Jour~ml ol .4~iall Earth 5,c*,m<s. Vol. 15, No. I, pp 55 5% 1997 I 1997 Else~iel Science Lid Pergamon .M rights leserved. Printed in Great Britain Pll: S0743-9547(96)00080-3 1367-912(I 97 $17.00 + 0.01) The occurrence of laumontite in volcanic and volcaniclastic rocks from southern Sumatra Anthony Hall* and Steve J. Mosst *Dept. of Geology, Royal Hollo~ay, Universit~ of London, Egham, Surrey, TW2(i 0EX; +SE Asia Research Group. Dept. of Gcolog3. Royal Holloway, University of London. Fgham, Surrey, TW20 0EX; now tit School of Applied Geology, Curtin University of Technolog3,, Perth, 6001, WA, Australia ( Recuired 12 ,S'epte#nDrs 1996: ,:lCCUl,lUd/br publication 10 December ! 996~ Abstracl Laumontite has been discovered in Tcriiary and Quaternary volcanic and ~olcaniclastic rocks of the Gumai Mountains, soulhern Sumatra (Indonesia). Descriptions arc given of three particularly laumontite-rich samplc~. The laumontite in these rocks is considered tt~ he :l product of hydrothermal alteration ratherthan of weathering or metamorphism. ~ 1997 Elsevier Science Ltd Introduction mountains trend NW SE and are cut by' a series of SW to N E flowing rivers. Figure 2 summarises the Zeolites are a common constituent of volcaniclastic lithostratigraphy of the area. The mountains are cored rocks which have undergone diagenesis or hydrothermal by late Jurassic to early Cretaceous sediments, volcanics alteration. As part of a wide ranging reconnaissance of and granodiorite and tire surrounded by Tertiary and potentially zeolite-bearing sedimentary successions, we Quaternary continental and marine sedimenls and have exainined the mineralogy of volcanic sequences in volcanics. southern Sumatra, Indonesia. The zeolites which occur in lavas and elastic volcanic Description o/ Tertiarl" lctumenlHte-bearolg rolc,~.mics o~ rocks form mainly by the alteration of an original glass the Ounlai Mountains phase, and can include about 5 or 6 common zeolite species and a number of other less common ones. The A selection of volcanic and ~olcaniclastic specimens particular zeolite species which are present depend partly were exalnined by X-ray ctifl'r;tutometry. The only zeolite on the composition of the parental igneous rocks and found in these rocks was laulnontite, which was present partly on the conditions of zeolitization, i.e. tempera- ill about a third of' the samples examined. ture, pressure, and hydrological conditions tn an c, pen Votcanics from two horizons were found Io contain hydrological system, clinoptilolite and mordenite are the laumontite: they are the Eocene Kikim Pormtltion and zeolites most commonly present if the parental glass is Quaternary deposits from tile Bukit Balai volcanic acid-intermediate in composition, whereas phillipsite centre. The Kikim Formatimt, the oldest Terliary rocks and chabazite are commonly present if the parental glass encountered on the margins of the Gumai Mountains, is rnore basic or alkaline. Large deposits oI" zeolite-rich are a sequence of volcanic breccias, andesiiic lithic and pyroclastic rocks occur in several countries, and are ~'rystal tuft~ and lavas. Many of the deposits can be increasingly being exploited for a variety of economic described as epiclastic. These are suggested by Van applications (Ming and Mumpton 1995) Gorscl (1988) and Gafoer ~,l ,w!. (1992) to be Eocene in In this note. we describe some new discoveries of age. laumontite in Sumatra, and discuss their significance in Good exposures of the Kikim Formation tire present relation to the various possible paragcneses of this along the Must river wtllex in the northwest Gumai mineral, i.e. its conditions of' formation. Mountains and in the Sating river on the northeast margin of the mountains (Moss 1995) (Fig. 21. The ~ample richesl in laumontitc, SJM83,94, is from the Laumontite in South Sumatra upper part of the Kikim F~:rmation along tile Saling river at location 103 20.()7"1 , 03 47.94'S. In c, ulcrop, Geological hack<ground decimetre beds of reddish-purple lithic tuff-breccia are interbedded with purple coloiaed claysiones. Bedding is The samples examined are from the Gumai mountains ,:menlated 120 147 NE, in comrnon with the entire in South Sumatra Province. This area lies oil the eastern Fertiary sequence along lhe Saling river section which flank of the Barisan volcanic arc which extends the dips and youngs toward the northeast. length of the island, and is close to several Quaternary Unconformable ttpon the Tertiary formations of the to Recent volcanic centres such as the active Gunung Gumai Mountains are several Quaternary formations Dempo and the dormant Bukit Balai, Bukit Kaba and ~Fig. 2). Along the Must rivet valley andesitic to basaltic Bukit Condong centres (Fig. 1). ava flows, tufts, agglomerales and volcaniclastics The Gumai Mountains are located on the western derived from Bukit Balai volcanic centre overlie Tertiar~y edge of the South Sumatra basin (Figs 1 and 2) to the formations of the NW margin of the Gumai Mountains west of the towns of Lahat and Muaraenim. The t Vig. 2). Samples S,IM38i94 and SJM40/94 are from 5"~ 56 A. Hall and S. J. Moss ~,",,. ~hwBukit Q~ Q % w W Co.do.g/ Q- I, ~ ~s, / Bukit ~ ) Q ~ Q ~ Q A Ba~i~f~l ' Muaraenim \ Bukit \ Q °engmau ~x ~ 4°S • --Mts. Q 4r Gunung ur~ Dempo Q Q \ IlL Q ~ Major faults ..... .... Mt. : ~,, " --- "-) ....... Q = Quaternary , ( sediments & volcanics v \ > \~ ~ Mi~ene granite Indian Q E q u a I o r I_~ ..... a .... Eocene to Miocene sediments & volcanics Ocean ~ " ~ ' ~ Jurassic. Cretaceous Lake . 4 ~ IL~ 102 ° 0 80km 103 ..... 50-O;m - ,,7 5os ',, 5"S I I , i , [______' ~:Q 5°s-_ Fig. 1 .... Fig. 1. Location of the (JUlll:lJ Mountains in lhc western part of Soulh Stilll:ltra. volcaniclastics and lava flows from the Bukit Balai Specimen SJM40/94 is from a lawl flow' and has the deposits to the southwest of Ulak Dabuk village, at appearance of a typical andesite from Quaternary 102 58.67'E, 03 41.35'S and 102 57.74'[!. 01 42.75% volcanics associated with the Bukit Balai volcanic centre respectively. I Fig. 2). It is homogeneous in texture and contains abundant, soft, white, rectangular, idiomorphic phe- Description ~/ the /¢lto~lolllllc-/~'orillg S¢lt/Ip/d> nocr.vsts up to 5 mm across, in a iine-grained, dark grey.' ~:!roundm;_tss. The predominant white phenocr~sts are of Within three samples investigated laumontite is a plagioclase, pseudomorphed from 0 to ]()()'!G by major constituent. These were an andesitic lava and two laumontile. A few small euhcdrai phenocrysts with the fragmental rocks with mainly volcanic clasts. These characteristic 8-sided shape of pyroxene arc entirely rocks have a completely unmetamorphoscd appearance pseudomorphed by calcite. The matrix shows flow with no penetrative deformation. They shm~ no vetoing alignment of small laths of plagioclase, and X RD reveals and no recrystallization is visible in hand specimen. The Ihe additional presence of laumontite, minor smectite main feature of the rocks are as follows. and zoisite in the matrix. Sample SJM83/94 is a lithic titff breccia fronl a A chemical analysis of samplc SJM40J94 (laNe 1) sequence of bedded volcanic breccia, tuffbreccia and shox~s that it has a broadly andesitic compositiom but claystones from the Eocene Kikim Formation (Fig. 2). with abnormally high Na and low Mg conlents. The The sample is poorly sorted and has sub-angular immobile trace element ratios (Zr'Ti(): = 0.1)188; clasts up to 1 cm across. The most common clast types NbY = 0.14) place the rock within the andesite field of are soft, pale grey lava and a hard red laka. "lhe matrix Winchester and Floyd's (1977i magma discrimination is a medium grey colour. Laumontite occurs in all diagram. the clast types and the matrix, but the clasts and matrix are mostly too fine-grained or too impregnated by hematite to locate much of the laumonlite in thin Discussion of the paragenesis of section. the laumontite Specimen SJM38/94 is a conglomeratc with poor sorting and rounded clasts in an abundant greenish-grey The circumstances leading to the formation of matrix of sand grade. The sample is flom Quaternary laumontite are less clear than for many of the other deposits from the Bukit Balai volcanic centre. The zeolites and, unlike the more commonly occurring outcrop wets heavily weathered and the rock is poorly zeolites, laumontite has not been produced in laboratory lithified and crumbly. The clasts are composed of several experiments by the alteration of natural glass. Moreover different types of lava, including some with fresh there are many regions in V¢]lich altered volcaniclastic pyroxene phenocrysts and some with abundant vesicles. locks are rich in zeolites without containing laumontite. The sand-grade matrix is mainly composed of single Nevertheless, laumontite doe,, occur in the altered crystals of pyroxene and plagioclase. Laumontite occurs volcanic rocks of some -eg ons and various explanations as a cementing material between these crvslals. of its origin have been proposed. Accounts of individual Occurrence of laumontite in :olcanic and volcaniclastic rocks 57 9~4~RJYlION E • 103°15+I ~x .1" ' 1~0 I i ~ 2(Jkm South Sumatra g2/:/:L~IEI(i(,' /./:/; *-" " K k rl ¢t~,ml~tl t~ wrr~'n~ ~,! / //.-: /"/~ "" Basin SJM40/93. '1A ."//./:/',% ,-R;,/./(/.( .,(/: ,.1 ,\ Bungamas /: ?, i~ Bukil Baiai volcanics ~..['~. "-:<~.: ;,:., ~ ~, 7 sediMi°cenemenlco Plioccnc L...~- ~~ IS ..... i1 PRE<FERTIARY L........... {~ TalangAkar F..... nor, "~-"Q: "; ";: "~.:/. ",.~. SJM83/94 BASEMENT ~.~,~,c~rr,,cuxn ~m ~ ~--s ~*a,ers ~,~,'~ / i~ ,l, , / l~ ,< ..< .,.L ...'. i{ ,~ SSB.SO~ISLII4A~ABI,~I~, OH¥ffiI~UKiT~AIyVOCCARIC KikimFormation ~"~': //' / </: 1/': /././: l/J'/ [~ Quaternaryvolcanics & ] ''/A, Kikim Formation L~ sediments I .....<': -~ Jurassic CrclaceousBasement ~] Qualemaxyrhyolite, I docile & andesiteflows ] 103o00, 10~ ~ 15' Fig. 2. Geological sketch map and lithosiratigraphy of the (}urnai Mountains. Adapted and modified lrom (iafoer ct a/ (1992) and Moss (1995).