Jul 1, 1990 - Abstract. Tikus is the most important granite-hosted primary tin-tungsten deposit on the tin island, Belitung. The bulk of the ore is located in ...
EconomicGeology Vol. 85, 1990, pp. 691-713
Greisenizationand Albitization at the Tikus Tin-Tungsten Deposit, Belitung, Indonesia MICHAEL
O. SCHWARTZ
Bundesanstalt fiir Ceowissenschaften und Rohstoffe,P.O. Box510153, D-3000 Hannover51, Federal Republicof Germany
AND SURJONO Directorate of Mineral Resources,Bandung,Indonesia Abstract
Tikus is the mostimportantgranite-hostedprimary tin-tungstendepositon the tin island, Belitung.The bulk of the ore is locatedin greisenzoneswhichenclosevery irregularlyshaped bodiescomposedalmostexclusivelyof quartz ___ cassiterite___ wolframite.A smallvolumeof ore is alsolocatedin albitizedgranitewhich gradesinto albitite. Detailedgeochemical studiesindicatethat the greisenandalbititewere formedby alteration of a K feldsparmegacrystic medium-grained biotite granite.Greisenizationproducedan enrichmentin Sn (avg 3,000 ppm), W (1,110 ppm), Fe203, MnO, Bi, Cs, Cu, F, Li, Pb, Rb, and Zn, aswell asa depletionofNa20, CaO, andSr. Albitizationis characterizedby an enrichment in Na20, CaO, A1•O3,Cu, F, Pb, Sr, and Zn, aswell asby a depletionof K20, MgO, SiOn, and Ba. However, Sn (2,100 ppm) and W (914 ppm) were concentratedonly with moderate albitization(albitizedgranite)whereasextremealbitization(albitite) is characterizedby low concentrations
of these metals.
CO•-H•O(-CH4-salt) fluid inclusionswith highly variable CO2/HsO ratios are the most abundanttype in the quartzof the greisenaswell asin the coarse-grained quartz___ cassiterite ___ wolframitebodiesenclosedin it. They homogenize in the 200øto 340øCrangeandindicate minimumtrappingpressuresof 0.7 to 2 kbars.CO•-(CH4) inclusionswith densitiesof 0.53
to 0.99 g/cma withouta visibleH•O phaseare lesscommon.Aqueousinclusions withouta visible CO• phaseare subordinatein greisenand enclosedquartz bodies(0.5-8 equiv wt % NaC1)but are the onlytype observedin albitizedgranite(1-18 equivwt % NaCI). The CaCI•/ NaCI ratiosare highin the albitizedgranite(up to 1:1); the homogenization temperaturesare in the 150 ø to 355øC range. It is assumedthat an evolutionaryrelationshipexistsbetweengreisenizationandalbitization. Na and Ca were releasedthroughfluid-silicatereactionsduringgreisenizationand the fluids lost CO2(-CH4) with falling temperature.The resultingNa-Ca-enrichedaqueousfluidsdepositedalbite and fluorite by replacementreactionsin the granite surroundingthe greisen. The majordepositionmechanism for ½assiterite andwolframitewaspH increaseandconcurrent temperaturedecreasein both greisenand moderatelyalbitizedgranite(with secondarymuscovite and topaz). Strongalbitization(albitite) did not increasethe fluid pH, and therefore, did not precipitatecassiteriteor wolframite. Introduction
of unweatheredtin-tungstenmineralizationin greisen and associatedalbitized granite.
THE Indonesiantin island Belltung constitutesthe southernmostpart of the southeastAsian tin belt Outline Geologyof Belitung Island and the which extends through peninsular Malaysia and TanjungpandanPluton Burmaasfar asnorthernThailand.Belitungproduces Three-quartersof Belitung Island is made up of a about5,000 metric tonsSnper year, mostlyfrom alluvial deposits.Tikus is the mostimportantprimary sequenceof Carboniferousto Lower Permian sanddeposithostedby granite on Belitung.It produced stones,shales,and chert (Adam, 1960; Osberger, approximately500 metric tonsof both Sn and WO• 1968; Fig. 1). Minor basalticsillsoccuron the north between 1916 and 1920, when the mine was flooded. coast.During the Triassic,thissequencewasintruded The undergroundmine hasnot been accessible since by gabbroicto graniticplutonicrocks(Aleva, 1960; then,but the Indonesian miningcompanyP. T. Timah Priem et al., 1975; Joneset al., 1977). carried out a 2,200-m drilling programin 1984 and The 900-km• Tanjungpandan plutonwhichhosts 1985. The availabilityof this fresh drill core makes the Tikus depositoccupiesthe northwestcorner of Tikus a unique place in southeastAsia for the study BelitungIsland(Fig. 1). It is composed of severalvao36•-0 • •2•/90/• 063/69•-•23$3.00
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rieties of biotite granite,of which a K feldsparme- been a time gap betweenthe coolingof the biotite gacrysticmedium-grained biotitegraniteis the most graniteandthe actionof the mineralizingfluidsthat abundant.Syeniteandgabbroconstituteabout1 per- is slightlygreaterthan the analyticaluncertaintyof cent of the area of the pluton (B. Lehmann, writ. the measurements. commun.). During the Neogeneand Quaternaryperiodsof The related tin depositsare generallyquartz-cas- erosion, cassiterite was concentrated in alluvial desiterite(sulfide)veinswhichintersectthe metasedi- posits.Tikusis the mostimportantsourcefor alluvial in the areaof theTanjungpandan pluton.Two mentarycountryrock.Primarytin deposits withinthe deposits graniteterrainarelesscommon.They consistof grei- large placerswhich originateat Tikus in the center sen-bordered veinswarmsandgreisen,of whichTikus of the plutonextendnorthandsouthwest,respectively is an example.Replacementdepositsin reactivetuff- (rig. 1). aceousbedsalsooccur(SchwartzandSurjono,1990). Geologyof the Tikus Deposit The Rb-Sr age of the biotite graniteof the Tanjungpandan plutonis215 ___ 3 Mawithaninitial87Sr/ The Tikus depositconsistsof severalgreisenized S6Srratio of 0.714 ___ 0.0005 (errorchronfor 8 points zonesin granitewithinan areaof 500 >< 100 m and 100 >< 80
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m (SurjonoandClarke, 1982). Recentdrillingshowed that the greisenextendsto at least-130 m. The greisen enclosesquartz bodieswith coarse-grained cassiteriteandwolframite(Groothoff,1916). According to old mine plans,the quartz bodiesmeasureup to severaltens of meters.The recent drilling did not intersect them, probably because they have been mined out. For this reason,the investigationof the quartz bodieswasrestrictedto dump material. The greisen grades laterally into muscovitized granitewhich exhibitsweak plasticdeformation.Toward the surface,the greisenzonebecomesnarrower and gradeslocally into stronglyweathered megacrysticmedium-grained granitewith brecciationfeatureswhichare hardlyrecognizable dueto the intense supergenedecomposition near the surfaceof the deposit.R. G. Taylor (oral commun.)proposesthat the major greisenzone may be a disguisedbreccia.This is difficultto prove becauserelict texturesare scarce in the core samplesof the greisen;if they are present, they exhibitthe sameweakplasticdeformationasthe surroundinggranite (seebelow).
Biotite granite
The most abundantrock type in both the Tikus area and the Tanjungpandanpluton is a megacrystic medium-grainedbiotite granite. It has a mediumgrainedmatrixwith subhedral,occasionally normally zonedplagioclasewith An3.2to Anlo (10-30 vol %), K feldspar(10-30%), quartz (25-35%), and brown biotite (4-10%) whichispartly chloritized.Muscovite is subordinate(3-4%) and occursmainlyasan alteration productof plagioclase.About 5 to 30 percent K {•ldspar megacrystswith a grain size of 10 to 60 mm are enclosedin the medium-grainedmatrix. K feldsparis often perthitic and showscross-hatched twinning.
Fluorite (up to 1%) occursin interstitialposition between quartz and feldspar.Other accessoryminerals are allanitc (up to 0.5%), apatite, zircon, xenotime, and monazite.
The rock exhibitsa texture characteristicof plastic deformation,with bent plagioclase lamellae,distorted microclinetwinningandCarlsbadtwinsin K feldspar,
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