Sudbury Abstracts

A.Y. Jackson Lookout – Sudbury 2018-12

A view across the eroded and deformed Sudbury crater along Highway 144 from the south rim to the north rim.(Natural Resources Canada and Ontario Geological Survey)


2017 October – Trace element study

Hf isotope evidence for effective impact melt homogenisation at the Sudbury impact crater, Ontario, Canada


We report on the first zircon hafnium-oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets. The differentiated melt sheet at the 1.85 Ga, originally ca. 200 km in diameter Sudbury impact crater, Ontario, Canada, yields a tight range of uniform zircon Hf isotope compositions (εHf(1850) of ca. −9 to −12). This is consistent with its well-established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised. We propose that the heterogeneity in other isotopic systems, such as Pb, in early-emplaced impact melt at Sudbury is associated with volatility-related depletion during the impact cratering process. This depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post-impact assimilation of country rock, whereas the systems of more refractory elements preserve initial homogeneities. Zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks. However, they display a marked offset approximately one-third up the melt sheet stratigraphy that is interpreted to be a result of post-impact assimilation of 18O-enirched rocks into the base of the cooling impact melt.

Given that impact cratering was a more dominant process in the early history of the inner Solar System than it is today, and the possibility that impact melt sheets were sources of ex situ Hadean zircon grains, these findings may have significance for the interpretation of the early zircon Hf record. We speculate that apparent εHf-time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust.

We also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at Sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself.

2017 May – Volcanism evidence

Protracted volcanism after large impacts: Evidence from the Sudbury impact basin

Guyett, et al

Photomicrograph of a vesicular green shard from the Onaping Formation of the Sudbury impact basin. Image credit: Paul Guyett, Trinity College Dublin


Morphological studies of large impact structures on Mercury, Venus, Mars, and the Moon suggest that volcanism within impact craters may not be confined to the shock melting of target rocks. This possibility prompted reinvestigation of the 1.85 Ga subaqueous Sudbury impact structure, specifically its 1.5 km thick immediate basin fill (Onaping Formation). Historically, breccias of this formation were debated in the context of an endogenic versus an impact-fallback origin. New field, petrographic, and in situ geochemical data document an array of igneous features, including vitric shards, bombs, sheet-like intrusions, and peperites, preserved in exquisite textural detail. The geochemistry of vitric materials is affected by alteration, as expected for subaqueous magmatic products. Earlier studies proposed an overall andesitic chemistry for all magmatic products, sourced from the underlying impact melt sheet. The new data, however, suggest progressive involvement of an additional, more magnesian, and volatile-rich magma source with time. We propose a new working model in which only the lower part of the Onaping Formation was derived by explosive “melt-fuel-coolant interaction” when seawater flooded onto the impact melt sheet in the basin floor. By contrast, we suggest that the upper 1000 m were deposited during protracted submarine volcanism and sedimentary reworking. Magma was initially sourced from the impact melt sheet and up stratigraphy, from reservoirs at greater depth. It follows that volcanic deposits in large impact basins may be related to magmatism caused by the impact but not directly associated with the impact-generated melt sheet.

Plain Language Summary

Meteorite impacts are a key process in shaping the surface of the Earth and other planetary bodies. Recent studies on Mercury, Venus, Mars, and the Moon suggest that large impact structures are related to volcanism. We have studied the geological materials filling one of the largest and best preserved impact structures on Earth: the Sudbury impact structure in Ontario, Canada. Detailed investigation of the petrology and composition of the rocks has revealed that protracted, submarine, explosive volcanic activity took place after the impact. We propose that the volcanic activity was initially fed by crustal melts directly produced by the impact, but that with time, volcanic activity was progressively fed by magmas originating at deeper levels within the Earth.


  • The deep bowl was formed 1.85 billion years ago when a large meteorite exploded as it hit the Earth’s atmosphere.
  • The basin is not only filled with remnants of melted rock from the surface, but also contains a mixture of volcanic rock fragments.
  • Those fragments had altered with time.
  • Published in the Journal of Geophysical Research: Planets, the changes were not only caused by volcanic activity right after the meteorite impact, but also by volcanism caused by magma coming from deeper levels within the Earth later.
  • The finding suggests large impacts, such as those that took place during a short period after the Earth was formed, can be followed by intense, long-lived, and explosive volcanic eruptions.

2008 – precise dating of the impact

Sub-million-year age resolution of Precambrian igneous events by thermal extraction–thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet

Donald W. Davis


A new approach to zircon dating is described that potentially offers a considerable improvement in the accuracy of 207Pb/206Pb ages on a wider variety of samples and at less cost than that generally available from conventional techniques. Zircon is first preheated in a vacuum to evaporate Pb from altered domains, leaving predominantly Pb with isotopes that preserve the primary age of the sample. Refractory Pb is removed from the zircon by thermal extraction at higher temperature directly into a silica-melt ionization activator within a thermal ionization mass spectrometer. This produces strong ion beams that allow207Pb/206Pb isotope ratios to be measured to high precision (±10−4) with almost negligible contamination from common Pb. Isotope fractionation appears to be more reproducible than with conventional solution analysis, allowing routine age determinations with precision of ~±0.2 m.y. for the Precambrian. As a test application, zircon from a noritic boundary phase of the Sudbury impact melt gives 1849.53 ± 0.21 Ma, while a phase from several hundred meters higher in the noritic layer is resolvably younger at 1849.11 ± 0.19 Ma (95% confidence errors). The enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating.

2016 – Zircon formation – Chemostratigraphy

Differentiated impact melt sheets may be a potential source of Hadean detrital zircon

Gavin G. Kenny, Martin J. Whitehouse and Balz S. Kamber

Author Affiliations Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland Department of Geosciences, Swedish Museum of Natural History, 104 05 Stockholm, Sweden


Constraining the origin and history of very ancient detrital zircons has unique potential for furthering our knowledge of Earth’s very early crust and Hadean geodynamics. Previous applications of the Ti-in-zircon thermometer to >4 Ga zircons have identified a population with reltively low crystalliztion temperatures of ~685 °C. This could possibly indicate wet minimum-melting conditions producing granitic melts, implying very different Hadean terrestrial geology from that of other rocky planets. Here we report the first comprehensive ion microprobe study of zircons from a transect through the differentiated Sudbury impact melt sheet (Ontario, Canada). The new zircon Ti results and corresponding Tzirxtln fully overlap with those of the Hadean zircon population. Previous studies that measured Ti in impact melt sheet zircons did not find this wide range because they analyzed samples only from a restricted portion of the melt sheet and because they used laser ablation analyses that can overestimate true Ti content. It is important to note that internal differentiation of the impact melt is likely a prerequisite for the observed low Tzirxtln in zircons from the most evolved rocks. On Earth, melt sheet differentiation is strongest in subaqueous impact basins. Thus, not all Hadean detrital zircon with low Ti necessarily formed during melting at plate boundaries, but at least some could also have crystallized in melt sheets caused by intense meteorite bombardment of the early, hydrosphere-covered protocrust.

Article: In the summer of 2014, with the support of the Irish Research Council (IRC) and Science Foundation Ireland (SFI), the team collected thousands of zircons from the Sudbury impact crater, Ontario, Canada – the best preserved large impact crater on Earth and the planet’s second oldest confirmed crater at almost two billion years old.

After analysing these crystals at the Swedish Museum of Natural History in Stockholm, they discovered that the crystal compositions were indistinguishable from the ancient set.

Chemostratigraphy of the Sudbury impact basin fill: Volatile metal loss and post-impact evolution of a submarine impact basin

Edel M. O’Sullivana, Robbie Goodhuea, Doreen E. Amesb,Balz S. Kambera

Abstract The 1.85 Ga Sudbury structure provides a unique opportunity to study the sequence of events that occurred within a hydrothermally active subaqueous impact crater during the late stages of an impact and in its aftermath. Here we provide the first comprehensive chemostratigraphic study for the lower crater fill, represented by the ca. 1.4 km thick Onaping Formation. Carefully hand-picked ash-sized matrix of 81 samples was analysed for major elements, full trace elements and C isotopes. In most general terms, the composition of the clast-free matrix resembles that of the underlying melt sheet. However, many elements show interesting chemostratigraphies. The high field strength element evolution clearly indicates that the crater rim remained intact during the deposition of the entire Onaping Formation, collapsing only at the transition to the overlying Onwatin Formation. An interesting feature is that several volatile metals (e.g., Pb, Sb) are depleted by >90% in the lower Onaping Formation, suggesting that the impact resulted in a net loss of at least some volatile species, supporting the idea of “impact erosion,” whereby volatile elements were vaporised and lost to space during impact. Reduced C contents in the lower Onaping Formation are low (<0.1 wt%) but increase to 0.5–1 wt% up stratigraphy, where δ13C becomes constant at −31‰, indicating a biogenic origin. Elevated Y/Ho and U/Th require that the ash interacted with saline water, most likely seawater. Redox-sensitive trace metal chemostratigraphies (e.g., V and Mo) suggest that the basin was anoxic and possibly euxinic and became inhabited by plankton, whose rain-down led to a reservoir effect in certain elements (e.g., Mo). This lasted until the crater rim was breached, the influx of fresh seawater promoting renewed productivity. If the Sudbury basin is used as an analogue for the Hadean and Eoarchaean Earth, our findings suggest that hydrothermal systems, capable of producing volcanogenic massive sulphides, could develop within the rims of large to giant impact structures. These hydrothermal systems did not require mid-ocean ridges and implicitly, the operation of plate tectonics. Regardless of hydrothermal input, enclosed submarine impact basins also provided diverse isolated environments (potential future oases) for the establishment of life.

Sudbury Post Impact:

Stage 1: Slumping of initial crater rim created a multi-rim structure containing an impact melt pool. Backwash of tsunami-like waves triggered by the impact washed over the sides of the crater, interacting explosively with the melt sheet. Fragmented melt cooled and solidified to form the Sandcherry Member.

Stage 2: The Sandcherry Member covered the cooling melt sheet, as vents continued to feed melt to the surface of the fill. Water filled the crater during a catastrophic collapse of the Sandcherry Member and lower Dowling Member, with which the melt continued to interact, forming the contact and lower units of the Dowling Member. Activity diminished over time, leading to a decrease in vitric shard size up section. Argillitic muds were deposited in the outer reaches of the structure, and transported by debris flows to the lower Dowling Member as the crater rim began to erode. Life began to colonise the surface waters, eventually causing anoxia in the deeper water column. Organic matter settled through the water column, scavenging many particle-reactive elements including Mo, V, Cr, and Co, causing a reservoir effect.

Stage 3: (A) Continued volcanism constructed the Middle and Upper Dowling Member. The final breach of the crater rim at the NE and SW apices allowed seawater to surge over the rim, replenishing the water column with transition metals and sulphate. Hydrothermal deposition was temporarily re-established, forming the Vermilion Formation VMS deposits. (B) Volcanism eventually ceased, and communication with the open ocean allowed the transport of sediment into the basin, constructing the Onwatin Formation.

Comet craters – literal melting pots for life on Earth

Representative samples across the basin fill were analysed for their chemistry and for carbon isotopes, and they revealed an interesting sequence of events.

The first thing that became evident was that the crater was filled with seawater at an early stage, and remained sub-marine throughout deposition. Importantly, the water in the basin was isolated from the open ocean for long enough to deposit more than 1.5 km of volcanic rock and sediment.

The lower fill is made up of rocks that formed when the water entered the crater whose floor was covered by hot impact melt. Fuel-coolant reactions deposited volcanic rocks and promoted hydrothermal activity. Above these deposits, reduced carbon starts to appear within the basin fill and the volcanic products become more basaltic.

Previously the puzzling presence of carbon in these rocks was explained by washing in from outside the crater basin. However, the new data show that it was microbial life within the crater basin that was responsible for the build-up of carbon and also for the depletion in vital nutrients, such as sulphate.

“There is clear evidence for exhaustion of molybdenum in the water column, and this strongly indicates a closed environment, shut off from the surrounding ocean,” added Edel O’Sullivan.

Only after the crater walls eventually collapsed did the study show replenishment of nutrients from the surrounding sea. These sub-marine, isolated impact basins, which experienced basaltic volcanism and were equipped with their own hydrothermal systems, thus present a new pathway to synthesis and concentration of the stepping stones to life.

Research paper: Chemostratigraphy of the Sudbury impact basin fill: Volatile metal loss and post-impact evolution of a submarine impact basin

2015 – Onaping Intrusion – Distal Ejecta – Impact Melt

Electron microprobe analysis of Sudbury breccia from the Creighton and
Coleman mines, Sudbury Ontario: Constraints on the extent of hydrothermal modification of magmatic footwall Ni-Cu-PGE sulphide mineralisation.
J.W. O’Callaghan, G.R. Osinski, R.L. Linnen, P. C. Lightfoot
The Sudbury Igneous Complex (SIC) represents the remains of a ~200-km diameter impact crater, formed 1.85 billion years ago. Ore deposits associated with the impact structure can be subdivided into contact, footwall and offset types, and represent the end products of magmatic and hydrothermal processes. Footwall Ni-Cu-PGE deposits are hosted within zones of Sudbury breccias (pseudotachylitic), near to contact ore deposits at the margin of the SIC. Our study focuses on the application of ICP-MS and electron microprobe analyses of a suite of Sudbury breccia samples collected proximal to active mines in the North and South Ranges of the Sudbury Basin. We investigate subtle geochemical variations to mineralogy in barren and mineralised breccia matrix from different lithologies, and use this information to constrain the relative roles of magmatic and hydrothermal processes in the modification of primary Sudbury breccia and better understand the footwall ore deposits.
Our research has identified ilmenite-magnetite with titanite rims within breccia <10cm from footwall mineralisation in the McCreedy East 153 orebody at Coleman Mine. This assemblage is consistent with both high ƒO2 and high ƒH2O conditions. Samples from near the footwall orebodies at Creighton Mine contain heterogenous titanite that may be associated with later metamorphic alteration. In Sudbury breccia samples from Creighton mine, we also observe evidence of partial assimilation of host granite in hand-specimen and have identified two species of titanite, one showing Al/Fe substitution indicative of the presence of F-OH at the time of formation, and a second low Ti, low Al/Fe variety that may have been derived from the host granite. For the first time we also report chlorine zonation in amphibole at Creighton mine, similar to zonation features reported in the Fraser mine in the North Ranges.


The Basal Onaping Intrusion in the North Range: Roof rocks of the Sudbury Igneous Complex

Denise ANDERS, Gordon R. OSINSKI, Richard A. F. GRIEVE, and Derek T. M. BRILLINGER Meteoritics & Planetary Science 50, Nr 9, 1577–1594 (2015)


The 1.85 Ga Sudbury impact structure is one of the largest impact structures on Earth. Igneous bodies—the so-called “Basal Onaping Intrusion”—occur at the contact between the Sudbury Igneous Complex (SIC) and the overlying Onaping Formation and occupy ~50% of this contact zone. The Basal Onaping Intrusion is presently considered part of the Onaping Formation, which is a complex series of breccias. Here, we present petrological and geochemical data from two drill cores and field data from the North Range of the Sudbury structure, which suggests that the Basal Onaping Intrusion is not part of the Onaping Formation. Our observations indicate that the Basal Onaping Intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components. Increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the Basal Onaping Intrusion. Planar deformation features within quartz clasts of the Basal Onaping Intrusion are indicators for shock metamorphism and, together with the melt matrix, point to the Basal Onaping Intrusion as being an impact melt rock, by definition. Importantly, the contact between Granophyre of the SIC and Basal Onaping Intrusion is transitional and we suggest that the Basal Onaping Intrusion is what remains of the roof rocks of the SIC and, thus, is a unit of the SIC and not the Onaping Formation.

Discovery of distal ejecta from the 1850 Ma Sudbury impact event

William D. Addison, Gregory R. Brumpton, Daniela A. Vallini, Neal J. McNaughton, Don W. Davis, Stephen A. Kissin, Philip W. Fralick, Anne L. Hammond GEOLOGY, March 2005

Sudbury Impact Distal Ejecta at Hillcrest Park, Thunder Bay Ontario – 2013.

ABSTRACT A 25–70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca. 1878 Ma and 1836 Ma. The Sudbury impact event, which occurred 650–875 km to the east at 1850 ± 1 Ma, is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features, are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features.


J. R. Weirich, G. R. Osinski, A. Pentek, J. Bailey 45th Lunar and Planetary Science Conference (2014)

Introduction: The Sudbury Structure has been interpreted to be a 200-260 km diameter impact crater that formed at about 1.85 Ga. The 60 by 27 km Sudbury Igneous Complex (SIC) is the deformed and eroded remnant of the original impact melt sheet. Sudbury Breccia (SB) is an ubiquitous rock type found exterior to the SIC.

SUDBURY DISTAL EJECTA (fallout from the Sudbury impact)

The Sudbury impact produced a red hot rain of glowing glass and shattered melted rock, that fell on the Thunder Bay area 1.85 billion years ago. It formed a layer over a metre thick of this hot debris. The last debris to fall was fine dust, and it took as long as a year to fall, scattering a dusty film right around the world. Some evidence of this has been discovered in the Thunder Bay area. The breccia is sandwiched between Gunflint Iron Formation and sedimentary strata of the Rove Formation. (Addison et al 2005 & 2010).

The Gunflint chert (1.88 Ga[1]): is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age.

The Rove Formation: is located in the upper northeastern part of Cook County, Minnesota, United States, and extends into Ontario, Canada. It is the youngest of the many Animikie layers, a layer of sedimentary rocks. Before the Rove sediments were laid down, during the Archean Eon, the Algoman orogeny added landmass along a border from South Dakota to the Lake Huron region; this boundary is the Great Lakes tectonic zone. Several million years later a thin layer of hypervelocity impact ejecta from the Sudbury impact event was deposited on the older, underlying, Gunflint Iron Formation, and the Rove was then deposited on top of the ejecta; it is estimated that at ground zero the earthquake generated by the meteor impact would have registered 10.2 on the Richter scale. During the Middle Precambrian a shallow inland sea covered much of the Lake Superior region and formed the Animikie Group, layers of sedimentary rocks overlying 2700-million-year-old Archean rocks. The Rove Formation is the youngest of the many Animikie layers.

The complex products of impact range from angular fragments of preexisting rocks and partially melted, recrystallized, or glassy fragments, to spherules that condense from vapor in the ejecta cloud (much like hail stones form in rain clouds). The shock wave produced by impact transports ejecta away from the site of impact at velocities of miles per second. On Earth the shock wave would produce giant tsunamis. The force of the currents on the bottom of shallow ocean basins would disrupt the layering and other features of sediments accumulating on the sea floor and probably even some of the sea floor itself. The layer of sediment that would accumulate after the tsunami had passed would be a very complex mixture of disrupted sediments and the ejecta material. Oxidation and hydration would further alter impact ejecta. At Hillcrest Park in Thunder Bay is a ten to twenty foot-thick layer over the Gunflint Iron Formation that fits the now accepted criteria for impact ejecta transported and deposited in a tsunami surge. This exposure had been described and dismissed by earlier geologists as a “chaotic mess” at the top of the Gunflint Iron Formation (Weiblen 2007).

On the track of the elusive Sudbury impact: geochemical evidence for a chondrite or comet bolide

Joseph A. Petrus, Doreen E. Ames andBalz S. Kamber Article first published online: 8 DEC 2014

Abstract Siderophile and lithophile trace element data for 69 samples from the Sudbury impact crater fill (Onaping Formation) and quartz diorite offset dikes help constrain the sources of the established moderately elevated platinum group element signature associated with the impact structure. The siderophile element distribution of the crater fill requires contributions from bulk continental crust, mafic rocks and a chondritic component. A mantle component is absent, but the involvement of mid to lower crust is implied. After considering post-impact hydrothermal alteration, melt heterogeneity and mafic target admixture, the projectile elemental ratios were determined on a more robust data subset. Chondrite discrimination diagrams of these ratios identify an ordinary or enstatite chondrite as the most probable source of meteoritic material in the Sudbury crater fill. However, the relative and absolute siderophile element distributions within the impact structure as well as bolide size models are congruent with the bolide being a comet that had a chondritic refractory component.

Differentiated Impact Melt Sheet

Ann M. Therriaut, Richard A F Grieve ARTICLE in ECONOMIC GEOLOGY 97(7):1521-1540 · OCTOBER 2002

Abstract The Sudbury structure, Ontario, is the remnant of a 1.85 Ga old impact crater, which originally had a diameter of 200 to 250 km. The Sudbury Igneous Complex occurs within the Sudbury structure. The Sudbury Igneous Complex is a 2.5- to 3.0-km-thick, similar to60- X 27-km elliptical igneous-rock body, which consists of four major lithologies (from top to bottom) traditionally termed “granophyre,” “quartz gabbro,” “norite,” and “contact sublayer” (sulfide- and inclusion-bearing noritic rock). with the exception of the latter, all these lithologies are continuous across the structure. Modal analyses reveal that, following the IUGS system of nomenclature, quartz gabbro samples are in fact quartz monzogabbros, a few of the norite samples are quartz gabbros, and most norite samples are quartz monzogabbros. In view of these observations, and in order to clarify the nomenclature, an updated terminology is proposed (from top to bottom): upper unit, middle unit, lower unit, and contact sublayer. The bulk composition of the Sudbury Igneous Complex, from North Range data, is granodioritic. Continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the Complex behaved as a single melt system. All the Sudbury Igneous Complex lithologies have the same light to heavy rare earth element (REE) ratio and an overall pattern of increased light REE and depleted heavy REE. The occurrence of primary hydrous minerals (homblende and biotite), deuteric alteration, and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in H2O Moreover, the granophyric and other far-from-equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase. The Sudbury Igneous Complex differs from traditional layered mafic complexes in the following aspects. It has an overall intermediate composition, a hydrous nature, a crustal isotopic signature, normative corundum, and an unusually large volume of granophyre. The Sudbury Complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical, and therefore, mineralogical layering. Reported here for the first time, and similar to those found in impact melt rocks elsewhere, are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts. The well-known ore deposits of the Sudbury region are directly related to the genesis of the Sudbury Igneous Complex. Some ores precipitated from the Sudbury melt, whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex. It is concluded that the Sudbury Igneous Complex is the best exposed and only well-documented, to date, terrestrial impact melt sheet to have differentiated.

2014 – Geology References

Sudbury Geology References

A Field Guide to the Geology of Sudbury, Ontario – 2009

The Geology and ore deposits of the Sudbury Structure – 1984

Bedrock Geology of the Regional Municipality of Sudbury. Map Series no P3187 – 1998


The Sudbury structure shows that large volumes of subsurface magma can be generated by impact. The Sudbury structure is a large (D ~200 km, 1850 Ma), deformed and eroded impact crater, whose central region was occupied by melt. An eight-year multidisciplinary study by Stoffler et al. (1994) concluded that the impact excavated deep into the crust, almost to the mantle (~30 km), before collapse and rebound. The present eccentric shape is due to subsequent tectonism. The melt (> ~12,000 km3), possibly superheated, formed by impact melting of crust within just a few minutes. The magma differentiated by gravity settling of crystals and immiscible sulfides to produce hundreds of metres of noritic cumulates (norite is a type of gabbro). Early formed pyroxene and sulfides were swept into basal depressions to form mineralised norite, overlain by slowly cooled igneous-textured rocks with differentiated compositions (Stoffler et al, 1994).

There is no record of volcanism at Sudbury but it may have been spectacular. The high temperatures implied by coexisting immiscible melts and mafic magmas are comparable to those of many large igneous intrusions, representing the mid- to upper-crustal reservoirs feeding surface volcanism. At Sudbury, the presence of pseudotachylites (veins of shock-induced glassy rock), contact zone breccias and an array of peripheral shock features is well established. Any mantle melt component is thought to have been small, but could have been delivered almost instantly via crust-spanning dykes with rapid post-crater closure (Price 2001).

Rapid closure of fractures may explain the absence of feeders in impact-induced melt bodies such as Sudbury. The Sudbury nickel deposits are crudely concentrated around the margins of the impact cavity and form the largest nickel mining district ever mined. The source of the nickel could be the impactor in terms of mass balance, although isotope data suggest a crustal source for the accompanying sulphur. In other impact craters, there is evidence for associated downwards and outwards injection of magma, forming dykes, breccias, and pseudotachylites, and for the establishment of vigorous hydrothermal systems. The Sudbury rootless impact melt, the likelihood of superheat, and the formation of immiscible sulphides are valuable lessons for mainstream igneous petrology and global ore prospecting. (Adrian P. Jones 2005)