Summary:
River Valley is a contact-type PGE mineralization deposit (Peck et al., 2001; Kaykowski et al., 2018). Mineral exploration at River Valley has traditionally focused on the contacttype PGE mineralization in the known zones. However, presence of several highly-anomalous assays of surface grab samples of outcrops within higher portions of the River Valley Intrusion stratigraphy suggests that opportunities may exist for notable reef or stratiform-type PGE mineralization, or narrow, high-grade breccia zones internally within the Intrusion
Mineralization
Disseminated, bleb, fragment, interstitial and veinlet chalcopyrite-pyrrhotite-pentlandite mineralization (<1% to 5% sulphide minerals, up to 10 g/t Pt+Pd) occurs in autolith-rich gabbronorite breccia within 5 to 50 m of the basal contact of the River Valley Intrusion. The mineralization is spatially associated with gabbro-norite cumulates and autoliths that are elsewhere poorly represented in the igneous stratigraphy of the River Valley Intrusion.
Sulphide Minerals and Assemblages
The sulphide mineralogy is predominantly pyrrhotite, chalcopyrite and pentlandite, with minor amounts of pyrite and millerite and rare bornite, cubanite, mackinawite and arsenopyrite. Typical sulphides and sulphide mineral associations are shown in Figures 7.16 and 7.17. Large bodies of sulphide are variably altered and replaced by fine-grained secondary amphibole, minor quartz and biotite, which gives many of the bodies irregular contacts or resulted in larger sulphides being reduced in size to small patches of disseminated grains within the alteration silicates. Locally, pyrrhotite altered to pyrite and pentlandite to millerite. Chalcopyrite is also present and may have recrystallized textures. Similar sulphide assemblages, alteration and recrystallization textures have previously been reported associated with PGM mineralization at the Lac des Iles Complex, northwestern Ontario (Djon and Barnes, 2012) and in the Bushveld Complex, South Africa (Smith et al., 2011).
Platinum Group Minerals
Holwell et al. (2014) identified a total of 172 PGM in ten polished thin-sections from mineralized (>1 g/t Pd) samples of mafic inclusions, amphibolite inclusions, and gabbroic matrix from River Valley. Each individual grain was classified by compositional type and textural association (Table 7.1). The vast majority of minerals were Pd and Pt phases; with a few grains of platarsite, hollingworthite and laurite (the latter two phases being the sole carriers of Rh and Ru). Minerals containing Os or Ir were not identified. The PGM identified were grouped as: (1) Pd, Pt tellurides; (2) Pd antimoarsenides; (3) Pt arsenides; (4) Pd arsenides; (5) Pd stannides; (6) PGE sulpharsenides; (7) Pt, Pd sulphides; and (8) Au- and Ag-bearing minerals. Each occurrence was also classified by its textural association: that is, enclosed in other PGM or enclosed in silicates, at silicate-silicate boundaries, enclosed in base metal sulphides, at sulphide-silicate boundaries, or sulphide-sulphide boundaries.
PGM Grain Size and Relative Area Analysis According to Holwell et al. (2014), the grain sizes of the PGM ranged from < 1 µm to 41 µm, with most < 10 µm in their longest dimension. The long and short axes of each mineral grain were measured in micrometres. Relative proportions of the various mineral phases and mineral species type (Table 7.2) are based on an estimation of area by using the long and short axes of each grain to approximate each to the area of an ellipse, as described by Holwell et al. (2006). This procedure produces data which accurately reflect the relative proportions of each mineral type within an assemblage and is preferable to proportions of mineral type by number of grains, which can be biased by a relatively large amount of very small grains.
PGM Assemblages
Overall, the assemblage is fairly restricted with only six minerals being identified more than five times. The vast majority are Pd phases, with fewer Pt minerals, and rare other phases. Pd-Pt tellurides make up 60% by area of all the minerals identified, with kotulskite (PdTe), making up 58% by area of all the minerals identified, with a single grain of moncheite (PtTe2) accounting for the other 2%. Sperrylite (PtAs2) is common and Pd arsenides, Pd antimoarsenides, and Pt-Pd sulphides also make up a small, however, significant proportion of the overall assemblage. The sulpharsenides hollingworthite (RhAsS) - platarsite (PtAsS) - ruarsite (RuAsS) series carry all of the Rh and most of the Ru observed, however, are relatively minor. Electrum is very rare with only four grains identified. Texturally, most PGM identified occur as single-phase grains and although the majority of grains were associated with silicates. Note, however, that all mineral grains identified were located less than a mm away from sulphides. In other words, PGM grains were not found in areas free of sulphides.
In detail, the PGM assemblages vary with host rock type. The different mineral assemblages are not preferentially associated with the matrix or a particular inclusion type, however, rather with the type of alteration the rocks underwent. The hydrothermally altered rocks host Pd antimoarsenides and Pd arsenides; however, these minerals were not observed in the metamorphosed rocks. There is almost twice the amount of Pd-Pt tellurides in the metamorphic textured rocks than in their hydrothermally altered equivalents.
PGM in Mafic Inclusions The mafic inclusions can be divided into those that show hydrothermal alteration and those that show recrystallized metamorphic textures (Holwell et al., 2014). Both show a very restricted assemblage made-up of tellurides, arsenides, and antimoarsenides (Figure 7.18A-C). In the hydrothermally altered samples, the majority of the PGM are Pd antimoarsenides (Figure 7.18C) with only 28% Pd-Pt tellurides, whereas in the metamorphosed inclusions, 64% are Pd-Pt tellurides and there are no Pd antimoarsenides.
PGM in Amphibolite Inclusions
The PGM assemblages in the amphibolite inclusions are similar to those in the metamorphosed mafic inclusions. They are dominated by Pd-Pt tellurides, with minor PGE sulpharsenides, and a single grain of the Pd stannide (atokite; Pd3Sn) and Pd antimoarsenides. The kotulskite grains in amphibolite inclusions commonly form laths and are found as inclusions in silicates and sulphides. Although the PGM in the amphibolite inclusions have the most varied number of associations out of all rock types, the minerals commonly have a close proximity to sulphides.
PGM in the Matrix
The rock matrix can also be subdivided into samples that have been altered hydrothermally and those that have undergone metamorphic recrystallization. The difference between the metamorphic and hydrothermal assemblages and associations of matrix rocks is almost identical to those observed for the mafic inclusions. The most abundant PGM type is Pd-Pt tellurides (Figures 7.18G-I) with almost double the amount in metamorphosed than the hydrothermal matrix rocks. There are no Pd antimoarsenides in the metamorphosed matrix and only 0.03% Pd arsenides, whereas these two mineral types combined make up 28% of the hydrothermally altered matrix rocks.