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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 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 -bearing sedimentary successions, we Quaternary continental and marine sedimenls and have exainined the 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 , 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

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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

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Fig. 2. Geological sketch map and lithosiratigraphy of the (}urnai Mountains. Adapted and modified lrom (iafoer ct a/ (1992) and Moss (1995). laumontite occurrences given in standard mineralogy Qualernary and Tertiary ha~ been tropical rain forest. texts (e.g. Deer el al. 1963; Gottardi and Galli 1985) It is a region in which deep chemical weathering affecls describe evidence for three distinct parageneses for all rock types to a considerable depth, and many surface laumontite occurring in altered igneous rocks: weather- exposures show the efl'ects of hiteritization. ing, hydrothermal and metamorphic. "lhese lhree It would not be unreasonable to propose lhat the possible parageneses will be considered in the light of the low-lemf~erature secondary which are replacing awtilable evidence. mosi of the original high-temperature minerals of these volcanic and ,,olcaniclastic rock are the products of (1) Wea,qu, rin

the Sumatran occurrence as being due to the large fluid SJM40/94, from Quaternary deposits, deep burial and flows that characterise hydrothermal occurrences else- heating can be discounted. SJM83/94 from the Eocene where. The rocks are not veined, and the laumontite Kikim Formation potentially could have been quite occurs mainly in lithologies with a low permeability. deeply buried and hence heated. However, the Gumai In fact the andesite (sample SJM40/94) is not obviously Mountains are on the western margins of the South vesicular and would have been a massive rock with Sumatra basin and the Tertiary section is much reduced negligible porosity before alteration; the laumontite in in thickness in comparison to the basin centre, being in this rock is essentially replacive (e.g. pseudomorphing the order of 1 1.5 km thick. Elsewhere on the margins phenocrysts) rather than cavity-filling. of the South Sumatra Basin, low values of organic Despite this, there are similarities between the altered maturity (generally, less the 0.8% vitrinite reflectance) andesite and the type of alteration known as trod the results of apatite fission-track studies suggest "propylitic". Characteristic features of propylites are a burial temperatures less than 10() C (Moss and Carter drab green colour, and the development of the mineral 1996). The second reason is that the alteration of the assemblage epidote/zoisite + chlorite + albite ~ calcite andesite {Table 1) was non-isochemical in nature, since (Beane 1982; Hulen and Nielsen 1986; Bartos 1987). il involved a redistribution of alkali ions, which is much Laumonite is not usually considered to bca normal more compatible with hydrothermal origin. constituent of propylites, but Giggenbach (1984) has predicted that it could occur at tow lcmperatures (~ 100-200"C). Conclusions

(3) Metamorphic Of the three possible parageneses of laumontite in igneous rocks, chemical evidence argues against a The status of laumontite as a metamorphic mineral weathering origin of the Sumatran occurrences, and was first recognised in New Zealand, where zeolites, geological evidence argues against a rnetamorphic including laumontite, occur throughout the thick origin A hydrothermal origin is considered the most Triassic sedimentary succession of the Southland p,obable, although the absence of veining within the syncline. On the basis of studies in this region, samples is interesting to note. The localised occurrence Coombs et al. (1959) proposed the name "zeolile ['acies'" of laumonlite is also consistent with an origin by for conditions of metamorphism beyond the reahn of hydrothermal activity rather than weathering or diagenesis in sediments but below those of the metamorphism. The resembhmce between the alleration pumpellyite and greenschist facies. Sub- present in the Gumai Mountains and the propylitic sequently, "'zeolite facies" metamorphism has been ~lteration associated with porphyry copper deposits in recognised in Japan and other circum-Pacific regions. lhe southwestern United Stales suggests that the igneous The zeolite facies has not been universally accepted. bodies in this region should be considered as potential P T conditions intermediate between those of diagenesis exptoralion targets for hydrothewnal mineralization. and the greenschist facies must be very widely attained, since greenschist facies rocks are so common, 'and yet ~lcknowlt,d~,,cmenl~ SJM acknm~ledges the financial support there are many regions in which no developmenl of t'.I" thc Universit> of London Somheast Asia Research Group zeolites is recognised prior to the onset of the greenschist and sponsoring SE Asia Consnrtium companies ~.RCO, facies. ('anadian Occidental, Exxon. IASMO, Mobil and Union Experiments on the stability of laumontite in the I exas. Dr Bona Situmorang, formerly the Head of Research presence of quartz under water-saturated conditions l)ivision of LEMIGAS, lakarI;~ Indonesia, is lhanked tbr (Liou 1971; Cho el al. 1987) suggesl thai under a arranging tieldwork permits. We are indebtcd to Nick Walsh l~r the chemical analysis given m dUs paper and lo Andy geothermal gradient of 30 /kin taumonlite ,aould be Mc('arthv and Diane Camelon h',l Iheir proof reading skills. stable in the temperature interval of 170 :~00 C (and at a lower temperature), but laumontite could occur at lower temperatures if the geothermal gradient were higher. However, free silica is not present in the REFERENCES rocks from Sumatra and heulandite does not occur, so Eartos P..I. (1987) Quiruxilca, Peru: mineral zoning and tmling of walhock aheration relati'< t{} Cu Pb Zn Ag vein-lill deposition. the experimental evidence is a poor guide lo possible Ec:m. (;col. 82. 1431 1452. formation temperatures. Miyashiro (1994)pointed out Ecanc R. E (1982) Hydrolhernml theration in silicate n)cks. In the signiticance of the equalion: 4cb'an{ ~,~ i1~ 1/i(, Geolo£) <~1 :h,. Porplu'rv Cot>per Del~o.~its. .~mtlnr{~/ern North America. l edilcd h,, Titlcy S. R.) I Ini\crsity of laumontite + CO: Al-i/i}n~i Press, Tucson. := calcite + kaolinite + 2quartz + 2H,O, ('apdcc~mmle 1.. (1952) Laumontitc du Pla des Areillan-, (Pyrcnees- Oriemalcs). B~dl~,lm Soci{;t~ lt,,,,t ,in, Natm'elh, Toa/~,u.~c 87, and suggested that laumontite could not occur if the 299 t()4. chemical potential of CO, was high, its place being taken ('ho M. Maruyama S. and liou I (i. (1987) An experimental by calcite and clay minerals. However, some of the inxeMigation of heulandite-laumontilc equilibra al I000 t~l 2000 bar laumontite-bearing rocks from Sumalra do contain P,~,. ('ontrih. Mineral. Pelrof 97, 43 50. traces of calcite. ('o,)nlhs I). S.. Ellis A. J.. F~fe W S ~md Taylor A. M. (1~59) The zeolile iacies, v, ith commems Dn fl~c ~nlcrpretation of h', drolherma[ There are lwo main reasons for not considering the synlhesis Geo~himica el ( u,rmuch/m~u 4~la 17, 53 I07 Sumatran occurrences of laumontite as metamorphic in l)cer \~. A.. Itm~ie R A. and /LIS~,DIa[I] .I. (1963) Rock-t'?~l'mi¢lg origin. The first is that they occur within a succession .]ltll('rCl[S. I0[. 4. k)'amework St/h~:/'s Ix)ngmall:-,, [.oildoll. which has not been subjected to a high temperaturc at l:a urc (,. { I '4921 Prim'i/de tllld 4ptdi{ a:~, m ~ c~/ IIlor~:(lllic (l~ (:{/'~'l~li.Sl~'l' any time since deposition. For samples S,IM38 94 z~nd Macmillan. Nc'~ York. Occurrence of laumontite in volcanic and volcaniclastic rocks 59

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