Lead Isotope Signature Of The Proterozoic Sediment-Hosted Base ...

Ceol. Bull. Univ. Peshowar, Vol. 25, pp. 59-65, 1992MOHAMMAD TAHIR SHAH1, RALPH I. THORPE2 &SHAMIM AHMED SIDDIQUE3.I NCE in Geology, University of PeshawarGeological Survey of Canada, Ottawa, Ontario, CanadaDepartment of Geology, University of South Carolina, USAABSTRACT: Protero~oic base metal deposits in the rocks of ihe Indian plate occur alongits northern margin at the Parang and Lahor propert ies near Besham. 711 ese are stratiforrnexha(ative type deposits which are highly deformed and regionally metamorphosed. Leadisofope szudies were carried out on these deposits. Lead isotope ratios yield model ages cf2120-2199 Ma (Prozerozoic) for both he Parang and Lahor deposits. The assumed minitnulnage of ,nineralization is 2077 Ma, as suggested by the secondary isochron. These deposirsare ,nore primitive in term of age and have lower p values relative to the well knownsediment-hosted deposizs elsewhere in the world. The non-radiogenic isotopic cornpositionand the homogeneity of the ore lead in bolh Pazang and Lalror ores suggest a well-mixedcommon source and siinilar processes for the formation of these ares.INTRODUCTIONSediment-hosted base metal (Zn, Pb, andCu) mineralization in the rocks of the Indianplate occurs along its northern margin in thefootwall of the Main Mantle thrust (MMT)around Besham. The rocks of the Indianplate, south of MMT, have a complex tectonometamorphichistory with polyphasedeformation and metamorphism. These rockshave been divided into the Besham, Pazangand Karora groups. The first two groups arepart of the basement complex and the third iscomposed of a cover of metasediments thatunconformably overlie the basement gneisses(Fletcher et a]., 1986; Treloar et al., 1989).The base metal mineralization is confined tothe Pazang group (Fig. 1), a supracrustalsequence of metasediments, which has beenthrust over the basement gneisses of theBesham group (Fletcher et al., 1986).The rocks of the Pazang group areseperated into Pelitic Formation, SulfideFormation, and Metaquartzi te/Meta-arkoseFormation from bottom to top (see Fletcheret al., 1986; Shah, 1991). The two areas,Lahor and Pazang, which are of prime importancefor their base metal mineralization,are situated about 1370 meters and 1680meters above sea level on western and easternside of the Indus River, respectively(Fig. 1). The Sulfide Formation contains theZn-Pb mineralization and constitutes the mostdistinct portion of the Pazang group in bothof these localities. The base metal mineralizationin the Besham area has been consideredto be of different genetic types: (a) skarntype (Ashraf et a]., 1980), (b) derived frommetamorphitic rocks and remobilized duringthe process of anatexis (Butt, 1983), and (c)stratiforrn exhalative type (Fletcher et a1 . ,1986; Shah, 1991).The lead isotope compositions af oresprovide a powerful tool in solving the gtoblemsof petrogenesis and metallogenesis. Theisotopic composition of lead in galena remainsunchanged after its initialcrystallization because of its ex trernely lowU/Pb and Th/Pb ratios. The lead isotoperatios of galena are, therefore, used to calculatethe model ages of the deposits in whichgalena crystallized.Various lead evolution msdels have beenproposed in the past (see Holmes, 1942;Houtermans, 1946; Stacey and Kramets,1975; Afifi et al., 1984; Deb et al., 1989).The Plumbotectonics model is another irnportantcontribution by Doe and Zartrnan

EXPLANATION .Besham Group~elitic, Psammiticschist and Gneisses. . calcareous, Graphitic......Banded GneissMica schist withquartz nodulesGraphite augen gneissand pegmatite gr~eissPazang GroupSulfide-rich Quartzite,Carbonate, Calc-silicateClinopyroxeni te andQuartzo-feldspathic rocks2) Pb, Zn & Fe sulfidemineralizationL-J FoliationA Compositional layering/ Lineation crenulationA Village, Poak in rnelwsLahor Granitic Complex- - Fault-Contacts-- -. Inferred contacts- .- Karakoram highway. -. . . . . . . . . . . StreamsFig. 1. Geological map of the area around Besham, northern Pakistan.60

Fig. 2. =07pb/2wPb diagram for the massive sulfide ore leads from the Lahor and Pazang prospects. The primarygrowth curves are after Staccy and Kramers (1975) and Cumming and Richard (1975).(1979) and Zartman and Doe (1981) in thesystematics of Pb isotopes. In this modelthey presented parameters for three geologicenvironments or reservoirs (1) mantle, (2)upper crust, and (3) lower continental crust,which have appreciable amount of U, Th,and Pb. They interpreted another geologicenvironment (orogene) which is responsiblefor the gross terrestrial isotopic patterns.The plumbotectonics model, therefore, dealtwith restricting the fields for average modernmantle, upper crust, lower crust andorogene lead.The Stacey and Kramers (1975) modelyields reasonable age estimates and is particularlyapplicable for galena lead fromPrecambrian massive sulfide deposits thatare syngenetic with the host rocks (see Staceyet al., 1977). The Proterozoic model hasyielded reasonable estimates of model agesfor the sediment-hosted base metal depositselsewhere in the world ( Deb et al., 1989).Both these models are, therefore, used to getreliable estimates of the ages of the Beshambase metal deposits and to correlate thesedeposits with the host metasediments.RESULTS AND INTERPRETATIONThree samples from Lahor and two samplesfrom Pazang massive sulfide ores, severalkilometers apart, were collected from undergroundworkings for Pb-isotope studies. Thelead isotopic compositions were measured inthe mass spectrometry laboratory of the Departmentof Physics, University of Alberta,Edmonton, Canada. The ratios are normalizedto the National Bureau of Standard 981common lead standard, The accuracy of theanalyses for Pb isotope composition of galenais in the range of 20.The Pb-isotope analyses, along with computedmodel ages and their p values, arelisted in Table 1 and plotted in conventionalPb-isotope diagrams in Figures 2-6. Thelead isotope ratios are non-radiogenic andvery similar in both the Lahor and Pazangprospects, This isotopic uniformity and the207Pb/204Pb ratios, which are close to 15.10,are characteristic of well mixed sediments.This isotopic homogeneity of the ore leadsin both the Pazang and Lahor deposits suggeststhat (a) these ores were formed bysimilar processes, and (b) the lead wasderived from a common source which wasisotopically well mixed. The homogeneousand non-radiogenic nature of the studiedgalena is in accord with the pattern for sediment-hostedexhalative deposits (Large,1981).

Fig. 3. m8PbPPb diagram far the massive sulfidc oreleads from the Lahorand Pazangprospects . Theprimary growth curves are after Stacey andKramers (1975) and Cumming and Richard(1975).The model ages of the Pazang and Lahordeposits have been calculated (Table 1) accordingto the Stacey and Krarners (1975)model and the Proterozoic model of Deb etal. (1989). The Proterozoic model yieldsages about 70 million years higher than thoseof the Stacey and Kramers model. Averagemodel ages of 2 120 2 34 Ma (according tothe Stacey and Kramers model) and 2 199 &35 Ma (according to the Proterozoic model)are obtained for these deposits. No isotopicages have yet been determined for the hostand underlying basement gneisses: however,a Proterozoic age is assigned to both the hostrocks and the underlying basement gneisseson the basis of field observations (see Fletcheret al., 1986; Treloar et al., 1989). The orebodies and the host rocks were subjected toFig- 4- An expanded plot of the isotopic compositionsof the Lahor and Pazang massive sulfide oreleads.thermal and regional metamorphism, but it isunlikely to say that the isotopic compositionsof the deposits were affected bymetamorphism in a way to completely resetthe age.Doe and Zartman (1979) found that leadderived from most upper crustal sedimentaryrocks (that have not been involved in orderived from granulite or high grade metamorphicrocks) by lateral secretion processesplot close to or above the Stacey and Karamers(1975) evolution curve in the 207Pb1204Pb vs206Pb120JPb diagram and close to or below theevolution curve in their '08Pb/204Pb vs 706Pbl?04Pb diagram. The lead isotope analyses forthe Pazang and Lahor ore bodies plot veryclose to the evolution curve in 207Pb1'04Pb vs206Pb1204Pb diagram (Fig.2) and above theevolution curve in 208Pbl'04Pb vs 206Pb/104Pbdiagram (Fig.3) of Stacey and KramersTABLE 1. LEAD ISOTOPE COMPOSlTIONS AND MODEL AGES OF STRATIFORM ZINC-LEAD DEPOSITSFROM LAHOR AND PAZANG PROSPECTS, BESHAM, NORTHERN PAKISTAN1 a2oaPbPMPb 207PbP03Pb 2oRPb/?hiPb Model age p Model age p- - - - - - - - - I - - - - -Lahore deposit 14.821 15.106 34.581 2154 9.69 2234 10.16Pazang deposit 14,844 15.096 34.5 17 2124 9.58 2201 10.02Model ages and p values @resent day YJPMPb of the lcad source) according to (1) the Stacey & Kramers (1975) model. and (2) the adjusted Proterozoic model of Dcb st al. (1989).

Fig .5. Isotopic composition of Lahor and Pazang massive sulfide ore lead in relation to growth curves ofplumbotectonic model (version ii, Zartman and Doe, 1981). Approximate primary isochrons arc afterJohansson and Rickard, 1985. Other sediment-hosted deposits also shown for comparison are taken fromDeb et al. (1989). These are: (1) Koongie Park, (2) Broken hill, (3) Mt. Isa, and (4) McArthur River inAustralia (Gulson et al., 1983; Gulson, 1984, 1985); (5) Sullivan in Canada (LeCoutcur, 1973); (6) Balmatdistrict in United States (Fletcher, 1979; Fletcher and Farquhar, 1983,); (7) Gamsberg and (8) Aggencys inSouth Africa (Koeppel, 1980); (9) Rampura Agucha and (10) Rajpura-Dariba in India (Deb et al., 1989).(1975). One explanation for the slightlyhigher 208Pb/204Pb values could be that theyreflect derivation of the lead, in part, from asource terrane that contained rocks subjectedto granulite facies or high grade metamorphismat asignificantly earlier time in thePrecambrian (see Wedepohl et al., 1978).Such a metamorphism causes a preferentialloss of U relative to Th and, therefore, yieldshigh Th/U ratios (Doe and Zartman, 1979).The high 208Pb/204Pb ratios, therefore, implylead evolution in a source material with ahigher average ~h/Pb ratio than assumed in~tacey and Kramers model or an episodicincrease of the Th/Pb ratio as a result of highgrade metamorphism.Four out of five data points form a verylinear array with a slope of 0.258 on the plotof 207Pb/204Pb vs 'O6PblX4Pb (Fig.4). Thislinear relationship implies that a small radiogeniccomponent was added to those samplesat a commen time. Tf this line is consideredto be a true secondary isochron, then thecalculated 2077 Ma (tain) age is a minimumage for the source of the radiogenic leadcomponent, most likely the host sedimentaryrocks, and also for the mineralization if thisis indeed syngenetic. This is also the maximumage for remobilization and addition ofa radiogenic lead component. If an age of2200 Ma is assumed for the mineralizationand for the source of the small radiogeniclead component added to at least most of theore specimens, two stage calculation yieldsan age of about 1950 Ma for the radiogenicaddition and probabTe metamorphicremobilization of the ore. It can be concludedfrom the Pb-isotope data of the Beshamore bodies that: (a) the component of ordinarylead was separated from its sourcereservoir 2120 f 33 Ma (according to themodel of Stacey and Kramers, 1975) or 2 199f 35 Ma (according to the Proterozoic modelof Deb et al., 1989); (b) small componentsof radiogenic lead were then subsequentlyadded to the common lead sometime after2077 Ma. The data further suggest that therewas probably little or negligible time differencebetween the age of the associated rocksand the time of mineralization.

Fig .6. Isotopic composition of Lahor and Pazang massive sulfide ore lead in relation to growth curves of theplumbotectonic model (version ii, Zartman and Doe, 1981).In terms of the general plumbotectonicmodel of Stacey et al. (1977), Doe andZartman (1979), and Zartman and Doe(1981), the Pazang and Lahor lead isotopedata plot between the evolution curves foraverage orogene and average mantle on the'07Pb/'04Pb vs 206Pb/204Pb diagram (Fig.5) andbetween average orogene and average lowercrust on the "O"Pb/'04Pb vs 106Pb/104Pb diagram(Fig.6). The model ages by theplumbotectonics model are greater than 2000Ma, which is consistent with the calculatedmodel ages given in Table 1 for these depos-:,nI La.The lead isotopic compositions of theBesham ore bodies are strikingly differentfrom those of Mississippi Valley Type (MVT)deposits. The206Pb/204Pb ratios are > 19 forMVT deposits and < 15 for Besham orebodies, and similarly the 208Pb/204Pb ratiosare > 39 for MVT deposits and < 35 for thewhere in the world.CONCLUSIONSThe Lead isotopic compositions give Proterozoicmodel ages for the base metal depositsof the Besham area. The non-radiogenic natureand isotopic uniformity of the ore leadare suggestive of an isotopically well mixedsource for these deposits.Acknowledgment: Dr. George Cumming ofthe Department of Physics, University ofAlberta, Edmonton (Canada) is gratefullyacknowledged for determining the lead isitopiccompositions of the Besham base metaldeposits. We are very thankful to the SarhadDevelopment Authority (Mineral Developmentwing) for providing field assistance tothe senior author. 'Besham ore bodies. In order to compare theREFERENCESisotopic compositions of the studied depos- Afifi, A., Doe, B.R., Sirns, P.K. & Delevaux,its with other type of deposits elsewhere in M-H.. 1984. U-Th-Pb isotope chronolog~ ofthe the pb-isotope compositions of sulfide ores and rocks in the Early Proterovariouswell known sediment-hosted basezoic metavolcanic belt of northern Wisconsin.Econ. Geol., 79, 338-353.metal deposits have been plotted in FigureAshraf, M., Chaudhry, M.N. & Husain, S.S.,5- The Besham deposits are more primitive 1980. General geology and signifiinterm of ages, and also in terms of deriva- cance of the Lahore ~ranite and rocks of thetion from a source with a low p value, than southern ophiolite belt in the Allai-Kohistanare the other sediment-hosted deposits else- area. Geol. Bull. Univ. Peshawar, 13, 207-

213.Butt, K. A,, 1983. Petrology and geochemicalevolution of Lahor pegmatite complex, northernPakistan, and genesis of associated Pb-Zn-Mo and U mineralization. In: The Granites ofHimalaya, Karakoram and Hindu Kush (F. A.Shams, ed.). Inst. Geol. Punjab Univ., Lahore,309-323.Cumrning, G. L. & Richards, J. R., 1975. Orelead isotope ratios in a continuously changing-.- earth. Earth. Planet. Sci. Lett., 28, 155-171.Deb, M., Thorpe, R. I., Cumming, G. L. &Wagner, P.A., 1989. Age, Source and StratigraphicImplications of Pb Isotope Data forConfirmable, Sediment-hosted, Base MetalDe~osits in the Proterozoic Aravalli-DelhiOrogenic Belt, Northwestern India. Precam.Res., 43, 1-22.Doe, B. R. & Zartman, R. E., 1979.Plumbotectonics 1, the Phanerozoic: In: Geochemistryof hydrothermal ore deposits (2ndedition), (H. L.Barnes, ed .) Holt, Rinehartand Winston, New York. pp.22-70.Fletcher, I. R., 1979. A lead isotope study oflead-zinc mineralization associated with thecentral metasedimentary belt of the GrenvilleProvince. Unpubl. Ph.D. thesis, Univ. Toronto.Fletcher, I. R, & Farquhar, R. M., 1982. Theprotocontinental nature and regional variabilityof the central metasedimentary belt ofthe Grenville Province. Lead isotope evidence.Canad. J. Earth. Sci., 19, 239-253.Fletcher, C. J. N., Leake, R. C. & Haslam,H.W., 1986. Tectonic setting, mineralogy,and chemistry of a metamorphosed strati formbase metal deposit within the Himalayas ofPakistan. J. Geol. Soc. Lond., 143,521-536.Gancarz, A. J. & Wasserburg, G. J., 1979.Initial lead of the Amitsoq gneiss, WestGreenland and implications for the age of theearth. Geochim. Cosmochim. Acta, 41,1283-1301.Gulson, B. L., 1984. Uranium-lead investigationsof minerals from the Broken Hill lodesand mine sequence rocks. Econ. Geol., 79,476-490.Gulson, B. L., 1985. Shale-hosted lead-zincdeposits in northern Australia: Lead isotopevariations. Eco. Ged., 80, 2001-20 12.Gulson, B. L., Perkins, W. G., & Mizon, K. J.,1983. Lead isotope studies bearing on thegenesis of copper ore bodies at Mount Isa,Quensland. Eco. Geol., 78, 1466-1504.Holmes, A., 1946, An estimate of the age of theearth. Nature, 157, 680-684.Houtermans, F. G., 1946. Die Isotopenhaufigkeitenim naturlichen Blei und das Alter desUrans. Naturwissenschaften, 33, 185-186.Johansson, A. & Rickard, D., 1985. Some newlead isotope determinations from the Proterozoicsulfide ores of Central Sweden.Mineral. Depos., 20, 1-7.Koeppel, V., 1980. Lead-isotope studies ofstrati form ore deposits of the Namaqualand ,NW Cape Province, South Africa and theirimplications on the age of the Bushmanlandsequence. Proceedings of the Fifth QuadrennialInternational Association on the Genesisof Ore Deposits Symposium, Schweizerbart'sche Verlagsbuchhandlung Stuttgart., 195-207.Large, R. R., 1977. Chemical evolution andzonation of massive sulfide deposits in volcanicterrains. Econ. Geol., 72, 549-572.Large, D. E., 1981. Sediment-hosted submarineexhalative lead-zinc deposits, A reviewof their geological characteristics and genesis.In: Handbook of strata-bound andstratiform ore deposits (K. H. Wolf, ed.), 9,469-507.LeCouteur, P. C., 1973. A study of Lead Isotopesfrom mineral deposits in SoutheasternBritish Columbia and in the Anvil Range,Yukon Territory. Unpubl. Ph.D. thesis. Univ.British Columbia.Shah, M. T., 1991. Geochemistry, mineralogy,and petrology of the sulfide mineralizationand associated rocks in the area around Beshamand Dir, Northern Pakistan. Unpubl. Ph.D.thesis. Univ. South. Carol. Columbia.Stacey, J. S., Doe, B. R., Silver, L. T. &Zartman, R.E., 1977. Plumbotectonics 11 A,Precambrian massive sulfide deposits. In:Geochronology and the problems of ore formations(S. F. Karpenko, ed.; in Russian).US. Geol. Sur. open-file rep. # 76-476, 26p.Stacey, J. S. & Kramers, J. D., 1975. Approximationof terristrial lead isotope evolution bya two-stage model. Earth. Planet. Sci. Lett. ,26,207-221.Treloar, P. J., Coward, M. P., Williams, M. P.& Khan, M. A., , Basement-cover im-brication south of lg8$ th Main Mantle Thrust,north Pakistan: Geol. Soc. Amer. SpecialPap., 232, 137-152.Zartman, R. E. & Doe, B. R,, 1981.Plumbotectonics - the mode. Tctonophysics,75, 135-162.