MORDENITE DEPOSITS AND ZEOLITE ZONATION IN TI-IE I-IORN MOUNTAINS AREA. SOUTH-CENTRAL ALASKA Occurrence ol'morrlenite, /~e~ilandite,und 1aun.lontite lbr-nzecl by burial nzetar71orptzisn1 of'tnurine ~~itrictu/]s. T11e esterlsioc mortlenitc drposils nzay lzuvc f~c.onomic1niporlat1c.e. STATE OF ALASKA Jay S. IIamrnond, C;ovc~t.~lor. (;LI~ 11. hIartin, C'o~tl~~~issiot~c'r,I)rp(. (I/' :Vulut.cll ll'cso~r~.cc~s Itoss (:. Schat't', Slalc Gcologisl CONTENTS .\l)stt-:Ic.t. ........................................................... Illt rotluclio~i..................................................... P~1rl)Osc............................................................. '\cl<llowlt~~lg1~l11~~11I............................................ B;~ck~ro~~tltl........................................................ c I7 his st~ldy........................................................... Lor;~tion01' aro;~.................................................. (:cancsr;il ~cv)lo~yoI' ;rrt,:~ ...................................... S:lnll)lillg.. ..................................... -. .................. M;illl)ing ............................................................... I,;~l)or;llorys[.~l(lic-s ............................................. MORDENITE DEPOSITS AND ZEOLITE ZQNATION IN THE HORN MOUNTAINS AREA, SOUTH-CENTRAL ALASKA photomicrograph ;~ndllis h(~lpfultlisc-nssions duritlg the‘ cc:llrsr of this work. hIy thanks art, also txstrntit~dto M.S. Extc.nsivc5 mortlcnit.ci-rich I~~l'l'soc.c~c~.ring wit11 t11(~ Itoh~riso~~for proparalio~l of thin soclions. Fillally, I 'I'alhe,ctn;~ I'ormution in lh(, Horn hIount;~ins;irt>;c 01' ~vish lo thank Irly so~ls.Mikc a~idHill II:~wkins, \rrho south-central Alaska ;irtS ol' cornn~c,rcial gr:itlcs. '1'111. stln~rdas my I'icbld i~ssist~~~lts. 1)rolwrties ol' tlir ~nortlrnitr~with rc.gurtl to cornnicrc.i;~l rccl~~irc,~ncntsnc~cltl I'urlh(>r st.~icly. l'hc zc~olitc~sin thcb arcs;1 wcBrc*l'orn~t,tl 1)) I)i~riiil tliagc~nc~sis;lntl rcx~ion:~l nic~t.;unorphism ol' lav:t ant1 Tllv zcolitcs ~ni~lcbr;~lsarc, of intt'rc,sl as important volcanic elc~lritustlcspositc~cl in a r~ugt~osynclin:rltrou~h. i~lcll~strial~nincbrals of' us(> ill a varic>l>. of fit,lcls. 'l'llc3 IIculandil(~:tntl I;~~~montitc~zonc%s suggcxst Illat. tll(' scltl- minc,relogical, c~homic~al,and inrluslrial propc,rtirls of thc irncnts wc>rc1sul),i~~ctc~d to :I tnaxiln~1111t(xnll)(bri~turcb ncl;lr %OO°C at watc,r I)rclssurc.s I'rom 0.5 to :i kilol)ars, wliic*h zc,olitcxs wrlro t)ric~fl~\.cl(~scribrcl hy Ilawkins (1973) 2nd corrc,spontls to I~uriuldcptlis ol' I to 10 hilomr~tc~rs. more r(bcc,~ltlyill two dr~tnilcclgct~c,ral rcvicw artic~ltxson Thcl occurrcLncc, of mortlc~nit.c~will~in tho li~~ill:~ntlitc~ thc usr and ~)rol~~~rtic~sof' ~c~olit(,s: hlu~npton ( 1973) and zonch is prol)al)ly tlucl lo lli(s I'ino-gr;~inc~tl11:1turc, 01' 11118 AIunson an:! Shtlppiirtl (197.1). 'l'liosc1 articl(~sshould br parent t.ul'fs, which c:ausc~d a high(.r silica ;tctivity th;~n co~~sult(~clI)!, t111:scb it~l(~rt~stotlill an ovcrvicw of' the‘ rlsewh(,rc in this /.o~ic,,Lht~rot)y 1)rotlucing ~mortic~tiitr. utility :uld (lc~onoi~iicv:~lut' of zc'olitc~s. i\nalcimc>, which occurs locally within the, mord(~nitc~ zoncb, may 1i:ivc~ I)c~clnSortn(,cl from t~iorclvnit(~01- I~(SII- lantlite lul'l's by the action 01' solictic,n:, IIi:tt wcsr1>marc, alkaline, Locally than c~lst~whcrc~in t.11c. /.oII(~. 'I'hcs l'ic.l(l I.cast I)y (:ull':c~ ulitl ot.ht.rs ( 1'17:;) giv(*h rtt- li;rI)l(. rcsults and sl~oultlI'incl uscx in zc.olit(1 ~)rohl)tscting. 'I'h(1 art,:t st~~tlic~ti(pl. 1) is localotl ul,out 12 ~nilcls INTRODUCTION north of' 'l'ahnc>t.ii Pass 1,otlgt~at mil? 12.1 of lhc5 (;lomi ilighway. 'l'l~cb arcs;l is ac~c~rssihlc~11y c~itht~roff-thcl-road PLJKPOSLC vc~liiclc3s,via roads to nclarby placc'r mincs; or hy float- c~quippc~tlfiscad-\ving airc*raft, \vhic41 can 1;wcI at So~~th 'l'htl primary purpost, of tllis stutly wah to invcstigat~' 1,akc on thc S~LI~~ICL~II~stro~iiiL>~ of tllc, stud) arc?I. thr ~nordcnitcdtlposits of tllc, IIorn hlount;~irls artha, upper Malanuska Valley, Alaska. A suhorcli~latopur- pose was lo preparc' a rcco~lllaissa~~ccgcbologic Inap of the arca sl~owi~lgthe geologic srtti~lgof tlic ~nord(b~~ittx 'I'hr sti~dyc.onsistvd of' invcbstigatit~gtht, zcolitization deposits. A final purposcl was to samplr otl1c.r arclas in of th(> lowor Juri~ssic'I'alkt~c>t~la formation in thcs IIorn lhr vicinity of lhcl study arra in stlarch of otllcr zcwlitcs hIounlai11s. 12s showll by (irantz (1965), splay faults drposits. This study is a conli~l~tatio~lof thi~l rr,portcld associaltld with thv Caribou l:ai~lt have‘ isolatcd ant1 previously (Hawkins, 1973). cssposcld a triangular sclction ol' thv 'I'nlkt~c~tn:~formation ill tiit> IIorn hlounl.ai~~sarc.:i. l'hc Talkcrtna forlnatioll in this arcLac,onsists lnainly of thick ~111itsof pyrocl;istic S;III~S~OIIOS,siltsto11(~, i111d I would likv to thank I).C. llarlman. 'I'.k:. Smith, ~untl claystono (co:rrsc, a11d fill(, watcr-laitl tuffs); a~idr~sitic N.C. Vtlach of ll~rAlaska Stat(%1)ivisioll of (;c5ologic;ll ;111d basaltic 1ava.c art3 also prc~sc~nt(pl. 2). Altho~~ghtllr and Grophysical Survrys for funding, logistic.al, iuitl formation is mostly marincl, ilo marinc, fossils wc3rc' zunaly tical support of this study. I also wish lo tl~a~~kIny found. LVood fragmt,~itsand casts wcro ol,st~rvotlill sointl coll(~aguc~,1)onaltl (:r?;brck, for his assii;tanc.c~ \r.illl LIII~~S.'rhv 'l':~lk~(<t~li~I'or~lli~tiotl as :I ~llol(~stxcsms to I)(' t>,pical of a c~i~gcosy~lc~lin:iId('posit. SPECIAL REPORT 9 locating and mapping tliis u~iit.The tuff, which supports little or no vrgclalion, forms an easily visible wliilr cap 'I'lle primary purposti of thc1 study was to detcrminrl on the ro~undvdhills at thr hratl of Albert Crrvk. In the extent and grade of 111(' mordrl~iitt>deposit in thc outcrop, tho tuft' co~lsislsof flat or partly c*onchoiclal area. To this end, fist-sized samples wrrr collrctc~clfrom fragmctits about 5 cm it1 tliamolt~r. IJnit ,Jlk-2 appears the numerous outcrops of tuffs, tuff:rcc~ous lvackes, and to br discontin~~ouslydistribut.rd \vithin map unit Jtk-3. lavas in the area. Plates 1 iuid 2 show thc localities Carcful srarcli rrvt,alcd no rxposurcs of tliis unit ~iortll- from which samples wrrt collectt>d. cast of the fllbert Crrrk cXxposures.IIowever. thc unit may rxtthnd to thr southwest side of Alfred Crrck, wllert> sample localitic,~79 :uicl 83 may be part ol' it. Morc tlrtailcd mapping is nredt~dhcrr. Because general mapping was of secondary im- Map unit Jtk-l consists mostly of volcmiic sandstones portance, the grologic map ihown in platc 2 is in- and basalt atid ;undvsilc flows. 'I'uffs are prrst~ntbut arts complete. Salient feat~~resof tht' map arc bast~don not as abutidiunt as in othor units. IJnit Jtk-1 forms tht) Grat~tz (1965)---particularly thc contact bc~lwc~c~nthc cliffs and the summits of tticb mounlai~lsat th(, hc,ad of Talkretna formation and thc 'I'~~xcdniformation, ;uid Alfrc,d and Albt.rt Cr(1t.k~. 'I'hr unit is c~stensivt~ly the faults ((lither mapped or infrrrcd): this rcport also zeolitizrd, iuicl contains abundant veins of chalky-wliitcx contains details of (J~1ater11at-ydcposits ~iotshown in lai~montitt,and largr~ 2- to 3.~111 vug fillings of pink plate 2. Mapping of the units within thtl 'Salk('clna Ilc~ul:uiditc~.ilt the head of Alfred Creek basil1 is a formation was done as part of tht, study. 'Sllr, map 25-foot-thic*k layer of 11iortlt~nilt~-richtuff; altho~lgh units shown in plate 2 arr based on hand-spccimr~i mapped as brlonging Lo 111iit Jtk-1, this tuff has brt>n properties and topographic c,xprr,ssion of thc units. dtsignated Jtk-2 LO draw attts~ltion to its ~norctenitc Map unit Jlk-5, lhc casternmost unit. consists mainly t~nriclimtxnt. of volcanic sandstont, (with some tuffs), and basalt and Zcolitv zonation (tliscussc~don p. 6) indicates that alidesite flows. Thr unit ii ~nodcratrlyallrrcd lo clay nit Jtk-l was tho most drrply buricd unit and is mirierals. Zcoliti~alion is not pronounced, although stratigrirphically t.h(> lowchsl unit of thosc mapped. Unit Jtk-5 is consid(~rc~dlo bc. the uppermost unit. 'rhis in- sands1onc.s. Unit Jtk-5 forms a st)rics of low, t~orthr~ast- forencr is supporter1 by lhc gelir.ral 20-degree south- trending ridges at thtx mo~~tllsof Albr,rt and North r~irstt~rlydip of Lhc litl~ologicutiits mapped. Creeks. l:i~ially, tllrl contact botwecln thc 'I'alkc~clna formation Map unit Jtk-4 is cl~aracterized b>, pu~i~ic~t~ousi~tltl utiits mapped I~c,rc ancl thcb 'I'~~srdniformalioli mapped vitric* tuffs with suborditlatc tuffaceo~~swackcs, t~nsalt, I)y (irantz (1965) was not dclerminrd in the prcssrnt and andesite flows. Zeolitization is morc tlxlcnsivr than st.~~tly.Conscquc'ntly, tho area in platc 2 bctwerti tho in unit Jtk-5, with h~~ulaliclilcvoid filli~lgsanti vcinlc.1~ norlllc~astc~rlylimits of tliis study and tllc 'l'~~scd~ii especially common in exposures of coarser lithologies formatio~lcontact of Ciralltz has brcn Irlfl blank. in tllc upper parts of Albert and North Creeks.