Metallogeny of tin in central Thailand: A genetic concept

Bernd LehmannInstitut für Angewandte Geologie, Freie Universität Berlin, Wichernstrasse 16, D-1000 Berlin 33, Federal Republic of Germany

Chamrat MahawatDepartment of lvlineral Resources, Geological Survey, Rama Vl Road, Bangkok 10400, Thailand

ABSTRACTTin mineralizalion in central Thailand is

associated with granitic rocks of the Thai-Burmese border range (westem group). Gra-nitic intrusions €st of Bangkok and near the(ampuchea border (eastem group) have notin. Fractional crystallization is the frmda-mental petrogenetic process thaa controls lheevolution of both groups of granites. The tin-bearing alkali-feldspar aplogranites of thewestem group display an extreme degree ofdifferentiation lhat has no petrological equiv-alent in the easlern group. These aplogran-it€s are the product of a combination ofmagmatic fmctionalion (primary fin enrich-menl tr€nd) modified by lluid interaction. Thelatter proces is responsible for a secondarygeochemical lin deficiency that is balanced byredishibution of ain in lracture systems-i.e.,tin mineralization. The pattern of tin deple-tion in the aplogranites provides ar indicalionof the tin polential in a given ore-formingwslem.

INTRODUCTIONTin deposih are generally associated in time

and spac€ with granitic inbusions (Ferguson andBateman, 1912; Taytor, 1979). Th€ SoutheastAsian tin provinc€ (Fig. l) coven parts ol In-donesia, Malaysia, Thailand, and Burma andcontributes about 40% of the current world tinproduction. Mineralization is associated withganites of both Permian-Triassic (mainly in In-donesia and Malaysia) and Cretaceous-Tertiaryage (particularly in Burma and Thailand). Cen-tral Thailand displays a remarkable polarity inthe distribution of tin mineralization. Graniticrocks east of Bangkok have no tin, whereas thegranites of the Thai-Burmese border range inwestern c€ntral Thailard host numerous andimportant tin ore deposits (Fig. l).

Figule l. Outline ot Southeast Asian tinprovlnce. Black ldangles locale maior tin andlin-tungsten ore deposits with moae than 5O0OI ol Sn metal content lor productlon plus re-sewea; cro$es indlcale larger glanite tei-ranes (adapted Irom UNESCO, 1985). Li e.k.rown tin deposits in central Sumath anono heast ol Vientiane (Laos) could eventuallyenlarge lraditional tin belt.

CEOLOOY, v. 17, p. 426-429, May t989426

- This circumstance Fovides a starting pointfor a petrographic-geochemical comf,arisonaimed at explaining the difer€nce betwe€n tin-bearing and tin-ba[en granites and the criticalfactors leading to the formation ofa tin deposit,

GEOLOGIC SITUATION ANDSAMPLING

Three different gradte lenanes have be€nsampled in an east-w€st transect across centralThailand.

1. Tin-barren, I-type gaoites of the Chan-thaburi region near th€ Kampuchea border(Permian-Triassic age). These rocks consislmaioly of K-feldspar megaq]Btic biotite endbiotite-homblende granites, with subordinateamouots of fayalite-hornblende granite andsome subunits of nonporphyritic biotiüe granile.The Chaothaburi intrusions form a comDosi-tionally exrended rock suite with SiO2 66 to 76wl% and metaluminous lo weakly peraluminouscomposition. They plot over th€ full length ofthermal minima from I to l0 kbar in the €xDer-imentally iovestigated graniric sysrem (T;rdeand Bowen, 1958; James and Hamilton, 1969).

2. The tin-baren, S-typ€ Rayong batholith,east of Batgkok, consists mainly of K-feldsparm€gacrystic biotite granite with variableamounts of s€condary muscovile. It has a Rb-Srisochron age of 221 :Lll Ma; 875r/8654 =0.7263 f6 (Nakapadungrat et al., 1984). TheRayong batholith shows little variation of majorelements. and it plots near the thermal minimumat I kbar pressure in the exp€rim€nially deter-mirted hydrous gmnite system.

3. Granites ofthe Thai-Burmese border ransein western central Thailand ur"

"oroorJdchiefly of K-feldspar megacrystic biotite gra{Nwith variable amounts of muscovite of subsbli-dus formation. They are intruded by smallalkali-feldspar aplogranite stocks. Modal com-positioN and major€lement conlent of tbeK-feldspar megaqystic ganites are similar totbose of the Rayong batholith; thefu ag€, how-ever, is Late Creiac€ous (BeckiDsale et al,,1979). The tin-tungsten mining areas of pilok inThailand and of oeighboring Hermyingyi inBurma are cent€red on apical parts of alkali-feldspar aplite stocl(s. The aplogranites are oftransitional magmatic-hydrothermal odgin; th€yhave the typical mineral assemblag€ quartz-microcline-albite-muscovite-tourmaline-gamet-fluo te-beryl. A Rb-Sr isochron age fromthe Hermyingyi stock is 59.5 l l.4 Ma:87Sr/ooSr, = 6.ttt -10 (P. Müllu, 1988. personalcommun.).

Mineralization consists of disseminations,stockworks, and veins with an ore associationcomposed mainly of arsenoplrite, chalcopyiite,sphalerite, py te, cassit€rite. and wolfram[e.Minor components are py{hotite, stannite, bis-muthinite, bismuth, molybdenite, and scheelite.The gangue ass€mblage is quartz-muscovrte-tourmalirc-b€ry1-fl uorite-apatite.

GEOLOCY, May 1989

TABLE 1. ARITHI'IETIC I.IEANS OF CflEI,IICAL DATA FOR GRAT.IITIC ROCKS OF CENTRATJTHAIIIAND AI{D BURI,IA

Loei Chantha-grano- buri

diorj.tes granites( n = 7 ) ( n = 2 e )

Rayong Botdergranite range

( n = 2 5 ) ( n = 2 0 )

Pilok Eernyingyiaplo- aplo-

granite gtanite( n = 1 7 ) ( n = 6 )

oxides (wt*)sio,Ti"O;A1,öarejojMnOr4gOCaONa?oKzö

L6r'

7 6 . L 4 7 6 . 0 00 . 0 3 0 , 0 3

1 3 . 1 6 1 2 . 8 30 . 5 2 0 . 9 9o , o 7 0 . 2 3o . 0 1 0 . 0 10 . 3 0 0 . 5 03 . 9 2 3 . 4 54 . 4 3 4 . 3 a0 . 0 4 0 . 0 1o . 7 3 0 . 9 8

6 3 . 3 40 . 5 9

1 5 . 8 64 . 9 90 . 0 92 . 2 04 . 6 23 . 4 32 . 9 9o . L 7

? o . 2 ao . 4 t

\ 4 . 3 43 . 3 8o . 0 6o , 4 62 . 0 63 . 7 63 . 9 6o . 0 90 . 6 8

7 2 . 6 00 . 3 1

1 3 . 4 02 . O ao . 0 50 . 6 6! . 2 62 . 7 45 . 0 00 . 1 6o . a 3

4766 62 0

62 6

t-54 9

L 0a 12 4

a2 54 2

a 6

7 3 . 4 2o . 2 5

1 3 . 5 31 . 8 80 . 0 6

1 . O O2 . 9 2

0 . 0 90 . 8 8

255a 5

< 1 55

3 42 4

aa 7

4 4 21 46 54 3L 9l 96 3

L49

8 9

Trace elenents (ppn)Ba 538ce 45cr 29cu 20La <20NbNiPbRbSnStThU

v

71 51 19 9

33 8 1

1 0< 5

1 0 92 2

l 35

< 1 53 01"1

99 0

6252 7L 72 T3 1

< 1 5r o 2

6 95 3

9 5

<t-51 a8 13 6<58 1

9797 2

2 4< 1 51 9 01 3 5

7 5

9 3

3 6 87 9

< 1 51 53 9

98

3 42 0 7

71 1 8

72 45 16 a

256

a 2

zn 49zr L47

D . r . 6 4

Note: , Analyses by X-ray f luorescence spectrometry.D.I . is Thorhton-Tut t ]e d i f ferent iat ion inalex.

Fe2O3 is totä l i ron.

Eto

100t0R b / S r

Figure 2' Rb/sr-sr variarioi diagram rorcentrar rhairand granitic rocks. stars rocate position orayerage low-Ca and high-Ca granites (furekian and Wed+ohl, 1961).

€- High-Ce grrnlt*

T I N _ B E A R I N G

O L o E l

E Ct l NTHAaUi l

a aoiDGß i^NoE

a t t lox

V HENIYIIICYI

427

EXTRAPOLATEDTIAGTATIC TREND

t

EXTRAPOLATEDMAGIIATIC TRE D

RESULTSChemical data for each rock group are given

in Table 1. In both the tin-batren granitic roclsfrom eastem c€ntral Thailand aod the tin-bearing granitic rocls from the westem part,trace-element trends can b€ defined that implyfractional crystallization as the dominant petro-geretic proc€ss controlling magmatic evolutioo.This is deduced from linear correlation trends inlogJog plos of various prairs of compatible andincompatible elements. The log Sr vs. log Rb/Srdiagram in Figure 2 defines a geo€ral fractiona-tion patt€n of the different magma series. Thehydrothermally overpritrted populations ofsamples from the Pilok atrd Hermyingyi aplo-granites form an extension of the general hendof magmatic differentiation to extreme levels,indicative of a Rb-Sr system that has not beendisturbed by fluids from extemal soulc€s. Thisdiagram, of cou$e, provides no evidence ofcog€netic relations between geographically sepa-rate rock groups, but it does give informationabout the relative degree of magmatic evolutionof defined groups of granit€s and points to thefact that tin mineralization is associated onlywith the mo6t fractionated rocls of these serrcs.

Th€ tin content of the rock in relation to thedifferentiation indices Rb/Sr and weight p€rcentTiO2 is shown in Figure 3. The samples fromthe Rayong and border ratrge ganitcs display asystematic trend that can b€ interpreted in termsof progressive magmatic enrichment of tin(Lehmann, 1982) which is statistically signifi-cant at a confidenc€ level of >99.9% with acorrelation coefficient rRuzs,-sn = 0.73 andhq, 5, = 0.63, respe€tively (53 samples). TheChaithaburi sample group has a relatively largescatter attributed to the margin of analyticalerlor near the tin detection limit. but thes€ datacould also be lltted to the pallern of progr€ssivetin enrichment during magmatic fractionation.The alkali-feldspar aplogranite samples fromPilok and Hermyingyi are characteriz€d by scat-ter distributions. The b€havior oftin in thrs casereflects the inlluence of exlemal faclors in anopen system. Th€ tin conc€ntration of thesesamples is anomalously low relative to their d€-gree of difterentiation, int€rpreted to be due toremoval of tin by fluid interaction. This mobil-ized tin is th€n d€posit€d in fractur€ systems thatare also the channels for fluid movement.

Figure 3. TiO2.Sn (upper part) and Rb/Sr-Sn(lower part) varialion diagtams lor cenlralThailand granites. Star indicates averagecrustal cgmposition (Taylor and McLennan,1985). Diagonal-rule area is region ol conpo-silions below analytical delection lor Sn. Ha-chured line separales least altered granitesamples lrom Pilok and Hermyingyi aplogran-ile samples lhat are nodilied by lluid inlerac-tion, Note tin deticit between exlrapolatedmagmalic trend and lluid.modilied aploglanitelield.

Eo

o

Eo.

co

0.1TiO2 (wt%)

ro|.)^'

c{oFoorto^

oo|

!taCJ

ataGo

Rb/Sr

GEOLOGY, May 1989

Figure 4. Metal logenicmodel lot tin minetaliza.lion. Magmatic endchmentoltin as lunction ol degreeol lractionalion given fa-vorabl€ bulk dist butioncoefficient Dsr <1 at lowoxygen fugacily, and min-eralizalion as result olsubsequent tluid inlerac.tion with highly evolvedmagmalic or hansilionalmagmatic-hydrothetmalsystem.

A geochemical balance of the system can bemade when the tin mineralization is taken intoaccount. The original me3n content of tin ol th9Pilok aploganites can b€ estimated from thegeneral magmatic correlation trend in Figure 3.A primary content of 85 ppm Sn in the magmaresults (standard deviation 35-269 ppm). Con-pared to the actual mean content of tin, which is27 ppm (range 0-64 ppm Sn), rhere is a differ-ence of 58 ppm Sn. This figur€ provides a basisfor estimation of the potential for tin ore of thePilok mining area, in which there is a volume ofabout I *0.5 km3 ofalkali-feldsprr aplogranite.The tin potential of 1.5 r0.7 x lO5 t (tonnes) Sn,calculat€d on this theoretical basis, comparesreasonably well with the cumularive productionplus reserve frgures for this ar€a, which totalapproximately 5 x loatSn.

MODELFigure 4 incorporates the principles of this

simple two-stage model of tin metallogenesrs:fractional crysrallization under conditions inwhich the bulk distribution coefficient for tinDsn (crystals/melt) < I (khmann, 1982) leadsto Sn levels high enough to load a cogenetic-magmatic and/or an externally derived fluidphase with sumcient metal to form ore-grademineralization. It is a requirement of this modelthat tin deposits be restricted to very small vol-ume fractions of large-scale ganite systems. Avolume estimate bas€d on the geologic circum-

stanc€s of the Pilok area and assuming perfe{tconditions of lractional crystallization with anideal value of Dsn = 0 leads to a minimum totalmelt volume of 28 al4 km3. A more realisticestimate fo. natural conditioos would give aoorder of magnitude of about 100 km3.

Tb€ bulk distribution co€flicient for tin secmsto be dep€ndent otr oxygen fugacity (Ishihaü,1977). Tin granites are generally ilmenite-bearing and have low Fe3*/Fe2* ratios, whichsugg€sls a more incompatible behavior of Sn2-as compared to Sn4*. Molybdenum, in contrast,has an opposite DMo -/o2 relation (Tacker andCandela, 1987), which provides an exilanationfor the overall bipolar metal distribution in thep€trogenetically related Sn-W-Mo ore-depositspectrum.

Oxygen fugacity also plays a critical role forthe hydrothermal mobility of tin, with the solu-bility of cassiterite near the quartz-fayalite-magnetite buffer about three log units greaterthan at the hematite-magnetite buffer (Eugster,1986). Fluid-rock interaction in highly fraction-at€d lowy'o2 granitic rocks may produce a veryefficient removal of tin, leading to anomalouslylow background levels of Sn, balan@d by hy-&othermal tin €nrichment in zones of exuemechemical focusing. Therefore, granites affectedby large-scal€ hydrothermal tin depletion maybe regarded as good choices for prosp€cting oftin deDosits.

REFERENCES CITEDB€ckinsalq R.D., Suensilpon& S., Nakapadungrat, S.,

and Walsh, J.N., 1979, Geochronology and geo-chemistry of granite magmatism io Thailand inrclation to a plat€ tectonic modet Ceological So-ciety of lrndon Journal, ,t. 136, p.529 540.

EugsFr, H.P., 1986, Minerals in hor water: AmericanMineralogis! v. 71, p.655 673.

Feryuson, H.c., and Baleman, A.M., 1912, ceologicfeatures of tin deposits: Economic Geology, v. ?,p.209-262.

Ishihara, S., l9?7, The magoerite-sed€s and ilmenite-series granitic rocks: Mining Geology, v. 27,p. 293 305.

James, R.S., and Hamilton. D.L.. 1969. Phase rela-tions in thesy5hm NaAlSirO8 KAlSilO8 CaAl2Si2Os-SiO2 al I kilobar waier vapoir iresure:Contributions to Min€ralogy and P€trology,v. 21, p. l l l -141.

Lehmann, 8., 1982, Merallogeny ofrin: Magmatic dif-ferentiation veßus geochemical heritags E{o-nornic Geology, v.77, p. 50 59.

Nakapadun$at, S., Beckinsalq R.D., and Suensil-pong, S., 1984, ceochronology and geology ofThai granites, r't Proceedings, Conferen@ onApplications of Geology and the National De-velopmenr: Bangkok, Chulalongkom Univenity,p.75 93.

Tacker, R-C., and Candela, P.A., 1987, Partitioning ofmolybdenum betwe€n magnetite and melt: Apreliminary experimental study of partitioning ofore metals betw€€o silicic magmas and crrstallinephases: Economic ceology, v. 82, p. 1827 1838.

Taylor, R.c., 1979, ceology oftin deposits: Amster-dam. Elsevier. 543 o.

Taylor. S.R.. and Mct-€;nan, S.M., 1985. The conu-nental crust Its composition and evolution:Oxford. Blackwell. 312 D.

Turekian, K.K., and Wedepohl, K.H., 1961, Distribu-tion of the elements in some major units of theEa(ht crusr ceological Society of America Bul-letin, v.72, p. 172-202.

Tonle. O.F., and Bowen. N.L.. t958. Origin ofgranitein the light of exp€rimentral studi€s in the systemNaAlSilOs-KAlSirOs-SiO2-H20: CeologicalSociety of America Memoir 74. 153 o.

UNESCO, 1985, Metallogenic map of south and eastAsia l:5.000.000: Paris and Tokyo. Commissionfor the Geological Map of rhe World, 4 shee6.

ACKNOWLEDMENTSThis study is part of the Federal Republic of Ger-

many Thailand Technical Coop€ration project ."Tin-bering and tin-barren granircs/prinary dn deposilsin SI Asia." Chemical dara were provided byBundesanstalt für Geowissenschaften und Rohstoffe inHannover. We thank Somboon Khosilanont for helDwith rhe field work, and Chrislopher Halls, UlrichPeters€n, and Kevin Shelton for critical reading of themanuscnpt.

Manuscript re€eived October 13, 1988Revised manuscript rec€ived January 9, I 989Manuscript accept€d January 18, 1989

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GEOLOGY, May 1989