Depositional environment and metamorphism of early Proterozoic iron formation in the Lüliangshan region, Shanxi Province, China

Precambrian Research, 39 (1988) 39-50

39

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

DEPOSITIONAL ENVIRONMENT AND METAMORPHISM OF EARLY PROTEROZOIC IRON FORMATION IN THE LOLIANGSHAN REGION, SHANXl PROVINCE, CHINA ZHU JINCHU, ZHANG FUSHENG and XU KEQIN

Department of Geology, Nanjing University, Nanjing (People's Republic of China) (Accepted October 2, 1987)

Abstract Zhu, J., Zhang, F. and Xu, K., 1988. Depositional environment and metamorphism of early Proterozoic iron formation in the Lilliangshan region, Shanxi Province, China. Precambrian IRes., 39: 39-50. The early Proterozoic iron formation of the Liiliangshan region is 18 km long and has undergone different degrees of metamorphism at different localities. Within this belt, the Yuanjiacun deposit is characterized by low grade metamorphism, diverse iron sedimentation facies, a well-exposed stratigraphic section and simple structure. It is an ideal case for studying the depositional environment and the metamorphic processes of Precambrian banded iron ores. Formed in a shallow water basin of a geosyncline, the Yuanjiacun iron formation some 1200 m thick comprises three sedimentation-ore formation cycles, each of which begins with coarse- to medium-grained clastic rocks (quartz sandstone or arkose), passes through fine-grained siltstone and argillitic rocks, and grades into pure colloidal chemical deposits represented by thin-bedded ferruginous quartzites. In each cycle from the bottom upwards, in general, there successively occur pyrite, siderite, iron silicates (including Fe-rich chlorite, minnesotaite and stilpnomelane), magnetite and haematite. These iron mineral facies are products of sedimentation and diagenesis to very low grade metamorphism stages. The spatial distribution of iron facies in stratigraphic sequence shows an increasing oxygen fugacity of the depositional environment during the course of each cycle. The main factors controlling the types of iron sedimentation facies and the valence state of iron are considered to be the proportion of sandy-argillitic component to colloidal chemical materials during deposition and the relative amount of organic matter in them. The abundance of organic matter results in the creation of a reducing environment and the formation of principally ferrous facies, whereas the lack of organic matter leads to the development of oxidizing conditions and the formation of ferric facies. Under a transitional oxidation-reduction medium, mixed ferrous-ferric facies dominate. The fracture-related regional metamorphism yielded progressive metamorphic zonation in the Liiliangshan region: the chlorite zone and the biotite zone of greenschist facies, and the almandite zone, staurolite zone and kyanite zone of the amphibolite facies. These metamorphic zones cross-cut banded iron ores and produce corresponding changes in the iron formation.

Introduction T h e L i i l i a n g s h a n G r o u p is o n e of t h e m o s t r e p r e s e n t a t i v e t y p e sections o f early P r o t e r o zoic g r e e n s t o n e f o r m a t i o n s a n d b a n d e d iron

f o r m a t i o n s in China. It c o n s i s t s o f a suite o f m a f i c - s i a l i c volcanics, s a n d y - a r g i l l i t i c sediments, carbonates and thin-bedded ferruginous q u a r t z i t e s t o t a l i n g over 15 000 m in t h i c k n e s s . T h e degree o f m e t a m o r p h i s m o f L i i l i a n g s h a n

40

l-klhllhaute 0

-'

Xian 0

'),

8eijing

•

J

/

<.J

O. -/

Shanxi I Prov i . . . . 'J

.i~'~'°~ nan

~0

~

U

~ S /

Zhengzhou

/

/YELLOW (

'

~

SEA

'~

Fig. 1. Locationmap of the workingarea. Group including the Yuanjiacun deposit is the lowest among the contemporary stratigraphic sequences in China. The geological section is well exposed and more complete, the structure is relatively simple, and the iron sedimentation facies are diverse, so that study of the Liiliangshan Group and the Yuanjiacun iron formation provides extremely important information about early Proterozoic sedimentation and volcanism, in understanding metamorphic processes and in the determination of the original nature of metamorphic rocks, as well as information on the depositional environment of banded iron ores. The working area is located 120 km westnorthwest of the city of Taiyan, Shanxi Province (Fig. 1). Part of the northern side of the Lfiliangshan anticlinorium of the Shanxi anticline, Northern China Platform, it is adjacent to the Jingle Paleozoic faulted basin on the east and is next to the Ordos syncline on the west. Structurally, this region is typical two layered, consisting of a folded Precambrian geosynclinal crystalline basement and a flat Paleozoic to Mesocenozoic platform sedimentary cover. Since the 1960s, a Research Group from Beijing College of Geology, Shen Qihan et al. from the Academy of Geological Sciences and Geological Team 218 and the Regional Survey Team from Shanxi Province have worked in this region. They have provided the framework for the stratigraphic subdivision of the Liiliangshan Group and the geological structure. This work

served as a fundamental basis for the study of the regional geology and stratigraphy of the area. They established the type section of the Liiliangshan Group in the Yuanjiacun district and interpreted that the attitude of NS-trending and steeply dipping east Liiliangshan Group is normal, i.e. top on the east side and bottom on the west side. Through our field work in May-August 1976, mainly based on the primary sedimentation structures such as cross- and inclined bedding, graded bedding, ripple marks, and spatial distribution of vesicles and amygdules in submarine mafic lavas, as well as the cycles of the Yuangjiacun iron formation, the stratigraphic sequence has been determined to be overturned (Zhang and Zhu, 1981). These primary sedimentation structures consistently indicate that the previous understanding of the bottom-top relationship was wrong, and that the correct relationship should be: top on the west side and bottom on the east side {Fig. 2). This suggestion has been proved and recognized by the systematic re-examination and comparison work of Regional Survey Team of Shanxi Province (Xu and Xu, 1985 ). The correct bottom-top relationship provides very important evidence for the establishment of the correct geological sequence of banded iron ores and for the discussion of the geological and physicochemical conditions of their formation. The Liiliangshan Group was traditionally interpreted by the Regional Survey Team as late Archean. K - A t radiometric dating on the muscovite from the schist of the Lfiliangshan Group and from the pegmatite cutting this stratigraphic unit gives an apparent age of 1960-1740 Ma (Regional Survey Team of Shanxi Province, 1972 ). This figure coincides with the most important early Proterozoic metamorphic event throughout the world, dated at about 1900 Ma (Zhang, 1984). From a combination of other geologic and lithologic features, the Lfiliangshan Group is probably early Proterozoic. Based on the understanding of the bott o m - t o p relationship and the geological age of

41

W

E

WENJIAGOU

HOUSHAN~

~

~

BOJISHA

Pt2 |

Ptt ly

Q

ripplemark

G gradedbeddn ig

Q

vesiclesand amygdules ~

crossbedding

Q n i cn il edbeddn i =' @

Ol I

It Km

sedimentationcycle

Fig. 2. Geological section of the Yuanjiacun area showing overturned primary structures (symbols of Precambrian stratigraphy are indicated in Table I).

TABLE I Generalized stratigraphictable of the Precambrian, Lfiliangshanregion,Shanxi Province Age and estimated date (Ma)

Stratigraphic unit

Symbol

Lithology

Thickness (m)

Late Proterozoic (600-1200)

Heichashan group

Pt3h

Arkose, conglomerate

1080

Middle Proterozoic ( 1500-1800)

Yejishan group

Pt2y

2617-5647

Lanhe group

Pt21

Sandy-argiUitic rocks intercalated with mafic volcanics Quartzite, sandstone, conglomerate, phyllite, marble

Lfiliangshan group Dujiagou formation

Ptlld

Rhyolite,rhyoliteporphyry, quartzkeratophyre Mafic volcanicswith interbedsof submarine pelite Phyllite,quartz sandstone Iron formation

3545

Early Proterozoic (1800-2500)

Late Archean ( > 2500)

Jinzhouyu formation

Ptllj

Peijiazhuang formation Yuanjiacun formation Nin~iawan formation

Ptllp

Qingyanggou formation

Ptllq

Jiehekou group

Ar2j

Ptlly Ptlln

Intermediate-acidic volcanics, tuff, iron formation, marble, phyllite Quartzite, leptite, schist Migmatite, gneiss, plagioclaseamphibolite, quartzite, marble

2566

2000-2500

880-2650 840-1200 2000-5000

4000 16 700

42 the Liiliangshan Group, and with reference to the research results of the Regional Survey Team of Shanxi Province, the simplified stratigraphic table for the Precambrian in the Liiliangshan region shown in Table I can be developed. G e o l o g y of the iron belt and s e q u e n c e of iron f o r m a t i o n The Liiliangshan Group is over 15 000 m thick and consists of a suite of spilite-keratophyre formations, terrigenous clastic formations, iron formations and shallow water carbonate formations. The ferruginous quartzites have been found in two stratigraphic units: the Yuanjiacun formation and the Ningjiawan formation. The former is predominant in importance. The Yuanjiacun iron-bearing formation is 18 km long and begins at the Yuanjiacun deposit in the north. It runs south and ends at Jianshan deposit in the south (Fig. 3 ). About 2 km south of the Jianshan deposit the large south-dipping left-lateral Xichuanhe River fault passes through in an east-west direction. The region has a kind of greenschist grade metamorphism but is higher grade in a fault-related regional metamorphic process (Wang et al., 1985). Progressive metamorphic zonation from north to south has been developed (see Fig. 7). The Yuanjiacun deposit is located at the northern end of this belt. Slightly metamorphosed Precambrian strata of NS strike dip steeply east at an angle of 60 °-80 °. Structurally, it is an overturned monocline. The middle part of the deposit is the iron ore horizons of the Yuanjiacun formation, underlain by the Ningjiawan formation on the east side and overlain by the Peijiazhuang formation on the west side. All these Proterozoic successions are unconformably covered by gently dipping Cambrian limestone (Figs. 3 and 4).

Detailed field measurement of stratigraphic sections in the middle part of the Yuanjiacun deposit in several localities, together with laboratory determinations and analyses, and incorporation of the results of Geological Team 218, makes it possible to subdivide the Yuanjiacun formation (1200 m thick) into three parts. A synthesized stratigraphic sequence of the Yuanjiacun iron formation is given in Table II. From the synthesized stratigraphic sequence of the Yuanjiacun iron formation, it is clear that the three parts of the Yuanjiacun formation practically reflect three sedimentation-oreformation cycles during the course of formation of the Yuanjiacun banded iron ores. The sedimentary rocks of each cycle begin with coarseto medium-grained clastics, i.e. very slightly metamorphosed quartz sandstone or arkose. Upwards, the grain size of the clastics becomes finer, and the iron and silica contents become richer. Passing through fine-grained sandstone, siltstone, sericite schist, chlorite schist, iron-rich silicate schist (ferruginous schist) and silicate-magnetite quartzite, the iron formation grades into nearly pure colloidal chemical sediments, represented by thin bedded magnetite quartzite and specular haematite quartzite. The haematite quartzite is the end member or the highest member of each cycle. A conformable but sharp contact between the haematite quartzite and the coarse-medium-grained clastics of the next cycle is usually found. Although there is some repetition or overlapping of sedimentary rocks or ores, the general tendency is consistent. Consequently, these three cycles represent three sedimentation-ore-formation sequences of transgressive character from a shallow water environment. The chemical composition of the rocks and ores of the Yuanjiacun iron formation is given in Table III. Each cycle begins with oSlitic ores and ferruginous schist. Upwards they pass through banded silicate-magnetite quartzite, and grade

43

iron ore body

J ~ ']

fault

0 1

Z i

4 I

6 I

Krn

Fig. 3. Simplified geological map of the Yuanjiacun-Jianshan area, Lliliangshan region (symbols of Precambrian stratigraphy are indicated in Table I).

into thin-bedded magnetite quartzite and haematite quartzite. Iron s e d i m e n t a t i o n f a c i e s

The iron mineral facies appearing in the iron formation of the Yuanjiacun deposit are diverse. They are mainly oxides (magnetite, haematite) and phyllosilicates (chlorite, minnesotaite, stilpnomelane) and, in lesser amount, carbonates (siderite, ankerite). Iron

sulphides (chiefly pyrite) are found occasionally. Magnetite is mainly found in magnetite quartzite and silicate-magnetite quartzite. Its grain size is not homogeneous from layer to layer, averaging 0.04 mm in diameter. Magnetite is idiomorphic or xenomorphic, usually in a mosaic texture with coexisting quartz, and thus forms thin iron-rich bands interbedded with thin silica bands almost free of iron. Very fine dusty magnetite is often included in quartz, showing colloidal features of deposition. Because of weathering and oxidation the magnetite in the surface region is mostly martitized. Haematite is mainly found in haematite quartzite. It is usually specular and fresh, stable against oxidation and weathering, and arranged parallel to schistosity. The laminae of specular haematite are usually very thin, from 10 -2 mm to 1-2 mm. The dusty variety is also very common. Iron silicates (chlorite, minnesotaite and stilpnomelane) have been determined by optical methods, X-ray powder diffraction and electron microprobe analysis. Chlorite is the most commonly distributed iron-bearing silicate mineral in the Yuanjiacun deposit. It is usually green to dark green (fl= 1.610-1.647). Compositionally it is ferromagnesian. Microprobe analysis shows the ratio [FeO]/( [FeO] + [MgO] ) to be 0.46-0.69. According to Hey's classification (Hey, 1954) the chlorite of the Yuanjiacun deposit mostly belongs to the turingite series. Generally speaking, the iron-rich variety is rich in Cr ( [Cr203] =0.4-2.6% ), but poor in Ca, and the magnesium-rich variety vice versa (see Table IV). Minnesotaite is found in minnesotaite-magnetite quartzite forming thin minnesotaite bands 0.5-2 mm thick. Under the microscope, the fresh mineral is a colourless transparent laminar aggregate resembling muscovite and talc. During oxidation and weathering it breaks down to form red limonite or goethite. The optical constants (fl=1.607)

44

w

•

\~,

E

3 P t i ly 1

I 1 ~ 1 Cambrian limestone

~

chlorite schist

basal conglomerate

•

Sericite schist

~///.L

=.

hematite

quartzite

quartz sandstone --~ arkose

magnetite quartzite silicate--magnetite

quartzite ~ 1

intermediate--mafic dyke

Fig. 4. Simplifiedgeologicalsection of the Yuanjiacundeposit. and [ F e O ] / ( [FeO] + [MgO] ) values (0.330.42) fall between those of pure talc and those of standard minnesotaite. The X-ray powder data show that the I and d values are close to those of standard minnesotaite of the Lake Superior Region (International Centre for Diffraction Data, 1980), but are significantly different from those of standard pure talc (see Tables V and VI ). Since crystallographically this mineral is close to standard minnesotaite, but chemically contains a considerable amount of MgO, it can be termed magnesian minnesotaite. The Yuanjiacun deposit is the first locality in China where this mineral has been reported in a Precambrian iron formation. Stilpnomelane has been found in tuff layers in the middle part of the deposit. It is more common within the ferruginous schist and in association with siderite horizons in the southern part of the Yuanjiacun deposit where postdepositional hydrothermal activity is stronger. It is dark brown or dark green in colour, with strong pleochroism under the microscope: fl - dirty brown or green-brown to dark green; a - brown-yellow or gold-yellow to green; fl= 1.60-1.70. Siderite and ankerite are mostly found in the

southern part of the Yuanjiacun deposit in drill holes, while on the surface they have changed to limonite or goethite. According to information provided by Geological Team 215, several layers or lenses have been found. The thickness is usually 5-20 m, and it is 70 m at a maximum. The iron carbonate beds are developed in association with carbonaceous chlorite schist, as well as being in ferruginous schist and silicate-magnetite quartzite. During the field measurement of the geological sections in the middle part of the deposit, the siderite beds were not seen. The reasons for this could be either surficial oxidation or lateral facies change. Within the sericite and chlorite schists a considerable amount of organic matter has been detected. Locally carbonaceous sericite and chlorite schists are present, sometimes with the development of disseminated or laminated pyrite. Shen et al. (1982) analysed free carbon in the iron formation of the southern part of the Yuanjiacun deposit. Organic carbon and graphite carbon are traced in all types of rocks and ores. Furthermore, the organic content in argillitic schist, ferruginous schist and siderite quartzite is relatively high, whereas in magnetite quartzite it is relatively low and in haematite quartzite it is negligible.

45 TABLE II Stratigraphic cross-section of the Yuanjiacun iron formation Peijiazhuang formation (Ptllp),overlyingsequence Sericiteschist Quartz sandstone

Yuanjiacun formation (Ptlly) Upper part, Ptlly~ 24 Haematite quartzite 20 m 23 Magnetite quartzite 93 m 22 Intercalated magnetite quartzite, sili- 47.4m cate-magnetite, quartzite, o~litic ores, ferruginousschist, chlorite schist, etc. 21 Quartz-chlorite schist and sericite 40m schist 20 Quartz sandstone 20 m Middle part, Ptlly2 19 Haematite-bearingquartzite 20.8 m 18 Haematite quartzite 49.8 m 17 Magnetite quartzite 124.7 m 16 Minnesotaite-magnetite quartzite 20.8 m 15 Intercalated oSlitic ores, ferruginous 59.1m schist, chloriteschist and sericiteschist 14 Magnetite quartzite with interbeds of 67.7m chlorite schist 13 Intercalated o~ilitic ores, magnetite 49.6m quartzite, silicate-magnetite quartzite, ferruginous schist and chlorite schist, locallywith tuff materials 12 Chlorite schist 30 m 11 Quartz-sericite schist 30 m 10 Quartz sandstone, quartz arkose 18.4 m Lowerpart, Ptlly1 9 Haematite quartzite 17.6 m 8 Magnetite quartzite 62.9 m 7 O~iliticores with interbeds of ferrugi- 39.8m nous schist 6 Magnetite quartzite 55 m 5 Coverlayer 40 m 4 Quartz chlorite schist 20 m 3 Sericite schist 15 m 2 Quartz sandstone with interbeds of 100 m chlorite schist and sericite schist Quartz arkose 25 m (Contact with underlying sequence of Ningjiawan formation, Ptlln, is covered)

The general features of the spatial distribution of iron sedimentation facies for the Yuanjiacun deposit are as follows. Scattered pyrite occurs only in the sericite and chlorite schists

with a b u n d a n t organic matter. Siderite lenses are developed mainly in association with carbonaceous schist, as well as in association with ferruginous schist and silicate-magnetite quartzite. Iron silicates are formed from the mixtures of the argillitic component and the iron-silica component. T h e y are mainly found in chlorite schists, ferruginous schists and silicate-magnetite quartzites. Spatially, iron silicate facies and iron carbonate facies can overlap each other, but usually the former lies above the latter. Magnetite begins to appear in ferruginous schists and silicate-magnetite quartzites, and the largest concentration of magnetite is reached in colloidally deposited magnetite quartzite. Specular haematite is mainly found in haematite quartzite and occupies the highest level of each cycle of iron formation. Its transition with magnetite facies is gradational. T h u s within a small area of the Yuanjiacun deposit all four types of iron sedimentation facies have been found. In stratigraphic sequence from the bottom upwards, the general order of their appearance is as follows: pyrite, siderite, silicates, and magnetite and haematite. Their spatial relationship with sedimentary rocks is shown in Fig. 5. This kind of vertical distribution of iron mineral facies is very similar to t h a t associated with the Kursk Magnetic Anomaly, U.S.S.R., described by Plaksenko et al. (1972); however, the trend found in the case of the Yuanjiacun deposit seems more impressive. It is also reported t h a t in the iron formation of the Cuyuna area, Lake Superior, carbonates and silicates are at the lower level of the stratigraphic sequence and oxides are at the higher level (Morey, 1972 ). Environment

of deposition

Corresponding to the successive vertical change in iron sedimentation facies, from bottom upwards, there is a gradational change in iron valence state from purely ferrous (pyrite, siderite, iron silicates), into mixed ferrous-ferric (magnetite), and finally purely fer-

46 TABLE III Chemical composition of rocks and ores from the Yuanjiacun iron formation

1 2 3 4 5 6 7 8 9 10 11 12 13 14

431-6 223-7 338-8 314-17 406-11 410-A 411-14A 406-57-1 406-52-1 409-1 402-1 408-4 404-3 406-21

Si02

Ti02

A1203 Fe203

FeO

MnO

MgO

CaO

Na20

K20

P20s

LOI

Total

65.39 46.56 73.98 39.23 65.82 42.89 30.00 46.86 65.98 46.13 44.49 48.99 45.99 42.91

0.60 1.27 0.21 0.58 1.03 0.60 0.10 0.12 0.20 0.03 0.06 n.d. 0.06 0.01

17.65 14.20 4.72 15.65 7.45 13.95 n.d. 3.18 5.04 0.88 n.d. n.d. n.d. n.d

1.56 15.82 5.56 12.58 10.69 9.66 1.07 3.39 5.19 8.60 5.08 5.69 0.68 0.34

0.06 0.08 0.11 0.03 0.10 0.02 2.28 0.03 0.09 0.02 n.d. 0.31 n.d. 0.12

0.70 7.74 1.53 6.13 5.04 6.20 1.17 1.49 2.24 1.33 0.13 0.16 n.d. 0.13

0.34 0.65 0.45 0.77 0.74 1.02 0.10 0.23 0.68 0.12 0.18 0.05 n.d. 0.02

0.11 0.32 0.03 0.04 0.03 0.03 0.14 0.05 0.04 0.06 0.06 0.08 0.07 0.06

5.34 0.62 0.03 0.03 0.03 0.06 2.24 0.03 0.03 0.08 0.03 n.d. 0.03 0.14

0.07 0.26 0.08 0.14 0.16 0.09 0.06 0.10 0.17 0.04 0.16 0.03 0.06 0.06

4.12 8.03 2.05 6.96 1.66 7.11 0.08 1.68 2.37 0.14 0.10 0.06 0.22 0.05

100.92 100.45 99.37 99.23 98.37 98.85 99.88 100.49 99.76 99.84 100.48 100.60 100.09 100.63

4.98 4.88 10.62 17.09 5.62 17.22 62.46 42.33 17.83 42.41 50.22 45.23 52.98 56.73

n.d., not detected. 1, carbonaceous sericite schist; 2, carbonaceous chlorite schist; 3-6, chlorite schists; 7, oSlitic martitized quartzite; 8, oSlitic chlorite magnetite quartzite; 9, chlorite magnetite quartzite; 10, minnesotaite magnetite quartzite; 11, 12, magnetite quartzite; 13, 14, haematite quartzite.

TABLE IV Chemical composition of Fe-rich chlorites from the Yuanjiacun deposit (by microprobe analysis) Compound MgO FeO A1203 Si02 Cr203 CaO [FeO]/( [FeO] + [MgO] )

14.67 31.42 23.01 30.43 0.49 0.68

15.04 30.24 22.72 31.59 0.40 0.67

TABLE V

Chemical composition of minnesotaite from the Yuanjiacun deposit (by microprobe analysis) Compound MgO FeO SiO2 Cr203 [FeO]/( [FeO] + [MgO] )

22.16 15.11 62.71 0.41

23.97 11.55 64.48 0.33

20.28 14.56 59.52 5.63 0.42

12.08 27.28 20.19 37.81 2.63 0.69

17.27 17.25 16.96 22.57 1 9 . 6 1 19.49 14.49 39.48 0.07 0.02 0.59 1.19 0.50 0.63

19.35 16.55 22.04 40.95 1.12 0.46

17.43 19.25 21.45 39.86 1.44 0.56 0.52

ric (haematite). This sequence reflects an increasing oxygen fugacity during the course of d e p o s i t i o n o f i r o n f a c i e s , i.e. f r o m a s t r o n g l y r e ducing environment through a moderately-reducing-oxidizing environment to a strongly oxidizing environment. Two key factors controlling the oxidation-reduction potential of the depositional environment are commonly considered. One factor is t h e o x y g e n a v a i l a b i l i t y , w h i c h is d e t e r m i n e d by water depth and proximity to shoreline. G e n e r a l l y s p e a k i n g , m o r e o x y g e n is a v a i l a b l e i n the zone of a near-shore shallow water sea basin

47

T A B L E VI X-ray powder data of minnesotaite from the Yuanjiacun deposit Yuanjiacun (this paper)

I d

6 3.10

5 2.66

4 2.19

7 1.81

Lake Superior~

I d

50 3.17

20 2.65

20 2.204

10 1.818

8

4

7

1

5

1.69

1.45

1.177

1.055

1.016

5 1.692

10 1.450

20 1.176

15 15 1 . 0 5 1 1.018

aInternational Centre of Diffraction Data (1980).

Rock type Sandstone Saricite schist Chlorite schist

Mt--bearinl[ schist I~--Qul~zile Hm Quartzite Nature of Sediments Clastic

Chemical Fe mineral facies Sulfide Carbonate

O

Silicate Oxide {

Mal[netite Hematite

Ore Structure Oolitic Thin--banded

Fig. 5. Spatial distribution of iron mineral facies in sedimentation-ore formation cycle of the Yuanjiacun deposit.

influenced by wave activity. The other factor is the content of organic matter and the presence of sulphate-reducing bacteria. The existence of rich organic matter and its protection from oxidation create a reducing environment. The presence of abundant sulphate-reducing bacteria forms a medium rich in H2S. In their interpretation of conditions of sedimentation of iron facies, James (1954), Borchert (1960) and others strongly emphasize the role of water depth and nearness to shoreline of a particular sedimentation locality. They sug-

gested that in the near-shore region and the shallow parts of the sedimentation basin oxides occur. In the further and deeper parts silicates and carbonates occur. In the relatively far and deeper parts sulphides occur. Gastil and Knowles (1960) studied the spatial distribution of iron mineral facies in the Wabush Lake area of eastern Canada. From northwest to southeast there successively appear specular haematite, magnetite, and iron carbonate and silicate. They came to the conclusion that the northwest side with haematite facies was the shoreline side during iron deposition. Plaksenko et al. ( 1972 ) studied the sequence of iron formation at the Kursk Magnetic Anomaly, U.S.S.R., and pointed out that the types of iron minerals are controlled by the content of organic matter. The following facts should be taken into account. (1) The ferrous and predominantly ferrous minerals, such as pyrite, siderite and iron silicates, are spatially closely associated with argillitic materials, where abundant organic matter is usually detected. (2) The iron oxide facies are developed in an environment in which sandy-argillitic materials are hardly reached and the organic content is low (especially for haematite facies). In a shallow sea basin the gradient of oxygen fugacity of seawater is expected to be negligible. Hence James' model (1954) seems inappropriate for the Liiliangshan region. The key factor controlling the oxygen fugacity during precipitation of iron facies seems to be the rel-

48

oxide facies ~

silicatefacies ~

F'~sulphide focies~argillite

~

carbonatefacies siltstone

j~

sandstone

Fig. 6. Sketch showing the environment of deposition of iron formation in the Liiliangshan region.

ative amount of organic matter at the locality of deposition. The factors of seawater depth and nearness to shoreline might be more significant and more universal for the oSlitic iron ores of the Phanerozoic. The environment of deposition deduced for the early Proterozoic iron formation in the Liiliangshan region is illustrated in Fig. 6. In the seashore zone, rapidly precipitated sediments are coarse, medium- to fine-grained sand and silt; along the gentle or flat sea bottom towards the sea basin, the grain size of the clastics becomes finer and finer. Sand and silt are replaced by argillitic materials, the maximum proportion of which is reached in chlorite schist; still further, the argillitic sediments gradually give way to colloidal chemical precipitates of iron-silica composition. The organic matter builds up in argillitic sediments and thus creates a reducing environment. The source of organic matter, is thought to be plankton which lived in stagnant water at a certain distance from the shoreline nearly free of wave action. The gradation from argillitic sediments to colloidal chemical sediments has led to different iron facies. In the sequence from bottom upwards they are as follows; pyrite; siderite; iron silicates; magnetite; haematite. The factors controlling the types of iron facies and iron valence state are suggested to be the relative proportion of sandy-argillitic materials to

colloidal chemical iron-silica materials and the content of organic matter in sediments.

Metamorphism The banded iron ores of the Yuanjiacun formation run south from the Yuanjiacun deposit to the Jianshan deposit for some 18 km. The degree of metamorphism intensifies from north to south. Based on the minerals that appear during metamorphism of argillitic and sandy-argillitic rocks of the Peijiazhuang formation and the Yuanjiacun formation, the following two metamorphic facies and five metamorphic zones can be subdivided (Wang et al., 1985): the greenschist facies are the chlorite zone and the biotite zone; the amphibolite facies are the almandite zone, the staurolite zone and the kyanite zone. The Yuanjiacun deposit and the Jianshan deposit are located at the northern end in the chlorite zone and at the southern end in the kyanite zone respectively {Fig. 7 and Table VII). The metamorphic facies and zones cross-cut the strike of metamorphosed rocks and are roughly parallel to the Xichuanhe fault. They could be caused by Xichuanhe-fault-related regional metamorphism. The Xichuanhe fault dips south. On its foot wall (northern) side are developed the high-pressure-sensitive minerals

49

2

0

N

YUANJIACUN =

l

.~! f

.f

.~of f

f,Xx

_

f

J

J

/

f

i

~;--0 ........

f

,/

_ ~ . / / / / / / / ' . . ~ ff--

~ . / / / / / / / / / L ~

i': .I

,,o,,"

I

•

~

//""

..

~ , ~ ° \'~* ~ o ~ ~ ~

-I

I

J......

J~O~ f / f

4 km

go.-7o,7

.....

.....

-'-2

s

(

"

Q

Fig. 7. Metamorphic facies and zones of the Liiliangshan region (slightly modifiedafter Wang et al., 1985). such as kyanite, chloritoid and locally glaucophane. However, on its hanging wall (southern) side are formed high-temperature minerals

such as andalusite and sillimanite.Migmatization and granitization are also widely developed south of the fault. The iron mineral assemblage of the Yuanjiacun deposit is mainly magnetite + haematite + siderite + chlorite + minnesotaite + stilpnomelane ( + pyrite). The three phyllosilicates are formed from the argilliticmaterials rich in iron and silica.There is no c o m m o n opinion on the condition of their formation. Gruner (1946), Mel'nik (1972) and Morey (1972) classified them as minerals of sedimentation stages. James (1955) interpreted them as products of low grade metamorphism. Because it is impossible to draw a sharp boundary between the sedimentation stage, the diagenesis stage and the low grade metamorphism stage, there is no serious problem in putting them into the class of minerals of sedimentation, diagenesis and initialstage of metamorphism. The mineral assemblage of the Jianshan deposit is magnetite + grunerite + cummingtonite. These minerals are associated with different kinds of schists containing kyanite, staurolite, almandite, biotite, muscovite and graphite. Siderite and specular haematite are basically absent. The oSlitic ores are also not found. Provided that the original iron formation and

~ABLE VII

Comparison tableof Metamorphic faciesand zonesofthe Ltlliangshanregion Metamorphic facies Greenschist facies

Amphibolite facies

Metamorphic zones

Mineralassemblage Argilliticmaterials

Ironfacies

Chloritezone

Chl +

Mt + Hem + Fe-Chl + Min + Stl + Sid

Biotitezone

Bi+Mu+Q+Ab

Almandite zone

Alm+Bi+Mu+Q

Staurolitezone

St+Alm+Bi+Mu+Q

Kyanite zone

Kya+Alm+Bi+Mu+Q

Ser + Q

Mt+Gru+Cum

Chl,chlorite;Ser,sericite;Q, quartz;Bi,biotite;Mu, muscovite;Ab, albite;Alm, almandite;St,staurolite;Kya, kyanite;Mt, magnetite;Hem, haematite;Fe-Chl,iron-richchlorite;Min, minnesotaite;Stl,stilpnomelane;Sid,siderite;Gru, grunerite; Cum, cummingtonite.

50 the c o u n t r y rock of the J i a n s h a n and the Yuanjiacun deposits were almost the same, the mineral assemblage of the J i a n s h a n deposit can be understood as products of amphibolite grade metamorphism. T h e main changes of iron minerals can be generalized as follows. Magnetite recrystaUizes and becomes coarser. Its grain size increases from 0.04 m m to 0.1-0.2 m m on average. In addition, the a m o u n t of dusty magnetite decreases. Accompanying the magnetite, the average quartz grain size increases from 0.01-0.05 mm to 0.1-0.5 mm. T h e phyllosilicates iron chlorite, minnesotaite and stilpnomelane are d eh y dr at ed to form grunerite and cummingtonite. Specular haematite disappears. Un d er conditions of increasing temperature and, more importantly, in the presence of reducing agent it tu r ns into magnetite. Siderite also disappears, either changing into grunerite through metamo r p h i c reactions or decomposing to form magnetite. Wi th respect to the c o u n t r y rocks associated with b an d ed iron ores, quartz sandstone t ur ns into quartzite; siltstone and sericite schist change into quartz-muscovite, muscovite, twomica and kyanite schists; chlorite schist changes into biotite, g r u n e r i t e - c u m m i n g t o n i t e , almandine and staurolite schists; carbonaceous schist changes into graphite schist. These features can be used as t r u s t w o r t h y evidence in the study of m e t a m o r p h i c processes and the d eter min ati on of t he original nat ur e of metamorphic rocks.

Acknowledgements T h e authors are greatly indebted to the Metallurgical-Geologial Corporation of Shanxi Province who provided facilities in doing field work. Senior students Wei Wenjun, Liu Tiebing, H u J in s h en g and some others took p a r t in field an d laboratory study. St af f members LinChengyi, Zhao Meifang and Zhang Gendi helped in mineral determinations and chemical analyses. Mrs. Gao Xiuyin drew the illustrations. T h e y are acknowledged.

References Borchert, R., 1960. Genesis of marine sedimentary ores. Trans. Inst. Min. Met., 69: 261-279. Gastil, G. and Knowles,D.M., 1960.Geologyof the Wabush Lake area, Labrador and eastern Quebec,Canada. Geol. Soc. Am. Bull., 71: 1243-1254. Gruner, J.W., 1946. The mineralogyand geologyof the taconites and iron ores of the Mesabi range, Minnesota. Officeofthe Commissionerof the Iron Range Resources and Rehabilitation, St. Paul, Minnesota. Hey, M.H., 1954.A new reviewof the chlorites.Mines Mag., 30: 277. International Center for Diffraction Data, 1980. Mineral Powder Diffraction File Search Manual. James, H.L., 1954. Sedimentary facies of iron-formation. Econ. Geol., 49: 253-293. James, H.L., 1955. Zonesof regional metamorphism in the Precambrian ofnorthern Michigan. Geol.Soc.Am. Bull., 66: 1455-1488. Mel'nik, U.P. and Siroshtan, R.E., 1972.Physico-chemical conditions of metamorphism of iron-silica rocks. In: N.P. Semenenko (Editor), Proceedings of symposium on the Geologyand Genesisof Precambrian Iron-Silica and Manganese Formations of the World, pp. 221-234 (in Russian). Morey, G.B., 1972. Iron mining areas of Mesabi, Gunflint and Cuyuna, Minnesota. In: N.P. Semenenko (Editor), Proceedingsof symposiumon the Geologyand Genesis of Precambrian Iron-Silica and ManganeseFormations of the World, pp. 204-221 (in Russian). Plaksenko, N.A., Koval', E.K. and Shchegolev,E.N., 1972. Precambrian iron formation of Kursk Magnetic Anomaly. In: N.P. Semenenko (Editor), Proceedingsof symposium on the Geology and Genesis of Precambrian Iron-Silica and Manganese Formations of the World, pp. 76-85 (in Russian). Regional Survey Team of Shanxi Province, 1972. Report of GeologicalMap, Jingle sheet, 1: 200 000.Unpublished. Shen, B., Song, L. and Li, H., 1982.An analysis of the sedimentary facies and the formation condition of the Yuanjiacun iron formation, Lanxian County, Shanxi Province, China. J. Changchun College Geol., Supplement, 31-50 (in Chinese). Wang, D., Ma, R., Wang, S. and Mou, W., 1985. Studies on fractural regional metamorphism with related migmatization and granitization. J. Nanjing University, 21: 537-544 (in Chinese). Xu, C. and Xu, Y., 1985. Redivision and designation concerning Archean strata in the Liiliang Mountains. J. Hebei CollegeGeol., 3:56-64 (in Chinese). Zhang, F. and Zhu, J., 1981. On the stratigraphic sequence of Liiliang Group, Lanxian region, Shanxi Province. J. Stratigraphy, 5:90-96 (in Chinese). Zhang, Q., 1984.Geologyand Metallogenyof the Early Precambrian in China. Jilin People's Publishing House (in Chinese).