Formation of Fragment-Rich Pseudotachylite in Large Terrestrial Impact Structures
Pseudotachylite zones in impact structures are dike-like bodies, which consist of angular and rounded host- rock fragments enveloped by a fine-grained to glassy matrix that crystallized from a melt. Popular models of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process. However, structural and geochemical characteristics of pseudotachylite bodies at Sudbury and Vredefort indicate that these processes were active at different times and locations. We show that pseudotachylite zones are effectively fragment- and melt-filled tension fractures that formed due to differential rotation of target rock during later stages of cratering. Pseudotachylite characteristics can be accounted for by a single process, i.e., drainage of superheated, low-viscosity impact melt into tension fractures.
Mineralogy of Post-Impact Hydrothermal Deposits at the Haughton Impact Structure, and Implications for Microbial Colonization
Hypervelocity impacts into H2O-rich solid planetary bodies have the potential to generate hydrothermal systems. The 23 km diameter Haughton impact structure hosts a well-exposed and well-preserved example of a post-impact hydrothermal system. Haughton is also the only known terrestrial impact structure in a polar desert environment, making it an excellent analogue for Mars. The predominantly cold and relatively dry climate has resulted in excellent preservation despite its age (39 Ma) . Field studies at Haughton have identified several localities of hydrothermal mineralization distributed throughout the crater : 1) intra-breccia hydrothermal vugs featuring hydrothermal breccias, flowstone/layered textures, dominated by calcite and marcasite that is frequently associated with sulphate and oxyhydroxide weathering products; 2) intra-breccia selenite vugs (crystals up to 1 m in length); 3) intra-breccia calcite dominated vugs; 4) isolated Fe-altered "gossan" type pipe structures around the crater rim, some of which contain near-surface sulphate-dominated deposits morphologically similar to silica sinter deposits in epithermal hydrothermal systems; and 5) pervasive quartz-carbonate veining of shattered country rock, around the edge of the central uplift. Hydrothermal minerals studied here include primary and secondary/weathering phases. X-ray diffraction has revealed diverse minerals from the hydrothermal deposits, including travertine (hydrothermal calcite,) silica minerals (quartz and chalcedony), Fe-sulphides (marcasite, pyrite), Fe and Ca sulphates (rozenite, jarosite, butlerite, fibroferrite, gypsum), Fe-oxyhydroxides (limonite-goethite), clays, and potentially zeolites. Mineralogy may provide important boundary conditions for microbial activity. Clays and zeolites may play a role in prebiotic organic synthesis. The mineral and chemical diversity observed at Haughton demonstrates the presence of various redox couples, which may have supported an equal diversity of microbial metabolic pathways. Sulphides and sulphates in particular may play important roles in microbial metabolism, by providing multiple redox states (e.g. Fe2+/3+, S2-/0/6+) for electron transport. Detailed mineralogical characterization may allow for the identification of the physiochemical conditions relevant to microbial metabolism. Fractionation during precipitation of sulphates and sulphides can also affect S-isotopic systematics, therefore an understanding of the mineralogy and paragenesis of S-bearing phases is necessary for the interpretation of possible S-isotope biomarkers in these deposits. The presence of chemical species conducive to varied metabolic activities is a prerequisite for microbial colonization of post-impact hydrothermal systems. Furthermore, hydrothermal minerals or their weathering products may preserve a chemical or morphological record of biological activity. The potential for microbial colonization of post-impact hydrothermal systems has significant implications for planetary exploration and astrobiology.  Lee, P. & G. R. Osinski,. Met. & Pl. Sci., 2005. 40 (12): p. 1777-1787;  Osinski, G.R., et al.. Met. & Pl. Sci., 2005. 40 (12): p. 1859-1877.
Assessing Crater Floor Morphology and Post-Impact Rotation of the Central Sudbury Impact Structure, Ontario
Knowledge of the crater floor morphology is paramount for understanding the formation and localization of Cu- Ni-PGE resource deposits of, and quantifying the effects of post-impact deformation on, the Sudbury Impact Structure. This is, however, significantly hampered by the lack of (publicly available) data on the orientation of contacts of the Main Mass of the Sudbury Igneous Complex (SIC), generally interpreted as the relic of a deformed impact melt sheet. Based on high-resolution topography data and mapped traces of contacts at surface, we determined the orientation of the base and the top of the synformal SIC at 117 stations. This allowed us to calculate internal thickness variations of the SIC and, thus, the variation of crater floor morphology and post-impact tilt of SIC contacts. Our results indicate that crater floor morphology varies up to 400 meters on the hundred-meter to kilometre scale. However, we also identified a distinct topographic high of 1500m above average crater floor over a lateral distance of about 20 km in the North Range of the SIC. This topographic high is spatially associated with an enhanced thickness in sulphide-rich zones at the base of the SIC on either side of the high. Our orientation data of SIC contacts agree with interpretations in which the SIC is ponded in the north, due to the possible presence of a peak ring, but also with an onset of post-impact deformation during its solidification. Besides constraining models of mineral resource deposit formation, our data bear significantly on the primary cause for observed variations in the width of the thermal aureole imparted by the SIC on its host rocks.
Deformation of the Eastern Sudbury Basin Revisited
Previous structural studies of the eastern Sudbury Basin have concluded that the state of strain in the steeply inclined Sudbury Igneous Complex (SIC) is incompatible with non-cylindrical folding. Consequently it was argued that the igneous complex cannot represent a deformed impact melt sheet as proposed by many recent authors. Using a combination of structural mapping, thin-section and orientation analysis of mineral fabrics as well as fault-slip analysis of brittle shear-fractures, we have re-evaluated the inventory of structural features developed within the eastern SIC and its immediate host rocks. In the NE-lobe of the igneous complex, characterised by maximum curvature, mineral foliations oriented axial-planar to the bisecting plane of the lobe are consistent with differential shearing, i.e. distortion of the SIC. The presence of concordant magmatic and low-grade metamorphic mineral foliations suggests that deformation started during cooling and solidification of the SIC and continued under regional metamorphic conditions. Directions of bulk shortening deducted from brittle shear-fractures are concordant to finite shortening directions indicated by mineral foliations at the same locations, suggesting similar deformation regimes in the ductile and brittle field. The distribution of ductile and brittle structures points to a transition in deformation style from continuous pervasive deformation in sedimentary rocks overlying the SIC to discontinuous deformation within the more competent SIC and its underlying crystalline footwall rocks. We, therefore, argue that the lack of pervasive ductile strain fabrics in the eastern SIC, previously interpreted to indicate a lack of solid-state strain, is a consequence of the mechanical strength of crystalline SIC and basement rocks under low-grade metamorphic conditions. The available structural and geometric data indicates that the SIC was deformed together with its host rocks. The distribution and orientation of mineral fabrics as well as the kinematics of brittle deformation are compatible with large- scale non-cylindrical folding of the eastern SIC. We conclude that the structure of the eastern Sudbury Basin is compatible with an impact melt sheet origin of the Sudbury Igneous Complex.
Geochemical Investigations of the Monturaqui Impact Crater, Chile.
The well-preserved Monturaqui Impact Crater lies in the Salar de Atacama (N Chile) at 3000 m elevation. Unlike larger impacts, this 460 m diameter crater did not form a large melt sheet but has lobes of impactite fragments on the flanks. The simple target lithology is Jurassic granite with mafic dikes, overlain by a thin Tertiary ignimbrite. XRF and ICP-MS analyses of whole target rock samples from inside and outside the crater and electron microprobe analyses of mineral and melt phases were used in least-square mixing calculations. The average modal mineralogy in the target rocks was from a calculated least-squares analysis that matched the whole rock analyses with the averaged analyses of the mineral phases: granite (40% plag, 34% qtz, 20% kspar, 5% bio, 1% apatite and trace magnetite); ignimbrite (62% frothy glass, 22% qtz, 10% plag, 3% bio, and 3% magnetite). Melt rocks are either light grey ductile clasts with biotite and feldspar, or dark brown impactite with metal spherules. Electron microprobe profiles show heterogeneous melt glass compositions over 10's of microns with four distinct components found in different samples. Impactites also contain lechaterlierite (melted quartz) and maskelynite (melted plagioclase). These melts are heterogeneous and poorly mixed within individual impactite clasts: Group 1A has low SiO2 (51%), low K2O (2%), and high FeO (25%); Group 1B has intermediate SiO2 (55%), K2O (2.5%), and FeO (20%); Group 2 has high SiO2 (66%), high K2O (4%), and low FeO (5%). The fourth impactite melt is solely sourced from melted ignimbrite: Group 3 has the highest SiO2 (73%), K2O (5%), and the lowest FeO (0.9%). Groups 3 and 1B are the most volumetrically abundant. The origin of the melt is not a simple combination of the target rock lithologies. These four main melt rock types clearly do not match the composition of the granite, due to the high FeO in the melt rock. To resolve the Fe enrichment, preliminary least-square mixing calculations were conducted to match the melt compositions by mixing the granite and an iron meteorite. These calculations indicated the melt compositions might contain 12% - 32% of the iron meteorite. Impactite contains Fe-Ni spherules, which may show a direct relationship to the impactor. Bunch and Cassidy (1972) suggested that the impactor was an octahedrite iron meteorite, implying low S content. In addition, Bunch and Cassidy found sulfide spherules in the impactite. Our analyses show S enrichment in impactite melt of ∼1400 ppm. Possible explanations for high S could be from a sulfide-rich impactor, the granite, or caliche deposits with gypsum. Work is in progress to resolve the full compositions of the target rocks and melt to determine the possible impactor and whether non-modal melting of the minerals contribute to the melt. References: Bunch & Cassidy (1972), CMP 36, 95-112.
Rock Magnetic Investigations of the Exmore Section From the Eyreville Core, Chesapeake Bay Impact Structure
The Chesapeake Bay impact structure (35 Ma), the largest known impact crater in the United States, was subject of a deep drilling by the USGS and ICDP in 2005-2006. The Eyreville core was drilled into the crater moat of the inner crater zone close to the central uplift and included three holes with a total depth of 1766 m, penetrating through a complete section of post-impact sediments and impact-related lithologies (Gohn, et al., 2008). Hereby, we present the results of our rock-magnetic studies (e.g. temperature-dependent magnetic susceptibility, hysteresis, FORC) of the core samples from Exmore section. The Exmore, sedimentary breccia which contains the clasts of sediment and crystalline rocks, forms the uppermost impactite unit of the Eyreville core and has been subdivided into several sub-units (Powars, et al., 2008) such as oscillation resurge deposits and debris-avalanche deposits. The rock-magnetic investigations, so far, have indicated that these sub-units are magnetically poorly distinguishable. The magnetic susceptibility of the whole section is dominated by paramagnetic behavior (κ < 10-4 SI). A small ferromagnetic component is, however, occasionally present revealing the intrinsic pattern of the susceptibility. This pattern appears to be characterized by rhythmic variations of oxidized clay (higher susceptibility), diamicton and sand (lower susceptibility) layers, respectively. Based on thermo-magnetic and hysteresis measurements, the ferromagnetic component in diamicton and clays is caused by low concentrations of titanomagnetites. Additionally, the hematite is present in clays, possibly reflecting the oxidation of primary minerals. Furthermore, the magnetic susceptibility exhibits the slight change in the pattern at level ∼ 620 m; with more variations below and less variations above. This level matches with a boundary of one the sub-units. In order to find the cause for this change in susceptibility and reveal any additional differences in the sub-units the more detailed investigations are currently ongoing. References Gohn, G. S., Koeberl, C., Miller, K. G., Reimold, W. U., Browning, J. V., Cockell, C. S., Horton, J. W., Kenkmann, T., Kulpecz, A. A., Powars, D. S., Sanford, W. E., and Voytek, M. A., 2008, Deep Drilling into the Chesapeake Bay Impact Structure: Science, v. 320. no. 5884, p. 1740- 1745. Powars, D. S., Catchings, R.D, Gohn, G.S., Horton, J. W., Edwards, L. E., and Daniels, D. L., 2008, Geophysics and deep coreholes reveal anatomy and complex infilling of the central crater, Chesapeake Bay impact structure, U.S.A.: Conference on Large Meteorite Impacts and Planetary Evolution IV, Vredefort Dome, South Africa, Abstract 3087.
Shock Metamorphosed Zircon From the Chesapeake Bay Impact Crater
Petrographic classification of impactites crucially hinges on the identification of impact melted components. The correct classification of impactites allows the interpretation of their formational setting. Zircon is a versatile, refractory indicator for shock pressures between ∼20- >90 GPa and temperatures above ∼1700°C. Between 20-40 GPa, zircon transforms to the high pressure polymorph reidite, which is stable up to ∼1200°C but reverts to zircon above this temperature. At 1700°C / ∼90 GPa, zircon decomposes to ZrO2 + SiO2. Characteristic granular textures result from annealing of decomposed zircon and reidite. Compared to most other shock metamorphosed minerals, shocked zircon is relatively resistant to secondary alteration. The identification of impact melt particles in the USGS-ICDP Eyreville drill cores into the Chesapeake Bay impact crater is hampered by variable degrees of alteration and the relative scarcity of inclusions of lithic clasts with recognizable shock metamorphic features. Most importantly, though, textural and compositional characteristics of altered impact melt particles can be easily confused with shale and clay fragments that did not experience high degrees of shock metamorphism. Zr-bearing phases were identified by systematic analyses of impactite thin sections under the scanning electron microscope in high contrast mode. Mineral species were then identified by Raman spectroscopy. Relative degrees of shock pressure and post-shock temperature regimes experienced by the host materials could be derived from the presence of reidite-zircon grains in shock stage II and III materials (∼35-60GPa, ∼300- 1200°C). These lithic clasts suggest little thermal annealing from post-shock temperatures, yet most other mineral components are pervasively altered to phyllosilicates. Shock stage IV (>60GPa, T>∼1500°C) melt particles were identified by the presence of clasts of zircon that are variably decomposed to granules of baddeleyite (ZrO2). Two pods of impact melt rock with hypo- and holocrystalline melt matrices contain clasts of unshocked zircon, granular textured zircon, and variably decomposed zircon. Reidite was not found, likely due to severe annealing. A comparison with the kinetics of decomposition of zircon and the thermal equilibration of impact melts suggests that decomposition of zircon must have occurred due to shock pressures >90GPa during the opening of the transient cavity at Chesapeake Bay.
Evaluating the Stable Isotopic Composition of Impact-Induced Hydrothermal Carbonates at the Haughton Impact Structure, Devon Island, Nunavut, Canada
Approximately sixty of the two-hundred impact structures discovered on Earth show evidence of impact-induced hydrothermal alteration . However, only about a dozen impact craters have been examined by stable isotope techniques, which are very useful tools for understanding other types of geothermal environments on Earth. The scientific purpose of undertaking stable isotope studies at impact craters is twofold: firstly, to identify physical and geochemical processes involved in hydrothermal activity such as fluid source, temperature and potential for ore deposits; and secondly, to search for new niches for life that are significant to astrobiology . The Haughton impact structure (23 km, 39 Ma) situated on Devon Island, in the Canadian Arctic, is an ideal case study since its hydrothermal system is relatively well-exposed and unaffected by external geological or biological processes, mainly due to its polar location. A variety of post-impact hydrothermal carbonate precipitates have been identified at the Haughton impact structure . In this study, we have characterised deposits by means of x-ray diffraction, x-ray fluorescence and stable isotope ratios of carbon and oxygen. Unlike the previous isotopic study that investigated carbonates of highly shocked clasts from impact breccias , the present study focuses strictly on carbonates of hydrothermal origin. The hydrothermal carbonates are predominantly calcite with minor dolomite; other significant hydrothermal phases include marcasite and selenite although isotopic analyses were not performed on those. The hydrothermal carbonates are found within four distinct spatial settings: (1) as fracture fillings in rocks of the central uplift; (2) as veins within lithologies across the displaced zone and rim; (3) as vug fillings within the impact-melt breccias, and (4) in hydrothermal pipe structures located exclusively on the crater rim. To fully understand the evolution and paragenesis of the post-impact hydrothermal system, carbonate minerals of unshocked target rocks (i.e., dolostone and limestone) and impact-generated lithologies (i.e., impact-melt breccias) were also analysed for reference. The majority of impact-induced hydrothermal samples analysed have similar isotopic compositions. The carbon isotope values range from +10 to +14‰(VPDB), while oxygen isotope values range from -2 to -7 ‰(VSMOW). Thus, the isotopic signatures are consistent with a cooling depleted meteoritic fluid source, and are similar to hydrothermal carbonates from Popigai (Russia) and Lockne (Sweden) impact structures. Also, we also note that the search for the origin of life on Earth and possibly on other planetary bodies is a key driver for Mars exploration. Although Mars is now extremely cold, current pieces of evidence indicate that water ice and possibly liquid water are extensively distributed in the sub-surface of Mars; therefore, extremophilic organisms could have survived within hydrothermally active impact craters, leaving behind unique isotopic signatures. Naumov, M.V., Geofl., 2005.5:165-184; Hode, T., et al., Astro., 2008. 3(2):271-289; Osinski, G.R.,et al., Met.and Pl. Sci., 2005. 40(12):1859-1877. Martinez, I., et al., Earth and Plan. Sci. Let., 1994. 21:559-574.
Post-Impact Crater Floor Fragmentation and its Importance for Localizing Cu-Ni-PGE Resource Deposits at Sudbury, Canada
The Sudbury Basin, the central portion of the Sudbury Impact Structure, consists of the synformal Main Mass of the Sudbury Igneous Complex (SIC), the relic of an impact melt sheet that is overlain by melt breccias and post-impact sedimentary rocks. Formation of the Basin is generally attributed to large-scale orogenic folding. The south western portion of the Basin is dissected by a series of prominent reverse faults that separate individual segments of the SIC. The segments differ markedly by their apparent thickness and strike separations of lithological contacts on the reverse faults (up to 5 kilometers). This indicates that localized reverse faulting contributed significantly to the shape change of the SIC and its host rocks. Reverse sense-of- shear is corroborated by the presence of metamorphic mineral shape fabrics with L-S geometry that are associated with the South Range Shear Zone, a first-order ductile thrust transecting the Basin longitudinally. Knowledge of the effects of reverse faulting and associated tilt of SIC segments is paramount for localizing Cu- Ni-PGE deposits at segment bases. Structural analysis of the south western Sudbury Basin revealed that metamorphic mineral shape fabrics are genetically related to the reverse faults. This allows us to apply a simple graphical method devised by Schwerdtner (1998) to constrain the possible range of shear strain vectors on the reverse faults. Furthermore, kinematic analysis of the structural inventory is used to identify the cause for 1) the variable strike separations along reverse faults, 2) the variation in apparent thickness of SIC segments, 3) the likely orientation of shear strain vectors on the fault planes and 4) the orientation of the base of SIC segments. The structural configuration of juxtaposed SIC segments suggests that they occupied different positions within the impact melt sheet with respect to the crater margin prior to km-scale translation on reverse faults.
New K-Ar Dating of Impact-Generated Melt From the Carswell Structure (Sasakatchewn, Canada): Evidence for a Mesoproterizoic Impact Event
The Carswell impact structure is situated in northwestern Saskatchewan, in the Paleoproterozoic Athabasca Basin. It is a 39 km-wide complex impact structure with an 18 km diameter central uplift made of crystalline basement rocks. The age of the impact event has never been accurately constrained: a large range of ages has been found, ranging from 115 to 515 Ma, though the distribution of shock metamorphism features and impact breccias, restricted to the basement rocks, suggests a much higher age (possibly pre-Athabasca). Because several occurrences of uranium have been discovered at the unconformity between basement and sediments as well as in some breccias from the central uplift, it is essential to determine the age of the impact event and its possible influence on the mineralization. Three samples of impact breccias were collected from the southern part of the central uplift, two in drill-cores (CLU02-00 and CLU4667-92) and one from an outcrop near the unconformity between the Athabasca Group sediments and the crystalline basement (CLU02-Af). Shock metamorphism features observed in the samples are consistent with high P and T conditions of ca. 60 GPa and more than 1500°C, sufficient for the resetting, by the impact event, of the K-Ar isotopic system of any mineral present. Thirteen measurements were conducted in different separates of the breccias. A mixture of melt and fine grained host rock fragments gave weighted mean ages of 1194 ± 15 Ma (sample CLU02- 00), 1092 ± 54 Ma (CLU4667-92) and 511 ± 12 Ma (CLU02-Af); pure melt yielded an age of 1026 ± 25 Ma; and feldspar directly in contact with the melt a mean at 1116 ± 36 Ma (the latter two both in sample CLU4667-92). The lowest age obtained of ca. 510 Ma is consistent with previous ages determined by K-Ar and Ar-Ar methods, but the rest of our data ranging between 1.1 and 1.2 Ga were previously unknown. We interpret the highest age obtained (1194 ± 15 Ma; sample CLU02-00) as the minimal age for the impact event. The presence of a chloritized material in the matrix of sample CLU02-Af could explain the lower ages of the two other drill-core samples, assuming a local and late hydrothermal event partly resetting the argon isotopic system. Moreover, the main stage of uranium mineralization hosted by the Carswell structure has been evaluated between 1050 and 1150 Ma by the U-Pb dating method, which is significantly lower than for the other unconformity-type uranium deposits elsewhere in the Athabasca Basin, for which primary mineralization is dated at 1350 to 1590 Ma. We suggest that either the U-Pb isotopic system of uranium occurrences within the impact structure has been reset by the thermal impact effect or that secondary recrystallization of uranium minerals happened consecutive to the impact event, facilitated by fracturing and brecciation. Considering these new isotopic K-Ar measurements, the Carswell impact event is thought to be of Mesoproterozoic age and is the only impact structure of this age observed outside Australia.
Genesis of Ni-Cu-PGE Mineralization Associated with the 1.85 Ga Sudbury Impact Event
Ni-Cu-PGE mineralization associated with the 1.85 Ga Sudbury impact structure can be attributed to 1) a large impactor, generating a large mass of superheated crustal melt, 2) pre-existing sulfide mineralization in the impact site, permitting metal-poor impact melt to reach saturation in sulfide and react with sulfide melt prior to crystallization, 3) thermomechanical erosion of footwall rocks, generating embayments and sulfide-inclusion- rich Sublayer, and locally remelting sulfides to generate large amounts of very fractionated sulfide melts and fluids, and 4) slow cooling, permitting even unrefined sulfide melts to fractionate. 'Offset' ore systems occur in radial/concentric quartz diorite dikes and breccia belts, cross-cut impact breccias, have barren inclusion-free margins and mineralized inclusion-rich cores, and were emplaced after the impact melt cooled enough to achieve sulfide saturation and mechanically erode footwall rocks but early enough to preserve near-original compositions. Some are fractionated (Cu-PPGE-poor to Cu-PPGE-rich) concentrically, consistent with capillary infiltration of fractionated sulfide melts into surrounding rocks. Others are fractionated downward through to Ag-Au-PPGE-Pb-Zn-Sn-Bi-As-Te-rich mineralization. Compositional variations and masses can be modeled by partial fractional crystallization (PFC) of MSS +/- ISS. 'Contact' ore systems occur in footwall embayments and are hosted by Sublayer and footwall breccias. Ores are typically less fractionated over mafic lithologies and more fractionated over felsic lithologies. Field relationships, textures, mineralogy, and geochemistry suggest that sulfide melts infiltrated and mechanically- eroded footwall rocks; variations in geochemistry and S-Pb isotopes confirm local derivation of many components in Sublayer. High density, very low viscosity sulfide melts wetted and percolated into fractured footwall rocks, filling and widening fractures, and ultimately surrounding, isolating, and buoying-up individual fragments. Cu-rich melts penetrated farther because of greater densities and wetting abilities, and lower solidus temperatures. Fragments generated earlier were completely incorporated into Sublayer; those generated later were only partially rotated and melted. Compositional variations and masses in slightly to moderately fractionated contact systems can be modeled by PFC of MSS +/- ISS. 'Footwall' ore systems occur up to 100s of metres below contact systems and are hosted by cataclastic/pseudotachylitic breccias. The large masses of very Cu-PPGE-rich mineralization and partitioning constraints preclude models involving FC or PFC (not enough Cu-PPGE-rich melt), equilibrium crystallization (not enough Cu-PPGE in the final melt), or sulfide-sulfide liquid immiscibility (Ds too small). Geological, thermal, and fluid dynamic constraints suggest derivation by dynamic remelting of contact systems. The rate of thermomechanical erosion along the basal contact initially exceeded the rate of heat conduction into footwall rocks, allowing even Cu-PPGE-rich melts to penetrate only short distances into footwall rocks but supercritical aqueous fluids derived by dehydration/melting of footwall rocks were able to dissolve Au-Pd-Pt-Bi-Te and infiltrate farther. With cooling, the rate of erosion declined and contact metamorphic isograds expanded, permitting deeper infiltration of Cu-PPGE-rich sulfide melts into brittle fractures, overprinting earlier-formed mineralization. Compositional variations and masses in strongly fractionated contact-footwall systems can be modeled numerically as a multi-stage remelting process or analytically as a zone-refining process.
Initial Composition of the Sudbury Igneous Complex - Evaluation of Existing Models
The Sudbury Igneous Complex (SIC) includes quartz-diorite Offset Dikes as well as melt-rich breccias of the Onaping Formation and represents the largest impact melt systems known on Earth. The Main Mass of the SIC, the impact melt sheet, is unique because it is differentiated into compositionally different layers and hosts world-class Cu-Ni-PGE mineralization at its base. Discussion of melt differentiation, emplacement and associated mineralization is still controversial. A major challenge in this discussion is the lack of knowledge of the initial composition of the impact melt sheet because geochemical modelling largely depends on the definition of the starting composition. Weighted averages of the differentiated melt sheet may be close in composition to the initial impact melt composition, although there is evidence for assimilation of target rocks prior or during sub-liquidus fractionation of the melt sheet. It has also been suggested that the initial impact melt is similar in composition to that of Lower Crust, the most unaltered vitric composition of the Onaping formation, and the average Offset Dike. Based on the current knowledge on cratering dynamics inferred from numerical modelling of the Sudbury impact, it is unlikely that Lower Crust and Onaping models constitute the primary composition of the impact melt. Our recent studies on the Worthington Dike and other Offset Dikes confirm that the Offset Dike melts were already contaminated to various degrees by assimilation of host rock prior to injection of impact melt into target rocks. Consequently, the Offset Dikes can not represent the pristine impact melt, despite their similarity to certain averages of the SIC or the vitric composition of the Onaping formation. We discuss various models on the initial impact melt composition.