SOME
RESULTS OF THE STUDY ON THE CHARACTERISTICS OF IP FREQUENCY DOMAIN ANOMALOUS
FIELD IN SOME ZONES AND BODIES OF COPPER
TĂNG ĐÌNH
Institute of
Geosciences and Mineral Resources (VIGMR), Thanh Xuân, Hà Nội.
Abstract: The frequency-domain IP, together with
made-in-Canada generator T3 and receiver V5 having 17 frequencies of 0.125-8192
Hz, is one of new geophysical technologies recently imported to Việt
I. INTRODUCTION
The
frequency-domain IP with made-in-Canada generator T3 and receiver V5, having 17
frequencies of from 0.125 to 8192 Hz, is one of new geophysical technologies recently
imported to Việt
The
purpose of this paper is to present some results of study on characteristics of
IP frequency domain anomalous field obtained from Tà Phời Cu ore zones and
bodies in
II. BRIEF OUTLINES ON CHARACTERISTICS OF
GEOLOGY AND MINERAL RESOURCES IN TÀ PHỜI AREA,
The
Tà Phời copper ore area is a potential area, in which the InterGeo Division is
carrying out a project entitled "Assessment
of potential on copper ores and other mineral resources in the Tà Phời area, Cam
Đường town,
-
Stratigraphy: In the study area, there are mainly
distributed metamorphic sediments of the member 2 of the Sin Quyền Formation,
dolomite marble of the Đá Đinh Formation, terrigenous sediments of the member 1
of Cam Đường Formation.
- Magmatism: Distributed intrusive
magmatic rocks in the study area are composed mainly of granites formed in the
phases 2 and 3 of the Po Sen Complex and diorite, lamprophyre of unknown in age
intrusive complexes. Their composition includes diorite, biotite hornblende
diorite, quartz diorite and dykes, such as diorite aplite, lamprophyre. Copper
ores are closely related to those intrusive magmatic bodies.
- Tectonics: Metamorphic rocks of
the Sin Quyền formation were strongly folded and compressed consequently
resulting in the formation of different in dimensions synclines and anticlines
with their axes stretching in NW-SE direction. Copper ores are usually
distributed in altered diorites and metasomatic rocks in the cores of
anticlines.
-
Mineral resources: In the prospecting area, sulphide and Cu, Au
mineralizations are distributed in alteration zones of fine- to medium-grained
diorites, quartz-bearing diorite, leucocratic diorite, biotite-plagioclase,
plagioclase-tremolite metasomatic rocks, quartz-feldspar-biotite schist,
graphite-bearing quartz-biotite schist. Ores are characterized by disseminated,
veined, disseminated vein structures. The main ore-forming minerals are pyrite,
chalcopyrite, cubanite, less pyrrhotine, while the secondary ore-forming
minerals are malachite, covelline. The main ore-bearing rocks are metasomatites
with actinolite, horn-form quartz-biotite schist, fine-grained hornblende
diorite.
III. IP FREQUENCY DOMAIN SPECTRAL CHARACTERISTICS OF ROCKS AND ORES IN TÀ PHỜI AREA
For the first time in Việt Nam, the IP spectral
characteristics of Cu, Pb-Zn and Au ores and rock samples of some deposits have
been studied by Sip-Fuch Version 02-06-2000 equipment with frequency band of
from 1.4 mHz to 12 kHz installed in the Institute of Geophysics, Clausthal
Technical University, FR of Germany. 57 rock and copper ore samples have been
taken from the Tà Phời area and then sent to
1. Graphite-bearing quartz-biotite schist is
represented by sample TP.1a has IP spectral characteristics which are shown on
the Fig. 1; its resistivity value varies from 125.9 Ω∙m at the frequency of
0.09 Hz to 100 Ω∙m at the frequency of 11.18 Hz. Value of polarization phase is
gradually increasing from 111.4 mrad at the frequency of 0.09 Hz up to 166 mrad
at the frequency of 11.18 Hz; PFE effect gradually increases from 9.8 % at two
frequencies of (0.09-0.7) Hz to 13.6 % at two frequencies of 1.4-11.18) Hz; metal
factor M gradually increases also too from 84.309 S/m at two frequencies of (0.09-0.7)
Hz to 135.608 S/m at two frequencies of (1.4-11.18) Hz.
2. Graphite-bearing plagioclase-biotite gneiss
is represented by sample TP.9a. The IP spectral characteristics are shown on
the Fig. 3, resistivity value at the frequency of 0.09 Hz is 117.1 Ω∙m
decreasing to 100.3 Ω∙m at the frequency of 11.18 Hz. The value of polarization
phase gradually increases from 56.6 mrad at the frequency of 0.18 Hz up to 60.8
mrad at the frequency of 2.92 Hz and decreases to 56.9 at the frequency of
11.18 Hz; the PFE effect gradually increases from 6.7 % at two frequencies of (0.09-0.7)
Hz to 8.6 % two frequencies of (1.4-11.18)
Hz; the metal factor M gradually increases also from 60.483 S/m at two
frequencies of (0.09-0.7) Hz to 86.056 S/m at two frequencies of (1.4-11.18)
Hz.
3. Cu ore-bearing plagiogneiss biotite is
represented by sample TP.7a. Its IP spectral characteristics are shown on the
Fig. 2; the resistivity parameter at the frequency of 0.09 Hz is 200.1 Ω∙m
decreasing to 168 Ω∙m at the frequency of 11.18 Hz. The value of polarization
phase gradually decreases following the increase of frequency: at the frequency
of 0.18 Hz its value is 98.6 mrad, but it is only 64.7 mrad at the frequency of
11.18 Hz; PFE effect is nearly constant following the increase of frequency,
which equals 10.5 S/m at two frequencies of (0.09-1.46) Hz and 10.6 S/m at two
frequencies of (0.73-11.18) Hz.
4. Diorite is represented by sample TP.4a. Its
IP spectral characteristics are shown in the Fig. 3; its resistivity is high
which decreases from 2554 Ω∙m at the frequency of 0.09 Hz to 2499.6 Ω∙m at the
frequency of 11.18 Hz. Value of polarization phase is small and less varies in
the interval of from 6.9 to 7.1 mrad, at the frequency of 1.46 Hz its value is
6.0 mrad; PFE effect is small and nearly constant from 1.3 % at two frequencies
of (0.09-1.46) Hz and 1.2 % at two frequencies of (0.73-11.18) Hz.
5. Cu ore-bearing altered diorite is
represented by the sample TP.4d. Its IP spectral characteristics are shown on
the Fig. 3 its resistivity parameter decreases from 28.5 Ω∙m at the frequency
of 0.09 Hz to 24.3 Ω∙m at the frequency of 11.18 Hz. Value of polarization
phase also gradually decreases from 135.8 mrad at the frequency of 0.18 Hz to
108.1 mrad at the frequency of 11.18 Hz; but the PFE effect gradually increases
from 7.5 % at two frequencies of (0.09-1.46) Hz up to 11.6 % at two frequencies
of (0.73-11.18) Hz.
Cu ore samples taken from Tà Phời area have the
same polarized spectrum as that of Cu ore samples taken from Sin Quyen, Ban
Phuc and Ba Vi copper deposits, which are presented on the Fig. 4.
The above-mentioned study results allow us to
come to some conclusions as follows:
1. The value of polarization phase and
polarization effect of Cu ore-bearing rocks are inversely proportional to
frequencies, that means these values gradually decrease following the increase
of frequencies from low to high values.
2. The polarization effects of graphite-bearing
rocks are directly proportional to frequencies, while their phase values are
inversely proportional to frequencies in the range of from 11.70 to 750 Hz and
are directly proportional to frequencies in the range of from 0.0057 to 0.09
Hz.
3. The polarization effects of rocks associated
with sulphidic gold are inversely proportional to the frequency; their phase
values are also inversely proportional to frequencies in the range of from 0.73
to 93.75 Hz and are directly proportional to frequencies in the range of from
0.0057 to 0.09 Hz.
The frequency domain IP spectral
characteristics can help us not only in finding Cu ores, sulphidic gold and
graphite, but also in distinguishing each from other among them. This is the
main advantage of the frequency domain IP method in comparison with the time
domain IP one.
IV. IP FREQUENCY DOMAIN ANOMALOUS CHARACTERISTICS OF Cu ORE ZONES AND BODIES IN TÀ PHỜI AREA
In order to establish the IP frequency domain
anomalous characteristics of sulphide Cu ore zones and bodies in Tà Phời area,
the IP frequency domain sounding have been carried out on 7 lines. The sounding
was done according to 2D model by Wenner-Schlumberger array for all of lines.
In
order to determine the receiving and transferring frequencies, the IP
measurements have been done on outcrops of rocks and ores with frequencies of
from 2-3 to 213. The results show that at the Cu ore
outcrops the resistivity value is small, phase value is high and inversely proportional
to frequency in the range of 8 - 0.125 Hz; and the characteristics of the rocks
are opposite; graphite-bearing schist holds high phase value, which is directly
proportional to frequency in the range of 1Hz - 0.125 Hz; so, in order to study
deeply and to find anomalies of maximum phase, the following frequencies are
chosen for measurement on all lines in the study area: 2, 1, 0.5 and 0.125 Hz.
Measurements
are subjected to quantitative analysis according to 2D model by software
RES2DINV and RESIX IP2DI.
The
results of pilot studies on the lines are as follows:
1. Line T3
The
results of quantitative analysis of frequency domain IP sounding data with
phase values at frequencies 1Hz, 0.5Hz and 0.125Hz are presented on Figs. 6a,
6b, and 6c. The results show that:
-
Resistivity model: Models of
resistivity values with resistance values of from about some to over 4000 Ω∙m
have been divided into 2 clear zones: a zone from pile -5 of the line to the
end of negative wing has resistivity values mostly higher than 10 Ω∙m related
to the graphite-bearing schist; a rest zone has resistivity values mostly
higher than 100 Ω∙m related to Cu ore-bearing altered diorite.
- Phase value model:
On the section of phase value model corresponding to frequency of 0.125 Hz,
there has been determined anomalous zone which is controlled by from pile -5 to
pile 30, existing at the depth of 100 m with resistivity values of 50-1000 Ω∙m;
phase values are higher than 35 mrad, related to ore bodies 4-8, which were
controlled both on the surface by trenches, outcrops and at the depth by
boreholes LK.12 at pile 9, LK.13 at pile 0 and LK.15 at pile 15. In all these 3
boreholes have met mineralizations, among them, in the borehole LK.13 has met
the ore at the depth interval of from 39 to 52 m; in the borehole LK.12 has met
the ore at the depth interval of from 102 to 110 m with ore content in each
sample reaching over 1 % (stratigrapghic column of the borehole is demonstrated
in the Tab. 1).
According
to geological data, in the distance from pile -6 to pile 25 there are
distributed 5 ore bodies (No 5-8) in the form of seam dipping to the west.
According to geophysical data, ores in
this area is in very complicated forms, the ore bodies cannot be recognized
clearly, for instance, the ore body 5 that is cut by the borehole LK.13 and the
ore body 6 met in the borehole may be only the same.
2. Line T5
The
results of quantitative analysis of IP sounding data on the line 5 with
following values. The results show that:
-
Resistivity model: Models of
resistivity values analyzed at different frequencies are similar to each other,
having resistivity values varying from some to over 1000 Ω∙m, which are divided
into 2 essential zones: the first from pile 0 to the end of negative wing,
holding very low resistivity values mainly less than 10 Ω∙m related to
graphite-bearing schist; the second holding higher resistivity values, mostly
over 100 Ω∙m, related to Cu ore-bearing altered diorite.
-
Phase values model: At the
section of phase values model at the frequency of 0.125 are determined 2 main
anomalous zones related to Cu ores.
*The
first anomalous zone is controlled by from pile -3 to pile 15, existing at the
depth of 100 m with resistivity values of 100-500 Ω∙m; values on polarization
are over 7 %; phase values of over 35 mrad are related to ore bodies 6 and 7.
The borehole LK.16 (pile 8 of the line) has cut ore at the depth of 25-27 m.
This anomalous zone has no anomaly at the frequency of 1 Hz.
*The second anomalous zone is controlled by
from pile 17 to pile 40, existing at the depth of more than 50 m with
resistivity values of 100-500 Ω∙m; phase values are over 35 mrad. This
anomalous zone is related to ore body 8.
3. Line T2
There are determined 2 main anomalous zones
related to Cu ores:
- The first anomalous zone is controlled by
from pile -10 to pile 13, existing at the depth of 100 m with resistivity
values of 50-500 Ω∙m; phase values are over 35 mrad. This anomalous zone is
related to ore bodies 4-7. At the pile 8, the borehole LK.5 has cut ore at the
depth of from 13 to 98 m; at the pile 3, the borehole LK.6 has cut ore at the
depth of from 23 to 130.5 m; at the pile -3, the borehole LK.7 has cut ore at
the depth of from 10.8 to 53.5 m.
- The second anomalous zone is controlled by
from pile 15 to pile 37. This is possible to observe its existence to the depth
of about 100 m with resistivity values of 50-500 Ω∙m; phase values are over 35
mrad. This anomalous zone is related to ore bodies 7 and 8.
The results of study on the lines T1, T4 and T6
indicate that the IP frequency domain anomalous characteristics on those lines
are similar.
phase value at the frequency of 0.5 Hz
Figure 5c. Results
of quantitative analysis of sounding data on the line T3;
phase value at the frequency of 0.125 Hz
Table 1. Stratigraphic column of the borehole LK.12, line T3
V. CONCLUSIONS
1.
The results of study on application of frequency-domain IP on lines in Tà Phời area
show that this method is of high effectiveness. Boreholes into anomalies
drilled on all lines have cut the ore. The ore-cut depth relatively corresponds
with distribution depth in the space. Zones and bodies of Cu ores are
characterized by following IP frequency domain anomalous characteristics:
-
IP spectral characteristics of Cu ore-bearing rocks are different from that of
graphite-bearing schist and non-Cu rocks. Cu ores hold low resistivity, but
high phase value, high polarization frequency effect and high metal factor; the
phase value and polarization frequency effect are inversely proportional to
frequency;
-
Cu ores reflected on anomalous zones hold high polarization value, high phase
value, normal resistivity value, normal metal factor and the difference between
phase values at frequencies 0.125 and 1 Hz yields positive number.
-
Graphite-bearing schist with sulphide mineralization reflected on anomalies
yields small resistivity value, high polarization value, high phase value, high
metal factor; the difference between phase values at frequencies 0.125 and 1 Hz
yields negative number.
2. The results of study on the application of frequency-domain
IP for prospecting of Cu ores in Tà Phời area allow to control the surface
distribution area as well as the distribution of ore bodies at the depth.
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