Mineral resources of Kerala: A tectonic perspective M Santosh and E Shaji

Santosh M, Shaji E MineralresourcesofKerala:Atectonicperspective

MineralresourcesofKerala:Atectonicperspective

MSantosh1,2andEShaji3

1SchoolofEarthSciencesandResources,ChinaUniversityofGeosciencesBeijing,Beijing100083, China2FacultyofScience,KochiUniversity,Kochi7808520,Japan3DepartmentofGeology,UniversityofKerala,Kariyavattom,Trivandrum,IndiaEmail:[email protected]

Abstract.TheKeralaregionformsthewesternsegmentoftheSouthernGranuliteTerrainandpreservesthe records of major geological and tectonic events from Mesoarchean to late NeoproterozoicCambrian, correlatingwithglobalsupercontinentcycles.Ironoresandgoldarethemetallicresourcesassociatedwiththe oldest events in this region. The Paleoproterozoic and Neoproterozoic Cambrian magmatic and metamorphiceventsweretheprincipalsourcesoftheheavymineralsandsalongtheKeralacoastcarrying richrareearthdeposits.TheCambrianmetamorphismandcrustalmeltingalsogeneratedawidevarietyof gemstone,graphite,andminormolybdenitemineralization.Prolongedweatheringanddenudationledtothe accumulationof heavyminerals, goldandgemstones in placer deposits. Worldclass chinaclay, limited bauxitedepositsandligniteareamongtheotherresourcesinKerala.

Keywords: SouthernGranuliteTerrain;Keralaregion;Tectonicevolution;MineralResources,India

Introduction

TheKeralaregionformsthesouthwesternsegmentoftheIndianPeninsula. Geologically,theregionfallsoccurssouthoftheArcheanDharwarCratonwithintheSouthernGranuliteTerrain(SGT;seereviews inSantoshet al., 2009; Collins et al., 2014), a collageof crustal blocksdominated by high grade metamorphic rocks and magmatic complexes, dissected byshear/suture zones (Fig. 1). Fromnorth to south, the Kerala region covers parts of theMesoarcheanCoorgBlock(Santoshet al., 2013a), thedominantlyNeoarcheanNilgiri Blockincludingthesuprasubductionzonecomplexesalongitssouthernmargin(Samueletal.,2014;Santoshetal., 2013b),thenorthwesternandsouthwesternsegmentsoftheMaduraiBlockdominatedbyNeoarcheanPaleoproterozoicandMesoNeoproterozoicrocksrespectively(Plavsaet al., 2012; Collins et al., 2014), andtheLateNeoproterozoicCambrianTrivandrumBlock(Santosh et al., 2007, 2009). The Moyar, Bhavani, PalghatCauvery andAchankovil ShearZones,someofwhichareidentifiedastracesofmajoroceanicsutures(Collinsetal., 2007;Santoshetal.,2009)passthroughtheKeralasegmentoftheSGT.

Intermsofglobaltectonicsandthemajorsupercontinentcycles(Fig.2),theearliestcrustalgrowthintheKeralaregionhasbeentracedbacktoMesoarchean(dominantly3.2Ga, also recycled 3.6 Ga; Santosh et al., 2013a), vestiges of which are well preserved in theKasargodandKannoordistrictsformingpartoftheCoorgblock,andinterpretedasanexoticmicrocontinentwhichdidnotwitnessesanyofthelatertectonothermalevents(Santoshetal.,2013a). ThesesegmentswouldcorrespondtopartofthenucleioftheoldestsupercontinentUrproposedbyRogersandSantosh(2004).Themajorepisodeofcrustalgrowthoccurredataround2.5Ga,withextensivejuvenilemagmatismandpartlyreworkingoftheancientcrust, followedimmediatelybyhighgrademetamorphismintheearliestPaleoproterozoic,similartotheevents recorded in the adjacent Tamil Nadudomain(e.g., Clarket al., 2009). Recent investigationsreportedsuprasubductionophiolitesandfelsicvolcanictuffsfromAttappadi,

ShajiE&PradeepkumarAP(Eds)2014MineralResourcesofKeralaTrivandrum:DeptofGeologyUnivofKeralaISBN97881923449041


Fig.1GeologicalandtectonicframeworkofsouthernPeninsularIndiashowingthemajorcrustalblocksand interveningshear/suturezones(afterCollinsetal.,2014;Santoshetal.,2014).TheoutlineofKeralaState isalsoshown.

alongthesouthernmarginof theNilgiri BlocksuggestingactivesubductionaccretionintheNeoarchean(Santoshetal.,2013b,Praveenetal.,2013).ThenextmajortectoniceventinthisregionwasduringlatePaleoproterozoic,around2.0Gainvolvingbothjuvenilemagmatismand



extensivecrustalrecycling(e.g.,Krneretal.,2012;Plavsaetal.,2012).Theimprintsoflate

Fig.2(a)CompilationofglobalzirconUPbagesandtheircorrelationwithmajorsupercontinentassemblies (modifiedfromRoberts,2012). (b)MajorgeologicalandtectoniceventsintheKeralaregionthroughtime correlatedwithglobalevents(afterZhaiandSantosh,2011).



Paleoproterozoic events are preserved in thedifferent crustal blocks including Madurai andTrivandrum,andfurthersouthintheNagercoilBlockinTamilNadu.Itisbecomingincreasinglyevident that a largepart of thebasement rocks in this regionareof Paleoproterozoic age, floatingwithinthePanAfricanOceanofreworkedcrustalcomponents,theformationoftheprotoliths of which coincided with the birth of the Columbia supercontinent (Rogers andSantosh,2002;Meert,2012;Nanceetal.,2014).Followingthisevent,thereisalongperiodof quiescence, with only sporadic evidence for any Mesoproterozoic and early Neoproterozoicactivity. However, starting mid Neoproterozoic (Ediacaran) magmatic activity flared up invariousblocksinrelationtodifferenttectonicprocesses,involvingbothplatemarginandintraplate,culminatinginthelatestNeoproterozoicCambriancollisionaleventwhenthewholeregion witnessed high grade metamorphism associated with the assembly of Gondwana(Santoshetal.,2009;Collinsetal.,2014).Pegmatitesandmaficdykesarethepredominantmagmatic representatives of the Phanerozoic world in this region, associated with postcollisionalextensioninCambrianOrdovicianandsubsequentyoungerriftingeventsrelatedtothedisruptionofGondwana. AvasttractofyoungersedimentarycoverfringespartsofthewesternmarginoftheStage.

MineralresourcesofKerala:asynopsis

Hereweprovideabrief summaryof thesalientmineral resourcesof thestateof Kerala inrelationtothemajorgeologicandtectoniceventsthroughtimeasschematicallyshowninFig.3.The ore reserve estimates and localities mentioned in the sections beloware basedon theinformation given in the official homepage of the Department of Mining and Geology,GovernmentofKeralaat:http://dmg.kerala.gov.in/index.php?option=com_content&view=article&id=55&Itemid=61ArcheanearlyPaleoproterozoic.TheoldestmetallicmineraldepositofeconomicsignificanceinKeralaisiron,mostlyassociatedwithmetamorphosedBandedIronFormations(BIF),andpresentlyoccurringashighlyfoldedanddeformedbandedmagnetitequartzitebands.DepositsofthistypeoccurinKozhikodeandMalappuramdistricts, andatotalreserveof84milliontonnesofironwithorecontinentvaryingfrom32to41%hasbeenestimated. AmongthemajoroccurrencesarethoseofEleyettimala,Naduvallur,Nanminda,Cheruppa,AlamparaandKorattimala,andthelargestoneisatAlamparawithatotalof35.2milliontonFeatagradeof35.2%Fe. MinormetaBIFsalsooccurinseveralotherlocalities,suchasthoseofAttappadi (Santosh et al., 2013b). The meta BIFs in Kerala dominantly represent oceanic sediments,possibly generated through seafloor hydrothermal processes, accreted onto the continentalmargin during subductioncollision tectonics during Archean early Paleoproterozoic, andsubsequentlymetamorphosed,asinferredfromrecentgeochronologicalstudiesoftheassociatedrocks(e.g.,Santoshetal.,2013b;Praveenetal.,2013).

Thenextmajormetallicdepositisgold,mostofwhichareconfinedtotheWyandGoldfieldinNorthKerala.ThegoldmineralizationoccursalonganumberofzonesfromGudalurinthe east extending westwards into Kerala where Nilambur in Malapparuam district andAttappadyValleyinPalakkaddistrictareconsideredprospective.Goldintheseregionsoccursinthree principal settings: (1) primary lodegold mineralization in quartz veins traversingPrecambrian crystalline rocks, (2) supergene gold associated with laterites in weatheringprofiles; and(3)placergoldassociatedwithstreamgravels(Nairetal., 1987;SantoshandOmana,1991;Santoshetal.,1995). AccordingtotheinformationfromMiningandGeologyDepartment, also compiling sources from the Geological Survey of India, prospecting hasestablished0.55milliontonnesofgradeof4g/tonneofgoldinMaruda,and0.08milliontonnesofgoldwithanaveragegradeof12.98g/tatKottathara.ExplorationthroughtestpitscarriedoutinplacerdepositsofNilamburvalleyalongtheriversPunnapuzhaandChaliyarpuzhashow



reservesof2.5millioncubicmeterofplacerswith0.1gm/m3ofgold,leadingtoaprojectionof30millionm3ofplacersforthewholearea.Theprimarylodegolddepositsoccurinquartz

Fig.3SchematicillustrationshowingthemajortectoniceventsintheKeralaregionthroughtimeandthe correspondingmineralization.



sulphideveinsmainlytraversingamphibolites(metavolcanics)asinNilambur.Therehasbeennodirectdatingonthetimingofthegoldmineralization,butbasedonzirconUPbdataonthe

Fig.4IndiaSriLankaMadagascarEastAntarcticainEastGondwanashowingthemajorgemfields(after MenonandSantosh,1995)

hostamphibolites(ourunpublisheddata),aNeoarcheanhistoryisinferred.FluidinclusionandstableisotopestudiesonthegoldmineralizationinNilambur(Santoshetal.,1995)showthat theore fluids hadsubstantial CO2 content, and that the fluids were sourced frommagmasderivedfromsublithosphericsources.Thesourceofgoldandsulphidesmightberelatedtothedehydrationanddecarbonationofthesubductedsedimentsandoceaniccrust,withthemetalsmobilizedthroughheatinputfrommagmaticunderplatingandfluidflux,leadingtostructurallycontrolled gold mineralization and carbonate alteration. Minor skarntype tungstenmineralization(scheelite)hasalsobeenfoundintheAttappadivalley.

MagnesitereserveinKeralaisestimatedas0.037milliontonnesandoccursmostlyintheMulliSalayurregionofAttappadiinPalakkadDistrict,withanaveragerecoveryof100kg/m3.



Steatite/talcoccursinseverallocationsaroundtheThalasseryTalukofKannurdistrictandthetotalreservesareestimatedas7.94milliontonnes.Themagnesitemineralizationismainlyveintype and locally massive, and is possibly related to CO2rich alteration of olivinebearingultramafic rocks in a suprasubduction mantle wedge. The steatite/talc is also a hydrousalterationproductofNeoarcheanultramaficrocks.

Metamorphism and accretion of continental shelf sequences have also producedcrystallinelimestonesinsomeplacessuchasthoseofWalayar,usedasrawmaterialforcementindustry.

LatePaleoproterozoic. TheLatePaleoproterozoictectonicsintheSouthernGranuliteTerrainincludingtheKerala regionmainly witnessedtheproductionandemplacement of felsic arcmagmaspossiblyinconvergentmarginsettings.TheserockshavelaterbeensubjectedtohighandultrahightemperaturemetamorphismintheLateNeoproterozoicCambrianandthereisnomajormineralizationassociatedwiththeserocks. However,theserocksalsocontributeasthesourceofilmeniteandrareearthbearingmineralsinplacerdepositsalongtheKeralacoast.

LateNeoproterozoicCambria. Severalfelsicplutons,someofthemwithalkalineaffinities,wereemplacedinthedifferentcrustalblocksoftheSouthernGranuliteTerrainduringthemidNeoproterozoic (EdiacaranTonian), but these do not carry any significant economicmineralizationexceptminorrareearthminerals.TowardstheendofProterozoicandthedawnofCambrian,allthecrustalblockssouthofthePalghatCauverySutureZonewitnessedhighgrademetamorphism,extensivecrustalreworkingandemplacementoffelsicmagmas(Santoshetal.,2009). MeltinginvolvedbotholderbasementaswellaslargevolumesofcontinentaldetritusdepositedasshelfsequencesinoceanbasinsthatclosedduringGondwanaassembly,suchastheKhondaliteBeltinTrivandrumBlock.Theorogenesiswasaccompaniedbyavariety ofmineralizationsourcedmostlyfromcrustalcomponentsandincludesgemstones,graphite,molybdeniteandrareearths.

ThegemstonemineralizationinKeralaislargelyhostedbycomplexlyzonedpegmatites,mostofwhichwerederivedbymeltingofaluminouspeliticprotoliths(Menonetal.,1994).ThegempegmatitefieldinsouthernKeralaandadjacentTamilNaduisdefinedbyanellipticalareaof70x35sqkm.ChrysoberylcatseyeisthemostvaluedamongthegemstonesrecoveredfromsouthernKerala.Sapphire,topaz,gemvarietyofberyl,tourmaline,garnetandfluorapatitearealsorecoveredassemipreciousstones.MenonandSantosh(1995)correlatedthegemstonemineralization in Kerala with those of Sri Lanka and Madagascar within the CambrianGondwanaassemblyandproposedamajorPanAfricangemfieldinEastGondwana(Fig.4). ThemajorgemrecoveryinKeralaisfromplacerdeposits(streamgravels)derivedbyweatheringoftheprimarysource.

Graphite in Kerala is dominantly associated with metamorphosed pelitic rocks andassociated lithologies. Radhika et al. (1995) distinguished distinct varieties of graphite inspecific genetic settings. These include: (1) disseminated and lumpy graphite occurring asstratabound deposits within khondalites (granulite facies metapelites) formed through theconversionofbiogenicmaterialduringhighgrademetamorphismandcharacterizedbylightercarbonenrichment;(2)coarsegraphiteflakesandflakyaggregatesinveins,pegmatitesandmeltpools which were precipitated by the reduction of CO2fluids that infiltrated fromexternalsources,andcharacterizedbymantlelikecarbonvalues;and(3)shearzonehostedgraphitemineralization resulting from precipitation fromCO2rich fluids with mixed carbon isotopesignature. TheeconomicgradeveintypegraphitewasearlierminedfromseverallocalitiesinTrivandrumdistrict.FlaketypeofgraphiteoccursinTrivandrum,Kollam,Kottayam,IdukkiandErnakulamdistricts.Someoftheseshowgoodbeneficiationfeatures,withahighrecoveryof fixed carbon (about 85%). The total ore reserves of graphite in Ernakulamand Kottayam



districtsareestimatedasaround6milliontonnes. Weatheringandlateritisationofthehostrockshasleftthegraphitemineralizationrecoverablewithease.

Molybdenite is associated with some of the Late Neoproterozoic Cambrian alkaligranitessuchastheAmablavayalgraniteinWynaddistrict(Santoshetal.1988a,b).Highly

Fig.5OrefluidevolutionduringtheformationofmolybdeniteintheAmabalavayalgranite(afterSantosh etal.,1988b).

coarseflakesandflakyaggregatesofmolybdenitehavebeenrecoveredfrompegmatitesandquartzveinsassociatedwiththegranite.Santoshetal.(1994)reportedpreciseReOsdatingof molybdenite fromAmbalavayal which constrain the timing of metallogeny as ca. 567 Ma.Santoshet al. (1988b) carried out detailed fluid inclusion studies and traced the ore fluidevolutionatAmbalavayal(Fig.5)withfluidsofmoderatetohighdensity(0.750.85g/cm3)andsalinity(15wt%NaCl)markingthemainorestage.

ThegranulitefaciesmetamorphismduringLateNeoproterozoicCambrian,particularlythoseofpeliticprotoliths,generatedanumberimportantmineralsincludingmonazite,ilmenite,rutile, zircon, leucoxene, and sillimanite, and the subsequent weathering and denudationprocessesconcentratedthesemineralsinthebeachsandsalongthewesterncoastgivingrisetothe rich heavy mineral sands. The heavy mineral sand and China clay (kaolin) deposits



contributetomorethan90%ofthetotalvalueofmineralproductioninKerala.ThemineralsandsintheChavaradepositareestimatedtocontain127milliontonnesofheavyminerals.Ilmeniteaccountsfor80milliontonnesfromthetotalreserveof1400milliontonnesofrawsand. ThecoastaltractinAlappuzhadistrictalsocontainsextensivedepositsofsilicasand.

Bauxiteoccursincloseassociationwithlateriteinseverallocationsalongthewestcoast of Kerala and deposits of economic significance are limited to a few locations in Kollam,Trivandrum,KasargodandKannurdistricts.ThetotalbauxitereservesintheStateareestimatedat12.5milliontonnes,withthelargestoneinNileswaramwithareserveof5.32milliontonnesofgradearound45%Al2O3andlessthan5%SiO2.

ThekaolinitedepositsofKundarainthezonebetweenTrivandrumandKollamdistrictsarewellknownfortheirworldclassquality(Nakagawaetal.,2006).ChinaclayhasalsobeenidentifiedinazoneextendingfromKannurtoKasargoddistricts.Theestimatedreserveis172milliontonnes,mostlyofsedimentaryandresidualorigin.LigniteoccursinmultipleseamsinKasargoddistrict,andareserveof250milliontonneshasbeenestimated.BothChinaclayandplasticclayareassociated.

Summary

Insummary,althoughtheKeralaregionwitnessedgeologicandtectonicprocessesfromtheearlyhistoryof theEarth toRecent, themineral resources are limitedprincipally becauseof theextremelyhighgrademetamorphismthatoverprintedtheregionduringGondwanaassemblyinCambrian and erased much of the earlier lower temperature (hydrothermal) records. Inaddition,extensiveweatheringanderosionoverprolongedgeologicaltimescaleshaveremovedtheupperlevelsofthecrust,exposingtherelativelybarrendeeprootsofthecontinent.Inspiteof the scarcity of metallic mineral deposits, Kerala holds some of the world class mineralresources including the rich rareearth bearing beach sands and the high quality kaolinite.Furthermore,thereisalsopotentialtoprospectforgoldinregionswhereyoungerfelsicmagmasextensivelyinvadedtheoldersubductedaccretedbasementandcausedlithosphericdestructionandpossiblemobilizationoforebearingmaterials, byanalogywiththeworlds richest golddepositintheJiaodongPeninsulainNEChina(GoldfarbandSantosh,2014).Thepaleosuturesand junctions of microblocks (Li and Santosh, 2014) might also be prospective for goldexploration.

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Mineral resources of Kerala: A tectonic perspective M Santosh and E Shaji

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