Evidence for Sulphur Loss in the Marathon PGE-Cu Deposit, Ontario

R.G. Ruthart, R.L. Linnen, I.M. Samson, D.J. Good

Purpose

The purpose of this study is to characterize and determine the process(es) which formed the PGE rich, low sulphur W Horizon mineralization at the Marathon Cu-PGM deposit. Two key processes have been identified in the formation of the W Horizon. The first is the formation of an immiscible sulphide liquid within the magmatic system to collect base and precious metals. A second process is required to upgrade the metal content the initial sulphide liquid. The results of this study show that multistage sulphur dissolution upgrading in an open system can explain the mineralization found in the W Horizon.

Geology

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 (Heaman and Machado, 1992).

Methodology

To examine the mineralization at the Marathon PGM-Cu deposit a detailed petrographic and geochemical study was undertaken using samples from selected diamond drill holes (DDH) that encompass a range of mineralization styles that are outlined in Table 1. Samples for whole rock geochemical analysis and thin sections were taken from each DDH through the mineralized horizons and the surrounding unmineralized rocks.

The sulphide minerals in the W Horizon are chalcopyrite and bornite with trace amounts of pentlandite. Bornite occurs as exsolution lamellae within chalcopyrite grains, and pentlandite as granular masses with chalcopyrite exsolution lamellae within larger grains of chalcopyrite (Fig. 3b-c). The exsolution of bornite and pentlandite is interpreted as evidence of sulphur loss from the initial sulphide liquid which collected Cu, Ni and PGE. As sulphur is removed the Cu-Fe-Ni-S system will be driven towards Cu and Ni enriched minerals.

The sulphide minerals in the Main Zone samples are chalcopyrite, pyrrhotite and pentlandite ± minor bornite. Pyrrhotite is typically found as the core of larger chalcopyrite grains, and pentlandite occurs either as exsolution flames in pyrrhotite or as granular grains within chalcopyrite (Fig. 3d).

Chalcopyrite grains within the mineralized zones are commonly mantled by magnetite. It differs from primary magmatic magnetite in that it lacks ilmenite exsolution lamellae and is devoid of Ti and V, having a nearly a pure Fe-end member composition (Fig. 3e-f). This texture has been interpreted as evidence of S loss.

Petrography

The Two Duck Lake gabbro hosting the W Horizon mineralization is a fresh, pristine gabbro that only contains small localized pockets (millimeter to decimeter scale) of hydrous minerals (chlorite, hornblende, actinolite and calcite). The sulphide minerals are dominantly interstitial between silicate grains, preferentially in contact with plagioclase, then olivine, clinopyroxene, and hydrous silicate phases.

Geochemistry

Whole rock S and Se contents were determined to investigate S loss. Sulphur and Se are both siderophile elements and incompatible in silicate melts. During crystallization of a silicate melt or the formation of a sulfide liquid it is expected that the S/Se ratio will remain constant. The removal of S through is expected to lower the S/Se ratio, as S is more mobile than Se (Barnes et al, 2009). Average S/Se values are ~800 for the W Horizon, ~1980 for the Main Zone and ~1700 in unmineralized samples (Fig 4). The lower S/Se found within the W horizon is evidence that sulphur loss has occurred.

R Factor Modeling

The sulphide metal content, or tenor, is the ratio of metals to S. The sulphide tenor has been calculated using the method of Kerr (2001, 2003) and is shown in Table 2. The average Pt and Pd tenor in sulphide in the W Horizon is over 30 times greater than the Main Zone.

Collection of metals by an immiscible sulphide liquid in a closed system can be modeled by the R factor (Campbell and Naldrett, 1979). The R factor model uses the ratio silicate magma to sulphide liquid (R) and the partition coefficient (D) of the metal into sulphide liquid to calculate the sulphide metal content. The sulphide metal content observed in the W Horizon is too high to be accounted for by the R factor model.

Conclusions

The W Horizon of the Marathon PGM-Cu deposit shows evidence for enrichment by sulphur loss processes. Petrography of the W Horizon has shown that the dominant sulphide mineral is chalcopyrite, and it commonly contains exsolution lamellae of bornite and pentlandite. In addition chalcopyrite grains within the mineralized horizons are commonly rimmed by Fe-end member magnetite. The lack of pyrrhotite in the W Horizon, exsolution textures and magnetite rimming are all interpreted as textural evidence of sulphur loss. Whole rock geochemistry has shown that the ratio of S/Se within the W Horizon is lower than the Main Zone mineralization and unmineralized samples. The lower S/Se ratio is interpreted as evidence of sulphur loss. The pristine and unaltered texture of the Two Duck Lake gabbro shows that the mineralization has not been formed in a hydrothermal system. Accounting for the high Pt and Pd tenor in sulphide using the closed system R factor model is not possible. The multistage dissolution upgrading model of Kerr and Leitch is able to model the Pt and Pd tenors observed within the W Horizon at reasonable R factor values.

References

Barnes, S.-J., Savard, D., Bédard, L. P., & 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(6), 647-663

Campbell, I. H., & Naldrett, A. J. (1979). The influence of silicate: sulfide ratios on the geochemistry of magmatic sulfides. Economic Geology, 74(6)

Good, D. J., & Crocket, J. H. (1994). Genesis of the Marathon Cu-Platinum-Group Element Deposit, Port Coldwell Alkalic Complex, Ontario: A Midcontinent Rift-Related Magmatic Sulfide Deposit. Economic Geology, 89(1), 131-149

Good, D. J. (2010). Applying Multistage Dissolution Upgrading and 3d-GIS to Exploration at the Marathon Cu-PGE Deposit, Canada. Abstracts, 11th International Platinum Symposium, 1-4.

Heaman, L. M., & Machado, N. (1992). Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contributions to Mineralogy and Petrology, 110(2), 289–303

Kerr, A. (2001). The Calculation and Use of Sulfide Metal Contents in the Study of Magmatic Ore Deposits: A Methodological Analysis. Exploration and Mining Geology, 10(4), 289-301

Kerr, A. (2003). Guidelines for the calculation and use of sulfide metal contents in research and mineral exploration. Newfoundland Geol. Surv., Current Res, 3(1), 223–229

Kerr, A., & Leitch, A. M. (2005). Self-destructive sulfide segregation systems and the formation of high-grade magmatic ore deposits. Economic Geology, 100(2)

The process of multistage dissolution upgrading in an open system (Kerr and Leitch, 2005) was proposed as the mechanism for upgrading Cu and PGE at the Marathon deposit by Good (2010). A simplified cartoon illustrating the process is shown in Figure 5. In the dissolution upgrading process a new batch of metal-bearing sulphur undersaturated magma enters a magmatic system that contains immiscible sulphide liquid. Metals are partitioned from the new silicate magma into the sulphide liquid. Since the new magma is sulphur undersaturated, some of the sulphide and metals are dissolved back into the silicate magma. Dissolution of the sulphide liquid has increased (loaded) the amount of metals in the silicate magma, and increased the R factor, metal partitioning between the silicate magma and sulphide liquid occurs again. The total amount of sulphide liquid in the system is now smaller, and the sulphide metal tenor has been increased by the dissolution process. The proportion of sulphide liquid dissolved is described by the loss factor (L), and the R is divided into two parts, the incoming incremental R factor (R' ) and the outgoing inc

cumulative R factor (R ).cum

Figure 6 shows the Pd tenor in sulphide calculated using the closed system R factor model and multistage dissolution model. The parental magma contains 10 ppb Pd, D is 40,000 and loss factors between 0.25 and 0.005 are used. The closed system R factor model begins to reach its limiting value of ~363 ppm Pd in sulphide at R ~400,000 (10D). The dissolution upgrading model is able to achieve much higher values of sulphide metal tenor at lower R factor values. As the loss factor (the proportion of sulphide dissolved by each incoming batch of magma) increases, higher sulphide tenors are achieved at lower R factors, at the expense of the total mass of sulphide. Using a loss factor of 0.02 the average Pd in sulphide (~3,000 ppm) is achieved at R values of 475,000. cum

Mineralization is hosted by the Two Duck Lake intrusion, a medium to coarse grained olivine-bearing ophitic gabbro. The Two Duck Lake gabbro intrudes into the Eastern gabbro, which is a fine to coarse grained cumulate composed of plagioclase, clinopyroxene and olivine (Good and Crocket, 1994). Contacts between the Two Duck Lake gabbro and the Eastern gabbro are irregular and erratic, abundant dikeletes (centimeter to meter scale) of Two Duck Lake gabbro cut the Eastern gabbro. These relationships indicate the Two Duck Lake gabbro consists of multiple intrusive stages.

The W Horizon is a high-grade PGE horizon characterized by low sulphur content, low Cu/Pd and high Cu/Ni ratios (Fig. 2). Within the W Horizon PGE grades are variable, and commonly range between 2-50 ppm Pt + Pd. The Main Zone is a larger, but lower grade mineralization zone, which contains disseminated sulphides.

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Coldwell Alkaline Complex

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Figure 1. General geology of the Coldwell alkaline complex showing the location of the Marathon PGE-Cu deposit. Modified after Shaw (1997)

S, Me

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Figure 5. Simplified cartoon illustration of the multistage dissolution upgrading process. A) Sulphide liquid (yellow) has formed within the silicate magma (grey). B) A new batch of metal bearing sulphur undersaturated magma (brown circles) enters the system. Metals are partitioned from the new magma into the sulphide liquid, enriching the sulphide metal tenor. C) The two magmas have mixed (grey) to form sulphur undersaturated magma, some of the S and metals from the sulphide liquid are dissolved back into the silicate magma. D) Due to dissolution of the sulphide liquid, the R factor has increased and metals are again partitioned into the sulphide liquid from the silicate magma. The total mass of sulphide has decreased and it has a higher sulphide metal tenor than originally.

Table 1: Summary of metals and sulphide mineralogy in W Horizon Samples

Hole ID Length (m) Avg. Cu(%)

Avg. Pt(ppm)

Avg. Pd(ppm)

Sulphide Minerals

DDH-306 8.0 0.59 11.8 41.6 chalcopyrite, bornite,pentlanditeDDH-368 0.6 0.10 5 8.0 chalcopyrite, bornite, trace pentlanditeDDH-369 2.0 0.03 2.1 7.8 chalcopyrite, trace borniteDDH-369 4.0 0.1 3.1 7.2 chalcopyrite, trace borniteDDH-441 10.0 0.5 0.4 1.5 chalcopyrite, trace bornite, pent-

landite, pyrrhotite

Figure 3. A) Photomicrograph under cross polarized light showing the ophitic texture of the Two Duck Lake (TDL) gabbro. Large clinopyroxene (cpx) grain showing continuous extinction wraps around olivine (olv) and plagioclase (pl). B) Photomicrograph under reflected light showing chalcopyrite (ccp) with bornite (bn) exsolution and pentlandite (pn) in the W Horizon. C) Backscatter image showing pn with internal ccp exsolution lamellae within a larger grain of ccp. D) Photomicrograph under reflected light of Main Zone mineralization showing a core of pyrrhotite (po) rimmed by ccp. E) Photomicrograph under reflected light showing a grain of ccp with bn and pn exsolution lamella and a rim of Fe-end member magnetite (mag). F) Backscatter image showing primary magmatic magnetite with ilmenite exsolution lamellae.

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Figure 4. Plot showing S vs. Se. The W Horizon has a lower S/Se than the Main Zone, indicating that sulphur loss has occurred.

LegendUnmineralized

306 W Horizon

368 W Horizon

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441 W Horizon

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368 Main Zone

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Table 2: Sulphide Tenor

Sulphides of the Marathon deposit (normalized to 100% sulphides)

Ni Cu Pt Pd Au(wt %) (wt %) (ppm) (ppm) (ppm)

Range Avg. Range Avg. Range Avg. Range Avg. Range Avg.

W Horizon 0-13 1 5-60 32 60-26,690 2,657 133-12,574 3,285 6-1,242 182

Main Zone 0-6 1 2-46 26 1-574 70 5-1,285 107 3-5 12

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/Pd

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Figure 2. A) Plot showing Cu/Ni vs. Cu/Pd. The W Horizon and Main Zone mineralization appear as two distinct groupings, with Cu/Pd much lower in the W Horizon. B) Plot of Pt + Pd vs. S. The W Horizon Samples have higher Pt + Pd at lower S compared to the Main Zone

Legend

306 W Horizon

368 W Horizon

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369 W Horizon (2)

441 W Horizon

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368 Main Zone

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Figure 6. Plot showing the results of the multistage dissolution upgrading modeling. See text for details.

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(D = 40,000)(Xo = 10 ppb)(R’ = 100)inc