Lapilli tuff

--- fallback from a massive impact event

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Figures 1-2. Polished faces (front and back) of two large slabs of a remarkable airfall deposit, the distal ejecta of the second-largest known impact event to strike our planet, some 1850 million years ago *. Samples from the Thunder Bay area of northwest Ontario, Canada. Collected from the old Highway 588 site, southwest of Stanley, south of Kakabeka Falls, where the ejecta deposit sits upon the Gunflint Formation. Colourful cherty breccia was exposed here in 2005, and the site was still informative in 2012 (Addison and Brumpton, 2012). This site has since been erased by highway construction. See Table 1, at base of page, for sample allocation. The rock contains a high concentration of dark, fine-grained, partially coalesced lapilli, some just 5-10 mm in diameter, the largest 25x20 mm.

* OK, third-largest, and the second-largest recorded in Earth's rocks. Current consensus seems to favour an old idea, namely that the largest impact on Earth occurred in our planet's infancy, with such force that the Moon was hurled out of the fray. Lucky for us that the planet did not disintegrate, or we would ("might") be living on an asteroid now! See quick chronology of our world, at Rock of the Month 212, Table 1.



"Rock of the Month #213, posted for March 2019" ---

Sudbury impact ejecta:

debris from the hypervelocity strike of a kilometres-wide asteroid on what is today the nickel (copper, platinum...) mining district that developed along with the city of Sudbury, Ontario. The Sudbury mineralization was discovered in the 1880s. Initial development was hampered by the lack of a process to separate nickel from copper. Once this metallurgical problem was resolved, Sudbury went on to become one of the greatest sources of nickel on Earth (other key districts are Noril'sk-Talnakh in Siberia and Kambalda in Western Australia).

The Sudbury district is home to the only great mineral deposits associated with a known impact structure. It is now believed that the incoming body, by chance, struck a region of the Southern province of the Canadian Shield that was already notably enriched in magmatic nickel-copper-(platinum) ores. The ores are all intimately related to great volumes of igneous rock, formed or remobilised at the time of the impact. The ores vary from disseminations of sulphide grains to thin but bonanza-grade (exceedingly rich) veins of massive sulphide (the footwall veins). This month's "Rock(s)" are an addendum to the earlier article on Sudbury ejecta samples.

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Figures 3-4. Views of a rough, broken face of the airfall deposit, and another slab of this distinctive grey, orange-weathering lithology.


Rock from the sky: the rain of "distal ejecta":

The recognition of the Sudbury impact structure, and the belated identification of its distal ejecta near the opposite end of Lake Superior, was dealt with in the earlier article on lapilli. The discoverers described their field evidence of ejecta from the 1850 Ma Sudbury impact, including a layer 25 to 70 cm thick, identified near the contact of the Gunflint iron formation and overlying Rove Formation in northwest Ontario, and between the Biwabik iron formation and overlying Virginia Formation in adjacent Minnesota, USA (Addison et al., 2005). The layer contains accretionary lapilli, shocked quartz and feldspar. Zircon crystals from nearby tuffaceous horizons (volcanic ash) could be dated, bracketing the ejecta deposits between 1878 and 1836 Ma (see also Fralick et al., 1998). The Sudbury event occurred close to 1850 Ma, some 650 to 875 km to the east. Planar deformation features in common minerals such as quartz are well preserved at some sites but destroyed at others, particularly by a process of carbonate (rusty orange ankerite) replacement in these ancient rocks. Subsequent field work led to recognition of distal ejecta deposits in Wisconsin and Michigan, and so in every state and province around Lake Superior. The ejecta include coarse lapillistone, such as the samples shown here, with spheroidal accretionary lapilli 1-15 mm or more in size. Other clues, in the absence of ejecta such as the lapilli, are sedimentary features such as contorted banded iron formation and megabreccia. The lapilli have been found around and in the city of Thunder Bay, and west to Kakabeka Falls, and have been seen to overlie older sediments with truncated stromatolites (Jirsa et al., 2011). A further survey of three sites in Michigan, Ontario and Minnesota, some 500 to 980 km from the impact, found that the ejecta layer thins away from the impact site, with thicknesses of circa 200, 100 and 20 cm, respectively at those locations. The proximal equivalent, in the Sudbury basin itself, would be the Onaping Formation (Huber et al., 2014).

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Figures 5-6. Breccia samples from surface, collected on the west side of the North Range, on the northwestern margin of the Sudbury district. Left: Cartier granite, beside Highway 144, south of the railway halt of Cartier. Right: Levack gneiss, off Highway 144, northwest of the junction at the north end of Windy Lake, west of Onaping. Samples provided by the Ontario Geological Survey. The astute observer will note that, apart from a few fragments (clasts) of granitic aspect, this impacted "granite" no longer looks like granite, nor is the "gneiss" a decent textbook example of a rock formed in regional metamorphism! Much of each sample is an invasion of dark, fine-grained melt, the breccia containing clasts of the various target rocks.


Meanwhile, back in Sudbury, "ground zero"...

The Sudbury mines continue to be profitable, after more than a century, with essential adaptations to more sustainable, environmentally means of mining and smelting. The general public can gain an appreciation for the geology and mining of the region at two superb exhibits in the region, Science North and Dynamic Earth.

Research continues apace, at Laurentian University in Sudbury, and elsewhere. Olaniyan et al. (2014) add a recent analysis to the structure and geophysical signature of the Sudbury basin and underlying igneous complex. Anders et al. (2015) describe the Basal Onaping Intrusion in the North Range, considered to be the "Upper Contact Unit" of the underlying Sudbury Igneous Complex, rather than the base of the overlying breccias of the Onaping Formation. Isotopic studies (McNamara et al., 2017) consider the rapid evolution of the magma system beneath the crater excavated by the impact. While large impactors appear to be vapourised, their remains spread worldwide as largely dust-fraction particles, at the impact site there would be mixing of melts and assimilation of crustal wall rocks. The development of discrete convection cells is envisaged in the melt sheet atop the region following the impact. At some point would come the collapse of the transient cavity formed in the impact. The igneous complex represents a broad chemical spectrum of melts, including norite, quartz gabbro and granophyre.

The world-class Sudbury ore deposits appear to have formed suddenly, but does not seem to represent the impacting body. At the famed Meteor Crater in Arizona, a tiny structure in comparison to Sudbury, some tonnes of iron meteorite have been recovered, the Canyon Diablo iron. However, despite historic exploration in the early 20th century, no larger body of nickel was ever found beneath the crater floor. Relatively small (circa 3 metres to 3 kilometres wide) craters such as Meteor Crater, Henbury (Western Australia) and Whitecourt (Alberta) may preserve iron meteorite fragments milligrams to hundreds of kilograms in size. Individual stony meteorites as large as 3-4 tonnes have survived to Earth's surface, and there are numerous examples of single iron meteorites 1 to 60 tonnes in mass. But larger impacts will tend to leave larger craters and - beyond a certain threshold of energy - the impactor itself will vanish in a cloud of gas and dust.

The Sudbury area lies along a line of older igneous bodies, the East Bull Lake Intrusive Suite, several of which contain apparently low-grade disseminated Ni-Cu-PGE mineralization. It may well be that such mineralization was amongst the large volumes of crustal material melted and remobilized by the impact. Given this sudden gift of energy, the processes known to form nickel (etc) sulphide deposits in the Earth's crust (Barnes et al., 2017), including separation of metal-bearing sulphide melts from silicate magmas, permitted formation of billions of tonnes of mineralization, much of which would, over the past 130 years, prove economic.

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Figures 7-8. Two more samples, from the McCreedy West mine. These pieces (sawn at the Royal Ontario Museum) reveal more of the "Sudbury story". Left: Sudbury breccia, with cm-scale granitoid clasts in a dark, fine-grained matrix, cut by discontinuous chalcopyrite-rich sulphide veinlets. This rock is thus mineralized, but is waste, nor ore. Right: ore from a vein of massive chalcopyrite, the only mineral visible to the naked eye. Such rock contains over 30 weight percent base metals (copper, and a little nickel) and remarkably high levels (tens of parts per million, which amounts to one or more ounces per tonne) of platinum and associated precious metals.


Acknowledgements

This material was kindly provided by the office of the Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, in Thunder Bay. Special thanks to Greg Paju for his generous and dedicated help with sample acquisition and preparation. See footnote to Table 1 for field collection personnel. As per February's rock, the Acasta gneiss, great job! The samples in Figs. 1-3 are being shipped to the Museu Joias da Natureza, together with pieces of impactites from the rim of the Sudbury basin (Cartier granite, Levack gneiss, Figs. 5-6) and smaller samples from the author's collection, of subeconomic mineralized breccia and massive sulphide footwall vein from the McCreedy West mine in Sudbury (slices related to the examples in Figs. 7-8). As a bonus, individual pieces and/or related samples of all these samples are being distributed to the Royal Ontario Museum, that assisted with sample preparation, and the OGS offices in Thunder Bay.

References (n=9; see also ROM 159 article and references).

Addison,WD and Brumpton,GR (2012) Sudbury impactoclastic debrisites at Thunder Bay. Institute on Lake Superior Geology, volume 58 part 2, 219pp., trip 1&13, 2-26, Thunder Bay, ON.

Addison,WD, Brumpton,GR, Vallini,DA, McNaughton,NJ, Davis,DW, Kissin,SA, Fralick,PW and Hammond,AL (2005) Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology 33 no.3, 193-196.

Anders,D, Osinski,GR, Grieve,RAF and Brillinger,DTM (2015) The Basal Onaping Intrusion in the North Range: roof rocks of the Sudbury Igneous Complex. Meteoritics & Planetary Science 50, 1577-1594.

Barnes,SJ, Holwell,DA and Le Valliant,M (2017) Magmatic sulfide ore deposits. Elements 13 no.2, 89-95, April.

Fralick,PW, Kissin,SA and Davis,DW (1998) The age and provenance of the Gunflint lapilli tuff. Abs. 44th Annual Meeting, Institute on Lake Superior Geology, vol. 44 part 1, 125pp., 66-67, Minneapolis, MN.

Huber,MS, McDonald,I and Koeberl,C (2014) Petrography and geochemistry of ejecta from the Sudbury impact event. Meteoritics & Planetary Science 49, 1749-1768.

Jirsa,MA, Fralick,PW, Weiblen,PW and Anderson,JLB (2011) Sudbury impact layer in the western Lake Superior region. In `Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America' (Miller,JD, Hudak,GJ, Wittkop,C and McLaughlin,PI, editors), Geol.Soc.Amer. Field Guide 24, 544pp., 147-169.

McNamara,GS, Lesher,CM and Kamber,BS (2017) New feldspar lead isotope and trace element evidence from the Sudbury igneous complex indicate a complex origin of associated Ni-Cu-PGE mineralization involving underlying country rocks. Econ.Geol. 112, 569-590.

Olaniyan,O, Smith,RS and Lafrance,B (2014) A constrained potential field data interpretation of the deep geometry of the Sudbury structure. Can.J. Earth Sci. 51, 715-729.


Table 1. The specimens

Table 1. Specimen data
Specimen Mass (g)Mag-sus (x10-3 SI units), n=3-5
Fig. 1-2-left lapilli 1948 ___0.28-0.32
Fig. 1-2-right lapilli 2100 ___0.28-0.32
Fig. 3- rough _994 ___0.15-0.18
Fig. 4- thick slab 4700 * ___0.26-0.34
Fig. 5-Cartier 2896 ___0.27-0.31
Fig. 6-Levack _903 ___2.98-4.39
Fig. 7-McCW breccia TBD ___ TBD
Fig. 8-McCW footwall vein TBD ___ TBD

Notes. Figs. 1-4 samples collected by Mark Puumala and Greg Paju, OGS, Thunder Bay. Figs. 5-6 samples collected by Shirley Peloquin and Clayton Kennedy, OGS, Sudbury. Figs. 7-8 samples collected by Graham Wilson of Turnstone GSL. Uncorrected magnetic susceptibility readings measured on SM-30 unit, on flattest available surfaces. The samples in Figs. 1-3 and 5-6 are for the Museu. Fig. 4: Turnstone collection. Figs. 7-8: Royal Ontario Museum collection. * Measured on weigh scale, ± 100 g, the others on Kern balance, quoted to ± 1 g.


Graham Wilson, 14 November 2018, 10-14 December 2018

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