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

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A Field, Petrographic, and Geochemical Study of Gabbros and Related Rocks from the Sandy Islands Gabbro Complex, Wollaston Domain 1 A Field, Petrographic, and Geochemical Study of Gabbros and Related Rocks from the Sandy Islands Gabbro 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 granites (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 magnetite 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 granodiorites (to quartz (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 feldspar, plagioclase, 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 granodiorite, 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, pyroxene, 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 tourmaline. Hornblende grains partly re­ place biotite flakes, whereas titanite grains form a reac­ tion rim around magnetite
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