The petro-geochemical study has been showing that Permian-Triassic mafic-ultramafic volcano-plutonic association in the Sông Đà Zone belongs to two series, namely: high-titanium – high-alkali; and titanium-low, alkali-low and high-magnesium [7; Trần Trọng Hoà et al, 1995, Summing up report of the KT.01-04 Project, in Archives]. The first detailed description on the Upper Permian - Lower Triassic volcano-plutonic high-magnesium - low in titanium - very low in alkali rocks of the Viên Nam Formation* distributed in the Nậm Muội area was realized by Russian and Vietnamese geologists since 1990 with the correlation to Phanerozoic basalt-komatiite associations of other regions of the world (Barbeton and Gorgona areas) [6]. During the process of study on magmatism in the centre of the Sông Đà Structure – the Tạ Khoa Anticline (situated in over 50 km SE of Nậm Muội area) similar rocks have been found in this locality [4; Trần Trọng Hoà et al, 1995, in Archives]. These new research results have been affirming the large distribution of mafic-ultramafic rocks of basalt-komatiite type in the axial part of the Sông Đà Structure, contributing to create the basis for correlating magmatic associations and interpreting satisfactorily the model of formation and development of the Sông Đà Structural zone, that are, at the same time, new premises for investigating related mineralization (Cu, Ni, Pt group) as well as accompanying Au, Ag in the confines of this structure.

A part of the research results on the basalt-komatiite association and minerals of the platinum group was published by the authors [4, 5]. In this paper, the geological sections, petrographical and geochemical characteristics of basalt-komatiite formation distributed in the southwest limb of the Tạ Khoa Anticline (Đčo Chẹn - Tạ Khoa, Nậm Chim, Noong Xang, Bản Mong, Tạ Hộc areas) will be presented in details, aiming to clarify the above mentiond problems.

In the geological side, the studied area belongs to the southwest limb of the Tạ Khoa Anticline. The mafic-ultramafic effusives form a band stretching on about 50 km in the NW-SE trend from Tạ Chan - Tạ Hộc to Bản Tăng areas with the width varying from 2 to 8 km.

The authors of this paper have carried out the detailed study on the volcano-plutonic rocks of the Tạ Khoa Anticline in 2 main sections: 1) along the car road from Cò Nòi to Đčo Chẹn Pass (abbreviated as Đčo Chẹn Section); 2) along the new car road from Hát Lót to the Noong Xang Village. Besides, there still are short sections cross-cutting the Bản Khoa and Bản Phúc ultramafic intrusions in the Tạ Khoa area, and the mafic-ultramafic dykes and veins in Bản Mong, Tạ Hộc and Nậm Chim areas (Fig.1). The results of geological mapping and detailed study on the material composition of these volcanic, subvolcanic and plutonic rocks have been showing that they correspond completely to the high-magnesium, very low in alkali, low in titanium volcano-plutonic rocks of basalt-komatiite type that were described from the Nậm Muội area.

The survey was carried out along the car road from the Cò Nòi Fork to Tạ Khoa, cross-cutting the distributive area of volcanites on a length of nearly 10 km. On the composition side, volcanites in this section are very varied, from ultramafic (komatiite-peridotite), subultramafic (komatiite-pyroxenite, basalt-komatiite) to melanobasalt and basalt, but among them melanocratic rocks play the predominant role. Intercalated with volcanites, there are small lenses having the composition corresponding to plagioperidotite (plagiowehrlite). In addition, there still are dykes and veins of plagioperidotite (komatiite-peridotite) and ophitic gabbro, metadiabase interbedded in Devonian clay shale and limestone. Locally, sulphide mineralization bearing minerals of the platinum group (michenerite) has been met in plagioperidotite [5].

The existence of these intrusives together with their close spatial and compositional relation (see the next part) to mafic-ultramafic volcanites in this area shows that they are components of the basalt-komatiite volcano-plutonic association of the Đčo Chẹn area.

The Noong Xang - Tạ Hộc Section is simpler than the Đčo Chẹn one. It is composed mainly of tholeitic basalt of medium or slightly high melanocrate, also low in titanium, relatively low in alkali. The varieties of high-magnesium are rarely met. Dykes of diabase having the similar composition with basalt are widespread. Locally, in melanocratic diabase located near the contact with surrounding carbonate rock there is thick dissemination of sulphide. However, the content of elements of the Pt group is relatively low, and not any platinoid-bearing mineral has been found in these places.

In the upper course of the Bản Mong Stream, tremolitized, talcified and serpentinized mafic-ultramafic pyroxenite penetrates in the bedding surface of Devonian metamorphosed sediments (coarse-banded quartzite) accompanied by dykes of strongly amphibolitized gabbro. The Ni-Cu ores in pyroxenite veins in Bản Mong have the average Pt content of about 0.35 g/t [5].

These are small massifs of < 1 km² in size, distributed generally in the centre or eastern limb of the Tạ Khoa anticline, among them the Bản Khoa and Bản Phúc Massifs are the most detailedly studied [6; Trần Quốc Hùng, 1986, Report on the petrology and mineralization of some intrusive complexes in N Việt Nam, Archives of the Institute of Geology, Hà Nội; Trần Trọng Hoà, 1995, in Archives]. They are composed mainly of dunite and plagioperidotite.

Some searchers has been regarding the above said massifs as the product of the process of protrusion [9]. However, according to our observation in the field the level of serpentinization of the massifs is strongest in the massif margin; toward the centre the rocks are rather fresh, preserving their structure and texture, proving that they are crystallized from a primary magmatic fluid. The surrounding rocks of the Bản Phúc Massif are clearly hornfelsified. The authors have been considering these intrusive bodies as the manifestation of the neck facies of volcano-plutonic rocks of the Tạ Khoa area [4].

They are composed of vein rocks with the composition varying from ultramafic to mafic intercalated between quartzite and amphibolite of nearly vertical dip. The thickness of the dykes varies from 10-20 cm to nearly 10 m. In some relatively great veins in the mouth of Nậm Chim Stream occurs the zoning from the centre outwards: in the centre these dykes consist of komatiite-peridotite having the spinifex texture of olivine or disorderly arranged rodlike crystals, towards the margin there are olivine melagabbro, olivine-bearing gabbro, and in the contact with surrounding rocks there is the black, fine-grained and compact rock of olivine gabbro type. In sulphide-rich peridotite, paolovite has been found. The detailed study on the material composition has been showing that their evolution trend is similar to that of ultramafic rocks of the Nậm Muội and Tạ Khoa areas.

So, in the southwest limb of the Tạ Khoa Anticline, the high-magnesium, low in alkali and low in titanium mafic-ultramafic complex of basalt-komatiite type expresses oneself in 3 forms: 1) volcanites; 2) dykes and veins; and 3) intrusives of neck facies.

Dunite and plagioperidotite of the Bản Phúc, Bản Khoa and Bản Xang Massifs belong to the rock group having the highest MgO content in the studied area (MgO = 41.62%), very low TiO₂ (0.19%) and total alkali (Na₂O + K₂O = 0.8%). Rocks of these massifs are all strongly altered, almost all are serpentinized with a gradually weak level from the margin to the centre of the massif. In the rock, olivine (Ol) occupies up to 45-65%; the remaining 35-55% consists of pyroxene (mainly of Cpx). Based on the petrographic and mineralogic characteristics they are similar to plagioperidotite in the Nậm Muội area [4].

The main minerals of the rocks of the basalt-komatiite series in the Tạ Khoa Anticline consist of olivine and pyroxene; accessory minerals: chrome-spinel, ilmenite, disseminated sulphide (mainly that of nickel-copper), usually together with native copper, which, sometimes, has significant content in copper occurrences related to mafic volcanites of basalt-komatiite type. Olivine of idiomorphic phenocryst form has the composition corresponding to forsterite-chrysolite (fOl = 8.5 - 15.8%), the content of CaO = 0.2 - 0.6%; NiO = 0.2 - 0.4%. Olivine crystals of acicular form has the higher iron degree (fOl = 18.9 - 21%), lower NiO content (0.12 - 0.28%). Clinopyroxene consists mainly of augite high in magnesium, low in iron (fCpx = 15.6 - 17%), high in chrome (Cr₂O₃ up to 2.3%). Chromspinelite encrusted in olivine has varying composition: Cr₂O₃ = 34-51; Al₂O₃ = 17-31; TiO₂ = 0.3 - 0.6, while in the form of small crystals in the groundmass it has following composition (%): Cr₂O₃ = 17.1 - 29.9; Al₂O₃ = 5.1 - 16.8; TiO₂ = 0.9 - 6.0. According to the composition and alteration trend they are close to chromspinelite from basaltoids of the trap formation.

Komatiite-peridotite: consists of the rocks having the MgO content varying from 20 to 30%. They have porphyritic structure with phenocrysts mainly from Ol (up to 80%) and Cpx (15-20%). The groundmass is microdoleritic, in some places it is of chloritized glass structure. Especially, in some samples Ol grains are clearly elongated and arranged in bands. The most part of them are serpentinized, while clinopyroxene is uralitized. The rocks are mylonitized, therefore coarse grains of olivine and pyroxene are smashed and cemented by small scaly materials (amphibole). This rock group has high content of magnesium (MgO = 26-305), while the content of titanium (TiO₂ = 0.55%) and total alkali (Na₂O₃ + K₂O = 0.4 - 0.65) is very low.

Basalt-komatiite: consists of the rocks having porphyritic structure with the phenocrysts of nearly rounded, xenomorphic or elongatedly prismatic with MgO varying from 17 to 18%. They are strongly serpentinized. The groundmass is of glass structure with micro-grained Pl and Cpx microlites, in places of leaf form or radial. The content of MgO, TiO₂ and Na₂O + K₂O as well as of other chemical components is similar to that of basalt-komatiite of the Nậm Muội area (Table 1).

High-magnesium basalt (olivine basalt): often has amygdaloidal structure. The Ol phenocrysts are all chloritized, and there rest only remains of long rod-like Ol crystals. The groundmass consists mainly of glass and microlites of plagioclase + pyroxene. Average content (%): MgO = 9.61; TiO₂ = 1.02 and Na₂O + K₂O = 3.34.

Leucocratic basalt (andesitobasalt): consists of the rocks having the MgO component varying from 6 to 8%. It has porphyritic structure with the phenocrysts mainly of Pl; the groundmass is rich in glass. Similar to the above described rock group, leucocratic basalt has the same characteristics: low in titanium. Average content (%): MgO = 7.1; TiO₂ = 1.39; Na₂O + K₂O = 2.79, in which K₂O is usually £ 0.8.

Beside the above cited rocks, the dykes and veins in the Nậm Chim and Tạ Hộc areas still contain ophitic gabbro and diabase. Ophitic gabbro is slightly mylonitized. Actinolite and biotite (chocolate coloured) has secondary origin, and biotite is developed only in the group of clastic materials together with amphibole or just in the residual crystal face (Ol or Cpx) bearing iron (magnetite) as opacitized product. In diabase, there have been observed Pl crystals of elongated prism form, in many places arranged in radial form; the most part of Cpx are amphibolized, xenomorphic and squamose, rarely in the platy form.

In general, the above described rocks in the Tạ Khoa Anticline area are similar in the mineral composition and structural characteristics to the rock group of the Nậm Muội area. In the high-magnesium and Cpx-rich varieties radial and elongated assemblage similar to the "spinifex" texture of pyroxene has been observed.

Examining the chemical composition of the mafic-ultramafic rock group (Table 1) from the Tạ Khoa area, we can see that they are similar to those from the Nậm Muội area with the characteristics of basalt-komatiite associations. The TiO₂ content in the studied rocks varies, but in general, in low level: 0.19 - 0.69%, that increases only in leucocratic varieties or in the rock of dyke and vein forms (1.02 - 1.34%). The Al₂O₃ content largely changing: 2.47 - 14.7%; Na₂O + K₂O = 0.4 - 2.9%, particularly, very low K₂O (0.1 - 0.9%; dry component) are the characteristic outline of the rocks from sections of Tạ Khoa Anticline area, among them dunite and plagioperidotite of Bản Phúc and Bản Khoa Massifs have the lowest content of the above said components. Examining the K₂O-MgO and TiO₂-MgO correlations (Fig.2), as well as the CaO/Al₂O₃ (> 0.8), CaO/TiO₂ (> 10), Al₂O₃/TiO₂ (14-23) ratios in these rocks, we can see that they are rather close to those of the rocks from Nậm Muội area. However, mafic rocks from Tạ Khoa area is a bit richer in titanium and potassium than those from Nậm Muội area.

Table 1. Average chemical composition (% of dry weight) of rock groups
of the basalt-komatiite association from Tạ Khoa area

The komatiite-peridotite, basalt komatiite, komatiite-basalt, as well as plagioperidotite have rather high content of Ni, Co, Cu, Cr ore elements (Table 2). Their average content varies in a great interval: highest for Ni from komatiite-peridotite in Đčo Chẹn (1467 ppm) and from dunite and plagiowehrlite of the Bản Phúc, Bản Khoa Massifs (2205 ppm). The average content of Cr is frequently high, and highest in basalt-komatiite (2625 ppm). The Ni and Cr content of basalt and diabase is also very high: Ni = 106-541 ppm; Cr = 176-1085 ppm.

Table 2. Average content (ppm) of Ni, Co, Cr and V in rock groups
of the P₂-T₁ basalt-komatiite association from Tạ Khoa area

Note: 1-4) Đčo Chẹn Section: 1. Komatiite-peridotite (n=2); 2. Basalt komatiite (n=2); 3. Komatiite-basalt (n=4); 4. Olivine basalt (n=2). 5-7) Nậm Chim Dyke: 5. Komatiite-peridotite and pyroxenite (n=4); 6. Basalt komatiite (n=2); 7. Basalt and diabase (n=5). 8-9. Hát Lót - Tạ Hộc Section: 8. Komatiite-pyroxenite (n=2); Diabase dyke (n=6). 10. Dyke and vein at Bản Mong: Tremolitized pyroxenite (n=7). 11-13) Bản Phúc - Bản Khoa Massifs: 11. Dunite and wehrlite at Bản Phúc - Bản Khoa (n=22); 12. Komatiite metabasalt at Tạ Khoa area (n=4); 13. Metabasalt at Tạ Khoa area (n=3).

Mafic-ultramafic rocks in the Tạ Khoa Anticline, especially komatiite-basalt and olivine basalt, are rather rich in Rb, Nb, U, Zr, Th, as well as light rare earth elements (LREE) La and Ce (Table 3, Fig.3). The value of their absolute content is generally 10-40 times higher than that in the primary mantle. The content of Sr, Ta and Th in komatiite-pyroxenite is usually equal to the composition of primary mantle, while in komatiite basalt and olivine basalt their content is only a bit higher. Based on the distribution characteristics of rare earths (Fig.4), the studied rocks can be subdivided into 2 groups: komatiite basalt and olivine basalt, characterized by relatively distinct separation of distribution, richer in LREE than HREE. As for the curve of distribution of rare earths of komatiite-pyroxenite, it does not express the separation, even the HREE are richer than LREE (T.1610) and expresses a light min-max Eu.

The multipole distribution feature of rare earths as above described is also clearly expressed in the rocks of the basalt-komatiite association at Nậm Muội area [4, 6, Trần Trọng Hoà, 1995, in Archives]. However, in the overall, based on the content characteristics and content distribution of lithophilous and rare earth elements as well as Th, U, Ta and Hf that are correlated to the composition of primary mantle and chondrite (Fig. 3, 4), the studied rocks have the tendency to be close to formations originated from a very poor mantle.

Table 3. Content of rare and rare earth elements in basalt-komatiite rocks
from Tạ Khoa area

Note: 1-3) Komatiite-pyroxenite; 4) Komatiite-basalt; 5) Basalt-komatiite; 6) Olivine basalt. Rb...Y - analyzed by X-ray fluorescence method; La...U - analyzed by neutron activation method by the Analytic Centre of the Novosibirsk Institute of Geology, Geophysics and Mineralogy. Source of data [7].

Cu-Ni sulphide mineralization in the Permian-Triassic mafic-ultramafic rocks in the Sông Đà Zone is rather abundant. Related to the titanium-low volcanites of basalt-komatiite type from the Tạ Khoa Anticline, apart from the Bản Phúc Cu-Ni Mine and about 10 known occurrences, there are some new Cu-Ni occurrences found by us which are related to komatiite-peridotite from Đčo Chẹn Pass area, basalt-komatiite at Noong Xang Village and komatiite-basalt at Nậm Chim. In the Nậm Chim occurrence, the mineral bearing Pd - paolovite has been found in the disseminated sulphide ores having the content of Pt = 15 ppb and Pd = 11 ppb. In the compact sulphide ores of the Bản Phúc Mine and the Bản Mong occurrence (having following content: Pt = 15 ppb; Pd = 1330 ppb; Ag = 1490 ppb; Au = 53 ppb) sperrylite and michenerite have been found [4, 7, 8]. In addition, in the above said ores native gold, electrum, telluride of Ag, Bi and Ni (Ni/Co = 17.6), native copper and native silver have been met [7]. Besides, arsenide and sulfoarsenide of Co and Ni are widespread.

The detailed study on high-magnesium, titanium-low and alkali-low mafic-ultramafic volcano-plutonic formation distributed in the southwest limb of the Tạ Khoa Anticline has been allowing to affirm the similarity on material composition of these rocks with the basalt-komatiite association from Nậm Muội area (Thuận Châu District, Sơn La Province). These basalt-komatiite rocks, according to last investigation results, form a continuous band from Tạ Chan - Tạ Hộc (in the NW end) through Đčo Chẹn Pass to, possibly, Bản Tăng (in the SE end) in a length of > 50 km. They completely differ in paragenetic rock association and composition from the volcanite band of the northeast limb of the Tạ Khoa Anticline, which stretches from Mường La to Bắc Yên - Vạn Yên, as well as from those in Viên Nam, Đồi Bù. The Mường La -Vạn Yên volcanite band, according to last studies, is composed mainly of high-titanium and high-alkali basaltoids, which are rich in lithophilous elements and LREE, as well as Nb, Th, U, Ta [8], and close to mafic volcanites of the subalkaline tholeite or alkaline basalt series. They are close to trachybasalt, trachyandesite belonging to the upper part of the section of the "Viên Nam Formation" (with basalt-komatiite in the lower part) from Nậm Muội area [6]. This has been allowing to consider that, the existence of basalt-komatiite in the southwest limb of the Tạ Khoa Anticline proves this is the deeper eroded part of the Permian-Triassic volcano-sedimentary section in the Sông Đà Zone.

The independent and separated in space existence of the basalt-komatiite band of the Sông Đà Zone, such as the Nậm Muội and Tạ Khoa bands, needs to be drawn separatedly on geological maps at large scale. It has not only good significance for lithostratigraphic classification in geological mapping, but also for the orientation in the construction of the petrological and metallogenic models of the Sông Đà Zone. The investigation on minerals in this zone during last years has been showing that the mineralization of Ni-Cu and accompanying precious metals of the PGE group is related to high-magnesium mafic-ultramafic rocks of the basalt-komatiite type [4, 8], while mineralization of Au and Cu-Au is related to high-titanium, high-alkali volcano-plutonic rocks. The affirmation on the large distribution of these formations in the Sông Đà Zone, as presented above, means that the enlargement of prospecting area for Ni-Cu and accompanied PGE new occurrences similar to the Bản Phúc and Bản Mong Mines, which are most perspective in Ni-Cu and PGE mineralization in Việt Nam, is a noteworthy task.

It is also necessary to carry out deeper study on the geochemical and isotopic characteristics of the mafic-ultramafic volcano-plutonic rocks in the Sông Đà Zone aiming to gather fuller informations for the interpretation of the forming mechanism of the Sông Đà Depression, one of important geological structures of the East Asian continent.

This paper was completed with the financial support from the Fundamental Research Project No. 71.07.01 (in 2001). The author would like to express their deep thanks to the Correspondent Academician Poliakov G.V., Dr. Balykin P.A., Eng. Petrova T.E. and colleagues of the Magmatism Section, Institute of Geology, NCNST of Việt Nam for their collaboration in scientific research and for their help in the realization of this paper.

1. Dovzhikov A.E. (editor), 1965. Geology of North Việt Nam. Gen. Dept of Geology, Hà Nội (in Russian), 665 p..

2. Lacroix A., 1933. Contribution à la connaissance de la composition chimique et minéralogique des roches éruptives de l’Indochine. Bull. Serv. Géol. Indochine, 20/3 : 17-57. Ha-noi.

3. Naldrett A.J., 1984. Deposits of nickel sulphide: Classification, composition and origin. In: Origin of deposits, 2 : 254-265. Mir, Moskva (in Russian).

4. Ngô Thị Phượng, 1994. Permian-Triassic high-magnesium volcano-plutonic association in the Sông Đà Zone, NW Việt Nam. Ph.D. Thesis, Novosibirsk Institute of Geol., Geoph. Miner. (in Russian).

5. Ngô Thị Phượng,Trần Trọng Hoà, Hoàng Hữu Thành et al, 2000. Các khóang vật nhóm platin (Pt, Pd) trong các thành tạo mafic - siêu mafic đới Sông Đà (Minerals of the platinoid group (Pt, Pd) from the mafic-ultramafic formations in the Sông Đà Zone). TC Khoa học Trái đất, Hà Nội.

6. Poljakov G.V., Balykin P.A., Glotov A.I. et al, 1991. High-magnesian volcanites in Da River Zone. Proc. 2^nd Conf. Geol. Indochine, 2 : 247-264. Hà Nội.

7. Poljakov G.V., Nguyễn Trọng Yêm, Trần Trọng Hoà và nnk, 1996. Các thành tạo mafic - siêu mafic Permi-Trias miền Bắc Việt Nam (Permian-Triassic mafic-ultramafic formations in North Việt Nam). Nxb Khoa học kỹ thuật, Hà Nội, 173 tr..

8. Trần Trọng Hoà, Hoàng Hữu Thành, Ngô Thị Phượng và nnk, 1998. Các tổ hợp bazantoiđ cao titan Permi-Trias rift Sông Đà: Thành phần vật chất và điều kiện địa động lực hình thành (Permian-Triassic high-titanium basaltoid association in the Sông Đà Rift: Material composition and geodynamic forming conditions). TC Địa chất, A/244 : 7-15. Hà Nội.

9. Văn Đức Chương, Nguyễn Trọng Yêm, Trần Văn Thắng, 1994. Ophiolit đới Sông Đà (Ophiolites of the Sông Đà Zone). TC Các khoa học Trái đất, 16/1 : 1-11. Hà Nội.