Ameriean Mi*eralogistVol. 57, pp. eas-dSZ irszzl

I

ALPINE-TYPE CHROMITE IN NORTH BORNEO, WITHSPECIAL REFERENCE TO DARVEL BAY1

Cuenr,ps S. HurcHrsox, Department ol Geology, Uruiuersitg olMalaEa, Kuala Lumpwr, Malaysia.

Assrnesr

chromite occurs as layers and pods in dunite and serpentinite lenses withinmost of the peridotite outcrops of saibah. The crystals are anhedral, rounded,cataclastic, and unaltered.

INrnooucrroN

volume of chromite for economic exploitation. rn all the investigatedareas the chromite occurs in dunite and serpentinized dunite bodiesand lenses (forming about 5/o of the ultramafite total) which occuralways within Iarger peridotite outcrops.

lPresented at the Twelfth pacific science congress, canberra, Australia:August 24th, 1971. [I{utchison, l97l, (abstract)l. .:

&35

836 CHANLES S. HATCHISON

Legend

f .owER / / f f ieow,o ic \ ^ tP t r t

lERtARv -

l-xAFlc/IO I ULTRAI'AFIC

cRETAcEous l,uo'ot't-,1 ggrp,gt65

Fi:IIil I€lonorPhic oc.ooici.:r:;::l bosemtnl

a Moin occut,nca ofItJl chromite

-**o'. R t

Xino botongot

- l t ! {

Voll.t

... D E llr

PE iIINSUL A

Lohod

KAL IIIAI{TAN

SUIU SEA

Sondolon

OARVEL 8AI

Frc. 1. Outline geologic map of Sabah showing the main occurr^ence of

chromite in relation to thle crystalline basement and the alpine ultra,ma^fic,/mafic

complexes (modified a1ter Kiri, 1968). Sedimentary formations ere left unshaded'

emplacement than the other ultramafites of the region (Fig' 2)' They

have also shown (Hutchison and Dhonau, 1969 and 1971) that the

peridotite of Tabawan island was of a higher emplacement tempera-

ture and has caused a thermal auleole in the Silumpat Gneiss from the

regional greenschist, through almandine amphibolite, to hornblende

&]7ALPINE CHROMITE IN BONNEO

AJ

H

a

d

da

- N

; @H O

; :A X

R

bo

oo

a 5

N

g

11

9 0 -=E .

3 c -

9 sa

: - ; * c /- 7 - ) t 1 . [i 6 =

. < J K

6

83t3 CHARLES S. HUTCHISON

granulite faiies. By contrast, the younger Saddle island group ultra-mafite was emplaced in equilibrium with the greenschist facies condi-tions, and it is this emplacement alone in the Darvel Bay area thatcontains chromite ore.

Texruns

The chromite occurs as layers and bands, ranging in thicknessfrom a few millimeters to several meters, and as lenses, pods, andlayer-zones of disseminated crystals. The layers have been folded andboudinaged, but in several instances original cumulate textures arepreserved. On Katung-Kalungan island (Fig. 2), for example, thechromite bands are sharply folded, but even after folding they pre-serve excellent graded layering reminiscent of an originally stratiformorigin (Fig. 3). In the Labuk Valley, 800 meters east of Katai onBukit Lumisir, the distribution of narrow bands of fine grainedchromite in several dunite bodies suggests crystal settling (Collenette,19M) .

Generally, however, the distribution of chromite is tectonically

Frc. 3. Left: sharp fold in chromite-serpentinite layers. The chromite layersshow high relief on the natural outcrop surface. (Specimen J 1173 from Kalung-Kalungan island).

Center: End view of the same specimen on a polished surface showing gradedlayering of the chromite, inverted by folding.

Right: Chromite disseminations and segregations in serpentinite. The distri-bution probably resulted from tectonic emplacement, and the original layeringhas been largely obliterated. (Polished surface of specimen J 1187 from Lailaisland). The white bars represent 3 em on each adjacent specimen.

ALPINE CHROMITE IN BORNEO 839

controlled, and original stratiform structures Iargely obliterated (Fig.3). In some areas, for exarnple at Bidu-Bidu in the Labuk Valleyarea, the ultramafic foliation is not parallel to, and is superimposedon an original stratiform layering. Newton-Srnith (1962) took this toindicate that the chromite had a magmatic-settling origin beforetectonic emplacement.

Individual localities are listed under Table l.The chromite crystals from all localities in Sabah have rounded

anhedral outlines (Fig. a). All are cataclastic to some degree. Themassive podiform bodies are also brecciated and healed by the ser-pentinite matrix (Fig. a). Sabah chrornite is remarkably free fromhydrothermally altered rims as have been described by Mih6,lik andSaager (1968). The absence of compositional zoning is indicated bysimilar reflectivity values across crystals and by sharp and singleX-ray diffraction peaks. Irregular patches and zones of ferritchromiteas described by Engin and Aucott (1921) have been observed veryrarely in Sabah. From the whole collection, only two examples ofhigher reflectivity ferritchromite have been found (A and B in Fig, 4).

Tnecn Er,nlrnrrs rN Couurny Rocrs

Method

Ana,lyses were by comparison with U. S. Geol. Surwey PCC-I, taking Ti = 44,Cr - 2600, and Ni - 2300 ppm and W-t taking K - 5310, Ti - 6400 ppm.

Whole rock specimens, pulverised in a tungsten carbidelined Tema mill vesselto avoid contamination, were made into pressed pellets backed with boric acidafter the manner of Norrish and Chappel (1g67) and analysed on a Philips X-rayvacuum spectrometer. K' lines were used throughout and the pulse heightanalyser carefully set to ensure maximum peak to background ratios.

Operati,ng condttior*

Countingtime

Element Tube Crystal kV mA sec. PathSensitivity

Detector per l0 ppm

Cr PET 44 26 40 vacuum flow c. 2.5 counts/4 sec.

Cr LiF 44 26 10 vacuum flow c. 49 counts/I sec.

W LiF 44 26 10 air flow c. 11 counts/1 see.

W LiF 50 20 10 air scintill- 31 counts/ation 1 sec.counter

Background meafltrements were made by averaging the two readings made at-+ 1.7" 2 0

K

Ti

Cr

Ni

840 CHARLES S. HATCHISON

Il i t r* fI{* f, is +, ;* f,i+ ++ ii t* i iet* t* ies € € t ri qa si ,tE;i ;e ff ;e i; l i ru;i s ea;t es ss

P

3

EEE. EE.E.E. g EEEEE; ; ; E ! ; Y f i ;j j i . : H ! r , E ; . 1 f l ; h H : I i E f l # fH H H ; e - f r H ' * c H - * a H E ; d e H I H $ H $ $EEEff E E Ee! !eE 5e.E f ! F EE E E E

E S Rd d d

@ o o o o o o $ d @d d N o s s @ @ < ! ) s o

o oi d d c t . ;

" i A i o i d d . i A ; o i d A i o i d o i d d o ;

E

o <

s = ! s : P i € : € e ? b o @ d ; " : d N . t

+ c ' c ; d d d c i d c t d r ; d i d d N o + q i d ; d ; o i

;

o.g E R b F * 3 g 3 K te i s i d e P N f t sN * K : e

q q q C e t { e q q q Q 1s s $ s g s 3 g 3 3 s g ;

o c N 6 @ N

F N + { O s + r

$ oR S E S

s : 3 . , 3 R t s

q J 9 ? t " l " : t+ z o @ o @

o 0 6 + r

R E B ' ' D g S

= d E * *F " E I I P g $ o N N E . s a $ q B gE = = 9 - A 3 2 : I ! P ; - E v I I 2

ALPINE CHROMITE IN BONNEO B4lr

D

0 m m llsJ+J

F

F: |ERR|TCHR0XITExtr.ntx sEnPEilililrE

Frc. 4. Drawings from polished sections of chromite occurrerrces in Sabah. Thematrix is generally serpentinite. F - ferritchromite with considerably higherreflectivity than chromite, which is shown hatched. A : specimen 7182, accessoryhydrothermally altered chromite from the Ranau Luhan road, Kinabalu area.B - J 1186, a,ccessory chromite with small areas of ferritchromite from Lailaisland. C - J t27, rounded chromite grains from Bidu-Bidu in the Labuk Valley.D - BB 120, massive fractured chromite from Banggi island. E and F - NB4169, massive fractured chromite from the Kinabalu area, E from the edge andF from the interior of a layer.

In view of the apparent absence of chromite, except as a rreryminor accessory, in the ultramafic bodies of Sakar and the outerislands of Tabawan and Maganting (Fig. 2), a selection of rock speci-mens from all the Darvel Bay ultrarnafic outcrops $ras analysed forchromium and related elements. The results &re shown in Figure 5.Specimens I through 17, and 19 and 27 are ultramafites. At the sametime a number of Eilumpat Gneiss specimens, 18 and 20 through 26were analysed.

The roek contents of chromium, nickel, and titanium, when com-pared in relation to the potassium contents, show the sharp distinctionbetween the ultrarnafic rocks and the Silumpat Gneiss (Fig. 5). Ex-cluding number 27 f.rom this study because it is abnormally enrichedin potassium, we find that the Darvel Bay ultramafic rocks have thef ollowing average contents:

s

u2 CITARLES S. HUTCHISON

U L T R A M A F I C R O C K S

I

I2 l

o

IA

I

?o

II

o

o 4o o ?

l a

oIA

II

Il 5

l r ]

o 5

o 2

t 0

0 0 0 t %

o 6

o l

r l

l 5

l 7

T H O T E I T I C B A S A L T S

T H O L E I T I C B A S A L T SI J L T R A M A F I C R O C K S

600 1,000

Polossium p0rls per mrl l ion by qerqhl

0 0 % 0 l %

Frc. 5. Plot on logarithmic scales of the Cr, Ni, Ti, and K contents of someDarvel Ba.y rocks. The dashed vertical line is an arbitrary division between thepotassium-based fields of ultramafic rocks and tholeitic basalts. The SilumpatGneiss extends to the right from number 18, but 27 is an ultramafic rock anoma-lously rich in potassium. The Silumpat Gneiss is characterised by its richness inpotassium and titanium; the ultramafites by their richness in chromium andnickel. 1 = J 1080, Dunite from Tabawan.z - J 1095, Serpentinite fromTabawan. 3 - J 1191, Pyroxenite from Nipa-Nipa. 4 - J 1168, Peridotite fromMaganting. 5 - J 1103, Peridotite from Tabawan. 6 : J 1083, Peridotite from Ta-bawan. 7 = J 1101, Peridotite from Tabawan. 8 - J 1175, Peridotite fromKatung-Kalungan. I = J lzm, Serpentinite from Saddle. 10 - J 1245, :ullrw-basite from Silam. 11 - J 1329 Peridotite from Sakar. L2 - J 1182, Serpentinitefrom Baik. 13 - J 1200, Pyroxenite from Giffard. 14 - J 1319, Peridotite fromSaka.r. 15 - J 1f96, Pyroxenite from Enam. 16 = J 1088, Dunite from Tabawan.17 - J 12i5, ultrabasite from Sakar. 18 - J 1344, Silumpa,t Gneiss from Silam.19 - J 1185, Py'roxenite from Laila. 20 : I 1075, Silumpat Gneiss from Tabawan.2l = J 1324, garnet amphibolite from Sakar. 22 = J 1050, Silumpat Gneiss fromSilumpat. tJ - J 1060, Silumpat Gneiss from Silumpat. 2t4 - J 126L, SilumpatGneiss from Silam.25 - J !n, Silumpat Gneiss from Tabawan.26 - J 1337,Silumpat Gneiss from Sakar. 27 - J 1215., Pyroxenite from Enam.

ALPINE CHBOMITE IN BORNEO 843

K 74 ppm, ranging from l0 to 240 (excluding 2Z)Ti 412 ppm, ranging from 60 to ZB0Cr 2770 ppm, ranging from t4b0 to 56g0 (excluding 1g and2T)Ni 1531 ppm, ranging from 120 to 2Bb0 (excluding lg and,27).

When compared with the worldwide average ultramafic values ofGoles (1967): K - 200:t, Ti = 300, Cr = 2400, and Ni: 1500 =Lppm, the Darvel Bay ultramafic rocks seem remarkably low in potas-sium and high in chromium. This might be taken to indicate that theyrepresent a suite of rocks strongly residual in character and to havecome accordingly from a Mantle region from which much basalticmagma has already been withdrawn by partial melting.

It is interesting to note that although no segregations of ore occuron the islands of Sakar, Tabawan, and Maganting, yet the ultramaficrocks from these islands are often as rieh in chromium as the rocksfrom the Saddle islands. Saddle island Group specimens in Figure bare 3, 8, g,10,12,13, 15, 19, and 22. These do not differ significantlyin their distribution of Cr, Ni, Ti, and K from the ultramafic massesof other Darvel Bay areas. Of course, the speciniens that were selectedfor analysis did not contain layers and segregations of chromite. Suchlayers and segregations oecur only in the Saddle island Group and onMount Silam. Hutchison and Dhonau (1969) showed by structuraldata (summarised on Fig. 2) tha| the deformation of the Saddle islandGroup ultramafic body is later than elsewhere in Darvel Bay. Ac-cordingly it must be concluded that not all ultramafic emplacementsare of the same age and presumably not all from the same parb of thellantle.

The Saddle island Group ultramafites have come from a Mantlearea that had a considerable development of ehromite layering, andHutchison and Dhonau (196g and 1gZ1) showed that this was a coldemplacement. On the other hand the ultramafite of Tabawan was ofsufficient temperature to have raised the Silumpat Gneiss in a thermalaureole to as high as hornblende granulite facies.

Specimens 18 and 20 through 26 arc of Silumpat Gneiss, whichHutchison and Dhonau (1g6g and 1gZ1) believe to be metamorphosedoceanic basaltic crust. For the specimens analysed, the following aver-age contents have been found:

K 653 ppm, ranging ftom 220 to 1500Ti 5778 ppm, ranging from 1760 to 12690Cr 232 ppm, ranging from 50 to 240Ni 96 ppm, ranging from B0 to 2G0

These values cornpare reasonably well with the average worldwide

U4 CHARLES S: HUTCHISON

tholeitic basalt: K 5400 ppm with a lower limit of about 500 ppm(Manson, 1967) ; Ti 9900 ppm (Manson, 1967) ; Cr 218 ppm (Prinz,1967) ; Ni 85 ppm (Prinz, 1967). From a eomparison of these values,the Silumpat Gneiss can well be regarded as typically tholeitic incharacter. Metamorphism to amphibolite and granulite faeies hascaused a partial depletion in potassium and perhaps titanium. One ofthe specimens J1324, is of a Eilu,ntpat Gneiss inclusion in the ultra-mafic mass of Pulau Sakar. It is now a garnet amphibolite, andalthough it must have been considerably modified by the enelosingultramafic mass, its characters as shown in Figure 5 (No. 21) are stillthat of the Silumpat Gneiss. It is hard therefore to escape the con-clusion that the Silumpat Gneiss is distinct both chemieally anilmineralogically from the ultramafic bodies of the Darvel Bay area.Dr T. P. Thayer (personal commun.) has questioned our interpreta-tion that the Silumpat Gneiss is divorced in age and consanguinityfrom the ultramafic bodies because as he has noted elsewhere, Thayer(1967), he regards "the ultramafic and gabbroic parts of ophiolitecomplexes as being identical with, and part of, the alpine igneous rocksuite." However the problem has been to a large extent resolved byLeong (1971) who has reeently re-mapped large areas of the hinter-land of Darvel Bay. He has found that there is a distinct CrystallineBasement which includes the Silumpat Gneiss and is, by radiometricand stratigraphic evidence, of pre-Lower Triassic age' In additionthere are distinct occurrences of banded gabbros associated with theultramafic rocks and these form the peridotite-gabbro complexeswhich post-date the Crystalline Basement. The problem in Sabah hasarisen from the fact that the banded gabbros of the alpine complexeshave in the past been confused, especially by Fitch (1955, and 1970) 'with the banded amphibolite gneisses which constitute the SilumpatGneiss of the Crystalline Basement.

Crrnourrp Crrplrrstnv

A number of analyses of chromites separated from ultramafic rocksof Sabah have been found in the literature. These are given in Table 1together with a few analyses by the present author by X-ray fluores-cence. The conversion to unit cell cationic proportions is best illus-trated by an example: NB5452

Molecular Cationicweight proportions

WeightOxide percent

CrO, ,7, 54.5Alo, 1/2 8.9

75.0r50.98

0.7r700.1745

ALPINE CHROMITE IN BORNEO

Total iron expressed as

u5

FeOr rz,

Mgo

Total cationssio,Tio,

1 6 . 0 7 9 . 8 5 .2003 less .0003 for ilmeniteimPurity : .2000

16.0 40.32 .3968 less t+ X .0248 for serpentineimpurity : .3968 - .L122 : .2846

1 . 3 7 6 160.09 .074879 .90 .0003

4 . 50 . 0 3

For the basic formula Rs*16R,*sO32, then niultiply by 24/l.816l,giving (crp.6Al3.6Feo.s)ro (Fq.eMg5.6)so32. The total iron of B.b is first

P urifi catio n B e I ore Analg si.s

samples were crushed and sieved -120 + 280 u. s. standard, washed as oftenas necessary with water to remove fine powder, then dried. The very magneticfraction was collected in a Frantz Ferrofilter and. discarded. The powdlr was nowslowly passed through a Frantz isodynamic separator with a side tilt of 15" anda forward tilt of 25. (Rosenblum, lgbg) and the chromite was collected withinthe range + 0.1 A, and -0.5 A. The non magnetic fraction was usually serpentineor olivine. If the chromife fraction visibly contained some green maierial, it *u,re-nrn at 0.4 or 0.45 A.

X-ray F luorescence Analgtical M etho d,

'obtainable from crescent Dental Manufaituring co., 77ffi west 4zth st.,Lyons, Illinois, 60534, USA

846 CHARLES S. HATCHISON

then the simple X-ray dilution procedure is capable of giving satisfactory re-

sults. This is especially important because of the difrculty of putting chromite

into solution. one important point should be noted and that is that the Fe,Oe

calibration curve between the two standards had a negative slope, showing the

overall importance of ma,trix effects (Mitcheu and Kellam, 1968). Accordingly it

is possible that iron determinations by this method are less than perfect. But

it appears that Cr, Al, Mg, Si, and Ti determinations are good.

Instrumental Settings for the Elemental Determinations

Element Tube Crystal kV

Countingtime Average sensitivity |ot l/s in

mA sec. chromite

CrzOaAbOaMsoFerOsTiOrSiOz

w L i F 4 4 8Cr PET M 26Cr KA? M 26w L i F 5 0 8Cr LiF 44 16Cr PET M 26

10 603 counts Per sec.20 20 counts Per 2 sec.40 6 counts Per 4 sec.10 790 counts Per sec.10 6700 counts Per sec.20 88 counts Per 2 sec.

AII determinations were with a flow counter and vacuum path. The background

of each peak was determined at -F1.6' 20from the peak position for each element

except in thq case of iron where it had to be at -+2.3' because of the close posi-

tion of the m&nga.Dese K F peak.

Discussiwt,

The analysed chromites of Table I are shown on Figure 6. Themajority of sabah chromites fall in the field of aluminian chromite buta few are in the chromian spinel compositional range of Stevens(1944), and a few can be considered as of metallurgical grade. Figure7 shows that with only three exceptions (BW 68, MW 12(32)' and BB120) the Mg:Fe ratio in R'* lies between the small range of 4.7:3'3and 6.0:2.0. This remarkably small range in Ri- while Cr:Al rangesfrom 12.5:3.0 to 6.8:9.1 (Table 1) is remarkable. Thayer (1970) hasbrought attention to this very fact that as alpine-type chromites

become less and less rich in chromium, they become richer in alumi-nium and not in iron; indeed the iron characteristically remains ap'proximately constant. In contrast, stratiform chromites become pro-gressively richer in total iron as their chromium content falls. The

sabah chromites are in this respect typically Alpine. Another inter-

esting difference is that stratiform chromites chalacteristically have

a unimodal and restricted chromium range. Alpine type, or podiformchromites, have a much wider range of chromium content which is

characteristically bimodat (Thayer, 1970). The range of chromiumin the Sabah chromites (fig. 6) is considerably wider than in any

stratiform body and there is a suggestion of a bimodal chromium con-

ALPINE CHROMITE IN BORNEO

Cr,5 ( Mg, Fc lo Ora

u7

Fcr6 ( l rg, Fc )s 052

+ 1t " iJ "n. 0. .

MAGNESIO FERRITE

P i f r

,(3

.- rrn,ro.-lG R A D E S

' / li i',lr',iliJ----F\

-o x _

\c H R 0 I t A t l

S P I I I E L

r 0 9 E ? 6 5 4 3

A l + 3 c A T t o l { s p E R u N t T c E L L

Frc. 6. Distribution of Sabah chromites in the Cr,-Alr-FeruRt*ru triangularclassification. The actua^l cationic proportions are plotted in only a small portion(Ieft) of the complete triangle (shown right).

z A l r s ( 1 t19 , Fc l s 032

( C r * A l ) C A T I O N S P E R U N I T C E L Lt 2 r 0 8 6 4

( C r , A l ) , U M g r O , a

MAGNESIO CHROMITE

U()F

2 z

G,U

4 'atzoF

A <o

+N

a f i

JJU(J

F 6z.f

E.g r 4

oz.o| - .(.)o

E ^U

o 2 4 6 8 t 0 t 2CHROMITE + 3

( c r , A r ) t 6 Fe8 'o j 2 Fe ' -

CAT IONS PER UNIT CELL

t4 t6MAGNETITE

+a +)Fe, j Fea- 0 ,a

Frc.7. Diagram to show the rema,rkable lack of variation in Mg:Fer* ratio forthe same chromitm. Stratiform chromites show a much wider range,

{""

7,.61 ','ir6i

T\EVENS, /944)

848 CHARLES S. HUTCHISON

tent with one mode around 7.9 and the other around 11.3 chromiumcations per unit cell.

Pnvsrcer, Pnopnnrrus

Unit CeU Edge

Stevens (1944, Fig. 2) plotted percent CreOs against cell edge o inA and found, with a few significant exceptions, a good linear fit. Hisdata can be recalculated to give the following regression equation:

percent CrzO's in chromite : 301.85 (oA) - 2445.66

His data consisted of 25 measurements of chromites ranging fromCrsO3 0.0 to 59.5 percent but excluding two samples which had highferric iron contents. But I do not feel that this was a valid justificationfor their exclusion, for the difficulty of putting chromite into solutionfor analysis must cast doubt on chromite Fe3*:Fe2* determinations.

Stevens (1544) must have been fortunate in his choice of data, forthe Sabah chromites do not fit well on his curve. Indeed it is surprisingthat a linear relationship between Cr2O3 and o exists, for in thechromite unit cell (Cr, Al, Fet*)ro (Fe2-, Mg)3O32 there are five var-iables with Cr, Fe3*, and Mg of roughly equivalent ionic radius (0.63,0.64, and 0.66 A respectively) and Al (0.51 A) and Fe2* (0.74 A).Hence it should be expected that variation in Al and in Fe*2 wouldhave much more control on the unit cell edge than Cr. Again we wouldexpect to find a different relationship in stratiform chromites, wheretotal iron increases as Cr decreases, than in podiform chromites, inwhich aluminium increases as chromium decreases. In the present studyseveral chromites listed in Table 1 were measured for unit cell edse(o) on a diffractometer.

Chronrite mixed with an almost equal amount of potassiurn bromate wassmeared on a glass slide and run with CuKa radiation at 1/8" per minute from61.5 to 63.5' 2e. The 2e difference between KBrOe K at22l at 61.61 and thechromite K a,M4 at approximately 63.16' is added to 61.61 to give the preeisechrdmite 20 K at044. From this, a2 = 32 ffo*. The results are tabulated in Table1. Chromite peaks for a.ll Sabah specimens are sharp and well resolved indicatingthe homogeneous nature of each specimen and the gene.ral absence of hydro-therrnal alteration to ferritchromite.

The variation of o in A with Al and Cr cations per unit cell isshown on Figure 8. The Al content in R*3 is considered to be the mostimportant variant because of the small size of the Al cation. The ratioof Mg:Fe in R*' is likely to have a pronounced effeet also on o andthe value given against each point on Figure 8 is the number of Fe*2

ALPINE CHROMITE ]N BORNEO 849: :

computed in the unit cell. Since this has a large ionic radius it is likelyto affect the size of o. The points in solid circles are from this studyand those in open circles are from Stevens (1944). A considerablespread of values has been obtained (Fig. 8) and the spread from thecomputed best-fit linear correlations is not systematically related toFe2n contents as hoped. The regression equations for these data, bear-ing.in mind that the spread is considerable, are: .

Number of aluminium cations per unit cell : 407.30 - 48.60 (o in A)

Number of chromium cations per unit cell : 58.46 (o in A; - 473.88But these equations can be used only for very approximate determina-tions. Ferhaps the reason for the imperfect nature of the data ofFigure 8 is the oxidation state of iron. Many chromites may havesome of the iron in R2* oxidised to ferric iron. The change in ionicsize would be expected to distort the lattice. Be this as it may, theresults of Figure 8 show that at this time X-ray diffraction determina-tion of chromite unit cell edge (a) cannot be with certainty related tothe composition of the chromite and accordingly doubt must be cast onthe validity of Steven's (1944) relationship between CrzOe and o.

Reflectiuity

Polished specimens of 11 of the chromites listed oq Table 1 werecompared wiih SiC No. 88 (reflectivity given as 20.1G percenL at, a590 nm) on a Vickers microscope fitted with an EEL digital-readoutmicrophotometer. The results of several determinations on differentmystals within each polished specimen are given in Table 2. An ex-cellent correlation was obtained of reflectivity with the number ofaluminium cations per unit cell (Fig. 9) giving a computed regressionequation:

Number of aluminium cations per unit cell

: 27.93 - L.82 X reflectivity (at 590 nm)

The relationship given in Figure g appears to be a precise andquick method of determining Al content in chromites. However thisrelationship has been determined on alpine-type chromites in which,as chromium content drops, aluminium rises progressively. This samerelationship is not expected to apply exactly to stratiform chromitesin which the overall iron content rises with chromium fall-off. Forsuch chromites the slope would be expected to be ]ess steep or revenreversed.

A wavelength of 590 nm was used, so that a comparison could be

\ z . j\

, . r ) \ , ' t o

o .o 2 . 5 ' \

\\

\

C H R O M I U M C A T I O N S: 58 .46 x o f8 ) -473 .88

2 , 5

\ \ ' ' ' .3 ' o o \

3 '5o or . r \a 2 '4

2 . 8E2 . 5

\ .2 -s\

\

\3 . 5 ^ \

" I ' t \a \

3 ' 2 o ^ \" 3 . 1 \

tz.s

3 . 1a

\ o\ 4 ' 4

\\s.o\ 2 ' l

2 ' 8 o ' " "

) ) . . .

. 2 . 4

o 3 . 8 t . . a . ,- \

3 . 5 \ . .

o _ o o 3 ' l

' \ 3 ' 1 .

ALUMTNTuM cATtoNs 3 '2 - - \: 407 .30 -4g .60 x o (E ) 4 . .

850

8 ' 2 0 8 . 2 4

o Sleveds (1964)

CHANLES S. HUTCHISON

8 ' 2 8

o A

8 . 3 2

o Th is S tudy

E^ c )I

c=

q b

co

roEO

E.=

r r Er r o

-(-)

= 8(J

.=c= 7

<l)o-o

65E(-)E R= v

.=E=< 4

ALPINE CHNAMITE IN BORNEO

Tenru 2. Rprr,ncrrvrrv er wavnr,rwcrn EgO r.rnr (coupennn wrrH src no. gg) elvnVH Nunrssns FoB soME SeseH.Cgnordrrng.

85t

SPEC]I\,IENNUIIIBEB

NB s452

J 1186

J 11A7

J 224

BB 12O

J 22?

EW 105

J 8090

H 134

NB 4169

J 't 142

Ftrtrr Farrwrv oI

59ONM

C H B O M I T E

a ? 6 r ?

1 3 , 4 + , 4

4 2 A L 1

' 1 2 . 5 + , 1

1 ' 1 , 9 + . 1

1 1 . 4 + , 3

1 O , 7 t . 1a l 2 L a

F E F R I T C H B O M l T E

1 0 t r O

VICKERStlABDNESS

1380 * 90

14EO ! 20

1510 I 60

1430 + 40

1370 + 60

1290 ! 24O

14lO ! ' tzl

1320 + 80

14UO + 90

1450 + 170

1420 t 60

J 'l 186 10?0 + 50

made with the values of Mih6,lik and Saager (196g), who quoted asimilar range of values but did not try to rerate the reflectivity rccomposition. They noted that chromite grains in the Basal Reef of thewitwatersrand system often show higher reflecting borders as a resultof alteration in situ. The alteration results in an increase in iron andchromium, and a decrease in aluminium and magnesium: This artera-tion is now regarded by most workers as being hydrothermal (seeEngin and Aucott, 1971) and it gives rise to higher reflectivity varues.rndeed it resembles magnetite in polished section, and if we follow thescheme of Stevens (19,M) as shown in Figure 6, the hydrothermalalteration of the sabah chromites would be expected to put them intothe field of ferrian ehromite, which is now called by most peopleferritchromite.

Reflectivity values of the Sabahthose given by Mih6,lik and Saager

+

Frc. 8. Relationship between the unit cell edge (a, A) and the number of Aland cr cations per unit cell for 6 sabah chromites and l0 from stevens (1g44).The number beside each point is the number of iron cations in R*.

ferritchromite are higher than(1968). They given 13.6 percent

852 CHARLES S. HUTCHISON

NUMBER

t 2 l l

OFPER

+aCr*Fe

' " CATIONSUNIT CELL

1 0 9 8 7

t

k

E t 3

O)tr)

F

l 2

"-e

F

F(-) r ru l "ILLU.JE.

3 4 5 6 7 8 9 1 0NUMBER OF ALUMINIUM CATIONS

PER UNIT CELL' Fri. 9, Variation of chromite reflectivity at 590 nm wa,velength (compared

with Sic No. 88 of given 20.16 percent reflectivity) with change in aluminiumcont€nt.

as.their highest determination, but this study indicates a value of theorder of 20 percent (Table 2). This reflectivity is more characteristicof magnetite, and indeed Ramdohr (1968) shoIMS an alpine chromite(p. 929) rimmed progressively by ferritchromite and then by mag-netite.

Hardness

Eaidness-was determined using a 200 g load and 25 sec. contacttime on a teitl.Ainiload hardness tester calibrated against a hardened

ALPINE CHRAMITE IN BORNEO 853

steel standard. The values obtained are listed in Table 2. The figuresare considerably higher than those quoted by Mih6lik and Saager(1968), but hardness can be of little determinative use for indicatingchromite composition because at these high values the margin of erroris greater than the range of variability sought.

Ur,rnltrerrrm-Onrcrx eun EuplecEMENT

The wide range of chromium content of Sabah chromites in com-mon with all podiform chromites, the tendency to a bimodal chromiumcontent, and the absence of iron enrichment as chromium falls, allpoint to major and essential differences between Alpine-type andstratiform ultramafites. These differences must reflect a radicallydifferent ultimate origin. It seems reasonable therefore to presume thatAlpine-type ultramafites have evolved in the Mantle just as strati-form ultramafites have differentiated from a gabbroic magma in thecrust. For these reasons the hypothesis of McTaggart (1971) thatAlpine-type ultramafites were originally identical to stratiform typesas cumulates in basic magma ehambers high within the crust, sub-sequently subsiding during tectonism to form oold, fault-controlledintrusions downward into lower tectonic levels, cannot be accepted.

The layering of chromite within dunite, showing excellent grading(Fig. 3) indicates that the chromite layers owe their origin to "igneoussedimentation" just as in stratiform deposits, but this differentiationmust have taken place in the Upper Mantle. Partial melting of Mantlematerial to give basaltic magma, which eventually extrudes at oceanicridges, must, leave behind a strongly ultramafi.c residuum. The verylow potassium contents of the ultramafic rocks of Darvel Bay (Fig. 5)indicate this residual character.

The association of the Sabah chromite-bearing ultramafic rockswith gabbro bodies and highly metamorphosed (generally in alman-dine amphibolite but occasionally in hornblende grairulite facies)tholeitic metabasalts could be accomodated within an oceaiiic spread-ing zone with rising Mantle current and accompanying high heat-flow.The work of Karig (1971), further elaborated by Dewey and Bird(1971), shows that such ocean spreading zones not only occur in themain ocean ridges, but also in ridges behind the island arcs of thewestern Facific in marginal or inter-arc basins. Indeed Karig (1971)has designated the Sulu basin, which lies in the top right corner ofFigure 1, as an inactive or fossil marginal basin with high heat-flow.

The curvilinear outcrop pattern of the ultramafite bodies of Nor[h.Borneo (Fig. 1) suggest sheeL-like emplacements from the Mantle intothe crust, and the convex westwards pattern, from Labuk Valley to

854 CHARLES S. HUTCHISON

Darvel Bay, must be taken to indicate that the ultramafite sheet dipseastwards, implying a westwards and upwards thrusting of Mantlematerial into the Crust.

Acrlqowr,nocrrvrpwrs

I am grateful to Mr Leong Khee Meng of the Geological Survey of Malaysia,Kota Kinabalu, for making available several chromite specimens. I wish to thankMr Jaafar bin Eaji Abdullah for the photography, Mr S. Sriniwass, Mr ChingYu llay, and Mrs Eaidar binte Ludin for the drawings, Tengku fsmail binTengku Mohammad for assistance,in X-ray preparations, and Miss Anne Chongfor typing the manuscript.

Appendix. Description of chromite localities of Table 1

Banggi Island (Kapitangan)-as thin lenses of 5 to 10 cm thickness in a 1 mband of serpentinized dunite within peridotite. (Sungei Tengah)-as bands andpods up to l0 crn width in strongly faulted and sheared serpentinized dunite(Collenette, 1964) .

Malawali Island: strongly brecciated chromite-bearing serpentinite.Labuk Valley: (Porog)-as veins and disseminations, striking 40"/-90' in a

dunite belt 61 m wide. One of the larger veins is banded and 4.6 m thick, passinginto solid chromite 3.7 m thick. It is locally brecciated. fndividual bands vary upto 23 cm thick and crystals seldom exceed 3 mm (Collenette, 1964). (Bukit

Lumisir)-as sheared and mylonitized veins. One ore-body is of 4.6 X 1.8 m ofcoarse grained chromite. Dunite lenses contain layers, veins, and accessory chro-mite (Collenette, 1964). (Sungei Taguuk)-as pods and bands up to 15 cm thickin dunite lenses. (Tangulap and Kuun-Kuun)-as a 6 mm layer and disseminationsin dunite lenses. (Ruku-Ruku)-as a pod, 80 X 50 cm, and as thin stringers indunite within peridotite.

Saddle Island group: The thickest dunites and serpentinites of Sabah occurhere. As tatrular layers grading into disseminations, and sometimes as irregularbodies with sharp contacts against dunite (Collenette, 1964). (Katung-Kalunganisland)-as trumerous discontinuous bands a few mm to 6 cm thick in serpentinite'(Laila island)-as common discontinuous bands, 3 to 13 mm thick, loeally swellingto 60 cm. (Saddle island)-as a fractured band, 30 cm thick, increasing to 1 m,outcropping over a 3 m distance. (Nipa-Nipa)-as bands 13 mm to 5 cm thiekin an ore zone 5O cm wide.

Mount Silam: An ore body, 13 X 7 m X 60 em maximum thickness, dipsirregularly N.E. at 15". It has a skin (after sheared dunite) of schistose serpen-tinite 2 to 30 cm thick, weathering to a lateritic clay. The ore body and ser-pentinite envelope is thought to have bden detached from its parent dunite andintruded upwards into the overlying peridotite (Collenette, 1964).

Repnnnrvcns

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ALPINE CIIROMII'E IN BORNEO

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856 CHARLES 8. HUTCHISON

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Mantrccript recei,ued,, September 20, 1971; accapted lor publicati,on, Nouember 11.lnI.