RESEARCH PGM deposits: A new model

Before writing off that long-ignored nickel-copper-platinum prospect, consider a re-examination of the property in light of a new genetic model devised by Prof David Watkinson of Carleton University, in Ottawa, Ont. He and two of his colleagues (postdoctorate Richard Dahl and Ph.D student Henry Dillon-Leitch) recently completed a genetic model for Fleck Resources’ platinum group metals (pgm) deposit in the Coldwell alkaline complex, near Marathon, Ont. Their work was supported by a $55,000, 2-year Natural Sciences and Engineering Research Council operating grant and by research agreements with the Ontario Geological Survey.

Although magmatic sulphide liquid segregation is a well established theory of pgm concentration, the combination of company exploration work and some detailed scientific research by Watkinson suggests that an additional fluid phase is important in the process. Geological mapping and re-examination of drill core from the Coldwell alkaline complex show that pgm minerals here precipitated from hydrothermal fluids. Some of Watkinson’s research also indicates that hydrothermal processes were active in the concentration of pgm in the Lac-des-Iles complex, also in northwestern Ontario, and in the Bushveld complex, in South Africa. His model of pgm concentration should encourage a detailed re-examination of the geology of old nickel- copper-platinum propects and may lead exploration geologists to pgm deposits that have gone undetected.

Watkinson’s research on a genetic model for the concentration of pgm focusses on the role of the fluid phase in the Coldwell alkaline complex. Located in the Thunder Bay area of Ontario, this complex is a Proterozoic composite intrusion of layered gabbroic to syenitic rocks. Along its eastern boundary is an arcuate belt of layered gabbroic rocks regarded as a single intrusive unit. A distinct coarse- grained gabbroic to monzonitic intrusion, called the Two Duck Lake Intrusion, contains the copper-nickel-pgm mineralization. It cross-cuts the layered gabbroic unit, called the Eastern Coldwell Gabbro. The pgm @Pt/(Pt + Pd) = 0.22# occurs with very copper- rich sulphide assemblages @Cu/Cu + Ni=0.90# in a 46-million-tonne deposit grading 0.37% copper and 0.039 oz platinium and palladium per tonne. The deposit occurs within the coarse- grained to pegmatitic gabbro- monzonite of the Two Duck Lake Intrusion. This unit invades and has assimilated Archean metavolcanic rock near its base and the layered succession of the Eastern Coldwell Gabbro near its top. Assimilation and mixing of rheomorphic material is responsible for the abundant xenoliths, granophyric patches, aplitic dikes, potassium-rich minerals and deuteric alteration that are spatially related to concentration of the pgm in disseminated copper-rich sulphides. Concentration of pgm occurs when fluids, probably in part derived from metamorphic breakdown of assimilated metavolcanic xenoliths, are enriched in copper and pgm during interaction with earlier precipitated magmatic sulphides and react with ilmenomagnetite. The precipitation mechanism is possibly similar to that proposed for some epigenetic gold deposits found in iron-rich rocks.

According to Watkinson’s genetic model, pgm in the Marathon deposit were concentrated due to hydrothermal processes. The fluid concentrates the pgm in gabbroic complexes within the Coldwell Alkaline complex. Outlining of the major pgm-bearing unit, the gabbroic Two Duck Lake Intrusion, is the result of detailed geological studies. His model develops from mapping, structural analysis and logging of drill core from the Coldwell Alkaline complex and should encourage similar examinations of old platinum prospects by exploration geologists.

The massive gabbroic Two Duck Lake Intrusion, which is not part of the layered gabbro succession, is a north-south elongated dike-shaped unit of 130 m maximum width, continuously exposed for 1.5 km. It is discordant to the stratigraphy of the layered gabbro and to its mesoscopic layering, and it exhibits magmatic breccia and xenolithic zones along the eastern (bottom) and western (top) contacts. Along the eastern boundary, the coarse-grained gabbro unit intrudes foliated mafic and felsic Archean metavolcanics. Foliation is systematically subparallel to the intrusion, suggesting the gabbro was intruded along a pre-existing major fault or shear zone. This early structure has then been reactivated. Steeply dipping to the west in the northern part, the intrusion becomes more flat southward. Several phases of faulting affected the coarse-grained gabbroic intrusion. They are of variable intensity and consist essentially of a major north-northeast sub-vertical fault and a conjugate network of minor northeast- and southeast-trending faults and fractures. The relative offsets along these structures vary along strike and are responsible for some of the discontinuities of the intrusion.

The Two Duck Lake Intrusion is characterized by its very coarse- grained to pegmatitic texture, abundant biotite, apatite, deuteric amphibole, and clots of quartz-feldspar intergrowth closely related in space to sulphide disseminations. The pgm are concentrated in copper-rich sulphides; pgm minerals occur along the grain boundaries of sulphides with altered ilmenomagnetite. The “volatile”- bearing minerals contain high concentrations of chlorine and fluorine.

Detailed petrological and geochemical cross-sections have been carried out across the Two Duck Lake Intrusion. The aim has been to identify any petrological and geochemical stratgraphy or zoning, their relationships, and to enlighten petrological and geochemical relationships with host Archean metavolcanics on the eastern margin and Eastern Coldwell Gabbro on the western margin. A typical section consists from bottom to top of three main subunits. Firstly, a fine- to medium-grained hornblende gabbro to monzodiorite of ophitic to poikilitic textures (plagioclase, potassium-feldspar, augite, olivine, hornblende, biotite, apatite, chalcopyrite, cubanite and pyrrhotite) including numerous recrystallized and partially melted xenoliths of metavolcanic material, and numerous granophyric pods. Secondly, a coarse to pegmatitic olivine ferrogabbro to ferrodiorite of hypidiomorphic to pegmatitic textures (plagioclase, augite, olivine, iron- titanium-oxides, biotite and apatite) cross-cut by numerous pods of gabbro to monzodiorite pegmatites where granophyres may occur. Thirdly, a coarse- to very coarse-grained olivine gabbro to diorite with hypidiomorphic to poikilitic textures (plagioclase, augite, olivine, orthopyroxene, biotite, apatite, chalcopyrite, cubanite, pyrrhotite, pentlandite and iron- titanium- oxides) including numerous xenoliths and blocks of equigranular fine-grained olivine- biotite gabbro from the Eastern Coldwell Gabbro. Most of these inclusions did not suffer partial melting as did the xenoliths in the first subunit.

The layering of the Coldwell Eastern Gabbro results from differences in the relative concentration of plagioclase and mafic minerals. The lowermost layers are alternating (melanocratic and leucocratic) bands composed of plagioclase, clinopyroxene, magnetite, minor olivine, biotite and rare sulphides. Higher stratigraphic layers consist predominantly of magnetite with minor clinopyroxene and rare sulphides. The layering within this unit is not continuous. There is a diffuse contact between the layered gabbro and the fine- to medium-grained massive gabbro similar to the gabbro of the Two Duck Lake Intrusion. Immediately above the Coldwell Eastern Gabbro is a thick succession of coarse-grained augite syenite consisting of alkali feldspar, amphibole, clinopyroxene and magnetite.

Prof Watkinson’s thorough studies on the Coldwell Alkaline complex have resulted in a genetic model which focusses on the role of the fluid phase for the concentration of pgm. This model is supported by evidence from his past studies on the pgm concentration at Lac-des-Iles. Here he worked with former colleagues Greg Dunning and Ray Talkington in co-operation with geologists from the Sheridan Group of com
panies, Riocanex Inc. and Texasgulf Inc. Although the occurrence of pgm in many layered mafic-ultramafic intrusive complexes may result from primary magmatic processes, geochemical data for Lac- des-Iles indicate that hydrothermal processes for the mobilization and concentration of pgm is more common than previously recognized.

The Lac-des-Iles complex, approximately 80 km north-northwest of Thunder Bay, Ont., is a sulphide- bearing, layered, mafic-ultramafic body. The magmatic sulphide and pgm mineralization is commonly associated with mafic cumulates. The complex is intruded into Archean granitoid and gneissic rocks of the Wabigoon belt and is cut by granitoid rocks, emplaced during the Kenoran event, and sills and dikes of Keweenawan-type diabase. Late-stage deuteric alteration or, less likely, greenschist facies metamorphism caused by the emplacement of the Kenoran granites, significantly redistributed and concentrated the pgm. This late-stage geological process involving a fluid phase has altered primary silicates, redistributed the copper-nickel sulphides and produced pgm-bearing telluride, arsenides, bismuthotellurides and sulphides. Most of the pgm mineralization is contained in the western gabbro, which consists of interlayered cumulates of gabbroic noritic, pyroxenitic and minor anorthositic rocks, and intermixed granitic rocks.

Petrological and mineralogical studies of drill core and surface samples from Lac-des-Iles suggest that the highest pgm values are associated with once-pyroxene-rich rocks, now predominantly composed of amphibole. Most pgm minerals are associated with secondary sulphides and silicates, even where the pyroxene and plagioclase are not totally altered. These studies indicate that hydrothermal processes are active in the mobilization and concentration of pgm at Lac-des-Iles.

Prof Watkinson’s studies on Canadian pgm properties and similar work on the Bushveld complex, South Africa, all lead to the conclusion that magmatic processes are necessary, but not sufficient, to generate large pgm concentrations. Pegmatitic textures, deuteric alteration, hydrous minerals, fluid inclusions, and other features all suggest a major role for fluids as concentrating agents. REFERENCES Talkington, R.W. and Watkinson, D. H. (1984): Trends In The Distribution Of The Precious Metals In The Lac-Des-Iles Complex, Northwestern Ontario. Canadian Mineralogist, Vol. 22, p. 125-136. Watkinson, D. H., Jones, P. L. and Marshall, D. (1984): Platinum Group Elements In The Eastern Gabbro, Coldwell Complex, Northwestern Ontario, Ontario Geological Survey, Miscellaneous Paper 121, p. 36-41. Watkinson, D. H., Whittaker, P. J. and Jones, P. L. (1983): Platinum Group Elements In The Eastern Gabbro, Coldwell Complex, Northwestern Ontario, Ontario Geological Survey, Miscellaneous Paper 113, p. 183-191. Joyce Musial is a Toronto-based geologist and freelance writer.