Petrology and Geochemistry of Amphibolites of the MacLean Lake Belt, Sucker Lake Area, La Ronge Domain

R. Hasanie I and C. T. Harper

Hasanic , R. and Harper. C.T. ( 1997): Petrology and geochemistry of amphibol!tes of the Maclean Lake Belt, Sucker Lake area, La Ronge Domain; in Summary of Investigations 1997 . Saskatchewan Geolog1cal Survey, Sask. Energy Mines, Misc. Rep. 97-4.

1. Introduction

Amphiholitcs of the MacLean Lake Belt (MLB) have hecn variously interpreted to be derived from sedimentary, volcanic, and plutonic protoliths. This diversity of opinion in part reflects the high­metamorphic grade, the lack of preserved primary structures, and misinterpretation of the effects of early alteration of the amphibolites. The objective of this study is to investigate the nature and origin of these rocks in the Sucker Lake area through: field examination, pctrography, and whole rock and trace element geochemistry.

Sucker Lake, located about 185 km northeast of La Ronge, lies within the Sucker Lake-Fleming Lake project area described by Harper ( 1996 and this volume). Sample material for the present study was collected in both 1996 and 1997.

2. Field Relationships of the Amphibolites

The MLB is composed primarily of biotite-quartz­feldspar gneisses of psammitic to pclitic composition, minor amphibolites, and calc-si licate rocks, which together form the MacLcan Lake gneisses (MLg). Although the amphibolites mainly occur in the MLg, they are also intercalated locally with the presumed overlying McLennan Group arkosic assemblage in the transi1ion zone between the two groups. The amphiboliles arc commonly associated with calc­silicate rocks.

Johnston ( 1969, 1970) considered most of 1he amphibolites 10 be metabasalts having identified pillow structures in a few places norlh-northeast of Sucker Lake. Harper ( 1996, this volume) also considers the arnphibolites to be primarily of volcanic origin hased on the existence of pillows together with fragmental textures. intercalation of mafic to felsic varieties, and early volcanogenic calcareous alteration, now represented by pods and lenses of epidotc-quartz± diopsidc±plagioclase±calcite±garnet.

The arnphiboli tes arc typically dark green to black, tine to medium grained, generally equigranular and granoblastic, and composed of hornblende and plagioclase with variable amounts o f biotile, garnet, quartz, ep1dotc, diopside, and calcite . They can also

I Geology Dcparimcnt, Universi1y of Regina. Regina. SK S4S OA2.

Saskmchewan Geological Surve.v

have a banded or streaky appearance with green grey and/or yellow green layers alternating with the darker layers. The colour banding primarily reflects variations in hornblende and plagioclase content and the presence or absence of epidote and associated minerals. Coarser grain size develops in epidotc-quartz±diopsidc± plagioclasc pods and lenses, which locally become very abundant. These arc considered 10 be metamorphosed early alteration products. The amphibolitcs are similar 10 the mafic mctavolcanic rocks that occur along the southeastern margin of the Central Metavolcanic Belt (CMB). Whether they are distal equivalents of the CMB or represent a different period of volcanism is not clear.

3. Petrography A preliminary assessment of the petrology has been started, but additional samples have yet to be sectioned. The amphibolites consist primarily of hornblende and plagioclase, with variable amounts of diopside, epidote, biotite, garnet, cummingtonite, quartz, and calcite. Accessory minerals include: opaques (pyrite, pyrrhotite, magnetite, and ilmenite), sphene, zircon, and apatite. Chloritc and sericite arc common alteration products of hornblende and plagioclase respectively.

Hornblende occurs as blocky to elongate grains from 0.5 to 3 mm long with a strong preferred orientation in S 1, and has various plcochroic schemes from pale brown to dark brown, olive green to dark green, and straw yellow to olive green to brownish green. Poikiloblastic habit is common with plagioclase being the main mineral inclusions. One section , 9711-0129 from the amphiholite unit lying above the arkose unil north of Koliniak Lake, contained alternating layers of hornblende and well-twinned, colourless cummingtonite grains.

Plagioclase, 1ypically of andesine composition, occurs as both polysynthetically twinned and untwinncd grains from 0.3 to 2.0 mm diameter. Partial IO extensive alteration to scricite is common, and in some sections plagioclase shows marginal replacement by calcite.

Epidote and diopsidc occur as minor constituents in many sections as an alteration of hornhlcnde, with epidote commonly present as granular aggregates. Both minerals occur as major conslituents in calcareous

51

layers accompanied by plagioclase, quart7.. and calcite. Tablel • Major, trace, and rare earth element analyses of Interstitial quartz and calcite arc also minor amphibolites from the Sucker lAke area. Analyses by

constitue nts in some amphibolites. Bor,dar-Clegg ar,d Company /.Jd. Major elements i11 percerit; trace and rare earth element.~ iri ppm; and gold in

Biotite is a minor constituent , occurring in close ppb.

associatio n with hornble nde , and has a fox red Sample pleochroism. In garnet-spotted varie ties, garnet occurs No. 9633 -0009 -0017 -0030 -0057 -0063 as aggregates of and individual , typically subhcdral, gra ins up to 3 mm diameter which have grown at the

Si 0 , 48.59 50.56 48.80 48.98 48.10 expense of hornble nde and plagioclase. The garnets are T i 0, 1.20 0.78 0.96 1.10 0.98 commonly surrounded hy newly formed quart,. and Al,O, 14.52 14.74 14.06 13.75 13.43 plagioclase and conta in inclusions of quartz. Fe10/' 13.92 9.80 12.46 13.80 11.29

MnO 0.21 0. 16 0.18 0.19 0. 19 The accessory minerals, sphcne and apatite, are closely MgO 5.50 7. 13 6.89 6.43 5.20 associated with hornble nde, and common ly have CaO 11.1 1 10.25 11.53 9.5 1 15.26 e uhedral shapes. Sphene is relatively abundant, up to Na,O 2.02 3. 12 1.58 2.28 U!4 several percent in some rocks, and can attain several K,O 0.2 1 0 .49 0.3 1 0.54 0.37 millimetres length. Zircon is present in most sections as P,O, 0. 11 0.19 O.IO 0. 14 0 .06 0 .01 to 0.03 mm long inclusions in hornblende with LOI 0.98 1.00 1.78 0.74 1.74

characteristic, hut paler, pleochroic halos than found in Total 98.40 98.28 98.68 97.47 98.47

biotite . The opaque minerals a rc evenly disseminated Au <5 <5 <5 <5 7 throughout the rocks. Pyrite mainly occurs in cubic Ag 2.3 1.9 1.7 1.9 2.3 habit: pyrrhotite as anhcdral grai ns. Cu 20 5 38 68 245

Pb 49 73 22 8 11 Zn 86 93 65 105 93

4 . Geochemistry Mo 10 9 8 9 7 Ni 79 166 102 86 92

Analyses from five amphibolite samples collected in Co 41 37 42 37 45 1996 are reported; results from samples collected in Cd 0.2 0.2 <0.2 0.2 0.3

1997 are not yet availa ble. These samples were Bi <5 <5 <5 <5 <5 As <5 <5 <5 <5 <5 analyzed for major oxides, 34 trace clements, and three Sb <5 <5 5 <5 7

of the samples for rare earth clements (REE) (Table I). Ba 84 155 so 11 35 Analyses of mafic volcanics that occur along the Cr 143 307 186 137 158 southeast margin of the CMB arc included in the v 320 186 256 289 263 chemical discrimination diagrams below for Sr 135 156 150 137 208 comparison. y 25.8 15.S 17.7 18.0 14.0

Ga 14 13 14 16 14 The amphibolites show basaltic chemical s ignatures. Li 13 20 15 16 10

They are subalkaline , tho le iitic basalts (sec F igures 9 to Nb <l < I < I <I < l Sc 37 22 30 40 32 11 in Harper, this volume) of low to medium K Zr 61 85 39 61 45

character (Figure I ) on the K10 versus Si02 plot of Th 0.3 0.9 0. 1 na na Le Maitre (1989). On several basalt discrimination u <0. 1 0.7 <0. 1 na na diagrams o f Pearce and Cann ( 1973) and Pearce ( 1975), the amphibolites show chemical affinities to La 8.9 14.0 7.4 7.4 7.5 both ocean floor basalt and island arc tholeiitic basalt Ce 12.8 24.7 8.2 na na (Figures 2 and 3). Sample 9633-0017 consis tently plots Pr 2.4 3.7 1.5 na na in the calc-alka line basalt fields of Pearce and Cann Nd 9.2 16.2 6.8 na na ( 1973) and on the line between basalt and basaltic Sm 3.8 3.9 2.3 na na andesitc in Figure I. In some cases there is an overlap Eu 1.3 1.0 0.9 na na

Gd 3.7 2.2 1.8 na na with the mafic volcanics of the CMB, but in o thers Tb 0.7 0.5 0.6 na na there is a distinct separation as in the Ti versus Cr plot Dy 5.4 2.8 3.9 na na (Figure 3), implying that the amphibolites represent a Ho 1.0 0 .7 0.8 na na d istinc t magma source and certainly a differe nt tectonic Er 2.6 1.8 1.9 na na selling from the CMB mafic volcanics. The scatlcr of Tm 0.5 0.3 0.3 na na some of the CMB mafic volcanics may s tem from mild Yh 3. 1 1.5 1.9 na na alteration. Lu 0.4 0.3 0.2 na na

The chondrite normal ized REE plot (Figure 4) shows a Whole rock analy~es by borate fusion extraction and ICP-Mass Spec. ; trace clements by HF-HNO,-HCIO,-HCI

somewhat overlapping pattern for the amphibolites and extraction and ICP-Mass Spec. ; gold by fi re assay Atomic CMB mafic volcanics. The rela tively fl at profi les arc Ah~orption; REE by borate fusion extraction and ICP-simila r to patterns obtained by Watters and Pearce MassSpec; and FeiO;*=total iron. ( 1987) for metabasalts fro m the La Ronge Domain that arc characteristic of oceanic is land arc settings.

52 Summary of lnvesti!(cllions /997

5

4.5

4

3.5

3 ~ !. 25 i l<l

I I

"' I I

2 I I

' I I

15 I I I I

' I I

' I ' I I

(j

043 45

medum-K

low·K

• 55 65 75

Si02 (wt t )

79

Johnston. W.G.Q. ( 1969): The geology of the eastern portion of the Waddy Lake area, Saskatchewan; Sask. Dep. Miner. Resour., Rep. 127. 43p.

____ { 1970): The geology of the May Lake (east half) area. Saskatchewan; Sask. Dep. Miner. Resour. . Rep. 130. 36p.

Le Maim:. R.W. {1989): A Classification of Igneous Rocks and G lossary of Terms; Blackwell. Oxford, l 93p.

Pearce, J. A. ( 1975): Basal! geochemistry used to investigate post te<.:tonic environments in Cyprus; Tectonophysics, v25. p41-68.

Pearce, J .A. and Cann. J.R. ( 1973):

Figure I - K,O versus SiO, plot to differentiate basalts into low, medium, or high potassium series (from Le Maitre, 198 9). Solid diamo11ds==amphiholite and open diamo11d=majic volcanic of the Central Metavo/canic Rell.

Tectonic setting of basic volcanic rocks determined using trace clement analysis; Earth Planet. Sci. Lett., vi 9, p290-300.

5. Conclusions Preliminary conclusions are that the amphibolites of the MacLean Lake gne isses arc o f volcanic origin, they show h igh iron , low potassium, tholciitic hasalt chemistry and arc somewhat trans itional between ocean floor and oceanic island arc basalt a ffinity .

6. References Harper, C.T. ( 1996): La Ronge-Lynn Lake Bridge Project:

Su<.:ker Lake-Fleming Lake area; in Summary of Investigations 1996. Saskatchewan Geologi<.:al Survey. Sask. Energy Mines, Misc. Rep. 96-4, p66-78 .

Ttl 100

A

Sun. S.S. ( 1982): Chemical composition and origin of the earth's primitive mantle; Geochim. Cosmochim. Acta, v46, pl 79-192p.

Watters. B.R. and Pearce, J.A. ( 1987): Metavoleanic rocks of the La Ronge Domain in the C hurchill Province, Saskatchewan: Geochemical evidence for a volcanic arc origin; in Pharaoh. T.C., Beckinsale, R.D .. an<l Rickard. D. (eds.), Geochemistry an<l Mineralization of Proterowic Volcanic Suites, Gcol. Soc., Spec. Pub!. No. 33. pl67-l82.

B HroJ

15000

OF9 0. 8

LKT A, 8 CA8 A. C

00'---~-'--...... ~~-}--·~,oo...__~ ,~~~--'-,~----',~~~200'--~225-L-----',,., z, -1

Figure 2 - Basalt discrimination diagrams of Pearce and Cami (1973). A) Ti/100-Zr-Sr/2 and R) Ti versus Zr. CA B=calk­alka/i11e basalt; IAB=isla11d arc basalt; LKT=low potassium basalt; and OFB=ocea11floor basalt. Symbols as in Figure I .

Saslwtchewan Geological S11n •ey 53

10"5 ~---~-~--.--,.---..--,-,r-,--,----r-r--..-----,--,---,-"""T-",-.-,

LKT · Low Polanio.rn T~ei

OfB · Oce"" flool Basal,

l(O)'-----...i...---'--'--L-.._.L-..JL.-.L......_ ___ -'-__ ..__.___.....__,___._...._L...J 10 100 ,cm

c, (ppm)

Figure 3 • Ti versus Cr basalt discrimination diagram of Pearce ( 1975), which shows a clear distinction between the amphiboliJes of the Mac Lean Lake Belt and the ma fie volcanics along the southeast margin of the Central Meta volcanic Belt. Symbols as in Figure 1.

.. ..... E

"' :, ... 0

:r e .... .3

Figure 4 • Chondrite normalized REE plot of the amphibolites and mafic volcanics. Symbols as in Figure 1. Normalizing values from Sun (1982).

54 Summury of Inve.ni1-:a1io11s /997