Sudbury Breccia Research Articles

Genesis of Sublayer, Footwall Breccia, and Associated Ni-Cu-Platinum Group Element Mineralization in the Sudbury Igneous Complex

Abstract The Ni-Cu-platinum group element (PGE) sulfide mineralization in the 1.85 Ga Sudbury impact structure occurs in two discrete environments: (1) mineralization along or near the basal contact, including disseminated to semimassive mineralization in an inclusion-rich sublayer, semimassive to fragmental mineralization in the underlying anatectic footwall breccia, and veins and disseminations in the underlying pseudotachylitic Sudbury breccia, and (2) disseminated to semimassive mineralization in the inclusion-bearing quartz diorite phase of radial and concentric offset dikes that extend up to 20 km from the basal contact. The sublayer, inclusion quartz diorite, and footwall breccia all contain abundant mafic-ultramafic inclusions—local xenoliths derived from nearby country rocks, exotic xenoliths derived from unexposed upper-middle crustal target rocks and, locally, anteliths derived from an olivine melanoritic early border phase of the Sudbury Igneous Complex—but the quartz diorite margins of offset dikes contain very few local inclusions and no exotic ultramafic inclusions. The similar inclusion populations indicate a genetic relationship between the sublayer and inclusion quartz diorite and interaction between the sublayer and footwall breccia. But the sublayer is characterized by a cumulate noritic matrix, the inclusion quartz diorite by a noncumulate quartz dioritic matrix, and the footwall breccia by an anatectic felsic-intermediate matrix. The overlying main mass norite is very homogeneous in terms of Hf isotopes, indicating that the impact melt sheet was well mixed, but ores, sublayer, inclusion quartz diorite, and to a lesser degree overlying main mass norites vary widely in their Pb-S-(Os) isotope compositions. The majority of mafic-ultramafic inclusions, except for anteliths, contain no sulfide and exhibit no signature of Ni-Cu-PGE depletion caused by prior sulfide saturation, which indicates that the association between mafic-ultramafic inclusions and Ni-Cu-PGE mineralization is attributable primarily to the refractory nature of the inclusions and to a lesser degree their similar hydrodynamic behavior as the sulfide melt and, secondarily, to derivation of sulfides from the same sources as the inclusions. The very large Sudbury impact event volatilized much of the Pb-S-Zn-Cd-Se-Bi and significant amounts of Sb-Ag-Cu-Au-As from the target rocks. It generated large amounts of superheated impact melt and only minor fragmental debris during the compression and excavation stage, which is when the inclusion- and sulfide-poor marginal phase of the offset dikes was emplaced. Large amounts of debris, including sulfide-bearing Huronian basalts, Nipissing and East Bull Lake intrusions, and Archean mafic gneisses, were generated during isostatic rebound, formation of a central uplift, and collapse of the central uplift and crater walls, which is when the inclusion- and sulfide-bearing internal phase of the offset dikes was emplaced. Convective and gravity flow aided horizontal transport of residual exotic inclusions, local inclusions, and sulfide xenomelts into embayments and funnels to form the protosublayer. Olivine-saturated melts, generated by thermomechanical erosion of local olivine-bearing country rocks, locally crystallized an olivine melanoritic early border phase of the main mass, which was disrupted and preserved in the funnels of some North Range offset dikes. Continued thermomechanical erosion of country rocks enlarged dike funnels and exploited other fractures to generate footwall embayments and significant geochemical and Pb-S-(Os) isotope heterogeneities. As the rate of thermomechanical erosion decreased and the rate of heat conducted into the footwall rocks increased, contact ores and some offset ores fractionally crystallized to form residual melts that generated footwall vein systems.

“Sudbury Breccia” and “Huronian Breccia”: spatial, sedimentological, and structural evidence for separate distinct breccia-forming events

Rounded argillite clasts within the lower Gowganda Formation of the Huronian Supergroup near Whitefish Falls, Ontario, have been historically mapped as Sudbury Breccia, implying that their formation was initiated by the Sudbury meteorite impact event. Alternative genetic models proposed to explain the breccia at Whitefish Falls include formation through intrusion of diabase into wet sediment accompanied by soft-sediment deformation events. Outcrops in the Whitefish Falls area contain clear evidence for early post-depositional fracturing: flow of argillites into brittle fractured sandstones. Linking these geological processes suggests that the formation of the breccia at Whitefish Falls was generated by faulting of the Huronian sedimentary basin during the sedimentation of the Gowganda argillites. Using a GIS approach to compare the distribution of known breccia bodies with mapped lithology and structure, it is apparent that the term Sudbury Breccia has been applied to two types of breccias. First, true Sudbury Breccia, which is characterised by rounded heterogeneous clasts in an aphanitic matrix, is only found in proximity to the Sudbury Impact crater. The distribution of the second, primarily sediment derived, type of breccia, as seen at Whitefish Falls, is strongly associated with mapped faults and regional-scale basement discontinuities, as defined by gravity and magnetic data. Since this type of breccia is present throughout the entire Huronian sedimentary sequence, the term “Huronian Breccia” is more appropriate. This breccia is not the result of a single geological event but rather episodes of fault activity, as the geometry of the Huronian basin evolved over time.

Indicator mineral and till geochemical signatures of the Broken Hammer Cu–Ni–PGE–Au deposit, North Range, Sudbury Structure, Ontario, Canada

The Broken Hammer Cu–Ni–PGE–Au footwall deposit in the North Range of the Sudbury Structure in Canada consists of a shallow surface zone of vein-hosted and vein stockwork-hosted mineralization within Sudbury breccia developed in the quartz monzonite Levack Gneiss Complex. The surface of the deposit consists of a 2–120 cm wide chalcopyrite vein and numerous smaller veins dominated by chalcopyrite–magnetite–millerite with trace gold, platinum group minerals, tellurides, bismuthides and selenides. The Laurentide Ice Sheet flowed southward across the region depositing a sandy till that contains abundant sperrylite (hundreds of grains), chalcopyrite, pyrite and gold in the heavy mineral fraction down-ice of mineralization. Mineral liberation analysis of the <0.25 mm heavy mineral fraction of metal-rich till identified a broader suite of PGM and sulfides than visual identification methods. The <0.063 mm fraction of till displays a strong geochemical signature of the mineralization for Pd, Pt, Au, Cu and Ag and, to a lesser extent, Bi, Te and Sn; however, geochemical signatures are not detectable as far down-ice as indicator minerals. Till sampling has not been used for exploration in the Sudbury region because of the abundant outcrop and the use of geophysical and prospecting techniques. This study demonstrates that indicator mineral and till geochemical methods are useful exploration tools for the region. The presence of sperrylite and chalcopyrite in oxidized till indicates that even thin (<1 m) highly weathered till is an effective sample medium here.

Mineralogical and Geochemical Characteristics Of Sudbury Breccia Adjacent To Footwall Cu-Ni-PGE Sulfide Veins and Structures In the Creighton and Coleman Deposits

Abstract The Sudbury Breccia is an impactite that formed in the target rocks of the 1850 +1.3/–2.4 Ma Sudbury impact structure in Ontario, Canada. The breccia is interpreted to have formed during crater excavation or modification at the time of the impact event. Copper-Nickel-Platinum Group Elements sulfide melts moved away from contact mineralization at the base of the melt sheet to form veins and stockworks in the footwall. The distribution of both Sudbury Breccia and the later sulfide melts is partially controlled by pre-existing weaknesses in the footwall, such as lithological contacts; as a result, the two are often spatially associated. A combination of early magmatic-hydrothermal and late metamorphic fluids modified the sulfide mineralization to create broad haloes of metalliferous hydrous silicate minerals proximal to the Sudbury Breccia and in the footwall rocks. This study examines variations in the trace-element geochemistry of the breccia-matrix mineral assemblage developed adjacent to footwall mineralization in the North Range (Coleman Mine) and the South Range (Creighton Mine). The Sudbury Breccia is widely considered to have formed in a single event, contemporaneous with the meteorite impact, therefore it provides a relatively consistent baseline for studying subsequent metamorphic and hydrothermal processes, unlike the Archean Superior Province and Paleoproterozoic Southern Province, which experienced pre-impact alteration associated with tectonic and igneous activity. This study focused on examining the textural, chemical, and petrological changes recorded in biotites, amphiboles, chlorites, and Fe-oxides produced by late magmatic and post-magmatic sulfide mineralization and U-Pb age dates of titanite. Titanite-chlorite assemblages in the North Range footwall of the SIC overprint metalliferous assemblages adjacent to the McCreedy East 153 footwall ore body. They yielded an age of 1358 ± 78 Ma, which is interpreted to be associated with the waning effects of the 1450 ± 0.15 Ma Chieflakian orogeny. In the South Range at the Creighton Mine, the Sudbury Breccia hosting the Creighton Deep footwall ore body records a shift from ferro-hornblende to ferro-tschermakite amphibole. When coupled with variations in Ca/Ti in titanite and a U-Pb titanite age of 1616 ± 33 Ma, the assemblage is interpreted to reflect increasing temperature-pressure gradient towards shear zones that were active during the Mazatzalian orogeny, during which time the sulfide mineralization was remobilized. At both locations, biotite exhibits an increase in Tl content that relates to the Ni content of the host rocks and proximity to sulfide mineralization. This relationship may be produced by remobilization of metals during interaction between sulfide mineralization and hydrothermal fluids. Although not directly associated with sulfide emplacement, these signatures provide evidence for a larger geochemical halo around the mineral zone that may provide the basis for ranking the prospectivity of the footwall and possibly vectoring towards mineral zones.

The impact pseudotachylitic breccia controversy: Insights from first isotope analysis of Vredefort impact-generated melt rocks

Besides impact melt rock, several large terrestrial impact structures, notably the Sudbury (Canada) and Vredefort (South Africa) structures, exhibit considerable occurrences of a second type of impact-generated melt rock, so-called pseudotachylitic breccia (previously often termed “pseudotachylite” – the term today reserved in structural geology for friction melt in shear or fault zones). At the Vredefort Dome, the eroded central uplift of the largest and oldest known terrestrial impact structure, pseudotachylitic breccia is well-exposed, with many massive occurrences of tens of meters width and many hundreds of meters extent. Genesis of these breccias has been discussed variably in terms of melt formation due to friction melting, melting due to decompression after initial shock compression, decompression melting upon formation/collapse of a central uplift, or a combination of these processes. In addition, it was recently suggested that they could have formed by the infiltration of impact melt into the crater floor, coming off a coherent melt sheet and under assimilation of wall rock; even seismic shaking has been invoked. Field evidence for generation of such massive melt bodies by friction on large shear/fault zones is missing. Also, no evidence for the generation of massive pseudotachylitic breccias in rocks of low to moderate shock degree by melting upon pressure release after shock compression has been demonstrated. The efficacy of seismic shaking to achieve sufficient melting asa foundation for massive pseudotachylitic melt generation as typified by the breccias of the Sudbury and Vredefort structures has so far remained entirely speculative. The available petrographic and chemical evidence has, thus, been interpreted to favor either decompression melting (i.e., in situ generation of melt) upon central uplift collapse, or the impact melt infiltration hypothesis. Importantly, all the past clast population and chemical analyses have invariably supported an origin of these breccias from local lithologies only.Here, the first Rb-Sr, Sm-Nd, and U-Pb isotopic data for Vredefort pseudotachylitic breccias and their host rocks, in comparison to data for Vredefort Granophyre (impact melt rock), are presented. They strongly support that the pseudotachylitic breccias were exclusively formed from local precursor lithologies – in agreement with earlier isotopic results for Sudbury Breccia and chemical results for Vredefort pseudotachylitic breccias. A contribution from a Granophyre-like impact melt component to form Vredefort pseudotachylitic breccia is not indicated. The most likely process for the genesis of voluminous pseudotachylitic breccias in large impact structures remains decompression melting upon formation and collapse of the central uplift, during the modification stage of impact cratering.

Reconstructing the Geochemical Signature of Sudbury Breccia, Ontario, Canada: Implications for Its Formation and Trace Metal Content

The Sudbury breccia is an impact-generated pseudotachylitic breccia formed in the footwall of the 1.85 Ga Sudbury Igneous Complex in Ontario, Canada. The breccia is commonly developed in areas of contrasting country-rock types, and it represents a common host to footwall-style Cu-Ni-platinum group element sulfide mineralization, developed in the immediate footwall of large, contact-type, Ni-Cu–rich ore deposits at the base of the Sudbury Igneous Complex. Three models have been proposed for the genesis of the Sudbury breccia: (1) in situ frictional or shock melting of target rocks, (2) cataclasis of the target rocks, and/or (3) injection of allochthonous impact melt from the Sudbury Igneous Complex into the brecciated crater floor. This study reports new mineralogical and geochemical data for samples of Sudbury breccia and associated host rocks, both proximal and distal to mineralization, from six different locations around the Sudbury structure. A mixing model was used to construct the composition of the Sudbury breccia matrix, using a combination of country rock and quartz diorite from the offset dikes, which are believed to provide an estimate of the composition of the early melt sheet. We demonstrate that the breccia matrix can be successfully modeled from footwall lithologies with no requirement for a contribution from the impact melt sheet. Instead, we propose a dynamic, parautochthonous in situ melt model, where the matrix of the breccia was derived in part by high-temperature melting of footwall lithologies. These inclusion and crystal-laden melts were mixed and transported into tensile fracture zones that developed adjacent to weaknesses in the country rocks. Principal component analysis of Sudbury breccia indicates that the trace metal content within the breccia that developed distal from mineralization is associated with assimilation of mafic units in the footwall. In contrast, the matrix of Sudbury breccia developed closer to mineral zones has a contribution of metal derived from the fractionated sulfide melt and/or a contribution of hydrothermally remobilized metal. Thus, by comparing the trace metal content in Sudbury breccia with local footwall lithologies, baselines can be established that can help to identify anomalous metal contributions that may be derived from the periphery of a footwall mineral system.

Reconstructing the geochemical signature of Sudbury Breccia, Ontario, Canada: implications for exploration models

Sudbury breccia is a widespread lithology in the footwall of the 1.85 Ga Sudbury Igneous Complex in Ontario, Canada (Rousell 2003). The breccia acted as a pathway for the emplacement of evolved, Cu...

Preliminary Evaluation of Trace Hydrocarbon Speciation and Abundance as an Exploration Tool for Footwall-Style Sulfide Ores Associated with the Sudbury Igneous Complex, Ontario, Canada

Deposits of Cu-Ni-platinum group elements (PGE)–rich sulfides occur along the North Range footwall of the Sudbury Igneous Complex, Ontario, Canada. The deposits have been modified chemically, mineralogically, and texturally by multiple syn- to postmagmatic hydrothermal events and are hosted mainly within Sudbury breccia, a contact metamorphosed and hydrothermally altered impact breccia. The composition of C1 to C6 unsaturated and saturated hydrocarbons in bulk fluid inclusion volatiles in the matrix of the Sudbury breccia was investigated using in-line rock-crushing gas chromatography (GC) in order to compare the hydrocarbon composition of this rock type in breccia zones containing economic footwall sulfide deposits at the Morrison deposit (Levack mine, North Range, Sudbury, Ontario) with barren breccia zones. Trace hydrocarbons, possibly abiogenic in origin, occur throughout the Sudbury Igneous Complex footwall and country rocks investigated. However, there are differences in the composition and abundance of bulk hydrocarbons released from mineralized and barren Sudbury breccia when fluid inclusions are opened. Volatiles released from samples of Sudbury breccia from sulfide-mineralized and sulfide-barren environments showed the following: (1) higher average abundances of light saturated hydrocarbons (C1–C4) and an approximately 2× higher total abundance of hydrocarbons (mol/g of rock basis) associated with samples from the mineralized study area, and (2) comparable average unsaturated hydrocarbon abundances in both settings, but a larger range in unsaturated hydrocarbon abundances in the mineralized study area. Differences in bulk hydrocarbon abundance and composition reflect differences in the abundance and hydrocarbon content of fluid inclusions trapped in the barren and mineralized Sudbury breccia zones. In addition, the compositional modification of circulating fluids by processes and conditions unique to the mineralized Sudbury breccia environment may have occurred prior to inclusion entrapment (e.g., modification by mineral catalysts, differences in temperature and oxygen fugacity). Preliminary spatial analysis of the hydrocarbon distribution within and surrounding the Morrison deposit shows that the total hydrocarbon abundance in the Sudbury breccia does not change with orebody proximity within a single host breccia zone. However, volatiles trapped in the Sudbury breccia matrix near the top of the footwall deposit, where it is physically connected to contact-style (Ni-Cu-PGE) ores, have elevated unsaturated hydrocarbon abundances (e.g., propene) compared to other parts of the same breccia zone distal to this zone of physical connection. Samples of the PGE-enriched Sudbury breccia and mineralized veins containing alteration assemblages characteristic of the footwall ore styles have higher unsaturated hydrocarbon abundances and unsaturated/saturated ratios compared to Sudbury breccia and vein samples that are PGE poor. Localized enrichments in unsaturated hydrocarbons in Sudbury breccia zones may be the result of specific ore minerals and metal complexes (involving the PGE and group VIII metals) that acted as catalytic agents, modifying the composition of initially light saturated hydrocarbon species prior to entrapment in fluid inclusions. To apply the analysis of hydrocarbon compositions in trapped fluid inclusions to discriminating mineralized from barren zones of Sudbury breccia, samples from targeted areas were compared to samples of barren Sudbury breccia (“background”). Although a large data set is presented here to establish these compositional fields, analysis of smaller sample subsets from each environment yields a statistically significant separation between the two environments. Total hydrocarbon abundance, C3/C1 ratios, and unsaturated/saturated hydrocarbon ratios are useful indicators of sulfide ore proximity and may help delineate regions enriched in PGE.

Multi-stage hydrothermal processes involved in “low-sulfide” Cu(–Ni)–PGE mineralization in the footwall of the Sudbury Igneous Complex (Canada): Amy Lake PGE zone, East Range

The Amy Lake PGE zone is a “low-sulfide-type” Cu-(Ni-)PGE mineralization in the East Range footwall of the 1.85 Ga Sudbury Igneous Complex occurring in a 100-m-wide Sudbury Breccia belt that coincides with an impact-related major fracture zone (Bay Fault zone). Detailed hydrothermal alteration mapping, fluid inclusion, trace element, and stable isotope studies revealed a complex alteration and mineralization history in a multi-source, multi-stage Sudbury-related hydrothermal system. The two major stages of syn-Sudbury hydrothermal activity are characterized by similarly high-salinity, high-temperature fluids that are (1) locally derived from footwall granophyre bodies, and typified with high Ni/Cu and PGE/S ratios and high REE contents (magmatic–hydrothermal stage), and (2) a more voluminous Cu–Ni–PGE-rich fluid flux probably originated from the Sudbury Igneous Complex/footwall contact (hydrothermal stage). The second hydrothermal flux was introduced by brittle fractures in the area and resulted in a complex zonation of alteration assemblages and mineralization governed by local footwall composition. The Sudbury-related hydrothermal event was overprinted by shear-related epidote veining and calcite–chlorite replacement, both regionally present in the Sudbury structure. Based on analogies, the most important factors involved in the formation of hydrothermal low-sulfide mineralization are proposed to be (1) accumulation of PGE-enriched fluids, (2) large-scale brittle structures as conduits to these fluids, and (3) adequate host rock composition as a chemical trap resulting in sulfide and PGM precipitation. In environments meeting these criteria, hydrothermal PGE mineralization is known to have formed not only in the Sudbury footwall but also from mafic–ultramafic intrusions associated with primary magmatic PGE from several locations around the world.

The Significance of Partial Melting Processes in Hydrothermal Low Sulfide Cu-Ni-PGE Mineralization Within the Footwall of the Sudbury Igneous Complex, Ontario, Canada

We present new results and review existing data highlighting different aspects of the genetic relationship between partial melting and hydrothermal mineralizing processes in the contact aureole of the Sudbury Igneous Complex. At the basal contact of the igneous complex, in the footwall breccia, crystallized partial melt pods and veins, referred to as footwall granophyres are abundant and intrude all rock types including the breccia matrix, as well as massive Ni-Cu sulfide ore. The final crystallization of these melts was accompanied by the segregation of high salinity fluids dominantly in the temperature range of 450° to 550°C, as revealed by Ti-inquartz thermometry and studies on primary fluid inclusions. In mineralized parts of the footwall breccia there is clear evidence that interaction of partial melts, exsolved fluids, and preexisting magmatic sulfides occurred. In the deeper footwall of the Sudbury Igneous Complex, similar footwall granophyres occur as networks of veins intruding impact brecciated (Sudbury breccia) country rocks and have an intimate spatial association with hydrothermal, low sulfide assemblages highly enriched in Pt, Pd, and Au (with a unique assemblage of PGM, including malyshevite and lisiguangite). It is suggested that partial melting processes were widespread in the contact and proximal footwall environment of the igneous complex and they were important in providing high salinity magmatic fluids to a hydrothermal system responsible for redistribution of base and precious metals. We describe observations which may evidence the migration of partial melts from the contact into the deeper footwall. Furthermore, we suggest a cogenetic model for the precipitation of sulfide-hydrous silicate assemblages in close spatial association to footwall granophyres. We emphasize that the use of footwall granophyres in mineral exploration for Cu-Ni-PGE ores in the footwall of the Sudbury Igneous Complex is clearly justified. Mapping of footwall granophyre vein networks highlights areas that were proximal to the contact environment and point out structural pathways that may have been used by syn- or postgenetic mineralizing fluids in a given area. Widespread partial melting appears to have occurred in the footwall of other large mafic-ultramafic complexes (such as Bushveld and Duluth) and similar genetic relationships of partial melting and ore-forming hydrothermal processes may have possibly existed also in these systems.

Geochemical and microstructural evidence for in situ formation of pseudotachylitic Sudbury breccia by shock-induced compression and cataclasis

Pseudotachylitic breccias occur in the crater floor of large impact structures. Their origin is widely attributed either to shock melting and cataclasis during propagation of an impact shock wave, or to frictional melting and cataclasis during large slip displacements along crater collapse superfaults. By contrast, some models propose that the breccia matrices are allochthonous superheated melts from overlying impact melt sheets or from sites within the crater floor, which leaked into dilational fractures during collapse of the crater. In an effort to test these models, the compositions of pseudotachylite Sudbury breccias in the South Range of the Sudbury impact structure, Canada, were here compared to those of its associated impact melt sheet, the Sudbury Igneous Complex (SIC). The studied breccias occur along the contact between sandstone and a gabbro stock, and additional breccia bodies are located solely within these two lithologies. Major, trace element and lead isotope systematics suggest that the contact breccias are derived by mixing of comminuted gabbro and sandstone and that breccias hosted exclusively by sandstone or gabbro have compositions similar to their host rocks. The SIC has distinct chemical and isotopic compositions which argue strongly against the involvement of the impact melt sheet in the makeup of the breccias. Quartz and plagioclase in clasts and matrix of the breccias are transected by multiple microfractures and they have pervasive mosaic textures characterized by rolling and mottled undulatory extinction of micron-sized subgrain-like domains and new grains. These textures and strong local host control on chemical and clast composition of the breccias suggest that the breccias formed in situ by shock-induced compression and cataclasis of their host rocks during propagation of the shock wave.

Partial melting and melt segregation in footwall units within the contact aureole of the Sudbury Igneous Complex (North and East Ranges, Sudbury structure), with implications for their relationship to footwall Cu–Ni–PGE mineralization

We performed detailed field and drill core mapping of partial melting features and felsic rocks (footwall granophyres, FWGRs) representing segregated and crystallized partial melts within the contact aureole of the Sudbury Igneous Complex (SIC) in the 1.85 Ga Sudbury impact structure. Our results, derived from mapping within the North (Windy Lake, Foy, Wisner areas) and East Ranges (Skynner, Frost areas) of the structure, reveal that partial melting was widespread in both felsic and mafic footwall units up to distances of 500 m from the basal contact of the SIC. Texturally and mineralogically, significant differences exist between rocks formed by partial melting within and between localities. In general, however, melt bodies are dominated by different quartz-feldspar intergrowths (e.g. granophyric, graphic) and miarolitic cavities up to 5 cm in diameter. Major and trace element compositions of Wisner and Frost FWGRs imply that they crystallized from melts dominantly derived from partial melting of felsic Levack Gneiss and Cartier granitoid rocks, as well as from gabbroic rocks only at Frost. These results accord with our observations on in situ partial melting features and crystallized melt of microscopic scale in both felsic and mafic rocks. We conclude that partial melting occurred at a pressure of 1.5 ± 0.5 kbar and at temperatures up to 750°C in the Wisner area and up to 900°C in the Frost and Windy Lake areas. Segregations of partial melt into veins and dikes are present in all localities, and were promoted by deformation of the Sudbury structure in the Penokean orogeny as indicated by dominant strike directions. Whereas veins and dikes reflect brittle conditions during melt migration, sheared melt pods in the Sudbury breccia matrix indicate ductile conditions during their crystallization. Our results suggest a close genetic association of partial melting, melt segregation, and hydrothermal processes responsible for remobilization of Cu–Ni–PGE sulphides into and within the SIC footwall.

Footwall-type Cu-Ni-PGE Mineralization in the Broken Hammer Area, Wisner Township, North Range, Sudbury Structure

Footwall-type Cu-Ni-PGE (Platinum-group element) deposits are a major focus of recent exploration in the Sudbury mining camp, owing to their higher Cu and precious metal contents compared to orebodies situated along the lower contact of the Sudbury Igneous Complex. The most important footwall-type orebodies occur in the Onaping-Levack area of the North Range, but economic deposits are found along the East Range of the structure as well. Recently, promising occurrences were also discovered in Wisner Township of the North Range. In this paper we report results from two main footwall-type Cu-Ni-PGE occurrences (the Broken Hammer zone and the South zone) of Wisner Township. Both discoveries are situated in intensely brecciated zones of the footwall (Sudbury Breccia), 1,300 and 500 m, respectively, north of the present Sudbury Igneous Complex-footwall contact. The mineralization consists of (1) massive sulfide veins, (2) disseminated/replacement sulfides, and (3) silicate-quartz–dominated veins. Although massive sulfide veins account for the major part of the resource, sulfide-poor disseminations may have similarly high precious metal contents. Ore assemblages are dominated by chalcopyrite, millerite, magnetite, and/or pyrite, quartz, and hydrous silicates. These are accompanied by platinum-group minerals (PGM) and other trace minerals, which commonly occur as composite grains within sulfides and hydrous silicates. These include minerals typical of footwall-type deposits (e.g., merenskyite, michenerite, hessite), but also minerals found only in a few Sudbury deposits (e.g., clausthalite, sopcheite, naumannite, bohdanowiczite). An unnamed CuPdBiS 3 mineral is one of the most abundant PGM at Broken Hammer and might be a Pd-equivalent of muckeite (CuNiBiS 3 ). Statistical investigation of metal distribution patterns using grab sample and drilling data sets from both localities indicate important differences between the two occurrences and the various mineralization styles (e.g., increasing Cu/(Cu+Ni) and decreasing Pt/(Pt+Pd) ratio with increasing sulfide content; metal concentrations recalculated to 100% sulfides decrease by up to two orders of magnitude for all metals from the disseminated to the massive sulfides). Fluid inclusion studies revealed three fluid generations which are very similar to those described from well known footwall-type deposits of the Onaping-Levack area. Primary polyphase fluid inclusions in Cu-Ni-PGE–bearing assemblages represent a highly saline (30–40 wt % NaCl equiv) magmatic-hydrothermal system with a high temperature (min 450°–500°C) and a lower temperature (min 300°–350°C) stage. A later low-temperature Ca-rich fluid (min 100°–200°C) is most probably part of a post-Sudbury Igneous Complex regional fluid flow in which temperature was controlled by the geothermal gradient and not related to ore-forming processes. The fluid inclusion data indicate that mobilization of highly saline and relatively high temperature (400°–500°C) Sudbury Igneous Complex-related magmatic-hydrothermal fluids similar to those described from the Onaping-Levack embayment also occurred in the Wisner footwall. Although our studies do not rule out the possibility of an initial magmatic sulfide melt emplacement, they prove that magmatic-hydrothermal fluids played a significant role in the transport and deposition of metals.

The formation of the Sudbury breccia in the North Range of the Sudbury impact structure

In the North Range of the ca. 1.85 Ga Sudbury impact structure, massive bodies of Sudbury breccia cut across the Archean Levack gneiss complex and Early Proterozoic Matachewan diabase dykes. The complex underlies the Sudbury Igneous Complex (SIC), which represents an impact melt sheet that lined the floor of the impact crater. The breccia occurs as 1–100 m wide irregular bodies that contain rounded clasts of diabase and tonalitic and dioritic Levack gneiss within a fine-grained to aphanitic, black to dark grey matrix. Mineral chemical analyses and Rietveld analyses based on X-ray diffraction patterns show that the breccia matrix contains microclasts of magnesiohornblende, oligoclase, and quartz, surrounded by metamorphic andesine laths, actinolite, Kfeldspar, albite, and chlorite, which crystallized during cooling of the SIC. On binary oxide-SiO 2 diagrams, breccia values plot on a compositional mixing line between those of the more felsic tonalitic gneiss and the more mafic dioritic gneiss and diabase. The breccia has concentrations in rare earth elements (REEs) intermediate between those of the dioritic and tonalitic gneisses and a REE pattern with a negative slope that is less pronounced than those of the gneisses. This is due to the addition of a minor diabase component that has a slightly negative REE slope, suggesting that the breccia is a mix of comminuted tonalitic gneiss, dioritic gneiss, and minor diabase. Comminution and cataclasis of these source rocks occurred along pre-existing anisotropies and fractures that formed and were reactivated during the growth and collapse of the transient crater. The comminuted materials mixed together as large bodies of breccia that were injected in dilational sites that opened during the upward expansion and collapse of the crater.

Deformation of the Sudbury Structure: Paleomagnetic evidence from the Sudbury breccia

Paleomagnetic results from fine-grained samples of Sudbury breccia, collected from 28 sites around the northern half of the 1.85Ga Sudbury Igneous Complex (SIC), reveal the existence of two major components of magnetization, A and B. A, carried by magnetite, was formed close to the time of the SIC and has a direction that is related to the dip of the norite unit within the different ranges of the SIC. Component B, mainly carried by pyrrhotite, is a younger, secondary magnetization of more problematical origin, in part related to heating during the 1.24Ga Sudbury magmatic event. In the northwest part of the SIC, A is found in breccia clasts, intermediate norite, sublayer, Levack Gneiss and Matachewan diabase, indicating that it is due to baking by the SIC. Tilt-correcting the West range by 36°, the North range by 22° and the northern East range by 36°, about axes parallel to local strike of the norite, minimises the dispersion of the paleomagnetic data to give a pre-folding direction of 199.4°/78.8°. The paleomagnetic data are similar to those obtained by Morris [Morris, W.A., 1984. Paleomagnetic constraints on the magmatic, tectonic, and metamorphic history of the Sudbury basin region. In: Pye, E.G., Naldrett, A.J., Giblin, P.E., (Eds.), The Geology and Ore Deposits of the Sudbury Structure, Special vol. 1. Ontario Geological Survey, pp. 411–427] on the SIC, suggesting for the first time that the SIC and the surrounding basement were deformed (probably during the 1.9–1.7Ga Penokean Orogen) as a single unit. The un-folding of the paleomagnetic directions indicates that SIC limbs were already inclined, a result of the original intrusion configuration, or a syn-folding remanence. It is possible that the SIC was originally flatter if the distribution of paleomagnetic directions is the result of flexural flow folding. This explanation could also produce a similar distribution of paleomagnetic directions across the basin, but cannot be quantified due to the lack of measurable fold-related strain in breccia and SIC units around the northern half of the basin.

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The Sudbury Structure, formed by meteorite impact at 1850 Ma, consists of three major components: (1) the Sudbury Basin; (2) the Sudbury Igneous Complex, which surrounds the basin as an elliptical collar; and (3) breccia bodies in the footwall known as Sudbury Breccia. In general, the breccia consists of subrounded fragments set in a dark, fine-grained to aphanitic matrix. A comparison of the chemical composition of host rocks, clasts and matrices indicates that brecciation was essentially an in-situ process. Sudbury Breccia forms irregular-shaped bodies or dikes that range in size from mm to km scale. Contacts with the host rocks are commonly sharp. The aspect ratio of most clasts is approximately 2 with the long axes parallel to dike walls. The fractal dimension (Dr)=1.55. Although there appears to be some concentration of brecciation within concentric zones, small Sudbury Breccia bodies within and outside these zones have more or less random strikes and steep dips. Sudbury Breccia bodies near an embayment structure tend to be subparallel to the base of the Sudbury Igneous Complex. Sudbury Breccia occurs as much as 80 km from the outer margin of the Sudbury Igneous Complex. In an inner zone, 5 to 15 km wide, breccia comprises 5% of exposed bedrock with an increase in brecciation intensity in embayment structures. Sudbury Breccia may be classified into three types based on the nature of the matrix: clastic, pseudotachylite and microcrystalline. Clastic Sudbury Breccia, the dominant type in the Southern Province, is characterized by flow-surface structures. Possibly, a sudden rise in pore pressure caused explosive dilation and fragmentation, followed by fluidization and flowage into extension fractures. Pseudotachylite Sudbury Breccia, mainly confined to Archean rocks, apparently formed by comminution and frictional melting. Microcrystalline Sudbury Breccia formed as a result of the thermal metamorphism, of the North Range footwall, by the Sudbury Igneous Complex. This produced a zone, approximately 1.2 km wide, wherein the matrix of the breccia either recrystallized or, locally, melted. An overprint of regional metamorphism obliterated contact effects in the South Range footwall. The Ni–Cu–PGE magmatic sulphide deposits may be classified into four types based on structural setting: Sudbury Igneous Complex–footwall contact, footwall, offset, and sheared deposits. Sudbury Breccia is the main host for footwall deposits (e.g., McCreedy East, Victor, Lindsley). Sudbury Breccia locally hosts mineralization in radial (e.g., Parkin and Copper Cliff) and concentric (e.g., Frood–Stobie) offset dikes.

The tectonostatigraphic evolution of the Huronian basement and the subsequent basin fill: geological constraints on impact models of the Sudbury event

Abstract Whereas both meteorite impact and volcanic mechanisms have been proposed to explain the rocks and associated ore deposits within the Sudbury structure (1.85 Ga), few models take into consideration the paleogeographic constraints implied by the history of the region before, and following the “Sudbury event”. Huronian depositional style south of the Sudbury structure appears to have been strongly influenced by the early development of transtensional extensional basins. These basins were initiated by sinistral strike-slip movement along part of a Paleoproterozoic continental margin, which later evolved into a passive margin. Transition from a rifted strike-slip margin to a passive margin was accompanied by major slope failure and soft sediment deformation south of the Murray Fault. Diabase dikes were intruded into unconsolidated sediments at considerable depth, producing round-stone breccias, which are texturally similar to Sudbury breccia. Minor compressional folding occurred prior to emplacement of the Nipissing Diabase Suite at around 2.2 Ga. The abundance of diabase intrusions near the Sudbury structure suggests that the area may have been a focal point for magma emplacement. The post “event” fill of the Paleoproterozoic Sudbury Basin reflects initial deposition in a deep-water, anoxic setting. There is no evidence to suggest that sedimentation was influenced by the steep-walls of an impact crater. Carbonate in the basal, Vermilion Member is of sedimentary-exhalative origin and was not derived from a shallow marine shelf. Turbidites in the Chelmsford Formation show no evidence of centripetal fill as might be expected from a restricted, circular basin. They appear to have been emplaced along the depositional axis of an elongate foreland basin developed in front of the rising Penokean mountain chain. Penokean deformation continued after the “Sudbury event” and influenced strata well into the Archean Craton. Deformation of Huronian strata in paleovalleys north and west of the Sudbury structure does not appear to be genetically related to the “Sudbury event”. If “superfaults” were generated around the Sudbury structure in response to compression and crustal rebound following a meteorite impact, these do not appear to have caused offset of pre-existing steeply dipping contacts between the Huronian and underlying Archean basement. A new model, consistent with these observations needs to be developed.

CHLORINE ENRICHMENT AND HYDROUS ALTERATION OF THE SUDBURY BRECCIA HOSTING FOOTWALL Cu Ni PGE MINERALIZATION AT THE FRASER MINE, SUDBURY, ONTARIO, CANADA

An assemblage of hydrous, Cl-bearing alteration-induced minerals is present within 150 m of footwall-style Cu–Ni–PGE sulfide mineralization hosted by the Sudbury Breccia at the Fraser Copper zone, Fraser mine, Onaping–Levack area, Ontario. The pyroxene hornfels to hornblende hornfels contact-metamorphic aureole of the Sudbury Igneous Complex (SIC) contains less than 15 modal % hydroxysilicates. Near the mineralized areas, a diagnostic assemblage of secondary minerals of the albite–epidote hornfels facies is present in the matrix of the Sudbury Breccia; it contains 25 to 80% actinolite + chamosite as well as epidot e + sodic plagioclase (An2.8–42) + quartz ± titaniferous magnetite ± biotite ± K-feldspar ± titanite. The Cl content and Cl/(Cl + F) values of amphibole and biotite in this matrix are elevated within 150 m of mineralization, and increase toward known occurrences of sulfides. Parameters of K, Na, and A-site occupancy correlate with Cl content in actinolite in such matrix material. The value of Mg# is lower in biotite in the matrix near mineralized areas and is inversely correlated with Cl/(Cl + F) in the bioti te, reflecting Mg–Cl or Fe–F avoidance. Calculated temperatures of amphibole–plagioclase equilibration for the diagnostic hydrous assemblage are between 442 to 540 ± 75°C. The assemblage formed during a retrograde metamorphic event during or after footwall Cu–Ni–PGE mineralization, and after passage of peak conditions of contact metamorphism at the base of the SIC. The enrichment of Cl in this assemblage was the result of the increased amounts of free Cl relative to F in the equilibrating fluid , and favorable cation proportions for incorporation of Cl into the structure of hydroxysilicate minerals. The Cl-bearing assemblage accounts for a significant portion of the bulk halogen content of Sudbury Breccia matrix. It is unique to zones of Sudbury Brec cia that host mineralization, and hence may be used to identify prospective areas.

"Sudbury Breccia" at Whitefish Falls, Ontario: evidence for an impact origin

Sudbury breccias are unusual clast–matrix rock bodies formed in abundance around the Sudbury Igneous Complex, the most obvious manifestation of a major impact event at Sudbury. At Whitefish Falls, ~70 km southwest of Sudbury, similar breccias consisting of clasts of argillite and amphibolite dyke enclosed in a fine-grained matrix of host rock are developed in metamorphosed argillites of the Huronian Supergroup. Pre-brecciation brittle textures in the host argillite and breccia clasts, such as layer-parallel foliation offset by cataclastic fractures, suggest that the host rock was entirely competent prior to brecciation. One composite penetrative foliation and its associated ductile folding were also formed in the argillite host prior to brecciation. Post-brecciation ductile deformation produced a regionally dominant east–west-trending foliation, and two late-stage folding events, and indicate a syn-Penokean age of brecciation. The breccias at Whitefish Falls are enriched in ferromagnesian minerals compared to adjacent, embayed and partially digested, host rock. Flow-foliated breccia matrices surround a highly rounded clast phase. These features are characteristic of impact-related pseudotachylyte, formed during extreme cataclasis and friction melting of the impacted host rock. We propose that these breccias formed by injection of a high-strain, pseudotachylytic melt, triggered by the Sudbury impact event, and focused along a blind superfault, coincident with a post-Penokean high-strain zone.

Sudbury Breccia (Canada): a product of the 1850 Ma Sudbury Event and host to footwall Cu–Ni–PGE deposits

The Sudbury Structure, formed by meteorite impact at 1850 Ma, consists of three major components: (1) the Sudbury Basin; (2) the Sudbury Igneous Complex, which surrounds the basin as an elliptical collar; and (3) breccia bodies in the footwall known as Sudbury Breccia. In general, the breccia consists of subrounded fragments set in a dark, fine-grained to aphanitic matrix. A comparison of the chemical composition of host rocks, clasts and matrices indicates that brecciation was essentially an in-situ process. Sudbury Breccia forms irregular-shaped bodies or dikes that range in size from mm to km scale. Contacts with the host rocks are commonly sharp. The aspect ratio of most clasts is approximately 2 with the long axes parallel to dike walls. The fractal dimension (Dr)=1.55. Although there appears to be some concentration of brecciation within concentric zones, small Sudbury Breccia bodies within and outside these zones have more or less random strikes and steep dips. Sudbury Breccia bodies near an embayment structure tend to be subparallel to the base of the Sudbury Igneous Complex. Sudbury Breccia occurs as much as 80 km from the outer margin of the Sudbury Igneous Complex. In an inner zone, 5 to 15 km wide, breccia comprises 5% of exposed bedrock with an increase in brecciation intensity in embayment structures. Sudbury Breccia may be classified into three types based on the nature of the matrix: clastic, pseudotachylite and microcrystalline. Clastic Sudbury Breccia, the dominant type in the Southern Province, is characterized by flow-surface structures. Possibly, a sudden rise in pore pressure caused explosive dilation and fragmentation, followed by fluidization and flowage into extension fractures. Pseudotachylite Sudbury Breccia, mainly confined to Archean rocks, apparently formed by comminution and frictional melting. Microcrystalline Sudbury Breccia formed as a result of the thermal metamorphism, of the North Range footwall, by the Sudbury Igneous Complex. This produced a zone, approximately 1.2 km wide, wherein the matrix of the breccia either recrystallized or, locally, melted. An overprint of regional metamorphism obliterated contact effects in the South Range footwall. The Ni–Cu–PGE magmatic sulphide deposits may be classified into four types based on structural setting: Sudbury Igneous Complex–footwall contact, footwall, offset, and sheared deposits. Sudbury Breccia is the main host for footwall deposits (e.g., McCreedy East, Victor, Lindsley). Sudbury Breccia locally hosts mineralization in radial (e.g., Parkin and Copper Cliff) and concentric (e.g., Frood–Stobie) offset dikes.