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Volume 32 Issue 2
Apr.  2021
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Lingfen Guo, Yongqing Chen, Binbin Zhao. Application of Singular Value Decomposition (SVD) to the Extraction of Gravity Anomalies Associated with Ag-Pb-Zn-W Polymetallic Mineralization in the Bozhushan Ore Field, Southwestern China. Journal of Earth Science, 2021, 32(2): 310-317. doi: 10.1007/s12583-020-1352-4
Citation: Lingfen Guo, Yongqing Chen, Binbin Zhao. Application of Singular Value Decomposition (SVD) to the Extraction of Gravity Anomalies Associated with Ag-Pb-Zn-W Polymetallic Mineralization in the Bozhushan Ore Field, Southwestern China. Journal of Earth Science, 2021, 32(2): 310-317. doi: 10.1007/s12583-020-1352-4

Application of Singular Value Decomposition (SVD) to the Extraction of Gravity Anomalies Associated with Ag-Pb-Zn-W Polymetallic Mineralization in the Bozhushan Ore Field, Southwestern China

doi: 10.1007/s12583-020-1352-4
More Information
  • The Bozhushan Ore Field, located at the western margin of the South China Block, is an important area for Ag-Pb-Zn-W polymetallic mineralization which may be associated with the Late Cretaceous granitic magmaism. In this paper, the singular value decomposition (SVD) was effectively applied to decompose gravity data at scale of 1:50 000 within the Bozhushan Ore Field to extract deep ore-finding information. Two gravity anomaly images displaying different scales of the ore-controlling factors were obtained. (1) The low-pass filtered image may reflect the deeply buried geological structures, hidden intrusions and concealed ore bodies. The negative gravity anomaly may reflect the overall distribution of granite bodies in the Bozhushan Ore Field. One negative gravity anomaly area may correspond to the exposed part of the Baozhushan granitic intrusion and the other corresponds to the concealed part of the granitic intrusion. The granitic intrusions are the main ore-controlling factors in this ore district. (2) The band-pass filtered image depicts the shallow concealed geological structures and geological bodies within this study area. There are two obvious negative gravity anomalies, which may be created by the hidden granites at different depths at both northwestern and southeastern sides of the exposed granitic intrusion. Thus the two negative gravity anomalies are favorable prospecting areas for various type of polymetallic ore deposits at depth. The gravity anomalies extracted by using the SVD exactly reflect the distribution of the ore deposits, structures and intrusions, which will give new insights for further mineral exploration in the study area.
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    Chen, Y. Q., Zhang, L. N., Zhao, B. B., 2015. Application of Singular Value Decomposition (SVD) in Extraction of Gravity Components Indicating the Deeply and Shallowly Buried Granitic Complex Associated with Tin Polymetallic Mineralization in the Gejiu Tin Ore Field, Southwestern China. Journal of Applied Geophysics, 123: 63-70. https://doi.org/10.1016/j.jappgeo.2015.09.022 doi:  10.1016/j.jappgeo.2015.09.022
    Chen, Y. Q., Zhang, L. N., Zhao, B. B., 2017. Application of Bi-Dimensional Empirical Mode Decomposition (BEMD) Modeling for Extracting Gravity Anomaly Indicating the Ore-Controlling Geological Architectures and Granites in the Gejiu Tin-Copper Polymetallic Ore Field, Southwestern China. Ore Geology Reviews, 88: 832-840. https://doi.org/10.1016/j.oregeorev.2016.06.031 doi:  10.1016/j.oregeorev.2016.06.031
    Chen, Y. Q., Zhang, S. Y., Xia, Q. L., et al., 2006. Application of Multifractal Filtering to Extraction of Geochemical Anomalies from Multi-Geochemical Backgrounds: A Case Study of the South Section of "Sanjiang Ore-Forming Zone", Southwestern China. Earth Science, 31(6): 861-866(in Chinese with English Abstract) http://www.researchgate.net/publication/288067428_Application_of_multi-fractal_filtering_to_extraction_of_geochemical_anomalies_from_multi-geochemical_backgrounds_A_case_study_of_the_southern_section_of_'Sanjiang_ore-forming_zone'_southwestern_china
    Chen, Y. Q., Zhao, B. B., 2012. Extraction of Gravity Anomalies Associated with Gold Mineralization: A Comparison of Singular Value Decomposition and Bi-Dimensional Empirical Mode Decomposition. Advanced Materials Research, 455/456: 1567-1577. https://doi.org/10.4028/www.scientific.net/amr.455-456.1567 doi:  10.4028/www.scientific.net/amr.455-456.1567
    Cheng, Q. M., Agterberg, F. P., 2009. Singularity Analysis of Ore-Mineral and Toxic Trace Elements in Stream Sediments. Computers & Geosciences, 35(2): 234-244. https://doi.org/10.1016/j.cageo.2008.02.034 doi:  10.1016/j.cageo.2008.02.034
    Cheng, Q. M., Zhao, P. D., 2011. Singularity Theories and Methods for Characterizing Mineralization Processes and Mapping Geo-Anomalies for Mineral Deposit Prediction. Geoscience Frontiers, 2(1): 67-79. https://doi.org/10.1016/j.gsf.2010.12.003 doi:  10.1016/j.gsf.2010.12.003
    Cheng, Q. M., 2012. Singularity Theory and Methods for Mapping Geochemical Anomalies Caused by Buried Sources and for Predicting Undiscovered Mineral Deposits in Covered Areas. Journal of Geochemical Exploration, 122: 55-70. https://doi.org/10.1016/j.gexplo.2012.07.007 doi:  10.1016/j.gexplo.2012.07.007
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    Freire, S. L. M., Ulrych, T. J., 1988. Application of Singular Value Decomposition to Vertical Seismic Profiling. Geophysics, 53(6): 778-785. https://doi.org/10.1190/1.1442513 doi:  10.1190/1.1442513
    Huang, J. N., Zhao, P., 2015. Application of a Multi-Fractal Model for Identification of Cu, Au and Zn Anomalies in Western Yunnan, Southwestern China. Geochemistry: Exploration, Environment, Analysis, 15(1): 54-61. https://doi.org/10.1144/geochem2014-286 doi:  10.1144/geochem2014-286
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    Li, Q. M., Cheng, Q. M., 2004. Fractal Singular-Value (Egin-Value) Decomposition Method for Geophysical and Geochemical Anomaly Reconstruction. Earth Science, 29(1): 109-118(in Chinese with English Abstract) http://www.researchgate.net/publication/284672230_Fractal_singular-value_eigen-value_decomposition_method_for_geophysical_and_geochemical_anomaly_reconstruction
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    Liu, J. S., Zhang, H. P., Ouyang, Y. F., et al., 2007. Bainiuchang SuperLarge Silver-Polymetallic Ore Deposit Related to Granitic Magmatism in Mengzi, Yunnan. Journal of Central South University of Technology, 14(4): 568-574. https://doi.org/10.1007/s11771-007-0109-x doi:  10.1007/s11771-007-0109-x
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    Zhang, Y. H., Zhang, S. T., Tan, S. C., et al., 2016. The Genetic Relationship between the Large Guanfang W Deposit and Granitic Intrusions, in Yunnan Province, Southwest China: Evidence from U-Pb and Re-Os Geochronology and Pb and Sr Isotopic Characteristics. Ore Geology Reviews, 79: 332-345. https://doi.org/10.1016/j.oregeorev.2016.05.025 doi:  10.1016/j.oregeorev.2016.05.025
    Zhao, B. B., Chen, Y. Q., 2011. Singular Value Decomposition (SVD) for Extraction of Gravity Anomaly Associated with Gold Mineralization in Tongshi Gold Field, Western Shandong Uplifted Block, Eastern China. Nonlinear Processes in Geophysics, 18(1): 103-109. https://doi.org/10.5194/npg-18-103-2011 doi:  10.5194/npg-18-103-2011
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Application of Singular Value Decomposition (SVD) to the Extraction of Gravity Anomalies Associated with Ag-Pb-Zn-W Polymetallic Mineralization in the Bozhushan Ore Field, Southwestern China

doi: 10.1007/s12583-020-1352-4

Abstract: The Bozhushan Ore Field, located at the western margin of the South China Block, is an important area for Ag-Pb-Zn-W polymetallic mineralization which may be associated with the Late Cretaceous granitic magmaism. In this paper, the singular value decomposition (SVD) was effectively applied to decompose gravity data at scale of 1:50 000 within the Bozhushan Ore Field to extract deep ore-finding information. Two gravity anomaly images displaying different scales of the ore-controlling factors were obtained. (1) The low-pass filtered image may reflect the deeply buried geological structures, hidden intrusions and concealed ore bodies. The negative gravity anomaly may reflect the overall distribution of granite bodies in the Bozhushan Ore Field. One negative gravity anomaly area may correspond to the exposed part of the Baozhushan granitic intrusion and the other corresponds to the concealed part of the granitic intrusion. The granitic intrusions are the main ore-controlling factors in this ore district. (2) The band-pass filtered image depicts the shallow concealed geological structures and geological bodies within this study area. There are two obvious negative gravity anomalies, which may be created by the hidden granites at different depths at both northwestern and southeastern sides of the exposed granitic intrusion. Thus the two negative gravity anomalies are favorable prospecting areas for various type of polymetallic ore deposits at depth. The gravity anomalies extracted by using the SVD exactly reflect the distribution of the ore deposits, structures and intrusions, which will give new insights for further mineral exploration in the study area.

Lingfen Guo, Yongqing Chen, Binbin Zhao. Application of Singular Value Decomposition (SVD) to the Extraction of Gravity Anomalies Associated with Ag-Pb-Zn-W Polymetallic Mineralization in the Bozhushan Ore Field, Southwestern China. Journal of Earth Science, 2021, 32(2): 310-317. doi: 10.1007/s12583-020-1352-4
Citation: Lingfen Guo, Yongqing Chen, Binbin Zhao. Application of Singular Value Decomposition (SVD) to the Extraction of Gravity Anomalies Associated with Ag-Pb-Zn-W Polymetallic Mineralization in the Bozhushan Ore Field, Southwestern China. Journal of Earth Science, 2021, 32(2): 310-317. doi: 10.1007/s12583-020-1352-4
  • It has been demonstrated by recent studies that hydrothermal processes in the Earth's crust can result in ore deposits characterized by high concentrations of metals with fractal or multifractal properties (Turcotte, 1997; Agterberg, 1995; Mandelbrot, 1989). This property of the high concentrations of metals is termed a singularity, and the hydrothermal processes are considered as singular processes. Singularity can be defined and characterized in different ways. From a geological application point of view, singularity can be defined as a special phenomenon with anomalous energy release or material accumulation occurring within narrow spatial-temporal intervals (Cheng and Agterberg, 2009). Singularity is a generic property and phenomenon of nonlinear natural processes that often generate end products which obey fractal or multifractal distribution and can be described by the power-law model (Cheng and Agterberg, 2009). Power law relations may reflect self-similarity (scale independence) of the underlying, genetic processes (Agterberg, 1995).

    It is well known that geophysical and geochemical anomalies are crucial clues for searching buried ore bodies. But these anomalies at surface caused by buried ore bodies are usually indirect and weak due to the shielding effects of the cover layer. Moreover, these anomalies are affected also not only by the burial depth, scale of ore bodies and petrophysical-chemical properties of cover layers but also by the superposition of anomalies from geological bodies (or ore bodies) with different petrophysical-chemical properties, depths and sizes, which lead to complexity of these anomalies. Therefore, identification and extraction of the geophysical and geochemical anomalies associated with buried mineralization play an important role in deep mineral exploration (Chen Y Q et al., 2015; Zhao and Chen, 2011).

  • With the SVD, a data set X can be decomposed to be a series of eigenimages. The SVD can be used for signal and noise separation (Clifford, 2005; Vrabie et al., 2004). The eigenvalues derived by means of SVD represent fractal or multi-fractal distribution described with power-law function. Li (2005) used the multi-fractal SVD for feature extraction and anomaly identification for mineral exploration.

    The singular value decomposition (SVD) is a factorization of a rectangular matrix X into orthogonal matrices

    where U is a left eigenvector matrix, S is a diagonal matrix called singular value matrix and VT stands for a transposition of the right eigenvector matrix. The singular values of X are the positive entries of S which can be entered in decreasing order along its main diagonal and are equal to positive square roots of the eigenvalues (λi) of the covariance matrices XXT and XTX. Thus

    where r=rank (X), $\sigma_{1} \geq \sigma_{2} \geq \cdots \geq \sigma_{r}, \quad \sigma_{i}=\sqrt{\lambda_{i}} $.

    The singular value decomposition of X can be also written as follows

    where r is the rank of X, ui is the ith eigenvector of XXT, vi is the ith eigenvector of XTX, σi is the ith singular value of X, and uiviT is an m×n matrix of unitary rank called the ith eigenimage of X (e.g., the first eigenimage, σ1u1v1). According to Eq. (3), the original matrix X can be rebuilt with all of the eigenimages. Also if some specific eigenimags are selected, a sub-matrix can be reconstructed.

    The singular values obtained by SVD method have features as follows: (1) they are distributed in decreasing order along main diagonal; (2) they represent different weighting coefficients of eigenimages; (3) their squared values (i.e., λ) are equivalent to power spectral density values in Fourier space (Li, 2005).

    Relatively few eigenimages contain the most energy of data set X. The percentage of each eigenimage (Pi) can be calculated as the following formula (Li, 2005; Freire and Ulrych, 1988)

  • Various types of rocks with different density can lead to singularities of gravity anomalies. In fact, the reason that traditional data process methods based on linear theory like Fourier transform and geostatistics are unsuitable for processing singular data is just because of their nonlinear and nonstationary properties (Cheng, 2012, 2008). Singularity is of typical scale invariant and obeys fractal or multifractal distributions. The singular values can be estimated for every location of geological bodies and used for characterizing the anomalies caused by buried geological bodies (Cheng, 2012; Cheng and Zhao, 2011). This may be reasons why SVD is more suitable than Fourier transform in extracting geophysical and geochemical anomalies associated with mineralization from their complicated backgrounds. Gravity data play an important role in inferring deep- seated geological structures and delineating concealed geological objects such as buried intrusive bodies and ore bodies. Effective use of gravity fields, like other geophysical fields, depends on establishment of a set of signatures that characterize forms, sizes and depths, as well as masses of various geological objects and their relationship to mineralization (Pan and Harris, 2000). The most direct information acquired from gravity fields is the density of geological bodies. A high gravity value indicates the presence of geological objects with higher average density than the materials surrounding them. Conversely, a low gravity value indicates the presence of geological bodies with relatively low average density. Because of heterogeneity in the density of geological bodies formed during complicated geological processes, even the same lithological unit in different spatial locations can cause different gravity anomalies, whereas different lithological units can produce similar gravity fields. This non-unique correspondence can cause some difficulties in inferring deep-seated geological structures and in delineating concealed geological objects. Thus, intrinsic geological and geochemical information is required. The scale of gravity anomalies is related not only to the size, but also to the depth of geological bodies. The same scale and type of anomaly might be produced by different lithological units located at different depths. Different scales and types of anomalies are possibly associated with differences in both the lithology and buried depth of geological bodies. These complexities and difficulties mean that new information decomposition techniques are required to identify possible ore-bearing locations from huge amounts of geological data. Over the last ten years, we have fulfilled some significant explorations in that application of SVD and mluti-fractal methods in extraction of buried deeply mineralization information by decomposing geochemical and gravity data (Huang and Zhao, 2015; Chen and Zhao, 2012; Zhao and Chen, 2011; Chen et al., 2007, 2006).

  • The Bozhushan Ore Field is located at the western margin of the South China Block, about 150 km west from the giant Gejiu giant tin polymetallic ore field. Bounded on the northwest side by the NE-trending Mile-Shizong fault which is adjacent to the Yangtze Block, and bounded on the southwest side by the NW-trending Ailaoshan-Red River fault which is adjacent to the Indo-China Block (Fig. 1). The Bozhushan Ore Field includes one giant silver polymetallic deposit (the Bainiuchang Ag-Pb-Zn-Sn (W) deposits, 6 470 t at 95 g/t Ag, 172 Mt. at 2.46% Zn; 109 Mt. at 1.56% Pb, 8.6 Mt. at 0.12% Sn) (Liu et al., 2007) and one large tungsten deposits (Guanfang W, 10 Mt. at 0.5% W) (Zhang et al., 2016) as well as a few small ore deposits and ore occurrences (Fig. 2).

    Figure 1.  Geological maps showing the outline and the major tectonic units of southeastern Asia (a), and the Bozhushan Ag-Pb-Zn-W polymetallic ore field is situated (b) (modified after Chen et al., 2017). CB. Cathaysia Block; YB. Yangtze Block; SB. Sibumasu Block; TP. Tibet Plate; ICB. Indo-China Block; IP. India Plate.

    Figure 2.  Simplified geological map showing the spatial distribution of intrusive rocks and the various types of mineral deposits in the Bozhushan Ore Field and the surrounding areas (modified after Zhang et al., 2016).

    The Cambrian, Lower Ordovician, Devonian, Carboniferous, Permian and Triassic strata are preferentially exposed at the surface due to uplift associated with Yanshanian (Mesozoic) tectonic movements. Numerous faults exist in the Bozhushan area, including the NNE-trending and NE-trending and the NW-trending faults. The Bozhushan granitic intrusion with SHRIMP and LA-ICP-MS zircon ages ranging from 86 to 91 Ma may form in a collisional tectonic setting in the early stages of the Late Yanshanian Orogeny (Zhang et al., 2016) and be related to lithospheric extension and asthenospheric upwelling induced by the change of the paleo-Pacific Plate motion in Late-Cretaceous (Chen X C et al., 2015).

    The granitic intrusion is mainly composed of porphyritic granite, monzonitic granite, biotite monzogranite etc. Skarnization, marbleization and silicification commonly developed in the contact zone between the intrusions and wall rocks. Many skarn tungsten polymetallic deposits have been discovered within the metasomatic contact zone (Fig. 2).

    It has been illustrated from the granite and granite-porphyry in the Bainiuchang super-large silver-polymetallic district that the granite-porphyry has strong mineralization and locally forms industrial orebodies. The average content of the ore-forming elements Cu, Pb, Zn, Ag, Sn, As and Sb in the granite is 3.49-100 times higher than that of normal granites (Liu et al., 2007).

  • The SVD is used for analyzing the gravity data surveyed at scale of 1 : 50 000 within the Bozhushan Ore Field (Fig. 3). It has been illustrated by Fig. 3 that the Bozhushan Ore District displays an irregular negative gravity anomaly with a extension in approximately NW-SE orientation.

    Figure 3.  Original gravity data image of the Bozhushan Ore Field at scale 1 : 50 000.

    Freire and Ulrych (1988) defined low-pass XLP, band-pass XBP, and high-pass XHP SVD images terms of the ranges of singular value used.

    The choices of p and q depend on the magnitudes of the singular values themselves. One of the properties of singular processes is the resulting power-law distribution (Cheng and Agterberg, 2009). Here we will use the power-law to determine p and q. The main proposition supporting the non-linear theory and application of power-law models is that mineralization can result from some singular processes, and that mineral deposits can be regarded as the products of some singular processes, and that these singular processes may be characterized by power- law models (Cheng and Agterberg, 2009; Cheng, 2007). The square of the singular values (i.e., λ) corresponds to the spectral energy densities of eigenimages (Li, 2005). Thus, the sum of energy (i.e., a measurement of energy in spectral energy radius or scale) whose squared singular values are larger than λi can be written as follows (Li and Cheng, 2004)

    λ and E (or P) may represent a fractal or multifractal (Li, 2005; Li and Cheng, 2004; Li and Liu, 2003).

    And its relevant proportion (P) is

    or

    Because of power law, the curve in log-log plot of λ-E can be separated into several segments due to different slopes. And the break points are p and q (sometimes can be separated into more than 3 segments). The reconstruction of some specific eigenimages whose singular values in the same segment may be corresponding to specific geological processes.

    In this paper, the SVD is applied to decompose the gravity data surveyed at a scale of 1 : 50 000 of the Bozhushan polymetallic ore field, southeastern Yunnan. By extracting the gravity components of a certain frequency, we aim to reveal the spatial relationship between the deeply buried geological structure and mineralization in the Bozhushan Ore Field, and provide evidences for prospecting deeply concealed orebodies. The gravity data surveyed at grid of 500 m×250 m for this study is provided by Yunnan Geological Survey. The total data resolution is ±2.32×10-6 m/s2. The densities of the outcropped geological bodies in this area range from 2.40 to 2.63 g/cm3 for the biotite monzonitic granite and granite porphyry, vary from 2.66 to 2.74 g/cm3 for the carbonate and clastic rocks, 2.74 g/cm3 for the basalt and 1.70 g/cm3 for the Quaternary clay within the Bozhushan Ore Field (Zhong, 1992).

    The Bozhushan Ore Field is associated with the Late Cretaceous granitic complex. It has been illustrated by Fig. 3 that the Bozhushan Ore Field displays an irregular negative gravity anomaly with an extension in approximately NW-SE orientation. The negative gravity anomaly center spatially coincides with the outcropped Bozhushan granitic intrusion. The fact that the negative gravity anomaly extends northwestern towards the giant Bainiuchang silver polymetallic deposit illustrates that the granitic intrusions may plunge into the northwestern direction and it has been confirmed that the concealed granitic bodies were detected at about 400 m depth by drilling exploration in the Bainiuchang Ore Field.

    With the SVD method, p, q in Eqs. (5), (6) and (7) can be determined. The curve in the ln-ln plot for λ-E of the gravity data in the Bozhushan Ore Field can be divided to three segments on the basis of the different slopes, two break points, p=3, q=10 (Fig. 4). The gravity image reconstructed form the sum of the 1st and 2nd eigenimages (right segment) can be regarded as low-pass filtered image, which generally indicates the regional ore-controlling factors. The gravity image reconstructed form the sum of the 3rd to 9th eigenimages (middle segment) can be regarded as band-pass filtered image, which usually reveals the local ore-controlling factors.

    Figure 4.  The ln-ln plot of λ-E of the gravity data in the Bozhushan Ore Field, p=3, q=10, ln (λ3)=5.34, ln (λ10)=9.92.

  • Gravity data of the Bozhushan Ore Field have been reconstructed by using the program compiled based on MATLAB software, and different eigen subspace images are shown in Figs. 5 and 6.

    Figure 5.  The gravity component image reconstructed from the 1st and 2nd eigenvalues in the Bozhushan Ore Field.

    Figure 6.  The gravity component image reconstructed from the 3rd to 9th eigenvalues in the Bozhushan Ore Field.

    Figure 5 is the reconstructed gravity image from the 1st to 2nd eigenvalues, which is equivalent to a low-pass filtered image, and reveals the deeply buried geological bodies. The image shows a concentrated negative gravity anomaly zone, which may reveal the overall distribution of the Bozhushan granitic intrusion, including the outcropped granites and the buried granites. Taking -7 μm/s2 as critical value, the Bozhushan Ore Field is divided into two negative gravity anomaly areas (Ⅰ and Ⅱ). One negative gravity component anomaly (Ⅰ) almost exactly coincides with the outcropped granites in Fig. 5. Another negative gravity component anomaly (Ⅱ) may reveal the buried part of the Baozhushan granitic intrusion, which illustrates also that the granite intrusion plunges towards northwest and extends to the Bainiuchang Ore Field. The emplacement of granitic magma could not only form favorable metallogenic space, but also provide thermal driving force for the migration of ore-bearing fluid.

    Figure 6 is the reconstructed gravity image from the 3rd to 9th eigenvalues, which is equivalent to a band-pass filtered image, and may reveal the shallow concealed geological bodies. There are two negative gravity anomalies (Ia and Ib) with the gravity anomaly value ranging from -1.43 to -7.19 μm/s2. The negative gravity anomalies (Ia and Ib) may be created by the hidden granite bodies on both sides of the exposed granite body, where concealed ore bodies may be discovered. The positive gravity anomaly (Ⅱ) with gravity anomaly value more than 2.88 μm/s2 may be caused by carbonate rocks with basalt interlayers, which are favorable host rocks. Almost all the known tungsten polymetallic deposits are located in the inner contact zone around the outcropped granitic intrusion with weak gravity anomaly value ranging from -1.43 to 2.88 μm/s2. Silver polymetallic deposits are distributed in the outer contact zone, and antimony deposits are located farther away from the granitic intrusion. Therefore, the contact metasomatic zone between the granite intrusives (Fig. 5) and the wallrock of carbonate is the favorable ore-prospecting area.

    The geologic-gravity section has been constructed in Fig. 7 along the A-B section (see in Fig. 6) to explore further the spatial relationship among the granitic intrusion, orebody and local gravity anomaly. The low gravity areas correspond to the swelling area of the Bozhushan granitic intrusion. The high gravity area coincides with the areas with thickened carbonate rocks over the granitic intrusion, which indicates that the variations of local gravity anomaly are mainly affected by the fluctuating surface of the granitic intrusion. It is illustrated that these polymetallic ore deposits are spatially distributed in the order of the skarn tungsten deposits, epithermal lead-zinc deposits, epithermal silver polymetallic deposits and epithermal antimony deposits from the granitic intrusion outwards outside and from deep to shallow.

    Figure 7.  The geological-gravity section showing the relationship among the spatial distribution of granitic intrusive, deposits and the local gravity anomaly component in the Bozhushan Ore Field.

  • Based on the above-mentioned discussion, it can be concluded that: (1) The low-pass filtered image reflects the deeply buried geological structures and geological bodies. The negative gravity anomaly reflects the overall distribution of granite bodies in the Bozhushan Ore Field. One negative gravity anomaly area corresponds to the exposed part of the Baozhushan granitic intrusion and the other corresponds to the concealed part of the granitic intrusion. The granitic intrusions are the main ore-controlling factors in this area. (2) The band-pass filtered image depicts the shallow concealed geological structures and geological bodies within the Bozhushan polymetallic ore field. There are two obvious negative gravity anomalies that may correspond to the hidden granite bodies at different depths at both northwestern and southeastern sides of the exposed granitic intrusion. It is inferred that large area of positive gravity anomalies may be created from carbonate rocks containing basic rocks. The contact metasomatic belt formed by granite and carbonate rocks is a favorable area for exploring vorious type of ore deposits. (3) The gravity anomalies extracted by using the SVD exactly reflect the distribution of the ore deposits, structures and intrusions, which will give helpful insights for further mineral exploration.

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