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The lithospheric structure and deep processes of the Mesozoic mineral systems in east China: constrained from integrated geophysical data Qingtian Lü*, Guixiang Meng, Jiayong Yan, Jinhua Zhao, Xuejing Gong Sinoprobe Center, Chinese Academy of Geological Science No 26, Baiwanzhuang Road, Beijing, 100037, China *Corresponding author: lqt@cags.ac.cn

INTRODUCTION

The middle and lower reaches of Yangtze River metallogenic belt is one of the most metal endowed geological regions in east China, and host more than 70 medium and large iron, copper, gold deposits, which is distributed at seven ore districts (Figure 1; Pan et al., 1999). Tectonically, the region is located in the east side of suture zone between the North China Block (NBC) and the South China Block (SCB) as indicated by the Tan-Lu Fault. Radiometric dating (Li et al., 1989), paleomagnetic study (Zhao et al., 1987), and tectono-stratigraphic analyses (Yin et al., 1993) indicate that the NCB and SCB began to collide in the eastern part of the NCB in the Early Triassic. However, large amount zircon U-Pb dating results show that the extensive magmatism and metallogenesis occurred in early Cretaceous, range from 145 Ma to 120 Ma and peaked at 140 Ma and 125 Ma (Zhou et al., 2008). It is clear that the magmatism and related ore forming processes occurred in the intra-continental setting. The Andean type metallogeny and deep processes have been well understood, the questions of why so many metal ore deposits occurred in intra-continental setting, what are the deep dynamic processes that control the initiation and evolution of the mineral system, have interested ore geologists for decades. Deep mineral exploration has been the world trends, the three-dimensional (3D) geologic models can help us better

understand the architecture of a region and assist in predicting new discoveries at depth. Recently, 3D geologic model construction for orebody delineation and target prediction has been becoming a routine in the deep mineral exploration. Integrated geophysical data, in particular, the seismic reflection profiles are essential in constructing 3D geologic model. Supported by the SinoProbe project, we have conducted multi-scale, extensive deep exploration over the middle and lower reaches of Yangtze metallogenic belt and the major ore districts (Figure 1). The high-quality datasets available for terrane-scale analysis includes: broadband seismic, deep seismic reflection, wide-angle reflection/refraction and long-period magnetotelluric (MT); and these available for domain- and district- scale analysis include: high-resolution seismic reflection profiling, magnetotelluric (MT), 1:50, 000 scale gravity and aeromagnetics; and these available for mine- and camp- scale analysis include: AMT, CSAMT, TEM and SIP over several known deposits. In this paper, we present the results from terrane- scale and domain- scale deep exploration. The terrane- scale results provide the detailed information about the lithospheric architecture and deep processes, which help us understand the formation of the “sources” region of the mineral system, the district-scale results, however, provide the physical features of the “conduit” and “trap” site of the mineral system, which could help targeting in depth deposits.

LITHOSPHERIC ARCHITECTURE AND

DYNAMIC OF THE YMB Base on the recorded broadband seismic data, we have performed teleseismic tomography, noise tomography and teleseismic two-plane-wave tomography, receiver function profiling and measurement of shear-wave splitting parameters, and have obtained the three dimensional P- and S-wave velocity structure of the upper mantle (Jiang et al., 2013, 2014; Ouyang et al., 2014), the variations of major boundaries, e.g. LAB and Moho and also the upper mantle deformation (Shi et al., 2013). The deep seismic reflection (Lü et al., 2015) and refraction profile revealed the detailed multi-scale crustal “conduit” for fluid migration, and also the “footprint” left by the fluids. The major results and geologic interpretation are summarized as the following: 1. Teleseismic tomography shows that there exists two belt-parallel high-velocity and one low-velocity anomalies in the upper mantle beneath the YMB, called “sandwich” structure The lower-V body and the deeper high-V body, centered at

SUMMARY Financed by the SinoProbe, a national collaborative multidisciplinary Earth science research project in China, the authors has conducted multi-scale and integrated geophysical exploration across the middle and lower reaches of Yangtze Metallogenic Belt (YMB) in east China. The data range in scale from terrane, district to camp or mine. The methods included broad band seismic, reflection seismic profiling, magnetotelluric sounding and gravity and magnetic modelling. The results provide first-order insights into the physical and structural properties of the lithosphere and upper mantle beneath the YMB, and thus provide in depth understanding of the deep processes that control the initiation and evolution of intra-continental mineral system. Key words: Metallogenic belt; Reflection seismic; Crustal structure; Deep processes; Mineral system.

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150 km and 300 km respectively, dip slightly to the southwest (Figure 2). The shallower high-V anomaly is explained as the current lithosphere (crust), the deeper high-V one as the delaminated lithosphere, and the sandwiched low-V anomaly as the upwelling hot asthenosphere. The shear velocity model derived from noise tomography and two-plane-wave tomography is quite consist with that of the teleseismic P-wave velocity model, a clear low-velocity zone at ∼100–200 km depth is observed beneath the Middle–Lower Yangtze River Metallogenic Belt. The north NingWu and NingZhen ore districts are clearly characterized by the strongest low velocity anomaly in the uppermost mantle at ∼70–200 km depth. The depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes stronger from the southwest JiuRui ore district to northeast NingWu ore districts.

Figure.1 Map showing the layout and Location of deep exploration profiles in Middle and Lower Yangtze Metallogenic belt and major ore districts (white frame). 1- Major faults; 2- Permanent seismic stations; 3- Portable broad-band seismic stations; 4-MT sounding points; 5-Reflection seismic and MT profile；6-Wide-angle stations; 7-Wide-angle shot points. TLF-Tan-Lu Fault; XHF-Xiangshui-Huaiyin Fault; CHF-Chehe Fault; MSF-Maoshan fault; JNF-Jiangnan Fault; SDF-Shouxian-Dingyuan Fault; XMF-Xiaotian-Mozitan Fault; XGF-Xiangfan-Guangji Fault. 2. S-wave receiver function evidence the thinning of the lithosphere beneath the YMB to 50-70 km. The Moho depth varies significantly along the profile, and “mantle uplift” exists right beneath the YMB. SKS/SKKS shear wave splitting results show a belt-parallel azimuthal anisotropy right beneath the YMB, and the fast-wave polarization direction present a horizontal “sandwich” pattern, showing a systemic variation from south China Block, YMB to the north China Block. The lower crust of the YMB is different from that of its adjacent areas in structure on the receiver function profile. It possesses seismic anisotropy with direction also roughly parallel to the belt. 3. The deep reflection seismic data reveals strong NW-SE upper crustal contraction deformation, characterized as crustal-scale tight fold, thrust fault and nappe. The “crocodile”

reflection structures are found beneath the Ningwu volcanic basin, right beneath the YMB, indicating the decoupled deformation process of the upper and lower crust. This reflection patterns can be explained as evidence for an intra-continental orogeny, with lower crust and upper mantle subduction and imbrication and middle and upper crust thrust upward. Refraction seismic and MT data provide velocity and resistivity distribution across the YMB, and show a general agreement with the tectonic units. 4. Based on the results of the integrated geophysical exploration, combining with the recent geochemistry results, a geodynamic model is proposed for the formation of the world-class metallogenic belt, this model includes several key deep processes, e.g. the subduction of intra-continental sublithosphere, the thickening and delamination of crustal roots, the upwelling of asthenosphere, the melting of delaminated lower crust, the underplating of the melts to the crust-mantle boundary and subsequent MASH (Melting, Assimilation, Storage and Homogenization) processes. Either the melting of delaminated lower crustal material or the MASH processes may contribute to the formation of adakitic-like magma, which is considered to be the “sources” of the mineral system. The adakitic-like magma/fluids rose up along “crustal penetrating” thrust faults to the shallow crust, and reacted with country rocks to form mineral deposits.

CRUSTAL ARCHITECTURE AND 3D

GEOLOGIC MODEL OF MAJOR ORE DISTRICT We have performed high-resolution seismic profiling, MT sounding and 1:50000 gravity survey in the major ore districts of YMB, For example, in Lu-Zong ore district, five intersecting integrated geophysical profiles with seismic reflection and MT data and 1:50,000 gravity data were collected. In Tongling ore district, six parallel integrated geophysical profiles with seismic reflection and MT and 1:50,000 gravity data were collected as well. Similar work was done in the Ningwu, Anqing-Guichi and Jiurui ore district. With these datasets, combining with geology, drillholes and aeromagnetic data, the 3D geologic model for each mineral districts was constructed. In the following section, we take Lu-Zong ore district as example to show how the 3D geologic model could reveal the upper crust structure and aid for deep mineral targeting. Lu-Zong ore district mainly consists of a northeast elongated volcanic basin and surrounding Paleozoic to Mesozoic strata and Cenozoic sedimentary basin, due to the long tectonic evolution history, the deep structure and composition is very complicated. With the recent discoveries of the Nihe, Xiaobaozhuang iron deposits, and Shaxi porphyry copper deposits at a depth of over 700 m, the Lu-Zong ore district is listed as one of the top priority domains for deep mineral exploration. Based on the analysis of deep seismic data, a series of new discoveries regarding the architecture and composition were obtained (Lü et al., 2014) as summarized in the following: 1. The Lu-Zong ore district consists of four major crustal blocks, they are Shaxi uplift, Qianshan-Kongcheng depression in the west, Lu-Zong volcanic basin and “along-river” uplift in the east. The north-south crustal elements show the northward “step-type” uplift, juxtaposed by two step-like faults, the WNW-ESE trending Tangjiayuan-zhuanqiao Fault and the Lujiang-Huangguzha-Tongling (LHTD) Fault.

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2. Lu-Zong volcanic basin presents a non-symmetrical shape with four inward dipping boundary faults. The northern and eastern boundary faults are deep faults, which control the development and evolution of the Lu-Zong volcanic basin. There are three WNW-ESE trending faults and six NE-SW trending faults cutting over the ore district. From north to south, they are LHTD, Tangjiayuan-Zhuanqiao fault and Yijing-Taojiaxiang fault; from the west to the east, six faults are Tan-Lu, Chuhe, Luohe-Quekou, Zongyang-Huangtun, Taojiawan-Shijiawan and Changjiang thrust fault. 3. The formation and evolution of the ore district was mainly affected by the Yanshanian intracontinental orogeny, and experienced the Middle-Late Jurassic contraction and subsequent Cretaceous extension, possibly due to the paleo-Pacific NW-trending subduction. Some findings regarding the nature of the faults were first obtained, e.g. the Changjiang thrust fault is a thrust system in nature, LHTD is a SW-dipping detachment. Two Jurassic basins was found, surrounding the northeast and southeast of Lu-Zong volcanic, the authors inferred that it might be the product of post-collision extension of Indosinian orogeny during the middle and early Late Triassic.

Figure 2. Results of teleseismic tomography over the middle and lower YMB with perspective view of velocity anomaly contour as to be 0.5% and -0.5% respectively. (a) Looking toward northeast, and (b) looking toward southeast. Blue and yellow colours denote the higher and the lower anomalies respectively. The base map is the topography of the studied region. The red curved lines indicate faults.

Figure 3. Perspective view of the 3D geological model of Lu-Zong ore district constructed from 3D gravity inversion. (a) solid 3D geological model looking towards the northwest; (b) looking toward northeast with break-out section; (c) looking toward northwest with break-out section; (d) 3D geological model with observed gravity anomaly (top) and calculated gravity anomaly (middle).

4. The 3D geologic model of Lu-Zong ore district has been constructed (Figure 3) by the method proposed by Lü et al. (2013). The results define the geometry, depth, and physical properties of geological bodies at depths. The 3D visualization of the results assists in understanding the spatial relations between various geological bodies and the ore-controlling intrusions. The model has confirmed most previous knowledge, but also revealed new features of different folds and intrusions that are important for planning exploration at large depths. A number of deep targets have been chosen by combining the conceptual mineralization model in the district with the 3D geological model. One testing deep drilling has confirmed the uranium mineralization at depth from 1500 m to 1750 m.

CONCLUSIONS

A large mineral deposit or mineral system is the consequence of geological processes which operated at a range of scales, from global, regional to microscopic (Blewett et al., 2010). The lithosphere-asthenosphere boundary (LAB) beneath YMB is around 70 km and shallower than adjacent area. The P- and S-wave velocities are much slower beneath the mineralized YMB terrane than the less mineralized adjacent region. A southwest-dipping high-velocity body occurs at 300 km depth may represent the delaminated eclogite layer. The deep seismic reflection profile reveals abnormal crustal architecture beneath YMB, indicating the main tectonic mode has been compression, which was shifted to extension during early Cretaceous. All these geophysical features provide clues of that a series deep dynamic processes have been occurred, and these make a favorable environment to induce mineral system and thus mineral deposits. Domain scale deep exploration could provide detailed information regarding to the crustal structure, such as basement faults, upper crust domes, intrusions and folds. A number of deposits are located in the Lu-Zong above or adjacent to basement fault and intrusions, suggesting a relationship between the structure and mineralization. When the reflection seismic data are integrated with regional gravity and magnetic data, the 3D architecture of the upper crust can be mapped. This map provides not only information on the mineral system, but the deep targets for mineral exploration.

ACKNOWLEDGEMENTS

We would like to express our gratitude to Prof. Dong S.X. and some students from Jilin University who were involved in the whole process of seismic and MT data acquisition. Thanks are also given to Dr. Xue, A and Li B from Petrosound Geophysics Company of Beijing for their help in seismic data processing. This work is supported by the Ministry of Land and Resources of China under the Project SinoProbe-03, and the National Natural Science Foundation of China under Grant 41630320.

REFERENCES

Blewett, R S, Henson, P A, Roy, I G, Champion, D C, Cassidy, K F., 2010. Scale-integrated architecture of a world-class gold mineral system: The Archaean eastern Yilgarn Craton, Western Australia. Precambrian Research, 183: 230-250. Jiang, G.M., Zhang, G.B., Lü, Q.T., Shi, D.N., Xu, R., 2014. Deep geodynamics of mineralization beneath the Middle-

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Lower reaches of Yangtze River: evidence from teleseismic tomography. Acta Petrologica Sinica, 30(4):907-917 (in Chinese with English abstract). Jiang, G.M., Zhang, G.B., Lü, Q.T., Shi, D.N., Xu, Y., 2013, 3-D velocity model beneath the Middle–Lower Yangtze River and its implication to the deep geodynamics, Tectonophysics, 606:36-48. Li, S. G., Hart, S. R., Zhang, S. G., 1989. Timing of collision between the north and south China blocks- The Sm-Nd isotopic age evidence, Science in China (Series D), 32(11): 1393-1400. Lü, Q T, Liu Z D, Yan J Y, Tang J T, Wu M A, Xiao X, 2014. Crustal-scale Structure and Deformation of Lu-Zong Ore District: Joint interpretation from Integrated Geophysical Data, Interpretation, submitted. Lü, Q.T., Qi, G., Yan, J. Y., 2013. 3D geological model of Shizishan ore field constrained by gravity and magnetic interactive modeling: A case history, Geophysics, 78(1): B25-B35. Ouyang, L., Li, H. Y., Lü, Q. T., Yang, Y. J., Li, X. F., Jiang, G. M., Zhang, G. B., Shi, D. N., Zheng, D., Sun, S. J., Tan, J., Zhou, M., 2014. Crustal and uppermost mantle velocity structure and its relationship to the formation of ore districts in the Middle-Lower Yangtze River region, Earth and Planetary Science Letters, 408: 378-389.

Pan, Y., Dong, P., 1999. The lower Changjiang (Yangze/Yangtze River) metallogenic belt, Easter Central China: intrusion- and wall rock-hosted Cu–Fe–Au, Mo, Zn, Pb, Ag deposits, Ore Geology Reviews, 15: 177-242. Shi D.N., Lü, Q.T., Xu, W.Y., Yan, J.Y., Zhao, J.H., Dong, S.W., Chang, Y.F. and SinoProbe-03-02 team, 2013. Crustal structure beneath the middle-lower Yangtze metallogenic belt in East China: Constraints from passive source seismic experiment on the Mesozoic intra-continental mineralization, Tectonophysics, 606:48-60. Yin, A., Nie, S. Y., 1993. An indentation model for the North and South China collision and the development of the Tan-Lu and Honam fault systems, Eastern Asia, Tectonics, 12: 801-813. Zhao, X., Coe, R. S., 1987. Palaeomagnetic constraints on the collision and rotation of North and South China, Nature, 327: 141-144. Zhou, M., 2014. Crustal and uppermost mantle velocity structure and its relationship to the formation of ore districts in the Middle-Lower Yangtze River region, Earth and Planetary Science Letters, 408: 378-389. Zhou, T.F., Fan, Y. and Yuan, F., 2008. Advances on petrogensis and metallogeny study of the mineralization belt of the Middle and Lower Reaches of the Yangtze River area, Acta Petrologica Sinica, 24(8): 1665-1678.