A Field, Petrographic, and Geochemical Study of and Related Rocks from the Sandy Islands Complex, Wollaston Domain 1

C. Madore 2 and I.R. Annesley2

Madore, C. and Annesley, I.A. (1994): A field, petrographic, and geochemical study of gabbros and related rocks from the Sandy Islands Gabbro Complex, Wollaston Domain: in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 94-4.

The Sandy Islands Gabbro Complex is well exposed on Early Proterozoic(?) age). The main belt of Wollaston a series of northeast-trending islands in the west-central Group metasediments is composed of graphitic pelitic part of Wollaston lake near the eastern edge of the gneiss, metamorphosed iron formation, pelitic gneiss, Athabasca Basin (Figure 1 ). It comprises an intermedi­ calc-silicate gneiss, psammopelitic gneiss, psammitic ate to mafic southeastern component and a felsic to in­ gneiss, metaquartzite, and amphibolites. The Wollaston termediate northwestern component (shown as an Group rocks are complexly deformed, polymetamor­ approximate contact on Figure 2, after Chandler, 1978). phosed, and rest unconformably (and tectonically) on re­ worked, antiformal Archean granitoid gneisses. The Preliminary mapping and sampling was undertaken in Wollaston Group metasediments are intruded by meta­ 1993 for petrological and geochronological purposes as gabbros, porphyritic (1815 Ma), and pegmatites part of an industry-funded geological investigation. Geo­ (Madore and Annesley, 1993). The precise age of the chemical results indicated possible potential for Au and Cu mineralization. Further work was undertaken in 1994 to investigate the primary (igneous) and secondary (metamorphic) characteristics, the petrochemistry, the tectonic environment, and the economic potential of the Sandy Islands Gabbro Complex.

1. Geological Setting The major elements of the Sandy Islands Gabbro Com­ plex were documented by Chandler (1978) and Ray (1978). Chandler (1978) noted that the complex corre­ sponds to an "egg-shaped" aeromagnetic high, which is consistent with the abundance of in the vari­ ous lithological phases of the complex. A monzogabbro phase (sample A93-026a) of the Sandy Islands Gabbro Complex has been dated at 1828 ±3 Ma by U-Pb zircon geochronology (unpubl. data; Annesley, Madore, and Krogh). This age is identical, within analytical error, to a U-Pb zircon age of 1828 ±3 Ma (refined to 1829 ±1 Ma) 06• ,.. for a coronitic, hornblende-phyric monzogabbro in the Karpinka Lake area (Annesley et al., 1993).

The Sandy Islands Gabbro Complex is found within the c:J PhoMrv 10o< CO¥« Wollaston Domain, one of the major subdivisions of the D AfhobOl(:0 GfOI.IP Cree Lake Zone (Lewry et al., 1985; Gilboy, 1983). The ~ '#olt,o$tOft Gt 1MJP Wollaston Domain is a northeast-trending, orogenic fold ..,.,.,,.....- MojonJ'1~ / fa1.1H rone, and thrust belt that is fault-bounded (i.e. Needle Falls ,,... - - 0ommo bOIJ""°'Y Shear Zone) to the east with the Peter Lake Domain * ~ o, 111a" CI• Gotibro C.omplu and the Rottenstone Domain, and to the west borders ,o IOO ,,o the eastern margin of the Hearne Province hinterland. It comprises Archean continental crust and continental Figure 1 · Map of the Precambrian Shield of northern Sask· margin supracrustal rocks (i.e. the Wollaston Group of atchewan showing the location of the Sandy Islands Gabbro Complex.

(1) Funding by Cameco, Cogema Resources Inc., PNC Exploration (Canada) Co. ltd., Uraner.z Exploration and Mining ltd., and the Saskatchewan Research Council. (2) Saskatchewan Research Council, 15 Innovation Blvd.. Saskatoon, Saskatchewan, S7N 2X8.

148 Summary of Investigations 1994 ~

j () I 1 I I 8 I w 0 L , L A s r' O · N ! "~

··-·---·····------·-· i __

' ~ -.

103''30 ' 25' 20' 15'

Figure 2 - Map showing sample locations.

Wollaston Group is not known; however, an Aphebian pear to be layered locally, and possibly rhythmic lay­ age is assigned to the Wollaston Group on the basis of ered but the latter is difficult to discern because of a its apparent unconformable relationship with the underly­ well-developed vein network. The vein network consists ing Archean granitoid gneisses and its subsequent of cross-cutting granitic veins {Figure 5). In places, the metamorphism during the Hudsonian Orogeny. Lewry gabbros are metasomatized with complete replacement and Sibbald (1977, 1980) and Sibbald (1983, 1985) of primary mineralogy and textures (Figure 6). have documented a broadly defined stratigraphic se­ quence, comprising four main lithological units for the At some localities (A93-026, M94-004, Figure 2), gab­ Wollaston Group. More recently, Annesley and Madore broic phases are intruded by (to (1991) have introduced a relatively simple stratigraphic diorites), which are light to dark flesh pink mottled black sequence for the Wollaston Group, which is similar to on the fresh surface and dull flesh pink to dull light grey Early Proterozoic supracrustal sequences (e.g. the mottled flesh pink on the weathered surface. They are Chantrey Group and the Ramah Group) of the Rae holocrystalline, inequigranular-porphyritic, fine to coarse Province (Hoffman, 1988 and 1989). grained, hypidiomorphic-granular, massive to moder­ ately foliated, fresh to altered, magnetic, and relatively dense. The granodiorites are composed of quartz, K­ 2. Field Relations , , and biotite with subordinate titan­ ite, hornblende, and opaque minerals (magnetite and The Sandy Islands Gabbro Complex is well exposed sulphides). K-feldspar phenocrysts are up to 3.0 cm in and consists of discontinuous rounded outcrops. The length. Locally, biotite flakes are partially altered to chlo­ best exposures occur along the wave-washed island rite. At locality A93-026, the gabbro phase is more de­ shorelines. Heavy lichen covers the outcrops farther in­ formed (i.e. in part sheared) than the intrusive land. The gabbros are medium grey to greenish brown , which suggests possible emplacement of mottled black on the fresh surface, and weather dull the granodiorite during shearing. Late pegmatite dykes dark grey to rusty brown to dull medium brown mottled and veins crosscut the gabbro complex. black. They are holocrystalline, essentially equigranular to inequigranular-porphyritic, overall medium grained, hypidiomorphic, ophitic to subophitic, massive, fresh to 3. Preliminary Petrographic Observations retrograded, invariably magnetic, and dense (Figures 3 and 4). The gabbros are composed mainly of varying Ten samples were collected from the Sandy Islands proportions of plagioclase, , hornblende and bi­ Gabbro Complex. They represent three low-Ti02 gab­ otite, subordinate K-feldspar, opaque minerals, titanite bros (M94-001a, -001b, and -007a), four high-Ti02 gab­ and apatite, and minor quartz. The gabbro phases ap- bros (M94-002, -003, -004a1. and -004a2). a quartz

Saskatchewan Geological Survey 149 Figure 3 - Sandy Islands Gabbro illustrating a fine· to medium­ Figure 6 - Metasomatized gabbro is holocrystalline, equigranu­ grained subophitic texture. The rock is composed of plagio­ lar to inequigranular, very fine grained, massive to weakly foli­ c/ase, augite, biotite, hornblende, and titanomagneffte (Location ated, granoblastic in texture, and moderately altered. The min­ M94-001). eral assemblage consists of diopside, plagioclase, quartz, horn­ blende, titanite, and magnetite. The primary mineralogy and tex­ ture is completely obliterated by the pervasive metasomatism (Location M94-00S).

diorite (M94-004b), and two calc-silicate gneisses (M94- 004c and -005).

The low-Ti02 gabbros are holocrystalline, fine to me­ dium grained, inequigranular, sub-ophitic, and essen­ tially unaltered. They are composed of plagioclase, augite, biotite, titanomagnetite, minor amounts of quartz and pyrite, and accessory apatite. Plagioclase laths range from 0.85 to 6 mm in length and display albite and Carlsbad twinning. The plagioclase grains are ran­ domly oriented and are partly enclosed within augite grains (Figure 7) which range from 0.6 to 7 mm in di­ ameter, and have a green to salmon pink pleochroism. Rutile exsolution lamellae occur along the augite cleav­ age planes. Hornblende partly replaces augite and bi­ Figure 4 - Sandy Islands Gabbro displaying well-preserved otite along their grain boundaries. Biotite flakes, from subophitic texture, consisting of plagioctase, augite, horn­ 0.65 to 4.5 mm in length, form intricate intergrowths blende, biotite, titanomagnetite, pyrite, and quartz (Location with titanomagnetite grains, 0.5 to 4.5 mm in size, that M94·007). are composed of a magnetite host with ilmenite lamel· lae (Figure 8).

The high-Ti02 gabbros are characterized by a holocrys­ talline, inequigranular, fine- to medium-grained, hypidio­ morphic to subophitic texture. They are partly recrystallized, massive, and weakly to moderately al­ tered. The high-Ti02 gabbros differ from the low-Ti02 gabbros by the occurrence of titanite (primary and meta­ morphic), and the lack of augite. The igneous mineral assemblage is composed mainly of plagioclase, horn­ blende, biotite, quartz, magnetite, ilmenite, titanite, and pyrite. Accessory minerals include apatite and chalcopy­ rite. The primary assemblage is partly recrystallized in places and displays a granoblastic texture. The meta­ morphic minerals consist of hornblende, titanite, scapolite, and . Hornblende grains partly re­ place biotite flakes, whereas titanite grains form a reac­ tion rim around magnetite grains (Figure 9). Plagioclase grains are randomly oriented, range from 0.6 to 4.5 mm Figure 5 - Sandy Islands Gabbro showing a network of narrow in length, and show albite and Carlsbad twinning. De­ veins and fractures with metasomatic reaction rims (Location spite partial static recrystallization, the primary grain out­ M94-004). line of plagioclase is still well preserved. The grains are

150 Summary of Investigations 1994 weakly altered to saussurite and partly replaced by tricate intergrowths with biotite flakes and magnetite scapolite, locally. Hornblende grains are both primary grains. and metamorphic, range from 0.35 to 5 mm in size, and show intricate intergrowths with adjacent plagioclase, bi­ Calc-silfcate gneisses {or metasomatized gabbro) are otite, and magnetite. holocrystalline, equigranular to inequigranular, very fine to fine grained, recrystallized, massive to weakly foli­ The quartz diorite is holocrystalline, equigranular to in­ ated, and weakly altered. The primary mineralogy and equigranular, very fine to fine grained, strongly recrystal­ texture of the rock is completely obliterated by the per­ lized, massive, granoblastic, and weakly to moderately vasive metasomatism. A granoblastic texture is well de­ altered. The hypdiomorphic texture of the quartz diorite veloped (Figure 10), and the rocks now consist of is well preserved, despite the strong static recrystalliza­ plagioclase, quartz, hornblende, diopside, biotite, titan­ tion. The primary mineral assemblage comprises plagio­ ite, magnetite, and pyrite. clase, quartz, hornblende, biotite, magnetite, and apatite; metamorphic minerals include titanite and epi­ dote. Plagioclase and quartz grains form an equigranu­ lar, granoblastic texture, and their grain size ranges from 0.2 to 0.35 mm in diameter. Poikiloblastic horn­ blende, ranging from 0.35 to 0.75 mm in size, forms in-

O .SO .t\M

Figure 9 - Drawing from a photomicrograph of gabbro (sample M94·002) that is holocrystalline, fine to medium grained, hypdio­ morphic, and massive in texture. The igneous mineral assem­ Figure 7 - Drawing from a photomicrograph of gabbro (sample blage consists of plagioclase (Pl), quartz (Qtz), biotite (Bt), M94·001a) which illustrates the characteristic subophitic tex· hornblende (Hbl), magnetite (black), and titanite (Ttn). Titanite ture of the gabbro. The primary mineral assemblage consists grains are late-magmatic and fonn a reaction rim around the of plagioclase (Pl), augite (Aug), hornblende (Hbl), and magnet­ magnetite grains. ite (not indicated), with minor quar1z (Qtz).

Figure 10 - Drawing from a photomicrograph of calc-silicate gneiss (metasomatlzed gabbro, sample M94-004c) illustrating Figure 8 - Drawing from a photomicrograph of gabbro (sample holocrystalline, essentially equigranular, very fine- to fine­ M94-007a) showing primary magnetite (Mag) with ilmenite la· grained, granoblastic texture. The mineral assemblage is com­ me/lae (/Im). The magnetite grains display intricate intergrowths posed of plagioclase (Pl), quartz (Qtz), hornblende (Hbl), diop­ with adjacent biotite flakes (Bf) and plagioclase (Pl) grains. side (Di), magnetite (black), and titanite (Ttn).

Saskatchewan Geological Survey 151 4. Geochemistry can be divided into geochemical g roups on the basis of Ti02, Al203, Fe203, and P20 s contents. This approach Table 1 presents the major and trace element geochem- has been used by Bellini et al. (1986), Fodor (1987), istry for nine of the gabbros and related rock samples and others, in distinguishing two types of (a low- from the eastern part of the Sandy Islands Gabbro. The Ti02-P20s type and a high-Ti02-P20s type) in the gabbros (M94-001a, -001b, -002, ·003, -004a1, ·004a2, Parami flood province of Brazil. and -007a) show a relatively large degree of composi- tional variability. For example, T i0 2 varies from 1.01 to The low-Ti02 group of Sandy Islands gabbros repre- 3.10 wt percent and MgO from 3.39 to 5.26 wt percent. sents the most primitive compositions, as Minimum and maximum values for the other oxides shown by its high MgO, Ni, and Cr values, and its low also differ to the same degree. In general, the Sandy Is· values of alkali, incompatible, partially compatible, and lands gabbros show a broad continuum from composi- rare earth elements. tions with relatively high to relatively low MgO and CaO contents. In addition, there exists an inverse correlation between Al203 and Ti02, Fe203 (total Fe as Fe203), 5. Economic Potential and P20s. Similarly, there exists a positive correlation Some of the Sandy Islands gabbros contain elevated, between Al20a with most of the compatible elements ano malous values of Au, as high as 1110 ppb (Table (e.g. Cr). 1). They are generally depleted in Pt and Pd (1 O ppb or less, not presented in Table 1) and are also depleted in The above mentioned and other geochemical charac· Cu, Co, Cr, and Ni. These results are consistent with teristics of the Sandy Islands gabbros, combined with the calc-alkaline nature of the gabbros. petrographic observations, indicate that the gabbros

Table 1 • Geochemical data of gabbro and related rocks from the Sandy Islands Gabbro Complex. Major oxides in wt percent, trace elements in ppm, and Au in ppb.

Sample M94·001a M94-001b M94-002 M94·003 M94·004a 1 M94-004a2 M94·004b M94·005 M94-007a G G G G G G OD cs G

SiO:i 56.20 56.10 52.30 52.30 49.20 52.30 65.20 67.80 56.40 Ti02 1.10 1.01 3.10 2.16 3.07 2.80 0.70 0.43 1.38 Al20:i 16.47 17.06 15.24 14.97 12.19 14.62 14.59 12.71 15.82 Fe~3 (total) 8.42 8.71 11 .65 13.06 15.48 12.09 6.1 4 2.97 9.07 MnO 0.12 0.12 0.13 0.14 0.18 0.1 5 0.06 0.04 0.1 4 MgO 4.93 5.26 3.49 3.39 4.53 4.10 2.97 3.49 5.23 CaO 6.34 6.69 6.45 6.94 8. 13 7.14 3.21 6.04 6.57 Na20 3.46 3.28 2.30 3.20 2.46 2.77 2.29 5.36 3.26 K20 1.40 1.50 2.50 2.20 1.30 1.80 4.20 1.00 1.70 P20s 044 0.49 1.24 1.48 1.49 1.10 0.19 0.14 0.82 LOI 0.70 0.70 1.30 1.00 1.10 1.30 1.30 1.00 0.50 Total 99.58 100.92 99.70 100.84 99. 13 100.17 100.85 100.98 100.89

C(%) 0.09 0.05 0.10 0.07 0.17 0.1 5 0.06 0.07 0.09 s (%) 0.09 0.06 0.13 0.17 0.36 0.19 0.02 0.01 0.11

Ba (ppm) 850 786 603 990 687 1141 1239 211 872 Be 1.0 1.0 1.8 1.4 1.4 2.0 1.6 1.6 0.8 Co 30 3 1 32 30 47 35 12 8 33 Cr 130 130 10 10 40 50 130 100 120 Cu 17 18 36 17 75 37 7 1 19 Ga 24 22 25 28 20 21 22 13 24 La 34 3 1 60 80 53 52 32 31 42 Mo 5 5 5 5 7 5 5 8 5 Nb 13 9 35 22 23 25 10 11 13 Ni 82 90 31 9 54 48 40 24 88 Pb 37 3 23 3 2 2 2 2 2 Rb 35 3 1 103 56 49 69 166 28 40 Sc 13 14 14 17 23 20 13 8 15 Sr 852 870 379 729 622 727 104 47 883 Th 3 3 11 4 3 3 5 25 5 u 3.1 3.5 5.9 5.6 3.5 5.9 3.6 2.9 3.4 v 116 123 280 134 359 246 101 72 148 y 18 17 38 31 26 25 23 32 24 Zn 109 114 85 156 154 121 37 16 117 Zr 160 100 190 210 120 220 150 120 140 Au (ppb) 14 6 111 0 12 36 40 140 24 50

Note: G=gabbro, OD=quartz diorite, and CS=calc-silicate.

152 Summary of Investigations 1994 6. References Lewry, J. and Sibbald, _T.1.1. (19n): Variation in lithology and tectonometamorph1c relationships in the Precambrian base­ Annesley, I.R. and.Madore. C. (1991): The Wollaston Group ment of northern Saskatchewan; Can. J. Earth Sci. v14 and its underlying Archean basement: Final Report; Sask. p1453·1467. ' ' Resear. Coun., Publ. R-1230+C-91, 140p, (confidential). ____ (1980): Therrnotectonic evolution of the Churchill Annesley, I.R., Madore, C., and Krogh, T.E. (1993): U·Pb Province in northern Saskatchewan; Tectonophysics v68 zircon, titanite, and monazite geochronology from the Wol­ p45-82. ' ' laston-Mudjatik Domain Boundary, Karpinka Lake area, northern Saskatchewan; Geol. Assoc. Can./Miner. Assoc. Lewry,~-~-· ~ibbald, T.1.1., and Schledewitz, D.C.P. (1985): Can., Jt. Annu. Meet., May 1993, Waterloo, Prog. Abstr. Varrat1on 1n character of Archean rocks in the western Churchill Province and its significance; in Ayres, L.D., Thur­ Bellini, G., Comin-Chiaramonti, P., Marques, L.S., Melfi, A.J., ston, P.C., Card, K.D., and Weber, W. (eds.), Evolution of Nardy, A.J.R., Papatrechas, C., Piccirillo, E.M., Roisen­ Archean Supracrustal Sequences, Geol. Assoc. Can., berg, A., and Stolfa, D. (1986): Petrogenetic aspects of Spec. Pap. 28, p239·261. acid and basaltic lavas from the Parana Plateau (Brazil): Geological, mineralogical, and petrochemical relationships; Madore, C. and Annesley, I.A. (1993): Metamorphic pressure­ J. Petrol., v27, p915-944. temperature conditions of the basal Aphebian Wollaston Group, Hearne Province, northern Saskatchewan; Geol. Chandler, F.W. (1978): Geology of part of the Wollaston Lake Assoc. CanJMiner. Assoc. Can., Jt. Annu. Meet.. May Fold Belt, north Wollaston Lake, Saskatchewan; Geol. 1993, Water1oo, Prog. Abstr., pA-65. Surv. Can., Bull. 277, 25p. Ray, G.E. (1978): Reconnaissance geology: Wollaston Lake Fodor, R.V. (1987): Low· and high-Ti02 flood basalts of south­ (west) area (part of NTS area 64L); in Summary of lnvesti· ern Brazil: Origin from picritic parentage and a common gations 1978, Saskatchewan Geological Survey, Sask. mantle source; Earth Planet. Sci. Lett., v84, p423-430. Dep. Miner. Resour., Misc. Rep. 78-10, p25-34.

Gilboy, C.F. (1983): Sub-Athabasca basement geology project; Sibbald, T.J.I. (1983): Geology of the crystalline basement, in Summary of Investigations 1983, Saskatchewan Geologi­ NEA/IAEA Athabasca test area; in Cameron, E.M. (ed.), cal Survey, Sask. Energy Mines, Misc. Rep. 83-4, p28-31. Uranium Exploration in Athabasca Basin, Geol. Surv. Can., Pap. 82·11, p1-14. Hoffman, P.F. (1988): United plates of America, the birth of a craton: Early Proterozoic assembly and growth of Lauren­ ____ (1985): Geology and genesis of the Athabasca tia; Ann. Rev. Earth Planet. Sci., v16, p543·603. Basin uranium deposits; in Summary of Investigations 1985, Saskatchewan Geological Survey, Sask. Energy ____ (1989): Precambrian geology and tectonic history Mines, Misc. Rep. 85·4, p133-156. of North America; in Bally, AW., and Palmer, A.A. (eds.), The Geology of North America-An Overview, Geol. Soc. Arner., vA, p447·512.

Saskatchewan Geological Survey 153