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Article

Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation

by
Zhenghao Sun
1,2,3,
Kezhang Qin
1,2,3,*,
Yajing Mao
3,
Dongmei Tang
3,
Fangyue Wang
4,
Noreen J. Evans
5 and
Qifeng Zhou
6
1
Xinjiang Research Center for Mineral Resource, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China
2
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
3
Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
4
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
5
School of Earth and Planetary Sciences/John de Laeter Centre, Curtin University, Perth 6845, Australia
6
Institute of Mineral Resources Research, China Metallurgical Geology Bureau, Beijing 101300, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(7), 785; https://0-doi-org.brum.beds.ac.uk/10.3390/min12070785
Submission received: 6 May 2022 / Revised: 14 June 2022 / Accepted: 17 June 2022 / Published: 21 June 2022
(This article belongs to the Special Issue Minerals of Alkaline Igneous Rocks: Chemical and Isotopic Features as Tracers of Magmatic Processes)
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Abstract

:
Alkaline rocks are generally enriched in rare metals (e.g., Nb, Ta, and Zr) and rare earth elements (REE), but the key factors controlling Nb-Ta-REE enrichment remain unclear. The Boziguoer Nb (Ta-Zr-Rb-REE) deposit in Southwest Tianshan (northern margin of Tarim Basin) is China’s largest, with reserves of 0.32 Mt Nb2O5 and 0.02 Mt Ta2O5. It is an alkaline felsic complex 4.45 km in length and 0.5–1.3 km in width, composed of alkalic granite and syenite, which can be subdivided into syenite I and syenite II. The main minerals in each lithofacies are the same (albite, K-feldspar, quartz, arfvedsonite and aegirine). The Nb in the deposit is mainly hosted in pyrochlore supergroup minerals, ubiquitous in alkalic granite and syenite of the Boziguoer deposit. The wide variation in cations (Ca, Na, REE, U, Th) in the A-site further classifies the Boziguoer pyrochlore supergroup minerals as fluornatropyrochlore, fluorcalciopyrochlore and fluorkenopyrochlore. All Boziguoer pyrochlore supergroup minerals are Nb-rich and Ta-poor at the B-site and dominated by F at the Y-site. These cation occurrence illustrate a new mechanism of substitution in the Boziguoer pyrochlore supergroup minerals (2Ca2+ +Ti4+ +4Ta5+ = REE3+ +A-V + 5Nb5+, where A-V is the A-site vacancy). This substitution mechanism is different from that in the pyrochlore supergroup minerals from other rocks such as carbonatite and nepheline syenite, which are dominated by the replacement of Ba (Rb, Sr) with Ca+ Na + A-V. In addition, the substitution of REE (mainly La, Ce) for Ca in the Boziguoer pyrochlore supergroup minerals is likely a result of either REE enrichment or a change in the REE partition coefficient during the evolution of the alkaline magma. Both the pyrochlore supergroup minerals and their host rocks display negative large ion lithophile element (LILE; K, Rb, Sr, and Ba) anomalies, positive high-field-strength element (HFSE) anomalies and light rare earth element (LREE) enrichment with negative Eu anomalies. This is consistent with the crystallization of the pyrochlore supergroup minerals from the magma rather than from hydrothermal fluids, suggesting a magmatic origin. These findings indicate that the mechanisms of pyrochlore supergroup minerals crystallization in alkaline magma may be significantly different from those in carbonatite and nepheline syenite, and that magmatic differentiation processes may have played a role in the enrichment of the Boziguoer deposit by Nb.

1. Introduction

Niobium is a critical metal with many important uses [1,2], and while there are numerous studies focused on carbonatite- and granitic pegmatite-related Nb deposits [3,4,5,6,7], Nb deposits associated with alkaline rocks show the opposite trend. Thus, the key factors controlling niobium mineralization in alkaline rocks are relatively poorly understood. Alkaline rocks are commonly enriched in rare metals, high-field-strength elements (HFSE; Nb and Ta), rare earth elements (REE) and radioactive elements [8,9,10,11,12]. The main issue regarding Nb enrichment in these rocks is whether the minerals hosting the rare metal were crystallized at the magmatic stage or formed during post-magmatic alteration processes, when hydrothermal fluids remobilized and enriched the rare metal element cargo of the primary magma [13,14,15,16]. The Boziguoer Nb-Ta-Zr-Rb-REE deposit in Southwest Tianshan, NW China, was discovered in 1998 and is currently China’s largest Nb deposit. It is also a ‘type’ example of Nb enrichment associated with alkaline magma. Boziguoer contains reserves of 0.32 Mt Nb2O5, 0.02 Mt Ta2O5 and 0.7 Mt ZrSiO4, and associated REE reserves of 0.2 Mt with 0.64 Mt Rb2O [17]. Moreover, it is associated with a 1100-km-long belt of alkaline rocks in SW Tianshan and is contemporaneous with Permian Tarim Plume activity (~280 Ma). These features mean that the studying of the Boziguoer deposit has both economic and scientific significance. According to field observations, some studies have proposed that Boziguoer Nb mineralization was associated with magmatic differentiation processes [18,19], whereas others suggest that later hydrothermal activity played an important role [20]. Resolving this genetic paradigm will have implications not only for Nb mineralization in the Boziguoer deposit, but also for our understanding of the whole of the NW Tarim alkalic intrusion belt.
Pyrochlore supergroup minerals are common ore phases in Nb deposits [21,22,23,24,25] and are the most important minerals hosting Nb in the Boziguoer deposit [17,26,27,28,29]. Previous work discovered a new pyrochlore supergroup mineral (fluornatropyrochlore) in the Boziguoer deposit [20]. The chemistry of pyrochlore group minerals can be used to elucidate the crystallization environment, equilibrium melt properties and magma evolution [23,30,31,32,33,34,35,36,37,38,39,40,41].
This study uses major and trace element analyses of pyrochlore supergroup minerals from different lithofacies of mineralized rock from the Boziguoer deposit in an effort to resolve the genesis of Nb mineralization. The compositional results are further compared with those from carbonatite and nepheline syenite in order to investigate the evolution of pyrochlore supergroup minerals’ composition during magma differentiation.

2. Regional Geology

The Boziguoer alkaline felsic rock-related Nb-Ta-Zr-Rb-REE deposit is located in the Southern Tianshan orogenic belt (Figure 1b), which is an important part of the Central Asia orogenic belt (CAOB, Figure 1a). The CAOB is a giant suture zone formed by the formation, evolution and extinction of the Ancient Asian Ocean among the Siberian craton, Sino-Korea plate and Tarim plate. It is a typical accretionary orogenic belt, characterized by the development of a large number of magmatic complexes [42,43]. The superposition of Permian mantle plumes in the Tarim basin resulted in complex metallogenesis [44,45,46,47,48], and Carboniferous–Permian magmatic rocks with associated porphyry-epithermal Cu-Au and Cu-Ni sulfide mineralization, particularly well developed [43,49,50,51,52,53,54,55,56,57,58]. The Southern Tianshan region of China consists of the Central Tianshan block, Southern Tianshan orogenic belt and the northern margin of the Tarim block, from north to south. The Central Tianshan block is an ancient continental microplate, whose basement rocks are primarily composed of middle Neoproterozoic gneiss, schist and marble. The basement rocks are directly covered by Sinian carbonates and moraines. In the south, the Central Tianshan block and Southern Tianshan orogenic belt are bounded by the Yili-Centre Tianshan Suture Fracture zone (YCSF). The Southern Tianshan orogenic belt is located between the Yili–Kazakhstan and Tarim–Karakum plates, and extends to the border of Xinjiang and Gansu in China, through Uzbekistan, Tajikistan, Kyrgyzstan and Kazakhstan. It is a collisional orogenic belt formed during the subduction and closure of the Southern Tianshan ocean basin in the late Precambrian. It then evolved gradually, accreting the continental blocks on either side and the complexes between them [59]. The composition of the Southern Tianshan orogenic belt is very complex, including not only the Precambrian basement rocks and oceanic accretion material, but also continental margin/shallow marine sediments from the northern margin of the Tarim block. The northern margin of the Tarim block is separated by the Tarim northern fault. The interior of the Tarim block is covered by desert, and most of the bedrock at the northern margin was covered by unconsolidated or semi-consolidated sedimentary rocks or sediments in the Meso-Cenozoic, with sparse outcrops in the middle and easternmost parts.
The northern margin of the Tarim basin and adjacent areas were intruded by a series of alkaline rocks in the late Carboniferous–Permian period (Figure 1b), including alkaline granite, quartz syenite, syenite, aegirine syenite and aegirine nepheline syenite. These extend from Atushi in the west, to the east of Yuli, and are distributed in a belt approximately 1100 km long [60,61]. Regional geological work is relatively weak, and the mineral resources discovered are very limited, including some rare metal and REE deposits. The host rock types are mainly alkaline granite, syenite, carbonate and alkaline pegmatite (accompanied by gem and jade mineralization) [62].
The Boziguoer deposit is the largest in this alkaline rock belt and considered the most representative of the type. It is located 43 km north of Baicheng county, east of the alkaline rock belt, on the southern slope of Hariketao Mountain in the western section of the Tianshan Mountain Range. Intrusive rocks are not well developed in this region, with only the Boziguoer alkaline felsic rock developed in the central parts. Volcanic rocks are relatively well represented, and volcanic activity can be divided into three cycles: Silurian, Early Carboniferous and Early Permian.

3. Geology of the Boziguoer Large-Scale Deposit

The exposed strata in the Boziguoer mining area comprise the Silurian Qiongkusitai Formation (S3g), with gray marble, argillaceous siltstone, biotite quartz schist and crystalline limestone. Fold structures are not well developed, and the stratum can generally be described as having a monoclinal structure with an east–west strike and a northward dip. The Boziguoer complex is an east–west fish-shaped stock that intrudes into the upper Silurian marble (Figure 2). It is around 4.45 km long and 0.5–1.3 km wide, covering an area of approximately 2.6 km2. The complex is composed of biotite alkali feldspar granite (east) and alkaline felsic rock containing aegirine and arfvedsonite (west). The alkaline felsic rock is the main ore-bearing rock and is composed of multiple lithofacies. Our pyrochlore supergroup mineral-bearing samples were all collected from the alkaline felsic rock. The boundary between ore-bearing rock and biotite alkaline granite is the NNE-trending fault (F2), with an NWW-trending fault (F3) and Silurian Qiongkushitai Formation marble to the south (Figure 2).
The ore-bearing rock is an elliptic stock distributed in the central and western part of the ore district. It is approximately 1.2 km long, 1 km wide and around 1.2 km2 in area. The strike is NWW, and the dip direction is NNE at an angle of around 70°. The whole western intrusion was mineralized with Nb, accompanied by Ta, Rb, Zr and REE [14]. The ore-bearing alkaline felsic rock can be divided into three types: aegirine arfvedsonite alkaline feldspar granite (alkalic granite facies), aegirine arfvedsonite quartz alkaline feldspar syenite (syenite facies I) and aegirine arfvedsonite alkaline feldspar syenite (syenite facies Ⅱ) [17]. A detailed description of each is given below. There were no obvious lithologic boundaries and cross-cutting relationships between lithofacies noted in the field, so a gradual transition is inferred. The relative positions of different lithofacies can be seen from the sampling positions (Figure 2). Previous zircon U-Pb geochronology on ore-bearing rock showed that the mineralized rock was formed in the Early Permian (280.9–291.6 Ma [27]; 290.1 ± 1.4 Ma [63]). Alteration is relatively weak, mainly including chloritization, epidotization, fluoridation and limonitization. According to preliminary evaluation, the Boziguoer deposit has reserves of 0.32 Mt Nb2O5 at 0.0418 wt.%, 0.02 Mt Ta2O5 at 0.0028 wt.%, 0.64 Mt Rb2O at 0.0831 wt.%, 0.7 Mt ZrSiO4 and 0.2 Mt ΣREE at 0.0765 wt.%. While Nb mineralization dominates, Boziguoer is considered to be a super-large Nb-Ta-Zr-Rb-REE deposit [17,61].
Detailed petrographic features of the mineralized rock lithofacies are given in an earlier study [17] and summarized below. The alkalic granite facies (Figure 3a,b) has the highest content of quartz (20%–25%) and the least arfvedsonite (5%–10%) and aegirine (3%–5%). This massive, gray–white, medium–fine-grained rock also contains albite (35%–45%) and K-feldspar (20%–25%) [17]. Syenite facies I (Figure 3c,d) contains albite (45%–55%), K-feldspar (20%–30%), arfvedsonite (8%–15%), aegirine (5%–8%) and quartz (5%–8%). It has the same crystal size and massive structure as the alkalic granite facies but is slightly darker in color. Syenite facies II (Figure 3e,f) has the highest content of arfvedsonite (10%–15%) and aegirine (5%–8%) and the lowest content of quartz (0%–3%). It is the only rock with banding, formed by the alignment of dark alkaline minerals, with gray–white to light blue color.
The main minerals of each lithofacies are the same (albite, K-feldspar, quartz, arfvedsonite and aegirine; Figure 4a), but the relative abundance of minerals, crystal size and structure distinguish the lithofacies from one another. Quartz is mostly euhedral and subhedral. Albite occurs as euhedral, columnar and platy crystals with a composition of An <5.0 (unpublished data). K-feldspar is euhedral and columnar. Greenish blue, highly pleochroic arfvedsonite grains are allographic–subhedral columnar, and some are interstitial and enclose feldspar and quartz crystals. These grains have amphibole cleavage (56°). Green to black green aegirine is highly pleochroic, allographic–subhedral (0.1–0.5 mm) and always occurs with arfvedsonite. It also has pyroxene cleavage (87°). Semi-euhedral and euhedral 0.1–2.5 mm arfvedsonite grains are interstitial between alkali feldspar and quartz. The arfvedsonite and aegirine are intergrown, and their aggregates can form bands. The interstitial texture and inclusion of feldspar and quartz indicate that arfvedsonite and aegirine crystallized later than these minerals (Figure 4b,c).
Accessory minerals mainly comprise the pyrochlore supergroup minerals, zircon, astrophyllite, thorite, monazite, bastnasite, xenotime and fluorite. Most form either granular aggregates, single crystals between gangue mineral grains or are encased in early alkaline feldspar. The distribution of all accessory phases is disseminated, indicating late formation of the ore minerals. Combining our data (Table 1 and Supplementary Materials Table S1) with previous research results [28] shows that Nb and Ta are mainly hosted in the pyrochlore supergroup mineral, a small part of Nb occurs in astrophyllite, Rb and Cs are mainly hosted in astrophyllite, Zr is primarily found in zircon and REE are concentrated in monazite, xenotime and bastnaesite.
Table
Table 1. Chemical composition of pyrochlore group minerals from the Boziguoer Nb deposit.
Table 1. Chemical composition of pyrochlore group minerals from the Boziguoer Nb deposit.
NumberBZ-Pcl-1-1BZ-Pcl-1-2BZ-Pcl-1-3BZ-Pcl-1-4BZ-Pcl-1-5BZ-Pcl-1-6BZ-Pcl-1-7BZ-Pcl-1-8BZ-Pcl-1-9BZ-Pcl-1-10BZ-Pcl-1-11
RockAlkalic granite facies
F2.402.462.202.172.172.422.402.302.281.882.06
Na2O7.266.806.246.866.917.506.916.386.444.485.97
Ta2O53.273.693.823.813.683.943.902.073.533.103.05
SiO200000.0600001.190
Nb2O560.259.158.858.359.058.558.258.656.854.655.3
ThO20.130.230.220.130.280.250.270.820.850.430.69
UO24.654.665.615.775.354.514.354.445.819.088.45
CaO10.911.110.911.011.110.911.013.313.311.214.0
TiO24.925.235.235.075.155.055.816.777.007.167.83
La2O31.431.301.451.321.230.981.170.380.400.350.53
Ce2O33.373.473.413.203.303.753.441.881.861.111.70
Fe2O3000.010.11000000.290
Total98.5297.9797.8497.7898.2697.7897.5196.9998.2894.8299.61
Structural formula (∑B-site cations = 2)/apfu
Na0.890.830.770.850.850.930.840.770.780.540.73
Ca0.730.750.740.760.750.740.750.890.890.740.95
La0.030.030.030.030.030.020.030.010.010.010.01
Ce0.080.080.080.070.080.090.080.040.040.030.04
Th00000000.010.010.010.01
U0.070.070.080.080.080.060.060.060.080.130.12
∑A1.81.761.71.81.791.851.761.781.821.451.86
Nb1.711.691.681.681.691.691.661.651.611.531.58
Ta0.060.060.070.070.060.070.070.030.060.050.05
Ti0.230.250.250.240.240.240.280.320.330.330.37
Si0000000000.070
Fe0000.01000000.010
∑B22222222222
F=O0.480.490.440.440.430.490.480.450.450.370.41
A-V0.20.240.30.20.210.150.240.220.180.550.14
NumberBZ-Pcl-1-13BZ-Pcl-1-14BZ-Pcl-1-15BZ-Pcl-1-16BZ-Pcl-1-17BZ-Pcl-1-18BZ-Pcl-1-19BZ-Pcl-1-20BZ-Pcl-1-21BZ-Pcl-1-22
RockAlkalic granite facies
F1.891.762.012.081.782.071.871.210.771.01
Na2O6.324.874.455.735.846.016.036.035.865.88
Ta2O52.72.293.072.812.791.732.282.24.074.21
SiO20000.041.490.340.220.090.140.18
Nb2O555.657.856.856.153.857.257.457.757.256.8
ThO20.540.390.520.610.890.940.690.710.770.78
UO28.338.428.468.218.144.244.314.314.274.42
CaO14.214.114.413.513.115.816.316.315.815.6
TiO28.057.457.857.237.337.036.947.047.027
La2O30.320.10.190.410.520.650.590.540.580.72
Ce2O31.390.81.121.521.812.182.021.952.062.31
Fe2O30.010.210.050000.010.0200
Total99.3498.2298.8598.2497.4598.2798.6498.0798.5398.89
Structural formula (∑B-site cations = 2)/apfu
Na0.770.580.530.70.710.730.730.730.70.71
Ca0.950.930.950.920.881.061.091.091.051.04
La0.01000.010.010.020.010.010.010.02
Ce0.030.020.030.040.040.050.050.040.050.05
Th0.010.010.010.010.010.010.010.010.010.01
U0.120.120.120.120.110.060.060.060.060.06
∑A1.881.661.641.791.761.931.951.951.881.89
Nb1.571.611.581.611.521.621.621.631.61.59
Ta0.050.040.050.050.050.030.040.040.070.07
Ti0.380.340.360.340.340.330.330.330.330.33
Si00000.090.020.010.010.010.01
Fe00.0100000000
∑B2222222222
F=O0.370.340.390.420.350.410.370.240.150.2
A-V0.120.340.360.210.240.070.050.050.120.11
NumberBZ-Pcl-1-23BZ-Pcl-1-24BZ-Pcl-1-25BZ-Pcl-1-26BZ-Pcl-1-27BZ-Pcl-1-28BZ-Pcl-1-29BZ-Pcl-1-30BZ-Pcl-1-31BZ-Pcl-1-32BZ-Pcl-1-33
RockAlkalic granite facies
F2.012.251.711.721.882.011.610.730.711.051.76
Na2O6.426.395.595.585.065.375.745.815.815.635.79
Ta2O52.412.33.083.223.412.973.333.053.222.112.51
SiO20.220.662.851.711.870.390.180.110.090.20.1
Nb2O558.657.853.753.752.654.554.556.355.155.155.4
ThO20.170.290.590.680.940.730.860.840.810.70.79
UO26.736.068.158.978.519.368.798.598.648.88.39
CaO11.311.414.615.114.114.514.315.414.414.414.4
TiO25.075.167.537.497.27.757.537.417.557.647.53
La2O31.571.60.440.40.550.370.660.470.690.660.67
Ce2O33.363.471.321.221.741.261.931.151.981.991.95
Fe2O30.01000.020.0300.010.0200.020
Total97.8397.4199.5199.7797.9099.1399.4899.8398.9298.2299.32
Structural formula (∑B-site cations = 2)/apfu
Na0.80.790.650.670.610.660.710.710.710.70.71
Ca0.780.780.9310.940.980.981.030.980.980.98
La0.040.040.010.010.010.010.020.010.020.020.02
Ce0.080.080.030.030.040.030.050.030.050.050.05
Th000.010.010.010.010.010.010.010.010.01
U0.10.090.110.120.120.130.120.120.120.120.12
∑A1.791.781.731.831.741.821.891.911.891.871.89
Nb1.71.671.441.491.491.561.571.591.581.581.59
Ta0.040.040.050.050.060.050.060.050.060.040.04
Ti0.240.250.340.350.340.370.360.350.360.370.36
Si0.010.040.170.110.120.020.010.010.010.010.01
Fe00000000000
∑B22222222222
F=O0.410.450.320.340.370.40.320.140.140.210.35
A-V0.210.220.270.170.260.180.110.090.110.130.11
NumberBZ-Pcl-2-1BZ-Pcl-2-2BZ-Pcl-2-3BZ-Pcl-2-4BZ-Pcl-2-5BZ-Pcl-2-6BZ-Pcl-2-7BZ-Pcl-2-8BZ-Pcl-2-9BZ-Pcl-2-10
RockSyenite facies I
F3.113.083.13.613.153.193.133.223.363.21
Na2O5.124.693.462.155.285.435.325.315.485.65
Ta2O50.250.490.710.780.740.310.440.831.721.27
SiO21.211.051.092.110.881.352.951.330.510.78
Nb2O564.164.164.364.263.565.163.264.56362.1
ThO20.211.290.821.080.880.320.620.810.451.21
UO28.697.6610.1108.417.838.316.347.297.35
CaO9.429.118.818.288.899.058.969.368.158.1
TiO24.414.494.544.784.194.364.24.194.34.13
La2O30.390.530.260.290.180.690.530.310.711.11
Ce2O31.111.261.10.881.11.451.171.422.713.65
Fe2O30.420.380.230.390.020.060.060.040.310.35
Total98.4698.1598.4898.5697.1899.1398.9097.6297.9898.89
Structural formula (∑B-site cations = 2)/apfu
Na0.590.540.40.240.620.620.590.610.650.67
Ca0.60.580.560.50.580.570.550.590.530.53
La00.010.01000.010.0100.020.03
Ce0.020.030.020.020.020.030.020.030.060.08
Th00.020.010.010.0100.010.0100.02
U0.110.10.130.130.110.10.110.080.10.1
∑A1.331.271.130.91.361.341.31.331.351.43
Nb1.711.711.711.651.741.731.651.721.731.72
Ta00.010.010.010.01000.010.030.02
Ti0.20.20.20.20.190.190.180.190.20.19
Si0.070.060.060.120.050.080.170.080.030.05
Fe0.020.020.010.0200000.010.02
∑B2222222222
F=O0.580.580.580.650.610.590.570.60.650.62
A-V0.670.730.871.10.640.660.70.670.650.57
NumberBZ-Pcl-2-11BZ-Pcl-2-12BZ-Pcl-2-13BZ-Pcl-2-14BZ-Pcl-2-15BZ-Pcl-2-16BZ-Pcl-2-17BZ-Pcl-2-18BZ-Pcl-2-19BZ-Pcl-2-20
RockSyenite facies I
F3.173.033.283.063.152.993.023.213.183.2
Na2O2.755.354.693.034.083.954.825.94.115.45
Ta2O51.531.781.811.151.761.550.971.811.680.29
SiO21.280.750.532.690.51.060.70.860.481.36
Nb2O562.66361.762.761.763.96563.561.665.1
ThO20.780.430.551.831.10.671.990.41.170.29
UO21110.79.8910.510.311.46.336.7910.27.79
CaO8.418.278.47.88.418.78.168.358.399.01
TiO24.034.353.664.514.224.044.34.214.214.41
La2O30.550.441.770.490.50.230.510.580.480.72
Ce2O31.771.691.551.792.431.212.842.452.411.31
Fe2O30.220.150.240.20.190.110.070.110.140.03
Total98.1199.9198.0899.7198.2899.8098.6798.1398.0898.97
Structural formula (∑B-site cations = 2)/apfu
Na0.320.630.570.340.490.460.560.690.50.62
Ca0.540.540.570.480.560.560.520.540.560.57
La0.010.010.0400.0100.010.010.010.02
Ce0.040.040.040.040.060.030.060.050.060.03
Th0.01000.020.0200.030.010.020
U0.150.140.140.130.140.150.080.090.140.1
∑A1.081.361.351.011.281.21.261.391.281.33
Nb1.711.731.751.621.731.731.751.731.741.73
Ta0.030.030.030.020.030.030.020.030.030
Ti0.180.20.170.190.20.180.190.190.20.19
Si0.080.050.030.150.030.060.040.050.030.08
Fe0.0100.010.010.010000.010
∑B2222222222
F=O0.60.580.650.560.620.570.570.610.630.59
A-V0.920.640.650.990.720.80.740.610.720.67
NumberBZ-Pcl-3-1BZ-Pcl-3-2BZ-Pcl-3-3BZ-Pcl-3-4BZ-Pcl-3-5BZ-Pcl-3-6BZ-Pcl-3-7BZ-Pcl-3-8BZ-Pcl-3-9BZ-Pcl-3-10
RockSyenite facies II
F3.013.223.083.293.312.864.213.183.113.11
Na2O5.456.916.346.285.286.394.576.196.196.21
Ta2O50.640.850.910.861.141.771.890.621.521.28
SiO20.880.691.331.011.031.080.871.120.490.51
Nb2O564.965.867.167.365.965.167.966.865.866.6
ThO21.881.020.310.611.121.380.831.211.410.79
UO27.025.838.117.616.416.116.495.195.625.46
CaO1.281.611.171.311.521.411.251.481.521.69
TiO22.722.812.412.612.382.892.712.862.862.63
La2O32.012.121.220.771.682.350.622.111.191.95
Ce2O38.447.856.357.878.248.487.298.298.018.53
Fe2O30.110.130.160.080.250.020.030.010.360.14
Total98.3498.8698.5199.5598.2499.8698.6399.0198.1198.89
Structural formula (∑B-site cations = 2)/apfu
Na0.650.820.730.720.620.750.520.710.730.73
Ca0.080.110.070.080.10.090.080.090.10.11
La0.050.050.030.020.040.050.010.050.030.04
Ce0.190.180.140.170.180.190.160.180.180.19
Th0.030.01000.020.0200.020.020.01
U0.10.080.110.10.090.080.080.070.080.07
∑A1.11.241.071.11.041.180.851.121.121.16
Nb1.811.811.791.811.81.771.81.81.81.82
Ta0.010.010.010.010.020.030.030.010.020.02
Ti0.130.130.110.120.110.130.120.130.130.12
Si0.050.040.080.060.060.070.050.070.030.03
Fe000.0100.010000.020.01
∑B2222222222
F=O0.590.620.580.620.630.550.780.60.590.6
A-V0.90.760.930.90.960.821.150.880.880.84
NumberBZ-Pcl-3-11BZ-Pcl-3-12BZ-Pcl-3-13BZ-Pcl-3-14BZ-Pcl-3-15BZ-Pcl-3-16BZ-Pcl-3-17BZ-Pcl-3-18BZ-Pcl-3-19BZ-Pcl-3-20
RockSyenite facies II
F3.223.213.123.133.013.083.193.253.333.15
Na2O5.494.244.215.495.985.635.526.245.36.38
Ta2O51.411.130.981.520.930.711.380.61.120.92
SiO20.520.831.10.810.571.180.611.111.011.28
Nb2O565.367.667.666.965.766.665.366.865.967.2
ThO20.681.220.120.411.660.110.61.231.160.37
UO26.216.818.616.987.368.376.35.216.448.12
CaO1.781.521.421.471.511.421.821.531.51.2
TiO22.712.632.222.862.532.812.732.82.412.5
La2O32.591.691.191.692.111.782.572.231.71.31
Ce2O38.417.637.638.017.687.328.398.318.316.34
Fe2O30.180.070.090.140.120.230.140.110.260.18
Total98.4898.5998.3099.3799.1599.2098.5699.4298.4498.94
Structural formula (∑B-site cations)/apfu
Na0.650.490.490.630.710.650.660.720.620.73
Ca0.120.10.090.090.10.090.120.10.10.08
La0.060.040.030.040.050.040.060.050.040.03
Ce0.190.170.170.170.170.160.190.180.180.14
Th0.010.0200.010.0200.010.020.020
U0.080.090.110.090.10.110.090.070.090.11
∑A1.110.890.881.031.161.051.121.131.041.08
Nb1.811.811.821.791.831.781.811.81.81.79
Ta0.020.020.020.020.020.010.020.010.020.01
Ti0.130.120.10.130.120.130.130.130.110.11
Si0.030.050.070.050.040.070.040.070.060.08
Fe0.01000.010.010.010.0100.010.01
∑B2222222222
F=O0.630.60.590.590.590.580.620.610.640.59
A-V0.891.111.120.970.840.950.880.870.960.92
Pyrochlore supergroup minerals contain various types of elements. We analyzed the main elements, not trace elements (Pb, Nd, Sn, Sb, Y, Mg, Mn and W). The trace elements test (Table 2) was found to indicate a certain amount of Pb, Nd, Sn and W, and their combined oxide content can reach 1–2 wt.%. Therefore, the oxide content totals in this table are somewhat low.
Table
Table 2. Trace element content of pyrochlore supergroup minerals from the Boziguoer deposit.
Table 2. Trace element content of pyrochlore supergroup minerals from the Boziguoer deposit.
Sample No.BZ-Pcl-1-1BZ-Pcl-1-2BZ-Pcl-1-3BZ-Pcl-1-4BZ-Pcl-1-5BZ-Pcl-1-6BZ-Pcl-1-7BZ-Pcl-1-8BZ-Pcl-1-9BZ-Pcl-1-10
RockAlkalic granite facies
Li35.139.832.614.011.423.325.96.67.710.3
Sc0.190.260.100.090.120.070.340.330.160.25
V0.020.050.020.000.000.010.130.030.040.19
Cr15.72.881.103.522.1214.725.73.372.9411.7
Co0.040.090.110.060.060.000.070.060.050.05
Ni0.140.000.000.000.100.170.320.000.230.00
Cu7.131.881.531.081.151.263.102.070.976.21
Zn17780.130.26.0039.232.631610,63014737,803
Ga363342310298305328273228326317
Ge13612210910711311477.479.9105103
Rb3.277.223.341.402.804.495.1813.372.264.39
Sr633624678654672682370360366403
Y394119401507161523652173436450543918
Zr13128710412794.699.0245375227266
Nb389,676400,247401,359403,532399,678397,061375,443380,754381,211373,589
Mo10155.71051171111096.696.687.047.98
Sn881978847889906896587858663669
Cs2.848.472.121.414.022.054.526.864.726.81
Ba10.812.86.827.7010.379.7513.013.716.415.7
La6405557950354577498861583741318256034970
Ce19,18018,64117,26316,65617,60019,74211,24110,75516,50715,954
Pr2152205619051895198121671208124018141813
Nd7717710365696559705774483714408056525818
Sm13231126101910651138116765678110301139
Eu45.838.332.734.538.339.017.819.428.631.9
Gd648443405413476473324386511628
Tb11873.665.870.281.079.947.656.275.6101
Dy691384348377434427245279371537
Ho11958.253.057.066.265.932.637.846.573.6
Er38918115917520420386.8101110183
Tm42.620.215.818.922.822.77.169.528.6415.8
Yb22310975.597.413312936.155.641.587.0
Lu15.97.204.685.889.078.792.174.082.756.60
Hf26.155.914.113.013.915.412.945.040.315.4
Ta17,69714,00015,51514,18518,65217,98822,84526,34027,93324,964
W11811221167217821712164814218177741033
Pb14,31813,58613,83713,81314,61615,5019880865536359387
Th7261825260716238675468215190644475637306
U36,82437,37137,95638,00937,60538,93771,85182,54277,46173,864
Sample No.BZ-Pcl-2-1BZ-Pcl-2-2BZ-Pcl-2-3BZ-Pcl-2-4BZ-Pcl-2-5BZ-Pcl-3-1BZ-Pcl-3-2BZ-Pcl-3-3BZ-Pcl-3-4BZ-Pcl-3-5
RockSyenite facies ISyenite facies II
Li34.451.666.120.231.251.163.247.552.177.3
Sc0.210.160.300.290.180.370.310.430.240.33
V0.000.010.020.000.000.010.030.030.020.10
Cr13.23.742.133.785.2212.115.12.483.745.27
Co0.020.030.010.020.010.010.020.030.050.02
Ni0.040.000.000.000.100.120.110.050.250.02
Cu6.152.742.513.082.111.754.222.113.925.10
Zn14822578.297.855.274.592.310211397.5
Ga452551432392351312492531520602
Ge155172121112102131120122151136
Rb4.785.124.423.212.534.475.986.367.319.91
Sr733701698722688680751721702822
Y473123311811162323745920292522573261825
Zr313298253320253234322407351382
Nb432,610422,876446,103417,105423,114454,584463,879466,448435,879443,567
Mo74.358.567.274.5120113130122151102
Sn99295610221113985898112311029881223
Cs7.546.324.425.313.319.115.326.975.356.51
Ba9.316.215.328.325.559.3211.212.39.326.51
La7663659161124587500396128367755592298406
Ce22,45122,30120,64516,63217,60628,76327,95525,88729,60524,763
Pr2553245122611884200332153101286532482725
Nd9221849678546498710111,54810,631985411,1698479
Sm1573131112191082120519931679153017481548
Eu56.344.139.135.641.270.158.149.259.343.1
Gd781529479402485975659608711768
Tb14389.180.169.590.217211199.1118115
Dy8304684093964411056579524638561
Ho14271.262.959.365.318187.580.297.970.1
Er471220189181213589272.3241306163
Tm53.226.319.119.623.565.131.224.233.913.1
Yb27112892.199.1143329162.911519261.9
Lu19.18.535.96.2110.224.111.17.1413.43.92
Hf53.358.931.247.544.352.131.258.861.133.5
Ta13,44413,10515,96419,10420,48681017543920310,77811,541
W1271125315441631169217541812181517651475
Pb11,53410,53111,15712,54812,15413,52113,02214,55415,65814,987
Th6657689574588143802551038013748579856921
U74,65473,55169,53186,698102,598105,87478,54669,85472,45468,589
In all three lithofacies, the pyrochlore supergroup mineral grains primarily show an octahedral morphology or form irregular granular aggregates with imperfect cleavage. They are yellow, dark red or reddish brown under plane polarized light, and grain sizes range from 70 to 140 μm. The pyrochlore supergroup mineral’s grains are disseminated unevenly throughout other major minerals (Figure 4d–f), and as with other accessory phases, textural evidence indicates that they crystallized relatively late, after the crystallization of most other magmatic minerals, such as alkaline feldspars, quartz, arfvedsonite and aegirine.

4. Samples and Analytical Methods

We collected samples from three mineralized lithofacies and observed the texture and occurrence of the pyrochlore supergroup minerals using optical and scanning electron microscopy (SEM) combined with back-scattered electron (BSE) images. The major oxides of the pyrochlore supergroup minerals in alkalic granite facies, syenite facies I and syenite facies II were analyzed in 33, 20 and 20 grains, respectively. The trace element content was determined and compared to the trace element composition of the host rocks (previously published [17]).
Major oxide analysis was undertaken using wavelength-dispersive spectrometry on an EPMA-1600 electron probe at the Tianjin Geological Research Center, operating at an accelerating voltage of 15 kV with a 12 nA beam current, 5 μm beam, peak counting time of 20 s and background time of 10 s for major elements. A program based on the ZAF procedure was used for data reduction. Fe2+–Fe3+ redistribution from electron microprobe analyses was carried out using the general equation of Droop (1987) [64] for estimating Fe3+. The experimental results were corrected using ZAF, and the 1σ uncertainty was <10% when the element content was greater than 0.1 ppm.
Trace element abundance in the pyrochlore supergroup minerals was determined using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) applied to polished thick sections, at the In Situ Mineral Geochemistry Lab, Ore Deposit and Exploration Centre (ODEC), Hefei University of Technology, Hefei, Anhui Province, China. These analyses were undertaken using an Agilent 7900 Quadrupole ICP–MS coupled to a Photon Machines Analyte HE 193-nm ArF Excimer Laser Ablation system equipped with a SQUID signal smoothing device. Helium was used as carrier gas and mixed with Ar makeup gas via a T-connector before entering the ICP. Each analysis was performed at a spot diameter of 30 μm, laser pulse frequency of 8 Hz and with a laser energy of ~2 J cm−2 for 40 s after measuring a gas blank for 20 s. Standard reference glass materials GSE-1g, GSD-1g and BCR-2G were used as external standards to plot calibration curves using preferred element concentrations for the USGS reference glasses from the GeoReM database (http://georem.mpch-mainz.gwdg.de/. Accessed on 1 June 2022). Off-line data processing was undertaken using the ICPMS Data Cal software package [65], and trace element compositions of oxide minerals were calibrated against multiple reference materials using 57Fe for internal standardization. The analytical uncertainty of the trace elements determined during this study is 10%.

5. Analytical Results

5.1. Major Element Composition of Pyrochlore

The pyrochlore supergroup mineral’s general formula is A2−mB2×6−wY1−n [31,66]. The A-site is occupied by large cations such as As, Ba, Bi, Ca, Cs, K, Mg, Mn, Na, Pb, REE, Sb, Sr, Th, U and Y. The CaO content of the Boziguoer pyrochlore supergroup minerals ranges from 10 to 16 wt.% in the alkalic granite facies, 7 to 9 wt.% in syenite facies I and 1.1 to 1.7 wt.% in syenite facies II (Table 1). The REE are mainly La and Ce, with La2O3 and Ce2O3 respectively ranging from 0.1 to 1.6 wt.% and 0.1 to 3.5 wt.% in the alkalic granite facies, 0 to 2 wt.% and 1 to 3.6 wt.% in syenite facies I and 0.5 to 2.6 wt.% and 6.4 to 8.5 wt.% in syenite facies II. There is little difference in Na2O, ThO2 and UO2 content in the pyrochlore supergroup minerals from the three lithofacies.
The B-site accommodates smaller and highly charged cations such as Nb, Ta, Ti, Zr, Fe3+, Al and Si [36] and rarely W5+ [67]. The Nb2O5 content varies from 53 to 60 wt.% in the alkalic granite facies, from 62 to 65 wt.% in syenite facies I and from 65 to 68 wt.% in syenite facies II. The TiO2 content in the alkalic granite, syenite facies I and syenite facies II ranges from 5 to 8 wt.%, 4 to 5 wt.% and 2 to 3 wt.%, respectively. Most analyses yielded low content of Ta2O5, SiO2 and Fe2O3 (Table 1).
The Y- and X-site anions can be O, OH and F. The F content in the three lithofacies varies from 1 to 2.4 wt.% in the alkalic granite facies, 3 to 3.6 wt.% in syenite facies I and 3 to 4.3 wt.% in syenite facies II.
The structural formula for the Boziguoer pyrochlore supergroup minerals has been calculated in terms of two cations in the B-site [68]. Vacancies are common in the A- and Y-sites. The calculated results show that the A-site has vacancies (A-V) varying from 0.05 to 0.96 apfu in the alkalic granite facies, 0.57 to 1.1 apfu in syenite facies I and 0.76 to 1.15 apfu in syenite facies II. There is a strong negative correlation between Nb and Ti, Ta and Ca (Figure 5a–c). Such trends indicate that substitution is Nb for Ti, Ta and Ca. There is a positive correlation between Nb and F, La + Ce and A vacancies, but Na and Nb are not correlated (not shown). The variation plot between substitution mechanisms (Figure 6) shows a trend with a slope of −1.0006, indicating that it can quantitatively describe the substitution mechanism of the pyrochlore supergroup minerals in the lithofacies at Boziguoer.

5.2. Classification of Pyrochlore in the Boziguoer Deposit

The results show that the Boziguoer pyrochlore supergroup minerals belong to the general pyrochlore supergroup (Figure 7). Five types of pyrochlore group minerals (pyrochlore, microlite, roméite, betafite and elsmoreite) have been defined, based on the atomic proportion of B-site atoms Nb, Ta, Sb, Ti and W [66]. All varieties in the Boziguoer deposit belong to the pyrochlore type, with Ti varying within roughly the same range as that in pyrochlore supergroup minerals from pegmatites associated with alkali granites and albitized granites (Figure 7). They are also similar to pyrochlore supergroup minerals from nepheline syenites, which also have very low Ta concentrations (atomic Nb:Ta ratio ranges from 10 to 1000). The low Ta content is the most conspicuous geochemical difference between pyrochlore supergroup minerals in alkaline rocks and those from carbonatite and granite pegmatites (Figure 7).
Pyrochlore subgroup minerals are further classified based on the dominant valences at the A- and Y-sites [20,66]. In the Boziguoer deposit, the A-site in the pyrochlore supergroup minerals is occupied by Ca, Na and vacancies (Figure 8), while the Y-sites mainly host F anions (WF/% is 0.71–4.21, the average is 2.56). Thus, fluorcalciopyrochlore, fluorkenopyrochlore and fluornatropyrochlore were identified in Boziguoer. The pyrochlore supergroup minerals in the alkalic granite facies are mainly fluorcalciopyrochlore, with a small amount of fluornatropyrochlore. Syenite facies I and syenite facies II mainly contain fluorkenopyrochlore, and the syenite facies II grains have more A-site vacancies and higher Na/Ca than those from syenite facies I (Figure 8).

5.3. Trace Element Composition of Pyrochlore

Trace element data for Boziguoer pyrochlore supergroup minerals are given in Table 2. The primitive mantle-normalized trace element and chondrite-normalized REE patterns are similar to those of the host rock (Figure 9), displaying negative large ion lithophile element (LILE; Rb, Sr and Ba) anomalies, and positive high-field-strength element (HFSE) and light rare earth element (LREE) anomalies. Negative Eu anomalies occur in both the minerals and host rock, but the normalized total trace element and total REE element content in the pyrochlore supergroup minerals is higher than that in the host rock. Rb and Ba are more depleted in pyrochlore supergroup minerals than in the host rock, but contains more U, Th, Nb and Ta. The REE and trace element distribution characteristics of the three lithofacies are roughly consistent. However, the total amount of REE and trace elements in the two syenite facies is slightly higher than that in the alkalic granite facies rocks.

6. Discussion

6.1. Substitution of Elements in Pyrochlore Supergroup Minerals

There are several elemental substitution mechanisms established for the pyrochlore supergroup minerals from carbonatite and nepheline syenite [25,32,33,37,38,68,72,73,74,75,76,77]; however, there are few studies on the pyrochlore supergroup minerals in alkaline felsic rocks. The substitution of elements in the Boziguoer pyrochlore supergroup minerals involves the new mineral, fluornatropyrochlore. Therefore, it is worth comparing the substitution mechanisms in the pyrochlore supergroup minerals from Boziguoer with that from other rock types.
During magmatic evolution, the pyrochlore supergroup minerals from carbonatite become progressively enriched in Na, Ca, F and Nb and depleted in Ta, Th, REE, Ti and U [33,37]. Therefore, the pyrochlore supergroup minerals’ Nb/Ta ratio in carbonatite increases with the degree of magmatic evolution. Similarly, pyrochlore supergroup minerals from Boziguoer are enriched in Nb and F and depleted in Ta and Ti with magmatic evolution. However, there is a difference between Boziguoer and carbonatite pyrochlore supergroup minerals with respect to the elements occupying the A-site. Pyrochlore supergroup minerals from carbonatites from the Lueshe, Niocan and Bond Zone, as well as nepheline syenite from the Lovozero, plot in all three fields on a Ca-Na-A vacancy ternary plot: magmatic, hydrothermal and supergene (Figure 8). The compositional evolution from calciopyrochlore and natropyrochlore toward kenopyrochlore (black arrow in Figure 8) is related to the exchange of Ba (Rb, Sr) for Ca+ Na and the consequent vacancy in the A-site [34,36]. Similar trends can be found in the Bingo pyrochlore supergroup minerals [32], which are a product of weathering, and in those from the Kola carbonatites [73], derived from supergene or low-temperature hydrothermal alteration. At Boziguoer, the trend from calciopyrochlore and natropyrochlore toward kenopyrochlore (red arrow in Figure 8) is related to the exchange of REE (La, Ce) for Ca, and the consequent vacancy in the A-site (Figure 6). Therefore, an evolutionary trend for the pyrochlore supergroup minerals crystallized from the Boziguoer alkaline felsic magma can be defined (red arrow to red circle in Figure 8). This compositional evolution is in contrast to that defined for the pyrochlore supergroup minerals crystallized from carbonatites and nepheline syenites. During magmatic evolution at Boziguoer, the variation in elements in the pyrochlore supergroup minerals changed as follows: B-site—Nb increased, Ti and Ta decreased; A-site—Ca decreased, La + Ce and A vacancies increased. Given this substitution of elements in the pyrochlore supergroup minerals during Boziguoer magma evolution controlled by the A- and B-sites, and on the basis of maintaining the charge balance (Figure 7), we propose the following substitution mechanism:
2Ca2+ + Ti4+ + 4Ta5+ = REE3+ + A-V + 5Nb5+

6.2. Genesis of Pyrochlore Supergroup Minerals from the Boziguoer Deposit

The pyrochlore supergroup minerals can form either by crystallization from a magma or during hydrothermal alteration. Magmatic pyrochlore supergroup minerals occur in pyroxenite, calcite carbonatite, silicate xenoliths and syenite. Hydrothermal pyrochlore supergroup minerals occur in hydrothermal veins within the complex, whereas supergene pyrochlore supergroup minerals accumulate within the lateritic profile [34]. The pyrochlore supergroup minerals in the Boziguoer alkaline complex is disseminated, rather than intergrown with hydrothermal veins and in the lateritic profile. Thus, the pyrochlore supergroup minerals in the Boziguoer deposit are likely a magmatic product. Their chemical composition also indicates a magmatic origin, as outlined below.
Firstly, Lumpkin and Ewing (1995) [31] argued that A-site large cations can be useful for the identification of chemical variation in pyrochlore supergroup minerals because these large cations are related to host rock evolution and alteration. LILEs are highly active elements and their content increases with fluid participation. The pyrochlore supergroup minerals’ LILE content at Boziguoer is extremely low, with the primitive mantle-normalized trace element plot showing negative anomalies of LILE (K, Sr and Ba) (Figure 9), and the results of EPMA tests were all below the detection limit, but the pyrochlore supergroup minerals LILE of hydrothermal deposits can reach 0.71 apfu of Sr, 0.48 apfu of Ba and 0.19 apfu of K [78]. These characteristics suggest that the degree of fluid activity was extremely low.
Secondly, while pyrochlore supergroup minerals are generally enriched in REE, and REE partition coefficients suggest that the LREE are more compatible than HREE [79]. This explains the high REE concentration with the enrichment of LREE in naturally occurring magmatic pyrochlore supergroup minerals. The pyrochlore supergroup minerals in Boziguoer (Figure 9) display enrichment in LREE, indicating a magmatic origin. Moreover, their primitive mantle-normalized trace element and chondrite-normalized REE patterns are similar to those of their host rocks (Figure 9), indicating that the studied pyrochlore supergroup minerals were the product of magmatic crystallization.
Thirdly, the pyrochlore supergroup minerals are able to strongly fractionate trace elements during magmatic evolution. They efficiently fractionate Zr from Hf and Nb from Ta, according to the differences among DZr, DHf, DNb and DTa [79]. The Boziguoer pyrochlore supergroup minerals feature positive anomalies of Nb and Ta, and negative anomalies of Zr and Hf, which are not seen in the host rock. These fractionations provide further evidence that the Boziguoer pyrochlore supergroup minerals are magmatic in origin.
The conditions of crystallization can be discerned from the pyrochlore supergroup minerals’ composition. Under oxidizing conditions, DTh is higher than the corresponding DU, resulting in U and Th fractionation in silicate magmas [79]. The content of U and Th in the host rock at Boziguoer is similar, indicating that the concentrations of those elements in the magma were similar when the pyrochlore supergroup minerals crystallized. However, the latter are enriched in U over Th (Figure 9), indicating that that the magma was also somewhat reduced at the time of the pyrochlore supergroup minerals’ crystallization.
Compared with the host rocks, the pyrochlore supergroup minerals’ REE pattern has a tetrad effect. Figure 10 shows the τ coefficients of the pyrochlore supergroup minerals and their host rocks. The calculation method of τ coefficients is derived from [80], where τ is each of the four tetrad coefficients, with positive values indicating a negative parabola, i.e., a ridge, or M-type tetrad, and negative values indicating a positive parabola, i.e., a trough, or W-type tetrad. The tetrad behavior is clear in the pyrochlore supergroup minerals but less clear in host rocks. It indicates that the magma may have undergone sufficient differentiation when the pyrochlore supergroup minerals crystallized. The LREE tetrads, τ1 and τ2, are strongly positive in all pyrochlore supergroup minerals. One possibility is that LREE was gradually enriched in the residual magma.

6.3. Implications for Alkaline Magma Differentiation and Nb Enrichment

In carbonatitic and alkaline magma systems, Nb is more soluble than Ta [81], which could explain the occurrence of Ta-rich pyrochlore supergroup minerals in primitive magmas and Nb-rich, Ta-poor pyrochlore supergroup minerals in more evolved, late-stage magmas. The pyrochlore supergroup minerals in the Boziguoer deposit are Nb-rich and Ta-poor, suggesting a relatively high degree of evolution for the ore-bearing rocks. Moreover, progressing from the alkalic granite facies to syenite facies I and then to syenite facies II, pyrochlore supergroup minerals evolve towards the Nb-rich pyrochlore end-member (Figure 7). On the other hand, the total amount of REE in the two syenite facies is slightly higher than that in the alkalic granite facies (Figure 9), even though the latter is less evolved. Our previous study of aegirine and arfvedsonite from Boziguoer also shows the same evolutionary trend [30], in contrast to the trend seen in calc-alkaline rocks, where the degree of evolution of granite facies is generally higher than that of syenite, monzonite and anorthosite facies. The dark minerals in Boziguoer are aegirine and arfvedsonite, which are likely products of late magmatic evolution [82,83,84]. The Bowen reaction sequence also indicates that dark alkaline minerals in alkaline rocks usually crystallize later than feldspar and quartz. This, combined with petrographic observations (Figure 4b,c), suggests that the sequence of crystallization of the main minerals in Boziguoer is from albite, K-feldspar and quartz, to aegirine and arfvedsonite. Syenite facies II has the highest content of arfvedsonite and aegirine, indicating that it was the latest and most evolved lithofacies. The alkalic granite facies has the least amount of arfvedsonite and aegirine, placing it first in the evolutionary sequence. The most advanced lithofacies have the most evolved pyrochlore supergroup minerals with the highest Nb content, suggesting that Nb in these minerals can indicate the degree of magmatic evolution, and that fractional crystallization is likely the key mechanism controlling the difference in pyrochlore supergroup minerals composition in different lithofacies of the Boziguoer deposit.

7. Conclusions

The disseminated pyrochlore supergroup minerals in the Boziguoer deposit can be classified as fluornatropyrochlore, fluorcalciopyrochlore and fluorkenopyrochlore. They are enriched in Nb and depleted in Ta, with extremely low content of LILE.
A new substitution mechanism for the pyrochlore supergroup minerals is proposed based on the association between Ti, Ta and Ca, and the Nb, La + Ce and A vacancies, i.e., 2Ca2+ + Ti4 + +4Ta5+ = REE3+ + A-V + 5Nb5+.
The sequence of crystallization for the main minerals in the Boziguoer deposit is albite + K-feldspar + quartz → aegirine + arfvedsonite → pyrochlore supergroup minerals. The content of Nb in the pyrochlore supergroup minerals is likely associated with magmatic evolution. The content of Nb, Ta, Zr and Hf in the pyrochlore supergroup minerals and their host rocks suggests that pyrochlore supergroup minerals fractionation may cause a decoupling between Nb and Ta and between Zr and Hf.
There is a significant difference between the elements hosted in the A-site of the pyrochlore supergroup minerals from Boziguoer alkaline rocks and that from other rocks (e.g., carbonatite and nepheline syenite). The substitution of Ba (Rb, Sr) for the Ca+ Na + vacancy in pyrochlore supergroup minerals from carbonatite and nepheline syenite is related to hydrothermal and supergene processes, whereas the replacement of REE (La, Ce) with the Ca + vacancy in the pyrochlore supergroup minerals from the Boziguoer deposit is likely related to alkaline magma evolution.

Supplementary Materials

The following are available online at https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/min12070785/s1, Table S1. Trace element content of astrophyllite from the Boziguoer deposit.

Author Contributions

Conceptualization, Z.S. and K.Q.; field investigation, Z.S and K.Q.; methodology, Y.M.; software, F.W.; validation, Y.M., D.T. and Q.Z.; resources, K.Q.; data curation, Z.S.; writing—original draft preparation, Z.S.; writing—review and editing, K.Q. and N.J.E.; visualization, Y.M. and D.T.; supervision, K.Q.; funding acquisition, K.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Natural Science Foundation of China (41830430), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP, Grant No. 2019QZKK0806) and the Key Deployment Project of CAS (ZDRW-ZS-2020-4-1).

Data Availability Statement

Not applicable.

Acknowledgments

Geological Engineers Decheng Ma, Chen Li and Xianfeng Wei from the 701 Geological Team of Xinjiang Nonferrous Geoexploration Bureau provided valuable assistance in the field survey. Senior engineer Haiming Xu from the Chinese Academy of Geological Sciences introduced the local situation and guide. We are also grateful to Hu Guo at Tianjin Geological Research Center for the EPMA analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Tectonic units and location of Southern Tianshan region in Xinjiang, NW China (modified from [58]); (b) distribution of the Boziguoer deposit in the Southern Tianshan region and coeval alkaline felsic rocks in the Tarim north rim (modified from [60,61]).
Figure 1. (a) Tectonic units and location of Southern Tianshan region in Xinjiang, NW China (modified from [58]); (b) distribution of the Boziguoer deposit in the Southern Tianshan region and coeval alkaline felsic rocks in the Tarim north rim (modified from [60,61]).
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Figure 2. Simplified geologic map of the Boziguoer intrusion and sample locations (modified after [28,29]).
Figure 2. Simplified geologic map of the Boziguoer intrusion and sample locations (modified after [28,29]).
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Figure 3. Hand sample images and microphotographs of main rocks of the Boziguoer intrusion. (a,b) = Alkalic granite facies; (c,d) = syenite facies I; (e,f) = syenite facies II. Abbreviations: Q = quartz, Ab = albite, Kf = K-feldspar, Aeg = aegirine, Arf = arfvedsonite.
Figure 3. Hand sample images and microphotographs of main rocks of the Boziguoer intrusion. (a,b) = Alkalic granite facies; (c,d) = syenite facies I; (e,f) = syenite facies II. Abbreviations: Q = quartz, Ab = albite, Kf = K-feldspar, Aeg = aegirine, Arf = arfvedsonite.
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Figure 4. Microphotographs showing the textures of Boziguoer alkaline granite ((a,d) = alkalic granite facies, (b,e) = syenite facies I, (c,f) = syenite facies II). Arf = arfvedsonite, Aeg = aegirine, Ast = astrophyllite, Qz = quartz, Or = orthoclase, Ab = albite, Pcl = pyrochlore supergroup minerals.
Figure 4. Microphotographs showing the textures of Boziguoer alkaline granite ((a,d) = alkalic granite facies, (b,e) = syenite facies I, (c,f) = syenite facies II). Arf = arfvedsonite, Aeg = aegirine, Ast = astrophyllite, Qz = quartz, Or = orthoclase, Ab = albite, Pcl = pyrochlore supergroup minerals.
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Figure 5. Variation in selected elements (apfu) in pyrochlore supergroup minerals from Boziguoer deposit. (a) Ti is negatively correlated with Nb, R2 is 0.88; (b) Ta is negatively correlated with Nb, R2 is 0.50; (c) Ca is negatively correlated with Nb, R2 is 0.81; (d) F is positively correlated with Nb, R2 is 0.71; (e) La+Ce is positively correlated with Nb, R2 is 0.56; (f) A-vacancies is positively correlated with Nb, R2 is 0.61.
Figure 5. Variation in selected elements (apfu) in pyrochlore supergroup minerals from Boziguoer deposit. (a) Ti is negatively correlated with Nb, R2 is 0.88; (b) Ta is negatively correlated with Nb, R2 is 0.50; (c) Ca is negatively correlated with Nb, R2 is 0.81; (d) F is positively correlated with Nb, R2 is 0.71; (e) La+Ce is positively correlated with Nb, R2 is 0.56; (f) A-vacancies is positively correlated with Nb, R2 is 0.61.
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Figure 6. Variation in selected elements (apfu) in pyrochlore supergroup minerals from Boziguoer deposit.
Figure 6. Variation in selected elements (apfu) in pyrochlore supergroup minerals from Boziguoer deposit.
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Figure 7. Triangular Nb–Ti–Ta pyrochlore classification scheme (modified by [30,40,66,69,70]).
Figure 7. Triangular Nb–Ti–Ta pyrochlore classification scheme (modified by [30,40,66,69,70]).
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Figure 8. Ternary plot of Ca, Na and A-site vacancy (modified from [36]). The magmatic, hydrothermal and supergene fields of pyrochlore supergroup minerals (black circles) are from [36]. The black arrow indicates the compositional evolution of pyrochlore supergroup minerals from carbonatite. The magmatic field of pyrochlore supergroup minerals from the Boziguoer deposit (red circle) is proposed based on the results presented in this study. The red arrow indicates their compositional evolution. The pyrochlore supergroup mineral data for carbonatite are from [23,31,37,41]; data for nepheline syenite are from [35].
Figure 8. Ternary plot of Ca, Na and A-site vacancy (modified from [36]). The magmatic, hydrothermal and supergene fields of pyrochlore supergroup minerals (black circles) are from [36]. The black arrow indicates the compositional evolution of pyrochlore supergroup minerals from carbonatite. The magmatic field of pyrochlore supergroup minerals from the Boziguoer deposit (red circle) is proposed based on the results presented in this study. The red arrow indicates their compositional evolution. The pyrochlore supergroup mineral data for carbonatite are from [23,31,37,41]; data for nepheline syenite are from [35].
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Figure 9. Chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns for Boziguoer pyrochlore supergroup minerals and the host rock. The chondrite and primitive mantle values are from [71].
Figure 9. Chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns for Boziguoer pyrochlore supergroup minerals and the host rock. The chondrite and primitive mantle values are from [71].
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Figure 10. Tetrad τ coefficients for pyrochlore supergroup minerals and their host rocks. The first to fourth tetrad coefficients are shown, respectively.
Figure 10. Tetrad τ coefficients for pyrochlore supergroup minerals and their host rocks. The first to fourth tetrad coefficients are shown, respectively.
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Sun, Z.; Qin, K.; Mao, Y.; Tang, D.; Wang, F.; Evans, N.J.; Zhou, Q. Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation. Minerals 2022, 12, 785. https://0-doi-org.brum.beds.ac.uk/10.3390/min12070785

AMA Style

Sun Z, Qin K, Mao Y, Tang D, Wang F, Evans NJ, Zhou Q. Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation. Minerals. 2022; 12(7):785. https://0-doi-org.brum.beds.ac.uk/10.3390/min12070785

Chicago/Turabian Style

Sun, Zhenghao, Kezhang Qin, Yajing Mao, Dongmei Tang, Fangyue Wang, Noreen J. Evans, and Qifeng Zhou. 2022. "Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation" Minerals 12, no. 7: 785. https://0-doi-org.brum.beds.ac.uk/10.3390/min12070785

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