Geological Association of Canada [GA]

 CC:701B  Wednesday  1030h

Terrestrial Impact Structures: Cratering Processes, Mineral Resource Deposits, and Environmental Consequences II

Presiding:  U Riller, McMaster University; G Osinski, University of Western Ontario; R A Grieve, NRCan


Origin of Pseudotachylite Matrix in Target Rock of the Sudbury Impact Structure, Ontario, Inferred From Geochemical Analysis

* Al Barazi, S (, Freie Universitaet Berlin, Institut fuer Geologische Wissensachaften, Malteserstraße 74- 100, Berlin, 12249, Germany
* Al Barazi, S (, Museum für Naturkunde Berlin, Invalidenstraße 43, Berlin, 10115, Germany
Riller, U (, McMaster University, School of Geography and Earth Sciences, 1280 Main Street West, Hamilton, ONT L8S 4K1, Canada
Hecht, L (, Freie Universitaet Berlin, Institut fuer Geologische Wissensachaften, Malteserstraße 74- 100, Berlin, 12249, Germany
Hecht, L (, Museum für Naturkunde Berlin, Invalidenstraße 43, Berlin, 10115, Germany

Archean and Paleo-Proterozoic target rocks of the 1.85 Ga Sudbury Impact Structure host fragment-rich pseudotachylite (locally known as Sudbury Breccia). The pseudotachylites vary from millimetre-wide veins to hundreds of meter wide fragment-rich zones and are often associated with lithological contacts and faults. In general, the pseudotachylitic matrix is fine-grained to aphanitic and hosts sub- to well- rounded lithic fragments derived from the immediate host rock. The origin of the matrix as well as the mechanism of pseudotachylite formation in large impact structures is debated. Based on previous geochemical analyses, the pseudotachylitic matrix has been interpreted mostly in terms of in-situ, cataclastic milling or friction-induced melting of the host rocks, despite the notion that the matrix cannot be solely derived by melting of the host rock. Our study aims to elucidate to what extent pseudotachylitic matrix formed from the immediate host rock or other sources of melt. Pairs of pseudotachylite matrix and immediate host rock samples from various target rock types were analysed for major and trace element contents. Whole rock chemical analyses were complemented with microprobe analyses (defocused beam) of matrix material to minimize the effect of host rock fragments on matrix composition. Our results indicate that the pseudotachylitic matrix is generally depleted in SiO2 and K2O, but is enriched in Fe2O3, MgO, CaO, and TiO2 with respect to granitoid host rocks. We also observed that the pseudotachylite matrix is characterized by enrichment in Cr, regardless of the host rock type. Collectively, this points to the presence of an allochthonous, i.e., more mafic, melt component, in addition to melt derived from local host rocks. Sills of Cr-rich Nipissing Gabbro bodies are ubiquitous in Paleo- Proterozoic and Archean target rocks at Sudbury. The contribution of melts from this rock type can account for the observed enrichment of Cr in the pseudotachylite matrix. In addition, various calculations and estimates of the initial impact melt composition show elevated Cr contents (about 107-190 ppm) compared to most pseudotachylitic host rocks. Therefore, injections of superheated (>1800° C) impact melt into dilation zones and mixing with local host rocks can explain the Cr-enrichment in the pseudotachylite matrix at Sudbury.


Transport Mechanisms for the Suevite of the Ries Crater, Germany: Insight From Fractal Analysis.

* Meyer, C (, Museum of Natural History Berlin, Invalidenstrasse 43, Berlin, 10119, Germany
Jebrak, M (, Universite du Quebec a Montreal, 201, Avenue du President-Kennedy, Montreal, QC H2X 3Y7, Canada
Stoeffler, D (, Museum of Natural History Berlin, Invalidenstrasse 43, Berlin, 10119, Germany

The suevite of the 14.35Ma, 25km wide Ries crater in southern Germany occurs in 3 different geological settings: 1) the crater suevite in the central crater cavity inside the inner ring, 2) the outer suevite on top of the continuous ejecta blanket, 3) dikes in the crater basement and in displaced megablocks [1]. The mechanisms of transport of fragments in the suevite remains poorly understood. In [2] the following processes are discussed: 1) "aerial" transport in a gaseous medium, 2) ground surging in a turbulent flow, 3) interaction of a temporary melt sheet in the central crater with surface water leading to "phreatomagmatic" explosions and subsequent aerial transport. In order to decipher the problem we measured the size distribution of clasts in several drill core sections: Noerdlingen, inside the inner ring (300m suevite); Enkingen, at the inner ring (80m suevite); Woernitzostheim, between inner ring and crater boundary (80m suevite); and Otting, outside the crater (9m of suevite). The drill cores were studied by digital stereometric analysis. Grain sizes of lithic clasts and melt particles were measured on the plane surface of the half core. For the Otting drill core thin sections were investigated at the same sampling depths as the investigated half cores. SEM studies have been performed at various depths. Thereby, the particle size distribution was measured in a size range of four orders of magnitude. The size distribution of the investigated samples obeys a power law. By plotting log r with r=square root(A*B) (A=length of the long axis, B=length of the axis normal to A) against Nc (number of clasts whose sizes are greater than r) the slope (D) of the curve represents the fractal dimension [3]. The results are: Noerdlingen: D(r>1mm): melt particles=2.05; rock particles=1.83; Enkingen: D(r>1mm) upper part: melt particles=0.98; rock particles=1.90; D(r>1mm) lower part: melt particles=1.49; rock particles=1.70; Woernitzostheim: D(r>1mm) upper part: melt particles=2.46; D(r>1mm) lower part: melt particles=1.70; Otting: D(r>1mm): melt particles=1.89; rock particles=1.93; total=1.96; D(0.1<r<1mm): melt particles=1.53; rock particles=2.37; total=1.45; D(0.005<r<0.05mm): total=1.29 At the Otting site the high D values of the lithic clasts imply a high energy single stage fragmentation for the microscopic particles, such as explosive fragmentation, and a secondary process where the finer grains were lost. The microscopic melt particles either were formed in a lower energy "fragmentation" process or the lower D values are a result of disruption of a large body of melt [3]. Subsequently, the melt was dispersed and abraded due to clast-melt interactions during transport. As a phreatomagmatic explosion is usually followed by a pyroclastic flow [4] our results for Otting seem to be in good agreement with [2] who proposes an aerial transport in a gaseous medium as the secondary process. The question for the kind of the primary and possible secondary processes in the other cores could only be solved by microscopic analysis. However, the increasing D values from Wörnitzostheim to Otting sites seem to imply a secondary milling and sorting of the particles during transport towards the outer crater [5]. [1] Stoeffler et al. (2009), LPSC, XL, abstr. [2] Artemieva et al. (2009), LPSC, XL, abstr. [3] Rousell et al. (2003) Earth-Sc. Rew. 60, 147-174 [4] Houghton & Schmincke (1989) Bull. Volc. 52, 28-48 [5] Horwell et al. (2001) J. Volc. Geoth. Res. 109, 247-262


Origin of Suevites at the Popigai Impact Structure, Russia

* Sukara, R E (, Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada
Osinski, G R (, Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada

During the late Eocene, ∼36 Ma, a probable chondritic body ~8 km in diameter, impacted the north- eastern edge of the Anbar Shield in northern Siberia [1], resulting in the formation of an impact structure ∼100 km in diameter, known today as Popigai. The original target site was covered by Proterozoic to Permian sedimentary rocks, consisting of conglomerates, quartzites, dolomites and limestones, and terrestrial sandstones, while the basement lithology was comprised of Archean crystalline basement rocks [2]. This study focuses on so-called suevite deposits of the Popigai structure. We note that this impact glass-bearing breccia is one of the least understood type of impactites. We have studied 12 thin sections and the corresponding number of samples collected from the vicinity of Rassokha River and Sakha-Yurjage Creek and conducted detailed petrography and scanning electron microscopy (SEM). The presence of at least two distinct types of glass has been identified, one exhibiting lighter colour and being a component of the groundmass, while dark brown glass comprises large clasts often with angular and elongate morphologies. The light glass contains both shocked and unshocked quartz, but the former generally does not exceed 10% by volume. In contrast, dark glass exhibits flow textures and, importantly, contains abundant preferentially oriented clasts of heavily shocked quartz with planar deformation features (PDF). In addition to variable amounts of glass, the groundmass contains quartz, clay, pyroxene, plagioclase and crystallites in addition to limited flow textures. Furthermore, the percentage of the groundmass varies considerably, even within individual thin sections and changes relative to the glass grain content, and can range from more than 50 vol% to barely noticeable. Together, these results show that the groundmass of these suevites in many instances is not clastic, but comprises a series of impact melt phases (cf., the Ries structure [3]). We, therefore, suggest that these impactites should be referred to as particulate clast-rich impact melt rocks following the definitions of Osinski et al. [3]. In terms of mode of emplacement, it is important to note that these suevites lie below large km-scale bodies of coherent impact melt. This indicates that these suevites were never airborne but flowed along the base of the transient cavity. Thus, our research of suevites on a micro scale indicates multi stage formational dynamics, which complements the complex nature [2] and formational mechanism of the Popigai crater on the macro scale. Refs: [1] Tagle, R. and P. Claeys (2004) An ordinary chondrite impactor for the Popigai crater, Siberia, Geochimica et Cosmochimica Acta, 69(11). [2] Vishnevsky, S. and A. Montanari (1999) Popigai impact structure (Arctic Siberia, Russia): Geology, petrology, geochemistry, and geochronology of glass-bearing impactites, Large Meteorite Impacts and Planetary Evolution II, GSA, pp. 19-60. [3] Osinski et al. (2004) The nature of the groundmass of surficial suevite from the Ries impact structure, Germany, and constraints on its origin, Meteor. and Planet. Sci., 38(11), 1641-1667.


Mechanisms of Ni-Cu-PGE Sulphide Ore Formation in the Impact-generated Sudbury Melt Sheet

* Lightfoot, P C (, Peter C Lightfoot, Global Technical Services, Vale, Highway 17 West, Sudbury, Ont P0M 1N0, Canada

Petrological and chemical variations in melt sheets provide important information about the scale of melt generation, convection, and differentiation mechanisms. Moreover, they provide a record of the S-saturation history of the melt and the potential for development of economic accumulations of Ni-Cu-platinum group element (PGE) mineralisation. The world-class Ni-Cu-PGE sulphide ore deposits at Sudbury appear to be the product of sulphide saturation and gravitational settling in a superheated crustal melt sheet. The ore deposits occur at or near the basal contact of the Main Mass of the Sudbury Igneous Complex (SIC). Contact ores are associated with the Sublayer Unit which is an inclusion-rich norite and granite breccia; the mineralisation consists of pyrrhotite, chalcopyrite, and pentlandite; there are significant variations in Ni-tenor of the sulphides between different embayments, but Cu/Ni is close to 1. Contact ores are sometimes spatially associated with or linked to footwall ore bodies; footwall ore bodies are typically stockworks of chalcopyrite, pentlandite, millerite, and bornite; these have elevated Cu/Ni and PGE relative to the contact ores. A third style of mineralisation is associated with radial and concentric quartz diorite (Offset) dykes at the outer margin of the SIC; these ores contain pyrrhotite, chalcopyrite, and pentlandite associated with inclusion-rich quartz diorite. Some large embayment structures (e.g. Creighton) and offsets (e.g. Copper Cliff) are heavily mineralized; other equally large embayments (e.g Trill) or extensive offsets (e.g. Foy) contain one to two orders of magnitude less known sulphide mineralization. The Main Mass of the SIC is differentiated upwards from norite through quartz gabbro to granophyre. Geochemical models show that the Main Mass norites of the SIC comprise a sequence of norites that is increasing depleted in Ni, Cu, and PGE through a large portion of the norite stratigraphy. The largest known ore deposits are developed adjacent to the thickest part of Main Mass norite stratigraphy in the South Range; barren and weakly mineralised contacts tend to occur where the Main Mass norites are thinner in North Range settings; a thicker unit of Main Mass norite is developed adjacent to the Levack and Coleman troughs where large deposits are located. The thickest Main Mass norite stratigraphy has basal disseminated sulphides that are metal-rich compared to the sulphides developed in areas where the Main Mass norites are thinnest. The scale and quantity of sublayer in an embayment structure may not be a good empirical predictor of mineral potential. Importantly, the Sudbury Main Mass and Offsets shows some fundamental differences in petrology and chemistry between the north and the south of the complex; these differences are found in the fingerprints of detailed chemostratigraphic variation through the norites. Both the chemostratigraphic variations in degree of metal depletion through the straigraphy and the abundances and ratios of major and trace elements record significant differences between the northern and southern sectors of the melt sheet. These features point to different country rock contributions to the initial melt, and they help to constrain the nature and extent of the convection and differentiation processes. Interestingly, the differences between North and South Range Main Mass rocks were not destroyed by convection on the scale of the melt sheet. Critically, it is these variations have regional and local implications for ore genesis models and exploration at Sudbury.


Heterogeneity in the Sudbury Impact Melt Sheet and Implications for Ni-Cu-PGE Ore Forming Processes

* Darling, J (, University of Bristol, Department of Earth Sciences Wills Memorial Building Queens Road, Bristol, BS8 1RJ, United Kingdom
Hawkesworth, C (, University of Bristol, Department of Earth Sciences Wills Memorial Building Queens Road, Bristol, BS8 1RJ, United Kingdom
Lightfoot, P (, Vale Inco Technical Services, Highway 17 West, Copper Cliff, ON P0M 1N0, Canada
Storey, C (, University of Bristol, Department of Earth Sciences Wills Memorial Building Queens Road, Bristol, BS8 1RJ, United Kingdom
Tremblay, E (, Vale Inco Technical Services, Highway 17 West, Copper Cliff, ON P0M 1N0, Canada

Whilst impact melts have often been regarded as relatively homogeneous, recent studies have highlighted geochemical heterogeneity in melts from impact structures with variable target lithologies (e.g. Kettrup et al, 2003). However, the degree to which heterogeneity can be preserved in more voluminous impact melt sheets is unclear. The 1.85Ga Sudbury Igneous Complex (SIC) is the largest preserved terrestrial impact melt sheet and has been intensively studied, partly due to it's vast Ni-Cu-PGE sulphide deposits, offering a well constrained system in which to test variability. Isotopic variations in the lowermost units of the melt sheet have previously been identified (e.g. Dickin et al, 1996), which indicate broad scale differences between the two exposed limbs of the SIC. In this investigation we have undertaken an intensive study of Pb isotope variations throughout the southern limb of the SIC. Our results show significant and systematic lateral and stratigraphic variations, preserved on a number of scales. These data reflect the preservation of melt cells of differing composition, and indicate that the melt sheet was not efficiently homogenised by convective processes. Our results place additional constraints upon the formation of sulphide ore deposits along the basal margin of the SIC. These resulted from segregation and gravitational settling of dense immiscible sulphide liquids following sulphur saturation (Keays and Lightfoot, 2004). By linking lateral variations in the Pb isotope ratios of sulphides from basal norites to stratigraphic changes, an indication of the relative timing of sulphide segregation is obtained. Cells of norite that reached sulphur saturation early have the strongest metal depletion signatures and appear to correlate spatially with the largest known ore deposits. The identification of significant heterogeneity and multiple sulphide forming events provides new insights into the development of melt bodies such as the SIC.


A Study of Impact Related Rocks Along the Whistle Offset Dyke in the North Range of the Sudbury Impact Structure

* Bygnes, L C (, Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Rd., Sudbury, ON P3E 6B5, Canada
McDonald, A (, Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Rd., Sudbury, ON P3E 6B5, Canada
Lafrance, B (, Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Rd., Sudbury, ON P3E 6B5, Canada

The Whistle offset dyke originates at the Sudbury Igneous Complex-Footwall contact and is interpreted to be contemporaneous with the ca. 1.85 Ga Sudbury event. The dyke is heterolithic, containing at least three lithologies: metabreccia (MTBX), inclusion quartz diorite (IQD), and quartz diorite (QD). Two types of MTBX are noted: 1) Oligomictic breccia, which is confined to areas of monzogranite or intermediate gneiss and found to only contain clasts of the proximal host rock and 2) Polymictic breccia, which is more widespread, and contains clasts of all rock types present. QD is distinguished by the presence of amphibole spherulites up to 2 cm in diameter. The presence of MTBX clasts in QD suggests that it was emplaced later. IQD has a fine- to medium-grained QD matrix with up to 35% clasts. Although the matrix appears to be that of QD, acicular amphiboles are rare. Recognition of IQD as veinlets that crosscut both QD and MTBX implies that it developed last. The contact between QD and IQD is gradational over mms, suggesting rapid emplacement of IQD before cooling of the QD. In some areas, the MTBX is cut by veins of Cu-Ni-(PGE) mineralization. As part of a detailed study focused on characterizing mineralized vs. barren MTBX, the following observations have been made: 1) Clasts in the MTBX exhibit a granophyric texture that is concentrated towards clast boundaries, along with thin rims of polygonal, recrystallized quartz and feldspar; 2) Interstitial quartz is observed in the matrix; 3) Angular, broken, and rotated clasts or grains are absent and there is a lack of undulose extinction; 4) Amphiboles are intact, chlorite appears to be primary, and feldspars have undergone little seritization. As proximity increases, amphiboles are partially to nearly completely altered to chlorite and plagioclase laths observed distal to the sulfide veins are replaced by sericite. Textures observed in MTBX suggest a partial melting origin for MTBX as opposed to cataclasis and frictional melting. As textures are similar for mineralized and barren MTBX, alteration provides a useful indicator for proximity of mineralization.


Emplacement of the Trill Offset Dike, Sudbury Impact Structure, Canada

* Klimesch, L (, Museum fuer Naturkunde Berlin, Invalidenstraße 43, Berlin, 10115, Germany
* Klimesch, L (, Freie Universitaet Berlin, Institut fuer Geologische Wissenschaften, Malteserstr. 74-100, Berlin, 12249, Germany
Hecht, L (, Museum fuer Naturkunde Berlin, Invalidenstraße 43, Berlin, 10115, Germany
Hecht, L (, Freie Universitaet Berlin, Institut fuer Geologische Wissenschaften, Malteserstr. 74-100, Berlin, 12249, Germany
Riller, U (, McMaster University, School of Geography and Earth Sciences, 1280 Main Street West, Hamilton, ON L8S 4K1, Canada

Quartz-dioritic dikes, so called Offset Dikes, are part of the Sudbury impact melt system. Sulphide mineralization in these dikes is enriched in PGE compared to most Ni-Cu-PGE footwall-type deposits of the Main Mass of the Sudbury Igneous Complex, a differentiated impact melt sheet. However, origin and differentiation of the dike rocks as well as mode and timing of their emplacement are still debated. This project provides the first detailed petrogenetic study of the recently identified Trill Offset Dike and seeks to elucidate dike emplacement and differentiation. For this purpose, samples from the Trill Offset Dike were analysed using thin-sections, SEM, microprobe, XRF and ICP-MS. Our analyses indicate that the Trill Offset Dike consists of four texturally distinct varieties: Quartz diorite poor in host rock fragments and sulphide inclusions (QD), inclusion-rich quartz diorite (IQD), glassy quartz diorite (QDg) and spherulitic quartz diorite (SQD). Similar to other Offset Dikes at Sudbury, the Trill displays a general zonation with the mineralized IQD in the dike centre and the QD at the dike margins. The QDg and SQD occur mainly as dike offshoots and apophyses or demarcating dike margins. Mineral and whole rock chemistry show that assimilation and local contamination of host rock as well as multiple injections of melt contributed to the compositional variation and zoning of the Trill. Our data indicate also that four dike rock varieties were emplaced during three temporally distinct phases of melt injection, whereby assimilation occurred likely at the base of the impact melt sheet prior to injection of each injection phase. Moreover, emplacement of the Trill started shortly after the generation of the impact melt sheet, occurred at temperatures close to sulphide segregation, but ended before sub-liquidus temperatures of the impact melt sheet was reached. Based on petrologic considerations from other Offset Dikes at Sudbury, emplacement of the Trill occurred likely up to ten thousand years after impact. Hence, the formation of the Trill Offset Dike may result from floor fracturing accomplished by dilation of target rock due to long-term crustal re-equilibration.