SHORTER COMMUNICATIONS 423
dispersed within chalcopyrite grains. They have also been noted to project into fractures in the associated cubanite blades.
Chalcocite, associated with minor amounts of covellite, has formed along cracks and fractures of chalcopyrite, as a secondary product.
Conclusion: This account of the textural features of the various minerals in the Deri-Ambaji ores, clearly indicates that the ore minerals and the gangue silicates in the enclosing host rocks were together involved in a metamorphic episode (cf. Vokes, 1969)-a vital fact overlooked in the previous studies. This, combined with the relict banding of the ore minerals, suggests a possibility of syngenetic origin of the ores with subsequent modification during metamorphism. The different aspects of this problem are being worked out so that a viable conclusion may be reached.
Acknowledgements: The author is grateful to Shri S. P. Nautiyal for providing facilities for field work and to Shri N. C. Shekhar for his warm hospitality at Ambaji. Thanks are also due to Dr. S. C. Sarkar, Jadavpur University and Dr. S. Varadarajan, Delhi University, for critically reading the manuscript and offering valuable suggestions.
REFERENCES
COULSON, A. L., (1933) The geology of Sirohi state, Rajputana. Mem, Geol. Surv. Ind., v. 63, pt. I.
HERON, A. M. and GHOSH, P. K., (1938) The geology of Palanpur, Danta and parts of Idar states. Rec. Geol. Surv, Ind., v. 72, pt. 4, pp. 367·412.
SHEKAR. N.
c.,
MOOKHEY, R. C. and BHAN, S. K., (1968) Ambamata lead-zinc-copper deposit,Banaskantha district, Gujarat. Abstract, Colloquium 011exploration and development of lion-ferrous metals in India, Geol. Surv. Ind., p. 22.
STANTON, R. L., (1964) Mineral interfaces in stratiform ores, Trans. Inst . Mining Met., v. 74, pp.45-79.
- - and GORMAN, H., (1968) A phenomenological study of grain boundary migration in some common sulphides. Econ. Geol.,v,63, pp. 907·923.
- - (1972) Ore petrology, McGraw-Hili, New York, 713 p.
VOKES, F. M., (1969) A review of the metamorphism of sulphide deposits. Earth Sci. Reviews, v,5, pp. 99-143.
PETROCHEMISTRY AND STUDY OF THE THERMAL METAMORPHIC PROCESSES IN THE CALCAREOUS ROCKS AROUND KODAG, DISTRICT 'PALAMAU, BIHAR
A. V. ULABHAJE
Mining Geology Section, Banaras Hindu University, Varanasi
Introduction: The present work deals with the chemical and mineralogical changes involved during the metamorphism of calcareous sediments. Three type localities (Fig. 1: Location map) where calcareous formations of Archaean age are welI exposed, were selected for this study. The reason for including these three areas lies in the fact that the changes due to different sets ofpoT conditions superimposed
differ-ent thicknesses due to uneven erosion accompanied with the overal1 plunging nature of the major fold axis.
Field work: Based on the preliminary knowledge of the above three and the ad-joining regions (from the unpublished reports of the earlier workers) detailed sampling and collection of material was undertaken in successive field trips. Although the method of sampling varied according to the geological setting and the nature of the outcrops, an effort was made in all the three areas to obtain samples that truly represented outcrops both with respect to the mineral variations and their volume. The resulting samples were studied petrographically, and subsequently a portion of the material supposed to include the minimum requisite number of samples was chemically analysed.
On the basis of the petrographic examination, the marbles of the three areas were grouped in the following associations. The associations were further divided into stages in accordance with the relative abundance of the mineral constituents.
Khalari: (I) Calcite-quartz-felspar association ( I. Calcite-felspar-phlogopite
J
rr.Calcite-felspar-quartz-tremolite Stages : III. Calcite-quartz-Ielspar-phlogopite\ IV. Calcite-quartz-felspar-phlogopite-tremolite
(2) Calcite-tremolite-phlogopite-diopside association
( I.Calcite-phlogopite-tremolite-quartz
I
n.
Calcite-tremolite-phlogopite-chlorite-quartz I III. Calcite-phlogopite-diopside-quartz Stages1
IV. Calcite-tremolite-phlogopite-felsparI V. Tremolite-felspar-calcite
LVI. Tremolite-calcite-diopside
Demu: (1) Calcite-tremolite-diopside-felspar-quartz association
r
n.I. Calcite-tremolite-chlorite-perthiteCalcite-tremolite-quartz-perthite-diopside~ Ill. Calcite-perthite-tremolite-diopside Stages I IV. Calcite-perthite-diopside
I V. Calcite-diopside-perthite-muscovite LVI. Calcite-quartz-perthite-diopside
(2) Diopside-felspar-quartz·calcite association ( I. Diopside-microcline-calcite-quartz
~ II. Diopside-plagioclase-calcite-tremolite-phlogopite-microcline Stages 1HI. Diopside-plagioclase-calcite-tremolite
LIV. Diopside-microcline-pJagioclase-calcite
Kodag: (I) Calcite-tremolite-serpentine-forsterite association ( I. Calcite-tremolite-felspar
I II. Calcite (with Dolomite)-serpentine-forsterite-chlorite Stages oj Ill. Calcite-serpentine-forsterite-chlorite-diopside-spinel
IIV. Calcite-serpentine-forsterite-diopside-tremolite-spinel-chlorite L V. Calcite-serpentine-forsterite-chlorite-tremolite
(2) Calc-magnesian silicates association
( I. Chondrodite-calcite-spinel-tremolite-chlorite-forsterite
I
n.
Diopside-calcite-tremolite-serpentine-felsparStages -\ HI. Grossularite-calcite-diopside-felspar-tremolite-scapolite-sphene
IIV. Talc-clinozoisite-diopside-sericite-felspar L V. Diopside-phlogopite
SHORTER COMMUNICATIONS 425
synthetic standards of known concentrations were used. The trace elements were determined on a grating spectrograph with a log sector device and the resulting spectrograms were read on a viewer comparator. Quantitative determinations were thus made. Here again working curves on the basis of R. u. powders were prepared at tbe beginning of this work. The data so made available were tabulated following the same order as already found out on the basis of the modal analyses of these rocks.
Itwas discovered that the mineral associations and tbe mineral assemblages correlated well with the chemical data, and therefore the subsequent interpretations and figures were prepared on that very basis.
e-:
.,
ii
i...,
• I ... :240 \ \
u, P. \._.,.
• LOCALITI[~ OF wOfllJ':
~ RIV[fl: OR NAL'"
- ' - ' - ' - ' STATE eOlJNO.lp'I'
, - - 01SHUCT~O\JMO""'t'
• II I•II RAtL .... TLINES 'wi"
Figure 1. Location map of the area.
R I B A G H
R
LO . "20
I I
If·
The chemical trends: The entire series of chemical changes can be studied by
426
existent in the previous two areas and therefore, even the increase ofSiO~marked in this association is meagre and slow. Beyond the initial stage which coincides with the maximum pressure of CO., the changes are marked by a progressive increase of SiO. and a decrease of CO.; that is, a change similar in nature to that found in Demu and Khalari. In the association (2) i.e. the calc-magnesian silicates association of
the Kodag area, the changes are very instructive and show that many of the appearing phases were metastable. The most important conclusion reached on the basis of the chemical changes in this association is that for a certain time the system so much gains SiO. that the other constituents become almost negligible. Beyond this stage the stability is brought in, though at a much lower level of SiO~. A comparison of the overall chemical changes in the three areas shows that H.O and the pressure of CO. form the most important parameters in controlling the mineral paragenesis of the carbonate systems.
Distribution and behaviour of trace elements: The distribution patterns of the
trace elements in these rocks shows that some trace elements show increase or decrease with advancing stages of mineralisation, some remain more or less constant under varying conditions or behave in an erratic manner, and some trace elements show an affinity towards certain stages of mineralisation.
The above pattern of distribution when studied in relation to the geochemistry of the elements on the one hand and their present behaviour on the other shows that there are two primary factors that 'have played a dominant role in controlling the distribution of the trace elements in the different calcareous associations. These are:
(i)elements on account of their ionic radii and valence have got access to the then existing lattice structures and (ii) such elements like Cr and W which have formed complex molecules of their own have their independent gradients. But the more significant conclusion as regards the present trace element study relates to the restrict-ed application of the rules so far formulatrestrict-ed by Goldschmidt (1937), Ringwood (1955), Ramberg and De Vore (1952) that the behaviour of the trace elements is controlled by their corresponding major elements or electro negativity . The present findings clearly demonstrate that the trace elements do not necessarily follow their corres-ponding major elements on the basis of their ionic size or valence, nor does the electro-negativity of the system play an important role. On the other hand,it is the nature of the chemical gradient, whether smooth or steep, that is most effective in deciding which trace element will get the preference and the appropriate site. Briefly,
it may be stated that the behaviour of the trace elements as found in the magmatic rocks or during metamorphism of non-calcareous sediments is different than the one seen in the calcareous sediments.
One important conclusion may be added and that is, unlike the magmatic rocks and the non-calcareous metamorphites where there is a tendency for the mafic and felsic trace elements to be seggregated in different structures, in the present instance, no such tendency is exhibited by the mafic and the felsic trace elements. On the other hand, invariably elements of the mafic group especially occur in different associations rather than in the same.
Relationship of the chemical and mineralogical data: The chemical data have
SHORTER COMMUNICATIONS 427
deficient .amounts to the stoichiometric proportions; and obviously in such cases,
the analytic results do not tally with the modal data. Further the correlation brings
about different metastable and arrested phases which might have been in existence
during the evolution of these rocks. The important conclusion arrived at in this
connection is that the mineral paragenetic events fall in a progressive chain in anhydrous systems while they overlap one another in presence of water.
Interpretation of the results: On the basis of the chemical and mineralogical data the present results have been critically interpreted in the light of recent
experi-mental and field findings. After ascertaining the approximate original composition,
the nature and magnitude of the chemical changes have been found out and the
chemical gradients determined. In this connection the relative role of the different
physico-chemical and geological factors have been considered and it has been shown that the factors, namely, the open and closed nature of the chemical system, the chemical gradient, and the presence of water have played a dominant role in bringing about the diversity of results.
The present results have shown that temperature accompanied or unaccompanied by pressure plays only an indirect role in disturbing the equilibrium and setting up a chemical gradient which alone becomes responsible for subsequent reactions and
mineral paragenesis. The transfer of material includes movements of different
cations towards the negative chemical potential through the process of diffusion accentuated by the presence of a vapour phase which always accompanies the
carbo-nate reactions. The role of water has been shown to be dominant in two ways:
namely, lowering the temperature range of the reactions, and formation of hydrous
silicates. The chemical changes have been correlated with the open or the closed
nature of the system as well as the geological settings and level. The range of
temperature except in the case of rocks from Demu has been found to be more or less similar.
In conclusion it may be noted here that the results obtained in thermal and regional metamorphic conditions in the case of calcareous rocks cannot be ascertain-ed on the basis of mineral assemblages or associations which reflect only the reconsti-tuted part of the latest history but by connecting a series of chemical changes right upto the original composition; in other words, it is the chemical gradient which can serve to differentiate the mineral assemblages of the calcareous rocks formed under
different metamorphic conditions. Secondly, it must also be noted that at least in
the present areas, neither syntexis nor reciprocal reactions and metasomatic activities
have played any significant role. On the other hand, all the present changes mark
several cycles of distribution of cations under the then existing chemical gradients
and involving large scale migrations in some instances. Thirdly, unlike metamorphic
activities pertaining to pelitic, psammopelitic and psamitic sediments those met with here need not necessarily mark a linear progression, but many a times there are fluctuations short or long enough to bring about a reversibility under certain
condi-tions. Finally the present results demonstrate beyond doubt that in the absence of
chemical data relating to unaffected, or for that matter the least affected, rocks under study, not much additive contribution can be made on a quantitative basis to our present knowledge of metamorphism of calcareous rocks.
Among the wider applications of the present work, a few significant ones may
be noted here. First, the grade and the facies classifications referring to P-T
428
concepts giving an idea of mineral paragenesis under one set of given conditions, and that this concept cannot be generalised to bring in its fold rocks from widely separat-ed areas and those of originally diverse chemical composition. Second, a resurvey on chemical lines of the qualitative petrographic data concerning metamorphism of calcareous rocks be made so as to evolve a working basis for investigations on such
problems.
REFERENCES
RAMBERG, H., (1952) The origin0/Metamorphic and Metasomatic rocks, University of Chicago Press, 912 p.
RINGWOOD, E., (1955) Principles of trace element distribution etc. Geochim. et, Cosmochim. et. Acta,v, 10, pp. 189-202; pp. 242-254.
ON THE OCCURRENCE OF BURIADIA HETEROPHYLLA (FEISTM.) SEWARD AND SAHNI IN THE RANIGANJ STAGE (UPPER PERMIAN)
OF INDIAN LOWER GONDWANA
MANJU BANERJEE
Botany Department, University of Calcutta, Calcutta
Introduction: Buriadia Seward & Sahni, a dominant and characteristic plant fossil of the Karharbari Stage of Indian Lower Gondwana is rarely reported from the Barakar Stage (Ganguly, 1959) and its occurrence was not until now reported from the rich fossiliferous beds of the Raniganj Stage. Feistmantel (1881, p. 122, pI. 47A, figs. 20, 22, 24 and 19) described a few coniferous shoots and a cone scale from the Kamthi beds of Raniganj Stage as Voltzia heterophylla Brongn. Only one of these coniferous shoots (No. 5360, Feistm.; 1881 PI. 47A, fig. 22) and the cone scale (No. 5357, Feistm., 1881, PI. 47A, fig. 19) kept in the G.SJ. museum, Calcutta have been found on re-examination to be too poorly preserved for identification on the . basis of external morphology; the other specimens of Feistmantel having been mis-placed were not available. Hence, the identification of the specimen as Voltzia
heterophyllais doubtful and it is preferable to recognise it as coniferous shoot. Two species of Buriadia have been reported so far. B. heterophylla (Feistm.) Seward& Sahni has been recently investigated in detail by Pant & Nautiyal (1967).
B. fragilis, the other species has been described by Maithy (1970).
Material and method: The specimen preserved on a greyish sandy shale has a carbonised crust almost throughout the length of the preserved shoot. Macerated preparations with the carbonised crust have been made by HF and schulze solution followed by dilute KOH. ,
SpecimenNo.~PC.274.
Locality of collection-Pathargaria colliery, Bhurungia seam, Mahuda basin, Jharia coalfield. Bihar.
Age and horizon-Raniganj Stage, Upper Permian.
