Geological And Radio Metric Investigations On Some Minerols In Sinai :

Abdelazim Mahmoud M. A. Mohanna 

Umm Alaw1 area, cover1ng 500 km~ occupies an impor~an~por~10n or t.nebasemen~ rocks ot’::>ou~hsmai . Dur1ng t.het’1elds~udy, ~he rock uni~s ot’~his area could be subdivided t nt,oseven prominen~ divis10ns; V1Z, me~agabbro and gneissosediori~e, quar~z diori~e-granodior1~e- adamel11~e associa~ion,quar~z monzoni~e, pegma~1~1c grani~e (~hese rocks represen~~he pre-r1ng dyke), Ru~ig volcan1cs, r1ng dykes (rhyodac1~e,porphyri~1c quar~z syeni~e w1~h syen1~e and medium grainedquar~z syeni~e ) and alka11 t”eldspar grani~e ( ~hese rocksrepresen~ ~he r1ng-dyke ).The me~agabbro and gneissose diori~e are ~he oldes~rock uni~s; ~hey are coarse grained, grey ~o black in colourand form low h111s 1n ~he landscape They exhibi~ bo~hgrada~ional con~ac~ rela~ion and sharp, t’aul~ con~ac~s wi~h~he surrounding rocks.As regards~o ~he quar~z dior1~e-granodior1~e-adamelli~eassocia~10n, ~he quar~z dior1~e is grey, medium ~o coarsegrained, speckled w1~h black amph1bole crys~als wh1ch usuallyshow pret’erredorien~a~10n grading la~erally t.o a pronouncedt”o11a~10n.Con~ac~s be~ween quar~z dior1~e and surroundingrocks are sharp, grada~10nal and some~1mes t”aul~con~ac~s. Thegranodior1~e and adamel11~e have very coarse and mediumgra1ned ~ex~ures; and are reddish whi~e ~o wh1~1sh grey incolours. They t’orm low h111s closely assoc1a~ed wi~h ~heme~agabbro, quar~z dior1~e and quar~z monzoni~e. The ou~cropsor ~hese rocks exnrbat- marked ext’01ia~10nwea~hering wi~h welldeveloped boundr1ng.- 87 -QuarLZ monzoniLe 1S medium LO coarse grained, grey LOreddish grey 1n colour and rorm low, h1ghly weaLhered hillsW1Lh some alLered zones. ConLacLs WiLh Lhe surrounding rocksmosLly exh1b1L sharp relaL10ns and are rrequenLly delineaLedby tau I LS.The pegmaLiL1c graniLe is very coarse grained,leucocraLic and rorm low, highly weaLhered hl11s WhlChcommonly dissecLed by dyke swarm; specially 1n Lhe norLh andnorLheasLern parLs or Lhe sLudy area.The RULig volcan.lcs represenL the base or GebelKatherlna. They are medium LO r.lnegra.lned, grey LO reddishgrey and black, acid Lo lnLermediaLe and assciaLed wiLhpyroclasL.lc sedimenLs. They show angular and subhorizonLalcontacLs W1Lh the granlLO.ld rocks.The ring dykes are composed or rhyodaclLe, porphyrlL.lCand m.lcrographic quarLz syeniLe and syeniLe. They 1nvade Lheabove menLioned rocks 1n Lhe sLudy area and have a greyishblack, red and wh.lLish red colours. The conLacL beLween Lher-a rig dykes and surrounding rocks are moest.Lytau Lt, cont.ac t..The alkali reldspar graniLe is medium LO coarse grainedand vary rn colour tr-om pink, buf’f’LO pinkish yellow. IL’Sboundary WiLh Lhe oLher rocks is always sharp. Th1S rock uniL.lnLrudes all Lhe above menL10ned rock uniLS and perLain LO Lheyounger graniLes or Lhe LaLe Orogenic graniLes or Lhe EasLernDeserL.110SLor the gran.lLu.ld rocksalLeraL1on, Thererore, Lhe use 01’exh1b1L some sorL orLhe major and some Lrace- 88 -element.s will be geochemically rnsn gn t tcant, for- t.he1nt.erpret.at.ionof’ t.ect.on1cset.t.1ng,The ¥, Nb and Th as wellas some ot.her t.race element.s have 1ndicat.ed t.hat. t.he st.udiedrocks raIl mainly 1n t.he r1eld or ..w1t.h1n plat.e granit.e ”andpar-t.a aLj y in ..vo.Lcan rc arc gr-ani t.e ..Wh1Ch are ot’ dit’t’erentt.ect.onicset.t.1ng.The d1vers1t.y or t.ect.on1C1nt.erpret.at.ionmaybe at.t.r1but.edt.om1x1ng 1n t.he.upper mant.le during int.rusion.The Rb/¥+Nb di agr-amss , rnc Lude bot.h ri eLdsplat.e and volcanic arc ur1g1ns t’avour1ng mixingdur1ng t.he t’ormat.10not’ t.he present. granit.u1d rocks.or’ot’w1t.h1nmagmaUmm Alawi area is highly dissect.ed by a great. number of’t’ault.s,ot’ which 125 t.rue and Inter-r-ed t’ault.s were examinedand st.at.ist.1callyplot.t.edon rose diagrams. Eight. tau Lt, set.swere dist.inguished in t.he st.udy area E-W, ENE-WSW, NE-SW,NNE-SSW, WNW-ESE, NW-SE, NNW-SSE and N-S 1n decreasing orderof’ age.The dykes which 1nvade t.he various rock unit.s in t.hest.udy area were recorded, st.at.ist.1cally analyzed and plot.t.edon a rose diagram. The result.s ot’ t.his analysis show t.hat. t.het’1rst.phase of’ volcan1c1t.y t wh1ch rollowed t.he ext.rusion of’RUt.1g volcanics ) 1S most.ly aC1d1C and alt.ernat.es wit.h t.hebas1c and int.ermediat.edykes ( rejuvenat.10n along NE-SW ),They have t.he rollow1ng t.rends; NNE-SSw. NE-Sl!’and E-l!’ Thesecond phase has t.he to Llow rng t.rends NNW-SSE, NW-SE,£NE-wSl!’and l!’Nl!’-E1SnE a decreasing order ot’ abundance. Thet.h1rd phase (Lat.e Neogene dykes ) has only a N-S t.rend and isrepresent.ed by basalt.s and doler1t.es. Some sandst.one out.cropsw1t.h charact.erist.1cs ot’ Nub1a rac1es were recorded f’illing t.heN-S t’ract.ures.- 89 -f10SL U1 ’-’ttt=’ .l·u....... Ull..L•.•” ~ •• .-......•...•;radioactiv1ty and radioelements which a~ree well withknown world avera~es or these rock units, except alon~faults, shear zones and some contacts which commonlyhi~her radioactiv1ty.thethehaveAs stated above, the ~eochem1cal results showed thatthere 1S an upper mantle m1x1n~ or the ma~ma that has rormed-90-~hese rocks. The same conclusioncould be arrived Crom ~heresul~s oC ~he eU/eTh and Ucon~en~/Thcon~en~ra~ioswhich show~ha~ mos~ grani~ic rocks were origina~edCrom man~le derivedmagmas and mixed wi~h older cra~onic crus~s. Kos~ of the rockuni~s in ~he s~udy area have s~andard devia~ionsand varia~ioncoeCCicien~s higher ~han ~he corrosponding means. In aminori~y oC ~hem ~he varia~ion coeCCicien~ exceeds 100%These resul~s indica~e ~he presence oC more ~han ones~a~is~icalpopula~ion; a Cac~ which conCirms ~ha~ ~hese werederived Crom more ~han one magma .The U con~en~ was f·ound~o be generally higher ~han ~heeU ,which may be a~~ribu~ed~o ~he leaching oC ~he highlymobile U under disequilibriumcondi~ions.