Trace element signature of hydrothermal alteration assemblages (epidote, allanite, actinolite, titanite) in the footwall of the Sudbury Igneous Complex: A laser ablation ICP-MS trace element vectoring and fertility study

In one of the world's largest Ni-Co mineral districts, geophysical methods have been successfully employed for detecting the traditional Sudbury contact and offset types of Ni-Cu-PGE ore for over a century. Discoveries of low-sulphide, high-PGE tenor orebodies, defined in 2005 (i.e. McCreedy West PM zone), in the footwall environment to the Sudbury Igneous Complex (SIC), caused explorationists to focus on the detection of these precious metal-rich resources that are hosted in randomly distributed impact-derived breccia entirely within Archean and Proterozoic country rocks. These zones of Sudbury breccia host two styles of footwall-type deposits: a) high-sulphide vein-style Cu-PGE ores (i.e. Strathcona, McCreedy East 153 chalcopyrite veins), widely recognized with a magmatic-hydrothermal origin and b) low-sulphide disseminations, blebs, and stringers with high platinum group element (PGE) tenor and dominated by silicate assemblages of hydrothermal origin. Due to the absence of chalcopyrite or sulphide minerals, this second footwall ore-style is a challenge to detect in the field using the geophysical methods traditionally used for magmatic deposits and thus, identifying key hydrothermal alteration assemblages and developing discriminant mineral chemistry diagrams could yield criteria for these hydrothermal footwall ores. Proper classification is important for guiding future exploration for the low-sulphide metal-rich deposits in the footwall to the Sudbury Igneous Complex. The aim of the study was to establish typical element-associations and behaviours for the paragenetically different hydrothermal assemblages produced during the diverse post-impact magmatic-hydrothermal history of the footwall and hanging-wall units along the North and East ranges of the Sudbury structure to potentially detect a unique signature for alteration related to the high-tenor PGE mineralization. This TGI4 Ni-PGE Project data release contains geochemical databases of (a) laser ablation ICP-MS mineral chemistry of epidote, allanite, amphibole, and titanite, from 62 samples and (b) whole-rock geochemistry of least-altered host-rock types. Epidote and amphibole in the alteration assemblages exhibit hostrock- normalized REE plots with characteristic topologies that are best described with nLREE to nHREE relations, and though groups of alteration types are REE-enriched, other groups are REE-poor including the PGE- mineralized alteration assemblage. Thus REE contents and patterns are not discriminants for detection of low-sulphide, high-PGE mineralization. Trace element partitioning occurs between texturally coeval minerals, such as epidote and amphibole (REE, Pb, Bi, Sn: shows affinity to epidote; Co, Ni, Zn: shows affinity to amphibole), as well as titanite and amphibole (REE, Sn, Zr, Nb, Yb, Th, U: affinity to titanite; Co, Ni, Zn: affinity to amphibole). This scavenging phenomenon greatly affects the element distribution of the mineral pairs; therefore, conclusions drawn on the trace element concentrations of a single mineral should be avoided. The trace element concentration of epidote/amphibole may be influenced by (1) the parental fluid composition (e.g. REE, U, Th, Ni, Pb, Sn), (2) the host rock, particularly mafic rocks (e.g. As, Zn) and/or (3) the crystal structural properties of the minerals (e.g. Mg and Sr in epidote). The pathfinder elements Ni, Pb, Sn, and Co in epidote and amphibole are the most reliable elements to fingerprint distinctly the PGE mineralizing alteration in the footwall. Both epidote and amphibole of mineralized sulphide-silicate and generally barren extensional assemblages show systematic differences in the concentrations of key pathfinders among different locations along the North and East ranges.