PETROPHYSICAL CHARACTERIZATION OF ORES AND ROCKS RELATED TO Cu, Au, AND Pb-Zn DEPOSITS
IN VIỆT NAM

TĂNG ĐÌNH NAM

    Institute of Geosciences and Mineral Resources (VIGMR), Thanh Xuân, Hà Nội.

Abstract: The results of study on petrophysical parameters of ores and rocks taken from Cu, Au and Pb-Zn deposits show that: different ores and rocks have different petrophysical characteristics. Non-ore or without graphite rocks have small phase value and the phase value is directly proportional with frequency; the graphite-bearing rocks have high phase value, which is inversely proportional against frequency in a range of from 11.7 to 750 Hz and is directly proportional with frequency in a range of from  0.014 to 1.46 Hz; sulphide-gold quartz has high phase value, which is inversely proportional against frequency in a range of from  0.73 to 93.75 Hz and is directly proportional with frequency in a range of from  0.0057 to 0.09 Hz. Cu ores have high phase value and perhaps, the phase value is inversely proportional against frequency. Pb-Zn oxidic ores have small phase value, which is directly proportional with frequency; Pb-Zn sulphide ores have high phase value and depending on whether high Zn content or high Pb content, they have different IP spectral characteristics. Pb-Zn sulphide ores bearing higher Zn content have a higher resistivity value, high phase value, which is inversely proportional against frequency in a range of from 0.0229 to 46.9 Hz; Pb-Zn sulphide ores of high phase value with predominant Pb ore have small resistivity value, high phase value, which is inversely proportional against frequency in a range of from 0.366 to 750 Hz and directly proportional to frequency in a range of from 0.0057 to 0.118 Hz.

I. INTRODUCTION

The determination of petrophysical parameters of ores and host rocks is very important to determine the combination of geophysical methods for mineral prospecting, as well as to choose parameters for modelling the ore deposits and their classification.

We have measured the density, magnetic susceptibility and IP spectral parameters of ore and host rock samples from different ore deposits and occurrences in Việt Nam, such as: Tà Phời gold-copper ore occurrence and Sin Quyền copper porphyry mine, Lào Cai Province; Bản Phúc nickel-copper mine, Sơn La Province; Nùng Lào, Khuôn Tòng and Nông Tiến lead-zinc mines, Tuyên Quang Province; Chợ Đồn lead-zinc mine, Bắc Cạn Province; Ba Vì  sulphide copper mine, Hà Tây Province; and Xà Khía sulphide-quartz gold mine, Quảng Bình  Province.

The grain density has been measured by the instrument Quantachrome Ultrapycnometer 1000 Version1.62. 

The spectral IP parameters have been measured by the equipment SIP-Fuchs Version 02-06-200 at the Institute of Geophysics of Clausthal Technical University in the FR of Germany. The instrument is used in the frequency range of from 1.4 mHz up to 12 kHz. It consists of two remote units recording the current and voltage signal. The measured data are transferred to the base unit where the apparent resistivity and the phase shift are determined. The SIP-Fuchs is connected to a computer which enables a real-time visualization of the results. The sample holder consists of two water tanks with platinum current electrodes. Tap water is filled in the tanks with a resistivity of approximately 100 W×m. Cylindrical samples with a diameter of 20 mm and a minimum length of 35 mm are fixed in a tube placed between the tanks. Rings of platinum or chlorided silver wire surrounding the samples are used as voltage electrodes.

II. TÀ PHỜI GOLD-COPPER ORE OCCURRENCE

The Tà Phời gold-copper occurrence is situated in Tà Phời village, 10 km NW of Cam Đường Town, Lào Cai Province. The prospect is located 10 km NW of Cam Đường Town.

1. Geological and mineral resource features

In the study area, sulphide mineralization and Cu, Au ores often occur in altered zones of fine- to medium-grained diorite, quartz-bearing diorite, leucocratic diorite, biotite plagiogneiss, tremolite-plagioclase metamorphosed rocks, biotite feldspar quartz schist, graphite-bearing biotite quartz schist. These rocks were hydrothermally altered, actinolitized, quartzized, chloritized, migmatitized, sericitized, epidotized, calcitized, tremolitized, etc...

The analytic results of metallogenic, grinding and panning studies show that the ore forming association in Tà Phời area consists of chalcopyrite, cubanite, pyrite, molybdenite, magnetite, gold and graphite.

The ore structures consist of disseminating, nest; small vein, thin band, and fracture filling vein ones. The secondary minerals include covelline, chalcosine, goethite, hydrogoethite, and limonite. Their structures are edge, network and pore ones, and pseudomorphic texture.

- Copper ore in altered diorite: includes samples No TP4c, TP4d, TP10a and TP10b with density values varying from 2.76 to 2.79 g/cm3. Magnetic susceptibility varies from 87 to 209 ×10-6 CGS. Occurred small resistivity values at frequency 3 Hz vary from 27 to 95 Ω∙m    .  Phase values at frequency 3 Hz vary from 54 to 117 mrad.

- Sulphide-Au ore in leucocratic diorite: includes samples No TP6a, TP6b, TP10a and TP10b with density values varying from 2.71 to 2.73 g/cm3. Magnetic susceptibility varies from 396 to 630 ×10-6 CGS. Occurred resistivity values at frequency 3 Hz vary from 1306 to 1514 W×m. Phase values at frequency 3 Hz vary from 76 to 85 mrad.

- Sulphide-Au ore in altered biotite dioritogneiss: includes samples No TP2a and TP2b with density values varying from 2.70 to 2.71 g/cm3. Magnetic susceptibility varies from 33 to 47 ×10-6 CGS. Occurred resistivity values at frequency 3 Hz vary from 472 to 560 W×m.  Phase values at frequency 3 Hz vary from 44 to 68 mrad.

- Sulphide-Au ore in altered quartz diorite: includes sample No TP3a with density value of 2.68 g/cm3. Magnetic susceptibility is 112 ×10-6 CGS. Resistivity value at frequency 3 Hz is 427 W×m. Phase value at frequency 3 Hz is 50 mrad. The rocks containing sulphide-Au have phase values inversely proportional against frequency in the range of from 0.73 to 93.75 Hz and directly proportional to frequency in the range of from 0.005 to 0.09 Hz. The IP characterization allows distinguishing the Cu ores, Au-bearing sulphide ores and graphite. This is the main advantage of the frequency-domain IP over the time-domain IP. The use of time-domain IP for prospecting Cu ores in the areas of protometamorphic rocks containing graphite is impossible to distinguish anomalies whether caused by Cu ore or graphite. In the other hand, it is easy to distinguish the anomalies caused by Cu ore, by sulphide or by graphite as well impossible.

According to the available data, the ore bodies usually are in the form of short prism of complicated to very complicated sharpness, while due to the surface data, the ore bodies are of some to 20 m in thickness, in some cases reaching up to 43 m, step dipping with dipping angle of 70-80o. The content of Cu varies from 0.3 to 2.64 % and the content of Au varies from 0.4 to 4 g/ton.

2. Petrophysical characterization of ores and rocks in Tà Phời area.

We have studied the petrophysical characterization of 25 ore and rock samples taken from Tà Phời area, which concretely include:

- Graphite-bearing quartz-biotite schist: includes samples No TP1a, TP1b, TP1c with density values varying from 2.66 to 2.68 g/cm3. Magnetic susceptibility varies from 134 to 145 ×10-6 CGS. Small resistivity values occur at frequency of 3 Hz, varying from 68 to 108 W×m. High phase values occur at frequency 3 Hz varying from 155 to 225 mrad.

- Quartz-muscovite schist: includes sample No TP8a, TP8b with density values varying from 2.78 to 2.79 g/cm3. Magnetic susceptibility varies from 145 to 244 ×10-6 CGS. Occurred resistivity values at frequency 3 Hz vary from 227 to 354 W×m.  Phase values at frequency 3 Hz vary from 27 to 32 mrad.

- Graphite-bearing biotite plagiogneiss: includes samples No TP9a and TP9b with density values varying from 2.71 to 2.72 g/cm3. Magnetic susceptibility varies from 120 to 122 ×10-6 CGS. Small resistivity values occurring at frequency of 3 Hz vary from 106 to 111 Wm. Phase values occurring at frequency 3 Hz vary from 45 to 61 mrad.

- Copper ore-bearing biotite plagiogneiss: includes samples No TP7a, TP7b with density values varying from 2.74 to 2.76 g/cm3. Magnetic susceptibility varies from 41 to 54 ×10-6  CGS. Resistivity values occurring at frequency of 3 Hz vary from 113 to 178 W×m.  Phase values occurring at frequency 3 Hz vary from 78 to 79 mrad.

- Plagioclase-tremolite metasomatite: includes samples No TP5c and TP5d with density values varying from 2.89 to 2.93 g/cm3. Magnetic susceptibility varies from 296 to 367 ×10-6 CGS. High resistivity values occurring at frequency of 3 Hz vary from 971 to 1263 W×m.  Small phase values occurring at frequency 3 Hz vary from 15 to 20 mrad.

- Copper ore-bearing plagioclase-tremolite metasomatite: includes samples No TP5a and TP5b with density values varying from 2.93 to 2.97 g/cm3. Magnetic susceptibility is 439 ×10-6 CGS. Small resistivity values occurring at frequency of 3 Hz vary from 40 to 174 W×m.  High phase values occurring at frequency 3 Hz vary from 77 to 149 mrad.

- Diorite: includes sample No TP4b with density value of 2.67 g/cm3. Magnetic susceptibility is 94 ×10-6 CGS. Resistivity value is 2307 W×m at frequency 3 Hz.  Phase value is 7 mrad at frequency 3 Hz.

 - Copper mineralization-bearing altered diorite: includes samples No TP4a with density value of 2.68 g/cm3. Magnetic susceptibility is 146 ×10-6 CGS. Resistivity value is 742 W×m at frequency 3 Hz. Phase value is 35 mrad at frequency 3 Hz.

III. SIN QUYỀN COPPER MINE

The Sin Quyền Copper mine is located in the NW of the Phan Si Pan structural zone, Bát Xát District, about 25 km NW of Lào Cai Town. It was discovered in 1961, then was detailed prospected and explored during the period from 1962 to 1982 years.

1. Geological and mineral resource features

The mine area has a complicated geological structure. The ore-bearing metamorphic rocks were attributed to the Sin Quyền formation with 2 parts:

- Lower part: two-mica schist, garnet-bearing mica-quartz schist, migmatitized disthene-two mica schist, graphite-bearing rock and paleo-pegmatite bodies.

- Upper part: quartz-mica schist, migmatitized plagiogneiss and tourmaline plagio-granitogneiss lenses, metasomatite, amphibole quartzite and marble. Most of the ore bodies are related to this assemblage. The granitoid of Po Sen Complex (γ ps) are composed of granite, migmatite, granitogneiss, granodiorite. Besides, there are also small blocks of granite, granodiorite of the Điện Biên Complex (γd db).

In the mine area, there are 3 main ore bands stretching nearly 40 km in the NW-SE trend. The western Lũng Thàng - Pin Ngân Chẹn band is copper - rare earths ore band. The middle Sin Quyền - Nặm Mít band is the main copper - rare earth ore band, including Sin Quyền, Cốc Mỵ, Suối Than mines. Located in the northeast, the Thung Sáng - Lũng Pô band is composed of small-sized copper veins that penetrate up into effusive rocks. The mineral composition of the Sin Quyền mine is abundant. The main ore-forming minerals (%) are: pyrrhotine 8-9, chalcopyrite 6-7, magnetite 25-30, pyrite 6-7 ... and others, such as sphalerite, galenite, rare earths, etc.

The results of measuring density, magnetic susceptibility and IP spectral characterization of Cu ores in Sin Quyền mine show that the content of Cu and Fe in the ores are very high, that make the density, magnetic suscepbility and phase values to be very high, while IP values to be small. The phase values are inversely proportional to frequency in a range of from 0.36 to 750 Hz.

The application of magnetic and frequency-domain IP methods to the investigation of Cu ore type in Sin Quyền mine is most effective.

- Altered quartz diorite includes samples No TP3b with density value of 2.66 g/cm3. Magnetic susceptibility is 111 ×10-6 CGS. Resistivity value at frequency 3 Hz is 311 W×m.  Phase value at frequency 3 Hz is 25 mrad.

The measurement results of density, magnetic susceptibility and IP spectral characterization of ores and rocks in the Tà Phời area show that the plagioclase-tremolite metasomatic rock has the highest density and magnetic susceptibility in comparison with that of other rocks. Graphite-bearing or Cu-bearing rocks have a small value of resistivity and high value of phase. The non-ore rocks or graphite have phase values directly proportional to frequency. Cu-bearing rocks have phase values inversely proportional against frequency.  Graphite-bearing rocks have phase values inversely proportional against frequency in the range of from 11.7 to 750 Hz. In the Sin Quyền mine, there are 17 ore bodies in the forms of vein, chain, lense, branched vein, lying in 4 cataclastic faulted zones in metamorphic rocks of the upper part of Sin Quyền Formation. They formed small-sized disseminated ore veins - zones with the length of 33-2875 m, and the thickness of 2,1-14 m. Investigation has been made to the depth of 570 m. The grade of Cu in the ore is from 0.1 to 11.58 %, in average: 1.03 %. The rare earth grade is 0.03 - 9.71 %, in average: 0.9 %.

2. Petrophysical characterization

We have studied the petrophysical characterization of 4 Cu ore samples from Sin Quyền mine including samples No Cusq1, Cusq2, Cusq3 and Cusq4 with density values varying from 3.64 to 4.2 g/cm3. Magnetic susceptibility is bigger than 200,000 ×10-6 CGS. Occurred resistivity values at frequency 3 Hz are small, varying from 1.5 to 22.6 W×m.  Phase values at frequency 3 Hz are very high, varying from 172 to 601 mrad.

IV. BẢN PHÚC COPPER-NICKEL MINE

The Bản Phúc Cu-Ni mine is situated at Mường Khoa village, Bắc Yên District, Sơn La Province. This is the biggest Cu-Ni ore mine in Việt Nam.

1. Geological and mineral resources features

The Bản Phúc Cu-Ni ore mine lies in a structure of thermal dome. At the core of the dome there are exposed the rocks of Tạ Khoa Formation (D1-2 tk) with the section composed of 3 parts: 1. Quartzitic sandstone and sericite schist; 2. Clay shale intercalated with mafic effusives, sericite schist complicatedly metamorphosed upward; 3. Thin bedded cherty shale, clay shale, calcaro-cherty shale. Conformably overlying rocks are cherty shale and calcaro-cherty shale of the Bản Cải Formation (D3-C1 bc). In the Tạ Khoa anticline, there is the Ban Xang, Bản Khoa, Bản Phúc ultrabasic intrusive massifs belonging to the Late Paleozoic - Early Mesozoic Complex, with mainly dunite, serpentinite closely related to Cu-Ni mineralization.

The mine includes two main ore bodies. The first one is in the form of vertical ore vein or follows the bed surface, lying at about 20-100 m outside of the ultrabasic intrusive massif. The ore structure is massive. The length of ore body is 640 m, the depth is 450 m, the thickness of the massive vein is 0.15-6.12 m, in average: 1.26 m and the average grade of Cu is 1.63 % and Ni – 6.42 %. The disseminated ore in the both sides are 0.58-19 m in length, in average: 7.15 m and average content of Cu is 0.75 %, Ni – 0.49 %. The main ore minerals of the major vein are pyrrhotine, chalcopyrite, pentlandite, magnetite, and the main minerals of the disseminated ore in the both sides are mainly chalcopyrite, with minor pyrite, sphalerite, galenite, native nickel ... The second ore body - infiltration ore, lying at the bottom of the ultramafic intrusive massif with irregular ore distribution, thick in the centre and thinner at two sides that is like the fan form. The ore has motley structure (dropping) with diameter of 1 cm, with disseminated small ore veins in the hornfels rock bed lying under the bottom of the intrusive massif. The average grade of Ni is 0.56%. The content of Cu and Co is very low. The ore minerals consist of pyrrhotine, pentlandite, chalcopyrite and magnetite.

2. Petrophysical characterization of Cu-Ni ores in the Bản Phúc mine

We have studied the petrophysical characterization of 4 Cu-Ni ore samples from the Bản Phúc mine, concretely as follows:

- Cu-Ni ore in ultramafic rocks: including samples No Cubp1 and Cubp2 with density values varying from 3.56 to 4.05 g/cm3. High magnetic susceptibility varies from 1131 to 1344 ×10-6 CGS. Occurred resistivity values at frequency 3 Hz varies from 450 to 1444 W×m.  Phase values at frequency 3 Hz varies from 83 to 139 mrad.

- Cu-Ni sulphide ore: including samples No Cubp3, Cubp4 with high density values varying from 4.23 to 4.49 g/cm3. Magnetic susceptibility varies from 402 to 536 ×10-6 CGS. Occurred resistivity values at frequency 3 Hz varies from 12 to 998 W×m.  Phase values at frequency 3 Hz varies from 71 to 234 mrad.

The results of measuring density, magnetic susceptibility and IP spectral characterization of Cu-Ni ores of the Bản Phúc mine show that the content of Cu in the ore is high, so the density values are very high, phase values are high. The phase values are inversely proportional to frequency in a range of from 0.029 to 49.6 Hz.

V. LŨNG CỦA COPPER OCCURRENCE

The Lũng Của Cu occurrence is located in Tan Linh commune, Ba Vì district, Hà Tây province.

1. Geological and mineral resources features

The surrounding rocks consist of basalt of spheric pillow, pisolitic and massive forms, andesit to basalt, andesite, porphyritic andesite in the lower part and agglomerate tuff, dacite, rhyolite, trachyte in the upper part of the Viên Nam Formation (P2 vn).

The ore body is vertically steeping and can be clearly observed with a length of 35 m, with the thickness of 2-3 m. Surrounding alteration is mainly argilitization, quartzation, dolomitization. Ore minerals consist of pyrite, chalcopyrite, bornite and less galenite, sphalerite, tetrahedrite. The vein mineral is quartz. Cu content varies from 1 to 24.9 %.

2. Petrophysical characterization of Cu ores

We have studied the petrophysical characterization of 2 Cu ore samples from Lũng Của occurrence, namely Cubv1 and Cubv2, with density values varying from 2.73 to 3.37 g/cm3. Magnetic susceptibility varies from 260 to 263 ×10-6 CGS. Occurred resistivity values at frequency 3 Hz are small, varying from 10 to 27 W×m.  Phase values at frequency 3 Hz are high, varying from 176 to 287 mrad.

The results of measuring density, magnetic susceptibility and IP spectral characterization of Cu ores from Lũng Của area show that Cu ores here have small resistivity values, but high phase values. The phase values are inversely proportional to frequency.

VI. XÀ KHÍA GOLD MINE

Situated in 3 km N-NE of Xà Khía village, Lệ Thủy District, Quảng Bình  Province, the Xà Khía gold mine is being a mining object of the Xà Khía Gold Mining and Processing Enterprise, under the Long Đại Industry-Forestry Company since 1996. From Oct. 1998 to Jun. 2002 the Vietnamese Resource Corporation LTD (Australia) has been carried out gold exploration there over an area of 50 km2.

1. Geological and mineral resources characteristics

In the regional tectono-structural plan of the study area, there is a riftogenous graben filled up with Triassic acidic volcanic material, overlying upon a lower Paleozoic basement Long Đại (D3-S1 ld), Đại Giang (S2 dg) and La Khê (C1 lk) Formations. The Xà Khía - Mu Me rift zone stretches NW-SE with a length estimated more than 10 km and average width of 100-500 m. Different types of Au-Ag, Pb-Zn mineralization are closely related to the formation and consequent activities of this superimposed rift zone.

There are three types of mineralization processes, such as:

- Sedimentary-fossilized copper (SEDEX type). This type is known in Lower Carboniferous carbonate sediments of the La Khê Formation and is represented by non-Au, Ag disseminated pyrite (3-5 %), which has no industrial value.

- Hydrothermal veins are in the form of thinny scattered veins of quartz-hematite, quartz-pyrite-carbonate, quartz-ankerite-pyrite, quartz-galene-chalcopyrite with anomalous industrial gold-silver content.

- Supergene enrichment in faulting areas: Representative for this type is Xà Khía gold mine. It is a small-sized mine with ore body in the form of kennel, 70 m long, 10-15 m wide, 30-40 m deep; Au ore grade: 2-7 g/t, maximum: 10-12g/t.

2. Petrophysical characterization of Au ores

We have studied the petrophysical characterization of 6 Au ore samples from Xà Khía mine as follows:

- Quartz-sulphide gold ore: including Auxk1, Auxk2, Auxk3, Auxk4 and Auxk5 with density values varying from 2.83 to 2.93 g/cm3. Magnetic susceptibility varies from 32 to 104×10-6 CGS. Resistivity values at frequency 3 Hz varies from 470 to 1594 W×m. Phase values at frequency 3 Hz varies from 97 to 152 mrad.

- Quartz-gold ore: including Auxk6 with density value of 2.64 g/cm3. Magnetic susceptibility is 96 ×10-6 CGS. Resistivity value at frequency 3 Hz is 3041 W×m.  Phase value at frequency 3 Hz is 12 mrad.

The results of measuring density, magnetic susceptibility and IP spectral characterization of Au ores from Xà Khía area show that sulphide-Au ores here have high phase values. The phases values are inversely proportional to frequency in a range of from 0.0057 to 0.09 Hz. Quartz-Au ore have high resistivity values, small phase values. The phase values are directly proportional to frequency.

VII. THANH CỐC - YÊN SƠN LEAD-ZINC MINE

The Thanh Cốc area is situated at the upper course of Đáy River, 40 km NE of Tuyên Quang Town, 30 km NW of Son Duong district centre.

1. Geological and mineral resources characteristics

The Pb-Zn ore veins in Thanh Cốc area are interbedded and intercalated with schist beds and altered rocks of Đạo Viện Lower Cambrian Formation (ε1 dv).  The ore is of hydrothermal origin, existing in the form of sulphide vein, chain, nest, prism, stock vein ore bodies of 50 to 500 m in length, 0.5 to 6 m in thickness.

Pb-Zn ores in Nùng Lào area exist in the form of complex of chain veins, elongated prism ore bodies, more than 500 m long, 0.5-6 m thick, that are distributed along faults and belt of quartz-sericite schist, calc-siliceous schist, sericite schist, quartz schist intercalated with thin interbeds of altered cataclastic limestone contacted with marbleized limestone. The ores consist mainly of Pb-Zn sulphides. Ore minerals are sphalerite and galenite. The content of Pb-Zn ores is from 5 to 53.6 %, mostly over 25 %.

The Khuôn Tòng ore bodies lie in a cataclastic zone of marbleized limestone of 2 to 4 m in thickness. Main ore minerals are sphalerite and disseminated galenite. Ore content is from 10 to 40 %, in average 20 %.

2. Petrophysical characterization of Pb-Zn ores in Thanh Cốc area

We have studied the petrophysical characterization of 2 Pb-Zn ore samples from Khuôn Tòng area namely Znskt1 and Znskt2 with density values varying from 4.02 to 4.07 g/cm3. Magnetic susceptibility varies from 131 to 169 ×10-6 CGS. Resistivity values at frequency 3 Hz vary from 738 to 1988 W×m.  Phase values at frequency 3 Hz vary from 133 to 188 mrad.

Pb-Zn ore samples from Nùng Lào area (NL1, NL2) have density values varying from 4.03 to 4.53 g/cm3. Magnetic susceptibility varies from 97 to 166 ×10-6 CGS. Resistivity values at frequency 3 Hz vary from 241 to 446 W×m.  Phase values at frequency 3 Hz vary from 97 to 143 mrad.

The results of measuring density, magnetic susceptibility and IP spectral characterization of Pb-Zn ores from Khuôn Tòng - Nùng Lào area show that Pb-Zn ores here have high densitty and high phase values. The phase values are inversely proportional to frequency in a range of from 0.0229 to 46.9 Hz. In Pb-Zn ore of Khuôn Tòng area, the Zn ore is predominant, and so its resistivity value is higher than that of Nùng Lào one.

VIII. NÀ BỐP LEAD-ZINC ORE MINE

The Nà Bốp Pb-Zn ore mine is situated in Bằng Lũng Commune, Chợ Đồn District, Bắc Cạn Province, 3 km north of Bằng Lũng village.

1. Geological and mineral resources characteristics

Involving in the structure of Nà Bốp Pb-Zn mine are mostly terrigenous sediments intercalated with carbonate rocks of lower part of the Cốc Xô Formation (D1 cx). The ores are mostly distributed in limestone. Overlying and underlying beds are quartz schists.

The origin of the ores is hydrothermal of medium temperature. Ore structure is vein, massive nest, thick dissemination, dissemination belt. Mineral composition of Pb-Zn sulphide ore is mostly composed of sphalerite, galenite, less pyrite, pyrrhotine.

In Nà Bốp area, there are 9 ore bodies with the length of some m to some hundred m, their thickness of from 1 to 20 m. Among the ore bodies, the No 1 is biggest in size, striking N-S, dipping to the east with the dipping angle varying from 25 to 55o. The thickness of this ore body is relatively stable, more than 10 m, stretching over 700 m. The content of Pb-Zn is from 5.5 to 30.58 %, in average: 10-12 %. The content of accompanying elements is low. 

2. Petrophysical characterization

We have studied the petrophysical characterization of 3 Pb-Zn ore samples from Nà Bốp area including Pbcd1, Pbcd2, Pbcd3 with high density values, varying from 6.74 to 7.02 g/cm3. Magnetic susceptibility varies from 3 to 72 ×10-6 CGS. Resistivity values at frequency 3 Hz are very small, varying from 0.1 to 4.6 W×m.  Phase values at frequency 3 Hz are very high, varying from 525 to 1122 mrad.

 The results of measuring density, magnetic susceptibility and IP spectral characterization of Pb-Zn ores from Nà Bốp - Chợ Đồn area show that rich Pb-Zn ores here have high density and very low magnetic susceptibility, very low resistivity values and very high phase values. The phase values are inversely proportional to frequency in a range of from 0.366 to 750 Hz and directly proportional to frequency in a range of from 0.0057 to 0.118 Hz.

IX. NÔNG TIẾN LEAD- ZINC ORE MINE

The Nông Tiến Pb-Zn ore mine is situated at Tràng Đá and Nông Tiến villages, about 2 km NE of Tuyên Quang Town. The mine is situated at an altitude of 338 m above sea level with an area of more than 1 km2.

1. Geological characteristics

According to data of the 1/200,000 Tuyên Quang Map sheet, the rocks of Nông Tiến mine belong to the upper part of the Formation (D1 dt2) which was divided into 3 members.

- Member 1: Thin-bedded calcareous clay with some lenses of sandstone and quartz-sericite schist grading upwards into grey-green, ash-grey, normally foliated limestone. Pb-Zn ocidic ore is in the form of vein accompanying with baryte.

- Member 2: Mostly terrigenous sediments with clay-sericite schist and lenses of thin-bedded limestone and some veins, nest of baryte.

- Member 3: Mostly carbonate sediments interbedded with sericite-chlorite-calcite schist, calcitized dark-grey calcareous schist and Pb-Zn oxidic, sulfide ores in the form of vein and pseudobeds which were and are being exploited.

The Nông Tiến Pb-Zn mine includes mostly oxidic Zn ores distributed on the surface in three zones of more or less 100 m in width, 300-500 m in length. These zones are separated by an interval of 150-200 m with 2 to 4 ore bodies (in the form of vein, lense, nest) distributed along tectonic faults of the same direction with that of the host rocks, that is why they are in the form of stratiform ore bodies. The ore bodies are mainly distributed in carbonate rocks, some veins are developed nearby the boundary with terrigenous rocks interbedded in the zone of baryte sandstone.

The ores are distributed at the depth of more than 100 m. The sulphide content increases downward, and baryte ore strongly decreases. There are known 2 ore zones: eastern and central ones with some veins of baryte.

2. Petrophysical characterization

We have studied the petrophysical characterization of 2 Pb-Zn oxidic ore samples from Nông Tiến area, namely Zntd1 and Zntd2, with density values varying from 3.19 to 4.26 g/cm3. Magnetic susceptibility varies from 409 to 647 ×10-6 CGS. Resistivity values at frequency 3 Hz vary from 806 to 2188 W×m.  Phase values at frequency 3 Hz are very small, varying from 4 to 6 mrad.

Pb-Zn sulphide ore (Znstd3) has a density value of 7.52 g/cub.cm, very small magnetism, very small resistivity value at frequency of 3 Hz (5 W×m), very high phase value at frequency 3 Hz (558 mrad).

Characterization of Pb-Zn ores from Nông Tiến area, Tuyên Quang Province shows that Zn oxidic ore here have high resistivity values and small phase value. The phase values are directly proportional to frequency. Rich Pb-Zn sulphide ore has a very great density, very small magnetic susceptibility, very small resistivity value and very high phase value. The phase values are inversely proportional against frequency in a range of from 0.366 to 750 Hz and directly proportional to frequency in a range of from 0.0057 to 0.118 Hz.

X. CONCLUSIONS

The above presented results of measuring petrophysical parameters of ores and rocks taken from different areas show that different ores and rocks have different petrophysical characteristics. Non-ore or without graphite rocks have small phase value and the phase value is directly proportional with frequency.

The graphite-bearing rocks have high phase value; the phase value is inversely proportional against frequency in a range of from 11.7 to 750 Hz and is directly proportional with frequency in a range of from 0.014 to 1.46 Hz.

Quartz-sulphide gold ores have high phase value, the phase value is inversely proportional against frequency in a range of from 0.73 to 93.75 Hz and is directly proportional with frequency in a range of from 0.0057 to 0.09 Hz. Cu ores have high phase value and perhaps, the phase value is inversely proportional against frequency. Pb-Zn oxidic ore have small phase value and the phase value is inversely proportional against frequency.

The Pb-Zn sulphide ores have high phase value and depending on whether high Zn content or high Pb content, they have different IP spectral characteristics. The Pb-Zn sulphide ores with higher Zn content have a higher resistivity value, high phase value and the phase value is inversely proportional against frequency in a range of from 0.0229 to 46.9 Hz.

The Pb-Zn sulphide ores of high phase value with Pb ore predominant have small resistivity value, high phase value and the phase values are inversely proportional against frequency in a range of from 0.366 to 750 Hz and directly proportional to frequency in a range of from 0.0057 to 0.118 Hz.

REFERENCES

1. Hallof Ph.G., 1974. The IP phase measurement and inductive coupling. Geophysics, 39/5.

2. Hallof Ph.G., 1974. The IP phase measurement and inductive coupling. Geophysics, 39/5.

3. Pelton W.H., W.R. Sill, B.D. Smith, 1983. Interpretation of complex resistivity and dielectric data. Geophysics, 29/4.

4. Pelton W.H., H. Ward, P.G. Hallof, W.R. Sill, P.H. Nelton, 1998. Mineral discrimination and removal of inductive coupling with multifrequency IP. Geophysics, 43/3 : 588-609.

5. Tang Dinh Nam et al, 2001. Scientific report on "Study on and establishment of complex of rational geophysical methods and setting up technological procedures for prospecting Pb-Zn ores in Vietnam" (in Vietnamese).

6. Tang Dinh Nam et al, 2003. Scientific report on "Pilot study on geophysical methods with new geophysical equipments, techniques and technologies which are able to find hidden burred ore bodies" (in Vietnamese).

7. Tang Dinh Nam et al, 2004. Spectral IP characteristics of rock and ore at the Tà Phời copper ore point in Cam Đường District, Lào Cai Province (in Vietnamese).

8. Tang Dinh Nam et al, 2005. Some results of study on application of spectral IP frequency domain sounding in the survey and prospecting of mineral resources in Vietnam. Proc. of the 4th Conf. on Sci. and Techn. Geophysics of Việt Nam (in Vietnamese).

9. Tang Dinh Nam, A. Weller, 2005. Some results of study on petro-geophysical characteristics of rocks and ores at some deposits of copper, gold and lead-zinc in Vietnam. Proc. of the 4th Conf. on Sci. and Techn. Geophysics of Việt Nam.

10. Vanhala H. and Peltoniemi M., 1992. Spectral IP studies of Finnish ore prospects. Geophysics, 57/12.

11. Vanhala H., 1992. Spectral IP studies of Finnish ore prospects. Geophysics, 57 : 1545-1556.

12. Weller A., 1996. Measurement of Spectral Induced Polarization for Environmental purposes. Envir. Geology, 27 : 329-334.