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Sudbury breccia is considered to have acted as a structural weakness in the footwall into which evolved ISS sulfide melts were emplaced. This was followed by a partial redistribution of metals into the breccia by fluids expelled from the cooling sulfide melt and subsequent hydrothermal remobilization of fluids by regional meteoric fluid systems in the footwall

(Abel et al., 1979; Coats and Snajdr, 1984; McCormick and Mcdonald, 1999; Hanley and Mungall, 2003; Hanley et al., 2005; White, 2012). Several studies on Sudbury breccia immediately adjacent to footwall Cu-Ni-PGE sulfide veins have noted its relative enrichment in metals compared with the adjacent footwall lithologies, particularly for Cu, Au, Pt and Pd (Morrison et al., 1994; Ames and Farrow, 2007).

Anomalous metal concentrations further away from the ore zones are difficult to reconcile with expulsion or remobilization of metals from sulfide melts. For example, Sudbury breccia near the Manchester Offset Dike has relatively elevated concentrations of up to 80 ppm Ni and 32 ppm Cu, despite the lack of a significant known mineral zone in the area. The geochemical modelling presented in the previous section implies that the trace metal content in Sudbury breccia could be partially derived from local country rocks. The robust principal component analysis of Sudbury breccia samples from the Creighton embayment and McCreedy 153 localities reveals a correlation between Mgo, MnO, FeO, Ni, Cr ± CaO versus SiO2 ± Al2O3, K2O that likely reflects the relationship between breccia matrix derived from

A B

C

Figure 2.9: Robust PCA PC1-PC2 correlation graphs of major elements and trace metals for A: Old Creighton Town and South Pump Lake Zone, B Creighton Deep and C: McCreedy 153 East. The co- ordinates sit within a circle of correlation. Loadings closer to the center of the circle (y, x = 0) indicate that not all the variation in that component is accounted for in PC1 or PC2. Loadings with similar co- ordinates indicate a positive correlation for those elements (e.g. MnO, MgO and Ni). See Abdi and Williams (2010) for details on PCA correlation plots.

more siliceous rocks versus more mafic rocks (Fig. 2.9A, B, C). Thus there appears to be a particularly strong association between Ni, Cr and the assimilation of mafic lithologies in the footwall. In the case of the Manchester sample site, Sudbury gabbro, a ubiquitous lithology in the South Range, provides the most suitable local metal source, in an area largely made up of quartz-feldspathic metasedimentary rocks. Studies by Al Barazi et al. (2009) on Sudbury breccia in the South West of the Sudbury basin, adjacent to the Worthington Offset Dike, also concluded that, aside from an impact melt component, Nippising (Sudbury) gabbro provided the best source for metal enrichments in the breccia. A degree of host lithology control on the breccia metal content is also demonstrated at Creighton Deep, where a Sudbury breccia sample with a Cu content of 97 ppm, (the average Cu concentration from this locality was 22 ppm), was found to include lithic fragments of altered metabasalt hosting discrete, very fine grained disseminated chalcopyrite and bornite. The general absence of mafic clasts in the breccia matrix can be explained by the preferential assimilation of mafic units, owing to the lower shear yield strength or thermal conductivity of minerals commonly associated with such rocks (e.g. biotite and amphibole) (Spray, 1992; Spray, 2010). In light of this, it would be expected that mafic footwall lithologies would have a significant influence on the geochemistry and trace metal content of Sudbury breccia, even in localities where it is a relatively minor component.

Compared with Ni and Cr, Cu in more distal breccia samples does not show a strong correlation with other elements (Fig. 2.9A). However, within mineralized zones at Creighton Deep and McCreedy 153 East, there is a second order principal component correlation (PC2) between Cu, Al2O3 and Na2O, inferring that Cu is controlled by a different process than that of the other trace metals (Fig. 2.9B, C). One explanation could be the occurrence of fine- grained disseminations of Cu-sulfide inclusions within sheet- and sorosilicates, such as chlorite, epidote and illite, which has previously been reported in aureoles surrounding ultramafic-mafic Ni-Cu-PGE intrusions and Cu-Au porphyry systems (Ahn et al., 1997; Suarez et al., 2009). It is possible that there is a similar association at Sudbury, resulting from hydrothermal interaction with the ore zones following their emplacement. For example, in Sudbury breccia from the McCreedy East 153 zone, post-mineralization, epidote-chlorite veinlets and biotite alteration patches are observed hosting subhedral, disseminated pyrite>>chalcopyrite grains. These post-impact epidote veinlets are associated with orthomagmatic- and basement-derived fluids and have been attributed to the localised remobilization of metals out of footwall ore zones (Molnar et al., 2001; Campos-Alvarez et

al., 2010; Tuba et al., 2014) and may explain the elevated Cu concentrations in felsic gneisses adjacent to sulfide veins in the 153 ore zone (up to 826 ppm Ni and 1240 ppm Cu), compared to distal host rock (8–50 ppm Cu). Combining the PCA results with the mixing model results indicates that the metal content of the breccia appears to be a continuum between assimilation of Ni-Cr bearing mafic units from the footwall and an aureole of silicate hosted, Cu inclusions that has developed near to the mineral zones, possibly reflecting hydrothermal remobilization.