-
tents, ranging between 22.57% and 27.67% (Table S5). The former has relatively higher SiO
2
content than the latter, ranging in 59.51%–67.35% (63.48% on average) and
58.06%–66.81%
(61.01% on average), respectively. The early plagioclase phenocryst in Sample ZK33 has
a lower Si content (58.06%–65.36%, 59.89% on average), showing that the magma had a
tendency of increasing acidity from early to late. Most plagioclases develop normal zoning
characterized by dark low-An rims (30 > An > 50, oligoclase) and light high-An cores
(
10 > An > 30
, andesine; Figure 10a–c), also suggesting the increasing magma alkalinity
and acidity along with its evolution [
20
]. One plagioclase phenocryst enclosed by the
graphite shows apparent occasional oscillatory zoning and has low a SiO
2
, high An, and rel-
Minerals 2022,12, 1458 12 of 23
atively small An spans (An = 32–42; Figure 10d), indicating a relatively stable neutral–basic
environment in which it crystallized.
Minerals 2022, 12, x FOR PEER REVIEW 12 of 24
Some orthopyroxene phenocrysts develop typical zonal structures, including ortho-
pyroxene, amphibole, and biotite from the core to the edge (Figure 3n). In the backscat-
tered electron photograph (Figure 9a), an orthopyroxene has high protrusion and devel-
oped cleavage at the core, but low protrusion and undeveloped cleavage at the edge, in-
dicating the orthopyroxene may have undergone metasomatic replacement and belongs
to transitional phase minerals. From the core to the edge, the Fe content decreases,
whereas the Mg content increases, showing the characteristics of reversely zoned struc-
ture. However, if it continues outward, the Fe content increases and the Mg content de-
creases (Figure 9b), suggesting that magma have experienced multiple mixing during the
formation of pyroxene.
Figure 9. (a) BSE images of an orthopyroxene phenocryst in graphite orbicule. (b) Compositional
variations along the profile of the Huangyangshan orthopyroxene phenocryst (based on the data
from Table S4).
4.2.4. Plagioclase
The plagioclase grains in both granites and graphite orbicules have similar Al2O3 con-
tents, ranging between 22.57% and 27.67% (Table S5). The former has relatively higher
SiO2 content than the latter, ranging in 59.51%–67.35% (63.48% on average) and 58.06%–
66.81% (61.01% on average), respectively. The early plagioclase phenocryst in Sample
ZK33 has a lower Si content (58.06%–65.36%, 59.89% on average), showing that the magma
had a tendency of increasing acidity from early to late. Most plagioclases develop normal
zoning characterized by dark low-An rims (30 > An > 50, oligoclase) and light high-An
cores (10 > An > 30, andesine; Figure 10a,b,c), also suggesting the increasing magma alka-
linity and acidity along with its evolution [20]. One plagioclase phenocryst enclosed by
the graphite shows apparent occasional oscillatory zoning and has low a SiO2, high An,
and relatively small An spans (An = 32–42; Figure 10d), indicating a relatively stable neu-
tral‒basic environment in which it crystallized.
Figure 9.
(
a
) BSE images of an orthopyroxene phenocryst in graphite orbicule. (
b
) Compositional
variations along the profile of the Huangyangshan orthopyroxene phenocryst (based on the data
from Table S4).
Minerals 2022, 12, x FOR PEER REVIEW 13 of 24
Figure 10. BSE images illustrating the petrographic characteristics of plagioclases in the granites and
the graphite orbicules and their An (anorthite composition) variation trends. The numbers represent
the An in plagioclase. (a,b) Plagioclase in granites showing high-An cores and low-An rims. (c) Pla-
gioclase in the light-colored core of graphite orbicules having andesine core and oligoclase rim. (d)
A plagioclase phenocryst surrounded by graphite having relatively high An and occasional oscilla-
tory zoning.
5. Discussion
5.1. Mineral Crystallization Conditions
Previous studies have shown that amphibole, biotite, pyroxene, and plagioclase can
be used to estimate their crystallization conditions, such as temperature, pressure, and
oxidation, and their applicable objects of amphiboles mainly constrain in calcic amphi-
boles with relatively high Ca contents [21,23,24]. Ye et al. [58] tried to estimate the tem-
perature and pressure of the sodic amphibole in the Huangyangshan pluton using multi-
ple formulas. However, the pressure calculation result was not reliable for the sodic am-
phiboles. The accurate temperature calculation result was more likely the closure temper-
ature for the element exchange to reach equilibrium other than the crystallization temper-
atures. Through amphibole-only thermometers, an estimated temperature result with er-
rors was obtained. In this study, only Type-IV and -V belong to calcic amphiboles. The
Type-IV amphibole occurs in the amphibole alkaline granite with fine granularity and
chloritization at prepheries. Type-V amphibole is mainly found in the cores of graphite
orbicules with an even grain size and straight edges. It is commonly euhedral and rarely
has obvious ductile deformation and directional arrangement, with the Si formula num-
ber less than 7.3 a.p.f.u. (6.88 a.p.f.u.–7.08 a.p.f.u.). All this evidence suggests a magmatic
origin of Type-IV and -V amphiboles [63,64]. They hardly develop oscillatory zones, indi-
cating their crystallized process can record the evolution of magma from melt to consoli-
dation [65]. In this paper, therefore, the quantitative estimation of these two types of am-
phibole are mainly carried out. Then, the characteristics of the magma system were de-
duced in combination with the relevant parameters of biotite and pyroxene.
5.1.1. Pressure
Most amphibole thermobarometers are only applicable for amphiboles with rela-
tively high Al and Mg/(Mg + Fe2+) values [21–23,66]. However, Type-IV and -V amphi-
boles have significantly low Al (0.89 a.p.f.u–1.15 a.p.f.u) and Mg/(Mg + Fe2+) (0.285 a.f.p.u–
0.476 a.p.f.u) values. For the calcic ferrohornblendes in the Huangyangshan deposit, there-
fore, we need a more universal estimation method.
Figure 10.
BSE images illustrating the petrographic characteristics of plagioclases in the granites
and the graphite orbicules and their An (anorthite composition) variation trends. The numbers
represent the An in plagioclase. (
a
,
b
) Plagioclase in granites showing high-An cores and low-An rims.
(
c
) Plagioclase in the light-colored core of graphite orbicules having andesine core and oligoclase
rim. (
d
) A plagioclase phenocryst surrounded by graphite having relatively high An and occasional
oscillatory zoning.
5. Discussion
5.1. Mineral Crystallization Conditions
Previous studies have shown that amphibole, biotite, pyroxene, and plagioclase can
be used to estimate their crystallization conditions, such as temperature, pressure, and
oxidation, and their applicable objects of amphiboles mainly constrain in calcic amphiboles
with relatively high Ca contents [
21
,
23
,
24
]. Ye et al. [
58
] tried to estimate the temperature
and pressure of the sodic amphibole in the Huangyangshan pluton using multiple formu-
las. However, the pressure calculation result was not reliable for the sodic amphiboles.
Minerals 2022,12, 1458 13 of 23
The accurate temperature calculation result was more likely the closure temperature for
the element exchange to reach equilibrium other than the crystallization temperatures.
Through amphibole-only thermometers, an estimated temperature result with errors was
obtained. In this study, only Type-IV and -V belong to calcic amphiboles. The Type-IV
amphibole occurs in the amphibole alkaline granite with fine granularity and chloritization
at prepheries. Type-V amphibole is mainly found in the cores of graphite orbicules with an
even grain size and straight edges. It is commonly euhedral and rarely has obvious ductile
deformation and directional arrangement, with the Si formula number less than 7.3 a.p.f.u.
(6.88 a.p.f.u.–7.08 a.p.f.u.). All this evidence suggests a magmatic origin of Type-IV and
-V amphiboles [
63
,
64
]. They hardly develop oscillatory zones, indicating their crystallized
process can record the evolution of magma from melt to consolidation [
65
]. In this paper,
therefore, the quantitative estimation of these two types of amphibole are mainly carried
out. Then, the characteristics of the magma system were deduced in combination with the
relevant parameters of biotite and pyroxene.
5.1.1. Pressure
Most amphibole thermobarometers are only applicable for amphiboles with relatively
high Al and Mg/(Mg + Fe
2+
) values [
21
–
23
,
66
]. However, Type-IV and -V amphiboles have
significantly low Al (0.89 a.p.f.u–1.15 a.p.f.u) and Mg/(Mg + Fe
2+
) (
0.285 a.f.p.u–0.476 a.p.f.u
)
values. For the calcic ferrohornblendes in the Huangyangshan deposit, therefore, we need
a more universal estimation method.
After experimental simulation, Mutch et al. [
67
] comprehensively considered the
effects of fluid, temperature, and oxygen fugacity on pressure and proposed a new calcu-
lation formula. It is widely used in the composition of symbiotic amphibole (Mg/(Mg +
Fe
2+
) = 0.32 a.p.f.u.–0.81 a.p.f.u.) and feldspar (An = 15–80), but the rock is required to
have a low variance mineral assemblage: amphibole + plagioclase + biotite + quartz +
alkali feldspar + ilmenite/titanite + magnetite + apatite. Most composition parameters of
the amphiboles in this study meet the above conditions and the mineral assemblages are
very consistent. Based on this method, the estimated pressure of Type-IV and -V amphi-
boles is 157–220 Mpa and the corresponding depth is 5.95–8.32 km (with a crustal density
of 2700 kg/m
3
). This might represent the magmatic emplacement depth at the time of
crystallization.
According to the Al
T
-based biotite geobarometer proposed by Uchida et al. [
68
], the
crystallized pressure of biotite in the graphite orbicules are mostly near 0, showing quite a
difference with the results of the amphibole pressure. Kang et al. [
69
] pointed out that the
calculation of granite diagenetic pressure has the following characteristics: If there exists
an amphibole + biotite mineral assembly in the granite and the amphiboles are crystallized
well, the calculated pressure of the amphibole will be reliable, whereas the biotite will not
be reliable. If no amphibole exists in the granite, there will also be a large error in the biotite
pressure results. The results of the biotite pressure will be reliable only if the amphibole
crystallized poorly and the biotite crystallized well. Therefore, this study tends to use
the amphibole pressure and depth as the formation pressure and depth of the graphite
orbicule cores.
5.1.2. Temperature
The cores of the graphite orbicules are characterized by dioritic lithoface rich in plagio-
clase and calcic amphiboles, meeting the requirements of some geothermometers
[70–72]
.
Using the thermometer proposed by Putirka [
72
] based on the correlation between the Si, Fe,
Mg, Ti, Na, known pressure, and temperature of amphibole, we obtained the crystallization
temperatures of 681–761
◦
C for Type-IV and -V amphiboles, with an average of 730
◦
C.
Their low crystallized temperature condition suggests a late stage of magmatic evolution.
Given that there is no thermobarometer for sodic and sodic–calcic amphibole, it can be
inferred that the formation conditions roughly through the comparison of its compositional
parameters. Generally, the Al content of the amphibole is closely related to the emplace-
Minerals 2022,12, 1458 14 of 23
ment depth and low Al indicates the shallow emplacement depth [
65
]. The crystallization
temperature has a strong negative correlation with the Si content, but negative correlations
with Al, Na, and Ti contents [
72
]. Type-I arfvedsonite always contains some ilmenite and
titanite inclusions, suggesting its high Ti content. From Type-I to -III amphiboles have
significantly higher Si, Na, and lower Al than Type-IV (Figure 4). In alkaline rocks, the
Na is much related to the amphibole compositions and magma components and cannot
be used to estimate the temperature. Therefore, we inferred that the cores of the graphite
orbicules were formed in a relatively high temperature and pressure condition compared
with the granite and enclaves and may have crystallized in an earlier stage of magmatism.
This is extremely consistent with the estimated temperature of the sodic amphibole (about
650 ◦C) and its late crystallization stage as reported by Ye et al. [58].
Under high temperature and high pressure, the content of Ti in biotite is closely
related to the temperature change, which is usually used as the basis for estimating tem-
perature
[73,74]
. Generally, biotite is Al-rich in peraluminous granite (S-type), Mg-rich in
calc-alkaline granite (I-type), and Fe-rich in alkaline granite (A-type). The biotite in the
graphite orbicules has extremely high Fe content (FeO = 23.47%–30.17%), showing typical
A-type granite features. In comparison, the biotite in enclaves has a lower Fe content
(FeO = 19.6%–21.87%) and higher Mg content (MgO = 9.02%–11.48%). These characteris-
tics are similar to the compositions of amphiboles, indicating they crystallized at similar
conditions. Based on the relation between the Ti content and crystallized temperature of
the biotite in metapelites, Henry et al. [
74
] proposed a Ti-based empirical biotite geother-
mometer. It was later proved to be equally applicable to granite [
75
]. The crystallization
temperature of the unaltered biotite in the enclaves is similar to that in the graphite or-
bicules, which yielded from 658 to 720
◦
C (686
◦
C on average). It is slightly lower than
Type-IV and -V amphiboles, suggesting they crystallized slightly later than the amphiboles.
The thermometer for pyroxene is applicable when the pyroxene and the melt are
in equilibrium. However, the pyroxenes in the studied graphite ore all occurred as phe-
nocrysts, which are not contemporaneous products with the amphibole alkaline granite.
Some other scholars put forward the estimation method of pyroxene thermometer through
experiments and thermodynamic principles, which is suitable for most rocks [
76
,
77
],
and requires the equilibrium parameter of symbiotic clinopyroxene and orthopyroxene
(KD) = 1.09 ±0.14
. Through calculation, there are five dipyroxene assemblages with K
D
be-
tween 0.98 and 1.16, satisfying this condition. According to the formula, the crystallization
temperatures of pyroxenes are 807–899
◦
C and 840–1012
◦
C, which are significantly higher
than the crystallization temperatures of amphibole and biotite. This is also consistent with
the pyroxene–amphibole–biotite crystallization order supported by the petrographic study.
5.1.3. Oxygen Fugacity
It is known that the oxygen fugacity of magma is positively correlated with the Mg con-
tent of amphibole and negatively correlated with the Fe content [
78
,
79
]. In addition, with the
enhancement of reducibility, Fe
3+
tends to change to Fe
2+
. As a result, the Fe# (Fe/(Fe + Mg))
and (Fe
2+
/Fe
2+
+ Fe
3+
) values of amphibole can reflect the oxygen fugacity of magma.
The higher these values, the lower the oxygen fugacity [
78
–
80
]. The results (Figure 11)
show that Type-I and -II amphiboles have extremely high Fe# (
0.951 a.p.f.u.–0.995 a.p.f.u.
)
and almost no Mg, indicating a very reductive condition in low oxygen fugacity. Type-
III amphibole has relatively low Fe# (0.484 a.p.f.u.–0.730 a.p.f.u.) and mainly falls into
the high oxygen fugacity field. Type-IV and -V amphiboles mostly have moderate Fe#
(
0.606 a.p.f.u.–0.741 a.p.f.u.
) and are plotted in the field of intermediate oxygen fugacity.
However, In Types I–IV amphiboles, all Fe elements exist in the form of Fe
2+
, indicating an
environment with very low oxygen fugacity. Besides, the (Fe
2+
/Fe
2+
+ Fe
3+
) value of the
Type-V amphibole ranges from 0.12 a.p.f.u. to 0.34 a.p.f.u. Such values are indicative of a
range of oxygen fugacity above the QFM buffer [
81
], suggesting the crystallization under
from low to moderate conditions.
Minerals 2022,12, 1458 15 of 23
Minerals 2022, 12, x FOR PEER REVIEW 15 of 24
biotite in metapelites, Henry et al. [74] proposed a Ti-based empirical biotite geothermom-
eter. It was later proved to be equally applicable to granite [75]. The crystallization tem-
perature of the unaltered biotite in the enclaves is similar to that in the graphite orbicules,
which yielded from 658 to 720 °C (686 °C on average). It is slightly lower than Type-IV
and -V amphiboles, suggesting they crystallized slightly later than the amphiboles.
The thermometer for pyroxene is applicable when the pyroxene and the melt are in
equilibrium. However, the pyroxenes in the studied graphite ore all occurred as pheno-
crysts, which are not contemporaneous products with the amphibole alkaline granite.
Some other scholars put forward the estimation method of pyroxene thermometer
through experiments and thermodynamic principles, which is suitable for most rocks
[76,77], and requires the equilibrium parameter of symbiotic clinopyroxene and orthopy-
roxene (KD) = 1.09 ± 0.14. Through calculation, there are five dipyroxene assemblages with
KD between 0.98 and 1.16, satisfying this condition. According to the formula, the crystal-
lization temperatures of pyroxenes are 807–899 °C and 840–1012 °C, which are signifi-
cantly higher than the crystallization temperatures of amphibole and biotite. This is also
consistent with the pyroxene–amphibole–biotite crystallization order supported by the
petrographic study.
5.1.3. Oxygen Fugacity
It is known that the oxygen fugacity of magma is positively correlated with the Mg
content of amphibole and negatively correlated with the Fe content [78,79]. In addition,
with the enhancement of reducibility, Fe3+ tends to change to Fe2+. As a result, the Fe#
(Fe/(Fe + Mg)) and (Fe2+/Fe2+ + Fe3+) values of amphibole can reflect the oxygen fugacity of
magma. The higher these values, the lower the oxygen fugacity [78–80]. The results (Fig-
ure 11) show that Type-I and -II amphiboles have extremely high Fe# (0.951 a.p.f.u.–0.995
a.p.f.u.) and almost no Mg, indicating a very reductive condition in low oxygen fugacity.
Type-III amphibole has relatively low Fe# (0.484 a.p.f.u.–0.730 a.p.f.u.) and mainly falls
into the high oxygen fugacity field. Type-IV and -V amphiboles mostly have moderate
Fe# (0.606 a.p.f.u.–0.741 a.p.f.u.) and are plotted in the field of intermediate oxygen fugac-
ity. However, In Types I–IV amphiboles, all Fe elements exist in the form of Fe2+, indicating
an environment with very low oxygen fugacity. Besides, the (Fe2+/Fe2+ + Fe3+) value of the
Type-V amphibole ranges from 0.12 a.p.f.u. to 0.34 a.p.f.u. Such values are indicative of a
range of oxygen fugacity above the QFM buffer [81], suggesting the crystallization under
from low to moderate conditions.
Figure 11. Fe# (FeT/FeT + Mg) versus AlT of the amphiboles from the Huangyangshan deposit (after
[80]). Symbols are as in Figure 4.
Figure 11.
Fe# (Fe
T
/Fe
T
+ Mg) versus Al
T
of the amphiboles from the Huangyangshan deposit
(after [80]). Symbols are as in Figure 4.
In mineralogy, Type-I arfvedsonite is an important petrogenetic indicator of alkaline
magma, which forms in the reducing condition. The Sayashk hydrothermal Sn deposit
occurring in the arfvedsonite granite in this area also reflects the low magmatic oxygen
fugacity [
82
]. A large number of metal sulfides and oxides such as pyrrhotite, chalcopyrite,
and ilmenite are developed in the graphite orbicules with rarely magnetite content, which
also reflects the characteristics of relatively low oxygen fugacity. However, magnetite is also
rare in the enclaves and the Fe
3+
content is very low in the component parameters (Table S1).
It does not show high oxygen fugacity characteristics, which is obviously ambiguous with
the high oxygen fugacity of Type-III amphiboles shown in Figure 11. Amphibole is the main
carrier of Mg in magma. During magma evolution, Mg is a refractory element that tends
to reside in residual melt, whereas Fe readily migrates into differentiated minerals [
83
].
During magma mixing, the intermediate-basic magma end member with higher Mg and Fe
contents evolved as the original melt. As a result, the Mg tends to remain in the amphiboles
in enclaves, showing lower Fe# characteristics. The Fe tends to migrate into felsic magma
to form minerals with higher Fe, such as arfvedsonite and ilmenite. Therefore, we used the
characteristics of Fe3+/(Fe2+ + Fe3+ ) as the criterion. The enclaves were formed in a lower
oxygen fugacity environment similar to the arfvedsonite granite.
Similar to those in amphiboles, the Fe
2+
, Fe
3+
, and Mg contents of biotite can also be
used to estimate the oxygen fugacity of the magma and fluids [
73
]. In the ternary Fe
2+
-
Fe
3+
-Mg plot (Figure 12a), the biotite in graphite orbicules is distributed near the NNO
evolution line showing low oxygen fugacity, indicating it formed in a relatively reducing
environment, which is consistent with the estimated results of amphibole. However, the
biotite sample in the enclaves was plotted below the QFM evolution line due to its very
low Fe
3+
value, showing extremely low oxygen fugacity. In the Logf(O
2
)–T diagram under
P
H2O
= 207.0 Mpa (Figure 12b), most samples are plotted near the NNO evolution line
including the biotite in enclaves. Considering that the Fe
3+
/Fe
2+
value is more sensitive to
oxygen fugacity but Figure 12b only considered the influence of the Fe# value, we tended
to conclude that the enclaves might be formed under more reductive conditions based on
Figure 12a.
Minerals 2022,12, 1458 16 of 23
Minerals 2022, 12, x FOR PEER REVIEW 16 of 24
In mineralogy, Type-I arfvedsonite is an important petrogenetic indicator of alkaline
magma, which forms in the reducing condition. The Sayashk hydrothermal Sn deposit
occurring in the arfvedsonite granite in this area also reflects the low magmatic oxygen
fugacity [82]. A large number of metal sulfides and oxides such as pyrrhotite, chalcopyrite,
and ilmenite are developed in the graphite orbicules with rarely magnetite content, which
also reflects the characteristics of relatively low oxygen fugacity. However, magnetite is
also rare in the enclaves and the Fe3+ content is very low in the component parameters
(Table S1). It does not show high oxygen fugacity characteristics, which is obviously am-
biguous with the high oxygen fugacity of Type-III amphiboles shown in Figure 11. Am-
phibole is the main carrier of Mg in magma. During magma evolution, Mg is a refractory
element that tends to reside in residual melt, whereas Fe readily migrates into differenti-
ated minerals [83]. During magma mixing, the intermediate-basic magma end member
with higher Mg and Fe contents evolved as the original melt. As a result, the Mg tends to
remain in the amphiboles in enclaves, showing lower Fe# characteristics. The Fe tends to
migrate into felsic magma to form minerals with higher Fe, such as arfvedsonite and il-
menite. Therefore, we used the characteristics of Fe3+/(Fe2+ + Fe3+) as the criterion. The en-
claves were formed in a lower oxygen fugacity environment similar to the arfvedsonite
granite.
Similar to those in amphiboles, the Fe2+, Fe3+, and Mg contents of biotite can also be
used to estimate the oxygen fugacity of the magma and fluids [73]. In the ternary Fe2+-Fe3+-
Mg plot (Figure 12a), the biotite in graphite orbicules is distributed near the NNO evolu-
tion line showing low oxygen fugacity, indicating it formed in a relatively reducing envi-
ronment, which is consistent with the estimated results of amphibole. However, the bio-
tite sample in the enclaves was plotted below the QFM evolution line due to its very low
Fe3+ value, showing extremely low oxygen fugacity. In the Logf(O2)–T diagram under PH2O
= 207.0 Mpa (Figure 12b), most samples are plotted near the NNO evolution line including
the biotite in enclaves. Considering that the Fe3+/Fe2+ value is more sensitive to oxygen
fugacity but Figure 12b only considered the influence of the Fe# value, we tended to con-
clude that the enclaves might be formed under more reductive conditions based on Figure
12a.
Figure 12. (a) Ternary Fe3+-Fe2+-Mg plot for the Huangyangshan biotite (after [73]). (b) Temperature
(T) vs. Log(fO2) for the Huangyangshan biotite (after [73]).
5.2. Magma Mixing and Evolution
The mixing of mafic and felsic magma is considered to be one of the most important
mechanisms for forming enclaves characterized by the igneous textural features, finer
Figure 12.
(
a
) Ternary Fe
3+
-Fe
2+
-Mg plot for the Huangyangshan biotite (after [
73
]). (
b
) Temperature
(T) vs. Log(fO2) for the Huangyangshan biotite (after [73]).
5.2. Magma Mixing and Evolution
The mixing of mafic and felsic magma is considered to be one of the most important
mechanisms for forming enclaves characterized by the igneous textural features, finer grain
size, and chilled margins in granites [
84
,
85
]. Up to now, some researchers have proved
that the dark inclusions in the Huangyangshan pluton suggest magma mixing resulted
from the injection of less differentiated mantle-derived intermediate–basic magma into
the highly differentiated alkaline granitic magma [
14
,
19
,
58
]. The present study provides
more evidence as follows: (1) The Type-II richterite phenocrysts at the contact between
the enclaves and granites have Ca, Na, and K contents similar to the Type-III fine-grained
richterite in enclaves (Figure 4c,e,f) but Mg and Fe contents and a crystalline form similar to
the Type-I arfvedsonite phenocrysts in granites (Figure 4b,d), suggesting clear transitional
characteristics. (2) Although the cores of graphite orbicules are characterized by neutral
rocks with high contents of plagioclase and mafic minerals, the biotites in them have the
feature of typical A-type granite (high Fe content). (3) The columnar and acicular shapes of
Type-III amphibole indicate a rapid quenching possibly during fluctuations of temperature
or water content of the magma [
55
,
86
]. (4) Amphiboles in dioritic-mafic rocks are commonly
rich in Ca and Mg, rarely appearing Na-rich amphiboles. However, the type of amphiboles
developed in dioritic enclaves is Na-rich richterite. (5) In the same magmatic system, the
stronger the acidity of the rock, the higher the content of the REE. However, REE contents of
Type-II, -III, and -V amphiboles in enclaves and graphite orbicules are significantly higher
than Type-I amphiboles in granites (Figure 6a), indicating their different magma sources.
(6) The pyroxene phenocrysts wrapped by amphibole and biotite indicates that they may
be the residue of mafic magma crystallization in the early stage of magmatic evolution. (7)
The reversely zoned structure developed in the pyroxenes indicate that the magma may
have experienced partial mixing of basic mantle-derived magma during its evolution and
then mixed with acidic crust-derived magma (Figure 9). (8) Plagioclases in orebody matrix
have obvious oligoclase to andesine cores and albite rims, indicating the trend of magma
transition from neutral to acidic. This partly reflects the contamination by a felsic magma.
The cores of graphite orbicules and the host granites have obvious different mineral
compositions, indicating they formed in different conditions. The type of amphibole in
the granites, enclaves, and graphite orbicules is sodic, calcic–sodic, and calcic amphibole,
respectively, indicating that their forming environment or parent magma was obviously
different. In Si-saturated alkaline magma, the evolution sequence of amphiboles is generally
from katophorite to winchite and then to ferrorichterite from early to late. Alkaline types
Minerals 2022,12, 1458 17 of 23
such as arfvedsonite and riebeckite are formed at the latest stage of crystallization [
87
].
With the evolution of magma, the quartz content and alkalinity of amphiboles will increase.
The existence of arfvedsonite in granites indicates that the magma has reached to the latest
stage. The Type-III ferrorichterite contains higher SiO
2
but lower (Na + K)
A
contents than
Type-I arfvedsonite. The geochemical characteristics of minerals and whole-rock show that
enclaves inherited the features of host A-type alkaline granitic magma [14,19], suggesting
that the enclaves have experienced sufficient magma intermingling during their formation.
Although showing the same dioritic rock characteristics dominated by plagioclase and am-
phiboles, the Type-III ferrorichterite in enclaves has more characteristics of crust source and
alkaline magma compared with Type-V ferrohornblende in graphite orbicules, indicating
the enclaves were formed in the environment with a higher magma mixing degree or in
the magma with higher replenishment of acid magma. In addition, the graphite orbicules
contain euhedral, coarse-grained amphibole, plagioclase, and clinopyroxene, whereas the
enclaves consist of fine-grained, from subhedral to anhedral ferrorichterite, and plagio-
clase, indicating that the cores of the graphite orbicules formed in a deep environment of
higher temperature and pressure. This is also consistent with the estimated amphibole
temperature and pressure mentioned above. In summary, it can be concluded that the
granites and the enclaves were formed in the latest magmatic evolution stage at the same
time. The crystallization temperature and pressure of cores of graphite orbicules are higher
and the characteristics of alkaline magma are weaker, indicating that they crystallized
relatively earlier.
The rebalancing process below the solidus will not significantly change the geochem-
ical characteristics of the amphiboles. Therefore, its composition, especially the REEs
and incompatible elements, can be used to explore the properties and petrogenesis of
the parent magma. Different types of amphibole in the Huangyangshan deposit all have
extremely negative Eu anomalies, which is consistent with the whole-rock compositions
(Figure 6a). It suggests that the magma experienced strong plagioclase crystallization and
differentiation when amphibole crystallization, which is similar to the characteristics of
acid rocks originated from mantle-derived magma [
88
]. Whereas the enclaves have rela-
tively weak negative Eu anomalies compared with these amphiboles, indicating that these
amphiboles crystallized later than plagioclase. The Ce is more incompatible than Pb [
60
],
so the lg(CE/Pb) value of the amphibole can reflect the degree of crustal contamination.
From Type-V to Type-I amphiboles, the lg(CE/Pb) value decreases (Figure 13), suggesting
the proportion of crust-derived acidic magma increased gradually. Besides, Sc, V, and Cr
are transition metals that are usually enriched in mafic, untra-mafic rocks, and depleted in
acidic and alkaline rocks [
89
]. The Sc, V, and Cr contents of Type-IV and -V amphiboles
are obviously higher than those of Type-I to -III amphiboles (Table S1), further showing
the melt differentiation was weaker during the formation of cores of graphite orbicules.
Therefore, the mixing of mantle-derived mafic magma has an important influence on the
evolution and differentiation of magma.
One of the critical issues is that the dioritic cores surrounded by a large amount of
graphite and the dioritic enclaves without graphite come from the same intermediate–basic
parental magma or not. Generally, the REEs are lower in mafic and untra-mafic rocks and
higher in acidic and alkaline rocks [
60
]. According to the above discussion, the parental
magma of enclaves and granites had experienced sufficient differentiation and mixing.
If the parent magma was from the same source, the amphiboles in the cores of graphite
orbicules were supposed to have lower REEs. However, the REE contents of Type-IV
and -V amphiboles are much higher than the others (Figure 6a). In addition, some early
crystallized coarse-grained clinopyroxene and plagioclase phenocrysts are developed in
the cores of graphite orbicules, whereas only fine-grained amphibole, plagioclase, and a
small amount of biotite occurred in the enclaves. The mineral assemblage of amphibole and
some biotite is a common cumulate result of andesitic melt [
90
,
91
]. The typical cumulate
rocks formed by basaltic melt are gabbro and the main minerals are clinopyroxene and
plagioclase [
92
]. Therefore, the parental magma of graphite orbicules cores may came from
Minerals 2022,12, 1458 18 of 23
basaltic melt, while the enclaves were more likely derived from andesitic melt. Thirdly,
some magmatic metal sulfides (pyrrhotite, chalcopyrite, and pentlandite assemblages) are
developed along with graphite. The Type-V amphibole in the graphite orbicules has a
higher oxygen fugacity (higher Fe
3+
content) and more mantle-derived components than
Type-III richterite in enclaves (Figure 11) and the biotite also has higher oxygen fugacity
(Figure 12). Therefore, it can be concluded that the enclaves and the cores of graphite
orbicules may not originated from the same parental magma. The basic parent magma that
formed graphite orbicules might be characterized by rich metal elements, rich volatiles and
high oxygen fugacity that were easier to crystallize sulfides. This may be one of the reasons
for the crystallization of graphite.
Minerals 2022, 12, x FOR PEER REVIEW 18 of 24
The rebalancing process below the solidus will not significantly change the geochem-
ical characteristics of the amphiboles. Therefore, its composition, especially the REEs and
incompatible elements, can be used to explore the properties and petrogenesis of the par-
ent magma. Different types of amphibole in the Huangyangshan deposit all have ex-
tremely negative Eu anomalies, which is consistent with the whole-rock compositions
(Figure 6a). It suggests that the magma experienced strong plagioclase crystallization and
differentiation when amphibole crystallization, which is similar to the characteristics of
acid rocks originated from mantle-derived magma [88]. Whereas the enclaves have rela-
tively weak negative Eu anomalies compared with these amphiboles, indicating that these
amphiboles crystallized later than plagioclase. The Ce is more incompatible than Pb [60],
so the lg(CE/Pb) value of the amphibole can reflect the degree of crustal contamination.
From Type-V to Type-I amphiboles, the lg(CE/Pb) value decreases (Figure 13), suggesting
the proportion of crust-derived acidic magma increased gradually. Besides, Sc, V, and Cr
are transition metals that are usually enriched in mafic, untra-mafic rocks, and depleted
in acidic and alkaline rocks [89]. The Sc, V, and Cr contents of Type-IV and -V amphiboles
are obviously higher than those of Type-I to -III amphiboles (Table S1), further showing
the melt differentiation was weaker during the formation of cores of graphite orbicules.
Therefore, the mixing of mantle-derived mafic magma has an important influence on the
evolution and differentiation of magma.
Figure 13. Plots of lg(Nb) vs. lg(Ce/Pb) (a) and lg(Ce/Pb) vs. lg(Nb/U) (b) of amphiboles from the
Huangyangshan deposit. Symbols are as in Figure 4.
One of the critical issues is that the dioritic cores surrounded by a large amount of
graphite and the dioritic enclaves without graphite come from the same intermediate‒
basic parental magma or not. Generally, the REEs are lower in mafic and untra-mafic rocks
and higher in acidic and alkaline rocks [60]. According to the above discussion, the paren-
tal magma of enclaves and granites had experienced sufficient differentiation and mixing.
If the parent magma was from the same source, the amphiboles in the cores of graphite
orbicules were supposed to have lower REEs. However, the REE contents of Type-IV and
-V amphiboles are much higher than the others (Figure 6a). In addition, some early crys-
tallized coarse-grained clinopyroxene and plagioclase phenocrysts are developed in the
cores of graphite orbicules, whereas only fine-grained amphibole, plagioclase, and a small
amount of biotite occurred in the enclaves. The mineral assemblage of amphibole and
some biotite is a common cumulate result of andesitic melt [90,91]. The typical cumulate
Figure 13.
Plots of lg(Nb) vs. lg(Ce/Pb) (
a
) and lg(Ce/Pb) vs. lg(Nb/U) (
b
) of amphiboles from the
Huangyangshan deposit. Symbols are as in Figure 4.
5.3. Implications for the Graphite Mineralization
During the evolution of magma, pyroxene, amphibole, and biotite in the Huangyang-
shan pluton crystallized successively with estimated crystallization temperatures of
840–1012 ◦C
, 681–761
◦
C, and 658–720
◦
C, respectively. Their sources also evolved from the
mantle source (pyroxene) to the crust–mantle mixed source (amphibole) and then to the
crust source (biotite), suggesting the temperature changing trend and evolution process of
magma. The early crystallized pyroxenes wrapped in the graphite orbicules were formed
in an intermediate–basic magmatic environment. According to petrographic observation,
the graphite is hardly direct contacts the pyroxenes, indicating that no graphite deposition
occurred before the high-temperature mafic magma endmembers injected into the acid
magma chamber. For melt or hydrothermal systems, graphite is usually deposited under
relatively reducing conditions or precipitated and crystallized due to the reduction in the
system [
93
,
94
]. The Huangyangshan pluton has significantly low oxygen fugacity and both
amphibole and biotite were formed in a low oxygen fugacity environment. The reduction
process of magma could be triggered by the mixing of mafic magma with the high oxygen
fugacity and alkaline felsic magma.
At present, Raman graphite geothermometers have been widely used in order to
determine the metamorphic grade and crystallization temperature of graphite [
95
–
98
].
Sun et al. [
17
] reported the crystallization temperature of the Huangyangshan graphite
calculated by Raman geothermometers to be 750–800◦C. It is higher than the temperature
of amphibole and biotite and lower than that of pyroxene, which is also consistent with
Minerals 2022,12, 1458 19 of 23
the petrographic observation. Sun et al. [
17
] also concluded the graphite crystallization
temperature is similar to that of magmatic graphite deposit around the world. Therefore, it
can be inferred that the Huangyangshan graphite might be crystallized at the end of magma
mixing and before entering the low oxygen fugacity environment of the magma condition.
In a low oxygen fugacity environment, carbon mainly occurs in theform of methane [
11
].
There are two main processes for the formation of graphite from methane: oxidation (CH
4
+ O
2→
C + 2H
2
O) and decomposition (CH
4→
C + 2H
2
). They both require the mixing of
oxidizing melt/fluid to drastically change the physical and chemical conditions of the sys-
tem [
26
]. However, no evidence of foreign melts/fluids mixing in the late magmatic stage
has been found so far, so the mineralization ability of the late magma needs to be further
confirmed. Nonetheless, the genetic relationship between the graphite mineralization and
magma mixing can still be identified by the coexisted sulfides with graphite and composi-
tional differences of amphibole and biotite in the granites and graphite ores. Combined
with the crystallization temperature of graphite, therefore, graphite mineralization might
be occurred from the late stage of magma mixing to the late stage of magmatism.
6. Conclusions
The amphibole in the Huangyangshan graphite deposit can be divided into five types.
It generally has low Al and high Ti, K, Si, and Fe contents and has REE patterns with
negative Eu anomalies and element distribution patterns similar to granites. The primary
biotite in the granites, enclaves, and graphite orbicules belongs to Fe-biotite, which has the
characteristics of high Si and Fe and low Al and Mg contents. The pyroxene phenocrysts in
graphite orbicules develop significant zonal structures and are characterized by high Si and
low Ca and Fe contents. The plagioclase phenocrysts are mainly oligoclase and andesine,
with positive and oscillating zonal structures.
The crystallization temperature of pyroxene, Type-V amphibole, and primary biotite
were 840–1012
◦
C, 681–761
◦
C, and 658–720
◦
C, respectively, suggesting their crystallization
order. The magma pressure and depth were 157–220 Mpa and 5.95–8.32 km, respectively.
Both the amphibole and biotite crystallized in a reducing environment with an extremely
low oxygen fugacity.
The element compositions of amphibole, biotite, pyroxene, and plagioclase suggest
that the Huangyangshan pluton has experienced significant mixing of mafic mantle-derived
magma and felsic crust-derived magma. The graphite mineralization might be occurred
from the late stage of magma mixing to the late stage of magmatism.
Supplementary Materials:
The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/min12111458/s1, Table S1: EPMA results of amphibole in the
Huangyangshan pluton; Table S2: LA-ICP-MS results of amphibole; Table S3: EPMA results of biotite;
Table S4: EPMA results of pyroxene; Table S5: EPMA results of plagioclase.
Author Contributions:
Conceptualization, X.S.; Data curation, X.S., J.L. and M.H.; Formal analy-
sis, X.S.; Funding acquisition, Y.R.; Investigation, X.S., Y.R., J.L., Z.S. and Z.L.; Methodology, X.S.;
Project administration, Y.R.; Software, X.S. and M.H.; Supervision, Y.R.; Writing—original draft, X.S.;
Writing—review and editing, Y.R. and J.L. All authors have read and agreed to the published version
of the manuscript.
Funding:
This research was funded by the Geological Survey project of the China Geological Survey
(No. DD20190159).
Data Availability Statement:
All data generated or used in the study are contained in the submit-
ted article.
Acknowledgments:
We sincerely appreciate Xiaolin Zhang and Chunjiang Wu from Xinjiang Branch
of China National Geological Exploration Center of Building Materials Industry and Chengyang
Wang from Institute of Disaster Prevention for their support of our fieldwork.
Conflicts of Interest: The authors declare no conflict of interest.
Minerals 2022,12, 1458 20 of 23
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