"Rock of the Month # 222, posted for December 2019" ---

Geochemical exploration for mineral deposits typically involves bulk analysis of sample types such as rock chips, soils or lake sediments. Generally the samples are analysed first for the element of interest (e.g., gold, copper, zinc and lead) and often also for associated elements (e.g., cadmium and lead with zinc; arsenic, antimony, tungsten or tellurium with gold...). In some cases, it makes sense to target the presence, and characteristics, of mineral grains derived from the orebody, and transported from the source by rivers or glaciers. A mineral species that can be detected far from its source is referred to as an "indicator mineral". A good indictor mineral will be tough, resistant to mechanical weathering, and so potentially found far from source, leaving a train of grains that can be traced back to its origin. Examples include gahnite (zinc-rich spinel) that may occur in metamorphosed zinc deposits, and certain kinds of garnet, found in the kimberlite bodies that may contain diamonds.

There has been much work on diamond indicator minerals, developed in tandem in South Africa and Russia, and later employed worldwide.

Kimberlites in Michigan

Quite some time ago, Bill Cannon and Mike Mudrey, experts on the geology of the American Great Lakes states, suggested that diamonds found in glacial drift across northern Michigan and Wisconsin could come from local kimberlitic rocks (Cannon and Mudrey, 1981). The Precambrian shield rocks ("cratons") of North America had stubbornly clung to any diamond endowment they might have, for a century since the great Kimberley finds in South Africa, and earlier finds in the rivers of Brazil and India. Still, Cannon and Mudrey were adding their voices to a number of others who, more or less quietly, were pondering the problem. Two early voices in particular should be noted: W.H. Hobbs (1899) and Archibald Blue (1900), who each had performed an analysis of the distribution of reported diamonds in Ontario and the Great Lakes states.

An early modern study of the Lake Ellen kimberlite (McGee and Hearn, 1984; McGee, 1986, 1988) noted that the Lake Ellen kimberlite contains garnet, pyroxene and ilmenite megacrysts, as well as lithic inclusions (xenoliths). There are xenocrysts and megacrysts of ilmenite (abundant, with 12.5-19% MgO), pyrope-almandine and Cr pyrope garnets (with up to 9.3% Cr₂O₃), Cr diopside (up to 4.5% Cr₂O₃), olivine (Fo₉₁), plus enstatite and phlogopite mica. Small eclogite nodules up to 3 cm in size contain jadeitic clinopyroxene with up to 8.4% Na₂O, as well as garnet, rutile, kyanite, sanidine K-feldspar and sulphide. The rocks provided evidence of the sampling, by the kimberlite, of heterogeneous upper mantle, but at that time no diamonds had been found.

The Lake Ellen kimberlite is northeast of Crystal Falls in the "UP" (Upper Peninsula) of northern Michigan. The Precambrian shield in Michigan is composed of two sutured Archean terranes on which were deposited lower Proterozoic passive margin and rift basin assemblages. The kimberlites intrude a Precambrian dome flanked by Paleozoic sediments, and display a structural control. Glacial till sampling and magnetic surveys account for all discoveries other than the initial outcrop find (Hoskin and Jarvis, 1993).

Beyond the actual kimberlitic bodies, which are recessive and so hard to find (as implied above: only one has been found in outcrop), the largest diamond found in the region was the 21.5 carat Theresa diamond, discovered by farmers in southeast Wisconsin in the nineteenth century. The first local kimberlite was discovered near Lake Ellen in 1968-1971. Some 25 kimberlites have been found since 1971 in the Upper Peninsula of Michigan and Wisconsin, most since 1983 (Anon, 1994, 1995). Jarvis and Kalliokoski (1988) examined 7 kimberlites in the state, all thought to be of Mesozoic, probably Jurassic, age. The kimberlites, Lake Ellen included, were not judged likely be be economically diamondiferous, and unlikely to be the source for large float (drift) diamonds found in Wisconsin.

The field trip guide of Carlson and Floodstrand (1994) advances the story. In the Lake Ellen area, the kimberlites are post-Ordovician intrusions, 1 to 20 acres (0.4 to 8.1 ha) in area, following a crude northwest trend through Iron, Dickinson and Menominee counties. This month's sample of the Lake Ellen kimberlite was exposed in a small pit. The Lake Ellen (1971) find is identified as a diatreme-facies kimberlite circa 8 ha in size. As seen in Figure 1, heavy mineral concentrates may easily be collected in the 20x12x5 m pit. Lake Ellen has a relatively high eclogitic garnet population. Exploration in the 1990s saw extensive sampling, with sample preparation at a laboratory in Crystal Falls, so that select heavy mineral grains could be picked and analysed, for comparison with known mantle rocks and kimberlites from elsewhere in the world. One larger body, a diamondiferous ultramafic lamprophyre, circa 50 acres (20 ha) in area, was identified within the limits of Kenosha, Wisconsin (Carlson and Adams, 1997). Small diamonds were found. The ilmenite chemistry is consistent with lamprophyre rather than kimberlite. During the mid-90s, it was established that diamonds occur in many of the Michigan kimberlites, but not, apparently, in economic quantities.

On a more technical note, Schulze and Hearn (2015) described a kimberlite dyke in southwest Pennsylvania. Some spinel has unusually high silica content, up to 0.59 wt.% SiO₂. They compared this to spinel xenocrysts from Lake Ellen and two localities in Wyoming, in a paper dedicated to the late spinel (chromite) expert, Queen's University professor Peter L. Roeder. Peculiar minor ultramafic bodies (dykes and sills, typically) have been reported elsewhere in central to eastern North America, including in upstate New York and nearby southeast Ontario.

Brief historical notes

According to Hobbs (1899), the first scientific reference to diamonds in the region was made by Kunz (1885). There had been a reported find of diamonds near Waukesha, Wisconsin in 1883. As of 1899, 17 macroscopic diamonds had been found in the Great Lakes region. The largest old find weighed 21.25 carats. This was the pale yellow Kohlsville diamond, 20x13x10 mm, found in kettle moraine on a farm in Wisconsin, and unearthed by a farmer while ploughing a field in the spring of 1886. The stones are concentrated near the moraines of the outer extent of the second great ice advance. Hobbs noted that "the material from which the diamonds were derived must clearly have been to the northward beyond the lakes, in the wilderness of Canada" (ibid., p.384). Blue (1900) noted that a range of peculiar reduced carbonaceous rocks had been found in Ontario, from graphite at the Silver Islet silver mine near Thunder Bay to anthraxolite at Balfour township, Sudbury. He ascribed more weight to the power of contact metamorphism than might a modern experimental petrologist, but from the time of Hobbs and Blue onwards, a few folk kept in mind the possibility of a diamond source in northern Ontario, Quebec, or Labrador. Diamonds were noted in south-central Indiana, and renowned gemmologist George F. Kunz collected rocks in the drift and forwarded them to Canadian experts for possible provenance (W.G. Miller, F.D. Adams et al.). Some of the float included jasper and jaspilite, as noted in the iron ranges north of Lake Superior, while Huronian quartzite was also a common constituent (Anon, 1906). Bell (1906) noted the occurrence of diamonds in glacial drift in the U.S.A., and possible provenance in Canada. The diamonds had been noted in a northwesterly trend roughly 600 miles (1000 km) long, from southwest Ohio through Indiana, Michigan and Wisconsin to Ontario. Early work on glacial striae (ice flow directions) led to speculation that the source was in the Labrador peninsula, but Bell disagreed since he noted that the striae were formed at different times, and that older tracks have been obliterated by more recent glacial action. The possibility of diamonds from sources in Ontario was firmly implanted (e.g., Sterrett, 1907, pp.1217-1226).

As the years passed, so the regional geology of Ontario was gradually discerned by geological survey and other geologists. Collins et al. (1926) described sedimentary iron ores near Wawa, at the east end of Lake Superior. Their work included reference to occurrences of altered ultramafic rocks that might be lamprophyre, or even kimberlite (ibid., pp.31-31), within the drainage of the Magpie river, from which, some six decades later, diamonds would be recovered, leading to a major staking and exploration rush that affirmed the presence of unusual Archean diamondiferous lamprophyres with numerous microdiamonds (but, once again, no immediate prospect of economic production). Kunz (1931) noted that the first bedrock discovery of in situ diamonds within host rock, in North America, was at Murfreesboro, Arkansas in 1905 (an unusual source, in a body of lamproite, now enshrined in the "Crater of Diamonds" park). Kunz also reported recovery of diamonds from placer gold workings along streams and rivers. He subscribed to a Canadian source. He noted that, about ten years earlier, a Canadian jeweller brought to him a rough broken diamond, just over 33 carats (6.6 grams), of little value as a gem but unusually interesting... "it was found in the digging of a railroad cut between Ottawa and Toronto". This was the mysterious "Peterborough diamond", origins unknown, but it may be noted that the Peterborough area is famous (at least amongst geomorphologists) as a playground of diverse glacial depositional landforms, especially drumlins and eskers. It is beyond the scope of this note to add a fuller history, but, a good thriller's-worth of adventures later, diamonds were indeed found in quantity, at source, in the James Bay Lowlands of Ontario, 90 km west of Attawapiskat (the Victor mine, Fowler et al., 2001) and in the Otish Mountains of central Quebec (Renard, where early mining plans, over a mine life of at least 11 years, at a mining rate of 6,000 tonnes of ore daily, factored in average annual diamond production of 1.7 million carats: Anon, 2011). As a footnote, those microdiamonds near Wawa showed that the diamond community had more to learn about the origins and occurrence of these storied stones (e.g., Sage, 2000). Sage describes the Sandor occurrence, an unusual dyke beside the east side of the Trans-Canada Highway north of Wawa, displays ultramafic lamprophyre dykes: the first Ontario occurrence of diamond in a non-kimberlite host, and the first Canadian discovery of diamonds in rock of Archean age.

References

Anon (1906) Diamonds. Can.Min.J. 27, 38.

Anon (1994) Crystal Exploration Inc.: continuing to evaluate diamond potential of Michigan's Upper Peninsula and in Wisconsin. Skillings Mining Review 84 no.1, 4-6 (12 November).

Anon (1995) Crystal Exploration Inc. - continuing to evaluate diamond potential of Michigan's Upper Peninsula and in Wisconsin. Skillings Mining Review 84 no.1, 5-6 (07 January).

Anon (2011) Thumbs-up for Stornoway's Renard project. Northern Miner 97 no.41, 20, 28 November.

Bell,R (1906) The occurrence of diamonds in the drift of some of the northern states. CIM Trans. (J.Can.Mining Institute) 9, 124-127.

Blue,A (1900) Are there diamonds in Ontario? OBM Rep. 9, 119-124.

Cannon,WF and Mudrey,MG (1981) The potential for diamond-bearing kimberlite in Northern Michigan and Wisconsin. USGS Circ. 842, 15pp.

Carlson,SM and Adams,GW (1997) The diamondiferous Six-Pak ultramafic lamprophyre diatreme, Kenosha, Wisconsin, U.S.A. Abs. 43rd Institute on Lake Superior Geology, vol. 43 part 1, 11-12, Sudbury.

Carlson,SM and Floodstrand,W (1994) Michigan Kimberlites and Diamond Exploration Techniques. Institute on Lake Superior Geology, vol.40, part 4, 15pp.

Collins,WH, Quirke,TT and Thomson,E (1926) Michipicoten Iron Ranges. GSC Memoir 147, 175pp. plus 2 maps.

Fowler,JA, Grutter,HS, Kong,JM and Wood,BD (2001) Diamond exploration in northern Ontario with reference to the Victor kimberlite, near Attawapiskat. Explor.Min.Geol. 10, 67-75.

Hobbs,WH (1899) The diamond field of the Great Lakes. J.Geol. 7, 375-388.

Hoskin,DJ and Jarvis,W (1993) Kimberlites in Michigan. In `Mid-Continent Diamonds' (Dunne,KPE and Grant,B editors), GAC Mineral Deposits Division, 160pp., 105-106.

Jarvis,W and Kalliokoski,J (1988) Michigan kimberlite province. Abs. 34th Annual Meeting, Institute on Lake Superior Geology, vol.34, part 1, 120pp., 46-48, Marquette, MI.

Kunz,GF (1931) Diamonds in North America. BGSA 42, 221-222.

McGee,ES (1986) Potential for diamonds in kimberlites from Michigan and Montana as indicated by garnet xenocryst compositions. GSA Abs.w.Progs. 18 no.6, 689, 99th Annual Meeting, San Antonio.

McGee,ES (1988) Potential for diamond in kimberlites from Michigan and Montana as indicated by garnet xenocryst compositions. Econ.Geol. 83, 428-432.

McGee,ES and Hearn,BC (1984) The Lake Ellen kimberlite, Michigan, U.S.A. In `Kimberlites I: Kimberlites and Related Rocks' (Kornprobst,J, editor), Elsevier, 466pp., 143-154.

Sage,RP (2000) The `Sandor' Diamond Occurrence, Michipicoten Greenstone Belt, Wawa, Ontario: a Preliminary Study. OGS OFR 6016, 49pp. plus 2 separate figures.

Schulze,DJ and Hearn,BC (2015) Mantle xenocrysts from the Masontown, Pennsylvania kimberlite: an ordinary mantle with Si-enriched spinel. Can.Mineral. 53, 767-773.

Sterrett,DB (1907) Precious stones. USGS Mineral Resources of the United States 23, for the calendar year 1906, 1213-1252.