Anatomy of the Archean Anshan iron ore belt in the North China Craton: A geophysical approach

Accepted Manuscript Anatomy of the Archean Anshan iron ore belt in the North China Craton: A geophysical approach Lin-Fu Xue, Chuan-Qi Dai, Ming Zhu, M. Santosh, Ze-Yu Liu PII: DOI: Reference:

S0301-9268(17)30096-7 http://dx.doi.org/10.1016/j.precamres.2017.04.004 PRECAM 4721

To appear in:

Precambrian Research

Received Date: Revised Date: Accepted Date:

23 February 2017 21 March 2017 2 April 2017

Please cite this article as: L-F. Xue, C-Q. Dai, M. Zhu, M. Santosh, Z-Y. Liu, Anatomy of the Archean Anshan iron ore belt in the North China Craton: A geophysical approach, Precambrian Research (2017), doi: http://dx.doi.org/ 10.1016/j.precamres.2017.04.004

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1

Anatomy of the Archean Anshan iron ore belt in the

2

North China Craton: A geophysical approach

3

Lin-Fu Xuea, Chuan-Qi Daia, Ming Zhua, M. Santoshb,c, Ze-Yu Liua

4 a

College of Earth Sciences, Jilin University, Changchun 130061, P.R. China

5

6

7 8

b

Department of Earth Sciences, University of Adelaide, SA 5005, Australia

c

School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China

9 10 11

* Corresponding author e-mail: [email protected]

12

Abstract

13

The Anshan region of the North China Craton hosts significant iron ore

14

reserves in the form of Archean banded iron formations (BIF). This study

15

focuses on NW–SE trending layered iron ore bodies identified using

16

aeromagnetic data with a 500 m point spacing and a new method that

17

combines structural analysis with the 3D inversion of magnetic susceptibility.

18

This approach has identified two prominent parallel magnetic anomalies within

19

the study area, with the bottom boundary of BIF body I estimated to be at a

20

depth of ~ 5600 m and the bottom boundary of BIF body II at ~ 5200 m. The

21

two BIF iron ore bodies are separated by granitoids associated with the

22

Archean Anshan micro-continental nucleus. The area close to this

23

micro-continental nucleus has undergone significant tectonic deformation,

24

including ductile shearing. This shearing is the main control on the distribution

25

of BIF iron ore belts in the Anshan area, in contrast to the previous view which

26

folding was thought to control the distribution of BIF. Combining our new data

27

with regional tectonic information yields a model where both folding and ductile

28

shearing controlled the distribution of the main BIF ore bodies in the study area.

29

This model provides insights into the distribution of iron ores in the Anshan

30

area as well as a guide for future deep exploration in this region.

31 32

Keyword: Banded iron formation; Geophysical imaging; 3D inversion of

33

magnetic susceptibility; Fold-ductile shear deformation; Anshan area.

34

1. Introduction

35 36

Precambrian banded iron formations (BIFs) are of considerable economic

37

importance and are the source of 90% of global iron production. Research into

38

Chinese BIF from the 1950s onwards has revealed that the majority of BIF in

39

China are located along the edges of the North China Craton (NCC), 80% of

40

which formed during the Neoarchean. These BIF are associated with

41

metamorphosed volcanic rocks and minor amounts of metamorphosed

42

sedimentary rocks. The majority of the BIF iron deposits in China are Algoma

43

type

44

metamorphism and deformation (Zhai and Windley, 1990; Zhai et al., 1990;

45

Shen et al., 1994, 2006; Zhang et al., 2011). In particular, the Anshan and

46

Benxi areas have become the focus of an increasing amount of research, as

47

these areas host more than 60% of Chinese iron reserves (Zhou, 1994; Li et al.,

48

2014). The effect of tectonic deformation on the spatial distribution of iron ore

49

and the three-dimensional geometry of belts of iron ore mineralization are key

50

research topics in these regions.

and

have

undergone

significant

and

complex

post-formation

51

It has been recognized that the spatial distribution and three-dimensional

52

geometry of Archean BIF type iron ore belts over the world are the result of

53

complex tectonic processes. Tectonic analysis indicates that folding and

54

shearing-related deformation, and the rheology of individual rock units exerted

55

important influence on the distribution of the BIF type iron deposits of the

56

Hamersley Basin of Western Australia (Campana, 1966; Powell et al., 1999;

57

Barley et al., 1999; Brown et al., 2004; Lascelles, 2006). Investigations of the

58

role of rock rheology and deformation during the formation of high-grade

59

BIF-type iron ore in the Quadrilátero Ferrífero region of Brazil have identified

60

significant rock flows or fluid activity associated with ductile shearing (Lagoeiro,

61

1998; Rosière et al., 2001; Hippertt and Davis, 2000; Siemes et al., 2003,

62

2008). The BIF within the Anshan and Benxi areas has been subjected to

63

intense deformation and the effects of large-scale magmatism, meaning that

64

these units are dismembered and dispersed throughout the surrounding

65

tonalite–trondhjemite–granodiorite (TTG) gneisses. This in turn means that

66

assessing the spatial distribution of this mineralization during mineral

67

exploration has proven difficult. The factors controlling the spatial distribution

68

of BIF-related iron ore bodies remain controversial, and include: (1) the

69

presence of a dome or anticline in the Anshan area dominated by Neoarchean

70

supracrustal rocks with a core represented by the Tiejiashan granite and limbs

71

that host iron ore belts (Li, 1977); (2) the presence of a NW-plunging

72

anticlinorium within the ancient basement of the Anshan area with an axis

73

located near Tiejiashan (Zhou, 1987); (3) the presence of ductile shear zones

74

that control the spatial distribution of BIF and associated iron deposits (Xu,

75

1991; Zhang and Wang, 1994); and (4) a new tectonic model outlined by Li et

76

al., who suggested that this region records a sagduction. The complexity and

77

multiple phases of formation of the BIF in the Anshan area have been outlined

78

previously (Liu, 1987). This research also determined that the spatial

79

distribution of BIF in the Anshan area is controlled by folding, where thicker

80

orebodies are located proximal to fold hinges. However, few studies have

81

examined the role of widespread ductile shear zones on the concentration of

82

iron ore during the structural transposition of the original sedimentary units in

83

this area. Some of the banded structures within magnetite quartzites in this

84

area may have formed by this type of structural transposition. In addition, the

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discontinuous iron ore belts within the Anshan and Benxi areas are hosted by

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three giant ductile shear zones that control the thickening, attenuation, and

87

breaking or pinching out of the ore bodies (Xu, 1991; Zhang and Wang, 1994).

88

Several of the ore bodies contain dark magnetite quartzite units with mylonitic

89

textures that have undergone grainsize reduction as a result of ductile

90

shearing. Optical microscopy indicates that iron-bearing minerals and quartz in

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these units have undergone intense deformation during ductile shearing and

92

show features that are indicative of recrystallization (Xu, 1987; Qu, 1988). The

93

surrounding rocks have also been converted to schists and mylonites as a

94

result of tectonic deformation. This indicates that although folding and ductile

95

shearing played important roles in the formation and spatial distribution of the

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Anshan BIF iron ore bodies, the exact distribution and extent of the ore bodies

97

in this region remains unclear. Several previous studies have explored the

98

characteristics of the iron deposits in the Anshan area, including their

99

exploration potential. For example, Fan et al. (2014) inverted interactively

100

gravity and magnetic anomalies to identify several synformal iron ore bodies.

101

This study also analyzed aeromagnetic and gravity anomalies along a profile

102

through a proven iron deposit within the Dong’anshan–Qidashan district and

103

predicted the presence of large iron ore bodies at depth beneath the Anshan

104

area.

105

Combining the three-dimensional geophysical imaging of BIF iron ore

106

belts with structural analysis could be a powerful approach to determining the

107

processes that formed these belts as well as providing useful guidance for

108

mineral exploration. This study focuses on dissecting the deep geological

109

structure of BIF iron ore belts in the Anshan area, including determining the 3D

110

geometry of these belts, the tectonic processes that formed these deposits,

111

and the prospectivity of this area for deep-seated iron ore mineralization. This

112

approach uses a three-dimensional susceptibility inversion method that

113

employs high-resolution aeromagnetic data in combination with structural

114

analysis. Our results confirm the role of folding and ductile shearing in the

115

development of iron ore bodies, with the distribution of iron ore in highly

116

deformed areas (e.g., the Anshan area) being controlled by ductile shearing,

117

whereas the distribution of iron ore in moderately deformed areas is controlled

118

by both folding and ductile shearing. Finally, the distribution of iron ore in

119

weakly deformed areas is controlled mainly by folding. This study outlines a

120

formation model of BIF iron ore belts that provides a basis for further

121

investigations into the BIF-related iron ore belts in the Anshan area and similar

122

ore belts in other areas, particularly with respect to the exploration for and

123

evaluation of deep-seated iron ore.

124 125

2. Geological background

126 127

The Anshan area is located in the eastern NCC and has a double-layered

128

crustal structure that consists of Archean crystalline basement and a

129

Neoproterozoic–Paleozoic

130

crystalline basement or micro-continental nucleus is dominated by the

131

Neoarchean Anshan Group (2.5 – 2.6 Ga), including the Yingtaoyuan (2530 –

132

2551 Ma), Cigou (2523 – 2571 Ma), and Dayugou formations, and a series of

133

tonalite–trondhjemite–granodiorite gneisses. The basement is overlain by

134

Paleoproterozoic greenschist facies (2469 ± 23 Ma) metamorphic rocks of the

135

Liaohe Group (Dai et al., 2013) that are different from the traditional

136

designation of this group (Li SZ et al., 2005, 2007, 2012). The majority of the

137

iron ore in this area is hosted by BIF within the Yingtaoyuan Formation of the

138

Anshan Group (Zhang, 1988; Zhou, 1994). The upper units within this

139

formation consist of sericite, chlorite, and biotite–quartz phyllites that are

140

unconformably overlain by units of the Neoproterozoic Qingbaikou System.

141

The lower part of the Yingtaoyuan Formation is dominated by chlorite, iron

142

ore–chlorite, sericite–chlorite, and biotite phyllites as well as sericite–quartz

143

schists. The Dayugou Formation contains felsic granulite and amphibolite units,

sedimentary cover

sequence. The Archean

144

lenses

of

magnetite-bearing

quartzite,

145

tremolite–epidote–diopside

146

unconformably underlain by the Tiejiashan granite (2.9Ga) that in turn is

147

cross-cut by the later Qidashan granite (2.5 Ga) (Fig. 1).

granulite

and

units.

layers The

of

Anshan

marble Group

and is

148

Two iron ore belts, the northern iron ore belt (ore belt I), The southern ore

149

belt (ore belt II), have been identified in the Anshan area (Fig.2), with the

150

northern iron ore belt (ore belt I) trending NW–SE (330°-340°) and dipping

151

near vertically (60°-85°). This belt contains (from north to south) the Qidashan,

152

Chentaigou, Wangjiapuzi, Hujiamiaozi, and Yanqianshan iron ores. Ore belt I

153

extends more than 14.5 km and contains main ore bodies with thicknesses of

154

100–300 m. The tops of these main ore bodies are characterized by an

155

oxidized hematite quartzite layer that has a thickness of a few hundreds of

156

meters. The main ore bodies beneath the oxidized ore belt are dominated by

157

magnetite and chlorite–magnetite quartzite units.

158

The southern ore belt (ore belt II) trends nearly E–W, although the iron

159

ores in this area trend NW–SE (330°-340°) and show significant variations in

160

dip (30°-80°). This belt contains (from east to west) the Dagushan, Heishilazi,

161

Dong’anshan, and Xi’anshan iron ores, with an overall extent of >2.5 km.

162

Single ore bodies within this belt have thicknesses of 90–300 m and a thin ore

163

body is also present within the units above the main Dagushan iron ore. The

164

belt is surrounded by phyllite and schist units with minor amounts of granulites

165

and amphibolites. The area contains a buried hematite–quartzite layer with a

166

thickness of 300 – 500 m as well as non-exposed magnetite quartzite and

167

amphibole–magnetite quartzite units, and minor amounts of chlorite–magnetite

168

quartzite.

169

This region also records widespread Neoarchean magmatic intrusive

170

activity, including the successive emplacement of the Lishan trondhjemite and

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Tiejiashan monzonitic granites. These intrusions define a TTG rock series that

172

includes quartz diorite, tonalite, granodiorite, and trondhjemite units, together

173

with calc-alkaline monzonitic granites. These intrusions disrupted and

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fragmented the Neoarchean Anshan group rocks that are preserved as islands

175

within the intrusive complex.

176

Three phases of Archean deformation have been identified in the study

177

area (Guo, 1994): (1) an initial phase of tectonic deformation during

178

2900–2800 Ma, including the generation of a NE–SW trending gneissosity

179

within granites as well as closed isoclinal folds with prominent axial planar

180

schistosity within supracrustal rocks; (2) a second phase at 2500 Ma that

181

generated NNW–SSE and E–W trending ductile shear zones and imparting

182

schistosity to the iron-bearing units along the ductile shear zones; and (3) a

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third phase at 2000 Ma involving shallow ductile shearing and overprinting

184

deformation. The unique tectonic history of this area means that the BIF

185

records several superimposed tectonic events that generated significant

186

internal deformation (Yang et al., 1983; Zhang et al., 1986; Xu, 1991; Zhang

187

and Wang, 1994). These three stages of deformation are consistent with the

188

known major tectonic events in the NCC that were associated with Archean

189

microcontinental amalgamation, Paleoproterozoic subduction and collision (Li

190

et al., 2007, 2012), and Mesozoic reactivation (Zhai and Santosh, 2011; Zhao

191

and Zhai, 2013; Yang et al., 2016; Yang and Santosh, 2015; 2017).

192 193

3. Data and processing

194

3.1 Aeromagnetic data

195 196

Two aeromagnetic surveys were undertaken in the Anshan region, the first

197

of which involved a survey with a distance between measuring points of ~2 km

198

whereas the second survey had a distance between measuring points of 0.5

199

km. Both surveys involved a slowly rising and falling terrain flight method with

200

an average altitude of 200 m in order to obtain high-resolution data, with the

201

altitude fixed using an equivalent source filtering noise-reduction approach

202

(Zeng, 2005).

203

Typically the data obtained from a magnetic survey are a set of magnetic

204

field measurements acquired within an above-surface 2D grid within the

205

volume of interest. These data are first processed to yield an estimate of the

206

anomalous magnetic field that is the result of the magnetically susceptible

207

material in the area. This is followed by inversion, an approach that aims to

208

derive quantitative information about the distribution of magnetically

209

susceptible material in the ground from the extracted anomaly data, meaning

210

that the extracted residual anomaly data are the main input for the inversion

211

program used. Buried magnetically susceptible material has a certain amount

212

of natural remanent magnetization, and the data used for the inversion indicate

213

the strength of the local magnetic field overprinted on the regional magnetic

214

field. Here, we assume that no remanent magnetization is present in the study

215

area and as such restrict our attention to the presence of induced

216

magnetization.

217

218

3.2 Physical properties of rocks

219

220

The magnetite quartzite units within the Yingtaoyuan Formation have

221

magnetic susceptibility values as high as 552,600 × 10−6 CGSM with remanent

222

magnetism values up to 3,052,800 × 10−6 CGSM. These values are much

223

higher than the values for the plagioclase amphibolite-hosted Fe-poor

224

quartzites of the Cigou Formation, primarily as the phyllitic rocks and

225

plagioclase amphibolites are either non-magnetic or are only weakly magnetic.

226

In addition, the banded magnetic quartzite within the Anshan Group has

227

magnetic susceptibility values of 3000–100,000 × 4π× 10−6 SI, whereas the

228

sedimentary cover is non-magnetic.

229

230

4. Aeromagnetic anomaly features

231 232

The Anshan region shows a clear high aeromagnetic anomaly zone,

233

although closer examination using a 1:50,000-scale aeromagnetic anomaly

234

map reveals that this anomaly is actually composed of the parallel NW–SE

235

trending high magnetic anomaly belts I and II (Fig.2). The relationships

236

between aeromagnetic anomalies and geological formations (Fig. 1) in the

237

study area indicate that magnetic anomaly belt I is delineated by the 0

238

aeromagnetic anomaly isoline and is closely related to areas with known iron

239

ore belts or BIF. In comparison, the region containing magnetic anomaly belt II

240

is covered by Quaternary formations. The Tiejiashan intrusions are located

241

between the two high magnetic anomaly belts, are associated with a region

242

with a low magnetic anomaly, and form the basement of the BIF in this area.

243

Reducing the magnetic anomaly data to a pole figure (Fig. 2) using areas

244

with known iron mineralization (Table 2) indicates that all of the known iron

245

mineralization in the study area is located in the region with a high magnetic

246

anomaly. The aeromagnetic anomalies associated with the southern ore belt

247

slightly overlap with the high magnetic anomaly belt II and are also associated

248

with known areas of mineralization that define an overall E–W trending belt,

249

despite the fact that the long axes of individual aeromagnetic anomalies

250

extend NW–SE to NNW–SSE. In addition, the eastern Tiejiashan granites are

251

associated with relatively closely spaced aeromagnetic anomaly contours,

252

whereas the western and northern parts of these intrusions are associated with

253

relatively widely spaced anomaly contours.

254

Reduced-to-pole aeromagnetic anomaly data for the study area yield

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different upward continuation (Fig. 3) heights that highlight the northeastern

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negative magnetic anomaly. This anomaly is associated with magnetic body I

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and suggests that this orebody dips to the NE. All of the mineralization within

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the Qidashan (32), Hujiamiaozi (35), Wangjiapuzi (33), Zhangjiawan (27),

259

Lazishan (26), Guanmenshan (36), and Yanqianshan (2) iron ores (Table 2) in

260

magnetic body I dip steeply to the NE or to the ENE. The 3D magnetic

261

susceptibility inversion model contains the same NE dip that is present in

262

magnetic body I. In addition, the high magnetic anomaly values associated

263

with magnetic bodies I and II disappear with increasing height, whereas the

264

northern magnetic anomaly remains consistently high, indicating that the

265

southern magnetic body I is located at a shallow depth whereas the northern

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magnetic bodies are located at a deep depth. The northern magnetic bodies I

267

and II coalesce at a height of 3000 m, suggesting they may be connected at

268

depth. These bodies form a circular magnetic high at a height of 9000 m and

269

do not close at a height of 20,000 m, suggesting that this reflects the regional

270

background magnetic field. The basement slopes and extends towards the NE,

271

with a low magnetic anomaly region in the northeastern and southwestern

272

parts of the study area (Fig. 3).

273

274

5. 3D inversion of magnetic susceptibility

275

5.1 3D inversion method of magnetic susceptibility

276 277

The geometry of BIF bands obtained using a 3D inversion of the magnetic

278

susceptibility data yields a 3D image that provides important information on the

279

evolution and distribution of iron ore in this region. The regional aeromagnetic

280

anomalies suggest that the 3D inversion of magnetic susceptibility reflects the

281

presence of an underground magnetic body. The 3D inversion program

282

operates by simultaneously minimizing the model objective function while

283

generating a synthetic dataset that adequately fits the collected data (Li and

284

Oldenburg, 1998). The model is then specified using a mesh of rectangular

285

cells, each with a constant susceptibility value, with magnetic responses

286

calculated for any location within the model volume. The airborne magnetic

287

data were also inverted in order to construct 3D susceptibility contrast models.

288

These data were reduced using standard regional–residual separation

289

techniques such as wavelength filtering (Li and Oldenburg, 1998), with

290

inversions performed until the synthetic data agree with the collected data

291

within a degree of misfit calculated using the statistical attributes of the data.

292

The model objective function is minimized by generating a magnetic anomaly

293

that is close to the reference model and is smooth in all three spatial

294

directions. We used the UBC-GIF inversion code, which incorporates a depth

295

weighting function that counteracts natural kernel decay (Li and Oldenburg,

296

1998). These data are first processed to yield an estimate of the anomalous

297

field generated by the magnetically susceptible material in the area. The goal

298

of the magnetic inversion is to obtain quantitative information about the

299

distribution of magnetic susceptibilities in the ground from the extracted

300

anomaly data, indicating that the input for the inversion program is assumed to

301

be the extracted residual anomaly data.

302 303

5.2 3D magnetic susceptibility inversion in Anshan

304 305

Buried magnetically susceptible material contains a given amount of

306

natural remanent magnetization. Our inversion incorporates the local magnetic

307

field separately to the regional magnetic field, allowing us to assume that no

308

remanent magnetization is present and enabling this modeling to focus on

309

induced magnetization. An upward continuation of 25,000 m is used as the

310

background of local field extraction with 1:50,000 local field magnetic anomaly

311

data inverted using the UBC Mag3D software package. This inversion used a

312

horizontal mesh subdivision value of 350 × 350 m with E–W profiles divided

313

into 162 grid cells and N–S profiles divided into 112 grid cells. Three of these

314

grids were extended with widths of 350, 700, and 1400 m, with an internal

315

depth subdivision of 175 m divided into 85 grid cells. The total number of grid

316

subdivisions is 1,685,040, including 168 E–W nodes, 118 N–S nodes, and 85

317

depth nodes.

318

The 3D magnetic susceptibility model for the Anshan area was obtained

319

after 60 iterations, yielding a 3D inversion map of magnetic susceptibility in the

320

Anshan area and an E–W cross-section. The map indicates that the Tiejiashan

321

granites are surrounded by high magnetic anomalies associated with magnetic

322

bodies I and II, and the 3D inversion model should conform to this. We

323

combined geophysical and geological data and selected a value of 0.14 as a

324

cut-off, with this value used during the 3D geological inversion of the magnetic

325

material within the study area (Fig. 4).

326

The 3D magnetic susceptibility inversion indicates that the top parts of

327

buried magnetic bodies I and II are located at a depth of ~300 m. The northern

328

magnetic bodies are located deeper than the southern bodies, with a bottom

329

interface burial depth of ~7000 m. Magnetic bodies I and II (Fig. 4) are

330

centered on the Tiejiashan granites, have a ring-shaped distribution, and are

331

located in the northwestern part of the study area. The 3D magnetic

332

susceptibility inversion indicates that magnetic body I dips to the NE with a dip

333

angle that is spatially variable. In comparison, the southern magnetic body

334

initially dips more gently, although this dip steepens to the north before

335

eventually becoming nearly vertical. The inversion also highlights a small

336

branch of southern magnetic body II that has a “U”-shaped connection with the

337

rest of the body before having a distorted central section with a middle domain

338

deeper than the end parts of this branch. Finally, the sections of magnetic body

339

II change from flat to elliptical with increasing distance from south to north.

340 341

6. Discussion

342

6.1 Formation mode of the BIF iron ore belt

343

6.1.1 Formation mechanism of the BIF iron ore belt

344 345

Different regions of the study area are associated with differently

346

deformed BIF, with magnetite quartzite in the Anshan area having a streaky

347

and banded structure that is defined by folded streaks and bands. The

348

deformation of quartz and magnetite within these units is also clearly visible

349

during optical microscopy. These rocks record ductile deformation and

350

recrystallization that has transformed the iron ore into a fine-grained

351

ferruginous–quartzose mylonite containing a well-developed foliation. Many of

352

the iron ore layers and over- and underlying units are also intensely deformed

353

and define ductile shear zones that impart a parallel banded structure to these

354

iron ore bodies. The bands within the ore bodies appear to be the original

355

sedimentary layers although they have been tectonically sliced. The

356

composition of this material, combined with structural and sequence

357

relationships, suggests that the original iron-bearing formation has been

358

essentially obliterated by multi-phase post-formation deformational events.

359

The Anshan area does not contain intensely developed fold structures

360

although mineralization in the Waitoushan area of the northern Anshan region

361

is associated with folds of various sizes (Yang et al., 1983). The BIF iron ore

362

belts in this area are considered to be tectonic deformation zones that record

363

strong ductile deformation in regions of compression, but have only undergone

364

folding in areas recording only weak deformation, with intermediate domains

365

that record moderate tectonic compression containing zones of folding of

366

various intensities as well as ductile shear zones.

367

The 3D magnetic susceptibility inversion has identified two NW–SE

368

trending tabular BIF bodies within the Anshan area (Figs. 4 and 5). The bottom

369

depth of these tabular bodies lies at ~7 km and these bodies have complex

370

internal structures relating to the intercalation of BIF with surrounding units.

371

These data suggest that the formation of the BIF iron ore belts is closely

372

related to both folding and ductile shearing. The location of BIF-related iron ore

373

belts is controlled by folding in areas with well-developed folds, whereas the

374

main control on the location of this type of mineralization is ductile shearing in

375

areas with more intense deformation. The BIF in the Anshan area clearly

376

records ductile shearing, suggesting that the formation of the BIF iron ore belts

377

was controlled mainly by this shearing.

378 379

6.1.2 Favorable sites for BIF iron ore belt

380 381

A composite multi-phase intrusion crops out between northern iron ore

382

belt I and the concealed northwestern iron ore belt II (Figs.1 and 2). This

383

composite intrusion includes the Chentaigou (3.3 Ga), Lishan (3.0 Ga),

384

Tiejiashan (2.9 Ga), and Dong’anshan (3.0 Ga) plutons (Wan, 2001), all of

385

which are older than the BIF in this area and are considered to be part of an

386

Archean microcontinent block. The formation of the ancient continental

387

nucleus and micro-continental blocks in this area might have occurred prior to

388

3.0 Ga, with dating of granite gneisses in the Anshan area indicating that the

389

Anshan micro-continental nucleus, one of the ancient Archean continental

390

nuclei of the NCC, formed at ~ 3.3 Ga (Wu et al., 2008). The formation of this

391

micro-continental block was followed by continental crustal accretion events at

392

2.9–2.7 Ga and tectonic deformation, metamorphic, and magmatic events at

393

2.5 Ga (e.g., the formation of the granitic Qidashan Pluton). The latter event

394

marks the splicing and cratonization of micro-continental blocks within northern

395

China during 2600 to 2500 Ma during a sequence of continent–continent,

396

continent–arc, and arc–arc collisions. Smaller arcs could also have been

397

present at this time that were not involved in the accretion and formation of this

398

micro-continental block collage, an event that is locally known as the Anshan

399

movement (Zhao et al., 1993; Bai et al., 1993; Cheng, 1994; Wu et al., 1998;

400

Zhai, 2010). The margins of the Anshan micro-continental nucleus underwent

401

intense deformation during the middle to late Neoarchean, as evidenced by the

402

presence of near-vertical schistosity within magnetite quartzite units as well as

403

broad and gentle folds, and small-scale ductile shear zones within zones of

404

weaker deformation. Areas with large-scale TTG intrusive magmatism

405

associated with significant tectonic deformation contain magnetite quartzite

406

units that also have a near-vertical schistosity. These rocks are largely

407

well-preserved despite the multiple pulses of magmatism (Fig. 6). However,

408

the BIF are more plastic and are more easily dismembered and destroyed,

409

meaning that composite fold–ductile shear zones are more widespread within

410

the northeastern and southwestern sides of the micro-continental nucleus, and

411

represent areas that were more favorable for the development of BIF ore belts.

412 413

6.1.3 Influence of the later structures on the BIF distribution

414 415

The southern ore belt (ore belt II) trends E–W although the majority of the

416

iron ore bodies within ore belt II trend NW–SE as a result of activity along the

417

Mesozoic Hanling–Pianling strike-slip fault. This southern ore belt does not

418

show a significant aeromagnetic anomaly within the map produced during this

419

study, with the upward continuation of the belt nearly disappearing at a height

420

of 3000 m (Fig. 3). This suggests that the iron-bearing lithological units (i.e.,

421

magnetic bodies) within this southern ore belt are shallow and small. The

422

southern ore belt also contains the aforementioned Hanling–Pianling Fault, a

423

feature that also cross-cuts and offsets the northwestern BIF. Some of the BIF

424

fragments within the southern ore belt also trend NW–SE, and the entirety of

425

this southern ore belt was the result of post-formation strike-slip movements

426

that affected the northwestern part of the BIF iron ore belts.

427

428

6.1.4 Formation mode of the BIF ore belt

429 430

The intense folding and contemporaneous intrusion of TTG rocks during

431

the late Neoarchean, and subsequent Mesoproterozoic and Mesozoic uplift

432

events dismembered and significantly denuded the BIF within the study area.

433

This means that the only BIF present in this area is relicts within the

434

widespread TTG gneisses, making it difficult to determine the original

435

distribution of these BIF. Existing models suggests that the preserved BIF in

436

this area was originally part of the folds, forming the cores or limbs of

437

preserved synclinal folds of parts of hook-shaped folds. The northern ore belt

438

has a monoclinal structure although the counterpart to this structure has not

439

been identified. Although the prevailing view is that the spatial distribution of

440

BIF and associated iron ore bodies is controlled primarily by fold structures,

441

these models cannot effectively explain the ductile shearing that is commonly

442

found in these iron formations, the fact that a tabular BIF body with an extent of

443

nearly 12 km does not contain any macro-scale folding, and the fact that

444

lenticular ore bodies occur intermittently throughout the ore belt. Here, we

445

combine our new data with previous research to develop a composite

446

fold–ductile shearing model that provides insights into the controls on the

447

spatial distribution of BIF iron ore belts (Fig. 6).

448

Significant NE–SW compression during the middle–late Neoarchean

449

affected the BIF in the study area. These ferruginous rocks behave plastically

450

at high temperatures, meaning that increased compression caused ductile

451

shear zones to develop along fold limbs. The initial stages of shearing were

452

associated with divergent ductile shear zone and fold axial surface directions

453

that changed with increasing compression, leading to the closing of folds and

454

the movement of the ductile shear zones towards being parallel with the fold

455

axial surfaces (Fig. 6). This sagduction-related process (Li et al., 2015a, b, c, d,

456

e, 2017) of folding and ductile shearing caused the squashing and expansion

457

of BIF within fold hinge zones, a process that can explain the hook-shaped or

458

tabular bodies in the study area. The limbs of folds that underwent intense

459

ductile shearing are also preferentially enriched with iron ore, as evidenced by

460

the high grade tabular bodies. Increased compression caused fold limbs to

461

become parallel, eventually forming closed folds with near-vertical axial

462

surfaces and a series of ductile shear zones. These structures further evolved

463

to large-scale ductile shear zones. This enhanced compression occurred

464

within the

465

micro-continental nucleus, forming areas of iron-enriched ductile shear zones

466

that correspond to aeromagnetic anomaly belt I as well as the northern ore belt

467

(Fig. 6).

northeastern

and southwestern domains

of the Anshan

468 469

6.2 Spatial distribution of the BIF

470 471

The cross-section provided in this study was constructed using drill hole

472

data and provides evidence of a structural configuration comprising

473

iron-bearing units within a closed fold. The BIF in this area is interlayered with

474

schists that decrease in frequency with increasing depth and are thought to

475

represent part of the original formation. This suggests that the iron ore bodies

476

form thick tabular-shaped units that extend to significant depths, indicating in

477

turn that this region is highly prospective for deep-seated iron mineralization

478

(Fan et al., 2013).

479

The fold–ductile shearing composite model for the Anshan area indicates

480

that the BIF ore belts are a series of synclines, anticlines, and ductile shear

481

zones, with different layers of iron-bearing units repeatedly appearing within

482

fold–ductile shear zones. The different iron-bearing layers and interlayers may

483

be the result of the same unit undergoing repeated folding and ductile shearing.

484

The original iron-rich BIF layers had transformed to discontinuous ore bodies

485

by the end of the Neoarchean as a result of intense folding and ductile

486

shearing. These iron ore bodies are generally located within composite

487

structural zones, and the bottom of the iron mineralization appears to be

488

dependent on the depth of development of closed folds and ductile shears.

489

Combining the 3D magnetic susceptibility inversion with the new genetic model

490

for the formation of BIF ore belts suggests that the BIF magnetic body

491

corresponding to aeromagnetic anomaly I (i.e., ore belt I) is tabular and formed

492

as a result of folding and ductile shearing. This BIF body has characteristics

493

that are indicative of formation within a ductile shear zone, and the shallow and

494

deep parts of the body have similar features, suggesting they formed as a

495

result of the same processes. The bottom of this body is located at ~5200 m,

496

suggesting this area has significant deep exploration potential. The BIF

497

magnetic body corresponding to aeromagnetic anomaly II (i.e., ore belt II) has

498

the same characteristics as iron ore belt I, with a bottom depth (~5600 m) that

499

again is indicative of significant deep exploration potential. In comparison, the

500

bottom boundary of the magnetic body in the southern ore belt (i.e., iron ore

501

belt II) is at a depth of ~3000 m. The fact that this belt consists of fragments of

502

the main ore belt that were dismembered during the Mesozoic suggests that

503

this belt has a shallower bottom depth with only limited potential for deep

504

exploration.

505 506

7. Conclusions

507 508

(1) The spatial distribution of BIF and iron ore bodies in the Anshan area is

509

controlled by folding and ductile shearing. Ductile shearing is the dominant

510

control on the distribution and geometry of BIF close to the Anshan

511

micro-continental nucleus. Both folding and ductile shearing have

512

enhanced iron ore grades. Potential iron ore bodies are generally located

513

within large-scale BIF-hosted ductile shear zones that record intense

514

deformation.

515

(2) The two tabular BIFs within the Anshan area extend NW–SE and dip to the

516

NE. The bottom of tabular body I is located at a depth of ~5200 m, whereas

517

the bottom of tabular body II is located at a depth of ~5600 m. These two

518

bodies developed separately within the northeastern and southwestern

519

sides of the Archean Anshan micro-continental nucleus. Both bodies are

520

associated with ductile shear zones and have significant deep exploration

521

potential.

522

(3) The southern ore belt consists of BIF fragments derived from the

523

northwestern ore belt and is cross-cut by strike-slip faults. These fragments

524

are generally shallow and have only limited potential for deep exploration.

525

Acknowledgments

526

We thank the editor and two referees for their valuable suggestions and

527

comments that improved our manuscript. Thanks to Prof. Sanzhong Li for his

528

kind comments on Fig. 6. This research is funded by a pilot project “Deep

529

Geological Survey of Benxi-Linjiang area” (Project No. 1212011220247), from

530

3D geological mapping and deep geological survey of China Geological

531

Survey.

532 533

References

534

Bai, J., Huang, X.G., Dai, F.Y., Wu, C.H., 1993. The Precambrian evolution of

535

China. Beijing: Geological Publishing House.

536

Barley, M.E., Pickard, A.L., Hagemann, S.G., Folkert, S.L., 1999. Hydrothermal

537

origin for the 2 billion year old mount tom price giant iron ore deposit,

538

Hamersley Province, western Australia. Mineralium Deposita 34(8),

539

784-789.

540

Beukes, N.J., Gutzmer, J., Mukhopadhyay, J., 2013. The geology and genesis

541

of high-grade hematite iron ore deposits. Applied Earth Science 112(1),

542

18-25.

543 544

Boyle, R.W., Davies, J.L., 2011. Banded iron formations. Geochimica et Cosmochimica Acta 37(5), 92-103.

545

Brown, M.C., Oliver, N.H.S., Dickens, G.R., 2004. Veins and hydrothermal fluid

546

flow in the mt. whaleback iron ore district, eastern Hamersley Province,

547

western Australia. Precambrian Research 128(3–4), 441-474.

548

Campana, B., 1966. Stratigraphic-structural-Paleoclimatic controls of the newly

549

discovered iron ore deposits of western Australia. Mineralium Deposita

550

1(1), 53-59.

551

Cheng, Y.Q., 1957. Problems on the genesis of the high-grade ore in the

552

pre-Sinian (pre-Cambrian) banded iron ore deposits of the Anshan-type

553

of Liaoning and shantung Provinces. Acta Geological Sinica 37(2),

554

153-180.

555

Cheng, Y.Q., 1982. Notes on the metamorphic series and metamorphic: belts

556

of various metamorphic epochs of china and related problems. Regional

557

Geology of China 1(2), 1-14.

558 559 560 561

Cheng, Y.Q., 1994. Outline of Regional Geology of China. Beijing: Geological Publishing House. Cui, P.L., 2014. Metallogenic Tectonic Setting, Metallogenic and Prospecting Models for Iron-Formation in the Anshan-Benxi Area. Jilin University.

562

Dai, Y.P., Zhang, L.C., Wang, C.L., et al., 2012. Genetic type, formation age

563

and tectonic setting of the Waitoushan banded iron formation, Benxi,

564

Liaoning Province. Acta Petrologica Sinica 28(11), 3574-3594.

565

Dai, Y.P., Zhang, L.C., Zhu, M.T., et al., 2013. Chentaigou BIF-type iron deposit,

566

Anshan area associated with Archean crustal growth: Constraints from

567

zircon U-Pb dating and Hf isotope. Acta Petrologica Sinica 29(7),

568

2537-2550.

569

Ding, W.J., 2010. Geochemical Characteristics and Genesis of band iron

570

formations from iron ore in Qian’An area. China University of

571

Geosciences(Wu Han).

572

Evans, K.A., Mccuaig, T.C., Leach, D., et al., 2013. Banded iron formation to

573

iron ore: A record of the evolution of Earth environments. Geology 41(2),

574

99-102.

575 576

Fan, J.J., 2011. Brief introduction of banded iron formation deposit. Engineering Construction 42(2), 20-28.

577

Fan, Z., Huang, X., Tan, L., Yang, X., Zhang, H., Zhou, D., Liu, Q., Cao, B.,

578

2014. A study of iron deposits in the Anshan area, China based on

579

interactive inversion technique of gravity and magnetic anomalies. Ore

580

Geology Reviews 57, 618-627.

581

Fan, Z.G., Huang, X.Z., Tan, L., et al., 2013. Geological Structure and Deep

582

Iron Deposits in the Anshan area. Geology & Exploration 49(6),

583

1153-1163.

584

Fu, J.F., Wang, E.D., et al., 2014. Element geochemical characteristics and

585

sedimentary palaeo-environment of the Yanqianshan iron deposit in

586

Liaoning Province. Geology in China 41(6), 1929-1943.

587 588 589 590

Goldich, S.S., 1973. Ages of Precambrian Banded Iron Formations. Economic Geology 68(7), 1126-1134. Gole, M.J., Klein, C., 1981. Banded Iron-Formations through Much of Precambrian Time. Journal of Geology 89(2), 169-183.

591 592 593 594 595 596

Goodwin, A.M., 1973. Archean iron-formations and tectonic basins of the Canadian Shield. Economic Geology 68(7), 915-933. Gross, G.A., 1980. A classification of iron formations based on depositional environments. Canadian Mineralogist 18(1), 215-222. Guo, H.F., 1994. The tectonic evolutionary succession of the Archean crust in the Anshan area. Regional Geology of China 1(1), 1-9.

597

Hammond, N.Q., Moore, J.M., 2006. Archaean lode gold mineralisation in

598

banded iron formation at the Kalahari Goldridge deposit, Kraaipan

599

Greenstone Belt, South Africa. Mineralium Deposita 41(5), 483-503.

600

Han, C., Xiao, W., Su, B.X., et al., 2014.

Formation age and genesis of the

601

Gongchangling Neoarchean banded iron deposit in eastern Liaoning

602

Province: Constraints from geochemistry and SHRIMP zircon U–Pb

603

dating. Precambrian Research 254(1), 306-322.

604

Heimann, A., Johnson, C.M., Beard, B.L., et al., 2010. Fe, C, and O isotope

605

compositions of banded iron formation carbonates demonstrate a major

606

role for dissimilatory iron reduction in ~2.5Ga marine environments.

607

Earth and Planetary Science Letters 294(1), 8-18.

608

Hippertt, J., Davis, B., 2000. Dome emplacement and formation of

609

kilometre-scale

610

Quadrilátero Ferrífero, southeastern Brazil. Precambrian Research

611

102(1), 99–121.

612

synclines

in

a

granite–greenstone

terrain

of

Hong, X.W., Pang, H.W., Liu, X.W., et al., 2010. Geological characteristics of

613

the Dataigou iron deposit in Benxi, Liaoning Province. Geology in China

614

37(5), 1426-1433.

615

Hou, K.J., Li, Y.H., Wan, D.F., 2007. Constraints on the Archean atmospheric

616

oxygen and sulfur cycle from mass-independent sulfur records from

617

Anshan-Benxi BIFs, Liaoning Province, China. Science in China 50(10),

618

1471-1478.

619

Huston, D.L., Logan, G.A., 2004. Barite, BIFs and bugs: evidence for the

620

evolution of the Earth’s early hydrosphere. Earth and Planetary Science

621

Letters 220(1-2), 41-55.

622 623 624 625 626 627 628

Isley, A.E., 1995. Hydrothermal plumes and the delivery of iron to banded iron formation. The Journal of Geology 103(2), 169-185. James, H.L., 1954. Sedimentary facies of iron-formation. Economic Geology 49(3), 235-293. James, H.L., 1983. Chapter 12 Distribution of Banded Iron-Formation in Space and Time. Developments in Precambrian Geology 6, 471-490. Klein, C., 2005. Some Precambrian banded iron-formations (BIFs) from

629

around

the

world:

Their

age,

geologic

setting,

mineralogy,

630

metamorphism, geochemistry, and origin. American Mineralogist 90

631

(10), 1473-1499.

632

Lagoeiro, L.E., 1998. Transformation of magnetite to hematite and its influence

633

on the dissolution of iron oxide minerals. Journal of Metamorphic

634

Geology 16(3), 415–423.

635

Lascelles, D.F., 2006. The genesis of the hope downs iron ore deposit,

636

Hamersley Province, western Australia. Economic Geology 101(7),

637

1359-1376.

638

Lascelles, D.F., 2012. Banded iron formation to high-grade iron ore: A critical

639

review of supergene enrichment models. Australian Journal of Earth

640

Sciences 59(8), 1105-1125.

641

Li, B., 2013. Gongchangling sedimentary metamorphism iron mine structural

642

control of iron in Liaoning Province. Shijiazhuang University of

643

Economics.

644

Li, H., Zhang, Z., Li, L., Zhang, Z., Chen, J., Yao, T., 2014. Types and general

645

characteristics of the BIF-related iron deposits in China. Ore Geology

646

Reviews 57, 264-287.

647

Li, H.M., Liu, M.J., Li, L.X., et al., 2012. Geology and geochemistry of the

648

marble in the Gongchangling iron deposit in Liaoning Province and their

649

metallogenic significance. Acta Petrologica Sinica 28(11), 3497-3512.

650

Li, H.M., Yang, X.Q., Li, L.X., Zhang, Z.C., Liu, M.J., Yao, T., Chen, J., 2015.

651

Desilicification and iron activation–reprecipitation in the high-grade

652

magnetite ores in BIFs of the Anshan-Benxi area, China: Evidence from

653

geology, geochemistry and stable isotopic characteristics. Journal of

654

Asian Earth Sciences 113, 998-1016.

655 656

Li, H.Y., 1977. The earth evolution and tectonic movement. The Journal of Jilin University (Earth Science Edition) 3, 67-89.

657

Li, J., Liu, Y.J., Jin, W., Li, X.H., Neubauer, F., Li, W.M., Liang, C.Y., Wen, Q.B.,

658

Zhang, Y.Y., 2017. Neoarchean tectonics: Insight from the Baijiafen

659

ductile shear zone, eastern Anshan, Liaoning Province, NE China.

660

Journal of Asian Earth Sciences in press.

661

Li, S.Z., Kusky，T.M., Wang, L.，Zhang, G.W., Lai, S.C.，Liu, X.C., Dong, S.W.,

662

Zhao, G.C., 2007. Collision leading to multiple-stage large-scale

663

extrusion in the Qinling orogen: insights from the Mianlue suture.

664

Gondwana Research 12(1-2),121-143.

665

Li, S.Z., Zhao, G.C., Liu, X., Dai, L.M., Suo, Y.H., Song, M.C., Wang, P.C., 2012.

666

Structural evolution of the southern segment of the Jiao-Liao-Ji Belt,

667

North China Craton. Precambrian Research 200-203, 59-73.

668

Li, S.Z., Zhang, Z., Sun, W.J., Dai, L.M., Zhang, G.W., 2015a. Precambrian

669

Geodynamics (I): From universal environment to proto-Earth. Earth Sci.

670

Front. 22 (6), 1–9 (in Chinese with English abstract).

671

Li, S.Z., Xu, L.Q., Zhang, Z., Sun, W.J., Dai, L.M., Guo, L.L., Cao, H.H., Zhang,

672

G.W., 2015b. Precambrian Geodynamics (II): Early Earth. Earth Sci.

673

Front. 22 (6), 10– 26 (in Chinese with English abstract).

674

Li, S.Z., Dai, L.M., Zhang, Z., Zhao, S.J., Guo, L.L., Zhao, G.C., Zhang, G.W.,

675

2015c.

676

Precambrian Geology. Earth Sci. Front. 22 (6), 27–45 (in Chinese with

677

English abstract).

678

Precambrian

Geodynamics

(III):

General

Features

of

Li, S.Z., Dai, L.M., Zhang, Z., Guo, L.L., Zhao, S.J., Zhao, G.C., Zhang, G.W.,

679

2015d. Precambrian Geodynamics (IV): Pre-plate regime. Earth Sci.

680

Front. 22 (6), 46–64 (in Chinese with English abstract).


681

Li, S.Z., Guo, L.L., Dai, L.M., Zhang, Z., Zhao, S.J., Zhao, G.C., Zhang, G.W.,

682

2015e. Precambrian Geodynamics (V): Origin of plate tectonics. Earth

683

Sci. Front. 22 (6), 65–76 (in Chinese with English abstract).

684 685

Li, Y., Oldenburg, D.W., 1998. 3-D inversion of magnetic data. Geophysics 63, 109-119.

686

Li, Y.H., Hou, K.J., Wan, D.F., et al., 2010. Formation Mechanism of

687

Precambrian Banded Iron Formation and Atmosphere and Ocean

688

during Early Stage of the Earth. Acta Geologica Sinica 84(9),

689

1359-1373.

690

Li, Z.H., Zhu, X.K., Tang, S.H., 2008. Characters of Fe isotopes and rare earth

691

elements of banded iron formations from Anshan-Benxi area:

692

implications for Fe source. Acta Petrologica et Mineralogica 27(4),

693

285-290.

694

Li, Z.H., 2009. Iron Isotope Geochemistry of Banded Iron Formations from

695

Anshan-Benxi Area, NE China. Chinese Academy of Geological

696

Sciences.

697 698

Liu, J., Jin, S.Y., 2010. Genesis Study of Magnetite-rich Ore in Gongchangling Iron Deposit, Liaoning. Geoscience 24(1), 80-88.

699

Liu, L., Zhang, L., Dai, Y., 2014. Formation age and genesis of the banded iron

700

formations from the Guyang Greenstone Belt, Western North China

701

Craton. Ore Geology Reviews 63, 388-404.

702

Liu, Q., Zhang, W., Li, W.Q., 2013. Developing model and prospect of the deep

703

iron deposits in Anshan-Benxi area, Liaoning Province. Geology &

704

Resources 22(4), 304-307.

705

Liu, R.Q., Cai, Y.T., Zhang, B.H., 1987. Tectonic style of the “Yingtaoyuan

706

Formation” near Anshan, Liaoning Province, with further discussion on

707

the chronological problem of this formation. Scientia Geologica Sinica 1,

708

24-38.

709

Ma, X.D., 2013. Neoarchean magmatism, metamorphism in the Yinshan Block:

710

Implication for the genesis of BIF and crustal evolution. Acta Petrologica

711

Sinica 29(7), 2329-2339.

712

Manikyamba, C., Balaram, V., Naqvi, S.M., 1993. Geochemical signatures of

713

polygenetic origin of a banded iron formation (BIF) of the Archaean

714

Sandur greenstone belt (schist belt) Karnataka nucleus, India.

715

Precambrian Research 61(1-2), 137-164.

716

Niu, S.Y., Sun, A.Q., Zhang, J.Z., et al., 2013. Structural analysis of the second

717

mining district of Gongchangling iron mine in Liaoning Province.

718

Contributions to Geology & Mineral Resources Research 28(2),

719

167-175.

720

Nutman, A.P., Friend, C.R.L., 2006. Petrography and geochemistry of apatites

721

in banded iron formation, Akilia, W. Greenland: Consequences for

722

oldest life evidence. Precambrian Research 147(1–2), 100-106.

723

Orberger, B., Wagner, C., Wirth, R., et al., 2012. Origin of iron oxide spherules

724

in the banded iron formation of the Bababudan Group, Dharwar Craton,

725

Southern India. Journal of Asian Earth Sciences 52(4), 31-42.

726

Powell, C., Oliver, N.H.S., Li, Z.X., Mcb. Martin, D., Ronaszeki, J., 1999.

727

Synorogenic hydrothermal origin for giant Hamersley iron oxide ore

728

bodies. Geology 27(2), 175.

729

Qu, F.X., 1988. The Proterozoic tectonic deformations and their effects on the

730

distribution of the iron ore deposits in Anshan area, Liaoning Province.

731

Contributions to Geology & Mineral Resources Research 3(2), 22-37.

732

Rosière, C.A., Siemes, H., Quade, H., Brokmeier, H.G., Jansen, E.M., 2001.

733

Microstructures, textures and deformation mechanisms in hematite.

734

Journal of Structural Geology 23(9), 1429-1440.

735

Rosière, C.A., 2004. The origin of hematite in high-grade iron ores based on

736

infrared microscopy and fluid inclusion studies: the example of the

737

Conceicao mine, Quadrilátero Ferrífero, Brazil. Economic Geology

738

99(3), 611-624.

739

Siemes, H., Klingenberg, B., Rybacki, E., Naumann, M., Schäfer, W., Jansen,

740

E., et al., 2003. Texture, microstructure, and strength of hematite ores

741

experimentally deformed in the temperature range 600–1100℃ and at

742

strain rates between 10−4, and 10−6s−1. Journal of Structural Geology

743

25(9), 1371-1391.

744

Shen, B.F., Zhai, A.M., Chen, W.M., Yang, C.L., Hu, X.D., Cao, X.L., Gong,

745

X.H., 1994. The Precambrian mineralization of China. Beijing:

746

Geological Publishing House 55-63.

747

Shen, B.F., Luo, H., Li, S.B., Li, J.J., Peng, X.L., Hu, X.D., Mao, D.B., Liang,

748

R.X., 1994. Geology and Metallization of Archean Greenstone Belts in

749

North China Platform. Beijing: Geological Publishing House 119-129.

750

Shen, B.F., 2012. Geological Characters and Resource Prospect of the BIF

751

Type Iron Ore Deposits in China. Acta Geologica Sinica 86(9),

752

1376-1395.

753

Shen, Q.H., Song, H.X., Yang, C.H., et al., 2011. Petrochemical characteristics

754

and geological significations of banded iron formations in the Wutai

755

Mountain of Shanxi and Qian'an of eastern Hebei. Acta Petrologica et

756

Mineralogica 30(2), 161-171.

757

Shi, J.X., Li, B.C., 1980. Origin of rich magnetite ores in the Gongchangling

758

area as evidenced by fluid inclusion studies from the Anshan-Benxi

759

region, Northeast China. Geochimica 33(1), 43-53.

760

Siemes, H., Klingenberg, B., Rybacki, E., Naumann, M., Schäfer, W., Jansen,

761

E., et al., 2008. Glide systems of hematite single crystals in deformation

762

experiments. Ore Geology Reviews 33(3), 255-279.

763

Song, B., Nutman, A.R., Liu, D., Wu, J., 1996. 3800 to 2500 Ma crustal

764

evolution in the Anshan area of Liaoning Province, northeastern China.

765

Precambrian Research 78, 79-94.

766

Sun, X.H., Zhu, X.Q., Tang, H.S., Zhang, Q., Luo, T.Y., 2014. The

767

Gongchangling BIFs from the Anshan–Benxi area, NE China:

768

Petrological–geochemical characteristics and genesis of high-grade

769

iron ores. Ore Geology Reviews 60, 112-125.

770

Taner, M.F., Chemam, M., 2015. Algoma-type banded iron formation (BIF),

771

Abitibi Greenstone belt, Quebec, Canada. Ore Geology Reviews 70,

772

31-46.

773

Wan, Y., Dong, C., Xie, H., et al., 2012. Formation Ages of Early Precambrian

774

BIFs in the North China Craton: SHRIMP Zircon U-Pb Dating. Acta

775

Geologica Sinica 86(9):1447-1478.

776

Wan, Y.S., Song, B., Liu, D.Y., 2001. Geochronology and geochemistry of

777

3.8-2.5 Ga ancient rock belt in the Donshan Science Park, Anshan

778

Area. Acta Geologica Sinica 75(3), 363-370.

779

Wan, Y.S., 2002. Geochemical characteristics of supracrustal enclaves in

780

Mesoarchean Tiejiashan granite in Anshan area and its geological

781

significance. Scientia Geologica Sinica 37(2), 143-151.

782

Wang, C., Huang, H., Tong, X., Zheng, M., Peng, Z., Nan, J., Zhang, L., Zhai,

783

M.G., 2016. Changing provenance of late Neoarchean metasedimentary

784

rocks in the Anshan-Benxi area, North China Craton: Implications for

785

the tectonic setting of the world-class Dataigou banded iron formation.

786

Gondwana Research 40, 107-123.

787

Wang, E.D., Xia, J.M., Zhao, C.F., et al., 2012. Forming Mechanism of

788

High-Grade Magnetite Bodies in Gongchangling, Liaoning Province.

789 790 791 792 793

Acta Geologica Sinica 32(2), 380-396. Wang, F.L., 2008. Geologic features and prospecting direction of the Anshan type iron-rich mineral. Jilin Geology 27(2), 45-51. Wang, S.L., 1986. The genetic types of rich iron deposits of Anshan group in Anshan-Benxi area. Mineral Deposits 5(4), 14-23.

794

Wang, W., Liu, S., Cawood, P.A., Bai, X., Guo, R., Guo, B., Wang, K., 2016.

795

Late Neoarchean subduction-related crustal growth in the Northern

796

Liaoning region of the North China Craton: Evidence from 2.55 to 2.50

797

Ga granitoid gneisses. Precambrian Research 281, 200-223.

798

Wang, Y.F., Xu, H.F., Merino, E., et al., 2009. Generation of banded iron

799

formations by internal dynamics and leaching of oceanic crust. Nature

800

Geoscience 2(11), 781-784.

801

Wang, Z.H., Zheng, J., 2010. Cause of Dataigou iron ore deposit and

802

prospecting marks. Journal of University of Science & Technology

803

Liaoning 33(4), 353-355.

804

Webb, A.D., Dickens, G.R., Oliver, N.H.S., 2003. From banded iron-formation

805

to iron ore: geochemical and mineralogical constraints from across the

806

Hamersley Province, Western Australia. Chemical Geology 197(1–4),

807

215-251.

808

Wu, F.Y., Zhang, Y.B., Yang, J.H., Xie, L.W., Yang, Y.H., 2008. Zircon U-Pb and

809

Hf isotopic constraints on the Early Archean crustal evolution in

810

Anshan of the North China Craton. Precambrian Research 167,

811

339-362.

812

Wu, J.S., Geng, Y.S., Shen, Q.H., Wan, Y.S., Liu, D.Y., Song, B., 1998.

813

Archaean geology characteristics and tectonic evolution of the

814

China-Korea Paleo-continent. Beijing: Geological Publishing House.

815

Xu, G.R., Chen, H.J., 1984. A preliminary study of komatiites in Anshan – Benxi

816

- Fushun region, Northeast China. Chinese Journal of Geochemistry

817

3(2), 128-141.

818

Xu, Y.G., 1987. Transformation of BIF-Type iron deposit in ductile zone,

819

North-east Anshan, Liaoning Province. Journal of Changchun College

820

of Geology 17(3), 283-292.

821

Xu, Z.Y., 1991. The origin and evolution of the banded structure in Archaean

822

iron ore body, Anshan area. Journal of Changchun University of Earth

823

Science 21(4), 389-396.

824

Xue, L.F., Li, W.Q., Zhang, W., et al., 2014. A method of block-divided 3D

825

geologic modeling in regional scale. Journal of Jilin University(Earth

826

Science Edition) 44(6), 2051-2058.

827

Yang, Q.Y., Santosh, M., 2015. Paleoproterozoic arc magmatism in the North

828

China Craton: no Siderian global plate tectonic shutdown. Gondwana

829

Research 28, 82–105.

830

Yang, Q.Y., Santosh, M., Collins, A.S., Teng, X.M., 2016. Microblock

831

amalgamation in the North China Craton: Evidence from Neoarchaean

832

magmatic suite in the western margin of the Jiaoliao Block. Gondwana

833

Research 31, 96–123.

834

Yang, Q.Y., Santosh, M., 2017. The building of an Archean microcontinent:

835

Evidence from the North China Craton. Gondwana Research,

836

http://dx.doi.org/10.1016/j.gr.2017.01.003

837

Yang, X.Q., Li, H.M., Li, L.X., et al., 2014. Characteristics of Fluid Inclusion, S,

838

H and O Isotope of Iron Deposit in Anshan-Benxi area, Liaoning

839

Province. Acta Geologica Sinica 88(10), 1917-1931.

840

Yang, X.Q., Zhang, Z.H., Duan, S.G., et al., 2015. Petrological and

841

geochemical features of the Jingtieshan banded iron formation (BIF): A

842

unique type of BIF from the Northern Qilian Orogenic Belt, NW China.

843

Journal of Asian Earth Sciences 113, 1218-1234.

844

Yang, X.Y., Liu, L., Lee, I., et al, 2014. A review on the Huoqiu banded iron

845

formation (BIF), southeast margin of the North China Craton: Genesis of

846

iron deposits and implications for exploration. Ore Geology Reviews

847

63(1), 418-443.

848

Yang, Z.S., Yu, B.X., Gao, D.H., 1983. Researches of tectonic deformation of

849

the metamorphic sedimentary iron-deposits in Waitoushan area,

850

Liaoning Province. Journal of Changchun University of Earth Science 2,

851

11-23.

852

Yu, S.X., Zhao, H.Z., Li, H.M., et al., 2014. Geological-geophysical prospecting

853

model of deep rich iron ore for No.2 mining area of the Gongchangling

854

iron deposit, Liaoning, China. Contributions to Geology and Mineral

855

Resources Research 88(10), 1889-1903.

856

Yvonne, W., 2007. 3D Modeling of Banded Iron Formation incorporating

857

demagnetization -–a case study at the Mussel white Mine, Ontario,

858

Canada. Exploration Geophysics 38(4), 1-5.

859 860

Zeng, H.L., 2005. Gravity and gravity exploration. Beijing: Geology Publishing House.

861

Zhai, M.G., Windley, B.F., 1990a. The Archaean and early Proterozoic banded

862

iron formations of North China: their characteristics, geotectonic

863

relations, chemistry and implications for crustal growth. Precambrian

864

Research 48, 267-286.

865

Zhai, M.G., Windley, B.F., Sills, J.D., 1990b. Archaean gneisses amphibolites,

866

and banded iron-formation from Anshan area of Liaoning Province, NE

867

China:

868

Precambrian Research 46(3), 195-216.

869 870 871 872

their

geochemistry,

metamorphism

and

petrogenesis.

Zhai, M.G., 2010. Tectonic evolution and metallogenesis of North China Craton. Mineral Deposits 29(1), 24-36 Zhai, M.G., Santosh, M., 2011. The early Precambrian odyssey of the North China Craton: a synoptic overview. Gondwana Research 20, 6–25.

873

Zhai, M.G., Santosh, M., 2013. Metallogeny of the North China Craton: Link

874

with secular changes in the evolving Earth. Gondwana Research 24,

875

275-297.

876

Zhang, B.H., Cai, Y.T., Zhang, W.B., Cui, W.Z., Zheng, J.Q., Liu, R.Q., 1986.

877

Structural deformation of the early pre-Cambrian rock groups in the

878

Anshan area, Liaoning Province. Journal of Changchun University of

879

Earth Science 2, 47-56.

880 881 882 883

Zhang, B.H., Qu, F.X., 1996. Control of polyphase folds and structural transposition. Geology & Prospecting 32(6), 12-16. Zhang, J., 2009. Research on Typical Mineral Deposit of Xi-Anshan Iron Ore and Deep Forecast to “An-Shan-Type” Iron Ore. Jilin University.

884

Zhang, L., Dong, C., Liu, S., et al., 2016. Early Precambrian Magmatism and

885

Metamorphism in Ural Mountain Area, North China Craton: SHRIMP

886

U-Pb Zircon Dating and Rock Geochemical Study. Geological Review

887

in press.

888

Zhang, L.C., Zhai, M.G., Zhang, X.J., Xiang, P., Dai, Y.P., Wang, C.L., Pirajno,

889

F., 2011. Formation age and tectonic setting of the Shirengou

890

Neoarchean banded iron deposit in eastern Hebei Province: Constraints

891

from geochemistry and SIMS zircon U-PB dating. Precambrian

892

Research, doi: 10.1016/j.precamres.2011.09.007.

893

Zhang, L.C., Zhai, M.G., Wan, Y.S., et al., 2012. Study of the Precambrian

894

BIF-iron deposits in the North China Craton: Progresses and questions.

895

Acta Petrologica Sinica 28(11), 3431-3445.

896

Zhang, L.C., Dai, Y. P., Wang, C. L., et al., 2014. Age, Material Sources and

897

Formation Setting of Precambrian BIFs Iron Deposits in Anshan-Benxi

898

Area. Journal of Earth Sciences and Environment 36(4), 1-15.

899

Zhang, P., Peng, M.S., Ouyang, Z.Z., Qiao, S.Y., 2012. Geological

900

characteristics and ore-searching guides of the iron deposits in

901

Anshan-Benxi area, Liaoning Province. Geology & Resources 21(6),

902

516-521.

903 904

Zhang, Q.S., 1988. Early crust and mineral deposits of Liaodong peninsula, China. Geological Publishing House.

905

Zhang, R.H., Wang, S.L., 1994. A new viewpoint about iron ore deposit:

906

Controlled by ductile shear zones at Waitoushan. Contributions to

907

Geology and Mineral Resources Research 9(4), 57-62.

908

Zhang, Z., Liu, J.M., Yu, C.M., et al., 2013. Application of integrated

909

geophysical prospecting methods in the evaluation of BIF deposits-a

910

case study in Inner Mongolia AohanqiSijiazi BIF deposits. Progress in

911

Geophysics 28(4), 2078-2084.

912

Zhao, G.C., Zhai, M.G., 2013. Lithotectonic elements of Precambrian

913

basement in the North China Craton: review and tectonic implications.

914

Gondwana Research 23, 1207–1240.

915 916 917 918 919 920

Zhao, Y.M., 2013. Main genetic types and geological characteristics of iron-rich ore deposits in China. Mineral Deposits 32(4), 685-704. Zhao, Z.H., 2010. Banded iron formation and related great oxidation event. Earth Science Frontiers 17(2), 1-12. Zhao, Z.P., Zhai, M.G., Wang, K.Y., Yan, Y.H., et al., 1993. Precambrian crustal evolution of the Sino-Korean Paraplatform. Beijing: Science Press.

921

Zheng, J.Q., Zhang, B.H., Cai, Y.T., Cui, W.Z., Zhang, W.B., Liu, R.Q., 1986.

922

Tectonic characteristics of the Archean Anshan group and their effects

923

on iron ore deposits in area of Beitai to Waitoushan, Benxi, Liaoning

924

Province. Contributions to Geology & Mineral Resources Research 1(1),

925

20-29.

926

Zhou, S.T., 1987. The petrochemical study of the Archean banded iron deposit

927

in Anshan-Benxi district, Liaoning Province. Bulletin of the Chinese

928

academy of Geological Sciences 16, 139-152.

929 930 931 932

Zhou, S.T., 1994. Geology of the BIF in Anshan-Benxi Area. Beijing: Geological Publishing House 1-277. Zhu, K., 2016. The formation and evolution of the Archean greenstone belt in the Anshan-Benxi area. Jilin University.

933

Figure captions

934

Fig. 1. Simplified geological map of the Anshan area. The inset shows the

935

location of the study area.

936 937

Fig. 2. A 1:50,000 scale reduced-to-pole magnetic anomaly map showing the

938

location of known iron ore deposits (triangles).

939 940

Fig. 3. Images showing the upward continuation of reduced-to-pole magnetic

941

anomaly data for the study area: (a) Location of the two main highly magnetic

942

anomalies associated with magnetic bodies I and II. (b) A 1000 m upward

943

continuation map showing a smoothing of the shapes of the main magnetic

944

bodies and the disappearance of some minor highly magnetic anomalies. (c) A

945

3000 m upward continuation map showing the coalescence of northern

946

magnetic bodies I and II, suggesting they may be connected at depth. (d) A

947

5000 m upward continuation map showing an enlargement of the coalesced

948

part of magnetic bodies I and II and the disappearance of all of the weaker

949

magnetic anomalies. (e) A 7000 m upward continuation map showing magnetic

950

bodies I and II forming a circular magnetic high. (f) A 9000 m upward

951

continuation map showing magnetic bodies I and II forming a circular magnetic

952

high. (h) A 20,000 m upward continuation map showing that the areas of

953

magnetic bodies I and II remain open.

954

955

Fig. 4. 3D magnetic susceptibility inversion for the Anshan area.

956 957

Fig. 5. E–W cross-section showing the distribution of iron formations within the

958

Anshan area.

959 960

Fig. 6. Model of the formation of BIF iron ore belts by folding and ductile

961

shearing within the Anshan area. (a) Initial formation of BIF during the

962

Neoarchean (2.6–2.5 Ga). (b) Initial folding of BIF in the Anshan area was

963

followed by significant compression near the Anshan micro-continental

964

nucleus, generating ductile shear zones (2.52–2.47 Ga). (c) Further intense

965

folding and ductile shearing led to the coalescence of several ductile shear

966

zones and the generation of wider ductile shear zones near the Anshan

967

micro-continental nucleus. This process concentrated iron mineralization

968

within the resulting ductile shear zones and formed the BIF-hosted iron ores in

969

the study area. This was followed by large-scale magmatism at the end of the

970

Neoarchean (2.5 Ga) that disrupted the BIF.

971 972

Fig. 7. Map and cross-section showing the location of iron ore in the study area.

973

The cross-section is based on drilling data from the Anshan area and shows a

974

structural configuration where iron-bearing units are hosted by a closed fold

975

that is indicative of intense deformation of the BIF in this region. Schistose

976

rocks are present as interlayers within the BIF, decreasing in number with

977 978

depth. The interlayers are generally thought to be part of the original protolith.

979 980

981 982

983 984

985 986

987 988

989 990

991 992

993

lithostratigraphic

main lithology

magnetic

remanent

susceptibility

magnetism

(4π×10−6SI)

(10−3A/m)

0–4100

0–3700

biotite–granulite

383–625

73–101

gneiss

80–57000

21–14900

magnetite

2000–61000

2900–479000

unit

Cigou formation

plagioclase amphibolite

quartzite Yingtaoyuan

phyllite

0–7360

250–3700

formation

magnetite

3000–100000

20000–50000

quartzite Table1.magnetic susceptibility of main rocks

994 995

number

iron ore

strike

dip

dip angle

32

Qidashan

305°-335°

SW,NE

70°-90°

35

Hujiamiaozi

145°-165°

NE-SW

80°-90°

33

Wangjiapuzi

310°-335°

SW,NE

70°-90°

27

Zhangjiawan

110°-290°

S-NE

65°-90°

26

Lazishan

100°-135°

NE

50°-80°

36

Gumenshan

275°-285°

NE

70°-85°

2

Yanqianshan

NNW

NEE

48°-90°

31

Dagushan

310°-315°

NE

60°-75°

38

Heishilazi

70°

NW-SE

75°-85°

37

Dong’anshanxiangqianyu

EW

S

25°-30°

24

Xi’anshan

NW-SE

NE

20°-47°

25

Dong’anshan

NW

NE

65°

996 997 998

29

Huolongzhai

30

Xiaolingzi

5

Zhanchigou

EW

NE

N

30°-80°

NW

42°-76°

NW

55°-80°

Table2. main iron mines occurrence features in Anshan

999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011

The 3D magnetic susceptibility model of Anshan is obtained using UBC Mag3D software. The 3D inversion results of magnetic susceptibility show that the buried depth of magnetic bodies. The model of fold-ductile shearing that controls the spatial distribution of iron formation and ore bodies is proposed, the composite fold-ductile shear zone is consequently easy to form in the northeastern or southwestern sides of the micro continental nucleus, where favorable development of the BIF ore belt occurs. Iron ore bodies are developed mainly along the composite structural zone. The bottom depth of the iron ore bodies depends mainly on the bending depth of the closed folds and the developed depth of the ductile shear zones.