SOME RESULTS OF THE STUDY ON THE CHARACTERISTICS OF IP FREQUENCY DOMAIN ANOMALOUS FIELD IN SOME ZONES AND BODIES OF COPPER ORE AT THE TÀ PHỜI AREA, LÀO CAI  PROVINCE

TĂNG ĐÌNH NAM, NGUYỄN TIẾN PHONG

            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 Nam. This method has outstanding advantages in comparison with that of time-domain IP. This is demonstrated that beside resistivity parameter and polarizability, the frequency-domain IP still provides information on phase values and metal factor as well as specifications of polarized spectrum in different frequency bands. Owing to these advantages, in interpretation of geological data, geophysical data will have higher confidence, in some comfortable cases these advantages can help to distinguish the nature of objects causing anomalies and anomalies reflecting different objects causing polarization as well.


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 Nam. This method has outstanding advantages in comparison with that of time-domain IP. This is demonstrated that beside resistivity parameter and polarizability, frequency-domain IP still provides information on phase values and metal factor, as well as specifications of polarized spectrum in different frequency bands. Owing to these advantages, in the interpretation of geological data, geophysical data become higher confident, and in some comfortable cases these advantages can help to distinguish the nature of objects causing anomalies and anomalies reflecting different objects causing polarization as well. The initial study results from pilot application of frequency-domain IP to Tà Phời Cu ore  area and Ba Xứ (Thái Nguyên ) and Nà Tùm  (Bac Kan) Pb-Zn ore areas allow the authors to set up  main characteristics of frequency-domain IP anomalies for the Pb-Zn and Cu ore zones and bodies at 3 pilot areas, as well as to do design procedures for measurement and analysis of frequency-domain IP data, serving for evaluation in exploration of polymetallic sulphide ores (lead, zinc, copper) in Việt Nam.

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 Lào Cai Province.

II. BRIEF OUTLINES ON CHARACTERISTICS OF GEOLOGY AND MINERAL RESOURCES IN TÀ PHỜI AREA, LÀO CAI PROVINCE

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, Lào Cai Province".

- 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. Ore bodies are in the form of veins, branched veins and lenses. The boundary between ores and surrounding rocks is not clear. The ores are mostly distributed at the top of the uplifted structures which is the crucial zone, creating favourable conditions for ore-bearing solutions to uprising and accumulating.

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 Germany for measuring their density parameter, magnetic induction index and frequency domain IP spectrum. Frequency domain IP spectral characteristics of some main rocks in the Tà Phời area are as follows:

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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