Comparisions between Creighton and Coleman Mine

Although some post-impact hydrothermal activity, such as the early, high temperature magmatic fluids, were regional (Marshall et al., 1999; Molnar et al., 2001), the variations between the mineral assemblages observed in Sudbury breccia at Creighton and Coleman Mine reflect the influence of more localized hydrothermal and metamorphic fluids. There are compositional variations between the tonalitic gneiss footwall lithologies at Coleman mine versus the mixture of granite and mafic metavolcanics at Creighton Mine, which may inhibit or encourage particular mineral assemblages to form.

At Creighton Mine, the spatial transition from ferro-actinolite → ferro-hornblende → ferro-tschermakite is indicative of increasing pressure conditions associated with shear zones that host remobilized footwall sulfide mineralization. Titanites at Creighton mine post-date the amphibole + biotite assemblage and do not appear to have modified the sulfides, though their geochemistry does indicate a temperature gradient towards shear zones, affirmed by their syn-orogenic Mazatzalian U-Pb ages. In contrast, chlorite is notably absent at Creighton Mine, which could be the result of prolonged higher temperatures and/or pressures in the South Range owing to the greater depth of the South Range prior to tectonic uplift and the closer proximity to the center of post-impact orogenic activity. This is also supported by the absence of mineral assemblages diagnostic of the SIC thermal aureole in the South Range, compared to the North Range, where the hornfels halo (e.g., pyroxene-hornblende, epidote- hornblende zones) has been preserved (Farrow and Watkinson, 1992; Prevec and Cawthorn, 2002; Hanley and Mungall, 2003). Amphiboles are relatively uncommon in the McCreedy 153 East deposit, despite being widely reported elsewhere in the North Range; this is possibly because of the more extensive chloritization observed in samples from this study (McCormick and Mcdonald, 1999; Hanley and Bray, 2009).

Titanite is ubiquitous at both Creighton and Coleman mine, however the estimated temperature of the fluids associated with the titanite formation (<350 °C) is too low for Zr to be used as a geothermometer, which is only effective in temperatures exceeding 600°C (Hayden et al., 2008). However, co-existing chlorite geothermometry is consistent with a low temperature (150 – 250 °C), metamorphic-hydrothermal fluid identified by several studies in the Coleman-Levack embayment (Molnar et al., 2001; Pentek et al., 2008; Tuba et al., 2014). Although some chlorites are clearly pseudomorphs of precursor biotite, the nickeliferous

species present adjacent to sulfide veins at Coleman Mine represent a more intense, albeit localized, alteration associated with brines remobilized along the same NW-SE structures as occupied by the sulfides. Based on the titanite age dates, the alteration appears to be a far- field effect of the 1.45 Ga Chieflakian orogeny. Fluids may have also been introduced along pre-existing structures such as the Pumphouse Creek Deformation Zone that transects the North Range, though the age of the structure is poorly constrained (Card 1994). A comparable Chieflakian chlorite-titanite assemblage was not observed at Creighton mine, but Szentpeteri’s (2009) study on the nearby Worthington offset dike (~20 km’s South West of Creighton) concluded that sulfides and precious metals were remobilized by a similar combination of saline shield brines and CO2-rich metamorphic fluids, which were concentrated along contemporaneous Chieflakian deformation zones. Combined with the Mazatzalian titianite from Creighton, this serves to confirm that the South (and possibly North) Range shear zones acted as conduits for multiple metamorphic-hydrothermal fluids, some of which are associated with modification of pre-existing sulfide zones.

In addition to the Ni contents of chlorite and amphibole, variations in Tl/Rb content in biotite also appears to be a potential vectoring tool towards mineralized zones at both localities. Substitution of Tl and Rb for K in biotite can be associated with primary magmatic fluids or lower temperature, epithermal fluids. Warren et al. (2015) also noted that Ni/Cr can be used to discriminate between biotites associated with mineralizing fluids and post-ore metamorphic fluids. Combining Tl/Rb vs Ni/Cr in biotite demonstrates a positive relationship at both Coleman and Creighton mine, though the signature is diluted at the latter site, possibly due to the greater mobility of Tl at lower temperatures, combined with the more prolonged, post-impact metamorphic activity experienced in the South Range. Although Tl is usually below detection limits in whole rock geochemical results, it has been noted that Rb and Ce decrease significantly in bulk geochemical results from mineralized zones (e.g., from 203 to 59 ppm Rb) (Fig. 3.6D). As Tl, Rb and Ce are all primarily hosted in biotites; this relationship could reflect Tl enrichment in biotites resulting in expulsion of Rb and Ce. Thus, Tl/Rb vs Ni/Cr may serve as a useful whole-rock or mineral-chemistry vectoring tool associated with magmatic-hydrothermal fluids that interacted with mineralized zones in the North and South Range, and complements the recent Ni-Cr-Cu biotite vectoring tool identified by Warren et al. (2015) at the Worthington offset dike. It is, however, important to note that at both localities the trace metal values within the hydrosilicate assemblages falls exponentially with distance from the sulfide occurrences indicating that, although

hydrothermal modification of the sulfides has taken place, the anomalous trace metal halo surrounding the ore zones may be spatially limited (possibly to within tens of meters). This may explain why there is an absence of a large, detectable sulfide or trace metal halo surrounding the massive contact-style deposits, as noted by Lightfoot (2007).