Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

2002 ◽  
Vol 107 (B8) ◽  
Author(s):  
Stephen A. Prevec
Keyword(s):  
Genetic Model ◽  
Thermal Evolution ◽  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex

On the depth of crater excavation and melting during the Sudbury impact: geochemical evidence from chilled impact melt dykes

2020 ◽  
Author(s):  
Alexander Kawohl ◽  
Hartwig E. Frimmel ◽  
Wesley E. Whymark ◽  
Andrejs Bite
Keyword(s):  
Continental Crust ◽  
Bulk Composition ◽  
Geochemical Data ◽  
Tectonic Deformation ◽  
Target Area ◽  
Upper Crust ◽  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex ◽  
The Impact

<p>The 1.85 Ga Sudbury Igneous Complex, Canada, is the remnant of a ~3 km thick impact-generated crustal melt sheet, caused by a 10-15 km large chondritic asteroid or comet that had left behind an impact structure of ~200 km prior to tectonic deformation und subsequent erosion. However, less is known about how deep the impactor penetrated the continental crust and where the source of the impact melt was. Mixing models including radioisotopes and trace elements on locally exposed country rocks have been used to evaluate their relative contribution to the impact melt. Based on this, Darling et al. (2010) have argued for shallow melting of the upper crust (UCC) only, either due to an oblique impact and/or a low-density bolide (comet). In contrast, the abundance of siderophile elements in impact melt-rocks was taken as evidence of a lower crustal source (Mungall et al. 2004), i.e. overlying rocks of the middle and upper crust must have been removed during the crater excavation stage. U-Pb age data on zircon xenocrysts also point to the involvement of rock types not exposed on surface (Petrus et al. 2016) in agreement with theoretical simulations, which have predicted a >20 km deep but unstable transient cavity (Ivanov & Deutsch 1999).</p><p>Large-scale (10s of km) and well-exposed impact melt dykes are a unique feature of Sudbury. The dykes are of granodioritic/quartz dioritic composition and are interpreted as clast-laden melt injections into the basement instantaneously after the impact (Pilles et al. 2018). Their vitric margins and distal extremities should therefore approximate the undifferentiated bulk composition of the Sudbury Igneous Complex prior to sulfide saturation. A compilation of published and new geochemical data of these dykes reveal a remarkably strong affinity (r<sup>2</sup> >0.989) to the average middle continental crust (MCC) as given by Rudnick & Gao (2014), especially in terms of major elements and fluid-immobile transition metals (Th, Zr, Hf, Nb, Ta, Ti, Sc, REE). The dykes are, however, significantly enriched in Ni, Cu and Cr, and to a lesser extent in V, Co and P relative to the typical UCC and MCC. A systematic loss of volatiles (Tl, Cd, Sn, Zn, Pb, Ag, Cs, Rb, Na, K, Ga, As) compared to either crustal model is not evident. These new observations favour a scenario in which the impactor and supracrustal rocks in the target area became vaporized and ejected. Shock melting affected predominantly the middle crust of the Canadian Shield. We also propose that the rocks that contributed to the impact melt were, on average, more mafic than the typical UCC and MCC. This is consistent with the report of exotic mafic-ultramafic xenoliths within the Sudbury Igneous Complex (Wang et al. 2018) and its anomalously high PGE concentrations (Mungall et al. 2004). (Ultra-)mafic rocks hidden at mid-crustal depth were a likely source of Ni-Cu-PGE-Co and gave rise to world class ore deposits presently mined at Sudbury. Such (ultra-)mafic intrabasement body might also explain the 1200 km<sup>2</sup> Temagami magnetic anomaly in the eastern vicinity of the Sudbury Complex.</p>

The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet

2002 ◽  
Vol 97 (7) ◽  
pp. 1521-1540 ◽  
Author(s):  
A. M. Therriault ◽  
A. D. Fowler ◽  
R. A. F. Grieve
Keyword(s):  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex

New insights into the structure of the Sudbury Igneous Complex from downhole seismic studies

2002 ◽  
Vol 39 (6) ◽  
pp. 943-951 ◽  
Author(s):  
David Snyder ◽  
Gervais Perron ◽  
Karen Pflug ◽  
Kevin Stevens
Keyword(s):  
Seismic Data ◽  
Seismic Profile ◽  
Seismic Profiles ◽  
Igneous Complex ◽  
Impact Melt ◽  
Vertical Seismic Profiles ◽  
Sudbury Igneous Complex ◽  
Common Depth Point ◽  
Deformation Models ◽  
The Impact

New vertical seismic profiles from the northwest margin of the Sudbury impact structure provide details of structural geometries within the lower impact melt sheet (usually called the Sudbury Igneous Complex) and the sublayer norite layer. Vertical seismic profile sections and common depth point transformation images display several continuous reflections that correlate with faults and stratigraphic boundaries logged from drill cores. Of four possible mechanisms that explain repeated rock units, late-stage flow or normal faulting that occurred within the last layers to cool and crystallize might best explain the observations, especially the most prominent reflectors observed in the seismic data. These results reaffirm previously proposed two-stage cooling and deformation models for the impact melt sheet.

High-Grade Magmatic Platinum Group Element-Cu(-Ni) Sulfide Mineralization Associated with the Rathbun Offset Dike of the Sudbury Igneous Complex (Ontario, Canada)

2020 ◽  
Vol 115 (3) ◽  
pp. 505-525
Author(s):  
Alexander Kawohl ◽  
Wesley E. Whymark ◽  
Andrejs Bite ◽  
Hartwig E. Frimmel
Keyword(s):  
Base Metal ◽  
Precious Metal ◽  
Quartz Diorite ◽  
Metal Sulfides ◽  
High Grade ◽  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex ◽  
Base Metal Sulfides ◽  
Platinum Group

Abstract Quartz dioritic impact melt dikes around the 1.85 Ga Sudbury Igneous Complex, locally referred to as offset dikes, are well endowed with respect to Ni-Cu-platinum group elements (PGE). However, only those dikes proximal (<6 km) to the main mass of the Sudbury Complex are mineralized at an economic grade and, in places, host world-class deposits. We report on a new discovery of such heavily mineralized offset dike at Rathbun Lake, about 15 km east of the currently known extent of the Sudbury Igneous Complex. There, a segment of amphibole quartz diorite is exposed at the contact between Huronian metasedimentary rocks and gabbro of the 2.22 Ga Nipissing Suite, xenoliths of which are abundant throughout the diorite and record textural evidence of partial melting. The mafic inclusion-bearing quartz diorite is the host of the Rathbun Lake showing, a small but high-grade PGE-Cu(-Ni) sulfide occurrence of hitherto controversial origin. A detailed petrographic and mineralogical characterization of this occurrence revealed a two-stage mineralization history. Disseminated to semimassive (net-textured) chalcopyrite ± loop-textured pentlandite ± magnetite containing Pd-bismuthotellurides and, more rarely, sperrylite and native gold—all of which are closely associated with base metal sulfides—are interpreted as magmatic. The semimassive sulfide averages ~40 g/t Pd + Pt + Au at a Cu/(Cu + Ni) of >0.9 and a Pd/Ir of >100,000. Mineralogy, ore textures, and mantle-normalized PGE + Au patterns match a specific type of Cu-rich mineralization in the Sudbury Igneous Complex known as footwall mineralization. By analogy with these footwall deposits, the occurrence is interpreted as having formed by downward percolation of a highly fractionated sulfide melt toward the bottom of a now largely eroded offset dike. The magmatic paragenesis was hydrothermally overprinted at lower greenschist-facies conditions to pyrite-chalcopyrite-violarite ± covellite ± millerite. This involved also local remobilization into pyrite-chalcopyrite veinlets and the liberation of precious metal minerals from their sulfide hosts. In contrast to base metal sulfides, most precious metal minerals were resistant to hydrothermal alteration, although corrosion of some grains is noted as well as their truncation by chlorite and epidote. Micron-scale X-ray mapping revealed a progressive replacement of magmatic Pd-Bi-Te minerals, where in contact with hydrous silicates, by Sb- and Hg-bearing Pd minerals such as temagamite, Pd3HgTe3. The timing and nature of this hydrothermal overprint remains uncertain, but a connection to later regional metamorphism and faulting seems most plausible. Our finding of magmatic PGE-base metal sulfide at Rathbun Lake suggests a new subtype of distal offset dike-hosted mineralization in an area so far not known for offset dikes. It opens up new opportunities in the search for unconventional ore deposits around the Sudbury impact structure and improves our understanding on the distribution of impact melt-derived dikes around complex craters.

scholarly journals The Pele Offset Dykes, Sudbury impact structure, Canada

2018 ◽  
Vol 55 (3) ◽  
pp. 230-240 ◽  
Author(s):  
Eric A. Pilles ◽  
Gordon R. Osinski ◽  
Richard A.F. Grieve ◽  
Adam B. Coulter ◽  
David Smith ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Optical Microscopy ◽  
Electron Microprobe ◽  
The Other ◽  
Impact Structure ◽  
Igneous Complex ◽  
Impact Melt ◽  
Sudbury Igneous Complex ◽  
Geochemical Analyses ◽  
First Time

The Offset Dykes are impact melt-bearing dykes related to the 1.85 Ga Sudbury impact structure. Currently, the dykes extend radially outward from—or occur concentrically around—the Sudbury Igneous Complex, which is the remnant of a differentiated impact melt sheet and the source of the dykes. The recently identified three Pele Offset Dykes intrude into the Archean rocks north of the Sudbury Igneous Complex. In this study, the Pele dykes are characterized for the first time by a combination of fieldwork, optical microscopy, electron microprobe analyses, and bulk geochemical analyses. The Pele Offset Dykes stand out from the other Offset Dykes at Sudbury in two significant ways: (i) All other known Offset Dykes consist of an inclusion-rich lithology in the center of the dyke and an inclusion-poor lithology along the margins. The Pele dykes, however, are only composed of the inclusion-poor phase. (ii) The Pele dykes—particularly the Central and Eastern dykes—have a more evolved chemical composition relative to the other Offset Dykes. These observations suggest that the Pele dykes were emplaced after the other known Offset Dykes during two injection events: the Western followed by the Central and Eastern Pele dykes.

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