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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
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MineralresourcesofKerala:Atectonicperspective
Fig.1GeologicalandtectonicframeworkofsouthernPeninsularIndiashowingthemajorcrustalblocksand
interveningshear/suturezones(afterCollinsetal.,2014;Santoshetal.,2014).TheoutlineofKeralaState
isalsoshown.
alongthesouthernmarginof theNilgiri
BlocksuggestingactivesubductionaccretionintheNeoarchean(Santoshetal.,2013b,Praveenetal.,2013).ThenextmajortectoniceventinthisregionwasduringlatePaleoproterozoic,around2.0Gainvolvingbothjuvenilemagmatismand
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MineralresourcesofKerala:Atectonicperspective
extensivecrustalrecycling(e.g.,Krneretal.,2012;Plavsaetal.,2012).Theimprintsoflate
Fig.2(a)CompilationofglobalzirconUPbagesandtheircorrelationwithmajorsupercontinentassemblies
(modifiedfromRoberts,2012).
(b)MajorgeologicalandtectoniceventsintheKeralaregionthroughtime
correlatedwithglobalevents(afterZhaiandSantosh,2011).
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MineralresourcesofKerala:Atectonicperspective
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
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MineralresourcesofKerala:Atectonicperspective
reservesof2.5millioncubicmeterofplacerswith0.1gm/m3ofgold,leadingtoaprojectionof30millionm3ofplacersforthewholearea.Theprimarylodegolddepositsoccurinquartz
Fig.3SchematicillustrationshowingthemajortectoniceventsintheKeralaregionthroughtimeandthe
correspondingmineralization.
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MineralresourcesofKerala:Atectonicperspective
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.
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MineralresourcesofKerala:Atectonicperspective
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
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MineralresourcesofKerala:Atectonicperspective
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
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MineralresourcesofKerala:Atectonicperspective
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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