Characterization of High-PGE Low-Sulphur Mineralization at the Marathon PGE-Cu Deposit, Ontario

Geological Setting

The Marathon PGM-Cu deposit is located on the north shore of Lake Superior within the Coldwell alkaline complex (Fig 1). The complex, which dominantly comprises gabbro and syenite, was emplaced at 1108 Ma as part of the Mid-Continent Rift System (Dahl et al. 2001).

The Marathon deposit has a measured resource of 94.3 million tonnes with an average grade of 1.1 g/t TPGM and 0.26% Cu. The largest zone of mineralization is the Main Zone containing disseminated sulphide mineralization. Within the deposit a high-grade PGE horizon, termed the W-horizon, has been recognized, and which is characterized by a low sulphur content, low Cu/Pd and high Cu/Ni ratios. The W-Horizon is hosted within the Two Duck Lake gabbro (TDLG), above the Main Zone of the Marathon deposit.

The TDLG has intruded into the Eastern gabbro, which is a fine-medium grained olivine, plagioclase and clinopyroxene cumulate. TDLG and Eastern gabbro contacts are often highly irregular and contain numerous irregular dikelets branching into the Eastern gabbro.

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Microprobe and LA-ICP-MS Analysis

Detailed Microprobe and LA-ICP-MS study of olivine and clinopyroxene was conducted through the mineralized zones in the sample suit in an attempt to identify magma pulses within the section (Figures 11 and 12). The major element olivine data does not show a high degree of variation through the section. In Horizon-1 there is a increase in Mg, Ni, and decrease in Mn, Fe and Ca.

From the LA-ICP-MS data the mineralized horizons have a distinct trace element signature. Horizon-1 is characterized by an increase in P, Cr, Ni and Cu, and a decrease in Ca and Cr. The results in Horizon-2 are less distinct due to the variability between samples, but show the same trends overall. From both the Microprobe and LA-ICP-MS data it was shown that the rims of olivine grains are depleted in Ca relative to the core through the sample suite.

Figure 8 Trace element whole rock geochemistry through M-07-368 Figure 9 Calculated ratios for M-07-368

Figure 10 Ratio plots for trace element geochemistry

Figure 11 Olivine major elements Figure 12 Olivine trace elements

Purpose of Study

The purpose of the study is to determine the process that concentrated PGE's in the W-Horizon. Three possible scenarios are being considered, individually or in combination: i) the horizon represents a magma conduit in which an immiscible sulphide liquid interacted with basaltic magma and extracted PGE from the silicate liquid in a dynamic system; ii) the horizon was in contact with a basaltic conduit, but the liquid flowing through the conduit was sulphur-under saturated such that sulphide dissolution resulted in very high PGE grades; or iii) the concentration of PGE is related to chromatographic separation by aqueous fluids during the compaction and late-stage crystallization of the igneous pile.

For this study, drill hole M-07-368 (Fig 3) was selected because it intersects the high-grade low-S mineralization (W horizon) and is not underlain by Main-Zone sulphides. A total of 74 samples were taken between depths of 105.3 and 184.1 m within a continuous unit of TDLG, with continuous samples taken between 129.0 and 147.0 m.

Two distinct mineralized horizons were delineated. The dominant sulphide in both horizons is chalcopyrite, Horizon-1 (129-131m) contains appreciable bornite and grades of 10 ppm Pt+Pd, 1000 ppm Cu and 1000 ppm S, and Horizon-2 (143-147 m) contains only trace bornite, but pyrrhotite is present and the grades are 1-2 ppm Pt+Pd, 1000-3000 ppm Cu and 2000-4000 ppm S. The average sample length is 0.3 m and the samples were analyzed for major and trace elements and precious metals.

Trace Element Whole Rock Geochemistry

Figures 8-10 show trace element results for the samples in M-07-368. A positive correlation between chalcophile elements and S and Se is shown in Figure 4. The multistage dissolution-upgrading process as proposed by Kerr and Leitch (2005) is predicted to produce low base metal to PGE ratios. This model may apply to Horizon-1 where Cu/Pd ratios are between 100-200 (Fig 9).

Work by Barnes et al. (2009) shows that sulphur loss can be recognized by low S/Se ratios and high Cu/S ratios (Fig 9-10). Assuming that S is more mobile than Se, S loss is expected to decrease the S/Se ratio. Horizons 1 and 2 are both characterized by low S/Se ratios, as low as 1100, which may indicate S loss. The same is expected for the Cu/S ratio, as the S is removed the Cu will be left behind increasing the ratio, this is shown in both Horizon-1 and Horizon-2.

The parallel trends of Cu/S and Cu/Se in Figures 10 also support a S-loss model. In Figure 10, the Horizon-2 Cu/S ratio is shifted to higher Cu values, potentially indicating S-loss. The relationships between Pd/S and Pt/S are shown in Figure 10. Horizon-1 is clearly distinguished from Horizon-2. It is possible that S loss has occurred in both horizons, but an additional process(es) may also be involved.

Figure 3. Detailed stratigraphy of drill hole M-08-368

Sulphide Phases

The majority of the samples contain a low abundance of sulphide minerals. Within Horizon-1 bornite is common as exsolution lamella in chalcopyrite (Fig 7a); pyrrhotite and sphalerite are rare. The PGM in this horizon occur as inclusions within sulphide phases or along the contact between sulphide and silicate grains (Figure 7c-f). Sulphides occur both along grain contacts and within silicate grains, replacement phases within plagioclase grains along cleavage is particularly common (Fig 7a). The Horizon-2 mineral assemblage includes chalcopyrite, pyrrhotite, pyrite and lesser amounts of bornite (as exsolution in chalcopyrite) and sphalerite. PGM within this zone occur as inclusions in sulphide and along sulphide grain boundaries. Replacement of chalcopyrite by magnetite is common along chalcopyrite grains in both Horizon-1 and Horizon-2 and is evidence of sulphur loss (Fig 7g-h).

Petrographic evidence from both zones suggest that a late stage sulphide liquid was present. Evidence includes flame-like exsolution of bornite within chalcopyrite (Fig 7e), sulphide filled fractures within silicate grains and sulphide melt inclusion trails cross-cutting silicate mineral grains (Fig 7i).

Silicate PhasesThe study drill hole is collared in the fine grained Eastern gabbro (Fig 5a).

Within the top 20 m there are thin bands (10-30 cm) of olivine magnetite cumulate (Fig 5b). From 58 to 182 m is a continuous unit of TDLG (Fig 5c-f). The basal unit is the foot wall rheomorphic breccia. The entire sequence is locally cut by quartz syenite dikes (Fig 5g).

The TDLG is a vari-textured sub-ophitic medium to coarse grained gabbro with local irregular pegmatitic pods (Fig 5d-g). The major minerals are anhedral clinopyroxene oikocrysts, which partially enclose subhedral to euhedral plagioclase and olivine grains (Fig 6 a, b). Minor mineral phases include interstitial primary magmatic Fe-Ti-oxides (magnetite with ilmenite exsolution lamellae), biotite and apatite. Local zones of late-magmatic quartz-K-feldspar graphic intergrowths and post magmatic chlorite and serpentine are also present.

Resorption textures are common throughout the TDLG, particularly plagioclase. These resorption textures are more common within the mineralized zones (Fig 6 c-f).

Fig. 4 Major Element geochemistry results for M-07-368

References

Barnes, S.-J., Savard, D., Bédard, L.P., and Maier, W.D. 2009. Selenium and sulfur concentrations in the Bushveld Complex of South Africa and implications for formation of the platinum-group element deposits, Mineralium Deposita, 44: 647-663.

Dahl, R., Watkinson, D.H. and Taylor, R.P., 2001, Geology of the Two Duck Lake Intrusion and the Marathon Cu- PGE deposit, Coldwell Complex, northern Ontario, Exploration and Mining Geology, v. 10, p. 51-66.

Good, D.J., 2010. Applying Multisatge Dissolution Upgrading at the Marathon Deposit, 11th International Platinum Symposium

Kerr, A. and Leitch A.M., 2005, Self-Destructive Sulfide Segregation Systems and the Formation of High-Grade Magmatic Ore Deposits. Economic Geology v. 100, p. 311-332 Murahwi, C., Sam Shoemaker, S., Gowans, R., Lemieux, J. and Jacobs, C., 2010, Technical Report on the Updated Feasibility Study for the Marathon PGM-Cu Project, Marathon, Ontario, Canada. 43-101 Report. 228 pages.

Major Element Whole Rock Geochemistry

Whole rock major element geochemistry was analyzed by X-Ray Fluorescence. The large scale trends for major elements shows a homogeneous gabbro (Fig 4). At both Horizon-1 and Horizon-2 there is a drop in CaO, and between the two horizons there is a steady increase in CaO indicating different magma pulses.

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Figure 2 Map of the study area.

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Interpretation

The model that best fits the data is that of multi-stage dissolution upgrading. As new pulses of sulphur under saturated magma pass through the conduit they dissolve sulphides already present there. As sulphur is dissolved by the magma the remaining sulphides are enriched with PGEs. The low S/Se and low Cu/Pd ratios in Horizon 1 and 2 are in accordance to this model. Textures of the sulphides in the mineralized horizons including magnetite rims replacing chalcopyrite (sulphur dissolution), bornite exsolution in chalcopyrite (copper concentrated in sulphides) and an increase in resorption textures in plagioclase are also in accordance with the model. The two horizons also show distinct major and trace element signatures in both olivine and clinopyroxene indicating changing magma composition in these horizons.

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Figure 1 General geology of the Coldwell alkaline complex. Modified after Shaw (1997).

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M-07-368

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