| Abstract | 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. |
| Summary | (Plain Language Summary, not published)
The Targeted Geoscience Initiative (TGI-4) is a collaborative federal geoscience program that provides industry with the next generation of geoscience
knowledge and innovative techniques to better detect buried mineral deposits, thereby reducing some of the risks of exploration. This is a report including full mineral chemistry databases for an activity in the Canadian Ni-Cu-PGE-Cr Ore Systems
Project of the TGI program. The objectives of this study focused on, investigating whether the trace element geochemistry of common hydrothermal alteration minerals (epidote, allanite, amphibole, titanite) found in Cu-PGE mineralized zones as well as
other environments can be fingerprinted. If so, the mineral chemistry acts as a proxy for fertility and possible vectoring for mineral exploration in challenging environments. |