Mid-Proterozoic Pb–Pb ages for some Himalayan base-metal deposits and comparison to deposits in Rajasthan, NW India

Precambrian Research 99 (2000) 171–178 www.elsevier.com/locate/precamres

Mid-Proterozoic Pb–Pb ages for some Himalayan base-metal deposits and comparison to deposits in Rajasthan, NW India S.C. Sarkar a, *, I.V. Chernyshev b, H. Banerjee 1 a a Department of Geological Sciences, Jadavpur University, Calcutta 700032, India b IGEM, Academy of Sciences, Moscow 109017, Russia Received 4 November 1998; accepted 6 August 1999

Abstract Lead isotope ratios from three base metal deposits, Rangpo, Gorubathan in the eastern Lesser Himalaya and Bageswar in the Lesser Himalaya of Kumaun have been studied. The first two deposits, generally stratiform and stratabound, occur within pelitic, psammopelitic/wacke type metasediments which are locally carbonaceous. The Bageswar deposit is hosted by silicified dolomitic carbonate rocks. The Pb-isotope ratios appear to be homogenous in two and heterogenous in one of the deposits. Model Pb–Pb ages obtained for the Rangpo and Gorubathan deposits are 1800 Ma and for the Bageswar deposit, 1550–1700 Ma. Dispersion in isotopic ratios reflect the geochemical heterogeneity, particularly with respect to m. Larger variation in the composition of Pb-isotope ratios in the Bageswar deposit may be explained by the participation of Pb from heterogenous sources. Pb-isotope characteristics of the Himalayan deposits are compared with those of a number of base metal deposits in the Delhi–Aravalli belt in Rajasthan, NW India. Ore lead from the deposits of both the Himalaya and of Rajasthan are enriched in 206Pb and 207Pb. However, lead in all cases plots on the evolution lines, corresponding to sources with higher U/Pb ratios. But the sources of lead in the Himalayan deposits had a lower Th/U ratio. The geologic observations as well as the Pb-isotope compositions are consistent with a sedimentary-diagenetic origin of the Himalayan base metal deposits. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Base metal deposits; Comparison with Rajasthan deposits; Lesser Himalaya; Model Pb-ages; Northwest India

1. Introduction Over recent decades earth scientists have taken more interest in the geology of the Himalayas. These studies pertain mostly to tectonics and accompanying magmatism and metamorphism. Their results shed light on the closing of the * Corresponding author. Fax: +91-33-473-1484. E-mail address: [email protected] (S.C. Sarkar) 1 Present address: INDAL, 1 Middleton Street, Calcutta 700017, India.

southern (Neo) Tethys and the formation of present-day Himalaya in the Tertiary. However, in the Himalaya there are older rocks and structures which resemble many of those found in the northern part of the Indian peninsula [Auden, 1935; Krishnan and Swaminath, 1960; Gansser, 1964, 1993; Valdiya, 1975]. The history of these rocks and how they relate, if at all, to the peninsular geology is still poorly understood. In this context lead-isotope studies of a few base metal deposits hosted within Proterozoic rocks of the Lesser Himalaya were undertaken to gain a better insight

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into the mineralization in the Himalaya, as well as to compare it with the similar mineralizations in Peninsular India, particularly in Rajasthan. From the North to the South, the Himalaya is divided into several tectonolithologic zones: the Trans-Himalaya (or the Lhasa block); the IndusTsangpo suture zone; the Tibetan Tethys; the High Himalaya (Central Crystalline Complex); and the Lesser or Lower Himalaya. Some researchers do not distinguish the Tibetan Tethys (Crawford, 1974) while some refer to the southern-most strip of the Himalaya, occupied by the Siwaliks and the equivalent, as the sub-Himalaya (Gansser, 1964) (Fig. 1). From the east to the west, the common Himalayan divisions are the Eastern Himalaya (including NEFA Himalaya, Bhutan, Darjeeling and Sikkim), Nepal Himalaya, Kumaun Himalaya and the Punjab Himalaya (including Kashmir Himalaya). The High Himalaya is separated from the Lesser Himalaya by the Main Central Thrust (MCT ). The Lesser Himalaya consists of sediments and lenticular bodies of gneissic rocks of Proterozoic, Paleozoic and locally Mesozoic age. It is thrusted over the molassic Tertiary Siwalik rocks of the Sub-Himalaya along a major discontinuity called the Main Boundary Thrust (MBT ). The southern margin of the Siwaliks is marked by another thrust surface called the Main Frontal Thrust (MFT ) (Fig. 1). Of the three ore deposits discussed here, two (Rangpo and Gorubathan) are located in the Lesser Himalaya of the Sikkim–Darjeeling area and the other (Bageswar) in a more or less corresponding stratigraphic position in the Kumaun Himalaya (Fig. 1).

2. Geology of the ore deposits The Rangpo and Gorubathan deposits are located at a distance of <100 km, while their distance from the mineralization site at Bageswar, Kumaun Himalaya is 750–800 km. All these are basemetal deposits with varying proportions of lead in the form of galena (Pb≥Zn≥Cu). The rocks hosting the ore deposits in the Rangpo and Gorubathan area belong to the Daling Group, an important unit in the eastern Lesser Himalaya. The sediments belong to the

psammopelitic and wacke type lithofacies. Metapelites are locally carbonaceous. Metabasites also feature in the association. The grade of metamorphism in the area containing the two deposits has not exceeded upper greenschist facies, whereas in the adjacent area of Darjeeling and Sikkim metamorphism of amphibolite facies has been attained. In the latter area the progressive metamorphism is thought to have occurred in two phases: the earlier one Precambrian and later, Tertiary, related to the main phase of Himalayan Orogeny (cf Ray, 1947; Sinha Roy, 1979; Searle et al., 1987; Barnicoat and Treloar, 1989; Windley, 1983; Neogi et al., 1998). Sparse isotopic data of Darjeeling metapelitic rocks reveal two distinct sets of ages: one upto at least 1100 Ma whole rock, and ‘mineral ages’ (Rb/Sr) (Crawford, 1974) and the other 18–91 Ma ( Ermenko and Datta in Ghosh and Singh, 1977). While the concentration of data in the range of 18–91 Ma may be attributed to remobilization resetting during the Tertiary metamorphism, the older ages point to a Precambrian event. Granite gneisses in the neighbourhood give ages belonging to both the Precambrian and the Tertiary (Acharya, 1979; Le Fort, 1975; Paul et al., 1996). In the Rangpo area there are two orebodies the ‘Bhotang Main Lode’ (Lode 1) and the ‘Subsidiary Lode’ (Lode 2). The former is traceable over a strike length of 250 m and proved through a vertical distance of 275 m. The ore bodies are generally concordant with the deformed rocks that host them [Mukherjee and Dhruba Rao, 1974; Sarkar and Banerjee, 1976(1986)]. Nevertheless, local translocation through flowage of ores during post-depositional intense deformation (and metamorphism) is noticeable. The immediate host rocks are carbonaceous phyllites, garnetiferous chlorite schists, chloritic quartzites (meta-wackes) and a band of amphibolite towards the hanging wall side of the ore zone. A crude zonation of the orebody with respect to Cu-, Pb-, Zn- and Fe-sulphides is noticeable: Cu- and Fe-sulphides dominate in the northeastern part while Pb–Zn are enriched in the southwestern part. No ‘wall rock alteration’ of any kind is observed. In the Rangpo deposit the principal ore minerals are pyrite, pyrrhotite and galena. Magnetite is important at places, where

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Fig. 1. A sketch geological map of the Himalaya and northern peninsular India (after Gansser, 1993). 1, Alluvium; 2, Pre-Gondwana basement rocks; 3, Gondwanas; 4, Mesozoic platform sediments on Indian Shield; 5, Deccan Trap; 6, Molasse-type sediments (SubHimalayan Siwaliks); 7, Lesser Himalayas; 8, Crystalline High Himalaya; 9, Tethyan sediments (platform); 10, Flysch facies; 11, Ophiolites (including ophiolitic melanges); 12, Trans-Himalayan plutons; 13, Tertiary and Quaternary volcanics. MFT, Main Frontal Thrust; MBT, Main Boundary Thrust; MCT, Main Central Thrust; SZ, Suture Zone. Ore deposits: +, Bageswar; #, Rangpo; $ Gorubathan.

magnetite-dominant layers may alternate with sulphide-dominant layers. But magnetite never occurs to the exclusion of the sulphides. Subordinate phases include tetrahedrite, cubanite, arsenopyrite

and glaucodot. Ores show annealing fabric except where affected by a later brittle type deformation (Sarkar and Banerjee, 1976). The Gorubathan deposit consists of two main

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orebodies, the Daling Kot Lode 1 and Lode 2 (Fig. 2). A magnetite–quartzite horizon also occurs above the mineralized zone, but not in physical contact. The orebodies are hosted by a rock association of sericitic chlorite–quartz schists, garnet (almandine)-bearing chlorite–quartz schists, carbonaceous phyllite and sub-ordinate basic volcanics. Compositional banding in the ore is common. Wall-rock alteration is absent. Principal ore minerals are magnetite, galena, pyrite and sphalerite. Minor phases are pyrrhotite and chalcopyrite. These ores are also annealed (Sarkar and Banerjee, 1976, 1986). In the Bageswar area of Kumaun Himalaya, particularly at Shisakhani, Channa Panni and Bilamu, lead sulphide mineralization occurs in the silicified dolomitic rocks of the Garhwal Series (Group) ( Krishnan, 1958), commonly believed to be equivalent to the Daling Group in the eastern Himalaya (cf Le Fort, 1975). The principal ore mineral is galena. Minor phases are pyrite and chalcopyrite. The ores are metamorphosed (Banerjee, 1977; Sarkar, unpublished work). The host rocks are metamorphosed to middle greenschist facies. Suggestions on the origin of the Rangpo and

Gorubathan deposits vary between epigenetic hydrothermal (Dar, 1968; Ghosh, 1975; Mukherjee and Dhruba Rao, 1974), to sedimentary-diagenetic (Sarkar and Banerjee, 1976; Ray, 1975, 1976). The first group of workers studied the Rangpo deposit, while Ray studied the Gorubathan deposit. The hydrothermalists sought the source of ore solutions in the neighbouring granite gneisses. Dar (1968) even suggested that the Rangpo deposit is an extension of the Singhbhum copper uranium belt. Sarkar and Banerjee (1976) and Banerjee (1979) favoured a sedimentary-diagenetic (SEDEX type) origin for both the deposits. The Bageswar deposit appears to be originally of ( Exhalative?) sedimentary-diagenetic origin (Sarkar et al., unpublished work).

3. Isotopic studies of lead Ten samples of galena, three from the Bageswar, four from Rangpo and three from the Gorubathan deposit were analysed in the Isotopic Geochronology Laboratory of IGEM, Russian Academy of Sciences, Moscow, for the isotopic

Fig. 2. A transverse section showing the mode of occurrence of the orebodies, Gorubathan deposit.

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composition of lead. Analyses were done in a single collector Mass Spectrometer, Model ML-1320, fitted with an automatic data-processing system (Chernyshev et al., 1983). In each case a galena crystal of ca 0.5 mm size was chosen. The grain was dissolved in water and the aliquot containing 0.3–0.5 mg Pb was loaded together with silica gel ionised from a Re-filament. Isotopic ratio measurements were corrected for fractionation using NBS standard SRM 981 as a reference. Standard deviation (2s) of the measurement of the ratios 206Pb/208Pb, 207Pb/204Pb and 208Pb/ 204Pb in the galena samples depended on the difference of mass and ranged between 0.1 (DM= 2) and 0.15% (DM=4).

4. Results and discussion Results of the analyses are shown in Table 1 and Fig. 3. In Table 1 the model ages are listed following the model of Stacey and Kramers (1975) and the ‘Proterozoic model’ utilised by Deb et al. (1989) in interpreting Pb-isotopic composition of the stratiform polymetallic deposits of Rajasthan, NW India. In the 206Pb/204Pb–207Pb/204Pb dia-

gram, data are compared with Pb-isotope growth curves of Stacey and Kramers (1975) (S–K, m =9.74 and W =36.84) and Zartman and Doe 2 2 (1981). The lead is characterized by high m (S–K 2 model ) with 208U/204Pb at 10.13–10.63. The isotopic composition of lead in each of the deposits is not homogenous. Variations as high as 0.75% were observed. As can be seen from Fig. 3, the model Pb–Pb ages [modified by Stacey and Kramers (1975)] for the Rangpo and Gorubathan deposits vary only between 1767–1819 and 1790– 1843 Ma, respectively. The mean model ages for these deposits 1789±22 Ma (Rangpo) and 1811±28 Ma (Gorubathan) agree with each other within the analytical uncertainty. One of the three analysed samples from the Bageswar deposit (sample No. 2) is enriched in 206Pb by ca 1.2%, compared to the two others and has a lower model age of 1553 Ma. The other two samples have similar 206Pb contents and ages older by >100 my than this ore ( Table 1). According to the isotopic composition, the samples from the sulphide deposits of Rangpo and Gorubathan on the one hand and those of Bageswar on the other, thus define two groups. The difference of the model ages

Table 1 Isotopic composition of lead in galena from the ore deposits of Bageswar, Rangpo and Gorubathan, Lesser Himalaya, India [standard deviation (2s) ranging between 0.1 (DM=2) and 0.15% (DM=4)] No.

Ore deposits

Specimen

1 2 3 4

Bageswar Bageswar Bageswar Rangpo

B6 - 1 B6 - 2 B6 - 3 NAFW/2/R/8

5

Rangpo

MAHW/17/B/7

6

Rangpo

NAHW/17/R/6

7

Rangpo

NAB-3/P/5

8 9 10

Gorubathan Gorubathan Gorubathan

D /6/4 2 K5/6/1 S/31/6/3

Characteristics

Carbonate-hosted PbSulphide deposit Stratiform ( locally stratified Metamorphosed polymetallic Sulphide deposits hosted by Pelitic/psammopelitic Metasediments

206Pb/ 204Pb

207Pb/ 204Pb

208Pb/ 204Pb

Stacey–Kramers model (1975) t

m

2

‘Proterozoic model’ (1989) Th/U

t

m (238U/ 204Pb)

16.240 16.424 16.236 15.818

15.551 15.555 15.541 15.400

35.956 36.051 35.941 35.439

1683 1553 1672 1779

10.67 10.48 10.61 10.20

3.88 3.80 3.87 3.79

1732 1608 1722 1839

10.84 10.68 10.79 10.46

15.822

15.410

35.489

1791

10.26

3.83

1850

10.51

15.876

15.457

35.250

1819

10.51

3.58

1872

10.73

15.805

15.386

35.402

1767

10.12

3.77

1830

10.39

15.948 15.858 15.830

15.511 15.428 15.420

35.606 35.502 35.509

1843 1790 1800

10.79 10.33 10.32

3.85 3.82 3.85

1889 1847 1857

10.97 10.58 10.56

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Fig. 3. Lead isotope evolution curves and isochron related to some Indian base metal deposits [compare Deb et al. (1989), Fig. 4]. S–K, Evolution curve according to Stacey–Kramers model (1975) ( m =9.74). A–D, Evolution curves according to Zartman–Doe 2 Plumbotectonic model (1981). A, Mantle; B, orogen; C, upper crust; D, lower crust. Sample numbers of the Himalayan deposits correspond to numbers in Table 1. Z, Zawar group of deposits; R–R–S, Rampura–Agucha, Rajpura–Dariba and Saladipura deposits. A–D, Ambaji and Deri deposits.

between these two groups seems beyond the range of analytical error. A dispersion of 0.1% in the obtained value of 206Pb/204Pb and 207Pb/204Pb is due to statistical uncertainty of the measurement and to variation in isotope fractionation, the overall uncertainty resulting in the variation of ca 10 Ma of the model age. This, however, be noted that the true uncertainty of model ages is usually very much greater than 10 Ma, particularly for not very large epicontinental deposits. Normally Pb isotope signatures can be interpreted with confidence in terms of model ages only for larger conformable deposits. The Pb isotope ratios are a function of the whole prehistory of the source rocks which may be more or less complicated. The dispersion of the Pb-isotope ratios, considered either with respect to a single deposit or all the deposits together, may reflect the geochemical heterogeny in the source rocks, particularly with respect to the U/Pb ratio. Such heterogeneities appear perfectly plausible if we consider that the sources of the detrital material constituting the Proterozoic sediments in the given areas ca 1800 Ma ago were Archean, or early Proterozoic

continental crust. The presence of source rocks having different values of m (238U/204Pb) satisfactorily explains the character of isotope ratios of lead in the ore deposits of Rangpo and Gorubathan. The source of Th/U ratio appears to have been rather normal on the whole. Based on the obtained data of 1800 Ma for the deposits of Rangpo and Gorubathan, we may broadly compare it with the probable age of the host rocks, keeping in mind the possible inaccuracy of the model itself. Form the Table 1 it may be seen that the difference in age obtained with the model of Stacey–Kramer and the Proterozoic model comes to ca 35 Ma. This suggests that the age of the Daling Group and the temporally equivalent rocks is much older than ‘Late Precambrian’, mentioned by Gansser (1993). It may be interesting to compare the Pb-isotope characteristics of the Himalayan deposits under discussion with those of the stratiform base metal deposits of Rajasthan NW India, deposited in an apparently different geological situation. Most of these occur in the Delhi–Aravalli belts ( Fig. 1) which is underlain or bordered by an Archean

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gneissic complex (Banded Gneissic Complex, BGC ). The Pb-isotopic data of the Rajasthan deposits (Deb et al., 1989) may be divided into three groups: 1. 1800 Ma (Rampura–Agucha, Rajpura–Dariba, Saladipura); 2. ~1700 Ma (Zawar group of deposits); and 3. ~1100 Ma (Ambaji–Deri). Comparison of the Pb-isotope signatures in the Himalayan ores under discussion and those in the Rajasthan deposits show that the groups (1) and (2) of the Rajasthan deposits and the Himalayan deposits, are both characterized by: high m values; and similar model ages. Sources of lead in the Himalayan deposits, compared to the above mentioned Rajasthan deposits, had lower Th/U values. The principal isotopic criterion of the source of lead is the value of the parameter m. The obtained positions of m of all the deposits under discussion lie above the orogene curve ( Fig. 3), which according to the Zartman–Doe model represents evolution of lead in the reservoir with the mean composition of rocks in the Earth’s crust. The value of m for the given deposits lies in any model between 10 and 11 which, strictly speaking, does not exclude participation of lead from deep magmatic source below the Proterozoic basin, that is, sources characterized by the values of the m is the range of 9.5–10. This could possibly be basic volcanic members, or their direct derivatives such as graywacke. However, the high values of m decidedly point to the fact that the principal supplier of lead to the statabound deposits was crustal rocks of granitic composition, a conclusion supported by the lithology of the host rocks. The possible participation of detrital granitic material of lower Proterozoic or Archean age in the formation of Daling Group of rocks is supported by the Proterozoic reconstruction. In conclusion it may be said that although the model lead ages of ore deposits is not very exact, that is, semiquantitative, in case of PalaeoProterozoic sediment-hosted deposits they give a reasonably correlatable age of the host rocks (Stacey and Kramers, 1975). A more accurate age of the host rocks could be obtained with the help of U–Pb systematics of authigenic zircon (detrital zircon will give the age of the source rock only).

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The ore metals, zinc and lead in particular, were probably leached from the basinal pile (sediments±volcanics) by the basinal brine during burial and diagenesis, in cases extending upto the early stage of metamorphism. The source of zinc should be Fe–Mg minerals and that of lead, K-feldspar(s). The hydrothermal fluid thus produced discharged into the sea-bottom, or advected along stratification, guided by cross-stratal faults. No doubt there are a number of geological features that are common both in the Lesser Himalaya and in the northern part of Indian peninsula. However, it need not be imagined that the ores in the two segments are co-genetic, although they are nearly isochronous. It may be relevant to mention that some workers on similar deposits have suggested that ore metals in the basinal sedimentary–volcanic package may not be adequate and that a part of the metals may be refluxed by a convective hydrothermal system in the tectonized (so rendering it more permeable) granitoid–granite gneiss basement rocks (Russell, 1988; Russell et al., 1981; Strens et al., 1987). A corollary of the above work is that since the basemetal mineralizations along the Lesser Himalaya are Proterozoic, they should not be viewed in the context of Himalayan orogeny or any tectonics leading upto it [cf Fig. 11(H ), Mitchell and Garson (1981)].

Acknowledgements The authors are obliged to the Sikkim Mining Corporation Ltd and the Geological Survey of India (GSI ) for help and co-operation during the field-work. Figure 2 was constructed with the help of drill-core data of the GSI. They are also obliged to the administrations of Jadavpur University, Calcutta, and IGEM, Academy of Sciences, USSR (now Russia) for encouraging this collaborative work. Financial support to SCS and HB were available respectively from the UGC–SAP Programme at the Department of Geological Sciences, Jadavpur University and the CSIR, New Delhi. H.B. continued the work during his stint with the GSI. Finally, but not least, the authors are thankful to Drs J.D. Kramers and M.J. Russell

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for their meticulous and constructive reviews of the manuscript

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