Lithogeochemical Studies in the Flin Flon - Amisk Lake Area

B.R. Watters 1

Watters, B.R (1990): Uthogeochemical studies in the Flin Flon - Amisk Lake area; in Summary of Investigations 1990, Sas­katchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report 90-4.

The geochemical study of metavolcanic rock units in the Flin Flon area, initiated in 1989, was continued during 1990. This study is being conducted in collaboration with the detailed bedrock remapping program being car­ried out in the Douglas Lake - Phantom Lake area (Thomas, 1989, and this volume) and the Mystic Lake area (Reilly, this volume).

Some aspects of geochemical data obtained for an ini­tial suite of mainly metavolcanic rocks and lesser in­trusive units from the Douglas Lake - Phantom Lake area (Thomas, 1989) are discussed below. Repre­sentative analyses from this suite were included in Wat­ters (1989). Analysis of about 200 more samples has also recently been carried out, but these data have yet to be processed. Rare earth element (REE) data for some of the initial suite has also been obtained and preliminary interpretations based on these results are in­cluded in this report.

1. Discussion of Results Although much of the work is still in a preliminary stage, data obtained so far has provided some clear indica­tions regarding the compositional character and geotec­tonic setting of the metavolcanic rocks. As reported in Watters (1989), it can be demonstrated that silica has been mobile to varying degrees in the metavolcanic rock units, thus classifications and interpretations based on Si02 can be ambiguous. Although such alteration is common in the area, it is not ubiquitous. Such observa­tions are consistent with the field evidence (Thomas, 1989) which indicates that many of the mafic pillowed volcanic units, for example, have been silicified.

Basaltic volcanic rocks from the Douglas - Phantom Lakes area, analyzed so far, are tholeiitic and of island arc affinity. This has been demonstrated by Watters (1989) and is also indicated by an AFM plot (Figure 1), various other interelement relationships and REE dis­tribution patterns, to be discussed below. It can be ar­gued that, in view of the possibility of alkali element mobility in these rocks, an AFM diagram would not produce a meaningful classification. The distribution of data points on this plot, with respect to the major clas­sification fields, however, is very similar to that on a Jen­sen cation plot (Watters, 1989) which does not include the alkali elements. While this does not altogether rule out the possibility of alkali mobility, it does suggest that it did not occur to the same extent as with silica.

Wherever possible, those elements which are regarded as being generally immobile are being used in the inter-

(1) Department ot GeolOgy, University of Regina. Regina. Saskatchewan

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,.•'•

FeO*

1.nu:e !!t .1 c

F1e!o • •

I \ ._ t

ca:c-alkal 1ne rield

MgO

Figure 1 - AFM (Na2')+K20- FeO• - MgO) plot for some metavolcanic and intrusive rocks from the Douglas Lake - Phan­tom Lake area. Field boundaries are from Irvine and Barager (1971). Full circles, metavolcanics; crosses, Boundary Lake In­trusion; squares, Bootleg Lake Granodiorite; X, Phantom Lake Granite.

pretation of these rocks. It is also recognized that the in­troduction of varying amounts of silica into many of the basaltic flow units would have modified the absolute concentrations of other major {and trace) elements to an as yet undetermined extent. For this reason, an at­tempt has been made, so far as possible, to base inter­pretations on variations of element ratios and on three­component diagrams. For example on a ternary plot in­corporating Y + Zr, Ti02x100 and Cr, the metabasalts clearly follow a tholeiitic trend as defined by Davies et al. (1979), having relatively lower (Y +Zr):Ti ratios when compared with calc-alkaline basalts {Figure 2).

The metavolcanic rocks are characterized by low Ti and low to moderate Zr contents and are confined almost ex­clusively to the volcanic arc field on Figure 3. Concentra­tions of the element Y are also typically low and, when plotted against Cr, provide further support for volcanic arc affinities {Figure 4).

The Zr-Ti-Sr discriminant plot (Figure 5), while including an element that may show a degree of mobility during metamorphism (i.e. Sr), provides an interpretation that is in agreement with the above, further suggesting that these mafic volcanic rocks are of arc tholeiitic character.

Summary of Investigations 1990

Ti0 2 *100

Y+Zr Cr

Figure 2 - Y+Zr- (Ti02x100) - Cr plot for some metabasaltic rocks from the Douglas Lake - Phantom Lake area. Trend lines are from Davies et al. (1979).

Flin Flon Ar ea - Vo lcanics

··- . . --.....

e 0. .9

"' I

10

::! ' , \

\ ,t ;t,"ljn··plat~

* ' '

Lavas .. ·.-1 \ f-

100 1000

Zr (ppm)

Figure 3 - Plot of Ti vs. Zr for metavolcanic rocks of the Douglas Lake - Phantom Lake area. Field boundaries aM from Pearce and Norry (1979).

Volcanic arc tholemic basalts typically have low con­centrations of K (Jakes and White, 1972) and this char­acteristic is also shown by the basaltic rocks from the Douglas Lake - Phantom Lake area, which show a range of 0.22 to 0.60 percent K20.

REE distribution patterns for basaltic rocks from the area are presented in Figure 6. Profiles for individual flow units have been grouped according to which mem­ber they have been assigned on the basis of field evidence {Thomas, 1989). While the REE profiles from the three members have similarly shaped patterns, vari­able degrees of total REE content allow fairly clear dis­tinctions to be made between them. All of the patterns

Saskatchewan Geological Survey

1000

L 100 u

Volcanic Arc

Basa lts ,. I

10 100

y (ppm)

Figure 4 - Plot of Cr vs. Y for metabasa/tic rocks of the Douglas Lake - Phantom Lake 81'18. Field boundaries are from Pearce (1982).

Ti I 100

Zr Sr/2

Figure 5 - Plot of Zr-Ti/ 100-Sr/ 2 for metabasaltic rocks of the Douglas Lake - Phantom Lake area. Field boundaries are from Pearce and Cann (1973).

are quite flat but the Creighton and Newcor Members' patterns are very mildly LREE depleted, whereas basal­tic rocks of the Bomber Lake Member are slightly LREE enriched. The patterns displayed by the three Members are characteristic of island arc tholeiites and indicate an origin for the basaltic magmas by melting of primitive mantle peridotite {Jakes and White, 1972; Cullers and Graf, 1984a).

REE profiles of rhyolitic units of the Myo Lake Member (Millrock Hill locality) show moderately enriched, slightly

45

<l> .µ ...... L -0 c o 10 .c u '-:,L. u 0 c:t

LI Co Pr Nd

Figure 6 - Rare earth element distribution profiles for basaltic rod units in tfM Douglas Lake - Phantom Lake area. ///// "' Creighton Member; 11 111 • Newcor Member; I I\ I\ = Bomtr er Lake Member.

concave-downwards patterns with no fractionation be­tween the LREE and HREE, but with very strongly developed negative Eu anomalies (Figure 7). Such pat­terns may indicate derivation of these magmas by frac­tionation of a tholeiitic basic parent magma involving plagioclase and clinopyroxene as the major fractionat­ing phases.

In view of the close spatial and temporal association be­tween these rhyolitic units and the basic units of arc tholeiitic character it is suggested that such a genetic link existed between the two rock types, the falsie mag­mas being derived by extensive fractionation of the volumetrically more significant tholeiitic magmas. Such a process would explain the REE characteristics and the relatively low Ki{) contents (2.10-2.62 percent) of the rhyolites is also consistent with their derivation from a low-K parent magma Derivation of the falsie magmas by partial melting processes is considered much less likely because of the probable lack of intermediate to fel­sic crustal source rock in what is considered to have been an evolving island arc.

<l> ...., ..... L 'Cl c 0 .c u

100

'- 10 :,L. u 0 a:

---

La C• Pr Nd S• Eu GO Tb Dy Ho Er T• Yb Lu

Figure 7 - Rate earth element distribution profiles for myotitic rock units of tfM Myo Lake Member in tfM Douglas Lake - Phan­tom Lake area.

46

REE distribution patterns for a single sample of the Phantom Lake Granite and for two samples of Boundary Intrusion, are presented in Figure 8. The former displays a strongly fractionated pattern with high LREE to HREE ratios and no Eu anomaly, a pattern typical of many granodioritic rocks (Cullers and Graf, 1984b). The two samples of Boundary Intrusion for which REE data have so far been obtained are basic in composition (48.10 percent Si02) and are strongly alkaline (5.24-5.29 per­cent total alkalis). The REE contents reflect these charac­teristics in that the profiles show moderately high total REE contents and moderately fractionated LREE to HREE with no Eu anomaly. Further data from a wider range of samples will be needed before reasonable inter­pretations can be made concerning the genetic sig­nificance of these characteristics.

Phlntc• Lak• Granite - -100 Boundary Intruslon { :.: :.:..:.

<l> .µ ..... (..

'Cl c 0 .c u

-..::~ .........

~ .....

'- 10 ~ u 0 a:

La C• Pr Nd

"~ . , ... :-,-·-·----.__ ___ _ ---- ---- -------

Figure 8 - Rare earth element distribution profiles for tfM Phan­tom Lake Granite and Boundary Intrusion.

2. Conclusions

1. Metabasaltic rock units in the Douglas Lake - Phan­tom Lake area, belonging to the Creighton Member, the Bomber Lake Member and the Newcor Member (after Thomas, 1989), display low potassium vol­canic arc tholeiitic characteristics, and have been variably silicified.

2. REE distribution patterns for rocks from the three members are similar in that they are flat with little or no LREE to HREE fractionation, generally have low total REE and no Eu anomaly. Small, but quite dis­tinct, differences in total REE contents, however, allow a distinction to be made between basalts from the different members.

3. Rhyolitic units of the Myo Lake Member display REE profiles consistent with an origin by fractionation of low-K tholeiitic parent magma of the type that gave rise to the basaltic flows of the other three members.

Summary of ln'IIJstigations 1990

3. References Cullers, R.L. and Graf, J.L. (1984a): Rare earth elements in ig­

neous rocks of the continental crust: predominantly basic and ultrabasic rocks; in Henderson, P. {ed.), Rare Earth Element Geochemistry, Elsevier, New York, p237-274.

____ (1984b}: Rare earth elements in igneous rocks of the continental crust: intermediate and silicic rocks • ore petrogenesis; in Henderson, P. (ed.), Rare Earth Element Geochemistry, Elsevier, New York, p275-316.

Davies, J.F., Grant, R.W.E. and Whitehead, R.E.S. (1979): Im­mobile trace elements and Archean volcanic stratigraphy in the Timmins mining area, Ontario; Can. J. Earth Sci., v16, p305-311.

Irvine, T.N. and Barager, W.R. (1971): A guide to the chemical classification of the common volcanic rocks; Can. J. Earth Sci., vs. p523-548.

Jakes, P. and White, A.J.R. (1972): Major and trace element abundances in volcanic rocks of orogenic areas; Geol. Soc. Arn., Bull., v83, p29-40.

Saskatchewan Geological Survey

Jensen, L.S. (1976}: A new cation plot for classifying sub­alkalic volcanic rocks; Ont. Geol. Surv., Misc. Pap. 66.

Pearce. J.A. (1982): Trace element characteristics of lavas from destructive plate boundaries; in R.S. Thorpe, (ed.), Andesites, J.Wiley and Sons, New York, p525-547.

Pearce, J.A. and Cann, J.R. (1973): Tectonic setting of basic volcanic rocks determined using trace element analysis; Earth Planet. Sci. Lett., v19, p290-300.

Pearce, J.A. and Norry, M.A. (1979): Petrogenetic Implications of Ti, ZI, Y and Nb variations in volcanic rocks; Contrib. Mineral. Petrol., v69, p33-47.

Thomas, O.J. (1989): Geology of the Douglas Lake. Phantom Lake area (Part of NTS 63K-12 and -13) ; in Summary of In­vestigations 1989, Sask. Geol. Surv., Misc. Rep. 89-4, p44-54.

Watters, B.R. (1989): Uthogeochemlcal studies in the Flin Flon • Arnisk Lake area and Glennie Domain; in Summary of In­vestigations 1989, Sask. Geol. Surv., Misc. Rep. 89-4, pSS-57.

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