relatively low figure. For ore zone this figure usually lies above 0.7 while shale-rich zones yield figures less than this. In one case interestingly (metreage 58-69) a negative correlation coefficient was obtained. The reason of this significant difference in the shale zones seems to be due to irregular enrichment of such zones causing considerable point to point variation.
This study provides a comparison between a bore hole sample (with assumed 100% core recovery) and a bulk sample collected from the same place. The volume of sample collected from an NX size bore hole of lcm length is 38.5 ern" while that from the channel of the dimensions mentioned above is 40 em", In terms of sampling, therefore, a channel could be taken as similar to a NX size bore hole.
Acknowledgement: The author is grateful to the General Manager, Bailadila Iron Ore Project, Deposit No. 5 for having kindly accorded permission to publish this paper. He is also thankful to his colleagues in the geological section of the project, who have collected the basic data for this study.
Reference
HAZEN SCOTT, W. JR., (1967) Some statistical techniques for analyzing mine and mineral deposit sample and assay data. Bull. 621, Bureau of Mines, U.S. Deptt, of the Interior.
(Jour. Geol. Soc. India, 17, 1976. 551-557)
Reconnaissance geochemical exploration for Copper
in the central part of the
Chitradurga
schist belt,
Karnataka, India
S. M. NAQVI, V. DIVAKARA RAO, K. SATYANARAYANA* AND S. M. HUSSAIN National Geophysical Research Institute, Hyderabad
Abstract
An area of about I IS Sq km was sampled on a 0.4 km spacing grid. The samples, collected from the bed rock, after removing the thin soil cover, were analysed for Cu, Zn, Pb and Co by the emission spectrograph using indium as internal standard. The Cu concentration map of the area shows nine zones of anomalous Cu content, the highest giving a value of 4000 ppm. Of these, four zones of highly anomalous Cu content (2000 ppm - 4000 ppm) are very promising and are recommended for detailed geochemical exploration.
Introduction
Sulphide mineralization in the central part of the Chitradurga schist belt has been traced over a distance of 35 km in a N-S direction from near Yarehalli to Chikkanna-halli. Although old workings for copper on the Belligudda hill near Ingaldhal were known since a long time, it was only recently the deposit has been taken up for commercial exploitation. (Radhakrishna, 1967, Radhakrishna & Sundararajan, 1972). The gravity and aeromagnetic data have indicated considerable thickness (8 km) of metavolcanics in the region (Naqvi, 1973, 1974). The geochemistry of the
basalts and metasediments showed considerable enrichment in Cu, as much as. 950 ppm copper (Naqvi and Hussain, 1972, 1973a, b). These studies encouraged taking up a reconnaissance geocbemical exploration for base metals in tbe area. Tbe preliminary results of this study are presented in this short communication.
Geological setting
The geology of tbe area has been described by various authors from time to time (Sampat Iyengar 1905; Naqvi 1973). The area largely consists of different types of metavolcanics and metasediments, which bave been folded and cross-folded into tight isoclinal folds. The metavolcanics are olivine/quartz tholeiites and andesites and are followed by sericitic phyllites with interbedded pyritiferous cherts (G. R. clays of Sampat Iyengar 1905). The available radiometric age data (Aswathanarayana 1964) of the mineralized zone and metavolcanics (Crawford, 1969) is suggestive of pre-tectonic, if not syngenetic, nature of tbe mineralization. This geological setting of the region is ideal for the occurrence of base metal sulphide volcanogenic deposits. From similar settings a large number of Cu-Zn deposits have been developed and being exploited (Sangster, 1972, Hutchinson, 1973, Roberts, 1975). The great thick-ness of the metavolcanic pile of olivine basalt-andesite in an eugeosynclinal environ-ment of early Precambrian (2.6 b.y.), an era, in which degassing of the protornantle and exhalation of the base metal sulphides is supposed to be rapid (Goodwin, 1974, Hutchinson, 1973), strongly suggests a more widespread base metal mineralisation, not necessarily restricted to Belligudda hill alone.
Sampling, analysis and the data
The soil geochemical exploration and its interpretation in the sbields is more complex than in the younger orogenic belts (Boyle et aI,1975). The soils over the Karnataka plateau are not necessarily residual in all cases. In most of the area under study, the soil cover is less than 1 m deep and below that a zone of weathered rock is encountered. The trace element geochemistry of the Ingaldhal copper deposit has indicated that the mineralization in the region is probably syngenetic massive volcano-genic type (Naqvi et al, 1976). This means that any geochemical exploration technique applied in the region should be oriented towards mapping primary dis-persions, resulting from the strata-bound lenses of chert-pyrrhotite-pyrite-chalcopyrite-spbalerite assemblages. Therefore, to avoid the possibility of encountering transport-ed soils in certain parts of the area and the complications resulting because of the rugged topography and furtber, in view of the large area to be covered, it was con-sidered that bed rock sampling would be more useful and informative than sampling of the soils. Moreover, the results of the geochemical exploration programme of Geological Survey of India as reported by Karunakaran (1970, p. 98) in the Ingaldhal and adjoining areas also suggested that bed rock sampling would probably yeild better results.
basaltic matrix. The analytical error in case of Cu was found to be 7%. The -details of the analytical procedure have been given by Naqvi and Hussain (1972) and Divakara Rao et al, (1976). The samples which yeilded Cu values of more than 500 ppm were repeated thrice in duplicates and the average was recorded at the sample location on the map. These values were used to prepare an isoconcentration map (Figure 1).
..
'I.·
"
COPPER CONCENTRATION IN THE BE" ROCK OF THE CENTRAL PART OF THE: CHITRADURGA SCHIST BELT
KARNATAKA -INDIA
I N 0 EX
---50- ~OPflm.COr-iTOUf'
_2~O_ 2~O,'jOO,7::'OppmCQNTOVR$
SAMPLE: I..OCATIONS r;=:=;:=:=J
)( HIGH vALUES
/ > AREAS TO BE r..t<EfltFOR
V DETA!LEDSruOIES
I.'
10'
"
Figure I. Bed rock Cu concentration map of the central part of the Chitradurga schist belt showing nine zones of anomalous Cu cotent, Zones A. C. D& E are recommended for detailed
geochemicalexploration. Location of the geochemical profiles given in figure 3 asaa', bb'. cc' and dd' is also shown.
The anomalies
The copper content of the 796 samples analysed varied between 10 to 4000 ppm; 35.2% of the samples contained~100 ppm and 29% population showed between 100- 200 ppm Cu. This showed that about 64% population contained~200 ppm
other rocks from the non-mineralized region showed that 70% of the samples contained::::200 ppm copper (Figure 2). Therefore 100 ppm can roughly be taken as regional background and 200 ppm as threshold of Cu concentration in the area surveyed. This method of background and threshold determination gives a rough idea of the anomalous parts of the surveyed region and the order of the anomalies mapped therein. The results of trend surface analysis and other statistical techniques applied to get the background and threshold from these data are being presented elsewhere. The background and threshold values in the region appear to be comparatively higher than some of the Cu deposits geochemically investigated (Boyle
et ai,1975). This probably reflects the disseminated nature of Cu enrichment in the metavolcanics and other rocks of the area.
ClJ ppm
200 600
I 1000I 1400I 1800I
32 2·0
>- A
o
Z 24 '·0
w
:::>
0 w
a:: 16 0
u,
0 0
~ 0 0
0
0 0v
8 ..
Figure 2. Histograms showing the frequency distri-bution of the Cucontent (A) in the entire area (796 samples) covered for reconnaissance geochemical exploration and (B) the rocks (89 samples) from non-mineralized area as estimated by Naqvi and Hussain (1972, 1973a, b). It shows that the background and threshold can be considered at 100 and 200 ppm. respectively.
anomaly is quite persistent and appears to be promising as it is near to the existing mine. In zone C (Fig. 1 and 3) three peak values of 2260, 1800 and 1600 ppm were recorded. These anomalies are also very persistent. Samples consist of pyritiferous {now limonite) cherts and metabasalts. In zone D concentrations of 4000, 2000 and 1250ppm were found along the strike ofpyritiferous chert bands. Similarly in zone E anomalies of the order of 1350, 1450 and 950 ppm were recorded on metabasalts and limonitic chert. Zones E, K and Jare found roughly parallel to the general strike of the region. This anomaly is found between Kallehadlu and Yarehalli villages over
2 . I c' ee' d' eel dd' Miles 1/2
1 . . 1_-'----''---.J-_~_'_'
i'
IiI bb/ I I I . I / I1
I I I I I.
i
I I i I i I \ / <.» I I I I,
f I roc't
I"
100 80 60 40 30d Q b
5000 4000 3000 2000 ddt 1000 800 600 400 300
Figure 3. Geochemical profiles from figure 1, aa' = profile over the Ingaldhal Copper Mine, bb' =profile over the southern part of the Belligudda hill, cc' =profile over the C zone, and dd' = profile over the zones D and E.
the second series of arcuate synclinal hills. The comparison of the geochemical profiles recorded over the existing mine (aa' in Fig. 3) and the unknown regions (bb', ee',dd' in Fig. 3) indicate the probability of encountering similar conditions at these locations.
type of deposits are found in various shapes and sizes in polyphase.deformational areas (Stanton, 1972). The anomalous zones over the central Chitradurga schist belt recorded over the metavolcanic, may be due to quite a few and separate lenses of Cu rich chert and pyrite-pyrrhotite. However, on the basis of such wide spaced sampling the exact positions of the mineralization cannot be demarcated and it is suggested that detailed geochemical exploration in the anomalous areas designated as A, C, D and E is undertaken as a prelude to drilling and exploratory mining.
Acknowledgement: The authors are grateful to Dr. Hari Narain, Director, National Geophysical Research Institute, Hyderabad, for his keen interest and permission for publication of the work presented here. We are indebted to Dr. B. P. Radhakrishna, who has guided and supported all our efforts in Precambrian geology and geochemistry. We express our gratitude to Mr. Dileep Kumar and Miss Aruna Sharma for their help in the analysis of the samples.
References
ASWATHANARAYANA,U., (1964) Age determination of rocks and geochronology of India: XXII
Int. Geol. Congo New Delhi, pp. 1-13.
BOYLE, R. W., BRADSHAW, P. VII'. D., CLEWS, D. R., FORTESCURE,J. A. C., GLEESON, C. F., HORNBROOK, E. H. W., SHILTS, W.• TAUCHID, M. and WALFE, (1975) Conceptual models in exploration geochemistry: The Canadian Shield. Jour. Geochern. Explor., v. 4,
pp. 109-213.
CRAWFORD, A. R., (1969) Reconnaissance Rb-Sr dating of the Precambrian rocks of the Southern Peninsular India: Jour. Geol. Soc. India, v. 10, pp. 117-167.
DIVAKARA RAo, V., NAQVI. S. M., SATYANARAYANA K. and HUSSAIN, S. M., (1976) Geo-chemical prospecting for copper in the Nuggihalli schist belt, Karnataka, India, Jour. Geol. Soc. India, v. 17, No.3, pp. 395-398.
GOODWIN, A. M., (1974) Precambrian belts, plumes, and shield development: Arner. Jour.Sci.,
v. 274, pp, 987-1028.
HUTCHINSON, R. W., (1973) Volcanogenic sulfide deposits and their metallogenic significance:
Econ. Geol., v. 68, pp. 1223-1246.
KARUNAKARAN, C., (1970) Evolution of the geochemical methods of prospecting for nonferrous metals in South India: Proceedings of the second seminar on Geochemical prospecting methods and techniques. Peradeniya, 10·20 September, 1970. Mineral Resourses Dept. Series, no. 38, pp. 95-102.
NAQVI, S.M., (1973) Geological structure and aeromagnetic and gravity anomalies in the central part of the Chitradurga schist belt Mysore, India: Geol. Soc. Amer, Bull., v. 84, pp,
1721-1732.
- - (1974) Review of the present status of the geochemical work on the Precambrian mafic rocks of India: Jour. Geol. Soc. India, v. IS, no. 4, pp. 380-389.
- - (1976) Physico-chemical conditions during Archaean as indicated by Dharwar geo-chemistry: In the Early History of Earth, John Willey.
NAQVI, S. M. and HUSSAIN, S.M., (1972) Petrochemistry of early Precambrian metasediments from the Central part of the Chitradurga schist belt, Mysore, India: Chern. Geol.,v, 10, pp. 109-135.
- - (1973a) Geochemistry of Dharwar metavolcanics and composition of the primeval crust of the Peninsular India: Geochim, Cosrnoehirn. Acta, v. 37, pp. 159-164.
- - (1973b) Relation between trace and major element composition of the Chitradurga metabasalts, Mysore, India, and the Archaean mantle: Chern.Geol., v. 11, pp. 17-30. NASH, T. J., (1975) Geochemical studies in the Park City DistrictII,sulphide mineralogy and
minor element chemistry, Mayflower mine: Econ, Geol., v. 70, pp, 1038-1049.
RADHAKRISHNA, B. P. and SUNDARARAJAN, V. M., (972) Investigation for copper in Mysore State, India: Spl. Publn, Dept. of Mines and Geology. Govt. of Mysore, 28p,
ROBERTS, R. G., (1975) The geological setting of the Matagani Lake Mine, Quebec: A Volcano-genic massive sulfide deposit,Econ. Geol.,no. 1, v. 70, pp. 1I5-129.
SAMPAT IYENGAR, P., (1905) Report on the survey work in Chitaldrug district:.Rec. Mysore Geol,
Dept.,v.6,
SANGSTER, D. F., (1972) Precambrian Volcanogenic massive sulfide deposit in Canada-a review:
Canadian Geol. Surv., Paper 72-22, 44 p.
STANTON, R. L.,(1972) Ore Petrology: McGraw Hill Book Co., New York, 713 p.
(Jour. Geol. Soc. India, 17, 1976, 557-561)
Rb-Sr Ages of
cordtertte-gneisses of
southern Karnataka
S. JAYARAM, V. S. VENKATASUBRAMANIAN Indian Institute of Science, Bangalore
AND
B. P. RADHAKRISHNA Geological Society of India, Bangalore
Abstract
The cordierite-gnelsses of southern Karnataka give Rb-Sr ages of 3010±90 rn.y. indicating jheir relationship to 3000 m.y. event in the Dharwar craton, for which evidence is available in a number of areas.
Introduction
The cordierite-bearing gneisses constitute an important but less studied lithological unit in the southern parts of the State with diverse lithological associa-tions. Their mode of origin and stratigraphical position is not clear. They have been interpreted as altered variants of the charnockite series (Jayaram, 1912) and as contact metamorphites (Sen, 1912). Rama Rao (1925, 1928) and Radhakrishna (1954) have studied these rocks and are inclined to classify them as metamorphosed supracrustals, probably also as 'basic fronts' of granitization episodes. Anantha Iyer and Narayanan Kutty (1975), on the basis of chemical and petrographic evidence regard these rocks as regional metamorphites, formed under probable P. T. conditions 650 - 750°C and 6 - 8 kb. Cordierite granulites also occur in the Rhodesian craton with similar lithological associations and Clifford (1974) provides evidence in support of the cordierite-bearing rocks of Transvaal predating the Swaziland Sequence, which includes rocks older than 3000m.y,
Geochronological studies could provide pertinent information in this regard and elucidate features of the early metamorphic history of the Dharwar craton. In the present work, cordierite-bearing gneisses in parts of Bangalore District of southern Karnataka have been selected for Rb-Sr geochronology studies.
