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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. Annesley 2 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 a series of northeast-trending islands in the west-central part of Wollaston lake near the eastern edge of the Athabasca Basin (Figure 1 ). It comprises an intermediate to mafic southeastern component and a felsic to intermediate northwestern component (shown as an approximate contact on Figure 2, after Chandler, 1978). Preliminary mapping and sampling was undertaken in 1993 for petrological and geochronological purposes as part of an industry-funded geological investigation. Geochemical 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. Early Proterozoic(?) age). The main belt of Wollaston Group metasediments is composed of graphitic pelitic gneiss, metamorphosed iron formation, pelitic gneiss, calc-silicate gneiss, psammopelitic gneiss, psammitic gneiss, metaquartzite, and amphibolites. The Wollaston Group rocks are complexly deformed, polymetamorphosed, and rest unconformably (and tectonically) on reworked, antiformal Archean granitoid gneisses. The Wollaston Group metasediments are intruded by metagabbros, porphyritic granites (1815 Ma), and pegmatites (Madore and Annesley, 1993). The precise age of the 1. Geological Setting The major elements of the Sandy Islands Gabbro Complex were documented by Chandler (1978) and Ray (1978). Chandler (1978) noted that the complex corresponds to an "egg-shaped" aeromagnetic high, which is consistent with the abundance of magnetite in the various 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) for a coronitic, hornblende-phyric monzogabbro in the Karpinka Lake area (Annesley et al., 1993). The Sandy Islands Gabbro Complex is found within the Wollaston Domain, one of the major subdivisions of the Cree Lake Zone (Lewry et al., 1985; Gilboy, 1983). The Wollaston Domain is a northeast-trending, orogenic fold and thrust belt that is fault-bounded (i.e. Needle Falls Shear Zone) to the east with the Peter Lake Domain and the Rottenstone Domain, and to the west borders the eastern margin of the Hearne Province hinterland. It comprises Archean continental crust and continental margin supracrustal rocks (i.e. the Wollaston Group of .. , 06• c:J PhoMrv 1 0o< CO¥« D AfhobOl(:0 GfOI.IP ~ '#ol t,o$tOft Gt 1MJP ..,.,.,,.....- MojonJ'1~ / fa1.1H rone, * ,,... - - 0ommo bOIJ""°'Y ~ o, 111a" CI• Gotibro C.omplu ,o IOO ,,o Figure 1 · Map of the Precambrian Shield of northern Sask· 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 ~ 8 w 1 () j I I I 0 L ,L A s I ' r O· N ! "~ ··-·---·····--- ----·-·i __ ' -. ~ 103''30 ' 20' 25' 15' Figure 2 - Map showing sample locations. Wollaston Group is not known; however, an Aphebian age is assigned to the Wollaston Group on the basis of its apparent unconformable relationship with the underlying Archean granitoid gneisses and its subsequent metamorphism during the Hudsonian Orogeny. Lewry and Sibbald (1977, 1980) and Sibbald (1983, 1985) have documented a broadly defined stratigraphic sequence, comprising four main lithological units for the Wollaston Group. More recently, Annesley and Madore (1991) have introduced a relatively simple stratigraphic sequence for the Wollaston Group, which is similar to Early Proterozoic supracrustal sequences (e.g. the Chantrey Group and the Ramah Group) of the Rae Province (Hoffman, 1988 and 1989). 2. Field Relations The Sandy Islands Gabbro Complex is well exposed and consists of discontinuous rounded outcrops. The best exposures occur along the wave-washed island shorelines. Heavy lichen covers the outcrops farther inland. The gabbros are medium grey to greenish brown mottled black on the fresh surface, and weather dull dark grey to rusty brown to dull medium brown mottled black. They are holocrystalline, essentially equigranular to inequigranular-porphyritic, overall medium grained, hypidiomorphic, ophitic to subophitic, massive, fresh to retrograded, invariably magnetic, and dense (Figures 3 and 4). The gabbros are composed mainly of varying proportions of plagioclase, pyroxene, hornblende and biotite, subordinate K-feldspar, opaque minerals, titanite and apatite, and minor quartz. The gabbro phases ap- Saskatchewan Geological Survey pear to be layered locally, and possibly rhythmic layered but the latter is difficult to discern because of a well-developed vein network. The vein network consists of cross-cutting granitic veins {Figure 5). In places, the gabbros are metasomatized with complete replacement of primary mineralogy and textures (Figure 6). At some localities (A93-026, M94-004, Figure 2), gabbroic phases are intruded by granodiorites (to quartz diorites), which are light to dark flesh pink mottled black on the fresh surface and dull flesh pink to dull light grey mottled flesh pink on the weathered surface. They are holocrystalline, inequigranular-porphyritic, fine to coarse grained, hypidiomorphic-granular, massive to moderately foliated, fresh to altered, magnetic, and relatively dense. The granodiorites are composed of quartz, Kfeldspar, plagioclase, and biotite with subordinate titanite, hornblende, and opaque minerals (magnetite and sulphides). K-feldspar phenocrysts are up to 3.0 cm in length. Locally, biotite flakes are partially altered to chlorite. At locality A93-026, the gabbro phase is more deformed (i.e. in part sheared) than the intrusive granodiorite, which suggests possible emplacement of the granodiorite during shearing. Late pegmatite dykes and veins crosscut the gabbro complex. 3. Preliminary Petrographic Observations Ten samples were collected from the Sandy Islands Gabbro Complex. They represent three low-Ti02 gabbros (M94-001a, -001b, and -007a), four high-Ti02 gabbros (M94-002, -003, -004a1. and -004a2). a quartz 149 Figure 3 - Sandy Islands Gabbro illustrating a fine· to mediumgrained subophitic texture. The rock is composed of plagioc/ase, augite, biotite, hornblende, and titanomagneffte (Location M94-001). Figure 6 - Metasomatized gabbro is holocrystalline, equigranular to inequigranular, very fine grained, massive to weakly foliated, granoblastic in texture, and moderately altered. The mineral assemblage consists of diopside, plagioclase, quartz, hornblende, titanite, and magnetite. The primary mineralogy and texture is completely obliterated by the pervasive metasomatism (Location M94-00S). diorite (M94-004b), and two calc-silicate gneisses (M94004c and -005). Figure 4 - Sandy Islands Gabbro displaying well-preserved subophitic texture, consisting of plagioctase, augite, hornblende, biotite, titanomagnetite, pyrite, and quartz (Location M94·007). Figure 5 - Sandy Islands Gabbro showing a network of narrow veins and fractures with metasomatic reaction rims (Location M94-004). 150 The low-Ti02 gabbros are holocrystalline, fine to medium grained, inequigranular, sub-ophitic, and essentially 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 randomly oriented and are partly enclosed within augite grains (Figure 7) which range from 0.6 to 7 mm in diameter, and have a green to salmon pink pleochroism. Rutile exsolution lamellae occur along the augite cleavage planes. Hornblende partly replaces augite and biotite along their grain boundaries. Biotite flakes, from 0.65 to 4.5 mm in length, form intricate intergrowths with titanomagnetite grains, 0.5 to 4.5 mm in size, that are composed of a magnetite host with ilmenite lamel· lae (Figure 8). The high-Ti02 gabbros are characterized by a holocrystalline, inequigranular, fine- to medium-grained, hypidiomorphic to subophitic texture. They are partly recrystallized, massive, and weakly to moderately altered. The high-Ti02 gabbros differ from the low-Ti02 gabbros by the occurrence of titanite (primary and metamorphic), and the lack of augite. The igneous mineral assemblage is composed mainly of plagioclase, hornblende, biotite, quartz, magnetite, ilmenite, titanite, and pyrite. Accessory minerals include apatite and chalcopyrite. The primary assemblage is partly recrystallized in places and displays a granoblastic texture. The metamorphic minerals consist of hornblende, titanite, scapolite, and tourmaline. Hornblende grains partly replace biotite flakes, whereas titanite grains form a reaction rim around magnetite grains (Figure 9). Plagioclase grains are randomly oriented, range from 0.6 to 4.5 mm in length, and show albite and Carlsbad twinning. Despite partial static recrystallization, the primary grain outline of plagioclase is still well preserved. The grains are Summary of Investigations 1994 weakly altered to saussurite and partly replaced by scapolite, locally. Hornblende grains are both primary and metamorphic, range from 0.35 to 5 mm in size, and show intricate intergrowths with adjacent plagioclase, biotite, and magnetite. The quartz diorite is holocrystalline, equigranular to inequigranular, very fine to fine grained, strongly recrystallized, massive, granoblastic, and weakly to moderately altered. The hypdiomorphic texture of the quartz diorite is well preserved, despite the strong static recrystallization. The primary mineral assemblage comprises plagioclase, quartz, hornblende, biotite, magnetite, and apatite; metamorphic minerals include titanite and epidote. Plagioclase and quartz grains form an equigranular, granoblastic texture, and their grain size ranges from 0.2 to 0.35 mm in diameter. Poikiloblastic hornblende, ranging from 0.35 to 0.75 mm in size, forms in- tricate intergrowths with biotite flakes and magnetite grains. Calc-silfcate gneisses {or metasomatized gabbro) are holocrystalline, equigranular to inequigranular, very fine to fine grained, recrystallized, massive to weakly foliated, and weakly altered. The primary mineralogy and texture of the rock is completely obliterated by the pervasive metasomatism. A granoblastic texture is well developed (Figure 10), and the rocks now consist of plagioclase, quartz, hornblende, diopside, biotite, titanite, magnetite, and pyrite. O .SO .t\M Figure 7 - Drawing from a photomicrograph of gabbro (sample M94·001a) which illustrates the characteristic subophitic tex· ture of the gabbro . The primary mineral assemblage consists of plagioclase (Pl), augite (Aug), hornblende (Hbl), and magnetite (not indicated), with minor quar1z (Qtz). Figure 8 - Drawing from a photomicrograph of gabbro (sample M94-007a) showing primary magnetite (Mag) with ilmenite la· me/lae (/Im). The magnetite grains display intricate intergrowths with adjacent biotite flakes (Bf) and plagioclase (Pl) grains. Saskatche wan Geological Survey Figure 9 - Drawing from a photomicrograph of gabbro (sample M94·002) that is holocrystalline, fine to medium grained, hypdiomorphic, and massive in texture. The igneous mineral assemblage consists of plagioclase (Pl), quartz (Qtz), biotite (Bt), hornblende (Hbl), magnetite (black), and titanite (Ttn). Titanite grains are late-magmatic and fonn a reaction rim around the magnetite grains. Figure 10 - Drawing from a photomicrograph of calc-silicate gneiss (metasomatlzed gabbro, sample M94-004c) illustrating holocrystalline, essentially equigranular, very fine- to finegrained, granoblastic texture. The mineral assemblage is composed of plagioclase (Pl), quartz (Qtz), hornblende (Hbl), diopside (Di), magnetite (black), and titanite (Ttn). 151 4. Geochemistry Table 1 presents the major and trace element geochemistry for nine of the gabbros and related rock samples from the eastern part of the Sandy Islands Gabbro. The gabbros (M94-001a, -001b, -002, ·003, -004a1, ·004a2, and -007a) show a relatively large degree of compositional variability. For example, T i0 2 varies from 1.01 to 3.10 wt percent and MgO from 3.39 to 5.26 wt percent. Minimum and m aximum values for the other oxides also differ to the same degree. In general, the S andy Is· lands gabbros show a broad continuum from compositions 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), and P20s. Similarly, there exists a positive correlation between Al20a with most of the compatible elements (e.g. Cr). The above mentioned and other geochemical charac· teristics of the Sandy Islands gabbros, combined with petrographic observations, indicate that the gabbros can be divided into geochemical g roups on the basis of Ti02, Al203, Fe203, and P20 s contents. This approach has been used by Bellini et al. ( 1986), Fodor ( 1987) , and others, in distinguishing two types of basalts (a lowTi02-P20s type and a high-Ti02-P20s type) in the Parami flood basa lt province of Brazil. The low-Ti02 group of Sandy Islands gabbros represents the most primitive magma compositions, as shown by its high MgO, Ni, and Cr values, and its low values of alkali, incompatible, partially compatible, and rare earth elements. 5. Economic Potential Some of the Sandy Islands gabbros contain elevated, ano malous values of Au, as high as 1110 ppb (Table 1). They are generally depleted in Pt and Pd (1 O ppb o r less, not presented in Table 1) and are a lso depleted in Cu, Co, Cr, and Ni. These res ults are consistent with the calc-alkaline nature of the gabbros. Table 1 • Geochemical data of gabbro and related rocks from the Sandy Islands Gabbro Complex. Major oxides in trace elements in ppm, and Au in ppb. Sample M94·001a M94-001b G G M94-002 G M94·003 M94·004a 1 M94-004a2 M94·004b G G OD G wt percent, M94·005 cs M94-007a G P20s LOI Total 56.20 1.10 16.47 8.42 0.12 4.93 6.34 3.46 1.40 044 0.70 99.58 56.10 1.01 17.06 8.71 0.12 5.26 6.69 3.28 1.50 0.49 0.70 100.92 52.30 3.10 15.24 11 .65 0.13 3.49 6.45 2.30 2.50 1.24 1.30 99.70 52.30 2.16 14.97 13.06 0.14 3.39 6.94 3.20 2.20 1.48 1.00 100.84 49.20 3.07 12.19 15.48 0.18 4.53 8. 13 2.46 1.30 1.49 1.10 99. 13 52.30 2.80 14.62 12.09 0.1 5 4.10 7.14 2.77 1.80 1.10 1.30 100.17 65.20 0.70 14.59 6.1 4 0.06 2.97 3.21 2.29 4.20 0.19 1.30 100.85 67.80 0.43 12.71 2.97 0.04 3.49 6.04 5.36 1.00 0.14 1.00 100.98 56.40 1.38 15.82 9.07 0.1 4 5.23 6.57 3.26 1.70 0.82 0.50 100.89 C(%) s (%) 0.09 0.09 0.05 0.06 0.10 0.13 0.07 0.17 0.17 0.36 0.1 5 0.19 0.06 0.02 0.07 0.01 0.09 0.11 SiO:i Ti02 Al20:i Fe~3 (total) MnO MgO CaO Na20 K20 Ba (ppm) Be Co Cr Cu Ga La Mo Nb Ni Pb Rb Sc Sr Th u v y Zn Zr Au (ppb) 850 1.0 30 130 17 24 34 5 13 82 37 35 13 852 3 3.1 116 18 109 160 14 786 1.0 31 130 18 22 31 5 9 90 3 31 14 870 3 3.5 123 17 114 100 6 603 1.8 32 10 36 25 60 5 35 31 23 103 14 379 11 5.9 280 38 85 190 111 0 990 1.4 30 10 17 28 80 5 22 9 3 56 17 729 4 5.6 134 31 156 210 12 687 1.4 47 40 75 20 53 7 23 54 2 49 23 622 3 3.5 359 26 154 120 36 1141 2.0 35 50 37 21 52 5 25 48 2 69 20 727 3 5.9 246 25 121 220 40 1239 1.6 12 130 7 22 32 5 10 40 2 166 13 104 5 3.6 101 23 37 150 140 211 1.6 8 100 1 13 31 8 11 24 2 28 8 47 25 2.9 72 32 16 120 24 872 0.8 33 120 19 24 42 5 13 88 2 40 15 883 5 3.4 148 24 117 140 50 Note: G=gabbro, OD=quartz diorite, and CS=calc-silicate. 152 Summary of Investigations 1994 6. References Annesley, I.R. and.Madore. C. (1991): The Wollaston Group and its underlying Archean basement: Final Report; Sask. Resear. Coun., Publ. R-1230+C-91, 140p, (confidential). Annesley, I.R., Madore, C., and Krogh, T.E. (1993): U·Pb zircon, titanite, and monazite geochronology from the Wollaston-Mudjatik Domain Boundary, Karpinka Lake area, northern Saskatchewan; Geol. Assoc. Can./Miner. Assoc. Can., Jt. Annu. Meet., May 1993, Waterloo, Prog. Abstr. Bellini, G., Comin-Chiaramonti, P., Marques, L.S., Melfi, A.J., Nardy, A.J.R., Papatrechas, C., Piccirillo, E.M., Roisenberg, A., and Stolfa, D. (1986): Petrogenetic aspects of acid and basaltic lavas from the Parana Plateau (Brazil): Geological, mineralogical, and petrochemical relationships; J. Petrol., v27, p915-944. Chandler, F.W. (1978): Geology of part of the Wollaston Lake Fold Belt, north Wollaston Lake, Saskatchewan; Geol. Surv. Can., Bull. 277, 25p. Fodor, R.V. (1987): Low· and high-Ti02 flood basalts of southern Brazil: Origin from picritic parentage and a common mantle source; Earth Planet. Sci. Lett., v84, p423-430. Gilboy, C.F. (1983): Sub-Athabasca basement geology project; in Summary of Investigations 1983, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 83-4, p28-31. Hoffman, P.F. (1988): United plates of America, the birth of a craton: Early Proterozoic assembly and growth of Laurentia; Ann. Rev. Earth Planet. Sci., v16, p543·603. _ _ _ _ (1989): Precambrian geology and tectonic history 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 Lewry, J. and Sibbald, _T.1.1. (19n): Variation in lithology and tectonometamorph1c relationships in the Precambrian basement of northern Saskatchewan; Can. J. Earth Sci. v14 p1453·1467. ' ' _ _ _ _ (1980): Therrnotectonic evolution of the Churchill Province in northern Saskatchewan; Tectonophysics v68 p45-82. ' ' Lewry,~-~-· ~ibbald, T.1.1., and Schledewitz, D.C.P. (1985): Varrat1on 1n character of Archean rocks in the western Churchill Province and its significance; in Ayres, L.D., Thurston, P.C., Card, K.D., and Weber, W. (eds.), Evolution of Archean Supracrustal Sequences, Geol. Assoc. Can., Spec. Pap. 28, p239·261. Madore, C. and Annesley, I.A. (1993): Metamorphic pressuretemperature conditions of the basal Aphebian Wollaston Group, Hearne Province, northern Saskatchewan; Geol. Assoc. CanJMiner. Assoc. Can., Jt. Annu. Meet.. May 1993, Water1oo, Prog. Abstr., pA-65. Ray, G.E. (1978): Reconnaissance geology: Wollaston Lake (west) area (part of NTS area 64L); in Summary of lnvesti· gations 1978, Saskatchewan Geological Survey, Sask. Dep. Miner. Resour., Misc. Rep. 78-10, p25-34. Sibbald, T.J.I. (1983): Geology of the crystalline basement, NEA/IAEA Athabasca test area; in Cameron, E.M. (ed.), Uranium Exploration in Athabasca Basin, Geol. Surv. Can., Pap. 82·11, p1-14. _ _ _ _ (1985): Geology and genesis of the Athabasca Basin uranium deposits; in Summary of Investigations 1985, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 85·4, p133-156. 153