DISCUSSIONS
Comment
(A comment on the paper 'Distribution of organic matter in the substrate of the Pennar river estuary, Andhra Pradesh' by K.R. Reddi published in the Journal of the Geological Society of India, v.46, July 1995, pp.83-86).
I would like to congratulate the author for his contribution on the distribution of organic content in the sediments of the Pennar ri ver estuary. However, there is a discrepancy and inconsistency in the paper. The author has not mentioned the sample collection procedure. From the text and fjgures as well, it is not possible to differentiate the samples of the monsoonal months from the samples taken in summer months. Author's statement that, "Organic matter was estimated in 65 bottom sediment samples collected seasonally at 15 stations over a period of one yearlt
(para 1, p.83) is ambiguous. How many samples out of 6S were collected in the monsoonal months?
There is a contradiction in the author's statements that "A rough inverse relationship is noted between the organic carbon (or organic nitrogen) and the weight percentage of the fine fractions of the sediments" (abstract, p.83) and "No estimations were made for such sediments that contain less than 10 percent of silt/clay fraction as their organic matter content is negligible" (Method of study para, p.83). Author's opinion in the abstract should uphold the fact that organic matter content is higher in the sediment samples containing less amount of silt/clay fraction. Organic C/Organic N ratio in the author's study averages 13.2 and the ratio in Trask (19321 study is 8.5. How is the author's value nearly 1112 times more than the usual ratio (p.85, line 2)?
On which basis the author states that "it is felt that organic matter in these sediments can be calculated approximately by multiplying organic carbon values with 1.7 and organIc nitrogen values with 22.4" (Conclusion para, p.85, lines 2,3 & 4). What is the scientific reason behind author's inference that ~'Organic matter is more in the monsoonal months than during the summer months (Conclusion para, p.85)? In the season of monsson, bacterial decomposition occurs t,o be maximum. Decomposition of organic matter mediated by bacteria creates anoxic conditions within a short distance beneath the sediment-water interface. The process changes the intermingled organic matter in the sediments into dissolved organic matter (Bricker, 1985). So in this season the organic matter associated with the sediments is minimum.
Besides, the author's judgement that .... prganic matter in the Coastal sediments is derived mostly from the planktonic and benthonic organisms and subordinately from the humic matter derived from the land through rivers" (Conclusion para, p.8S, lines 7,8 & 9) is mere a qualitative approach. Base-line studies of the organic components of the sediments with,the help of c-g-c-m-s (computerised gas chromatography -- mass spectrometry) system is capable of identifying materials derived from anthropogenic activities and natural compounds (Eglinton et al. 1975). Besides, fractionation and lipid analysis of the separate
fraalion of sediments may give more definitive information about input sources than analysis of unfractioned sediments (Thompson and Eglinton, 1978).
Department of Geology , Banaras Hindu University, Varanasi - 221005
References
AlAI SRIVASTAVA
BRICKER, O.P. (1985), Environmental factors in the inorganic chemistry of natural systems: The estuarine benthic sediment environment. In: Environmental Inorganic Chemistry. Irgolic, K.L. and Martell, A.E. (Eds.). Deeifield Beach, VCH Pub!., pp.135-153.
BGLINTON, G., SIMONElT, B.R.T. and ZORO, J.A. (1975). The recognition of organic pollutants in aquatic sediments. Proc. Royal Soc. London, B. 189, pp.415-442.
THOMPSON, S. and EGLINTON, G. (1978). The fractionation of recent sediment for organic geochemical analysis. Geochim. Cosmochim. Acta, v.42, pp.l99-207.
TRASK, P.D. (1932). Origin and environment of source sediments of Petroleum. Gulf Pub. Co., Houston, 323p.
Reply
Regarding the comment on the sample collection procedure, the author has indigeneously designed an aluminium container using simple principle (Sreenivas et al.
1991). Since estuary is divided as lower, middle and upper estuary and as such demarcation of samples collected were hj~edfully indexed. But the season-wise sample distribution differentiation from both text and figures as well is possible provided bar diagrams are drawn which were deleted according to the suggestion of the referee. A total of 18 samples were collected out of 65 during monsoonal months.
Author is thankful to the reader for underlining the fact that organic matter content is higher in the sediment samples containing less amount of silUclay fraction. Is the study area average 13.2 not 1112 times more than the usual ratio in Trask (1932) study (p.85, line 2)? We regret for the typographical error in the text as 22.4 instead of 14.5. The basis of our statement (Conclusion para, p.85, lines 2,3 & 4) is Trask (1932) and Rajiv Nigam and Jorn Thiede (1983). The reader's explanation for higher organic matter content in monsoonal months seems to be inconsistent with the present work. The main factor for the increase of organic matter during monsoonal months is the similarity in the settling velocity of both organic constitutents and finer particles (Trask, 1939). A parallel relationship has also been noticed in Vembanad lake (Murthy and Veerayya, 1972). Since the estuary receives marginal lithogenic flux and run-off during monsoon, a high rate of sedimentation and low dissolved oxygen content were anticipated which is on the other hand characterised by high organic matter content (Sebastian et
at.
1990). An abundant supply of organic matter in water column, relati vely rapid rate of accumulation of fine-grained inorganic matter and low oxy gen con tent of waters immediately above the bottom sediments would favour high organic matter in the bottom sediments (Sverdrup et at. 1942). The high organic matter may also be due to the higher input of sediments and vegetal matters during monsoon season by river.The author's aim of studying organic matter content was not to understand the several components associated with it, by using c-g-c-m-s (Eglinton et al. 1975) but to relate the
parameter to the microbiotal population dynamics and diversity in tenns of space and season. Therefore, it was not opted for.
Department of Geology K.R. REDDI
Sri Venkateswara University, Tirupati -517 502.
References
EGLTNTON, G., SIMONEIT, B.R.T. and ZcIRO, J.A. (1975). The recognition of organic pollutants in aquatic sediments. Proc. Royal Soc. London, B.189, pp.415-442.
MURllIY, P.S.N. and VERAYYA, M. (1972). Studies on the sediments of Vembanad Lake, Kerala State, PartI -Distribution of organic matter. Indian Jour. Mar. Sci., v.l, pp.45-51.
NIGAM, R. and THIEDE, J. (1983). Recent foraminifers from the inner shelf of the Central West Coast, India: A reappraisal using factor analysis. Proc. Indian Acad. Sci. (Earth Planet. ScL), v.92, pp.121-128.
SREENIVAS, K., RAJU, B.N., HONNAPPA ::md REDDI, K.R. (1991). Ostracoda in the Estuarine sediments, Pulicat Lake Estuary, East Coast, India. Jour. Geol. Soc. India, v.37, pp.492-499.
SVERDRUP, fI.U., JOHNSON, M.W. and FLEMING, R.H. (1942). The Oceans - their physics, chemistry and general biology, Prentice Hall, New York.
TRASK, P.D. (1932). Origin and environment of source sediments of Petroleum. Gulf Pub. Co., Honston, 323pp. _ _ _ (1939). Recent marine sediments. Amer. Assoc. Petrol. Geol., 428pp.
DISCUSSIONS 447
Comment
(Comment on the paper entitled, "Uraniferous organic matter in the sandstone-type uranium ore from Domiasiat, Meghalaya, India" by N. Krishna Rao et al. published in the Journal of Geological Society of India, vAS(4), 1995, ppA07-425).
Dr. Krishna Rao and his associates are to be complimented for their detailed petrographic study, along with high-quality photomicrographs, of the uraniferous organic matter (OM) in the sandstone-type uranium ore from Domiasiat, Meghalaya that led to identification of serveral forms and seven types of OM of type II kerogen containing uraninite of three generations. However, I would like to comment on the following points.
1. Nomenclature of sandstone: There is no consistency in nomenclature given for the sandstone hosting both uranium mineralization and OM. Thus, it is termed as quartz arenite-quartz wacke-arkosic wacke-subarkose (p.409), quartz arenite (p.420) and quartz wacke-subarkosic sandstone (p.423). This sandstone is actually a "feldspathic/quartz arenite" (Dhana Raju et al. 1989; Kaul and Vanna,_ 1990). The inconsistent nomenclature appears to be due to non-separation of authigenic cement (silica, OM, pyrite and marcasite) from detritus matrix, and when both are considered as "matrix", an "arenite" erroneously becomes "wacke".
2. Type of alkali feldspar: While the alkali feldspar of the sandstone is microscopi-cally identified as "orthoclase-microcline" (pA09), the authors have mentioned that, based on XRD study, the alkali feldspar in the Domiasiat quartz arenite is a highly disordered type close "to sanidine in composition (p.420). In fact compositionally all the three -sanidine, orthoclase and microcline - are one and the same {'KAlSi]Og"), whereas both sanidine and orthoclase are highly disordered, high temperature, monoclinic polymorphs having small and moderate 2V, respectively,"whereas microcline is the highly ordered, low temperature, triclinic polymorph with high 2V. Thus, what the authors presumed as '~sanidine" is most
probably "orthoclase".
-3. Uraninite or Pitchblende: The major uranium phase in OM is given as "Uraninite", particularly so "in the absence of the structural features characteristic of pitchblende" (p.410). However, the observations of the authors that "broad and diffuse refle~tions in the X-ray pattern of unheated lumpy OM (Fig.24A), lower value for ao of 5.41A and U02.
44 (p.415)", coupled with very low-temperature of uranium mineralization in
the area, all indicate that the major U-phase is "pitchblende" but not uranin1te, since the latter generally gives clear and sharp peaks in X-ray pattern with ao of ca.S.47 A and upto U02.:!}"
Although numerous values of reflectivity are given for OM, not even a single measurement is given for "Uraninite", as by % R it is easy to distinguish pitchblende (10-15% R) from uraninite (ca.17% R). The U-phase with the properties given in the paper should have less than 15% R, as earlier studies have shown it to be "pitchblende" with 11-12% Rat 546 nm (Dhana Raju et al. 1989; Kaul and Varma, 1990). It would have been better if the optical properties of 3 generations of "Uraninite" were given instead of identifying them based on grain size and loci alone.
4. Characteristics of OM: OM, based on microstructures, is identifed as of type I or II kerogen (p.419 and 423) and as type II kerogen (in "abstract", pA07). Generally, organic matter is broadly of "in situ" and "migratory" types, respectively, designated as "kerogen", and "bitumen", with "kerogen", in turn, classified as types I, II and III, based respectively, on its source - "algae", "planktons"· (marine) and "higher plants (mostly continental)" (P. Landais, Oral Comm., 1988). Viewed in this light, the type I or II kerogen,
with preference for type II (in abstract, p.407), identified by the a~thors implies that OM is "in situ" and derived- from planktons of marine environment or algae, whereas Kaul and Varma (1990) have ident~fied OM of both "jn situ" and "migratory" types. As the sandstone at Domiasiat hosting OM is shown as deposited in a "proximal braided channel of fluviatile environment" (Dhana Raju et al. 1989), the possibility of kerogen being of type II derived
from planktons of marine environment is a remote possibility. It would have been better if analytical techniques like rock-eval pyrolisis, NMR, and liquid and gas chromatography were used for classification of OM rather depending on its microstructures alone. Further-more, it is mentioned on p.423 that there is a "positive relation between reflectivity of OM . and the distance from and size, of uraninite inclusions" ,whereas in the plot (Fig.2SA) there
is actually negative relation bl~tween them.
5. High content of some eleme~ts of basic afffinity in OM: To explain the high content of elements like Co, Ni, V, Cr, Zn, Cd (of basic rock affinity) in OM, the authors have invoked" Sylhet traps" as a possible source, besides the Khasi granite batholith. It is difficult to accept the "Sylhet Traps" as a source for the above elements in O¥ of Domiasiat as the same is ruled out by the north to south palaeochannel direction and field setting· in the area, with" Sylhet Trap" being further south and in the downstream direction of sandstone hosting OM. Rather, the high content of these elements with basic affinity can be explained by basic to acidic provenance rocks for the OM-beraring sandstone of Domiasiat that include metabasic rocks like amphibolilte and pyroxene granulite of the ~rchaean Gneissic Complex, epidiorite and dolerite, besides the dominant South Khasi batholith, which is a high-Ca, 1-type granitoid with higher contents (2 to 9 orders that of average high-Ca granite) of these elements, e.g. Ni of 16 to 46 ppm, V of 21 to 128 ppm and Cr of 130 to 195 ppm (Dhana Raju et al. unpublished data).
Atomic Minerals Division, Nagarbhavi, Bangalore - 560 072.
References
R. DHANA RAJU
DHANA RAJu, R., PANNEER SELVAM, A. .and YIRNAVE, S.N. (1989). Characterization of the Upper Cretaceous Lower Mahadek sandstone and its uranium mineralization in the Domiasiat·Gomaghat-Pdengshakap area, Meghalaya, India. Exploration and Research for Atomic Minemls, v.2, pp.l-27.
KAUL, R. and VARMA. H.M. (1990). Geological evolution and genesis of the sandstone-type uranium deposit at Domiasiat. West Khasi Hills district, Meghalaya. India. Exploration and Research for Atomic Minerals. ~3,
pp.l-16. .
Reply
Our replies to the points raised are as follows :
1. Nomenclature of sandstone: The term 'quartz arenite-quartz wacke-arkosic wacke-sub arkose' gives the whole range of composition of the D<;>miasiat terrigenous sandstone, falling in the Fig.S.l of Pettij ohn et ai. (1987, p.145), while the terms used
elsewhere in the text are general terms falling within this range, and therefore we do not find any inconsistency. The matrix component of several samples examined by us did exceed 15% and was mostly cOlnposl!d of silty and clayey material (besides authigenic pyrite, marcasite etc), and bulk of the matrix component is not auth.igenic in origin. The sandstone is unconsolidated-partially consolidated, and therefore, a high component of "authigenic cement" does not arise.
2. Type of alkali feldspar: Sanidine is a homogeneous disordered alkali feldspar while orthoclase is nowadays universally interpreted as an intermediate stage of ordering
DISCUSSIONS 449
towards formation of microcline (Smith and Brown, 1988, p.209 and 218). Thus, sanidine is completely disordered, orthoclase is partially ordered and microcline is fully ordered.
It is difficult to distinguish between isolated grains of orthoclase and sanidine in sedimentary rocks such as sandstone from optical properties alone, while microcline is commonly identified by its characteristic twinning. Hence, the alkali feldspar phases in Domiasiat sandstone are identified as orthoclase and microcline .. That at least some of this "orthoclase" is in a highly disordered state, as suggested by its near zero triclinicity, is a personal communication by K.D.P. Singh and not an observation made by the authors. What followed is a logical conclusion that some of the fe~dspar is perhaps sanidine derived from an acid volcanic source.
3. Uraninite or pitchblende: Normally, the term pitchblende is used to describe the dense botryoidal variety of uranium oxide that is commonly found in low temperature vein deposits. Smith (1984) finds no justification for continuation of this usage as no real distinction has been recorded between uraninite and pitchblende. According to him "uraninite is easily identified by its XRD pattern, and all U02H show the same pattern, except for the changes in spaces due to composition". XRD studies identify any V02H phase as uraninite only and there is no JCPDS card existing on pitchblende. Though the V02 phase at Domiasiat is at a higher oxidation state, following the observation of Smith (1984), we preferred the term uraninite for it.
Reflectivity of "uraninite", particularly in sandstone-type deposits, is highly variable depending on oxidation state, presence of pits, microcracks and submicroscopic inclusions of other minerals and size of individual grains. In the authors ,. opinion, it would be erroneous to distinguisn uraninite and pitchblende based only on R%.
Identification of three generations of uraninite is not entirely based on grain size and loci, but mainly based on the sequence of diagenetic evolution recorded in the organic matter. The optical properties of different generations of uraninite were not reported in the paper, because we felt that it may not throw much light on their.genesis.
4. Characteristics of OM : We have recorded the presence of a secondary OM formed during the organic acid stage of OM diagenesis, which corrodes detrital silicates and clearly shows a cutting-across relationship with primary uraninite and organic structures; this type of organic matte,r definitely has a migratory habit. Except for this, we feel that, bulk of the OM showing characteristic structural features of the precedent biospecies is insitu. As most of the structures identified by us belong to liptinitic group, we consider that the OM is Type II kerogen. Regarding the observation that uraniferous sandstone was deposited in a proximal braided channel system (Dhana Raju et at. 1989), we are of the opinion that there
is clear over-printing of a marine affinity over a fluviatile system as explained elsewhere in our paper. Liptinitic affinity of OM, alongwith structures resembling those of the planktonic algae and authigenic species 'like apatite do indicate marine affinity of the present composition of the mineralized sandstone.
Yes, we do agree that detailed analysis of OM by rock -eval pyrolysis tests, NMR and GC would have given more authentic data for the classification.
We have recorded a clear antipathic relationship between reflectivity and distance of OM from uraninite. Reflectivity of OM increases with closeness to the uraninite. Inadvert-ently, the sentence as framed, conveys an opposite impression and authors thank Dhana Raju for bringing it to our notice.
S. High content of some elements of basic affinity in OM: Yes, if a simplistic view based on the present downstream location of Sylhet Traps in respect of Domiasiat sandstone is taken, the Sylhet Traps cannot be in the provenance·to account for the high content of elements of basic rock affinity in the Domlasiat OM. But the present disposition of Sylhet Traps, for that matter other formations of the area also, are the reoult of several overlapping
processes, faulting (Raibah and Dauki faults), uplift ofthe northern block, marine transgres-sion and regrestransgres-sion, erotransgres-sion and deposition. It is very difficult to visualize the geographical extent of Sylhet Traps during Cretaceous-Tertiary period. and the present areal extent may be only a fraction of its original expanse. Besides the suspected occurrence of sanidine, there are reports of occurrence of tuffaceous matter also in the lower Mahadek sandstone of Domiasiat. If confirmed, these will be additional evidences in favour of occurrence ofSylhet Traps in the provenance of M[ahadek sandstone. For the present, this possibility cannot be completely ruled out.
\Ve are thankful to Dhana Raju for the information on the high concentration of elements like Ni, V and Cr in the South Khasi Batholith; these data being unpublished, were obviously not available to the authors. However, high concentration of Zn, Pb, P etc. in the Domiasiat OM could not hav1e been derived from these rocks.
Ore Dressing Section
Bhabha Atomic Research Centre Hyderabad - 500 016
References
N. KRISHNA RAO
T.S. SUNILKUMAR
D. NARASIMHAN
DHANA RAJU, R., PANNEER SELVAM, A. and VIRNAVE, S.N. (1989). Characterization of the Upper Cretaceous Lower Mahadek sandstone and its uranium mineralization in the Domiasiat-Gomaghat-Pdengshakap area, Meghalaya. Exploration and Research for Atomic Minerals, v.2, pp.I-27.
PETIlJOHN, FJ., POTIER, P.E. and SIEVER, R (1987). Sand and Sandstones (2nd Ed.), Springer-Verlag, New York, 553p.
SMmt Jr, D.K. (1984). Uranium mineralogy. In: De Vivo, B., Ippolito, F., Capaldi, G. and Simpson. P.R. (Eds.), Uranium Geochemistry, Mineralogy, Geology, Exploration and Resource, Institution of Miningand Me tall urgy, London, pp.43-88.
SMITH, J.V. and BROWN, W.L. (1988)., Feldspar Minerals, voU (2nd Ed.), Springer-Verlag, Berlin. 828p.
j\NNOUN ClEMlENTS
NATIONAL SEMINAR ON ENVIRONMENTAL IMPACT ASSESSMENT OF SMALL-SCALE MINERAL RESOURCE UTILIZATION. on 6-7, October 1995 at the Department of Geology, M.L. Sukhadia University. Udaipur. For further particulars write to: Dr. P.S. Ranawat, Convener, Dept. of Geology, M.L. Sukhadia University, Udaipur - 313·001.
SYMPOSIUl\-1 ON RECENT DEVELOPMENTS IN ENVIRONMENTAL GEOSCIENCE. 9-10, Octobe.r 1995. at the Department of Geology, Sri Vcnkateswara University, Tirupati. For further particulars write to: Prof. E.A.V. Prasad. Dept. of Geology, Sri Venkateswara University, Tirupati -517 502.
INTERNATIONAL CONFERENCE ON NATURAL AND TECHNOLOGI· CAL COASTAL HAZARDS. 2·6, December, 1996 at the Geology Dept., Sri V cnkateswara University, Tirupati. For details write to: Prof. K.L. Narasimha Rao, Dept. of Geology, Sri Venkateswara University, Tirupati -517 502.
