Journal of Geoscience and Environment Protection, 2014, 2, 46-53
Published Online June 2014 in SciRes.
How to cite this paper: Dai, N. B. et al. (2014). Magnetic Method Surveying and Its Application for the Concealed Ore-Bo-
dies Prospecting of Laba Porphyry Molybdenum Ore Field in Shangri-La, Northwestern Yunnan Province, China. Journal of
Geoscience and Environment Protection, 2, 46-53.
Magnetic Method Surveying and Its
Application for the Concealed Ore-Bodies
Prospecting of Laba Porphyry Molybdenum
Ore Field in Shangri-La, Northwestern
Yunnan Province, China
Nguyen Ba Dai, Chuan Dong Xue, Kun Xiang, Tran Trong Lap, Qureshi Javed Akhter,
Shi Lei Li
Department of Earth Sciences, Kunming University of Science and Technology, Kunming, China
Email:, cdxue001@ali m
Received February 2014
Recently, a number of large molybdenum (-copper) deposits have been discovered successively in
the Laba area, Shangri-La county, northwestern Yunnan province. The investigation confirmed
that there is a superlarge porphyry-skarn hydrothermal vein type molybdenum-polymetallic-
metallogenic system with the total prediction reservoir of more than 150 mt molybdenum. The
porphyry intrusions contributed to the mineralization closely, the superficial little vein molybde-
num (-copper, lead, silver) ore-bodies are usually located in faults and fractures, and the deep
porphyry type ore-bodies occurred in the granodiorite porphyries, the skarn type ore-bodies oc-
curred in the contact zone intrused into Triassic limestone or Permian basalts. Laba ore block is a
new exploration area with great prospecting potential. In order to reduce the target area and
guide the further exploration work, the magnetic method measurement about 3.3 square kilome-
tres was carried out in the ore field. This paper presents an application of analyzing the horizontal
and vertical derivative, using Fast Fourier Transform (FFT) filter (FFT high-pass, low-pass, cosine
roll-off, suscepbility), calculated spectra frequency energy to predict the depth and intensity of the
apparent remanence magnetization of source (Hilbert). The calculated results and magnetic
anomalous show that the remanence anomaly is caused by the intrusions into the Triassic lime-
stone and Permian basalts with small anomalies, and the depth of located source is not great. We
have identified a number of positions to the three drilled well, the drilled result specify interpre-
tation with very high accuracy. The magnetic method is helpful to identify porphyry mineraliza-
tion, and judge the shape and depth of the concealed ore-bearing intrusive bodies under the simi-
lar geological condition.
Magnetic Method, Physical Property Parameters, Concealed Ore-Bodies Prospecting,
Laba Porphyry Molybdenum (-Copper) Ore Field, Northwestern Yunnan Province
N. B. Dai et al.
1. Introduction
In the Laba town and surrounding area about 100 square kilometres in Shangri-La county, northwestern Yunnan
province, there distributed large amounts of smaller copper-, lead-, and zinc-polymetallic deposits and minerali-
zation spots. Through much detail geological work during the past several decade, the advance of mineral re-
sources prospecting exploration is little, and the know ore deposits occurred in the not wide range area. Till to
2010, the No. KT1 molybdenum (-copper)-rich orebody in Tongchanggou ore block was firstly explored by Ge-
ology Investigation Institute of Yunnan Bureau of Geological Survey sponsored by Yunnan Copper Industry
Group Company (e.g., Li et al., 2012). After that, a number of molybdenum (-copper) deposits have been dis-
covered successively, and porphyry intrusions contributed to the mineralization. The superficial little vein or
veinlet molybdenum (-copper, lead, silver) ore-bodies are usually located in secondary fractures and the inter-
section part of different direction faults, and the deep porphyry type ore-bodies occurred in the monzonitic gran-
ite to granodiorite porphyries, the skarn type ore-bodies occurred in the contact zone intrused with Triassic
limestone or Permian basalts in a certain range. Up to now, the Laba molybdenum (-copper) deposit has become
a super large porphyry deposit with resources reserves of 63.5 mt Mo @ 0.11%, associated with 33.7kt Co@
0.22% and Au 14.7t @ 0.65ppm. The investigation confirmed that there is a super large porphyry-skarn hydro-
thermal vein type molybdenum-polymetallicmetallogenic system, and the total prediction molybdenum reservoir
will be more than 150 mt. The exploration and research work is still in progress, the reserves increase gradually.
The Laba ore block is a new prospecting exploration area with distance about 3 kilometres north to the Tong-
changgou ore block, and the prospecting potential is great. Field investigation shows that concealed granite in-
trusions may exist in the depth, and mineralization was related to magmatic intrusion. But the area covering
layer thickness, morphology of ore bearing rock buried depth, stratigraphy and structural rule is unknown, the
exploration work deployment is of poor effectiveness. In order to reduce the target area rapidly, and guide the
further mineral exploration and evaluation work effectively, the magnetic method measurement work (Eun-
Jung et al., 2011; Vanessa et al., 2013; Wang et al., 2012; Yang et al., 2011) for the concealed ore-bodies pros-
pecting was carried out starting from July 2012. Totally, we have surveyed finished the magneticzation survey-
ing of the Laba ore field more than 3 square kilometres. This paper puts forward a new data processing method,
and presents an application of analyzing the derivative horizontally and vertically, using Fast Fourier Transform
(FFT) filter for rectangular window applied to horizontal derivative. The aim is to calculate spectra frequency
energy to predict the depth and intensity of the apparent remanence magnetization of source (Hilbert) (Alanna &
Yára, 2009; Fabio, 2012; Ilya & Ahmed, 2009; Maysam et al., 2013; Pejman et al., 2011; Stocco et al., 2009),
and is applied to calculate magnetic data anomalous. The results show that, the remanence anomaly is caused by
the intrusion into the Triassic limestone and Permian basalts with small anomalies, and the depth of located
source is not great. We have identified a number of positions to the three drilled well, the result specify inter-
pretation with very high accuracy. And it can judge the shape and depth of the concealed ore-bearing intrusive
bodies. The magnetic method is helpful to identify porphyry molybdenum (-copper) mineralization under the
similar geological condition.
2. Geology Backgro und
The Laba molybdenum (-copper) polymetallic deposit is located in the western margin of the Yangtze block,
and is adjacent to the southern connection part of the Yangtze block and the Zhongdian island arc belt, which
belongs to the southern part of the Late Triassic Yidun magmatic arc. The Zhongza block and Jinsha River su-
ture are located to the northwest along the Geza faults zone, and the Garze-Litang suture zone to the northeast.
From a regional perspective, the Zhongdian arc is a significant Triassic porphyry and skarn copper-polymetallic
district (e.g., Hou et al., 2003), hosting several large deposits, such as Pulang, Xuejiping, Hongshan, Gaochiping
and Chundu, as well as many smaller deposits and occurrences under the setting of the Mesozoic arc. But re-
cently, the Late Cretaceous monzonitic granite to granodiorite porphyries with porphyry and skarn molybdenum
(-copper) mineralization (e.g., Li et al., 2012; Peng et al., 2014) and lamprophyre has also been identified in
Laba area. They exposed diabase dikes and sporadically in a small area. Acid intrusions in the area extend from
north to south and the belt passes through Yidun arc and enters the western margin of Yangtze landmass.
The Laba molybdenum (-copper) ore field covering an area of 10 square kilometres is located in Laba town of
Logi Xiang in Shangri-La county, northwestern Yunnan province, southwestern China. The deposit has two ore
blocks termed the Tongchanggou and the Laba blocks. The Tongchanggou ore block, located in the southeast,
N. B. Dai et al.
was discovered in 1985, and the exploration has not completed. The Laba ore block, to the northwest, was only
recently discovered and is under exploration.
The wall rocks are dominated by Middle Triassic Beiya group (T2b) limestone, and Permian Dam group (P2d)
volcanic rocks interbeded with dense shaped limestone, siltstone and sandstone, which underwent contact
metamorphism to hornfels and marble “Figure 1”. The main structure line orientation is N-NW, and rarely NE.
The ore field lies in the northern plunging segment of the NW-trending Baihua Mountain anticline with Permian
Dam group (P2d) basalt in core, and Triassic Beiya group (T2b) limestone in limbs, the east wing occurred 60˚ -
50˚ angle, the west wing is 280˚ - 300˚ and angle of 45˚ - 60˚Figure 1”. In this area, the main faults are F1, F2,
F3 and F4 fault. Among them, the F1 fault (Tongchanggou) trending NNE and cuting the anticlinal axis, is the
major structures, and also control the distribution of magmatic rocks and related ore deposits. ANNW trending
Yanshanianmonzonitic granite to granodiorite porphyry is exposed only in the center of the Laba ore block. The
outcrop area is approximately 200 m2 with length of 10 - 120 meters and width of 5 - 40 meters, its contact zone
with the surrounding sedimentary rocks is not clear. The porphyry is wholly mineralized and hydrothermal me-
tasomatism in the upper part locally displaying skarn type and hydrothermal vein type mineralization in wall
rocks, forming a porphyry minerogenetic series. The molybdenum (-copper) ore-bodies have been measured to
depth of from 100 to 850 meters.
3. Research Metho ds
3.1. Magnetic Data Survey
The magnetic survey work carried out in July 2012, using the Proton Geometric G856 device, which allows the
accuracy of 0.5 nT. The north-south direction is line survey, the spacing line to line of 100 m, the distance points
of 10m. Using the same accurate measure, it measured diurnal magnetic field. The magnetic field century give
by IGMR at July 2012 update, with altitude is 2400 m of similar altitude of diurnal magnetic field. Survey area
is smaller, but the great alter in topography, altitude changing from 2300 - 3200 m (Figure 2). Therefore, the
terrain adjustment is very important. The magnetic field is reduced by ±0.23 nT with alteration in 100 m alti-
The obtained results show “Figure 2”, the small anomaly of different intensity along north-south direction,
along F1 fault toward the Logi river, the adjacent coordinate 606100 to 607300 and 3069500 to 3071700, these
magnetic anomaly parameters from −650 nT to 200 nT.
Figure 1. The geological map of Laba molybdenum (-copper) ore field.
N. B. Dai et al.
Figure 2. Shangri-La, Laba anomalous magnetic map.
3.2. Method and Analysis Filter
The magnetometric study of igneous intrusions is thoroughly linked to the definition of the vector components
of the magnetic field:
TTo Ta
= +
Being T is the vector of total magnetization, To is the vector of induced magnetization and Ta is the vector of
remanent magnetization. The space is defined function f(x, y, z).
Measurement data processing gradually, eliminating diurnal variation, affecting the normal field and regional
background field, so that it can obtain the magnetic anomaly map of the study area. Then, using Butterworth fil-
ter, Lowpass filter, Highpass and Bandpass filter for data.
An analysis signal is:
As ..dx dxdz dz+=
where, dz is the vertical derivative, and dx is the horizontal derivative.
The Fourier transform of a space domain function f(x) is defined to be:
( )
( )
The reciprocal relation is:
( )
( )
e dx
where, ω is an angular wavenumber in radians per ground_unit (for x in ground_units). The wavenumber in cy-
cles per ground_unit (r) is simply ω/2π.
A line of data in the space domain can be thought of as a sequence of magnetic values at points along a
straight line, each point separated by a constant distance. Such a line can be transformed to and from the
wavenumber domain by use of a discrete (FFT). The equivalent data set in the wavenumber domain is com-
monly called a Transform. A Transform of a line consists of real and imaginary amplitude values as a function
of wavenumber in cycles per original distance unit.
The susceptibility filter calculates the apparent magnetic susceptibility of the magnetic sources using the fol-
lowing assumptions:
N. B. Dai et al.
• The IGRF has been removed from the data.
• There is no remanent magnetization.
All magnetic response is caused by a collection of vertical prisms of infinite depth and strike extent.
A susceptibility filter is, in fact, a compound filter that performs a reduction to the pole, downward continua-
tion to the source depth, correction for the geometric effect of a vertical prism, and division by the total mag-
netic field to yield susceptibility:
( )()()( )
( )
( )()
( )
( )( )
,2H K
sincos cos
Lk Fk
θπ ωθ
=⋅⋅Γ ⋅
Γ=+ ⋅−
where, h is the depth in ground_units, relative to the observation level at which to calculate the susceptibility. Ia
is pole reduction amplitude inclination. Inclination to which to use the phase component only in the reduction to
the pole. The default is
. If
is specified to be less then
, it is set to I. I is geomagnetic inclination.
D is geomagnetic declination. F is total geomagnetic field strength.
Each of these filters starts with a line of data in the space domain and produces a new line of data in the space
domain that is the result of applying the filter.
3.3. Hilbert Transform
The Hilbert Transform option does the Hilbert Transform by the means of FFT based on the following known
( )
] ]
( )()
F Hfx isgnw Ffx= −
where, F[f(x)] is the Fourier transform of f(x), H[f(x)] is the Hilbert transform of f(x), and:
Sgn(w) = w/|w| = +1 for w>0, = 0 for w=0, = -1 for w < 0.
Firstly, forward FFT transform of the input channel. Three output channels are created (this is only for the
none-array input channel case). They will have the same name as the input channel but extension _rand _i
for real and imaginary components of the transform, and “_w” for the wave number in radians/fiducial. (Note
that the trend has been removed before FFT) Note that the output values are the real and imaginary components
of the positive frequencies of the transform. Since we are dealing with the real-valued space domain problem,
the negative part of the spectrum is simply the conjugate of the corresponding positive part, i.e. h(-f) = [h(f)]*,
and is not included in the output. The fiducial number will be in cycles/fiducial. The wave number channel will
be in radians/fiducial.
Then, the one-dimensional Hilbert transform operator isgn(w) is applied to the FFT transformed data. Fi-
nally, the inverse FFT transform to obtain the Hilbert transform results into the output channel. For the real data
practice, it is suggested to remove trend line based on all data points (the default) before FFT process to prevent
the discontinuity from the data two edges. The removed trend will be replaced back in the same manner as it
removed after FFT. For the real data practice, it is also suggested to expand the data 10% before FFT process to
prevent the discontinuity from the data two ends. The first extend data by the user required % points, then fur-
ther extend to the number of the power of 2 for the FFT process. For instance, if the original data contains 60
points, then the values will be padded with 10%, or 6 points at the end, giving 66 points. This will then be ex-
tended to the next power of 2, or 128 points, to do the FFT.
The extended area will be interpolated by Maximum Entropy Prediction (MEP) method. MEP samples the
original data points to determine its spectral content. It then predicts a data function that will have the same
N. B. Dai et al.
spectral signature as the original data. As a result, the predicted data will not significantly alter the energy spec-
trum that would result from the original data alone.
4. Application Analysis Data and Interpretation
Study area is smaller, the terrain changes sharply, exposed three meaning. As we all know, remanence intensity
limestone value is very low, and ore bearing rock body can cause remanent intensity is high, so it can be based
on this to find ore bearing rock.
Magnetic anomaly map obtained shows “Figure 3”, the small scale anomalies are small to medium strength.
We conduct filtering process and derivation, respectively filtered derivation in the horizontal direction and the
vertical direction, and then the analytic signal strength. Signal intensity greater than 2 is mainly concentrated in
a certain range. The next task is to mark the range, calculated remanence magnetization intensity as based on
abnormal volume on the strength of the remanent magnetization, changes in the range of 0.005 - 0.2 cgs.
Then , the FFT Hilbert transform can be used to determine the prisms high remanence magnetization, the
depth and size of prisms caused by magnetic anomalies. The results showed that “Figure 4”, intrusive rock
mineralization caused by abnormal position in the range of 60-700m below the surface of terrain, extends the
scale of ore rock mass 50 - 300 m.
Through the study of geology, structure, mineralization regularity and geophysical survey work, the analysis
and interpretation of display, has found that 5 of concealed ore bearing intrusive rock branch. Borehole samples
display, metal element intrusive body containing molybdenum and copper mineralization. Combined with other
drilling data, identified 5 small scale ore bearing intrusive rock branch, ensure they are Mo-Cu ore body not
Through drilling compared with construction completed, the results of measurement data of magnetic method
and drilling results are basically consistent, accurate rate reached more than 85%.
5. Conclusion
1) By using FFT transform filter, in the horizontal and vertical derivative, and the use of FFT for Hilbert
Figure 3. The magnetic data interpretation anomalous of Laba molybdenum
(-copper) ore field..
N. B. Dai et al.
Figure 4. The concealed ore-bearing intrusion bodies of Laba molybdenum
(-copper) ore field.
transform processing magnetic data, to determine the intrusive rock size and depth, effective for small anomalies
size porphyry deposit and buried shallow, results and drilling revealed the coincidence rate reached above 85%.
2) According to the analysis and interpretation of results, other magnetic anomalies should be concealed
ore-body Mo-Cu instructions, can be verified.
This study is financially supported by the Doctoral Fund Projects of Education Ministry of China (20125314110006),
the Natural Science Fund Project of Yunnan Province (2009CI030) and the Science and Technology Project of
Yunnan Baofeng Mineral Resource Co. Ltd. We thank Mr. Shilei Li and other geological engineers for their
help during the field work. We sincerely thank Dr. Haixia Li and other reviewers for their comments and sug-
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