"Rock of the Month #144, posted for June 2013" ---

What is peridotite?

Peridotite is a common type of ultramafic rock, that is, an igneous rock rich in iron and magnesium, commonly derived from a deep ("primitive") source in the Earth's mantle. It is composed of essential olivine plus associated silicates (pyroxenes, plagioclase feldspar, etc), oxides and a range of accessory minerals. The ferromagnesian minerals (olivine, and sometimes the pyroxenes) are often variably replaced by secondary serpentine and very fine-grained magnetite.

This particular sample is extensively serpentinized, with the major minerals being, in decreasing order of abundance, optically identified as serpentine, relict fresh olivine, plagioclase feldspar, orthopyroxene, clinopyroxene and iron-titanium oxides. The latter include primary spinel (magnetite), embayed ilmenite, and secondary magnetite released from olivine upon alteration to serpentine. Accessory and trace phases include brown mica (phlogopite?), deep green chlorite, brown amphibole, a pale chlorite (penninite), apatite, tremolite, talc and traces of the sulphides pentlandite, pyrrhotite and chalcopyrite. The rock does not appear appreciably mineralized - and gives little indication that a fabulous orebody lurks nearby!

Ultramafic rocks of the Midcontinent Rift

The most visible and extensive products of magmatism in the Midcontinent Rift are relatively fine-grained, shallow intrusive or volcanic rocks. Examples are the Nipigon sills north of Thunder Bay, Ontario, and the basalts of the Keweenaw peninsula. By far the largest development of Keweenawan-age intrusive mafic-ultramafic rock resides within as many as 40 discrete bodies that comprise the Duluth complex. This crescent-shaped igneous domain extends west and north from the North Shore of Minnesota, underlying a region in excess of 5,000 km². Olivine-rich rocks tend to weather rapidly, and this factor plus locally-abundant glacial deposits often leave their occurrences recessive and hard to find, with limited outcrops. Thus relatively few such intrusions of Keweenawan age have been located beyond the Duluth complex. They may be largely buried under thin soil and drift (Eagle); totally buried by swamps and glacial till (Tamarack, Minnesota); or found beneath lakes with no real outcrop (the Thunder Bay North project at Current Lake, Ontario).

The existence of the host intrusion of the Eagle deposit was first brought to light by Klasner et al (1979), in a government report describing partially serpentinized peridotite of early Keweenawan age, outcropping at two sites on a 20-km-long zone of positive aeromagnetic anomalies in northern Marquette county. Most of the area is drift-covered, with minimal exposure. They identified the rock as a plagioclase lherzolite, with 40-50% olivine (Fo₈₀), 10-15% each of orthopyroxene and clinopyroxene, 5-10% plagioclase (labradorite), 4-6% Fe-Ti oxides (ilmenite and magnetite) and 1-2% sulphides (mostly pyrrhotite, pentlandite, chalcopyrite). The peridotite was found to be anomalously rich in copper and sulphur and was correctly judged to have potential for magmatic Ni-Cu sulphide mineralization. The intrusion underlies the Yellow Dog Plain, "a nearly featureless area covered by Pleistocene sand and gravel", and "nearly devoid of bedrock exposures". Kennecott's exploration work also uncovered further rocks of this type, the BIC peridotite. The BIC occurrence comprises two small intrusions in Baraga county, with outcrops on hillsides southeast of the town of L'Anse, east of the hamlet of Bovine on highway 41, and some 35 km west of Eagle (Rossell, 2008).

The Mellen complex near Ashland in northern Wisconsin (Fitz, 2011) contains a small amount of peridotite, but is mostly much more evolved, with much anorthositic gabbro and granite. The Mellen complex is also much larger and apparently somewhat later than the small, locally mineralized ultramafic bodies discussed here. Indeed, it may be the fourth-largest Keweenawan intrusive complex, after the Duluth complex, the Nipigon sills and the Coldwell complex (near Marathon on the north shore of Lake Superior).

Similar peridotitic intrusives occur in northwest Ontario south of Lake Nipigon, including the Seagull complex (Heggie, 2005; Middleton and Heggie, 2005); Current Lake intrusive complex (Goodgame et al., 2010); and Eva-Kitto intrusion (Laarman and Hollings, 2005). Economically significant occurrences or deposits, besides Eagle, have been identified at Seagull, Current Lake and Tamarack (further examples have long been known in the evidently later, larger bodies within the Duluth and Coldwell complexes, but these are a quite different style of host intrusion to the small bodies).

The Eagle nickel-copper deposit

The Eagle deposit was discovered in 2002, by a team employed by Kennecott Exploration Company of Salt Lake City, a wholly-owned subsidiary of London-based mining giant Rio Tinto. Government-sponsored academic research, geological survey reports, drill-core libraries and airborne geophysical surveys were important in recent discoveries in the region, including Eagle and Tamarack (Rossell, 2013) and Thunder Bay North. Finds of mineralized boulders both close (<1 km, Thunder Bay North: Goodgame et al., 2010) and distal (Eagle, >20 km: Rossell, 2013) encouraged explorers to persist in what, up to the 1990s, had been judged poorly-prospective terrain by most parties.

The Eagle peridotites display a trio of mineralogical features judged characteristic of rocks of similar inferred age and appearance across the Midcontinent Rift, namely: 1) abundant brown mica, a phlogopite or titanian biotite; 2) an intensely green chlorite (other chlorite species may also be present); and 3) anhedral, late magmatic overgrowths of a rich brown amphibole. At a minimum, these phases occur in ultramafites from Eagle, the BIC intrusion, Current Lake and Seagull (author's unpublished notes; see also Schandl, 2005).

Ware et al. (2008: see also Ding et al., 2008) quoted an age for the intrusion, based on the zirconium oxide mineral baddeleyite, from feldspathic peridotite and related rocks, of 1107.3±3.7 Ma, firmly within the early stages of the Midcontinent Rift. Dates on the region's smaller ultramafic bodies are as yet scarce and not of the highest precision, absent ideal crystals for U-Pb or Pb-Pb geochronology. However, they appear to all lie in the early phase of the rift, very roughly 1120 to 1105 Ma.

The July 2002 discovery of the Eagle deposit marked the culmination of more than 10 years' exploration by Kennecott in the early Proterozoic Baraga sedimentary basin (Rossell and Coombes, 2005). The discovery hole cut a remarkable 84.2 metres of massive sulphide averaging 6.3% Ni and 4.0% Cu. The eagle deposit occurs in the more westerly of two small intrusions known as the Yellow Dog peridotite. These bodies are not foliated and cut Penokean fabric, consistent with the inferred and later proven Keweenawan date. They are coarse peridotite and feldspathic peridotite, variably serpentinized, and "possible amygdules in the olivine poor phase(s) suggest a shallow level of intrusion" (Rossell and Coombes, 2005).

In 2007, the state of Michigan approved plans for an underground mine with decline access (Anon, 2007). The metric resources were then quoted at 3.6 MT grading 3.8% Ni, 3% Cu, 0.8 ppm Pt, 0.5 ppm Pd and 0.3 ppm Au: the tonnage was modest, but the base- metal grades stellar. By the time environmental permits were in place for the mine, the resource proved robust, adjusted to 3.6 MT with base-metal grades of 3.47% Ni and 2.93% Cu (Anon, 2010).

The mine is expected to enter production in late 2014. Rio Tinto has taken particular care with the environmental and social context of the project, the Rio Tinto Eagle mine. This is especially important because much of the Eagle ore is semi-massive sulphide, and thus has potential to develop acid mine drainage and attendant ecological threats, unless properly managed. Throughout the past two centuries, only a minority of mines in the three states abutting Lake Superior have worked sulphide orebodies: most of the mines have worked iron (oxides in banded iron formations) or copper (mostly native copper) ores, though there have been exceptions involving copper or copper-zinc sulphide, gold and other types of deposit. The mill construction has a workforce of some 350 contractors, and the payroll at mine and mill is expected to top 230 employees once mining commences.

On 13 June 2013, it was announced that Rio Tinto would sell the developing base-metal mine and associated Humboldt mill complex to Lundin Mining. The JORC-standard probable resource at Eagle is quoted at 5.2 million tonnes grading an average 2.93% nickel, 2.49% copper, plus 0.64 g/T platinum, 0.43 g/T palladium and 0.08% cobalt. Though the precious-metal grades are relatively modest, the high Pt/Pd ratio of almost 1.5 is consistent with other known mineralization in the Midcontinent Rift, and the PGE will be a significant byproduct of the high-grade Ni-Cu ore.

References

Anon (2007) Michigan approves Eagle mine. Northern Miner 93 no.45, 3, 31 December.

Anon (2010) Rio Tinto to build Eagle nickel mine. Northern Miner 96 no.21, 3,15, 12 July.

Ding,X, Ripley,EM and Li,C (2008) Geochemical and stable isotope studies of hydrothermal alteration associated with the Eagle deposit, northern Michigan. Abs. 54th Annual Meeting, Institute on Lake Superior Geology, vol.54 part 1, 80pp., 14-15, Marquette, MI.

Fitz,T (2011) Granitic, gabbroic, and ultramafic rocks of the Mellen intrusive complex in northern Wisconsin. Institute on Lake Superior Geology, volume 57 part 2, 184pp., trip 2, 163-184, Ashland, WI.

Goodgame,VR, Johnson,JR, MacTavish,AD, Stone,WE, Watkins,KP and Wilson,GC (2010) The Thunder Bay North deposit: chonolith hosted Pt-Pd-Cu-Ni mineralization related to the Midcontinent Rift. Abs. 11th International Platinum Symposium, Sudbury, 4pp.

Heggie,G (2005) Whole Rock Geochemistry, Mineral Chemistry, Petrology and Pt, Pd Mineralization of the Seagull Intrusion, Northwestern Ontario. MSc Thesis, Lakehead University, 364pp.

Klasner,JS, Snider,DW, Cannon,WF and Slack,JF (1979) The Yellow Dog peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. Michigan Department of Natural Resources, Report of Investigation 24, 31pp.

Laarman,J and Hollings,P (2005) Petrogenesis and PGE mineralization of the Eva-Kitto intrusion, northern Ontario. Abs. 51st Annual Meeting, Institute on Lake Superior Geology, vol. 51 part 1, 70pp., 32-33, Nipigon, Ontario.

Middleton,RS and Heggie,G (2005) Seagull intrusion, Ontario: a unique PGE-Ni-Cu setting. Abs. 51st Annual Meeting, Institute on Lake Superior Geology, vol. 51 part 1, 70pp., 47, Nipigon, Ontario.

Rossell,D (2008) Geology of the Keweenawan BIC intrusion. Institute on Lake Superior Geology, volume 54 part 2, 199pp., trip 7, 181-199, Marquette, MI.

Rossell,D (2013) The discovery of the Eagle Ni-Cu deposit and the Tamarack Ni-Cu-PGE prospect: examples of the benefits of public funded geo-science projects. In "Ni-Cu-PGE deposits in mafic-ultramafic rocks: insights and new discoveries", PDAC short course, Toronto, 381-398.

Rossell,DM and Coombes,S (2005) The geology of the Eagle nickel-copper deposit: Marquette county, Michigan. Abs. 51st Annual Meeting, Institute on Lake Superior Geology, vol. 51 part 1, 70pp., 54, Nipigon, Ontario.

Schandl,ES (2005) Petrographic data from the west-central Nipigon embayment, Lake Nipigon Region Geoscience Initiative. OGS MRD 156, 402pp. on 1 CD-ROM.

Ware,A, Cherry,J and Ding,X (2008) Geology of the Eagle project. Institute on Lake Superior Geology, volume 54 part 2, 199pp., trip 4 and trip 8, 87-114, Marquette, MI.