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GAINS AND LOSSES OF ELEMENTSRESULTING FROM WALLROCK ALTERATIONA QUANTITATIVE BASIS FOR EVALUATING LITHOGEOCHEMICAL ,SAMPLEBy D. A. Sketchley and A. J. SinclairThe University of British ColumbiaDepartment of Geological Sciences I,INTRODUCTION

Multi-element lithopeochemical analysesare increasingly widelyused in the explordlion fa r many types of gold deposits. To m u -imize the information gain from such datn it is imperative to apprec-iate the chemical nature of unaltercd country rock and altered wall-rock of variou s origin s, and to quanti fy hc gains and losses ofelements during the alteration process. An understanding of altera-tion history is important because many alteration zones are closelyassociated, spatially and genetically, with precious metal deposits.A number of methods has been proposed to quantify the pro-cedure for major and minor elements, including nn assumption ofconstantvolume,Berth 'sstandardcell,andconstantsilicatetrahedra (Poldervaart, 1953).The general assumption of constant volume is clearly incorrect.Barth's standa rd cell ;assumes that the num ber of oxygen atomsremains unchanged during metasomatism, whereas Pnldcrvaart as-sumed that the number o f silica tetrahedra is unchanged during thealteration process. Whatever the validity of these approaches oquantifying gains and losses, it is apparent that none of the preced-

alteration haloes developed around gold-quartz veins enclosed ining methods can be applied usefully in the case of carbonate-richbasic volcanic rock s. Such oss-gain situations can be dealt with bythe use of a procedure presented initially y Gresens ( I 967) and laterby Babcock (1973).GRESENS METASOMATIC EQUATIONof elements during metasomatism i n terms ofGresens derivedan ideal equation for calculating lossesnd gains

(1) Pdrenl and product rock compositions,(2) Specific gravities of the parent and product rocks,(3) Volume chang e during metasomatism.with an explanation of terms.where X, = loss or gain (grams of component n).

Gresens' equation will not be developed here but is reproducecX, = a[f,X,(G,iG,) - X,]

a = initial weight of rock A, commonly taken as 1 M Igrams so that X, will be weight per cent change of E .component oxido.X,, X, = weight fraction of component X in parent rock (A :G,, G, = specific gravitier, of parent rock (,4) and product rock

fv = volume ratio of product rock to ]parent rock .and product rock. (B).(B).

PROCEDURE( 1 ) Whole rock chemical analyses are required for both parent z.nd(2) Specific gravities are measured for both parent and prod~ct

product rocks to provid e values for X, and X,.

(3) Volume change during metasomatism is estimated as a propor-rocks to provide values for G, andG B.

tion of the volume of product rock to a unit volume of par:ntrock. An estimate of fv is obtained by examining the ratios ofimmobile elements such as TiO, and AI,O,. For example:fv = (TiO,), / (TiO,), = (A120JA / (AI@&

ABUNDANCE OF MAJOR ELEMENTS (WEIGHT PER CENT) FOR DDH 80-88,TABLE 6-7-1.ERICKSON GOLD MINES LTD.

Element - J H - I - J H - 2SO, . .. . 38.849.55AI203TiO,Fe203Mn OMgOCaO

80-88 80-88

. . . .. . . . . . . .. . . . . 11.40 12.071.00 1.02

. . 0.15 0.15. . . .. . .. . . . . . . . .

. . .. . 8.69 8.935.87 5.60. . .. . . . . . . . 1 1 . 2 1 10.40

. . 2.78.12LDI* . . . . . . . . . 17.28 15.22Total. . . . . . . . . . . . . . 91.64 96.41

. . . . . . . . .. . . .. . . . . .. . . .

Na,O . . . .. . .. . . 0.30 0.28K2OP A 0.12 0.07

. . . . . .. . .. . .. . .

- J H - 3 - J H - 480-88 80-8841.82 48.190.86 1.408.97 10.610.18 0.165.98 5.58

10.02 15.15

10.43 6.230.10 0.010.07 0.102.25 0.1713.70 7.6094.38 95.20

13.43 13.52 14.141.27 1.24 1.3210.38 10.87 11.330.19 0.17 0.165.83 7.14 7.314.36 11.19 10.400.01 1.40 2.1 I0.58 0.13 0.110.10 0.10 0.107.44 4.14 2.9696.19 96.71 97.84

~~ * LO1 = Total loss-on-ignition at 550C and IO0O"C.Analyses done at the Department of Oceanography, The University of British Columbia.~ _ _ _ _

__* This project i s a contribution lo the CanaddBritish Columbia Mineral Development Agreement.British Columbia Ministry of Energy. Mines and Pclrnleum Rcsuurcer, Geological Fieldwork. 1986, Paper 198i-I413

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4 . n

3 . 0

2 .0

1 .29t . 0

0 2

3 0

2 0 -

4 0

3 0

2 0

1 1 11 0

0 . 2

O Z L . , , , , , I , I , , , , , , I- 8 - 4 0 4 8 12 16 20

- I . , JWEIGHT % CHANGE

v~ = V o lu m echangewithconstant AI,O,. TiO,an d ZrVI = C o n s ta n t volume

V F l V , = Final volumellnit ia l volume

to weakly altered wallrocks (6). Sample 3 is abnormal in containing a quartz veinlet. Legend for lines representing weight per cent elem entFigure 6-7-1, Composition-volume diagrams for ix contiguous carbonatired basalt samples extending from the vein contactI ) outwardvariations as a functionof volume changes is shown for sample 1. Lines labelled V , are constant volume; lines labelledV, are the interpretedvolume changes based on Figure 6-1-2. Element gains and losses are thus the intersections of line V, with individual element lines.41 4

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Avalue of 1 indicates no volumechangewhereas > I indicatesvolume ncreasean d < I indicatesvolumedecreaseduringmetasomatism.AN EXAMPLE- ENNIE VEIN, ERICKSON MINEscribed by Sketchleyan d Sinclair, 1987, Ske tchle yetal ., 1984, andCarbonate alteration haloes developed at Erickson mine are de -Sketchley, 1986.Sevencontiguous amples extending from heJennie vein, through a carbonatized zone nd into adjacent unalteredcoun uy rock, were annlysed for whole rock chem isvy by X-rayfluorescence. Results are listed in Table 6-7-1. Using the analyticaldata or a single alteration sampleand he data for unalteredwallrock, it is simple to construct a composition-volume diagram(Babcock, 1973) as follows: foreach elemen tCresen s metasomaticequation is solved for any two very different values of fv , say 0.05and 3.0. Thus, two points are known on the composition-volumemetasom atic d iagram ;and can be joined by a straight line as inFigure 6-7-1. C omparable straight lines can he co nstructed for eachelement. An estimate of the volume change, fv. can he made fromimmobile elements and this canbe drawn on the graph parallel to thecomposition a xis. Intersections of the elemental straight lines withthe volume factor line provide graphical quantitative estimates ofthe loss or gain of all elements with volume change taken intoaccount. The losses and gains can he calculated more precisely byeach analysis of an unaltered ro ck, S ix such diagrams, representinguse of Gresens formula. Similar diagrams can be constructed for

VOLUME CONSTRAINTS

8.rnClnumber

- 3

- 1- 2- 4- 5- 7-6

A.A 1 2 0 3 TiO, Z rv e r a g e

CONSTRAINING COMPONENTS

the six altered rock analyses (Table 6-7-1) from he Jennie veinalteration halo,are shown in Figure 6-7-1. They illustrate :hevariable manner in which individual rocks have reacted to meta-somatism. The diagrams become somewhat (complicated w h esubstantial and variable lossesand gains have wcurred as in FigoreThis pattern is in sharpcontrast with the simplicity of Figure6-7-1(1 ) representing altered ock immediately adjacent to thew i n .6-7-1(6) which reflects only very minor metasomatic changes.data reportedhere, we attempted to estimate hevolume factor

Volume changes vary from one sampleo another. For the Jemieindependently for each sam ple using three separate immobile v w -ables, A1,0,, TiO, and Zr. Results shown in Figure 6-7-2A indka tethat the assumption of immobil ity of these h e omponents, al-though not perfect, is reasonably well satisfied. T he approximatevariation in volume changeoutward from thevain wall is shown inFigure 6- 7-2 8, In general, the amount of volume change decreu esoutwards from the vein toward unaltered country rock. The exctp-quartz vein explaining this anomaly.tion is a single sharp peak repmenting sample 3 which includes a

It is useful to exam ine individual elementss profiles of loss-g,dnversus position in an alteratio n halo (distan ce outward fro m w:indramatic gains of K,O and SiO, and losses of MgO, total Fe (aswall). Results for eight elements are shown in Figure 6-7-3 wh,:reFe,O,) and Na,O are apparent from he alter:uion haloes. Me r-estingly, our calculations suggest a major rearrangement f CaO inthe alteration halo, perhaps with a slight net loss.

] 1 CHANGEVOLUME10085 80

70 -60 -50 -40 -30 -20 -10 -0 -

- l o 1 number B.T-Tl--0 1 2 3 4 5 6 7 8DISTANCE TO VEIN (m)

+ = L o c a t i o n o f q u a r t zv e i n l e tv = Q u a r t zv e i nUB = U n a l t e r e d b a s a l t

Figure 6-7-2. A- olume factors (V final/V initial) cstimated forach of six carbonatized basalt samples (1 -6)and one naltered sample(7 ) for each of three relatively imm obile components. V olume factors (ratios) are estimated by the ratios of weight percentages for imm obileelements in unaltered tu altered samples, that is, Wt initiallWt final. B -Interpreted volume change accompanying alterations for sevencontiguous samples extending outwards from adjacent to Jennie vein ( I 1 to unaltered wallrock (7).415

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-2.0i1 2 3 4 5 8 7 4.0 Na,O3.0 -2 .0 -1.0 - V UB-1 2 3 4 5 8 7 -1 2 3 1 5 6 74.0 - 1 I 1

iii v0 --T o t a l F ea se 2 0 3 4.0 - A1203 4.0 - TiOp

0 3.0-za0 2. 0 -s

2.0 -.0 -I

3.0 - 3.0 -

l . 0 v

1.0 - 1.0-

iz -1.0 -1.01.0 --2.0 - E. -2.0 -. -2.0 - F.

12.0 --10.0 --8.0 -.6.0--4.0 -

-2.0 -- v0 - -

-2 .0i0 1 2 3 4 5 6 7

Legend1 = Location o fV = Vein

quartzveinlet

UB = Unalteredbasalt

DISTANCE TO VEIN (rn)Figure 6-7-3. Losses and gains of major oxides in six carbonatized basalt samples extending outwards from Jennie vein and expressed asweight pe r cent of unaltered basalt. Results shown are based on average volume changes of Figure 6-1-2.

416

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specific volatile materials such as H,O, C 0 2 and sulphur. Instead,The nature of OUT chemical data did not permit identification ofwe obtained a w eight m easuref "loss-on-ignition" (u ) I ) as shownin Figure 6-7-4 hut recognize the addition of H,O (sericite), CO,(carbonate) and S, (pyrite) to the alteration halo.

1816

20

I I I I I I I I I I0 1 2 3 4 5 6 7 8DISTANCE TO VEIN (rn)

Figure 6-7-4. Loss-an-ignition (LOI) or seven contiguous basaltsamples extending outwards from Jennie vein into unaltered basalt.Data from Table 6-7- I

DISCUSSIONAn understanding of element exchange during metasomatic pro-cesses nvolvingwallrockalteration sanessentialbase or heinterpretation of multi-element lithogeochemical data in preciousmetal exploration. Fora quantitive study of the kind described herewhole rock chemical analyses are required.t is important o realizethat most multi-element inductively coupled plasma (ICP) data setscannot b e used for such calculations as they are generally obtainedusing partial extraction techniques. A small group of well-selectedsamples analysed both by ICP and an appropriate whole rockmethod (for example, X-ray fluorescence)ill provide data to carryout the calculations ecommendedhereandalso will perm it anevalua tion of the extentof partial extraction inherentn the CP data.Both types of information are essential to a sound interpretation ofICP data.

CONCLUSIONGresens' metasomatic equation provides a useful procedure ti11examining gains and losses of elements in altered rocks. The pro-cedure utilizes whole rock ch emical analyses fix altered and UII-altered rocks and permits a quantitative evaluation f the effec ts ofmetasomatism without relying on peculiar constraintsuchas "coil-stantnumber of oxygenatoms"or"constantnum ber of silicatetrahedra".

following nformation:In the case of the Jennie vein the whole rock data provide llre( I ) Volume changes during alterationre most pronounced near hevein (approximately 30 per cent) and decrease outwardsow;ird(2) Addition of volatiles from the veino altered w allrock decreaszsunaltered wallrock.

(3 ) SiO, and K,O have been added throughou t the alteration halooutwards from the vein wall..

(4) Na,O, MgOand otal Fe as Fe,O,) havebeen ost fromwith only rare exception.

( 5 ) CaO has b een redistributed in the alteration halo such that nearthroughout the alteration halo.the veinCaO is bnormally high whereas furtherway CaO hasbeen lost.

(6) AI,O, and Ti 02 appear to have increased slightly in the haloalthough hese very minor changes may simply reflect ocalvariations in the original composition.ACKNOWLEDGMENTS

This paper is part of an extensiv e study of ,wallrock alteratiaonPetroleum Resources, he Natural Sciences and Engineering He-supported by the British Columbia Ministry of Energy, Mines ;ndsearch Council and Erickson Gold Mines Ltd.REFERENCESBabcock, R . S . (1973): ComputationalModelsofMetasomatic Ro-Gresens. R.L. (1967): Cornpo shon-Vo lume Relationsh ips of M8:t-

cesses, Lithos. Volume 6, pages 279-290.

Poldervaart, A. (195 3): Petrological Calculations in Metasomaticasomatism, Chemical Geology, Volume 2 , pages 47-65,

481-504.Processes, American Journal @ Scie nce, Volume 251, pa(,esSketchley, D.A. 1986):The Nature of CarbonateAlteration ofBasalt at Erickson Gold Mine, Cassiar, North-central BritlshColumbia, Unpublished M.Sc. Thesis, Department of Gc:o-S k e t c h l e y ,D . A .a n dS i n c l a i r .A . J . l 9 8 ? ) :M u l t i e l e m e n t

logical Sciences, The University ofBritish Columbia.Lithogeochemistry of Alteration Associate83 with G old -qu amEnergyMines and PetroleumResources, Geological Fidd-Veins of the Erickson Mine, Cassiar District,B.C. Ministq of

Sketchley, D. A. , Sinclair, A .J andSomerville, R. (1984): P.e-work, 1986, Paper 1987-1, this volume.liminary Repo rt on Wallrock Alteration, Erickson Gold Mine,Cassiar District; B .C . Ministry of Energy, Mines and 1%-troleumResources , Ge dog ic a lF ie ldwork ,1983, Paper1984-1, pages 219-232.

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