ON THE FORMING ORIGIN OF SAPPHIRE AND RUBY IN VIỆT NAM

NGUYỄN VIẾT ݹ, TRẦN TRỌNG HÒA¹, TRẦN TUẤN ANH¹,
NGϡ THỊ PHƯỢNG¹, NGUYỄN NGỌC KHϡI², HOÀNG HỮU THÀNH¹

¹Institute of Geological Sciences, Hoàng Quốc Việt Road, Cầu Giấy, Hà Nội.
²National University of Hà Nội, Nguyễn Trãi, Hà Nội.

Abstract: Inheriting the results of researches of Việt Namese and foreign geologists, combined with the field survey and laboratory investigations, the authors set forth the idea of regional metamorphic origin of sapphire and ruby of Việt Nam, though they are distributed in the metamorphic rock or in Cenozoic basalt. However, this is only a preliminary thought on this matter, more evidences must be looked for and especially appropriate research methods must be found out in the coming time.

The authors also recommend to carry out researches and expanded prospecting on these precious minerals in three main regions: (1) the region of metamorphic rocks reaching amphibolite or higher facies; (2) the region of wide distribution of intraplate basalt formed in vulnerable zones; and (3) the region of wide distribution of ultramafic rocks with indications proving that they have been brought from deep layers of the earth crust in the condition of strong shear and compression.

Sapphire and ruby in our country have been attracting the attention not only of businessmen and miners, but are also of many scientists, especially in respect of their forming origin.

Sapphire and ruby of commercial value have been discovered and extracted in all three regions of Việt Nam: the North, the Central and the South, and have been classified by worldwide researchers to the rank of sapphire and ruby of SE Asia together with those of Thailand, Myanmar, Cambodia and other countries. In the following, for simplification, we will refer to them in short as "sapphire".

The question of forming origin of sapphire in Việt Nam in particular, and in SE Asia in general, has been touched upon to in many works by Việt Namese and foreign scientists. However, there still exist many different opinions and interpretations. There is no surprise for different opinions, sometimes the opposite ones often occur during researches and approach to the truth.

In this paper we have no ambition to prove any new thing, but we would like only to raise some thoughts about the forming origin of sapphire in Việt Nam, wishing to contribute to the discussion on a very important issue in the prediction and planning for prospecting on this kind of rare and precious mineral of the country.

When dealing with the issue on origin of sapphire in our country, scientists often direct their attention to the deposits distributed in three different regions. For the Yên Bái and Quỳ Châu deposits their attention is concentrated on the origin related to such processes as regional metamorphism, metasomatism and pegmatite intrusion. Whereas for the deposits in the South, most of their attention is focused on Cenozoic basalt formations. Many scientists consider that the sapphire deposits of South Việt Nam have direct spatial relationship with basalt formations, while those in the North and Central regions have special relationship with metamorphic formations of ancient uplifted blocks.

The approaches to the issues of different researchers are also different. Some researchers are focused on the differentiation of formation types (metasomatic and pegmatitic, exogenous and endogenous, etc.), i.e. they pay attention to the mechanism of forming process of sapphire. Some other researchers are interested in more comprehensive causes, for example the causes of the environment that gave rise to the above forming mechanism. In other words, they look for the causes of the causes of the sapphire generation in concrete geologic formations. In certain respects, both approaches can be considered to have the aim of solving the problem related to the formation of sapphire. However, in our opinion, the second approach is more complete and comprehensive. All the set forth opinions, which are aimed at restoration of the sapphire forming process, that is after all only hypotheses. Therefore we must accept the argument which is more logical and have more convincing evidences.

Synoptically looking, it is possible to raise a remark that few people would reject, that commercial sapphire in our country is always distributed in areas of wide distribution of metamorphic rocks reaching the amphibolite facies and higher. In particular, the sapphire in the South, as mentioned above, has special relationship with Cenozoic basalt. But obviously these formations are also developed in the region of wide distribution of deep metamorphic rocks of the Kon Tum uplift in the form of cover of various thickness.

The relationship (first of all in space) between sapphire and metamorphic formations, in particular their distribution in the marble, crystalline schist members, in gneiss, etc. is not only seen in Việt Nam, but also in many places in the world, such as the South Ural mountain, East Pamir (Russian Federation), Mogok (Myanmar), Mangari (Kenya), Longido, Lossogonoi (Tanzania), Hokkaido (Japan), Afghanistan, Australian continent, etc.

The sapphire deposits in the regions of wide distribution of Cenozoic basic effusive rocks are also met in many places of the world, especially in our neighbouring countries such as Cambodia, Laos, Thailand, China, etc. Besides, these formations are also seen in Russian Federation, Australian continent and some other countries.

The above evidences show that the formation of sapphire in our country and in other countries of the world has certain common characteristics, which reflect a similar geodynamic regime in the evolution process of the earth crust, leading to the apparition of this kind of mineral. Therefore, investigation on the origin of sapphire in Việt Nam cannot be separated from the achievement of the worldwide scientists. In the meantime it will contribute to the clarification of problems paid attention by scientists in the world.

So far, on the origin of sapphire in Việt Nam there are some opinions of the following authors:

Ngụy Tuyết Nhung, Nguyễn Ngọc Tr­ờng and Phạm Văn Long [5], on the basis of paragenetic mineral association analysis, have come to the conclusion that the sapphire in Quỳ Châu is the product of the metasomatism between the leucocratic components of migmatite and carbonate rocks, which occur in the condition of regional metamorphism of amphibolite facies associated with selective melting process forming migmatite and pegmatite. Thus, here we can see that the forming mechanism is a metasomatic process, but the deep cause is the regional metamorphism.

Phan Trọng Trịnh et al. [8] in turns highly evaluate the role of the left strike-slip tectonic movements along the Sông Hồng fault zone during the Cenozoic period. The shear zone plays the role of a canal for conducting the solution and containing the ruby ore. According to these authors, the movement along the shear zone has also the ability to cause local melting and as a result, together with the active participation of the deep originating solutions, they make the exchange more active, and create sapphire at the boundary between the rock varieties rich in aluminium and those poor in silica. However, there still are some problems requiring further research, for example the metamorphic age of the Sông Hồng Group, the source of aluminium supply for the formation of sapphire, etc. But the model set forth by these authors have proved that the decisive role in the formation of sapphire is played by the extensive regional metamorphism, regardless whether it is the metamorphism caused by the subduction or by shearing movement.

When discussing the origin of sapphire in the alkaline basalt in some places including Việt Nam, A. A. Levinson and F. A. Cook [2] set forth a model which they call "metamorphic theory". According to this theory, sapphire in the alkaline basalt is of exotic origin, where the basalt plays only the role of "the carrier" which brings the sapphire from great depth to the land surface, while the sapphire itself is formed in the subduction zone where there are suitable temperature and pressure which transform the aluminium-high materials such as clay, bauxite, diaspore, etc. into corundum. It is the collision between the Eurasian and Indian plates, that is the factor creating sapphire in the deep layers of the Earth crust from which it is brought to the surface by alkaline basalt magmatic extrusion activities during the Cenozoic period in SE Asia. This model has been used by Phạm Văn Long [7] to explain the origin of sapphire in basalt in the South of Việt Nam.

Phạm Tích Xuân, Nguyễn Xuân Hãn [6] consider that the sapphire in the South was formed at great depth and was brought up during the extrusion process, especially the eruption of Cenozoic basaltic magmas, as components of ultramafic nodules.

In the worldwide scale, the opinions of scientists about the origin of sapphire of the types described in Việt Nam are also focused on several above mentioned types, although each of researchers has a different research method and approach. As regards the sapphire in basalt there is also an opinion of magmatic origin, i.e. sapphire is crystallized directly from the magma with suitable composition. The 4-magma model by Guo et al. (1992a, 1994) is an example. According to this model, sapphire is formed and brought to the surface through 4 stages, each of them corresponding to a type of magma: first, the magma rich in carbonate raising from the mantle, containing incompatible elements. This magma is mixed with the alkali-rich magma (granite) in the lower crust layer. As a result of mixture the third magma is formed with composition suitable for the formation of Al₂O₃. The extrusive activities of the fourth magma (basaltic magma) brings corundum to the surface. This model may absolutely occur in the Earth entrails, however it is too complicated and requires experimental researches. Phan Tr­ờng Thị [9] considers also that corundum could be crystallized from alkali-high basaltic magma based on the experimental model developed by Yagik and Onuma (1969). However, A. A. Levinson and F. A. Cook [2] puts forwards three reasons to reject the possibility for the corundum in the alkaline basalt to be crystallized from that magma: 1) Experimental research shows that corundum cannot be crystallized in an ordinary basic magma fluid. 2) The surface features of the corundum such as partial corrosion, regular shape, etc. indicate that the corundum crystals have been dissolved by alkaline basalt, i.e. it is not stable in this environment; 3) Mineral inclusions in corundum such as zircon, columbite, etc., cannot be crystallized from basaltic magma.

Reviewing some points of the opinions about the formation of sapphire, in combination with field survey in all three regions of the country, we consider that sapphire in Việt Nam has the origin from regional metamorphism, although on the spatial side they may occur in the distribution area of metamorphic rocks or basalt. This remark is based on some facts as follows:

1- Long ago ruby of large size has been found in a marble layer in Lục Yên mining area (Yên Bái) (Photo 1). Ruby-bearing marble has been also found in many places on the world, such as Myanmar, Russia, China, etc. Most of scientists consider that ruby is formed during the metamorphic process at the same time with the formation of marble. The apparition of ruby in the marble has rejected the opinion of some authors considering that in Lục Yên area the metasomatic process occurs between young intrusive magmatic rocks such as the syenite of Pia Ma Complex and surrounding limestone layers. However, if indeed such process occurs, then in space ruby must be distributed in the inner contact zone, i.e. in the inner margin of the intrusive massif, and not in the marble as it is seen, because the silican reducing process has caused the accumulation of Al₂O₃ just in the magmatic body.

2- In the area of Ia Knôp feldspar quarry (Buôn Ma Thuột) we can find sapphire (of low quality) which is in large crystals (with a length of 5-6 cm or more), idiomorphic, occurring in the leucocratic component with pegmatitic structure (corresponding to syenite) of the rock layer metamorphosed to amphibolite facies (Photos 2.a, 2.b, 2.c). Similarly, in some thin sections of garnet granulite in Van Lem area on the road from Đăk Tô to Tu Mơ Rông (Kon Tum province) occurs a considerable amount of corundum scattered in the rock and especially in garnet (Photos 3.a, 3.b). In the Sông Hồng metamorphic zone (North Việt Nam) corundum has been also found in metamorphic rocks of sillimanite facies [8]. According to published data, in the concentrate panning samples of crystalline schist and plagiogneiss in Kan Nack (Plei Cu) occurs sapphire of small size (Nguyễn Văn Trang, Trần Xuân Toản, 1994); corundum of 2–5 mm in size has been also found in the Đăk Pung mafic granulite (Phạm Bình, 1997). Overseas, ruby and sapphire have been also discovered in metamorphic rocks, e. g., red colour ruby has been found in oligoclase-biotite gneiss in China [1]; ruby has been also found in granulite in Tanzania [3], etc.

3- The regional metamorphic origin of sapphire in South Việt Nam has been proved rather clearly by studying a rock sample provided by Dr. Nguyễn Ngọc Khôi (Hà Nội National University). The sample was collected near Kon Tum town, Kon Tum province in the form of rolling stones. By visual examination the rock is of blue-gray color, solid, with relatively high density. On the blue-gray background stand out ruby grains of relatively large size (in some cases reaching 1 cm in length), of red, purple red colour, with a clear colour zonation: in the marginal part it is of pinkish purple colour, in the core it is dark red (Photos 4, 4.a, 4.b). The rock is composed mainly of sapphirine, fassaite and ruby. We name this kind of rock as Sapfascorite to emphasize its peculiarity and scientific significance, according to the model of corsilzirspite (corundum - sillimanite - zircon - spinel) which has been discovered in an alluvial sapphire deposit in Bo Phloi area (Thailand). The rock-forming minerals of sapfascorite have been analyzed by the Microprobe method at the Institute of Geosciences of the Siberian Branch of the Russian Academy of Sciences. It is notable that the mineral of this rock has allowed to confirm that they were formed in the condition of high temperature and pressure metamorphism. However, one of the problems related to this rock sample is to continue to look for its relationship with surrounding rocks. As the bedrock has not been discovered, it is very possible that these are xenoliths brought from a great depth by the basalt or a magma fluid, and the possibility of their formation just in the highly metamorphic rocks of the Kon Tum Uplift should not be excluded.

4- According to published documents (Nguyễn Xuân Bao et al., 2000), in the Làng Hồi area in South Việt Nam, where ultramafic rocks are widely distributed, corundum has been found in placer. Nguyễn T­ờng Tri et al. (1994) has discovered grains of corundum of smoke and blue colour in some altered pyroxenite samples of Hiệp Đức ultramafic complex. Recently, our direct researches show that corundum occurs in thin sections of serpentinized and talcized ultramafic rocks in the Hiệp Đức massif area (Photo 5.a, 5.b). The corundum forming mechanism in these rocks so far is still unknown. However, when studying the presence of corundum in the rolling stones of gabbro in Hokkaido areas (Japan) Morihita Tomiaki and Kodera Tadahiro [11] consider that corundum may be formed just from gabbro, one of the component of the Horoman ultramafic complex, due to its metamorphism in the high pressure condition by the reaction: Ca component in clinopyroxene = corundum + grossular. These authors regard this corundum as of high pressure type.

The metamorphic origin of corundum in mafic and ultramafic rocks perhaps needs further investigation.

Returning to the issue of spatial relationship between sapphire and Cenozoic basalt, the question is, what would be the supply source of sapphire if it is not crystallized directly from the magma and exists only as "passengers" in the basalt? In our opinion, the sapphire in basalt is supplied from three basic sources:

1) From the deep metamorphic zone according to the above mentioned A. A. Levinson's model;

2) From the old metamorphic layers of the Kon Tum Uplift;

3) From the mafic and ultramafic rocks containing corundum as described above.

However, if sapphire is taken by basaltic magma immediately from the metamorphic rock layer covered by it, why it is seen only in the alkaline basalt magma? Here, there are two ways of interpretation: firstly, the tholeiitic basalt is a variety of basalt richer in SiO₂ than alkaline basalt and its effusion process takes place more slowly due to its high viscosity, therefore when falling in this environment sapphire becomes unstable; secondly, according to geologic documents, in the section of the basalt cover in South Việt Nam there is usually an intercalation of basalts of various composition, i.e. in space they are always attached to each other. In the mean time, all mined sapphire deposits are of placer form. Therefore, it is difficult to confirm that sapphire is related only to alkaline basalt. The clarification of this issue will have a very important practical significance for further prospecting and exploration works.

In conclusion, according to our thought, metamorphism is the main forming process of sapphire in Việt Nam. It is the primary cause of other secondary processes, especially metasomatism, to form corundum. The initial aluminium supply sources for the forming process of corundum is very abundant and diverse: they may be pelite, old lateritic weathering crust, or may be aluminium-high magmatic rocks, and may be also aluminium sediments which truly existed in the past.

The above mentioned judgements on the origin of sapphire show that investigation and expanded prospecting on this precious and rare mineral should be concentrated first of all in three main areas: 1) area of metamorphic rocks reaching amphibolite or higher facies; 2) area of wide distribution of intraplate basalt formed in vulnerable zones; and 3) area of wide distribution of ultramafic rocks with manifestations proving that they were brought up from deep layers of the earth crust in the condition of strong shear and compression.

To terminate the paper, we would like to repeat that the above statements are only some preliminary thoughts, that needs further research for finding more evidences and, especially, for finding appropriate research methods to be applied in the coming time.

The authors would like to receive comments and discussion from scientists of different geological offices and organizations. This paper is completed under support of Project "Research on the forming conditions and distribution rules of precious minerals in relation to magmatic activities of Central Việt Nam and Tây Nguyên regions" – DTDL-2003/07 and the Basic Research Project grant No 71.31.04.

REFERENCES

1. Galibert O., Hughes Richart W., 1995. Chinese ruby and sapphire - a brief history. J. Gemmology, 24/ 7: 467 - 473..

2. Levinson A. A., F. A. Cook, 1994, Gem corundum in alkali basalt: origin and occurrence. Gemand Gemmology.

3. Mercier A., Debat P., Saul John M., 1999. Exotic origin of the ruby deposits of the Mangari area in SE Kenya. Ore geology Reviews, 14/ 2 : 83 - 104.

4. Ngụy Tuyết Nhung, Nguyễn Ngọc Khôi, et al., 1994. Crystallo-mineralogical characteristics and forming conditions of corundum in Việt Nam. J. of Geology, A/222 : 9 -16. Hà Nội (in Vietnamese).

5. Ngụy Tuyết Nhung, Nguyễn Ngọc Trư­ờng and Phạm Văn Long, 2002. Mineralogical characteristics of the ruby in Quỳ Châu (Nghệ An) and their relationship with its forming conditions. J. Sci. of the Earth, XVIII/3 : 34-41. Hà Nội (in Vietnamese).

6. Phạm Tích Xuân, Nguyễn Xuân Hãn, 1996. Petrology of ultramafic nodules and the issue of origin of gemstones the Cenozoic basalt in South Central region of Việt Nam. J. of Geology, A/236 : 14-34. Hà Nội (in Vietnamese).

7. Phạm Văn Long, 2000. Forming condition and origin of the sapphire in basalt of South Việt Nam. J. of Geology, A/258 : 35-45. Hà Nội (in Vietnamese).

8. Phan Trọng Trịnh, G, Giunali, Ph. H. Leloup, Hoang Quang Vinh, 1999. Forming origin of the ruby in Lục Yên area and along the Sông Hồng metamorphic zone. J. of Geology, A/245: 4-9. Hà Nội (in Vietnamese).

9. Phan Tr­ờng Thị, 1993. Corundum and zircon - bearing basalt in Southeast Asia. J. of Geology, B/1-2 : 57 - 72. Hà Nội.

10. Suthirat Ch., 2003. Review on genesis of Thai corundum: a clue for basaltic type corundum exploration. Proc. of the 2^nd Intern. Conf. on gem-minerals of Việt Nam. 159 - 163.

11. Tomoaki M., Tadahiro K., 1993. Finding of corundum - bearing gabbro boulder possibly derived from the Horonam peridotites complex Hokkaodo, Northern Japan. J. of Mineralogy, Petrology, and Economic geology, 93/2 : 52 - 63.