deposits and resource potential

NGU-BULL 436,2000- PAGE 175

Building stone of central and southern Ethiopia:

deposits and resource potential

HAILEYESUSWALLE,SINTAYEHUZEWDE&TOMHELDAL

Walle,H.,Zewde,S.&Heldal,T.2000:Buildin gstone of cent raland sout hern Ethiopi a:deposits and resource potential.Norgesgeol ogiskeundersekeiseBulletin436,175-182.

Thebed roc k geology ofEth io piaexhibitsavariety of rock typesthat canpotentiallybedevelo ped forbuild ing stone production.TheETHIONORprojecthas sofarincludedinvestigation ofsuchresourcesinthesouthernandcentral partsof Ethi opia.WithinthePrecambrian,there are severaldep ositsof pinkand greygranitoids,greyandwhite marble, soapstone andserpentinit e;anumber of thesedepositsarecurrentlyexploite dbyEthiopiancompanies.

Mesozoicsandstoneandlimestonearefound atseveralplacesintheeast-centralandcentralpartof thecountry,and inadditiontothesmallnumberofexistingquarryoperations,apotentialexistsforthedevelopme ntofnewdeposit s.

Basalt,tufts andigni mbrite areexte nsivelyusedfor localhousin g and construct ion in Ethiopia.Suchresources representapotentialfor low-costsupplyofanexcellentconstructionmat erial.andcouldbedevelopedfurt her.

Hail eyesusWalle& SintayehuZewde,EthiopianInstituteofGeol og ical Surveys,AddisAbaba,Ethiop ia.

TomHeldal,Norgesgeo/ogiskeundersekelse,N-7497Trond heim,Norway.

Introduction

Duringthelast 20 years,theproduction and useof building stone¹hasstead ily increasedworldwide,and todaystonehas reached apositi on as oneof theworld'smostimportantmin- eral resour ces.For many count ries, expor t of stone has becom easignifica nteco nomicact ivity.For others,therec- ognition of local sourcesof building sto ne has secured a steady supp lyof cheapand durable const ruct ion mat erials fordomesticpurposes.

Almostanytype of rock thatcanbeshaped and dressed toblocks andslabs can be considered as apotentialbuilding sto ne source.Most comm onl y exploite d are massive rocks suchas granite andot her igneou srocks,marble ,limeston e andsandst one,andslab rocks suchas slateand flagstone.For local housing and ot her const ruct ion, soft and/or easily cleaved rocks are preferred, sincetheycan be wor ked with sim p le tool s.On the internat io nal market ,however,the aes- theti cproperties of therocks(colo ur and st ruct ure)are far moreimportant, andgenerally thepricing of building st one depend son the excl usivenessof therock.Rarecolourssuch asblue,pure whi te, pure black, yellowandeme raldgreenare conside rab le higherpricedthanthemor e 'ordinary'colours.

For exam ple, prices (delivered port) for blocks of the low priced varietiesof Indiansiliceoussto neare from 300 to 600 USD/cbm,dependingon blocksize.Similarly, amiddleprice range isfrom 600 to 1000 USD/cbm,and high-pr icetypes from1000 to1400USD/cbm².ForNorw egi an'BluePearl'lar- vikite,thepricerangeis1000- 2000USD/cbm.For any indus- trial scaledoperatio non build ing sto ne,itis ofvita limpo r-

1. Theterm 'building sto ne'includes anytype of rock,shaped and dressedto blocksorslabs,used for const ruct io n.Buildi ng stoneis a more general term than the term 'dimension stone; which is restrictedtocut-to-sizeslabsandblocks.

2. ht tp/ /ww w.chariotinternational.com/priceListTiles.htm

tance that the depo sit can give com me rcial-sized blocks (minim um220x120x100 cm)and/orslabs of uniform qual- ity,wit h produ ct ion costsmatchin gthemarket price(Shad- mon1996).

Duringthelast decades,andespeciallydurin gthe1990's, systemat ic prospecting for building stone in Ethiopia has been carried out by both the EIGSand privat ecompanies, and a number of buildin g stone dep ositsthroug ho ut the count ry havebeen put into producti on.Thisisreflect edby the ext ensiveuse of Ethiopi an sto nein new buildi ngsin the capita land other cities.

Thus,an import ant goalwit hi n theETHIONORprojecr' is toinvest igate and evalu atethepotentialforlocat ing build- ingsto ne dep ositsin Ethiopia.Sofar,fieldwor khasbeen car- riedoutin the cent ralandsout hern part s of thecount ry,and the main result s are present ed in thi spaper.The north ern region sof Gonder,Tigr ay and Amhara wi ll be covered in the rem ainingpart of theproj ectperiod(1999-2000).

Geological background

Thebedrockgeology of Ethiop ia em bracesagreatvarietyof rocktypeswithina wideage range (Mo hr 1971,Kazmin 1972, Meng esh etal. 1996;Fig.1).Precambrianmet am or ph icand igneou srocks cover 23%of thecountryandincludesomeof themostinteresting buildingst onesources,suchasmarbles, granitoid sand soapsto ne.Thick successionsof Palaeozoic and Mesozoic sediments (25%) overlie the Precambrian.

Theseincludebuilding stonequalit y limeston es and sand- stones. A large partof thecountryiscovered byTertiaryand Quaternary volcani crocks (44%),and in theseareas,basalts,

3. The ETHIONORproject is an Ethi o pian-Norwegi an co-operation projectaimed at increasingtheknowled ge of mineralresourcesin Ethio pia.NGUisadvising EIGS wit hinvario us fieldsregardi ngmin- eral resources explo rationandgeologicalmapping.

NGU-BULL436,2000-PAGE176

N

HAILE YESUS WALLE, SIN TA YEHU ZEWDE&TOM HELDAL

DEPOSITS

1 Mora & BapuriMarbles 2 BulenardGanzJmarbles

3 BanxJa (Moye,Gewi& Ekonte)marbles 4 CehaneGran ite

5 MankushMarble

~ ~~~~~;~L~~~~e

8 HamaresaGranite 9 BabileGrante 10MelikaGran te

11KentichSerpentinitearosoapsto ne 12CaletiMarble

n

15Addis AbabaIgn m txl te 16Kombo lchGran te 17Wonchlt & ..lemaLimestone 18Ziglsoapstone

GENERAL~EDGEO LOGY Quaternary cover(undifferentiated) Quat ern ary-Tert iaryrift sediments&volcanics Te rtiary sedimentaryrock s

Tertiaryvolcanic rock s Meso zoi c sedimentary rocks Palaeozoicsedimentaryrocks ProterozoicmetamorphicRock s

• Archaeanrocks

Fig.1.Simp li fie dgeological map ofEt h io p ia with locali tiesof buildingsto nein the cent ra l and so u the rnpartsofth e country.

tuffs and ignimbrites areext ensively usedfor local hou sing andconst ruct ion.

ThePrecambrianrocks ofEthiop ia,upo nwhich young er sedimentaryand volcanic rockswere dep osited,areexposed intheeast-central(Harar),west-cent ral(Gojamand Wellega), north ern (Gondar and Tigray)and sout hern (Sidamo, Bale andIIlibabo re)parts of the count ry.Mostof the exposuresare found in theperipheralregion s, whereyounger rockshave beenrem ovedby erosion.

ThePrecambrianrocks can be divided into aLower,Mid- dle and Upp erComplex (Kazm in1972).The Lower Complex comp risespossibly Archaeangneisses,migmatites andgran- itoids, form ing abasemen ttothe volcanosedime ntarysue- cession s ofthe Middle and Upper Proterozoic Comp lexes.

These successionsessent ially show polyph asedefo rmation and low- to medi um-grade metamorph ism.However, the Proterozo ic rocks of nort hern Et hiopia, thou gh stro ngly defor med, areonlyweaklymetamorphose d.

A variety of igneo us rocks,predom inantly granito idsof Proterozoic toEarlyPalaeozoicage, occurasint rusive bodies wit hin the Precam brian metamo rphic units.Some ofthese have been emplacedpriorto,orcontemporaneouswit h,tee- ton om et amorph ic events; otherspostd atethese events.

TheLat ePalaeozoic(?)to Mesozoicsedimentary roc ksof Eth iopia were depositedduri ng a regional transgression of theIndianOcean,followed byLateMesozoic upli ft andero- sion(Kazmin1972).In thewestern-central part ofthe coun- try,thelower portionofthe Mesozoicsuccession isrepre- sented by theAdigratSandstone ofTriassic toJurassic age. Thisrests uncon form ably on thePrecambri an basement, or

slightly unconformab ly onlocallydeveloped Palaeozoicsed- imentary rocks. The Adigr at sandstone varies in thickne ss froma fewto 800 m,and consistsessentiallyof red toyellow, well-sorted quart z sandsto ne.The upperpart, however , isin placescalcareous,part icularly closeto the transition to the overlying limestones of theAntaloGroup. Thicklimestones are develop ed inthe middle part ofthis group.These vary from near-shore, oolit hic lim estones,through fossiliferous, pale limeston e and marl to black limestonesdeposited in deeperwate r.Inthe Harar area,a possiblecorrelativeto the AntaloLimeston e,theHamanleiSeries,exhibitsthickbedsof pale,calciticto dolo mi ti climesto ne.

A large porti on of CentralEt hiopia is covered by volcanic rocks, rangingfromthe extens iveplateau basaltswit hi n the Early to MiddleTerti aryTrapSeries,to Quaternarylavas, tuffs and igni mbrite.

Massive stone - marble, limestone and granite

In the explo itatio n of massive stone (d imensional stone).

large, commercial blocks are ext ract ed in the quarry and transportedtoaprocessingplant forfinalshap ing andfinish- ing to slabsand tiles.Themosthomogeneous and attractive typesofrocks maybe expo rted to other countriesas rough blocks.Generally,deposits suita ble for exploitation of mas- sivesto nesho uldbe very hom ogeneous,both in colour and instructure. Furtherm ore, theoccurrence of joints,veinsand ot her factor sthat contr ibute to a reducti on in the overall block yield,should be very small,except for extremelyhigh -

HAILEYESUS WALLE,SINTAYEHUZEWDE

s

priced (rare) rocks.The most obvious resource pot ent ial within the investiga ted regionsinEthiopialies inthe huge marble formations inthe westernpartofthecountry,andin diffe rentvarietiesofgranitesandgneiss,limest one and ser- pentinite/soapst on e.

Market evaluat ion of such deposits is difficult. Firstly, there are no object ive price listsavailabl eon the interna- tionalmarket, and prices can varygreatlydueto changesof fashionand personal taste ofthe cust omers.Secondly, very few produce rsact uallygo public wit h theirprices,andapic- ture of the market situation implies a needfor experience among the professionals involved in theevaluation ofstone deposits.Furthermore, the market for roughblocks is signifi- cantly different in compariso nwith finishedproducts. In the formercase,the customersare stone-prod ucingcompanies aroundtheworld,whilst the custo mersof finishedproducts areend-users(architectsandconst ructo rs).Thenotabledif- ferencein raw materialpricesfordifferent rock typesmay not be as obvio usfor fini shedproducts.Generally,however,the price difference betw eenhighl y att racti veand less att ract ive rocktyp esismuch higherfor raw blocksthan for finished produ cts. In this arti cle,we have used published and non - published price listsfor raw blocks ofcomparable typ es of stone as atool in marketevaluatio n. Dueto thesubjec tive character of suchinfo rmation, price indicat ionsgiven below mustberegarded as indicati veonly.

In Ethiop ia,the use and dom esticproduction of cut-to- size stonehave increased duringthe last decades,contribut- ing positivelyin the developm entof a high-qu ality ,domestic industry.For the major part,dim ensionstone usedin Ethio- pia is derived from domesticsourc es.Although limeston e and marblehavebeenproduc edfor a longtime,theproduc- tionand useofsili ceous rockshasonly recently sta rted,and

Fig.3.Detail ed geological map of the Ganzi marble deposit. Gojam (see Fig.1 forlocation).The mapil- lustrat esthe str uct ural complexity seeninsom eof the marble depos- its.

lOOm

NGU-BULL436,2000-PAGE 177

Fig.2.The Daletimarble(Wellega) cladding thefacadesof a modern buildi ng inAddisAbaba.

it iswithin that sect orwewouldexpect themost significant growthin the country's dim ension -ston eproducti on in the years to come.

Marble

The most interesti ng deposit s of marble are found in the weste rn part of Wellega (Daleti) and Goj am (Mora, Bulen, Mankush and Baruda).The area is quite remot e,and trans- portdistances to Addi sAbaba varybetween550 and 800 km, for the most part along non-paved roads.However,at the presenttime,transportcosts arest ill lowenoughforthemar- ble-producingcompaniestofind theproduction profitable.

Mostof themarbledepositsfound in these areas form an elevated morpholog y, ot hers have a flat, though well exposed,morphol og y.Predominant ly,the marblesare cal-

s

Geologicalcross-sections

Grey.banded calcitemarble,fi ne!mediwn -grained Whitecalcite marble withgrey bands,medium/coarse-grained White-grey,sulphide-bear ingdolomitic(?)marble

(outcropsshowndarkerin lithologies) ____ Lithologicalboundari es.observed

- Lithological boundaries.interpreted f i Strike!dip foliation/!vertical foliation

.. -

NGU-BULL436,2000-PAGE178

Fig. 4. Folded pre-tecto nic dykes (amphibo lite) in calcite marble, Manku sh,Goja m.

cit ic,butwhite togrey dol omiteoccuras layerswithi nthe calcite marblesor as bordering units.Thelatter has,until now,not been ofsignificantinter estasbuilding stone.

Several typ esof commercial marble occur in the area (Malis&Dejen e1983, Heldalet al. 1997). These include fine- tomedi um-grained, graph it icgreymarblewit hwhite bands, medium- to coarse-grainedwhite marble wit hgreybands (Fig.2)and severalsubordi nate typ essuch as pink,silicate- richmarble,pure white,fine-grained marbleandskyblueto green marble.The latter two types seem,both from field observatio ns and comparison with description of similar deposits in Norway,to be connectedto contact-metamor- phicaureoles surrounding gabbro int rusions.Grainsize var- ies from fine- to coarse-grained, and these variati onsare believedto betheresult oflocalvariations in metamorp hic conditio ns, probably caused by heat ing related to syntec- tonicintrusions,and grain-sizereductionduring lat er defor- mationepisodes,especiallyin high -strain zones.Inthelatter case,the marbleshave oftena schisto se, 'slabby'appearance, and are notsuited for productionofsquared blocks.Fold pat- ternsmay be verycomplexwith insome of themarbledepos- it s,especiallywherecompeten t layers of dolomiteareinter- banded with lesscomp etent calcite marble.An example fromone of thecomp lexlyfoldedGojamdepositsisshown in Fig.3.Measurementsoffoldaxesfurthe r show thatthe folds are non-cylindrical,demandingcare in estimatingsubsurface marblereserves.

Pre-,syn-and post-t ectonicdykesand veinsof acidicand basic composition are common in several of the marble deposits (Fig. 4). Parti cularly,the foliat ion-parallel ,folded dykescauseproblemsfor the ext raction ofmarble,sincethey infi lt rate a large part ofthe rock mass andarediff icultto pre- dict.Thus,insuch areas,theblockyieldand the possibilities of using sophist icated quarrying technology,such as dia- mond-wire sawing, arered uced.Furthermore ,closelyspaced jointscausesimilarproblem sin manycases.Howe ver,both the existing quarrying activity by several companies and recent investigations by the EIGS indicate a fairly goo d pot ent ialfor increasingthe production of low- tomedium- priced,grey andwhitemarblefromthe area.Furtherm ore, investigations suggest a possible development of highly

HAILEYESUS WALLE,SINTAYEHU ZEWDE&TOM HELDAL

exclusive typ esof marble,suchaspure white andsky blue varietie s.Thelatt ertwomay obtain very highpricesonthe intern ati onalmarket,butthedeposit s discovered todateare smallandinhomogeneous.

Granitoids and gneiss

Intrusiveigne ousrocksarecommonin the Precambrianof Ethiopia and range from granit ic bodies within the migma - tites of theLowe rComplex,to pre-,syn and post-tectonic intr usionsin the MiddleandUpperComplexes(Kazmin 1972, Heldaletal. 1997).The former groupispossiblyof Archaean age,and,at present,the National Mining Companyis exploit - ing deposits in theHarararea(Fig.5).The depositsare situ- atedclosetothe smalltown ofBabil e,and for m massiveboul- ders and small hills, giving good opportunities for the ext racti onoflarge volumes ofcommercial-sizedblocks.The Babilegranite is a medium- grained,pink to red 'schlieren'- granite witha variegated,veinedst ruct ure(Fig. 6),reflecting its close relation ships wit h the surroundi ng migmatitic gneisses.Bycomp arison with other commercial'schlieren' - granit es on theinternat ional market,alow- to medium -pr ice level is estimated . The struct ure of the granite may vary withinsmall areasbetween more or less gneissose(Fig 7),

Fig.5.Quarrying ofa giant boulde rintheBabilegranitedeposit,Harar.

Fig.6.Polished surface of the variegated Babile granit e (right)and flamed andhoned tiles of theporph yriticAngerGuten granite(left).

Hotel Sheraton,AddisAbaba.

HAILEYESUS WALLE,SINTAYEHUZEWDE&TOM HELDAL

Fig.7.Gneissoselayersin theBabilegranitedeposit.

giving possibilitiesforext racti onofseveral commercial st one types, butalso demandi ngcareinthepredi cti on of uniform future reserves.

Pre- and syntectonic granitoids within theMiddle and UpperComp lexescomp rise medium-grained,grey granodi - orites,fine-grained,pin kto redgranite,coarse-grained, pin k graniteand porph yrit ic,pin ktogrey granites,allesti mate d to belong to alow-to medium-pr ice levelatthe market.The plutons may showa penetrativefoliation throug houttheir entirethickness, or have a foliate d margin wit hamassive, non-foliated core.The AngerGuti granite, Well ega (some- times called the Guten granit e), is presently exploite d by Natio nalMini ngCompany. This is afoliated 'Augen'-granite, with large, pink phenocr ystsof microclin e in abrownish-grey ground mass of blotite, plagiocl ase,quartz and potassium feldsp ar (Figs.6and 8). Several depositsoccur in the area, forming smo ot h hills, wit h astee p foliation facilitatingextr ac- tion by drilling,blasting and wedging.Variationsin colour and st ruct ure may occur,especially due to heterogeneous distributionof the phenocrysts anda varying degree offolia- tion development.The phenocrysts arecommonly rounded and rotatedbyst rain.Aplitic veins of severalgeneration s are abundant,and may contributeto anincreasing wasteratioin someof thedeposit s.Joint spacing in the area varies signifi-

Fig.8.Rawblocksurface of theAngerGutengranitein thequarry (cut perpendicularto thefoliation).Drillholesareapproximately25mmwide.

NGU-BULL436,2000-PAGE179

cantly, but somesmall hillsshowa spacingof morethan 1.5 m on average,whichmay be suff icient forlarge blockpro- duct ion.

Severalother granite depositsmay be offuture interest.

These includethe Dehane granite, closeto Mora inGoj am. Thisis a coarse-grained,pink granite, forming huge hillsand ridgesof bould erdeposits,which by reconnaissanceappear tobeveryhomogeneou s in colou rand struct ure.Join tsare wide lyspaced,facili tati ng produ cti o n of largeblocks.Inthe sout h,pilot quarrying of fine-gr ained ,pin kish-g rey granite hasbeencarriedout nearKibre Meng istin theSidamoarea, where several small, syn-tecto nic(?)granite pluton s occur. However,thesegranit es are tightl y jointed,and locallythick kaolin zones are develop edin the weat hering profile.Thus, these granites are prob ablynot ofgreat interestasdimen- sion-stone prospect s.

Limestone

Calcitic limestone of dimension -ston e quality is predomi- nant ly found wit hi n the Jurassic Antalo limest one (centr al part of the count ry) and the HamanleiSeries(east-cent ral part).Thebest exposuresand the mostinteresti ng deposits of theAntaloLimestone are found in the cent ral part of the AbayValley, andsidevalleys such asthe Jema,Wonchit and Mugervalleys.

TheJema and Wonchit limestone deposit s occur in the bottoms of the valleys ofthesamenames.Thelowerpartof the lim estone unit is byfar themostinteresting, sincethis is the part wherethe bed thic kness reaches more than one metre(Wondaf reshetal.1993).The limestoneis essential lya calcareaou s,fossilifereou s sandst onewith poorlydevelop ed st ruct ure; colour variesfrom brow nto off-white. Joint spac- ing variesconsiderabl y in the area,where themoremassive parts of the depositsform small hil ls and plateaux.At the present time, these limeston e deposit s are not being exploited, due to difficultaccess(t heaccessroadisofpoor quality )and locallycloselyspaced joint s.

Largelimestone dep osits are also fou nd in the eastern part of Ethiop ia,in theHarar-Hak img ara areas.The Hakirn- garalimeston ehas bedsvaryingfromsome tensofcenti me-

Fig.9.Sawn limestonebedinaquarrywithin theHakimgaralimestone deposit, Harar.Notereduction fieldsintheupperpartofthe bed.

Scrpcntin itc NGU-BULL436,2000-PAGE180

tresto severalmetres in thickness(Schled eetal.1990;Fig9).

Generally, extract ionof commercial-sizedblocksis possible inthethicker beds, exceedingone metre in thickness,where thespacing of vert icaljointsiswide. Thelimeston e is partly fossiliferou s, and contain s abund ant stylol ites.The colour variesbetween yellowish-brown and dark grey, the latt er occurring inirregularlydist ributedreductio n patt erns.Quar- ryingoperat io nsare carriedout by boththeNation alMining Companyand theEthiopian MarbleIndust ry in thevicinity of Harar.Thelim eston eformshil ls and theareaisconsid eredto havealargepot ential foreasily accessib ledeposits.

Serpentinite and soapstone

These rocks arefound in the Upper Complexes of bot hthe Sidamo and the Gojam areas. Deposit s in the Sidamo- Kenticha andinthe Metekel-Zigiareas(Goj am)were visited during the projectperiod(Heldal et al.1997).Theybelongto the Upper Proterozoi c Complex of the Adola and Birbir groups respecti vely.

Serp ent init es(generally greenwithwhiteveins) are often referred to as 'green marb le' on the inte rnational market.

Such rockshavebeen usedin Europ e as decorative stone since ant iq uity, andwell-known typ esinclude severalItalian deposits,such as the'Verde Antiqu e'.Serpent init espredom- inantlybelong to low-to medium-price levels.Soapston es are geo logically tightly linked to serpentin ites,but are not thatwell establish edon the internationalmarket. Howev er, suchrocks areincreasingly used for firep laces and even as build ing stone in northern Europe, and there might be a futuremarket develop mentwhichwould favour the exploi- tation ofsuch deposits in many placesaround theworld.

In Kent icha,Sout h Ethio pia, isolated,lens-shapedserpen- tinite bodies occur within amphibol it e unit s. The serpen- tin ites are fold ed and foliated,and are enveloped by talc schists.Talc-richzonesarealso found inshearzones wit hin the serpe nti nites. The green serpent inites apparentlyseem highly fractu red at the surf ace,butfromsuperficialinvestiga- tio nsit is difficul tto separate penetrative joi nt s from selec- tive weat hering of carbonate veins. Thus, more detailed investigat ionsbycoredrilling and sampling arenecessary before any conclusions can be reachedconcerning sub-sur- facequalit yofthe serpentinite.

The talczones observed in Kent ichaessentially contain talc,chloriteand rustyspots that probably representweath- ered,Fe-tichcarbonate,suchas magnesite.Thedept h of this stainedweatherin g profile is yet notknown ,since only super-

Talcschistandsoa pstone

I

Fig. 10.Sketchshowing the occurrence of talc-schistand soapstone en- veloping deformedserpentinite bodies in the Kenticha area,Sidamo.

HA ILEYESUS WALLE,SINTAYEHU ZEWDE

s

Fig.11.Massive and homogeneoussoap ston eintheMet ekelZig i area, Gojam.

ficial sampling has been carried out. Further knowledge on this is of vital im porta nce bothfor investigationsofpot ent ial soapston e andindust rial talcdeposits.Thetalc-rich rocks are essentiallycoarse-grained,flaky and have a green to grey col- our. Bot h massive and schistose zones are observed, the former concentrated in fold hinge zones along the margins of the serpentinites,as illustrated in Fig. 10.These foldhinges are of special interest, sincethe deposits are thickened in these areas, and the soapsto nes have a more massive appearan cethan in the thinnedlimbs.Detrital ultramaficsare also describedfrom theSidamo area,but these depositshave notyet beeninvest igat ed regarding buildingstone.

The soapstonesin the Metekel Zigi areainGoj am occur in thetransition zonebetweentheult ramaficpart s of layered int rusion s(Grenne etal. 1998)andthe surroundingmetased- iments. As in Kenticha,interchanging massive(Fig. 11)and schistosesoapstone is found.Rustyweat hering seems,how- ever, to be lessintense than in Kenticha,andit shouldbe pos- sib le to obtainfresh rock almost at the surface.In conclusion , bot h areas have an interesting potent ial for soapsto ne deposits,though further investigations are necessary,espe- cially concerningthe depthof the weat hering of magnesite in Kenticha.

Construction stone for local housing

For localhousing and construction, easy workable rocks are requiredin preference to rocks with attractive aesthet icqual- ities.The stone is shaped and worked to finished products (hewnslabs andbuilding blocks)in the quarry and/orat the construction site.It is im portan t that the rocks can be worked wit hsim pletechnology to a low cost. InEthiop ia,there are long trad itionsin using Mesozoicsandstone and tertiaryvol- canic rocks for suchpurposes.Alsothe aforementionedHarar limestone is widely used.

Sandstone

Regard ing building stone,the best potential of sandstone lies withinthe thick,red bed seriesof theAdigratSandstone along an axisfrom Amboin the south,through the Abay val-

HAILEYESUSWALLE,SINTAYEHUZEWDE& TOMHELDAL

Fig.12.Wedgin g sandsto ne bedsinthe Amboquarries.

leysto Tigray inthe north.Exploitationmainlyoccursin the depositsnearthe tow n of Ambo (Figs.12 and 13).Here,the cross-bedded,red and whitesandsto neis worked to ashlars, split bricksand slabs mainly with the help ofsimple tools such as sledge hammers, wedges and crow bars (Biazen 1962,Heldal etal.1997).Productsare distributedthroughou t mostofEt hio pia, eventhou gh amajorpart of theproducti on is usedin the capita l.

Fig.13.Wallc1addedwit h the Ambo sandsto ne,HotelSherato n,Addis Ababa.

NGU-BULL436,2000-PAGE181

Fig.14.Wallconst ructio nwithvesicularbasalt near DebreMarkos.

Basalt, tuff and ignimbrite

Basalt is wide ly usedfor localconstr uct io n, both as buildi ng stone and aggregate,in those part s ofthe country where suchrocks are found(Fig.14). A large numberofsmallbasalt quarries are fou nd throug hout the Cent ral Highlands,and crud ely shaped pieces are worked with simple tools and manpower. In general,the basaltdepositsare not considered to be ofspecific intere st for more industrial-scaledbuilding stoneexploitati on,wit hthepossibleexceptionofcolumna r and vesicula rbasalt(Fig.15).The forme r coul d,withsimple technol og y,rep resent apot ent ialfortheprodu ct ion of split pavingsto ne,while thelatt er,duetoits excellentworkab ility, could beusedforashlarand brick.

Inand neartheRift Valley,there are sizeabledepositsof tuffs (vi tr ictuff, lithic tuffand crystaltuff) and ignimbrites.

These rocks are generally porous, soft to carve and easyto split,and for along time,ignimbrite and tuff have repre- sente d the most importantbuildingstones of Addis Ababa (Karst aedt & Wondafrash 1986; Fig. 16).Due to the high porosity, theserockshave excellent insul at ing properties.A num ber of small quarriesare worked in the vicinity ofthe capit al(Fig.17),mainlybyhand ,using simpletools. The ign- imbrites are somew hat harderto work than thetuffs, and therefor emore commonly usedforrubblethan ashlars. Due

Fig.15.Quarry in colum narbasalt nearthetownof Am bo.

NGU-BU LL436,2000 - PAGE182

Fig. 16.Ignimbriteusedfor the const ruction of theCatholicchurc hin Addis Ababa.

Fig.17.lgnimbrit equarry in Add is Ababa.

to their softness,suchrocks are,however,not verysuitabl e foruseas pavingst oneandstairs.

Tuff s and ignimbritedeposits are alsoexploi tedat several other places in the country.For instance,the famous,rock- hewn churc hesofLalib ela are carved in softtuffs,and these rockshave alsobeen employed more recentlyfor localcon- structi on inthearea.

Conclusions

Cent ral andSouth ernEthiopia exhibita variety of rocks that already are either developed as, or can be in the future, important building stone resources.The grey andwhite mar- bles ofthe western regions have alreadygained important industrial developm ents, and contri bute today in shaping theimage of buildingfacadesin Addis Ababa andot her Ethi- opiancities.There is stil laninterest ing potentialbothforfur- therdevelop mentofindustrial-sizedquarriesin homogene-

HAILEYESUSWALLE,SINTA YEHUZEWDE&TOM HELDA L

ous marble depositsand forfind ing moreexclusivetypesfor the expo rtmarket.

Extractionof granite isst illin its earlybeginn ings in Ethi- opia, but increasing knowledge of the resource potential combi nedwithimproved extractionmethods cou ldbenefita positivedevelopmentalsointhissector.However,the grani- toid deposits localised unti lnowincent raland southernEt hi- opiaare of suchatype that will meettoug hcomp etition on theinternat io nalmarket.

Both limestone and sandstone are ext ensivelyusedinthe domesticmarket. With inboththeAdigratsandstone andthe Antalo and Hamanl ei limestonesthere are stillpossib ilit ies for explorationof new resources,though thesewill be limited to the relatively small areaswhere these unitsareexpo sed.

The most important part of the Ethiopianbuilding stone producti on,atleastintermsof volume, isthe exploitat ionof volcanic rocksfor localhousingand,closetothe capital,for indust rial-scale constructionwork.Suchresourcesrepresent aconsid erabl e potentialfor low-cost supply of anexcellent constr uct ionmaterial,andcould be furtherdeveloped.

Acknowledgements

Theaut horswishtothank the NationalMiningCompanyand Ethiop ian MarbleInd ustryfortheir helpduring fieldwork.The helpfulcommentsof Ingvar Lindahl (NGU)and Chris Spencer (BRGM, Orleans)are much appreciated.

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