PYROPHYLLITE DEPOSIT IN TẤN MÀI, VIỆT NAM

J. KROOS1, J. KASBOHM1, LÊ THỊ LÀI2

1Institute of Geosciences , University of Greifswald, Germany
2Institute of Geological Sciences, VAST, Hà Nội

Abstract: The deposit Tấn Mài is situated along the marginal deep faults of a volcanic region in North Việt Nam. Pyrophyllite, kaolinite, dickite and alunite are products of hydrothermal alteration process affecting on the Mesozoic acidic volcanic rocks. The reserves are estimated as of some hundreds millions tons.

While pyrophyllite and dickite show an expansion during thermal processes, in opposite kaolinite has a  shrinkage  behaviour. That is why pyrophyllite and dickite are suitable raw materials for production of high voltage isolators and semiconductor.

A selectable exploration of pyrophyllite, kaolinite and dickite from Tấn Mài deposit is necessary to enhance the technical quality of the product, as well as refine upon its utilization.

15 selected samples from Tấn Mài deposit are investigated by macroscopic and light microscopic methods as well as by XRD, DTA/TG and TEM. Results are compared with one another. It could be considered the macroscopic field method and light microscopy are efficient methods for distinguishing pyrophyllite from kaolinite. The exploration processes can be also controlled by these methods too. However a clear distinction between kaolinite and dickite is only possible by DTA/TG.

Furthermore the results of XRD and  DTA/TG show the existence of two polytypes of pyrophyllite (2M1 -  and 1M- types)  in Tấn Mài deposit.

INTRODUCTION

Next to the kaolinite deposit, the pyrophyllite deposit in Tấn Mài has a rising importance to the Việt Namese economy. It was found deposit is found in 1974 in the mountainous area of the East Bắc Bộ region in the northern part of the Quảng Ninh Province, in 10 km south of the Chinese border. It lays there in 100 to 800 m above sea level. The result of detailed geological exploration work, which was carried out in the years 1980 to 1985, showed that the deposit has an exploitable reserve of more than 40 million tons of raw material [4, 5].

The polarization microscopy, XRD, TEM, SEM and  thermal analysis on the ore material from Tấn Mài deposit taken in the years 2002 and 2004 have been carried out at the Institute of Geography and Geology, Greifswald University, Germany, in order to find out a support method for a selected mining focused on pure pyrophyllite, a high value raw material for many inland industry branches and for export.

I. GEOLOGICAL SETTINGS

The deposit area is dominated by Paleozoic sediments (Tấn Mài formation of Ordovician and Silurian: sericite schist, quartz-schist and siltstone) and Mesozoic volcanites (Khôn Làng formation of Triassic: rhyolite, rhyolite-dacite, felsite and rhyolitic tuffs, conglomerate and limestone) and also sediments of the Nà Khuất formation of Triassic with conglomerate, sandstone, siltstone and shale. An intensive tectonic fault system shapes the landscape as well as the location of deposit. The different ore bodies, mostly in shape of lenses, are located along the marginal faults in the region (Fig.1).

1. Lower Tấn Mài subformation: sandstone, sericite schist, quartz - schist

2. Upper Tấn Mài subformation: Sericite schist, siltstone, sandstone.

3. Lower Khôn Làng subformation: Rhyolite, tuff, tuffaceous, siltstone shale, limestone.

4. Upper Khôn Làng subformation: sandstone, siltstone, schist.

5. Lower Nà Khuất subformation: Conglomerate, sandstone, siltstone.

6. Upper Nà Khuất subf: sandstone, siltstone, shale limestone.

7. Granite, granodiorite.

8. Lower Hà Cối subformation: Coglomerate, sandstone, silstone, shale.

9. Pyrophyllite, quartzite rock with alumite.

10. a/ Kaolin quartzite; b/ sericite quartzite.                                

11. Faults: a/ Supposed; b/ Defined.

Figure 1. Geological setting of Tấn Mài Deposit (Source: Trần Xuân Toản, 1983)

The metasomatose and the hydrothermal changes started at the end of Triassic times. The acidic volcanites covering siltstone built a resistant horizon for metasomatose.

II. MINERALIZATION

The mineralization is detected in certain zones after geological exploration. These different zones are distinguished as follows:

(1) pure pyrophyllite in the center

(2) pyrophyllite with a high amount of quartz up to 60%

(3) kaolinite-pyrophyllite rocks (40% of kaolinite)

(4) kaolinite-dickite rocks and

(5) alunite

These types of minerals are characterized by the content of Al2O3 . At the moment it is extraordinary complicated to describe the exact development of stage-by-stage genesis of the deposit. But it is known that alunite was found only in contact to the Mesozoic shales and siltstones while the pure pyrophyllite occurrence is located next to the effusives and pyroclastic material. The different mineralization stages is given in Table 1.

Pyrophyllite: After mineral composition 3 types of pyrophyllite can be distinguished:

Type 1: pure, quartz-free pyrophyllite with traces of kaolinite (95% of pyrophyllite)

Type 2: pyrophyllite with up to 60% of quartz (Fig. 3 a)

Type 3: quartz-free kaolinitic pyrophyllite (with 40% of kaolinite) (Fig. 3 b)

Type 1 is macroscopic one, described as a dense, white to dark grey or even green stone with low hardness. Type 2 is marked by small (mm-extension) quartz particles in grey to white-opaque stone. Another hint is the relict structure of the former rhyolite (Fig. 2).

Figure 2. SEM – photo of pyrophyllite. The original rhyolite structure can be seen

At least Type 3 can be seen due to the occurrence of dark grey spots (cm-extension) in a white opaque matrix.

In opposition to the macroscopic description these 3 types are to be observed by microscopy excellently. Under the polarization microscope pyrophyllite has white-yellow interferences, while kaolinite and dickite are mostly dark grey to black (Fig. 3).


 

 

 

 

 

 

 

 

Figure 3. Picture of polarization microscopy with pyrophyllite and quartz (a)
and pyrophyllite/kaolinite/dickite (b)

The distinctions between kaolinite and pyrophyllite and even the differences of all 3 types can easily be shown.

The 2M1 pyrophyllite was detected in XRD-phase analysis by the main reflection at 3.08 Å (100%), 9.21 Å (50%) and 4.58 Å (40%) as found in JCPDF-data files 12-203 (Fig. 5, 6). Nevertheless 1M-pyrophyllite (Fig. 4) is the dominating polytype. A different pyrophyllite generation is not yet proofed. These two polytypes of pyrophyllite are also seen by thermal analysis. They show reactions at 700°C and 870°C with a mass loss of 5-6% [6]. The pyrophyllite of Tấn Mài is developed as xenomorphic and sheeted particles by terms of morphology, seen by electronic microscopy in <2 μm fraction. That reveals a coincidence of pyrophyllite of Shokosan (Japan) and Robbins (USA). All phase analyses are represented in Tab.1.

Table 1. Different mineralization stages in Tấn Mài deposit [8]

Mineralization stage

Characteristic mineral assemblage

Forming temperature (oC)

Kaolin

Kaolinite, dickite and quartz

~260

Pyrophyllite

Pyrophyllite, kaolinite and quartz

~260-290

Silica

Quartz and pyrophyllite or kaolinite

260-290

Diaspore

Diaspore and pyrophyllite or kaolinite (Vein filling in siliceous zone)

~290-320

Alunite

Alunite, pyrophyllite and diaspore

~290-320

Figure 4. TEM – photo of pyrophyllite, 1M-Polytype

Kaolinite/Dickite: This stone shows a light transparency, a low hardness and has white opaque, yellow and often green colors. By means of XRD and thermal analyses (kaolinite ~550°C and dickite ~670°C) (Tab. 1) it is possible to make reports about the share of the minerals in the rock. It is also to be mentioned that the kaolin particles in <2 μm fraction mostly show pseudo-hexagonal morphology.

Alunite: The alunite-zones can be described macroscopically as light white to pink coloured harsh material with extreme low hardness and no structure while under scanning microscopy alunite shows a dense, coarse crystal structure. Peaks at 2.99 A (100%), 2.29 A (80%) and 1.926 A (70%) and especially in the area of 5.77 A (30%) and 4.96 A (60%) mark this clay mineral. In thermal analysis alunite has 2 endo-thermal reactions at 580°C and 800°C (Tab. 1, Fig. 4).

Hydrothermal alteration near granites or rhyolites are predestine to build ore deposits like “dissiminated or porphyry copper ores”. A hint for that are sulfides like pyrite in the area of alteration. According to [1, 3] alunite or crandallite occurrence in kaolin deposits can be used as indicator for more staged hydrothermal phases, ore phases as well as hypogenous and supergenous processes. In EDX-analyses of SEM and TEM microscopy a few crandallite crystals were found, nevertheless supergenous processes have just a small effect. It is supposed that there will be no further oring in Tấn Mài deposit.

Figure 5.  XRD - pattern of kaolinite with dickite

Figure 6. XRD-pattern of pyrophyllite with quartz and dickite

CONCLUSION

The deposit has been exploited since 1980 for inland consumption as raw material for ceramic and refractory production. According to [6,7], pyrophyllite can also be used in the synthesis of ultra hard materials and as substituting material for talc in cosmetic and pharmacy industry. Even in paper industry a market could be opened for pyrophyllite. [8] showed that it is possible to rise the degree of whiteness up to 10% with the help of combining wet grinding with high gradient magnetic separation. But the mineralogical analyses are not enough to make sure reports for a selected mining.

After the results of different analytic methods are compared, it could be considered the macroscopic field method and light microscopy are efficient methods for distinguishing pyrophyllite from kaolinite. That could be the way for starting a controlled selected mining to get a high standard of pyrophyllite material and it could be also the way to increasing prices on world stock markets. The exploration processes can be also controlled by these methods too. However a clear distinction between kaolinite and dickite is only possible by DTA/TG.

Further more the results of XRD and  DTA/TG show the existence of two polytypes of pyrophyllite (2M1 -  and 1M- types)  in Tấn Mài deposit.

REFERENCES

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9. Ullrich B., 1994. Dilatometrische Untersuchungen an Schichtsilikaten und Schichtsilikatgesteinen, Teil 1: Nichtquellfähige Dreischichtsilikate. Keramische Zeitschrift, 46 : 1-5.