PROCEEDINGS, Thirty-Fourth Workshop on Ccothermal Rcsewoir EnginccringStanford University. Stanford California. February 9-l l, 2009

SC;P-TR. I 87

ABSTRACT

Gedongsongo surfbce manifestations of the Ungarangeothermal prospect in Central Java, Indonesia, have

f,igtr tempeiaturi properties, 72 - 870C,2 - 6 pH and770 ppm of chloride. The stable isotopic ratios of6180 and 6D of t'wo adjacent manifestations show a

slightly different enrichment. The westem partreveals a rnore magmatic water influence than those

o[ the eastem one. This is supported by the presence

of relic manifestations as most occurs in the easternpart; whereas active fumaroles and steaming groundexist at westem part.

INTRODUCTION

Ungaran Volcano is located at northern part of the

Java volcanic arc (Figure l). Java itself is developed

by the north-south modem subduction system of the

Indian oceanic plate and Eurasian continental plate

(Hamilton,1979). Ungaran lies within 4

stratovolcanos range, i.e. Merapi, Merbabu,

Telomoyo and Ungaran (from south - north; Figure

1). Bemmelen (1949) describes the Ungaran volcano

as composed of augite-olivine basalt-andesite (Lower

Pleistocene), augite-olivine basalts (upper

Pleistocene), augite-hornblende(biotite) andesite

(upper Pleistocene and Holocene) and hornblende

andesite (Holocene). These three distinct

compositions are recognized as the products three ofmajor volcanic eruptions which were separated by

two collapses stnrctures (Bemmelen, 1949). This

stmctlre type rnight control surface manifestations at

Ungaran.

Some geothermal manifestations appear to surroundUngaran volcano, i.e. Kali Ulo, Diwak, Banaran and

Gedongsongo. In this paper, the author focuses onmajor manifestations that occur at Gedongsongowhich is situated at the southern flank of Ungaransummit. An update of manifestatior.r descriptions and

fluid origins are described.

Overview of Gedongsongo Manifestations of the Ungaran Geothcrmal Prospcct'Central Java, Indonesia : a preliminary account

Mochamad Nukman

Geophysics Study Program, Gadjah Mada UniversitySekip Utara, FMIPA - UGM

Yogyakarta, 5528 l, Indonesiae-mail: moch nukman@ugm.ac.id

Figure l; Localiryu o"f (lngaran GeothermalProspect, Central Java, Inclonesia

TTIERMAL MANIFES ATIONS

Thermal manifestations of Gedongsongo are in the

form of fumaroles, hot springs and steaming ground.The active thermal manifestations mostly occur in the

western part of a slnall creek on the southenr f'lank ofUngaran summit (Figure 2). Measured temperaturesmade using a thermistor at the verlt of fumaroles (size

0.75 x 0.50 m) reaches 850C (measured in November2008) with significant mass flow rate (Nukman, irrprep.) The fumarole discharges fiom steep ctiff;whereas steaming ground (45 x 30 m) rvith sirnilartemperatures coexists in higher topography. Thissuggests that flimaroles were fomted due to heatedground water which in bisected topography (Figurer

3). The temperature of hot spring (eastern part) andsieaming ground (southem part) ieaches about 400C

and 50uC, consecutively.

In the eastern part of the creek exist relicmanifestations (Figures 2 & 3). There are many dead

trees covered by sulphur and mud. The su\thur odorcan be recognized at very close distance, and thereare tiny fracfures over the ground also covered bysulphuric mud interpreted as relic 'rvarm or hot springchannel mouth.

E+.. ..-

The altered ignimbrite and andesitic roeks re,"'eals ahigh alteration intensiq- (0.8 of 1 AI of Brow,'ne,

1999) and per-vasive silica replacement is alsopresent: there trre not manv feldspar left. Relicepidote also appears at solre outcrops.

FLUID T}'PE AII} ORIGIS

1!,'ater samples ol nanif-estatlsn lvere cclL:cted, ililct.i

in rinsed pol3.'e!hene bcttles. ft'rliy tilled anci sealeri ici

avoirl any precipitation pria; analJ':;is pr*ces:e.:, Tlre

chemieal constiruent-c r!e.e an*l1zed b-_v the

Llhemistr:- Depafiment of {iGM ilndor:esia.r and

isotopic constiluents lveie measur*d b;; G\-5 LcucrHutt $iew Zealandl. Thc chernical corlpositi+n r-;

shown in Table L

The West Ung-01 sarnpL- u'as coilecr'ed fio;ttfunraroles at the s/e,\teri clilf: the Easl Ung-Oi w'as

collected from hrrt sprine located just a le.* lreters to

east r-'rl the \\,'est Un.e-O1 location. The chemical

cofltents are plotted Ltrr lernar-!' rliasran: ci {-:i=5D.;-

HCO3, to identifu fluid t1pe, i.e. maf iii.r-:, voleanic.

stearl heated anrl periphe ral tlpe iGiggecbaclr. 1 991 ).

The isotopic conrposilion of rr.'ater data is pi+iled in

5D versus 6'tO diagrani anii compared to nret:crlervater line of Craig i liJ6 I l to identi{}' fluid origin and

process, i.e, nreteoric. magmat!c, mixine. boiiing.vaporization and precipitation f Crar-s. 195 I ;

Truesdcll,l 977, Ellis & \'lahon,l 977i.

Tahle 1

('henicrtl nntl i.rotoltit' t oil.\liutrttt.\ tuitit; o .,.,i rtlGedangson.qo manilestation. L!ngaran -'E::othenna/prospect, Cenftal Java. Indonesiu,

local metectric v'ater icode: cald creek) ha:;

d''o : -7.7, 6 D = - 46 6 in o;,,,,

Figure 2: Lacaliq,n of surJaceGeelangsongo.

rnanilbttatiosrt aJ

,^ ___:{_:-"i*'r.i 3:15* s.-:.tr -.=i.i J.:rc:

iir_ : j+rf:ai:=i

: :r:! : +a-r:r:i.=', i: - !-+l !*ii-i : .: n.::":r.:i:!=l+.1,'_ -

, ,=i: k.i...j:rj

Figure 3. East-tt'est cross-section o,rler

Gedongsongo manifestatiotls (detailedexplanation tlescrihed in the text).

The distribution of manifestations shorvs a N-Saligrunent (Figure 2). parallel with steep cliff (andsmall creek) rvhich possibly f<rm.red due to a collapsestrueture L)r nomral l-aulting as commonly occur invi-rlci-Lnic regions. This permeal:le structure isconsidered to conroi the convective heat kanst-er. Asiliciflcierl fracture inrlicating a nornral fault is foundat lhe relic stearnrng ground al northern part {Figure2). There is no reiic epidote in this site.

.t

I -eication Wcst Une-ill E;rsi l:it

pll

Li

Na

K

Ca

Mg

CI

Sa)I

HCOI

SiL-):

7

{J. I

( l.8

54.1

t.l

404

?5-i.-l

i008.1

'lll.L)

6.?

| .o

62

i0.q

_i.?

?ir0. il

! se.6

a'to.\D

,6i{

s.l -4!).4

The West Ung-01 is more sulphate rich water with a

significant amount of chloride as illustrated in Figure

4. This composition $uggests a mixing process has

been occurring in region.

c^ HCO.

Figure 4: Gedongsongo water type based on SOa-

C|-HCO3 ternary diagram of Giggenbach199t.

The isotope data (Figure 5) is parallel with this

interpretation as the West Ung-01 shows enrichment

of 6180 relative to Craig meteoric water line, but does

not reaches the magmatic box area where 6180 is

higher (enrichment in l8O content than those 160 as

commonly ocour at igneous rock and in water

reaction contact with magma; Ellis & Mahon, 1977).

Although East Ung-02 is classified as matue water

type but has depleted 6180 and lies on the global

meteoric water line of Craig (1961).

Figure 2: Oxygen isotope positive shift of West Ung-0I relative to meteolic water. The

meteoric waterline is constructed basedon the global meteoric line of Craig(1 e6 t).

DISCUSSION

Gedongsono fluid type is classified as acid sulfate-chloride water which consistent with collapsedstructure processes. A slight magmatic fluid influenceis recognized in the western part of Gedongsongo,where the active manifesatotions exist; this suggest

that a mixing process is occurring in this region. Totest this interpretation, we plan additional samples

will be collected and analyzed (in particular forisotope) in the near future..

AKNOWLEDGfMENTS

Prof Pat Browne of Auckland Universiry is thanked

for suggestions through email correspondences. Ms.Valerie Claymore of GNS Lower Hutt and ChemistryDept of UGM are greatly appreciated for the isotope

measurements and chemical analyses, subsequently.My wife Lina and daughter Amelia are thanked foraccompanying me on the last field trip. This research

is partially flurded by LPPM (Research Division) ofUGM.

REFERf,NCES

van Bemmelen, R.W. (1949), The geology ofIndonesia, vol.lA, Martinuj Nujhoff, TheHague.

Browne, P.R.L. (1999), Hydrothermal Alteration,Lecture note 655.61i, Geothermal Institute,Auckland University.

Craig, H. (1961), "lsotopic variations in meteoricwaters", Science, 133, 1702-1703.

Ellis, A.J., Mahon, W.A.I. 1977, Chemistry andgeothermal systems, Academic Press, NewYork.

Giggenbach, W.F. (1991), Chernical techniques ingeothermal exploration: in Applications ofGeochemistry in Geothermal ReservoirDevelopment (ed, F.D'Amore), I 19- 144.

Hamilton, W. (1979), Tectonics of the lndonesiaregion, Gelogical Survey Professional Paper1078, US Govt Printing Office, Washington.

Hochstein, M.P., Browne, P.R.L. (2000), Srufacemanifestations of geothermal systems withvolcanic heat sources, Encyclopedia ofVolcanoes, Academic Press, 83 5-855.

50

n West UhS-01i East Ung-02

$,a.{,.

".^*"ty'

Hochstein, M.P., Sudarman, S. (2008), "History ofgeothermal exploration in Indonesia from1970 to 20A0" , Geothermics,3T ,220-226.

Truesdell, A.H., Nathenson, M., (1977), The effects

of subsurface boiling and dilution on the

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