C anadian Iy'I iner alo gis t,Vol. 12, pp.95-103 (1973)

Aasrnacr

The compositions of Cu and Cu,Fe.sulphides homa stratabormd deposit in southwestern Alberta havebeen investigated by means of the electron micro-probe, Althor."gh the compositions of the bomitesclosely approach Cu6FeSa, the associated Cu-sulphides- blaubleibender covellite and anilite - show a widerange of compositions, witlq however; certai4 Cu : Sratios (1.12 + 0.01, 1.32 * 0.M anal l.?6 s 0.ffi)numerically dominant. Simple statistical treatrnentsare used to assess and interpret the data from a totalol more than 350 analytical points. Optical observa-tions suggest that preferred values correspond to com-positions of phases osolved lrom pre-existing solidsolutions in the region ovellitechalcocite.

Ixrnooumrox

Stratabound and associated Cu mineralizationfound within the Precambrian Upper Appekun-ny, Grinnell, and Lower Siyeh Formations (Price1962) of the Lewis Series in southwestem Al-berta has been studied. The deposits are locatedwithin a sequence of continental margin, shallowwater, red-bed sediments and are confined almostentirely to horizons of white or green quarEiteand green or black argillite. The most significantshowings are restricted to the Upper Grinnelllormation, richer zones in some instances occur-ring adjacent to faults and the contact zones ofhighly metasomatized, Precambrian diabases.Bornite * covellite I digenite is the dominantCu-bearing assemblage but minor amounts ofwittichenite, chalcopyritg enargite and/or galenahave been observed. Mineragraphic studies re.vealed two distinct varieties of covellite. One ispale to medium blue in colour, weakly aniso-tropic and commonly associated with bornite.The other is dark blue, strongly anisotropic andoccurs principally filling fractures and replacingthe paler blue covellite. the bornite or the chal-

* Permanent address: Depamnerrt of Geology,University of Alberta, Edmontorq Alberta, CanadanT6G 2E1.

ELECTRONI MICROPROBE INVESTICATION OF COPPER SULPHIDESIT{ THE PRECAMBRIAN TEWIS sERIEs OF S.W. ALBERTA, CANADA

R J. GOBLEKir*Ia Explor*inn Limiteil, T - 8il0 - 109 Street

Ed,monton, Alberta, Can ada

D. G. W. SMmr{*Department of Ceolngg and Mineralog, Universitg of Adelaide

Adelaide, Sj. 5000, Awfralia

cocite. Both varieties of covellite remain bluewhen viewed in oil of refractive index 1.55'whereas'normal' covellite appears reddish-violet(Cameron 1961). This is a characteristic ofblaubleibender covellite (Sillitoe & Clark 1969).A microprobe study was undertaken to determinethe relationships between the two different co-loured varieties of covellite and to investigatethe composition and stoichiometry of these andother Cu-sulphides and Cu,Fe.sulphides present.

The analytical data obtained from the covel-lites indicate a considerable compositional scatter.The treafinent which follows shows how suchmicroprobe data may be evaluated by simplestatistical means to eliminate unreliable dataand to allow a more detailed interpretation thanone which simply accepts the probable presenceof a range of compositions.

Anelvrrcar. PnocsDuRE

The six samples studied were selected fromquartzites occurring within the Upper GrinnellFormation at the Yarrow Creek - SpionkopCreek copper deposit, southwestem Albenta. Onesample (number 1) was taken from a sulphide'bearing carbonate vein cutting a quartzite adja-cent to a diabase dike; the other five samplesare from sulphide-bearing quartzites.

Samples were mounted in epoxy, polished andcarbon-coated. Analyses were performed on anA.R.L. EMX microprobe. A 15 kv operating volt-age and a focussed beam were used throughoutthe analyses. Standards have tlle following com-positions :

Standard Lz 35.2/6Cu, 32.5/eS, 32.5/6Fe'(Synthetic)

Standard 2z 34.29%Fe, 46.01/6 As, 19.69% S,(Arsenopyrite)

Cu and S were determined against Standard Iand Fe against Standard 2 e,xcept where, as notedin the tables, Cu and Fe were determined si-multaneously, in which case Fe was determined

95

96 TI{E CANADIAN MINERALOCIST

against Standard 1. fusenic was determinedagainst Standard 2 and Se against pure metal.Ten 20 sec. counts were taken on every grainanalysed in each sample; five 50 sec. countswere made at standards peaks before and afterthe specimen readings. Background correctionsv'ere based on measurements either side of theanalpical lines used. Counting rates for Cu, Feand S present as nla,ior components were approxi-mately 600-1000 cps (on bornite covellite andchalcocite), 300 cps (on bornite), 500-750 cps(on chalcocite, bornite and covellite) respective.ly. A beam curent intograting system was usedto eliminate errors from short-term instrumentaldrift.

Full ZAF corrections to the average ba&-ground-corrected intensities for each grain weremade using the APL computer program of Smith& Tomlinson (1970), Low counting rates allowedthe dead-time correction to be neglected. Notall elements present were determined in somespecimens; in such cases the presence of un-analysed elements was confirmed. by a wave-length scan and the concentrations calculatedby differencg using a subroutine of Smith &Tomlinson (1970). When this procedure wasused it is indicated in the tables. Structural for-mulae were determined by another subroutine.

Rssurrs

The initial results for individual grains (afterthe ZAF correction procedure but unevaluatedas to reliability), together vdth the calculatedmineral formulae (based on a multiple of onesulphur) are given in Table l. Letters appendedto specimen numbers refer to the colour of themineral in reflected light and are keyed in thetable. Calculated average Cu:S ratios for thevarious Cu-sulphide grains analysed are plottedin the form of a histogram in Figure 1. The termCu-rylphide will be used henceforth in this paperto refer to the minerals covellite, digenite andchalcocite and does not include Cu, Fe-sulphidessuch as bornite and chalcopyrite.

Within individual Cu-sulphide grains there isa wide range of compositions, with individualpoints (i.e. individual 20 second counts) showingconsiderable variance from the mean for the tenpoints comprising the analysis of the grain. Thisis appatent in Figure 2 where Cu:S ratios forindividual points are plotted as a histogram(only those points for which Cu and S weredetermined simultaneously are included). Thecompositional spread is particularly great for in-dividual grains in specimens 1, 3, 5 althoughthis is not strikingly apparent in Figure 2 becausethe compositions for several grains are plotted

together. Lr contrast the Cu:S ratios of bornitepoints plot as tight groups (Fig. 3).

Compositions and Cu:S ratios for individualpoints were based upon the approximate formu-la: /e element at individual poinX:

Counts for element at individual point-:- Average counts for elements for lb pointsX /6 element for average of l0 points

This relationship holds true if compositions ofindividual points approximate the average com-

TABLE I. RESULT5 OF ELECTRON PROBE ANALYSES . AVERAGE

ND.sought but not detected* calculated as 100%-gelements ana'lysedi detennined agalnst standard larincludes 38.8% Bt. The apparent (uncorpected) concentra-

tion of Bl was deternined on an,lrl-itne as 36.43.bt inciudes 16.09 AsColour ln reflected light: db-dark bluag ]b-l.ight blue; w-wnlte; p-purptei o-orange; c-cream; y-yellol|lN0TE: Sb,Pb,Ag sought but not detected-ln representattvedb, lb, H, p grains

All concentrations determlned by the electron probe havebeen -corrected for-background, atonic number, irhsorptton,and ttuorescence effects.

EACH

]IMEN %A) ss r fe NDro ta I

Formulatb t77 .8r los .sv 178.7p 164.2o 35 ,0db | 6e.6!db I 7 l .01db | 71 .3 'd b l 7 2 . 1 idb | 66 .5 'r o I z+.a:\b 72.81Ib l 72 .8 (

W | 1 b . 9 1p l o r . lc )40 ,7d b | 7 r . 5 4db I 6s .6cdb | 70.0c'tbl7s.47p | 61 .53' l " - ' ' 'db | 68.95db I 68 .91' r b

l 77 . l el b I 69 .351b172.e3w 177.77w 177 .46p 1 62 .91v | 50 .6db l 72 . l edb t 69 .97lbl 67 .s7w ln . s+p I 63.94

Idbl 68.26dbl 68.56dbl 68.76oDl ou. / Idbl 67.e9ib l 78.21' r b178 .39' lb l

75.141w | 78 .151w 177.601p | 63 .701

122.061lzz.t't| 2 r . 10126.08t * ' -I 30.2grI 28.86r129.59t27 .93l a r < r

I 16:66-i zs.oe127 .33?2.3622.3025.8020.328.44*

25.5827.71*? 1 7 n

?n aA*

30.54?2.67*30.8827.8022.362? .6725.6228.2 |lz , r* lte:;i-l?6.56 |?? .12 Ia a . I J I

31.72* ll l . t l4* l32 .68 I3 2 . 1 i I3 r . e l I21.16*l22.50 |2 2 . 1 7 |22.04 |22.03 |25 .6e I

I 0.r1r

I r i . ' t6

126.2I 0.08r

I 0.08r

I

I o.r,f

lu.,,|

0 . "0.02f

10.76.rt t . o v

0.20r

0. l4f

1'1.21 )2-0 |0.26i I0.40.r

I10.50 I0.02r1NDI

o,*rlI

10.81 I

' l seI d <

l;GI As,BilAs,Bi.set ^t : '

1""lAslnsl"l*l A s

l lG.st .s"lI,*lA;lnslst " -

I As,Bil Se l

As rBiB i ,Se

SeAsAsAs,Bi

AsAs

As

As

AsAs,Bi

I ruu.u[| 8e.21I 99.8CI r 0 r . 4 iI 95 . rI

I r00.00I I00.00| 100 .90i l oo .061 98 , t 3| 100.00

98.50' t 00 .13

1 00.0599.27

I 00.211 00.04

I 00.00'101.56

101 .05100 .0199 .54

100.0099.45

100.00I00. 23100.73100. '13100 . l 3ee.73 le6.80l

I 00.00 |1oo.o0 Ie4.53 Iee.96 |

1 0 0 . 1 7 Iroo.oo I100.00 I'r 0l .44 |'100.82

|99.90 |'100 .001

1 00.89 1e7.3r I' r00.19

|99 .63 |

I00.20 I

t l L U r r o )' l cu;';;srl cu;';;s

l*i:iiie::;slI cu, , .sI 9,i l,i:l;lt.:iiI cu;';;sI cui':;sI I'i j;i:I ilt.::;I cu;:;;s

13fr:;l5il:::!:I cu, ,,si cu; ' ; ;sI l,; l;,1I uur !q)

I :u4.4 s leo . ss)qI uu3.5 sreo . sssq

lCu r r aSI cu;:;;s| 9ur.zz!l L u l . I 3 Jl C u '

" , S!u1 .76 :L lJ l

" r )Cuq.95Fe1 -q6Sr+Cu3 . oFeo . tAssa .2Cu'

" ,Scu , ' , l s II

L U r . o 5 |

cu, ' l is Icur' i ireo.rusu ICu, noS Icu , ' i ^ s ICu, ' ^ .S I

lL U l n a ) |

cu, '^ Is Icu i ' i is Icu, ' i .s I

IL U I r r ) J

IL U r z q ) |

cui',^s iCur]io're6.rrs* |

ELECTRON MICROPROBE IT]-YESIIGATION OF COP"ER SUI.P}IIDES

0.90 100 lr0 r20 1.30CttS

97

I

a 3

= ,zzE -o

I

2

€zzE Joo 6

I

2

e 3

€z q

z€ 5

l

2

€z 4z= 5Hq 6

I

2

Hi 35Iz ,z

EU )

l

2

d 3

zzE "gS 6

o.90 lm 1.10 120 1.30Ca:s

Frc. 1. Mean compositions of Cu-sulphide grains.

Fic. 2. C-ompositions of Cu-zulphide points at which Cu and S were determined simultaneously.

Frc. 3. Compositions of 'bomite' points at vrhich Cu and S were determined simultaneously.

Ftc. 4. Compositions of Cu-sulphid,e points at which Cu and S were determined simultaneouslyand with all anomalous points excluded,

Frc. 5. Compositions of %omiteo points at which Cu and S were determined. simultaneously andwith all anomalous points excluded.

Frc. 6. Compositions of anomalous Cu-sulphide points at which Cu and S were determined simul-taneously and compositions of Cu-sulphide points for whiih Cu was analysed and S was deter-

mined by difierence.

qR TTIE CANADI.{N MINEMLOGIST

position of the ten points. For widely varyingcompositions small errors may be introduced bythe fact that the size of, ZAF corrections dependsupon the Cu and S contents. The error is negli-gible for compositional ranges encountered with-in individual grains in this work.

The compositional spread demonstrated by Fi-gure 2 may arise from several sources other thansample inhomogeneity and therefore an attempthas been made to distinguish the more reliabledata. Possible sources of data spread other thantrue compositional variation include instrumentalfactors, surface irregularities and effects of thethird dimension being unknown. Instrumentaldrift was effectively eliminated by using a beamcur"ent integrating system which controls count-ing times, and by the measurement of standardintensities before and after specimen intensities.Whilst every attempt was made to eliminate sur-face effects by careful sample preparation, somescatter from this source may still remain. How-ever, the major source of variation not relatedto changes in mineral composition is likely tostem from the selection of analytical points : itis not possible to determine what lies beneaththe surface in a polished mount of opaque min-erals. Thus the excited volume may include ma-terial other than that selected for analysis (e.9.quara gangue, other sulphides etc.) A simplestatistical approach was adopted in attemptingto evaluate the data. Surface irregularities andinclusions of 'foreign' material in the e:<citedvolume would tend to give totals for the analysedelements that deviate appreciably frorn IAA%,Consideration of count rates and concentrationsfor Cu and S in the Cu-sulphides and the stand-ards showed that t}le combined statistical count-ing error should give totals of 100* approximate-ly 2.1/s. Analyses with totals lying outside thisrange were considered unreliable and discarded.Irr a case where the foreign material inadvertent-ly included in the excited volume was anotherCu-sulphide the analysis would not be rejectedon this statistical criterion and indeed at leastsome of the 'intermediate' compositions retainedby this screen may reflect such situations. In anattempt to evaluate the remaining data, a sta-tistical approach was again adopted. For a trulyhomogeneous grain the total counts (N) at anyindividual point should not deviate by more than2.5SVN from the mean total counts (N-) ob-tained from the ten points analysed (assumingthat the 99% confidence level is being aimedat and other sources of error have been eliminated). This test was applied to counts obtainedat every analysed point for both of the principalelements (Cu and S). Points thx lay outsidethese limits are hereafter termed 'anomalous'

(but not discarded). Note that where S wasobtained by difference, the analyses lryere setaside for the time being and were not includedin this statistical treatment.

Of the 70 bornite points evaluated by thesemethods, 4 are anomalous with respect to Cucounts, and 5 anomalous with respect to S counts.Three points are anomalous for both Cu and S(and give unreliable totals). Hence only 6 pointsout of a total of 70 are anomalous. In contrast,of the 259 Cu-sulphide points, 40 are anomalousin their Cu counts and 61 anomalous in S counts.Of these, 21 points are anomalous for both Cuand S, leaving a total of 80 anomalous pointsout of 259. Fifty-six of these points were discardedas unreliablq having totals outside the range100 * 2.1. Of these 56 points, 27 occurred in just3 grains. fu the conditions for the bornite andCu-sulphide analyses were the same, it is proba-ble that the majority of anomalous Cu-sulphidereadings reflect true sample inhomogeneity.

Data for the Cu-sulphides and bomites thatare not classified as anomalous are plotted ashistograms in Figures 4 and 5. For Figure 4, ifmore than half of any set of 10 readings froman individual grain was anomalous none of thereadings was plotted, since it is essentially mea-ningless to compare individual readings with anaverage in such a situation. It is evident from acomparison of Figures 2 and 4 that, as might beexpected, it is mainly the scattered intermediatedata that are anomalous.

Of the 26 Cu-sulphide grains analysed, 3 grainsgave totally unreliable analyses (the majority ofpoints giving totals of much less than 1007o),l9 grains gave analyses that were reliable buthad-some anomalous points that were apparentlyof different compositions and were consequentlynot included in Figure 4, and 4 grains gaveanalyses with numerous anomalous points (7 ormore out of 10) and appear to be composed ofmore than one type of Cu-sulphide (i.e. showedtwo or more different Cu:S values). These lattercases were examined to see if they could bedivided into groups of points each having valueswithin the statistical counting error for that group.A sample calculation (tabulated in Table 2)shows the original and evaluated data for a darkblue Cu-sulphide grain in sample 6. The evalua-tion of the data indicates that this particularset o[ points may represent sulphide compositions with Cu:S ratios of approximately 1.02 and1.10, or possibly l.0a 1.08 and 1.13.

Fiswe 6 is a histogram of anomalous Cu-sul'phide points combined with those points forv,hich Cu was analysed directly and S calculated.As might be anticipated there is a greater scatterof points, although comparison indicates that

ELECTRON MICROPROBE INVESTIGATION OF COP?ER SULP}IIDES 99

prefcred compositions in Figure 4 are also slight-ly preferred in Figure 6. It may be noted thatthe preferred composition at Cur.zsS is enhancedand an additional preferred composition is sug-gested in the range Cu1.s1-1.s6S.

shows data for all the anomalous points. Column3 gives the data for points at which S was notanalysed directly but calculated by difference.No data for whidr Cu/6 * S/e lies outside therange 100 + 2,1% are tabulated.

Examination of the results suggests that thereare three dominant varieties of Cu-sulphides pre-sent, with compositions Cur.rz*o.ogS, Cur.sz*o.oeSand Cur.zs+o,ooS. Various compositions interme-diate to these and between these and covellite(CuS) on the one hand and chalcocite (Cu2S)on the other, also s<ist ; Cur.oo*o.orS andCrrt.rr*o.orS are most prevalent. Figure 4 suggeststhat variation within the ranges given may bereal; for examplg in specimen 2, grains withcompositions near to Cur.zs*o.ooS actually havea bimodal distribution, indicating possible com-positions of Cur.za*o.orS and Cur.ro.lo.orS.

Cu,Fe-sulphides

Bomites in specimens I, 2, 4 5 and 6 closelyapproach stoichiometric CuoFeSa although slight-ly deficient in Cu and Fe. Since the Fe contenrof tlle standard was assumed from stoichiometry,little significance can be attached to the smalldeficiency in this element. Compositions of thesebomites vary within the range Cua.ss*o.rzFeo.ss*o.osS+ (see Fig. 3). Sample 3 contains abornite-like mineral tentatively identified asidaite but with a composition Cus.rzto.ozFeo.eo*o.oeSa. This is somewhat enriched in Cuand depleted in Fe compared to the mineral thatSillitoe & Clark (1969) consider sroichiometricidaite (CqFeSa). Sample 3 also contains an'anomalous' bornite which is very depleted inCu compared to the 'normal' bornites of othersamples. It has a composition approximatingCua.esFeo.esSa, although individual points varyfrom about CuaFeo.sSe to Cu6Fes.eSa (as S was

IltsRpRErarrotI

Cu-sulphides

Application of the methods outlined above tothe Cu-sulphide data indicates that certain Cu:Sratios appear to be numerically more significant.Column I of Table 3 indicates only points thatwere not classified as anomalous and for whichboth Cu and S were determined. Column 2

* anoma'lous- Statistical counting error for Cu=2.58.f 1 6557=t332 (99"1 conf idence each) anona-lous i f <16225, >16889.

** anomalous- Stat ist ical countinq error for.S=2.58f 13654=t301 (99% confidenie level ) anoma-' lous

i f <13353, >13955.t unrel iable- Lie outside range l00t2. l% defined

by using maxjmum posit ive and negative countingeffors for Cu and S.

Under "Total C,ounts S Peak" read 1.4S88*. aver-age = 13654.

TABLE 3. CoMPoSITIoNAL RAIIGES 0F ANALYSED Cu-SULPHIDES

LU:5 OTrcorrected t

Cu-sul phi des

Cu:Stanona lous'Cu-sul phides

Cu:S of Cu-sulphldesfor which S not

analysed

IotalNo. ofPoi nts

r .06$.02(e)L l2!0.03(39)

1.229.02(6)'1.3210.04(25)

1 .76S.06(7r )r .88$.02( 6 )

. 02$ .02 (8 )

. 06$ .02 ( 6 )

. I 2r0.03(4)

. I 6t0.01 (3)

.22&.02('t)

.32!0.04( 7)

. 43$ .05 (10

.65i0.04(4)

.76 1o.06 ( 7)

l 0210.02' I .06!0.02

1 .12 !0 .031 . 1 6 ! 0 . 0 1' t .22{.02

1 ,32 !0 .041 .4310 .051 .65 l0 .04I .76!0.06I . 88$ .02n t h a r [ 1 6

z)8)2316)21)o ,

4)?4)? l

928Jd

l 5I

1029

'corrected' Cu-sulphides are those polnts csmposlng agrain for whJch more than five points are not 'anona-ious' (for further detalls, see text)Note: The no. of points lylng in a range is indlcatedin brackets.

TABLE 2. STATISTICAL ANALYSIS OF DATA FOR TENINO ONE DARK lEN 6

ipec i -nen

l o r a lCountsCu Peal % C u

TotaiCountsS Peak % s

Total

lueTaqe

6db 16452 67 .56 14 ]08* 32 .98 100.54 1 .0316409 67.39 14284** 33.62 101.0' l 1.0116454 67.57 14298 33.42 100.99 1.0216321 67 .02 14278 33 .37 100.39 l .0 l17106* 70.25 13462 31,47 101.72 1.1316416 67 .41 ' t4045**

32 .83 I00 .24 1 .0416471 67.64 12704* 29.69 97.33f t .1516793 68.96 12008* 28.07 97.03+ 1.24I6678 68 .49 13715 32 .06 t00 .55 1 .0816466 67.62 13534 31.63 99.25 t.0BL6557 67.99 L3654 3L.9L 99.9L 7.-aB

POSSIBLE GROUPING OF POINTS OF SIMILARCOMPOSITIONS IN SPECIMEN 6db

5 simLLayooi,nte

6db 16471 67.64 12704 29.69 97,33 1.15r16793 68.96 12008 28.07 97.33 1,24.t1632t 67 .02 14278 33 .37 100.39 1 .0116409 67 .39 14384 33 .62 101.01 I .0 t16454 67.57 14298 33.42 100.99 '1.0216452 67 .56 t4 '108 32 .98 I00 .54 1 .03164 '16 67 .41 14045 32 .83 100.24 1 .04L64t0 67.39 L4223 33.24 L00.63 L.02

Statist ical counting error for Cu=330;stat ist ical counting error for 5=307(99% conf idence i imits-no anorulousreadi nos

16466 67.62 13534 3'1.63 99.25 1.0816678 68 .49 13715 32 .25 100.55 ' t .0817i06 70.25 13462 31.47 101.72 t.13

ht. for L6750 68.79 13570 3L.78 LA0.5L L,L0Tbore 6 Statistjcal counting emor for Cu=334;zointe statistlcal counting effor for 5=301

(99% confidence Ieve' l)

t00 TTIE CANADIAN MINERALOGIST

e-

' T -

Kv(onemonn+ Fmt,len) [il:h:le$T TCv(Ot femnn+Frcnze l , l97 l ) | | l .Bb-cv(Eorton&slinnei,l96z1 'r

k- F'lCv(onemqnn+Frozel,l97l)

| g

l lBb-Cv(ottmm+roal,l97l) lF lfBb-Cv(Ol ranmr fFrenze l , l9T l ) ln lnBb-Cr(OtttrortFFreueil9Tl) l- l 'Bb.Cv[orremm+Fmpt.l97l)

| |Bb-cv{silit@ & clo'k,19691

l(&rth,p"r...*.rcn)Bb-Cv(Otne|FroFmel, Pn)Bb{v(Sil l i toe & Clorl, 1969)

Bb-cv(sil l i ioe &clqt, 19691

|

Bb-Cv(Sillitoe & Clorl, 1969 )

SynBb{v (Frenzel,196l )

tFwall9n)

rF.*t,le7t) [tJ:-o"s

An (Clcrl & Sil l i loa,l97l b)An (Sil l i toe & Cbk,l969)

& S i l l i toe , ' l97 lb )& 5 i l l i toe, 1971 b')

Borton & Slinner, 1962)

(Eorton & Slinner,'1967)

t\)

iTrneABb-cv

lo*",,.*

t r a g

F?I?iFilI$ qex; r3I: FF' i d ;

a ; 7

!

@

.S }A Fl l r nt ' l ;a g . l f

: = + c

r . 6 [Sd F F

e Par 6_c oF T6 . ^

s !+ =q o

I3

ns ar l ' it r E ' := 1 iii 9. :t3 . d 3 -o a '

oo

Fig. ?. Compositions of Cu-sulphide points aletermined in this paper (c)- compared with compositions of Cu-sulphides determined, by other authors (a) and 'idealised' compositions (b).

ELESIIRON MICROPROBE INVESTIGATION OF COPPER SULPTIIDES 101

determined by difference this grain is not in-cluded in Figures 3 or 5).

The remaining Cu,Fe.sulphides were analysedmerely to check optical identifications. Theorange ('o') and yellow (V) gains of specimensI and 4 occur as irregular blebs associated withCu-sulphides. The cream ('c') grain of sample 2occurs as an irregular mass within a bornitegrain. The orange grain of specimen I is proba-bly chalcopyrite although it shows a slight Fedeficiency. The yellow grain of specimen 4 ispossibly tennantite but if so shows a markeddeficiency in fu and contains a minor amountof Fe. Both of these analyses total well belorrI00/s, and they are therefore somewhat suspect.The creanr-coloured grain of specimen 2 is ten-tatively identified as wittichenite although itscomposition (Cus.odio.esSs) is depleted in Birelative to the formula CusBiSs, quoted by Die.trich (1969) A spectrometer scan on tJris grainrevealed no other constituents, so it is unlikelythat tJle assumed Bi content is greatly in error(see also footnote to Table 1).

Drscussnon

Sillitoe & Clark (1969) found that anilite(Cur.roS) rvas converted to a digenite'type solidsolution even by gentle grinding at room tem-perature and thus, presumably, by polishing. It

Frc. 8. Grain of blaubleibender covellite showing paleblue (white) and dark blue (grey) exsolved phases.The latter shows lighter and darker bluq incom-pletely rasolved lamellae. Reflected light 100X oilimmersion objective ; magnification : X1360.

therefore seems likely that much of the Cu-sulphide reported here with a composition ofCur.za*o.oeS was originally anilite. Analyses inthe upper part of this range may perhaps repre-sent original digenite.

Cu-sulphides with compositions of Cur.rg*o.oaSand Cu:..s2*o.o.S are probably two distinct struc-tural varieties of blaubleibender covellite. Moh(personal communication, 1971) has found twosuch varieties eacJr giving distinct X-ray powderpatterns and having compositional ranges ofapproximately Cur.oz-r.zoS and Cur.oz-r.azS. TheCur.rr*o.osS and Cur.gz+o.oa,S reported in thispaper may be preferred compositions of thesetwo varieties of blaubleibender covellite. If thisis the case the compositions reported here maybe, represented in the manner shown in Figure 7where data of other workers are summarized inpart (a), the connpositions reported here in part

Frc. 9. Grain of blaubleibender covel-ite showing dark-er blue phase (grey) with pat&es of a lighter blue(white) phase intergrown. The former shows ex-tremely fine and poorly resolved, paler blug some-what sinuous lamellae. The NW-SE grain runningacross the photograph is due to the polishing com-pound used to buff the sample before photograph-ing. Reflected light, l00x oil imnersion obiective;magr.iflcation: X1360,

102

(c), and inferred 'ideal' compositions and mix-tures of ideal compositions suggested by thiswor\ in part (b).

Whether intermediate compositions representsolid solution of end members or whether tleyrepresent intergrowtJrs of the various phases wasnot apparent from the microprobe data. How-ever, an optical re-examination of the specimensusing a 100X oil immersion objective revealedthe presence of intergrowths of dark blue andlighter blue phases on an exFemely fine scale,ranging down to a fraction of a micron in size.Typical tadures are illustrated in Figures 8 and9. Re-investigation of one sample (number 2)with the electron microprobe showed that it wasnot possible to resolve areas of different com-position rrithin a grain by means of r-ray scan-ning pictures. However, on positioning the beamas closely as possible on carefully selected points,in the great majority of cases the following com-positions were obtained: lighter blus areas appa-rently free of lamellae - Cur.zs-r.ssS; darkerblue areas with very fine regularly oriented andspaced lighter blue lamella€ - Cur.re-r.zzS ;darkest blue areas either lree of lamellae or con-taining very few la-ellae - Cu:..0s-r.rzS. le thelast case it was possible to find only a few areasin which the beam could be placed with suffi-cient precision- It is therefore suggested that origi-nal compositions lying between chalcocite andcovellite have o<solved to give compositions closeto the opreferred' Cu:S ratios of L.I2, 1.32 and1.76. The ratios of L.l8-1.22, found principally

in sample number Z could possibly represent anintermediate composition stable at some highertemperature. There is no evidence concerningthe temperature at which the exsolution tookplace. However, in view of the previously men-tioned observation of Sillitoe & Clark (1969) thatanilite is converted to digenite by even gentlegrinding at room temperatwes, the possibilityshould be borne in mind that the exsolutionfeatures reported here could have been promotedby sample preparation techniques. The lowestCu:S ratios observed in the exsolution productscorrespond closely to those recorded for the darkblue covellite, described earlier as filling fracturesand replacing other Cu-sulphides.

In Figure 10 tl_re compositions of bornitgoanomalous' bomite, and idaite grains reportedii, this paper are plotted together with data onthese minsals given by previous investigators.The 'normal' bomites of this paper are similarto those of Sillitoe & Clark (1969), but containslightly more Cu (and perhaps less Fe). Theone 'anomalous' bornite encountered also ap-parently contains less Fe than those of Sillitoe &Clark (1969). The idaite (?) grain analysed isintermediate in composition between'anomalous'bornite and CugFeSe (idaite) but once againappears to contain less Fe than those idaitesdetermined by either Sillitoe & Clark (1969) orOttemann & Frenzel (1971). On the other hand,compositions in the region between bornite andanilite (or digenite) such as those reported by

A N

+o tom ic % CuFrc. 10. Triangular plot of compositions of Cu- and Cr:,Fe-sulphides determined in this

paper compared with compositions determined by other workers.

' this strdy (Cu-:ulf ides Indicoted qe-,rEpreseniingronga of voluas )

+ sil l i ioe & Clor! (t969)x Clck & Sil l i toe ( l9Zlo)

o Ottemin &Frewl(197o Frenzel & Otiamon'{19

+ von GehlenlP64)

r C lo rk (1970)o Ollemon & Frerel (l9Zl]o Frenzel & Ottamon'{ 1967)

y' lhereticol

ELECTRON MICROPROBE INVESTIGATION OF COPPER SULPHIDES 103

Morimoto & Gyobu (1971) have not been ob-served in the course of this study.

Acrnowrmcrmmrrs

One of us (D.G.W.S.) acknowledges financialsupport for this work from the National Re'search Council of Canada. We are grateful toI. P. N. Badham for critically reading the manu-scrip! and to R. D. Morton for helpful discus-sions.

RsFBxNcEs

Benrorv, PJ., In. & Srnnwn, B.I. (1967) : Sulphidemineral stabilities. Geochemistrg of tlydrothermalOre Deposits, H.L. Bames (ed.)" tlolt Rineharil &Winston, New York, 236-333.

Carrrnor, EN. (1961) : Ore MbroscopU. John Wiley& Sons, New York

Cranr, .4-H. (1970) : An oceurrence of tlle assem-blage native sulphur - covellite - 'Cur.sxFess.sxl

Aucanquilcha, Chilq Amo. Mineral. 55, 913-918.& Snr.rror, RI. (1971a) : A note on iron

bearing normal mvelling N. lb. Mineral. Monatsh.42+4%-

'(1971b) : Supergene anilite from MinaEstrella (Salado). Atacamq Chile, N. lb. Mineral.Monntsh. 515-523.

Dnrnrct, RV. (1969) : Mineral ?aDles. McGraw-Hill,New Yorlq 87 pp,

Fnrlran, G. (1961) : Der Cu-Llberschuss des blaublei-benden Covellins. N. !b. Mineral. Mortatsh. 199-204

& Orrruerv, J. (1967) : Eine Sulfidpara-genese mit kupferhaltigen Zonarpytite von- Nukun-itamu,/Fiji. Mineraliurn D eposita 1, 307-316.

Mon, Gll (1963) : Phase relations at low tempera-tures : blaubeibender covellites. Camegie h*t. Wash-Yearbook 63' 208-2W.

Monrr"roto, N. & Gvoru, A" (1971) : The compositionand stability of digenite. Ams. Mineral. 56' 1889-19m.

Ot:zuarr, I. & FnBN'zq G. (1971) : Neue lvlikro-sonden-Untersuihungen an Idaiq Covellin und blau-bleibendem Covellin N. lb. Mineral. IvIormtsh. S0-no

Pnre, R.A (1962) : Fernie map arear east half' Al-berta anil British Columbia. GeoI. Swu. Can." Pa'per 6l-4.

Surrro4 R.H. & C.unr; A.H. (19@) : Copper antlcopper-iron sulphides as the initial products of su-pergene oxidatioq Copia$ mining districq NorthernChile. Amer. Mi:rwal. 54, 16841710.

S*rmr, D.G.W. & Tonrr-rusow, M.C. (1970) : AnAPL language computer progrqYn for use in electronmicroprobe analysis. CornpuJer Contrib. 45t StateGeol. Suru"/Uniu. Karsas" % pp,

Von Ganr.nrr, K (1964) : Anomaler Bornit und seineUmbildung zu Idait und "Chalcopyrit" in deszen-denten Kupferinzen von Somms*lah1 (Spcsart).Fortschr. Mineral. 41, 163.

Marutscript receiued. Febrtnrg 1973" emended April1973.