Platinum-Group Element Mineralization in a Hydrothermal Cu-Ni Sulfide Occurrence, Rathbun Lake, Northeastern Ontario

Economic Geology Vol. 81, 1986, pp. 1272-1277 Platinum-Group Element Mineralization in a Hydrothermal Cu-Ni Sulfide Occurrence, Rathbun Lake, Northeastern Ontario WILLIAM F. ROWELL AND ALAN D. EDGAR Department of Geology, University of Western Ontario, London, Ontario, Canada N6A 5B7 Abstract Pt, Pd, and Au are concentrated in a hydrothermal Cu-Ni sulfide occurrence near Rathbun Lake, northeastern Ontario. The mineralization is associated with a sheared zone of the Wanapitei intrusion where hydrothermal fluids have altered relatively pristine gabbronorite to chidrite, sericite, quartz, epidote, and biotite. Chalcopyrite and pyrite, the principal sulfide minerals, occur as massive and disseminated mineralization. Bismuthian merenskyite and to a lesser extent kotulskite, michenerite, and temagamite are the Pd-bearing minerals, whereas Pt occurs primarily in sperrylite--64 percent of the Pd mineral grains are associated with altered silicates rather than sulfides. Although grains within sulfides may have been exsolved, the size and number associated with silicate suggest many were precipitated independently. Grab samples average 9 percent Cu, 0.2 percent Ni, 20,829 ppb Pd, 9,736 ppb Pt, and 3,053 ppb Au. Values for the other platinum-group elements are near background levels. No correlation is evident between Cu and the platinum-group elements or Au. The ratio of Pt + Pd/Os q- Ir q- Ru is much larger than for tholeiitic-hostedeposits of magmatic origin and probably reflects the relative solubilities of the platinum-group elements. Introduction ALTHOUGH most platinum-group element-rich sulfide deposits are considered to be magmatic (Naldrett and Duke, 1980), recent studies suggesthat platinum-group elements may be redistributed and concentrated by late-stage hydrothermal activity (cf. Kinloch, 1982; Piispanen and Tarkian, 1984; Talkington and Watkinson, 1984). Despite the evident mobility of some of these elements, very few platinum-group element-rich sulfide deposits are strictly of hydrothermal origin. This paper describes an occurrence of this rare type near Rathbun Lake, northeastern Ontario (Fig. 1). At Rathbun Lake hydrothermal fluids have utilized shears and faults in the Wanapitei intrusion to leach, transport, and concentrate platinum-group elements. General Geology The Rathbun Lake occurrence is at the south end of Rathbun Lake near the northeastern margin of the intrusion (Fig. 1). As most of the occurrence has been excavated, in situ examination of the mineralization is not possible. Descriptions of the subsurface geology (Koulomzine, 1955; Ogden, 1957) indicate that the sulfide mineralization is associated with extensive fracturing of the gabbronorite. Drill holes near the occurrence intersect several weakly mineralized shears and a north-northwest-trending fault is exposed in the shaft (Ogden, 1957). These structures are not apparent at the surface. However, gabbronorite exposed around the shaft collar is heavily sheared and slickensides are evident on some of the mineralized dump material. Massive sulfide mineralization occurs in a zone approximately 14 m long by 0.3 to 0.6 m wide. Disseminated sulfides around this zone increase the mineralized width to about 3.0 m. The zone strikes perpendicular to the contact at 135 ø and dips steeply The Wanapitei intrusion is ring shaped, centered to the north at 60 ø, with almost all sulfide mineralaround Portage Bay on the northeast shore of Lake ization confined to the gabbronorite. Wanapitei (Fig. 1). It intrudes massive and laminated Gowganda Formation graywacke. The petrology and Mineralogy and Mineral Chemistry geochemistry of the intrusion are described by Finn Twenty selected samples with sulfide mineraliza- (1981) and Dressier (1982). tion ranging from massive to sparsely disseminated Approximately 98 percent of the intrusion is com- were collected from dump material near the mine posed of medium-grained gabbronorite. Despite shaft for optical and geochemical study. This material lower to middle greenschist facies regional metamor- is considered to be representative of the in situ minphism (Card, 1978), much of the intrusion exhibits eralization. relatively pristine mineralogy and textures (Finn, Silicates 1981). Alteration is pervasive only where faults of the regional Onaping system have acted as conduits The mineralogy of the host gabbronorite has been for circulating hydrothermal fluids. extensively altered by hydrothermal fluids to chidrite, 0361-0128/86/582/1272-652.50 1272

PGE IN Cu-Ni SULFIDE, RATHBUN LAKE, ONTARIO 1273 N -J--I Olivine Diabase I ] Wanapitei Intrusion T I Gowganda Formation Contact FIG. 1. Geology of the Wanapitei Lake-Portage Bay area showing location of the Rathbun Lake platinum-group element deposit (modified from Dressier, 1982). Inset map shows location of map area. quartz, epidote, sericite, and biotite. Pyroxenes are tion. Accessory minerals include millerite, violarite, almost obliterated and plagioclase remains only as thin magnetite, goethite, pyrrhotite, covellite, and moalbitized rims enclosing highly saussuritized cores. lybdenite. Of these only millerite, violarite, and mag- Large fibrous masses of chlorite comprise much of netite are widely distributed. the gangue, except where sulfide mineralization is Medium- to fine-grained, subhedral to'euhedral sparse and epidote and saussuritized plagioclase are pyrite occurs primarily in massive sulfides. Large pymore prominent. Quartz, an accessory in unaltered rite grains often contain tiny inclusions of chalcopyrite gabbronorite, accounts for 10 to 15 percent of the and magnetite, less frequently platinum-group mingangue; generally, it is interstitial to other minerals erals, and rarely gold. In a few coarse anhedral pyrite and fills small fractures. Biotite occurs only in samples grains, containing abundant exsolved chalcopyrite, with minor sulfide mineralization and is usually par- the two minerals appear in an intergrowth texture tially altered to chlorite. indicative of simultaneous deposition at low temperature (Ramdohr, 1980). Sulfides Chalcopyrite is predominantly an interstitial min- Textures indicate that sulfide deposition occurred eral. In massive sulfides coarse chalcopyrite fills inby the metasomatic replacement of magmatic silicates terstices between pyrite, often slightly rounding many and to a lesser extent by fracture filling. Sulfides re- of the otherwise euhedral grains. Chalcopyrite in displace silicates along margins and cleavage planes and seminated sulfides fills tiny fractures and forms small occur as irregularly shaped granular dispersions ragged patches commonly associated with alteration within silicate grains. Magmatic textures due to im- silicates. miscibility between sulfide and silicate melts were not Millerite is present as fine anhedral grains, often observed. associated with chalcopyrite. In many Ni sulfide de- Chalcopyrite and pyrite comprise about 55 and 40 posits, millerite occurs as a secondary mineral in supercent, respectively, of the total sulfide mineraliza- pergene assemblages (Ramdohr, 1980) but is also re-

1274 W. F. ROWELL AND A.D. EDGAR ported as a primary mineral (cf. Hudson and Groves, Sixty-four percent of the Pd minerals occur in the 1974; Keele and Nickel, 1974). At Rathbun Lake, gangue, often in clusters which may include up to 40 millerite appears to be primary as there is no evidence grains (Fig. 2A). The remainder are associated with of it replacing any earlier sulfide phase. Approxi- sulfides, 18 percent at chalcopyrite-silicate interfaces, mately 25 percent of the original millerite has been 7 percent as inclusions within chalcopyrite, and 11 altered to violarite by supergene fluids. Although percent as inclusions in pyrite. Many of those assoviolaritization is widespread in some samples, few ciated with sulfides may have been exsolved. Howgrains have been completely replaced and thus mil- ever, it seems more likely that larger grains and grain lerite rimmed by violarite is a common texture. In a clusters in the gangue were precipitated indepenfew grains where replacement has been almost com- dently. plete, remnant millerite is present as fine needles. Merenskyite is a bismuthian variety, containing minor Sb (Table 1). Grains vary from <1 to 100 tim Platinum-group minerals in diameter, with the majority between i to 20 tim. Based on optical properties, 1,252 platinum-group Many of the larger grains are part of clusters in the mineral grains and 31 grains of gold were tentatively gangue (Fig. 2A). Kotulskite has an exceptionally large identified in 20 polished thin sections. As platinum- Sb content (Table 1) compared to analyses of this group mineral grains are difficult to discriminate by mineral from other localities (Cabri and Laflamme, optical means alone, the compositions of 45 were 1981). About 50 percent of the total kotulskite is aspositively determined by semiquantitative electron sociated with merenskyite and temagamite, either in microprobe analyses. Totals for the Pd minerals are exsolution textures or composite grains. Individual less than 100 percent (Table 1) because pure elements grains range from <1 to 40 tim in diameter. Michewere used rather than synthetic standards. However, heritc forms a few, relatively large (65-80 tim), ragfrom the calculated stoichiometry, merenskyite, ko- ged grains found in small groups in the gangue. It has tulskite, michenerite, and temagamite were identified. a minor Pt content (Table 1) similar to michenerite Optically, 70 percent of the grains are merenskyite, in the nearby Sudbury deposits (Cabri and Lafiamme, 20 percent kotulskite and 5 percent each are mich- 1976). enerite and temagamite. The presence of temagamite is of particu]ar interest Only a single sperrylite grain was identified opti- as it is a rare minera] hitherto reported only from the cally and this mineral was confirmed by electron mi- Stillwater Complex, Montana (Cabri and Lafiamme, croprobe analysis (Table 1). With a diameter of 250 1981), the New Rambler mine, Wyoming (Loucks and tim sperrylite is more than twice as large as any of McCa]lum, 1980), and the type loca]ity at the Temthe Pd minerals. Where the grain is bordered by sil- agami mine, Ontario (Cabri eta]., 1973). Compared icates and chalcopyrite, it is corroded, whereas a eu- with ana]yses from these loca]ities, Rathbun Lake hedral face embayed in pyrite remains unaltered. temagamite has slightly less Pd, Te, and Hg and These relationships indicate that this grain probably slightly more Sb (Table 1). All grains are less than 20 crystallized relatively early in the paragenetic se- tim in diameter and form part of biminera]ic or triquence. minera]ic assemblages which may include kotulskite TABLE 1. Means and Ranges of Electron Microprobe Analyses of Rathbun Lake Platinum-Group Minerals Elements weight percent Pd Pt Te Bi Sb As Hg Total Merenskyite 25.7 53.5 16.5 0.7 Range (23.3-31.0) (49.3-59.6) (12.5-20.4) (0.2-2.2) Kotulskite 36.5 0.3 33.9 17.7 7.9 Range (32.6-41.0) (n.d.-0.6) (27.4-41.0) (12.1-19.5) (5.1-18.6) Temagamite 34.0 40.4 0.3 1.5 Range (32.9-34.4) (39.3-40.9) (0.2-0.4) (1.0-2.5) Michenerite 23.4 1.5 35.1 40.8 0.7 Range (22.8-24.0) (1.5-1.5) (33.0-37.2) (40.6-41.0) (0.6-0.7) Sperrylite 56.7 96.4 96.3 20.3 96.6 (19.8-20.8) 98.7 42.7 99.4 Standards used Pt, Pd, S, As, Sb, Bi, Te, HgS Acceleration voltage 20 kv, sample current 400 n.d. = not detected

PGE IN Cu-Ni SULFIDE, RATHBUN LAKE, ONTARIO 1275 tario, to low-temperature subsolidus events near the end of a complicated magmatic history. Geochemistry To assess the platinum-group element enrichment, five samples with massive to weakly disseminated sulfide mineralization were analyzed for platinum-group elements and Au using a combined fire assay-neutron activation method. To facilitate comparisons with other platinum-group element-rich sulfide deposits, Cu and S were analyzed by XRF and Ni by direct current plasma. Additional analyses of Rathbun Lake samples were obtained from Koulomzine (1955). Values are comparable to those obtained this study (Table 2). O.07mm The results (Table 2) show that Pd is more abundant and homogeneously distributed than Pt. Gold contents vary considerably, and on average, are lower than Pt and Pd. The average Os, Ir,.Ru, and Rh values are in the lower parts per billion range. The abundance of chalcopyrite relative to millerite and violarite is such B that the average Cu/Cu + Ni ratio is well over 0.9. No correlation between Cu and the platinum-group elements and Au is evident since most platinum-group mineral grains are associated with silicates rather than sulfides. In Figure 3, platinum-grou p elements and Au contents at Rathbun Lake are compared with tholeiiticand komatiitic-hostedeposits on the basis of chondrite-normalized 100 percent sulfid e values. Although K tholeiitic-hosted sulfide deposits are characterized by Pt + Pd/Os + Ir + Ru ratios > 13 (Naldrett and Duke, 1980), the ratio of 1,120 for Rathbun Lake is far greater than for the other deposits (Fig. 3). Large ratios have already been noted in other hydrothermal deposits. At the New Rambler mine, Wyoming, the O.03mm Pt + Pd/Os + Ir + Ru ratio is 1,100 (McCallurn et al., 1976). The large ratio at Rathbun Lake supports the FIG. 2. A. Cluster of white merenskyite grains in quartz and suggestion of McCallurn et al. (1976) that in hydroehlorite gangue. B. Bimineralie grain of temagamite (T) and merenskyite (M) and single grain of kotulskite (K). C = ehaleopyrite. thermal deposits, such ratios may reflect the relative solubilities of platinum-group elements. Keays et al. (1982) propose that, relative to magmatic deposits, hydrothermal orebodies are depleted and merenskyite (Fig. 2B). Adjacent to the other in Ir, and to a lesser extent Pd, due to the relative platinum-group minerals, temagamite is creamy colimmobility of these elements. At both Rathbun Lake ored with a distinctive grayish hue. The Pd bismuthotelluride mineralization at Rath- and the New Rambler mine (McCallurn et al., 1976), Ir is low but Pd is relatively high. The New Rambler bun Lake appears to be characteristic of deposits deposit is widely accepted as being of hydrothermal where platinum-group elements have been concentrated by hydrothermal fluids. In the hydrothermal origin. IfRathbun Lake is also a hydrothermal deposit, as evidence suggests, perhaps the criteria of Keays et Cu ores of the New Rambler mine, Wyoming (Mcal. (1982) should be amended. Callurn et al., 1976), and Messina, South Africa (Mihalik et al., 1974), Pd also occurs only in compounds Summary with Bi and Te. Although the association of Pd with these volatile elements may be typical of hydrother- A hydrothermal origin is indicated by the minermal deposits, Cabri and Lafiamme (1976) attribute alogy and geochemistry of the platinum-group ele- Pd bismuthotelluride mineralization at Sudbury, On- ment mineralization at Rathbun Lake. The occurrence

1276 W. F. ROWELL AND A.D. EDGAR TABLE 2. Platinum-Group Elements, Au, Ni, Cu, and S Contents of Rathbun Lake Samples Ni Cu S Pt Pd Rh Ru Ir Os Au Sample no. (wt %) (ppb) This study R-1 0.22 R-2 0.22 R-3 0.26 R-4 0.16 R-5 0.36 0.22 19.9 23.4 1,800 23,000 17 <5 0.1 <3 9.1 19.4 74 35,000 14 20 0.3 6 8.3 18.4 18,000 37,000 8 <5 0.3 11 4.7 9.4 33,000 19,000 3 40 0.2 23 2.3 2.4 190 12,000 23 15 28.0 3 8.9 14.6 10,613 25,200 13 <17 5.8 <9 25O 890 1,300 950 2,8OO 1,238 From Koulomzine (1955) 3 0.21 3a na 5 0.50 5a 0.11 6 0.35 9 0.29 0.29 i (1 + 2) 0.24 7.03 na 3,375 23,125 na 17.26 na 2,750 29,687 na 5.51 na 1,187 23,125 na 19.92 na 656 5,313 na 7.25 na 2,812 16,875 na 1.31 na 2,187 5,000 na 9.71 2,161 17,188 9.32 14.6 9,736 20,829 13 17 11,250 63 6,250 5,854 5.8 <9 3,053 na = not available -- mean 1oo is located in an area of the Wanapitei intrusion where circulating fluids have altered the mineralogy of the host gabbronorite to chlorite, sericite, saussuritized plagioclase, epidote, and biotite. Pt occurs primarily in sperrylite and Pd in bismuthotelluride minerals which are considered to be characteristic of low-temperature hydrothermal deposition in some other deposits. Most platinum-group mineral grains are associated with silicates, suggesting that many were directly precipitated rather than exsolved from sulfides. The ratio of Pt + Pd/Os + Ir + Ru is much larger than for tholeiitic-hosted sulfide deposits of magmatic origin and reflects the relative solubilities of the platinum-group elements. Acknowledgments We thank R. L. Barnett and B. Marden for technical assistance. Financial support for this study was provided by the Ontario Geoscience Research Grants Program and the Natural Science and Engineering Research Council of Canada. September 12, 1985; February 17, 1986 REFERENCES FIG. 3. Chondrite-normalized 100 percent sulfide values vs. platinum-group elements + Au (after Naldrett and Duke, 1980) for Rathbun Lake and tholeiitic- and komatiitic-hosted platinumgroup element deposits (from Naldrett et al., 1980). Cabri, L. J., and Laflamme, J. H. G., The mineralogy of the platinum-group elements from some copper-nickel deposits of the Sudbury area, Ontario: ECON. GEOL., v. 71, p. 1159-1195. -- 1981, Analysis of minerals containing platinum group elements: Canadian Inst. Mining Metallurgy Spec. Vol. 23, p. 151-174. Cabri, L. J., Laflamme, J. H. G., and Stewart, J. M., 1973, Tem-

PGE IN Cu-Ni SULFIDE, RATHBUN LAKE, ONTARIO 1277 agamite, a new palladium-mercury telluride from the Temagami Loucks, R. R., and McCallum, M. E., 1980, Platinum group mincopper deposit, Ontario, Canada: Canadian Mineralogist, v. 12, erals in the New Rambler copper-nickel deposit, Wyoming: Inp. 193-198. ternat. Mineralog. Assoc. Mtg., 1 lth, Novosibirsk, Proc., v. 1, Card, K. D., 1978, Metamorphism of the middle Precambrian p. 200-218. (Aphebian) rocks of the Eastern Southern Province: Canada McCallurn, M. E., Loucks, R. R., Carlson, R. R., Cooley, E. F., Geol. Survey Paper 78-10, p. 269-282. and Doerge, T. A., 1976, Platinum metals associated with hy- Dressier, B. O., 1982, Geology of the Wanapitei Lake area, district drothermal copper ores of the New Rambler mine, Medicine of Sudbury: Ontario Geol. Survey Rept. 213, p. 131. Accom- Bow Mountain, Wyoming: ECON. GEOL., v. 71, p. 1429-1450. paniedby maps 2450, 2451, scale 1:31680 (1 inch to % mile). Mihalik, P., Jacobsen, J. B. E., and Hiemstra, S. A., 1974, Platinum- Finn, G. C., 1981, Petrogenesis of the Wanapitei gabbronorite group minerals from a hydrothermal environment: ECON. intrusion: A Nipissing-type diabase from northern Ontario: Un- GEOL., v. 69, p. 257-262. pub. M.Sc. thesis, Univ. Western Ontario, 112 p. Naldrett, A. J., and Duke, J. M., 1980, Platinum metals in magmatic Hudson, D. R., and Groves, D. I., 1974, The composition of vio- sulfide ores: Science, v. 208, p. 1417-1428. larite coexisting with vaesite, pyrite, and millerite: ECON. GEOL., v. 69, p. 1335-1340. Naldrett, A. J., Innes, D. G., and Sowa, J. M., 1980, Platinum group element concentrations in some magmatic ores in Ontario: Keays, R. R., Nickel, E. H., Groves, D. I., and McGoldrick, P. J., 1982, Iridium and palladium as discriminants of volcanic-ex- Ontario Geol. Survey Misc. Pub. 93, p. 171-178. halative, hydrothermal, and magmatic nickel sulfide mineral- Ogden, M., 1957, Unpublished report on Dolmac Mines Limited ization: ECON. GEOL., v. 77, p. 1535-1547. property, Rathbun Township, District of Sudbury: Ontario Geol. Keele, B. A., and Nickel, E. H., 1974, The geology of a primary Survey, Ministry of Natural Resources, Assessment File Remillerite-bearing sulfide assemblage and supergene alteration search Office, file 64-6035. at Otter shoot, Kambalda, Western Australia: E½ON. GEOL., v. Piispanen, M., and Tarkian, M., 1984: Cu-Ni-PGE mineralization 69, p. 1102-1117. at Rometolvas, Koillismaa layered igneous complex, Finland: Kinloch, E. D., 1982, Regional trends in the platinum-group min- Mineralium Deposita, v. 19, p. 105-111. eralogy of the critical zone of the Bushveld Complex, South Ramdohr, P., 1980, The ore minerals and their intergrowths: To- Africa: ECON. GEOL., v. 77, p. 1328-1347. ronto, Pergamon Press, 1205 p. Koulomzine, T., 1955, Unpublished report on Dolmac Mines Talkington, R. W., and Watkinson, D. H., 1984, Trends in the Limited property, Rathbun Township, District of Sudbury: On- distribution of the precious metals in the Lac-des-Iles-Complex, tario Geol. Survey, Ministry of Natural Resources, Assessment northwestern Ontario: Canadian Mineralogist, v. 22, p. 125- File Research Office, file 63-6035. 136.