FEATURES OF MANGANESE-BEARING STRATIGRAPHIC UNITS IN TRÙNG KHÁNH AREA (CAO BẰNG PROVINCE)

NGUYỄN CÔNG THUẬN, ĐỖ VĂN THANH,
NGUYỄN VĂN TƯ, NGUYỄN VĂN THUẬT

Geological Mapping Division of the North, Nguyễn Văn Cừ, Long Biên, Hà Nội

Abstract: The Trùng Khánh manganese deposit (Cao Bằng Province) of sedimentary origin was discovered long ago. The study on the manganese-bearing formation is being completed day by day. In previous studies the Bằng Ca and Tốc Tát Formations were considered as bearing manganese, but there still are different ideas on their age. The new data on the sediments bearing manganese collected during the geological mapping at 1:50,000 scale of Trùng Khánh sheet group (2000-2004) and some thematical studies were considerable. But it is to note that this is the first time the authors of the geological mapping at 1:50,000 scale have been describing the presence of manganese in the Lũng Nậm Formation (C₁ ln). Besides, the manganese concretions and deluvial manganse related to Quaternary formations of different origins have been mentioned in this area.

I. INTRODUCTION

Up to now, there are many studies in different levels on manganese-bearing formation in the Cao Bằng area. One of them is the geological map at 1:200,000 scale of the Chinh Si - Long Tân sheet [5] which has been giving a general view on this formation. In this work, the authors has described two stratigraphic units bearing manganse: Bằng Ca Formation, composed mainly of terrigeno-cherty sediments, and Tốc Tát Formation, composed mainly of striped and medium-bedded limestones bearing industrial manganese seams. On the basis of brachiopod fossils from Tốc Tát Formation, they are referred to Givetian and Frasnian.

The prospecting and assessement of manganese at 1:25,000 and 1:2,000 scales carried out by the Geological Party No.48 and No.105 during 1972 - 1976 almost only inherited the subdivision of Phạm Đình Long [5] and paid attention to industrial manganese in the Tốc Tát Formation, but manganese in terrigeno-cherty beds and manganese concretions were only a bit mentioned.

Later, some synthetic researches, such as: the revision of the East Bắc Bộ geological map sheet series at 1:200,000 scale, the projects on improvement of stratigraphy of Việt Nam, Upper Devonian - Lower Carboniferous stratigraphy, and some articles published in the Journal of Geology in which the presence of manganese in the Bằng Ca and Tốc Tát Formations has been recognized.

During the process of the geological mapping and mineral investigation at 1:50,000 scale of the Trùng Khánh Map sheet group, the authors from the Geological Party 202 have subdivided in detail on lithostratigraphy and biostratigraphy as well as determined the distributive rule of manganese-bearing formations in the geological map at 1:50,000 scale. Apart from the two Bằng Ca and Tốc Tát Formations, in this paper the Lũng Nậm Formation (C₁ ln) and Quaternary formations of different origins will be described (Table 1).

Table 1. Levels of study on manganese-bearing formations in Trùng Khánh area

Strati- graphic levels	Chinh Si - Long Tân map sheet at 1:200,000 [5]	Project KT.01.05 and some journal papers (1995-2002)	Geological map of Trùng Khánh sheet group at 1:50,000 scale (2004)
Q	Brief description of deluvial manganese ores in Bằng Ca, Hạ Lang areas	Not mentioned	Delineating the distributive rule and describing manganese ores related to Quaternary formations of various origins
C1	Attributed to Lower Carboniferous without mention on manganese	First description of Lũng Nậm Formation without mention on manganese ores	Recognizing the Lũng Nậm Formation and describing interbeds and lenses of manganese ores, determining their distributive rules
D3fm		Famennian Tốc Tát Formation containing manganese ore seams interbedded with carbonate beds
D3fr	Tốc Tát Formation containing manganese ore seams of industrial values interbedded with limestone	Frasnian Bằng Ca Formation containing manganese ore seams interbedded with terrigeno-cherty beds
D2gv	Bằng Ca Formation with thin seams and lenses of manganese ores in terrigeno-cherty beds

In previous works the Bằng Ca Formation has been considered as bearing manganese, but the feature of the sections bearing manganese concretions and structural components are not thoroughly studied. So, in some sections of manganese-bearing terrigeno-cherty layers there was confusion between the Bằng Ca and Lũng Nậm Formations.

II. FEATURES OF MANGANESE-BEARING FORMATIONS IN THE TRÙNG KHÁNH AREA

1. Features of Upper Devonian - Lower Carboniferous sediments bearing manganese

Upper Devonian - Lower Carboniferous sediments bearing manganese in the Trùng Khánh area consist of three formations: Bằng Ca, Tốc Tát and Lũng Nậm ones.

a. Bằng Ca Formation (D₃fr bc)

This formation was firstly mentioned by Bourret R. [1] corresponding to the Upper Bằng Ca Subformation [5]. It is composed of cherty shale, grey to black-grey clay shale interbedded with sericite-chlorite schist, thin-bedded calcareous chert of 1-20 cm in

thickness, interbedded with lenses of manganese and, in some places, lenses of limestone, the upper part is interbedded with manganese lenses of industrial value. The formation lies conformably upon limestone of Givetian age and under Tốc Tát Formation (D₃ tt). Calcareous chert and limestone contain the conodont Palmatolepis sp., coniconchia Styliolina sp. indet. and some bivalves, brachiopods.

The formation is described as follows:

Bằng Ca - Lũng Thoáng section composed of two members:

Member 1: conformably overlying limestone bearing the stromatoporoid Amphipora ramosa Phill., and brachiopod Stringocephalus of Givetian age. It is composed of black-grey cherty shale, calcareous chert interbedded with black-grey, but white-grey by weathering thin-interbedded chert bearing Coniconchia and brachiopod fossils. Thickness: 200 m.

Member 2: mainly of black grey, but yellowish-brown by weathering, manganese-bearing chert, chert interbedded with black-grey cherty clay, some thin ore manganese beds, bearing Coniconchia fossils. Thickness: 40-50 m. This member grades upward into striped limestone of the Tốc Tát Formation.

The total thickness of the formation reaches 240-250 m.

Lưu Ngọc - Bắc Võng River section, composed of three members:

Member 1: conformably lying upon Givetian limestone there is cherty shale interbedded with grey and yellow-grey by weathering chlorite-sericite schist. Thickness: 50 m.

Member 2: black-grey, thick-bedded limestone. Thickness: 30 m.

Member 3: cherty shale, bearing thin lenses (20 cm) of manganese ore. Thickness: 150 m. This member grades upward into striped limestone of the Tốc Tát Formation.

The total thickness of the formation reaches 230 m.

The other sections of the Bằng Ca Formation, such as Khau Liêu Pass section, are composed mainly of black-grey, weathered to yellow-grey cherty shale, bearing seams of manganese ore. The Kanh Ka section is of the same composition with thin beds of manganese ore and lenses of limestone, bearing the conodont fossils Palmatolepis haasi, Pa. cf. haasi, Polygnathus sp.,…

b. Tốc Tát Formation (D₃ fm tt)

The Tốc Tát Formation is largely distributed in Trà Lĩnh, Tốc Tát, Bản Khuông…areas, composed of striped limestone, some thin- to thick-bedded limestone interbedded with manganese ore seams. The section includes 2 members:

Member 1: black-grey, green-grey, red-brown striped, motley limestone, some light-grey, thick-bedded limestone. At the base of this member there is dull-striped, thin to medium-bedded limestone. Besides, in this member there are some very thin manganese ore layers without industrial value. Above limestone contains the conodont fossils Ancyrodella nodosa, Palmatolepis marginifera, Pa. glabra, Pa. distorta, Pa. haasi,…and brachiopods. Thickness: 30-320 m. This member conformably lies upon the Bằng Ca Formation.

Member 2: grey, white-grey, black-grey, striped, thin- to thick-bedded (0.2-1.0 m) limestone. In the middle part there is a seam of manganese ore of 0.15-3.0 m thick. In this member Palmatolepis glabra, Pa. trachytera, Pa. sigmoidalis and brachiopods have been found. Thickness: 50-110 m. This member grades upward into chert of the Lũng Nậm Formation.

The total thickness of the formation reaches 80-330 m.

Some concrete sections are seen as follows:

Búng Ổ section

Member 1: pinkish-grey, motley, striped limestone, interbedded with some thick-bedded (20-70 cm) limestone; bearing in abundance the conodont fossils Palmatolepis glabra, Pa. distorta, Pa. pectinata. Thickness: 130 m.

Member 2: limestone, cherty limestone, striped limestone interbedded with a seam of (thin- to thick-) manganese ore of 0.2-3.0 m in thickness (content: 30-55%), bearing the conodont fossil: Palmatolepis glabra, Pa. gracilis, Pa. sigmoidalis… Thickness: 110 m.

The total thickness reaches 240 m.

Lưu Ngọc - Bắc Võng River section

Member 1: white-grey, striped, motley limestone, 100 m thick.

Member 2: black-grey, striped white-grey, thin-bedded limestone interbedded with some chert bearing a seam of manganese ore (15 cm). Thickness: 70 m.

The total thickness reaches 170 m.

In the other sections, such as Khau Liêu, Kanh Kha Pass ones, the Tốc Tát Formation has the same composition bearing manganese ore seams of industrial significance and conodont fossils, such as Palmatolepis haasi haasi, Pa. gracilis, Pa. sigmoidalis.

c. Lũng Nậm Formation (C₁ ln):

The formation was established recently [2] in the core of the syncline in the Tốc Tát, Bản Khuông, Nộc Cu and Hát Phan areas. It is composed of black-grey chert, cherty limestone, limestone. The lower part of the formation contains thin manganese ore beds and lenses. The section consists of two members:

Member 1: black-grey, but white-grey by weathering, striped limestone bearing thin interbeds or lenses of manganese ore. Thickness: 50-100 m. Limestone and cherty limestone contain sponge spicules, the crinoid: Pentagonocyclicus sp., and conodont: Siphonodella sp.

Member 2: black, thin- to medium-bedded cherty limestone, irregular-grained limestone, organic limestone interbedded with some cherty shale, chert, bearing fossils of crinoids and the foraminifera : Tournayella discoidea; Conodonts: Paragnathodus sp., Siphonodella sosticha, Pseudognathodus triangulus, Ps. sp. Thickness: 20-200 m.

The total thickness reaches 70-300 m.

It is to note that the member 1 is just the manganese-bearing beds of the Lũng Nậm Formation. It always overlies the coarse-striped, thin- to thick-bedded limestone of the member 2 of Tốc Tát Formation and underlies limestone, cherty limestone, some thin-to thick-bedded chert containing in abundance crinoids, forams, conodonts of Early Carboniferous age. It differs from cherty-terrigenous beds which are rare in fossils (only a littlle Coniconchia) interbedded with manganese lenses of underlying Famennian Bằng Ca Formation. Terrigeno-cherty beds of the Lũng Nậm Formation are weathered to white-grey colour; especially cherty limestone of this formation contains sponge spicules, crinoids and weathered to soft, porous rock.

The formation lies conformably upon Tốc Tát Formation, and the upper part is covered unconformably (?) by the Bắc Sơn Formation. Some sections of the formation are described as follows:

Lưu Ngọc - Bắc Võng River section

Member 1: chert, cherty shale, calcareous chert, chert with thin seams and lenses of manganese ore; bearing in abundance fossils of sponge spicules and the crinoid Pentagonocyclicus sp.. Thickness: 50-100 m.

Member 2: black-grey to black, thin- to medium-bedded cherty limestone, uneven- grained limestone, organic limestone interbedded with some manganese-bearing chert, calcareous chert bearing in abundance fossils of sponge spicules and crinoids: Cyclocyclicus kuangtungensis, Pentagonocyclicus sp., conodonts: Pseudognathodus, Gnathodus, Paragnathodus, Siphonodella isosticha; forams: Tournayella discoidea, Parathurammina cushmani. Thickness: 70-100 m.

Ròng Tháy section

In the Ròng Tháy mine area, th Lũng Nậm Formation includes 2 members and lies conformably upon the Tốc Tát Formation.

Member 1: grey, black-grey, weathered to yellow-grey, white-grey chert, cherty shale with a little limestone bearing manganese lense; containing in abundance Crinoids fossils; thickness: 50 m. Manganese ore lenses have the thickness of some to 70 cm. Their Mn content reaches 0.55-19.3%, locally up to 54.58%.

Member 2: black, weathered to grey colour, thin- to medium-bedded cherty limestone, organic limestone, interbedded with a little chert; bearing in abundance crinoids and the forams: Endothyra sp., Spinobrunsia sp.,…

The features of the formation are similar in Búng Ổ, Nà Lum and Nộc Cu sections.

2. Manganese in Quaternary formations

Previously, the studies were mainly concentrated on deluvial manganese ores, situated in the primary ore areas. Based on recently received materials, manganese ores have been found in Quaternary sediments with following types:

Deluvial manganese ores: occuring in Tốc Tát, Bản Khuông, Bằng Ca, Phia Hồng areas. They occur in the deluvial cover and piedmont valleys with the sedimentary composition of rolling boulders of limestone, angular fragments of chert, sandstone, siltstone and shale. The distributive depth reaches 0-16 m.

Concretions of manganese ore: subdivided into the following origins: fluvial (aQ), deluvio-proluvial (dpQ) and fluvio-proluvial (apQ). Manganese concretions occur in Quaternary valleys, especially in karst valleys in the distributive area of Bản Cỏng Formation (D₂g bcg). Manganese concretions of fluvial formations (aQ) occur in the river terrace I and II. But it is worthy to pay attention that in the terrace II of Quây Sơn River in Đình Phong area there are manganese concretions layers of 0.7-2 m in thickness underlying the red-brown, yellow-grey silt clay layer of 0.7-2.7 m in thickness. Manganese concretions in the terrace I of Đình Phong area underlies yellow-grey silt and clay bed with the thickness of > 1 m; the manganese concretion bed has the thickness of > 1-2 m, locally it covers directly the surface of white-grey Givetian limestone of Bản Cỏng Formation. Manganese concretions in the proluvio-deluvial formation (dpQ) occur in enclosed limestone valleys, surrounded by the eroded limestone slope of mountain of Bản Cỏng Formation. The thickness of this formation is 0.5-10 m. Manganese ores occur under the cover surface from 1 to 2 m, mixed with boulder, breccia, pebble, grit, silt, clay. This type of manganese ore is of high prospect. Manganese concretions of fluvio-proluvial origin (apQ) have the same features as those of proluvio-deluvial origin, but occur in the places where the activity of surface runoff is constant.

3. Age of manganese-bearing formations

Based on new paleontological discoveries, the age of the manganese-bearing formations is determined more precisely in Trùng Khánh area.

a. Bằng Ca Formation (D₃fr bc)

The formation has been referred to Givetian by Phạm Đình Long [5] based on the supposition that, it has lateral facial transition to limestone bearing the bachiopods Stringocephalus burtini, Amphipora ramosa, Dendrostella sp. of Givetian age. Later, while studying on a stratigraphic thematic project, Nguyễn Đoá and Nguyễn Đình Hồng [4] have described the Coniconchia: Homoctenus aff. kikiensis and brachiopod: Desquamatia cf. zonataeformis from chert of the Bằng Ca area and dated the formation as Frasnian. As the result of the geological mapping at 1:50,000 scale by the Geolgical Party No.202, in the upper part of Bằng Ca Formation in Kanh Ka section, conodont fossils from limestone lenses in chert have been discovered, such as: Palmatolepis haasi, Pa. cf. haasi, Polygnathus sp., Hindeodella sp. of Frasnian age.

Morever, the lowest part of Tốc Tát Formation bears Late Frasnian component, that is similar to the section in Kanh Ka Pass where there is transitive boundary from Bằng Ca to Tốc Tát Formations bearing conodont. So, the dating of terrigeno-cherty formation bearing manganese as Frasnian is reasonable. However, in limestone lenses in the lowest part of the terrigeno-cherty layer of Bằng Ca Formation in South Hạ Lang, the authors has found the coral: Acantophyllum sp. of Givetian. Therefore, the presence of Givetian in the lowest part cannot be excluded.

b. Tốc Tát Formation (D₃fm tt)

Basing on the result of geological mapping at 1:200,000 scale and the collected brachiopod fossils: Lingula aff. striatula, Schizophoria bistriata, Praenaegonoconcha sp., Athyris sulcifera nalk., Cyrtospirifer sp., Echinoconchus striatus (Hall.)…, Phạm Đình Long [5] has referred the Tốc Tát Formation to Frasnian.

Since 1980 conodont fossil group has been studied. This is the global key and index fossil group. The researches of paleontologist-stratigraphers such as Nguyễn Đình Hồng, Nguyễn Đóa, Phạm Kim Ngân, Phạm Huy Thông, Tạ Hòa Phương… have sufficiently described the stratigraphic levels of conodont. Besides, forams has been determined, such as: Septatournayella sp., Uralinella sp., Uralinella turkestanica, U. angusta, U. bicamerata, Glomospira serenae, Septabrunsiina boukaert, Quasiendothyra communis, Q. kobeitusana, Q. konensis, Q. inflata, etc. The above fossils have been referred to Early Famennian to Tournaisian.

During the process of the geological mapping at 1:50,000, the authors of the Trùng Khánh Project have established conformable relation between Bằng Ca and Tốc Tát Formation in Độc Lập area (exposure TK. 101) where limestone of the lowest part of the formation contains the Late Frasnian conodonts Palmatolepis haasi; Spathognathodus sp., Hindeodella sp., Ancyrodella nodosa, Ancyrodella ioides; and the upper part yields the conodonts Palmatolepis triangularis, Pa. trachytera, Pa. gracilis gracilis, Pa. sigmoidalis, Siphonodella sinensis …and forams.

Based on the above fossils, the age of Tốc Tát Formation has been considered as Famennian of Late Devonian and Tournaisian of Early Carboniferous.

c. Lũng Nậm Formation (C₁ ln)

Phạm Đình Long [5] has described chert, calcareous chert at the High 100 in the Tốc Tát Mine area which have been referred to Early Carboniferous; crinoid fossils and sponge spicules. These sediments have been attributed to Lũng Nậm Formation during the geological mapping at 1:50,000 scale.

According to Tạ Hòa Phương and Đoàn Nhật Trưởng (2002), some limestone layers (5 m thick) in the Búng Ổ section yield forams of Early Tournaisian age, such as: Bisphaera malevkensis, Parathurammina stella, P. spinosa, P. cushmani. Chernyshinella (Birectochernyshinella) sp., Chernyshinella crassitheca…and Algae: Girvanella cf. ducii, Stacheoides sp., while limestone of the upper part of the formation yields the forams Tournayella discoidea maxima, Glomospira sp., Eoforchia moeller, Septabrunsiina sp., Palaeospiroplectammina tchernyshinensis, Paraendothyra cf. fortentosa, P. verkhojanica, and conodonts: Siphonodella sp., Pseudopolygnathus ex gr. triangulus of Late Tournaisian age.

In limestone interbedded with chert at the base of Kéo Quang Pass section, Tạ Hòa Phương and Đoàn Nhật Trưởng (2002) have found Late Tournaisian foram fossils: Tournayella discoidea maxima, Septabrunsiina sp., Septatournayella sp., and the Early Visean conodont Pseudognathodus homopunctatus. Therefore, the Lũng Nậm Formation has been dated as Early Carboniferous age (from Early Tournaisian to Early Visean).

4. Features of manganese ores of the formations

The data collected from the works of prospecting of the Geological Party No.202 has shown the features of manganese ores of the units.

a. Position and dimensions of manganese ore seams

In the Bằng Ca Formation, the manganese ore seams and lenses are concentrated in the upper part with the thickness of some mm to 60-70 cm, forming manganese-bearing chert beds of 3-5 m in thickness, such as in Bản Mặc, Hạ Lang, Bằng Ca areas.

In the Tốc Tát Formation, the member 2 contains one industrial manganese ore seam with the thickness of 0.1-3 m, (0.4 m in average). This seam contains nests of chert that increases its thickness. It lies stably in the mid of the member 2 and is interbedded with striped, thin- to medium-bedded limestone, as in Tốc Tát, Phia Hồng, Lũng Riếc…etc. areas.

In the Lũng Nậm Formation manganese ores occur in thin layer of 1-20 cm, and lenses of 40-70 cm in thickness. Manganese ore seams usually form chert layers bearing manganese with the thickness reaching some tens to thousands cm, even 10 m.

In Quaternary sediments concretion and deluvial manganese ores often underlie a cover of 10 cm - >1 m in thickness. Concretion and deluvial manganese ores occur in fluvio-proluvial, proluvio-deluvial, fluvial and deluvial formations. The seams bearing manganese has the thickness of some tens cm to 10 m.

b. Manganese content

Manganese in the Bằng Ca Formation is of low content, but SiO₂ is of high content. However, in some areas the content of ore lenses is high. Mn: 3.03 - 55 05%; in average: 15.8%; Fe: 0.75 - 21.05%; SiO₂: 2.58 - 87.7% (> 40% in average).

Manganese in Tốc Tát Formation: the Mn content varies 5.51-55.43%; in average: 31.35%. Fe: 0.53 - 26.14%; in average: 9.8%; SiO₂: 0.099-59%; even 85.02%; in average: 21.6%.

Manganese in Lũng Nậm Formation: the Mn content is low, silica content is high. Mn: 0.55% - 54.18%; in average: 10 - 15%; the Mn content in deluvial ore: 40 - 55.35%; in average: 46%; Fe: 5 - 9%; in average: 7%; SiO₂: 0.76 - 8.66%; in average: 4 - 6%; P: 0.04 -0.45. Content in m³: 4.17 - 647 kg/m³. Manganese content in concretions: (0.5 - 1 cm in size of grain): 10.68-18.9%; in average: 14.02%; Fe: 10.12 -15.32%; in average: 12.43%; SiO₂: 8.43-36.68%; in average: >20%. Content in m³: 30 - 625 kg/m³.

c. Ore mineral composition:

Manganese ores in the formations have the following minerals:

Manganese oxide: consisting mainly of pyrolusite (MnO₂); braunite (Mn₂O₃), hausmannite (Mn₂O₃)_, jacobsite (Mn, Fe)₃O_4;; golandite (Ba, Mn, Mn₆O_4), vredenburgite (3 Mn₃O₄2Fe₂O₃); franklinite (Zn, Fe, Mn) (Fe, Mn)₂O_4.

Manganese hydroxite: psilomelane (MnO₂ MnOn H₂O) is the main mineral in the ore, especially in oxidized zone; manganite (Mn₂O₃H₂O).

Manganese carbonate: manganocalcite (Ca, Mn)CO₃, rhodochrosite (MnCO₃).

Manganese silica: rhodonite (MnSi₂O₉)_.

The manganese ore of member 2 of the Tốc Tát Formation is composed of pyrolusite and psilomelane, the other minerals are of little quantity.

The above minerals are unevenly concentrated and depend on the thickness of ore bodies. The ore bodies of thick and tight lenses are composed mainly of pyrolusite and psilomelane; and the thin, soft and loose manganese seams interbedded with chert are composed mainly of manganite.

5. Outlines of distributive rule of manganese-bearing forrmations

In the Geological Map of the Trùng Khánh sheet group at 1:50,000 scale, the manganese-bearing forrmations are distributed in synclines or synclinorium. The core of synclines consists of rocks of the Bắc Sơn and Lũng Nậm Formations. The manganese ore seams are exposed in the limbs of synclines or synclinorium. These synclines has their axial plane of southeast-south direction (140^o-190^o /50^o-70^o), such as the ones in Tốc Tát - Bản Khuông area.

The primary manganese (industrial ore seams of high content) in limestone of Tốc Tát Formation occur in synclines and exposed in the limbs where there are beds of chert bearing sponge spicules (spongolite) of the Lũng Nậm Formation. “Spongolite” beds have been considered as marker beds lying over ore, so the presence of industrial manganese ores is possible when still exist these marker beds.

Manganese concretions and deluvial manganese ores almost occur in karstic valleys or in the mountain foot under the primary ore bodies. The manganese concretion bodies are concentrated in proluvio-deluvial, fluvio-proluvial sediments. They cover the surface of karstic valleys of limestone containing Stringocephalus of the Givetian Bản Cỏng Formation.

III. REMARKS AND RECOMMENDATIONS

The premilitary results of geological mapping and mineral investigation at 1: 50,000 scale of the Trùng Khánh sheet group has clarified the manganese-bearing stratigraphic units in the area, that are:

1) For the first time the presence of manganese in Lower Carboniferous sediments of the Lũng Nậm Formation has been recognized. The formation is distinguished clearly by lithologic composition, stratigraphic order and age with the other formations such as Bằng Ca and Tốc Tát, which has been mentioned before.

2) The presence of manganese ores in the Lũng Nậm Formation shows that a cycle of manganese formation starts from terrigeno-cherty beds bearing manganese of the Bằng Ca Formation, then carbonate bearing manganese of the Tốc Tát Formation, then terrigeno-cherty beds bearing manganese of the Lũng Nậm Formation and mainly of deep water facies of marine environment.

3) Concretionary and deluvial manganese ore related to Quaternary formations of different origins have been described and subdivided in detail.

4) Formerly, deluvial manganese ores in member 2 of the Tốc Tát Formation has been considered only of industrial value. Nowadays, with modern exploiting technology, all the manganese types in terrigeno-cherty beds as well as in Quaternary sediments have industrial values.

The paper has been completed with the contribution of technical cadres of the Trùng Khánh Project, who have overcome dificulties to collect the materials approving manganese-bearing formations, at the same time with the interest and help of the directorate of the Division. The authors express their deep thanks.

REFERENCES

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3. Nguyễn Công Thuận, Tạ Hòa Phương, 2002. Tài liệu mới về tuổi của phần chân hệ tầng Tốc Tát ở vùng Hạ Lang, Cao Bằng. (New data on the ages of the basal part of Tốc Tát Formation in Hạ Lang, Cao Bằng area). TC Khoa học, Đại học Quốc gia Hà Nội, 3 : 87-91. Hà Nội.

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