The Sudbury breccia matrix from the North and South Range typically has a similar macroscopic appearance, consisting primarily of a dark grey, microcrystalline, aphanitic matrix with entrained millimeter- to meter-sized clasts. Cataclastic varieties, with flow banded textures are also locally reported in the South Range and most distal outcrops of Sudbury breccia (Rousell et al., 2003), but they are not an important variant in the footwall and are not included in this study. Clasts are derived from the immediately adjacent footwall, and they can be subdivided into lithic fragments and smaller, individual mineral grains. Lithic fragments in the studied sample suite are consistently rounded to sub-rounded, an observation that is interpreted to be indicative of thermomechanical erosion in a high temperature melt, rather than lower temperature cataclasis, which should produce more angular clasts (Lin, 1999; O’Callaghan et al., in press). Mineral grains are sub-rounded to angular and, as with the larger lithic fragments, tend to be felsic in composition (i.e. predominantly quartz and feldspars). Mafic clasts and grains are notably absent, even where the breccia veins cut through diabases, or diorite gneisses. This is attributed to the preferential comminution of minerals commonly associated with mafic phases (e.g., pyroxenes and micas), which have lower fracture toughness and thermal conductivity (Spray, 2010). Closer to the SIC, the breccia matrix tends to exhibit recrystallization as a result of thermal metamorphism. This is primarily manifested as a coarsening in grain size to a sub-igneous texture (Fig. 3.3A, B, C, D) and, in some cases, alteration to a green-grey color. Aside from the increased presence of discrete disseminated sulfides, there is no definitive macroscopic features within the thermal aureole of the SIC to differentiate between breccia proximal and distal to mineralization.
3.6.1 Samples adjacent to the McCreedy East 153 Ore Deposit
Microscopically, the Sudbury breccia matrix distal to the SIC in the North Range is very fine-grained to vitric, with no discernible crystals and it commonly displays flow banding or compositional zonation at the vein margins (Fig. 3.3C). We use the term “vitric” here to describe particles that resemble or may have formed from a glass. However, no glass clasts were observed in the Sudbury breccia, as one would expect completel devitrification by subsequent hydrothermal and metamorphic activity, producing fine grained quartz, feldspars and hydrosilicates such as chlorite. Thin (<20 cm wide) chlorite selvages off-shooting from Sudbury breccia zones, which may represent devitrified melt, were observed both in this study and by Randall (2004). Contorted and partially melted quartz and feldspar within the matrix at the margins of Sudbury breccia zones indicate that temperatures
500µm 500µm A D 500µm C Qtz + Fldspr Ttn Ttn Qtz + Fldspr Bio All F E 500µm B
Figure 3.4: PPL thin section images of Sudbury breccia matrix from Creighton Deep and Coleman Mine, distal (A, C) and proximal (B, D) to mineralisation, displaying the progression from ‘cold’ to ‘hot’ breccia. Backscatter SEM images of two forms of titanites observed at each mine locality. At Coleman Mine, titanite is present as a reaction rim on ilmenite-magnetite grains (E) whereas at Creighton Mine the titanite is primarily an anhedral, subtly zoned overprint on pre- existing biotite (F). Abbreviations: quartz - Qtz; feldspar – Fldspr; amphibole – Amph; titanite – Ttn; ilmenite – Ilm; magnetite – Mag; allanite – All; chalcopyrite – Cpy.
exceeding 1000°C (Spray, 2010; O'Callaghan et al. in press). With decreasing distance from the SIC, the matrix shows greater degrees of recrystallization and alteration, with the growth of biotite, epidote, and amphiboles in a groundmass primarily comprised quartz, orthoclase and plagioclase (Fig. 3.3D). The selvages of intensely altered, dark green-black Sudbury breccia are immediately adjacent (< 0.5 meters) to the sharp-walled sulfide veins.
Biotite is present primarily as platy, subhedral, light brown grains that exhibit increasing degrees of chloritization with proximity to mineralization (Fig. 3.4A, B). Within ~20 m of sulfide veins the primary biotite is almost entirely replaced by chlorite and a secondary, deep- red-brown, anhedral, poikiloblastic biotite is developed. The alteration selvage adjacent to sulfide veins also hosts a primary, radial chlorite variety that is spatially associated with the sulfide veins. The remainder of the altered matrix consists of quartz, sericitized feldspars, calcite, epidote, pyrosmalite [(Fe,Mn)8Si6O15(OH,Cl)10] and rare pyroxene. Amphibole was primarily observed as very fine, colorless to green, acicular grains. Proximal to mineral zones, in addition to acicular varieties, amphibole is also observed as a continuum between partial replacement rims to complete overprinting of pre-existing pyroxenes by green, anhedral to subhedral clusters of actinolite-tremolite (Fig. 3.4C, 3.4F). Within the thin, more intense alteration selvage, amphibole appears to have been entirely replaced by epidote and a third chlorite species that forms ragged, anhedral clots that host titanite exsolution lamellae (Fig. 3.3B). Fe-oxides (ilmenite-magnetite) are generally very fine-grained clusters that increase in abundance towards mineral zones. Within <100 m of mineralized zones, ilmenite exhibits <5 µm magnetite exsolution lamellae and titanite alteration rims (Fig. 3.3E), an assemblage also noted by Gasparrini and Naldrett (1972) and McCormick et al. (2002a). The almost complete absence of ilmenite in more distal samples has previously been attributed to complete replacement by titanite and/or magnetite. These observations are generally in
agreement with previous studies (Farrow and Watkinson, 1992b; Morrison 1994; McCormick and Mcdonald, 1999; Hanley and Mungall, 2003; Hanley and Bray, 2009).
3.6.2 Samples Adjacent to the Creighton Deep Ore Deposit
Compared with the North Range, Sudbury breccia matrix at Creighton Deep is coarser grained, regardless of proximity to mineralization or the SIC (Fig. 3.3A, B). This may be a result of the different rheological conditions in the South Range or formation at greater depth, prior to post-impact uplift (Rousell et al. 2003). Sudbury breccia matrix in distal samples
100µm 100µm 100µm B A C D Secondary Bio Primary Bio/Chl Qtz + Fldspr + Amph Chl Amph Bio 50µm Amph Qtz + Fldspr Qtz + Fldspr Ttn 100 µm 100 µm Amph Amph Cpy Qtz + Fldspr Bio E F
Figure 3.5: (A) PPL thin section image of chlorite pseudomorphs replacing biotite in breccia matrix distal to mineralization at Coleman mine. Secondary biotite is present as a deep-red-brown, variety that can be poikiloblastic. (B) PPL images of chlorite with titanite lamellae parallel to cleavage planes in alteration selvages adjacent to footwall sulfide veins at Coleman Mine. (C) PPL image of acicular, colourless actinolite distal to mineralized zones. (D) PPL image of zoned amphibole-hornblende proximal to mineralization at Creighton mine, and nickel-bearing, blue-green ferro-tschermakite enclosing remobilized sulfide grains (E). (F) PPL image from Coleman mine of the larger, anhedral amphibole clots observed in samples proximal to mineralization. See figure 3.3 for explanation of abbreviations.
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(<200 m from mineral zones) is composed of quartz, plagioclase, K-feldspar, stilpnomelane, biotite, with rare epidote, allanite and very fine grained, acicular actinolite-tremolite. Biotite becomes more abundant and shifts from being light brown to dark brown to green in colour with proximity to the mineral zone at Creighton Deep, though its platy form and size remain relatively consistent. In contrast, stilpnomelane in the breccia matrix appears to be primarily associated with the presence of entrained Huronian gabbro or metabasalt clasts, in which this mineral is a common component. Amphibole is rare in Sudbury breccia distal to the Creighton Deep mineral zone, primarily being observed as very fine-grained, acicular grains (Fig. 3.4C). Within <200 meters of the mineralized zone a second, coarser-grained (>100
µm), prismatic and zoned amphibole, exhibiting a brown core and green rim is present (Fig. 3.4D). A third, pleochroic green-blue variety of amphibole is preserved as intergrowths with sulfide stringers within mineral zones (Fig. 3.4E). Titanite is common throughout the Creighton area and does not change in appearance or abundance with proximity to sulfide mineralization. Unlike at Coleman Mine, titanite is rarely found as a rim on euhedral magnetite grains, instead it is primarily present as late-stage, subtly zoned, anhedral clusters that overprint biotite and amphibole assemblages (Fig. 3.3F). Ilmenite is absent in the samples from the Creighton embayment and magnetite is only observed as very fine disseminated grains in samples <200 m from mineralization. No pyroxene or chlorite was observed in samples from Creighton Mine.