Discussion

DISCUSSION

Comment

11.

(Comments on the paper, ' Ore Mineralogy and Fluid Inclusion

Ch~racteristics

of Different Ore Associations from Malanjkhand Copper Deposit, M.P., India' By

M. K. Panigrahi,

B.

Misra and A. Mookherjee published in the Journal of the

Geological Society of India, Vol. 37, March, 1991, pp. 239-256).

Two aspects of the Malanjkhand copper deposit, ore mineralogy and fluid

inclusion characteristics, have been dealt with by the authors which, however, have

not been related to one another. In spite of the fact that separated sphalerite was .

not found suitable for fluid inclusion study, at least a broad idea of thermal regime

during the main stage copper mineralisation could have been presented. Before

reaching a conclusion that mineralisation occurred at temperatures similar to those

deduced from fluid inclusions in host quartz reef/veins, related thermodynamic

aspects are to be critically examined and an attempt has to be made to furnish some

supporting evidence. Based on the information given in the paper under discussion

here, some points have been brought out below which deserve attention of the

authors.

(1) So much of relevant information is hidden in hydrothermal alteration assemblages of the Malanjkhand copper deposit that it would have been worthwhile making use of this in developing a sequence of coherents event Jeading to the mineralisation. Some definite clues are provided by hydrothermal minerals occurring with are minerals in all the four ore associations. The presence of epidote indicates temperature of at least 250°C and quartz+ calcite veins and suspected presence of adularia (fresh K~feldspar replacing plagioclase megacrysts, quartzo-feldspathic veins and feldspar impregnations as mentioned by the authors) indicate boiling of hydrothermal fluid. Co~existence of epidote and adularia suggests that the fluid at a temperature of about 250°C had relatively high pH, may be due to partitioning of CO2 and H2S with the steam fraction (Browne, 1978; Absar, 1991).

(2) Extensive kaolinisation at the granite-quartz reef contact in the southern part could have occurred under the influence of a low pH bicarbonate - sulphate fluid formed due to con-densation of steam carrying C02 and H2S in near surface ground water bodies. Alter·· natively. as epidote and kaolinite form under contrasting pH conditions (Absar, 1991). there could have been at least two different phases of hydrothermal activity.

(3) The presence of alunite, a hydrothermal mineral forming from low pH solutions as in active geothermal systems of Japan (Browne. 1978), in the oxidation zone of the Malanj-khand deposit suggests that primary sulphide ore was oxidised to considerable depth in the southern part of the reef under the influence of relatively hot descending steam-heated waters. More intense fracturing in the southern part of the reef supporting deeper per-colation of solutions may have resulted due to overpressures caused by sufficiently large gas partial pressure over and above the hydrostatic pressure. The oxidation pattern is normal in the northern part of the reef.

(4) Co-precipitation of magnetite (II) and chalcopyrite during the main stage suggests tempe-ratures of about 250c_{C. At temperatures below 240°C, the magnetite stability field}

DISCUSSlON 443

involving such numbers should be carefully interpreted. In general, therefore, tempera-tures deduced from ore mineralogy are similar to those suggested by alteration minera-logy.

(5) Presence of aqueous bi-phase and mixed aqueous-carbonic inclusions indicates boiJing and separation of gases as also suggested by atteration mineralogy.

(6) Fluid inclusion homogenisation temperatures of 250°C and above, considered by the authors to correspond to an earlier barren phase of quartz precipitation are in fact similar to those indicated by mineralogical assemblages. The isochores intersecting at 225°C and 275°C are in complete disagreement of suggested average homogenisation temperatures of 160-1 BO°C. The fact that vapor!vapor+ liquid ratio of studied inclusions varies by an order of magnitude (0.03 to 0.3) suggests that inclusions have trapped various mixtures of liquid and vapor (+C02) or have leaked necked (Bodnar et al. 1985).

(7) Measured freezing points are anomalously low and consequently calculated salinity values are abnormally high. It appears that trapped fluid had very high C02 content but in the paper freezing points have been converted to salinity values without taking into consider-ation the effect of CO2 on freezing point depression (Hedenquist and Henley. 1985). It is rather impossible to imagine a hydrothermal fluid with 26 to 34% salinity. The Salton Sea geothermal brine, the most concentrated hydrothermal solution known from terrest-rial geothermal systems and epitherm,t1 deposits has 25% by weight of total dissolved solids.

Homogenisation and freezing temperature data on fluid inclusions need a

review in the light of work done by Hedenquist and Henley (1985) and Bodnar

et al.

(1985).

Mineralogy of hydrothermal and ore assemblages suggests

tempe-ratures of at least 250°C and that the fluid responsible for mineralisation was

boiling. Fluid inclusion data, in addition to boiling, indicate presence of

rela-tively Jarge amount of CO

2 •

Very Jarge variations in liquid to vapor ratio suggests

that inclusions have trapped mixtures of liquid, vapor and CO

2

in varying

propor-tions, thus making the interpretation rather difficult. Except for the' suspected'

presence of Iss-breakdown phases (which are described only in conclusions) there

ir.

DO

clear-cut indication for low temperature mineralisation at Malanjkhand. The

so-called suspected Iss-breakdown phases may also be explained by a later low

tem-perature event as indicated

by

the presence of kaolinite and alunite.

-Geothermal Division Geological Survey oj India Lucknow.

References

AHSAN ABSAR

ABSAR, A. (1991) Hydrothermal epidote - an indicator of temperature and fluid composition. Jour. Geo!. Soc. India, v. 38, pp. 625-628.

BODNAR. R. 1., REYNOLDS, T. J. and KUEHN, C. A. (1985) Fluid inclusion systematics in epithermal systems. In: Geology and Geochemistry of epithermll systems. Reviews in Economic Geology, v. 2, pp. 73-97.

BROWNE, P. R. L. (197S) Hydrothermal alteration in active geothermal fields. Earth and Planetary Sciences, v. 6, pp. 229-250.

OIGGENBACH, W. F. (1980) Geoth~rmll gilS equilibria. Geochim. Cosmochim. Acta, v. 44, pp. 2021-2032.

444 DISCUSSION

Reply

We thank Absar for his interest in our work. We, however, refrain from

commenting on several truisms and

~eneralities

in his statements and restrict

our-selves to specific points raised. Since textural evidence unequivocally suggests that.

precipitation of quartz spans over the entire mineralization spectrum and beyond,.

deduced changes in fluid characteristics (in quartz) with time does provide a 'broad

idea of the thermal regime during the main stage mineralization'.

Therefore~

Absar's contention, that the aspects of minera1ization and fluid characteristics.

'have nol been re1ated to each other', is untenable.

(1) Indeed, information lie hidden in all associated features in a deposit. Unfortunately. Absar seems to have 'retrieved' wrong information on the basis of a questionable premise that' presence of epidote is an unequivocal evidence of temperatures higher than. 250a_{C' (Absar, 1991). Epidote occurs at temperatures down to 190°C in the Salton Sea}

geothermal system (eho et ai, 1988 and Caruso et ai, 1988) and its stability down to· 150a_{C has been shown by Bowers et al. (1984). Equally tenuous is his contention that}

presence of calcite and epidote represents different alteration events; the two minerals. can happily coexist under favorable solution composition (Bowers et al. op. cit.). There-fore, constraining temperature by mere presence/absence of phases like epidote would be overtly simplistic and erroneous. Further, it is very difficult to visualize that the ubiqui~. tous occurrence of fresh K·feldspar in the whole body of the enclosing as well as the regional surrounding granites could have resulted from boiling of a fluid without leaving any imprint such as presence of vapor.rich inclusions in the precipitating minerals. Boiling is one of the mechanisms affecting the pH of the solution through volatile expul. sion, but not the only one as Absar would like us to believe; initial rock and fluid compositions and fluid/rock ratio are also major factors (Giggenbach, 1984) ..

(2) Kaolinization at Malanjkhand is very much a localized supergene phenomenon and is not.. associated with the primary mineralization or hypogene alteration events. This is corroborated by the occurrence of a supergene sulfide zone directly below the kaotinized, highly fractured reef a t its southern extremity.

(3) We have not detected any alunite in the oxidation zone at Malanjkhand. Even if it occurs in the southern part, it has no bearing on the primary mineralization. Field rela-· tions suggest that there had been a late, post·mineralization brittle deformation. as is evident from highly fractured nature of the post.mineralization basic and aplite dykes within the mineralized area. Therefore, the fracturing, which is all-pervasive postMmine-ralization and on a regional scale, should not be ascribed to local fluid overpressure.

(4) Actually, Giggenbach (1980) constructed the diagrams for the stability of magnetite at. different temperatures by the reaction:

FeS2

+

H2

+

(H20)

=

(FeO)

+

2 H2S

for which he fitted the equilibrium constant empirically as a function of temperature from. the well discharge data. He did not consider any mineral phase due to lack of thermo-dynamic data and activity-composition relationship in chlorites and epidotes at that time. However, the point is that there is no dearth of variables to be chosen for thermodynamic analysis of mineral·fluid equilibria. The same variable f_H2S can be considered with activity ratios of cationic species (Cu++. Fe++) to show the stability of magnetite-chal-copyrite (Brimhall, 1980). Bowers et al. (1984) and Panigrahi (1992) have demonstrated stability of Mt-Cp assemblages down to 200°C and less. 'Oxygen fugacity, even if dimiM nishingly (?) small', has its usual significance where phase assemblages are conc:erned. (5) Presence of coexisting aqueous and aqueous-carbonic inclusions, suggests many possibi.

DISCUSSION 445

aDd can happen at lower temperature conditions also (Bowers and Helgeson, 1983). This is not strictly • boiling'. Besides, we do oot have any unequivocaJ indication of boiling of the fluid (from fluid inclusion microthermometry)-such as homogenization to both vapor and liquid phase at same temperatures (Roedder and Bodnar, 1980).

«6) The range of

<

150-200"C in Figures 6-a and 7-a where the clustering of the points is observed, is basically the homogenization temperatures not corrected for pressure. The points hlling towards the minimum salinity and density ranges in Figures 6-a, band 7-a, b wouJd necessarily have considerable pressure corrections and hence, would shift towards higher temperatures. We did not get many intersection points of the isochores, but the two sets shown in Figure 8 cover density values of 0.954 to ] .059 within which most of the inclusions lie (Fig. 5). This gives ao idea about the actual range of tempera-ture of the are fluid which can be constrained between

<

150 to 275°C. The statement that 'isochore intersecting at 2250

and 275°C are in complete disagreement ... ' is, therefore, as unwarranted as it is untrue. In situations where boiling is negated, the pressure corrected temperatures (as obtained by isochore-intersections) are the actual tem-peratures of the ore fluid and their agreement/disagreement with the homogenization temperatures is irrelevant. It is also immaterial if the V/V+L ratio varies by one order of magnitude because the maximum absolute value is 0.3 which cannot be ca]]ed vapor rich. Besides. maximum number of inclusions fall in the low range of 0.03-0.04. There-fore, it cannot be concluded that the inclusions trapped various mixtures of liquid and vapor. Nor is there any evidence of leakage or necking down.

(7) The data presented in Figure 2 are only the T FMs and oot T M values. Perhaps Absar did not look carefully into Figure 3 where salinity ranges of 2-28% NaCI equivalent and 2-36% NaCl equivalent are shown for the reef and vein ores respectively; and in the reef (larger number of inclusions) almost all the inclusions lie within the 16 wt% NaCI equiva-lent. We are somewhat intrigued to learn that' it is impossible to imagine a hydrother-mal fluid with 26-34 wt% salinity'! Inclusion fluids of >60 wt% total salinity are en-countered in many porphyry-type ores (Roedder, 1971). If, as we postulated (Panigrahi.

1992), one of the fluid components is derived by exsolution from the granite (diluted by meteoric water contamination) it cao surely have the salinity range observed. The critic objects to our salinity calculations on the ground that the effect of dissolved CO₂ has not been taken into consideration. Table V of Hedenquist and Henley (1985) shows that the discrepancy in the measured (T m-mo) and calculated (T m-calc) values of freezing point depression is maximum for the maximum measured gas molality. At low gas molality the dissolved gas content has negligible effect on freezing point depression which is assumed to be due to dissolved solids only. More relevant, however, is the probable error due to the simplistic approximation to the NaCI-H20 system, an approximation which we also feel. gives inaccurate values for salinity. A more precise quantification must await refinement of the H20-COr NaCl system at low temperatures. Besides, Hedenquist and Henley (op. cit.) have also pointed out that dissolved C01(C02aq) in the solution is expected to be present in dissociated form (CO-,; /HCO;) and hence the effect cannot be considered to be the same as that due to C02-non-polar species. This parti-cular problem. the resolution of which requires precise PVTX data on the relevant systems is too well-known a lacuna to warrant overstretching. The lacuna notwithstanding, such gross estim':ltion of the fluid chemistry serves several other purposes-such as identifi-cation of the process of evolution of the fluid.

l.l.T .• Kharagpur 25-02-1992

°Jadavpur University, 'Calcutta

References

PANIGRAHI

MISHRA

MOOKHERJEE*

446 DISCuSSION

BOWERS,. T. S., and HELGESON, H.

c.,

(1983) Calculation of the thermodynamic and geological consequences of non-ideal mixing in the system H20-C02-NaCl on phase relations in geologic systems: Equation of state for H~O-C02-Nac1 fluids at high pressures and.

temperatures. Geochirn. Cosmochim. Acta., v. 47, pp. 1247-1275.

BOWERE, T. S., JACKSON, K. J., and HELGESON, H. C., (1984) Equilibrium Activity Diagrams for coexisting minerals and aqueous solutions at pressures and temperatuIes up to 5kb and

600°C. Heidelberg, Springer Verlag, 397. p.

BRIMHAJ_L, G. H. JR. (1980) Deep hypogene oxidation of porphyry copper potassium silicate protore at Butte, Montana; A theoretical evaluation of the copper remobilization hypo-thesis. Econ. Geol. v. 75, pp. 384-409.

CARUSO, L. J., BIRD, D. K., CHO, M. and LIOU, J. G. (1988) Epidote bearing veins in the State

2-14 drill hole: Implications for hydrothermal fluid composition. Jour. Geophy. Res.,

v. 93, 811, pp. 13123-13133.

CHO. M .• LlOU, J. G. and BIRD, D. K. (988) Prograde phase relations in the State 2-14 Well

metasandstones, Salton Sea Geothermal Field, California. Jour. Geophy. Res., v.93(Bl1), pp. 13081-13103.

GIGGENBACH, W. F. (980) Geothermal gas equilibria. Geochim. Cmmochim. Acta., v. 44, pp.2021-2032.

- - 1984) Mass transfer in hydrothermal alteration system: A conceptual approach. Geo-chirn. Cosmochim. Acta., v. 48, pp. 2693.2711.

GREENLAND, L. P. (1970) An equation for trace element distribution during magmatic crystalli-zation. Amer. Min., v. 55, p. 455.

HEDENQUIST. J. W. and HENLEY, R. W. (1985) Effect of C02 on freezing point depression measurement of fluid inclusions-Evidence from active 5ystems and applications to epi-thermal studies. Econ. Geol., v. 80, pp. 1379-1406.

PANIGRAHI, M. K. (1992) Copper-Molybdenum mineralization and associated granitoids at Malanjkhand, M.P., India. Ph.D. Thesis (Unpublished). lIT, Kharagpur, 153 p. PANIGRARI, M. K., MlsHRA. B. and MOOKHERlEE, A. (199]) Ore mineralogy and fluid inclusion

characteristics from different ore associations from Malanjkhand copper deposit, M.P., India. Jour. Geol. Soc. India, v. 37, pp. 239~256.

ROEDDER, E. (1971) Fluid inclusion studies on the porphyry.type are deposits at Binghilm, Uttah ; Butte, Montana and Climax, Colorado. Eton. Geo!., v. 66, pp. 98-120.

ROEDDER, E. and BODNAR, R. J. (1980) Geologic pressure determination from fluid inclusion studies. Ann. Rev. Earth Planet. Sci., v. 8, pp. 263-101.

Comment

2

(Comment on the paper entitled 'Microfossils from the Don-stromatolitic

Middle Proterozoic Vindbyan chert, Cbitrakut area, Uttar Pradesh',

By

S. Kumar

and Purnima Srivastava, published in the Journal of the Geological Society of India,

Vol. 38, No.5, November 1991, pp. 511-515).

In this papes, there are two microfossil forms

(Filamentous Form

'A',

DISCUSSION 447

.assemblages (with biostratigraphic zonation) of Sangrampur hill sediments

(Nau-tiyal, 1984, 1986).

Genus TrachysphaeridiumTimofeev, 1959, p. 28 (nom. nud.) Group Acritarcha

'was originally described by Timofeev (1959) as having thick, stout test' with

.~hagreen-like

surface, but no type species designated (Norris and Sarjeant, 1965).

Subsequently, Timofeev (1966, p. 36) designated Trachysphaeridium attenuatum

Timofeev, 1959 as the type species (with 150 to 160 urn overall size). The tests

,(or vesicle) of Trachysphaeridium commonly occur in the Proterozoic

~ediments/

metasediments as diagenetically modified: texture spongy, granulate, coriaceous or

reticulate, often with mineral grain impressions or perforations, with moderate

thick wall (about 1 ,u.m thick). So, a great many names have been applied to the

·same taxon, e.g., Leiosphaeridia Eisenack and Kildinella Timofeev, etc., to name a

few (Hofmann et al. 1979). All these genera (with different species) mainly have

thin to moderately thick walls (e.g., test wall 1 ,urn thick, with test diameter range

56 to 105 ,urn in Trachysphaeridium dalkiensis Nautiyal, 1988, MiddJe Proterozoic

.age). On the contrary, Kumar and Srivastava (1991) described a fairly thick

·(vesicle) wall as ranging between 2 to 7 ,u.m for the Trachysphaeridium sp. With

this morphological character, their said form

IDly b~

assigned either to the

·acritarch genus Vavosphaeridium Timofeev, 1956 (e.g., Vavosphaeridium

vindhyanen-.$is Maithy and Shukla, 1977).

.

So, the forms (with wide varying morphologi"cal characters) illustrated

as

Trachysphaeridium (PI. I, Fig. A-D, pp. 512-514, by Kumar and Srivastava 1991)

need a drastic revision.

.

.Department of Geology, University of Lucknow, Lucknow 226007, India

References

AVINASH CHANDRA NAUTIYAL

HOFMANN, M. J., HILL, J. and KING, A. F. (1979) 'Late Precambrian microfossils, southeastern Newfoundland. In: Current Research, Part B, Geol. Surv. Canada, Paper 79-1B,

pp.83-98. .

'MAITHY. P. K. and SHUKLA, M. (1977) Microbiota from the Suket Shales, 'Rampur, Vindhyan System (Late Precambrian), Madha Pradesh. Palaeobotanist, v. 23 (3), pp. 176-188. NAUTIYAL, A. C. (1984) Discovery of Algonkian (Upper to Middle) micro-organisms from Semri

Group at Sangrampur hill, Banda dist., India. Geosc. Jour.. v. 5 (1), pp. 81-86. - - (1986) Lower Vindhyan (Algonkian) microflora (microfaupa) and biostratigraphy

of.San-grampur hill, Banda dist.; Northern India. G~osc. Jour., v. 7 (l). pp. 1-22.. . . . . .

'NORRIS. G. and SARJEANT, W.A.S. (1965) A descriptive index of genera of fossil Dinophy~ae and Acritarcha. Paleontological Bull. 40, New Zealand, Geo!. Surv., pp. 1-72.

TIMOfEEV, B. V. (1959) Drevnesaja Flora Pribaltiki iee Stratigraficeskoe Zoacenie. Trud. vses. neft. nauc.- issled. geol.- razv. Inst, No. 129. pp. 1-319 (in Russian).

448 DISCUSSION

Reply

In his comment Dr. A. C. Nautiyal has doubted the identification of

Trachy-sphaeridium

and has quoted his earlier reports (Nautiyal, 1983; 1984; 1986) on

microfossils of Son Valley and Banda district, TJ.P.

The authors do not agree with Dr. Nautiyal's remarks. His main argument

is that since the thickness of the wall of spberiods is between 2 - 7,urn, they cannot

be identified as Trachysphaeridium. The identification of Trachysphaeridium is based.

on diagnostic characters given by Timofeev (1966). The diagnostic characters are:

wide diameter range, robust and thick wall and reticulate surface texture. Wall

may be smooth. wrinkled or cracked. Trachysphaeridium was originally described

from the material recovered by maceration technique. The form under discussion

from the Chitrakut area has been described from the petrographic thin sections of

black bedded chert, where the preservation of the form is excellent which enabled

us to measure the thickness of the wall. The form has been compared only with.

those forms which are reported from the petrographic thin sections by Knoll and

Calder (1983) from the Rysso Formation with diameter range of 10 - 210 ,urn and

by Knoll (1984) from the Hunnberg Formation with diameter range of 80 - 350 .urn.

The thickness of the wall has not been mentioned anywhrre except that it is thick

and robust.

It

is quite logical to accept a wall thickness of 7 ,urn as 'thick' for

spheroids of more than 200 ,urn.

Since the present report is based on the study of only petrographic thin

sections, the paper mentioned by Dr. Nautiyal vide Nautiyal (1983) was not

referred as the report is based on material recovered by maceration technique

where syngenecity and indigenousness of the recovered fossils are always a

matter-of doubt. The photomicrographs published by Nautiyal (1984 and 1986) are matter-of

such poor quality that nothing can be inferred and no conclusion can be drawn

on the basis of published photographs.

Department of Geology,

Luckllow University.

. Lucknow, U. P.

References

S. KUMAR

PURNIMA SRIVASTAVA

KNOLL, A. H. (1984) Microbiolas of the late Precambrian Hunnberg Formation, Nordaustlandet, Svalbard. Jour. Palaeon., v. 58, pp. 131-162.

KNOLL, A. H. and CALDER, S. (1983) Microbiotas of the Late Precambrian Rysso Formation, Nordaustlandet, Svalbard. Palaeontology, v. 26, pp.467-496.

NAUTlYAL, A. C. (983) Algonkian (Upper to Middle) micro-organisms from the Semri Group of Son Valley (Mirzapur district). India. Geosc. Jour. v. 4 (2). pp. 169-198.

- - (1984) Discovery of Algonkian (Upper to Middle) micro-organisms from Semri Group at Sangrampur Hill, Banda district, India. Geosc. Jour. v. 5 0), pp. 81-86.

- - (l986) Lower Vindhyan (Algonkianj microflora (microfauna) and biostratigraphy of Sang ram pur Hill, Banda district. Northern India. Geosc. Jour., v. 70), pp.1-22.