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Mafic intrusions in the footwall of the Sudbury Igneous Complex: Origin of the Sudbury impact melt sheet and its associated ore deposits
Abstract
The majority of the mafic intrusions and volcanic rocks in the footwall of the Sudbury Igneous Complex (SIC), which is believed to have formed from a melt sheet produced by the impact of a very large bolide, belong to one of two large igneous events, viz: the ∼2450 Ma East Bull Lake (EBL) suite of intrusions or the ∼2200 Ma Nipissing Diabase suite of intrusions. This has been established by U-Pb LA-ICP-MS dating of zircons from the newly recognized Frood intrusion, which hosts the Frood and Stobie ore bodies, and two of the Sudbury Gabbro intrusions, and comparison of the geochemistry of these intrusions with that of the EBL intrusive suite and the Nipissing Diabase suite. The age of the Frood intrusion (2421 ± 32 Ma) falls within error of other EBL-type intrusions whereas the ages of the Nairn (2203 ± 11 Ma) and Totten (2168 ± 11 Ma) intrusions fall within error of published ages for the Nipissing Diabase. Moreover, the primitive mantle-normalized patterns of the petrogenetic trace elements on extended spidergrams of the Nairn and Totten Sudbury Gabbros are similar to those of the Nipissing Diabase as are those of the Frood intrusion to other members of the EBL intrusive suite.
The Ni-Cu-PGE sulfide deposits at Sudbury are associated with the lower contact of the SIC, where a discontinuous inclusion-rich norite contains much of the mineralization at the contact, and an inclusion-bearing quartz diorite is directly associated with the mineralization of the radial and concentric offset dykes. The Sublayer rocks and the overlying unit of melanocratic norites provide strong evidence for a relationship between the ore deposits and both inclusions and melts of mafic-ultramafic target rocks. The most common pre-impact mafic-ultramafic target rocks are intrusions of the EBL and Nipissing Diabase suites, although there are also Archean-aged mafic-ultramafic enclaves in the gneisses beneath the North Range of the SIC. The Totten orebody in the Worthington Offset and the Kelly Lake ore body in the Copper Cliff Offset are directly associated with footwall rocks comprising Sudbury Gabbro and Nipissing Gabbro, respectively, whereas the Frood-Stobie Offset and the associated Frood and Stobie ore deposits are sandwiched in a breccia belt containing large rafts of amphibole megacrystic gabbro belonging to the extensively broken-up Frood intrusion. Olivine-bearing mafic and ultramafic rocks, some of which exhibit shock metamorphic features, but others do not, are common inclusion types and occur in diverse Sublayer environments. The unshocked melanorite and olivine melanorite inclusions share common geochemical characteristicsde, indicating that they were derived from a similar source. Their geochemistry is also similar to that of the Frood Intrusion as well as the East Bull Lake intrusion; this indicates that they may have been sourced from East Bull Lake-type intrusions that were incorporated into the melt sheet at the time of melt generation.
Mass balance calculations indicate that the felsic country rocks which account for 77% of the total volume of the SIC and contain 18 ppm Ni and 21 ppm Cu together with the sulfide-poor mafic country rocks which account for 23% of the SIC and contain 133 ppm Ni and 92 ppm Cu have too little Ni and Cu to account for the Ni and Cu contents of the quartz diorite, which provides an indication of the composition of the initial melt sheet.
Calculations using the R-factor equation of Campbell and Naldrett (1979) indicate that the magmas that formed the norites and the ore deposits had 137 ppm Ni and 121 ppm Cu. Mass balance calculations have been used to determine if the country rocks have sufficient Ni and Cu in them to generate the ore deposits. The weighted median Ni content of the sulfide poor (<200 ppm Cu) mafic country rocks is 133 ppm and hence very similar to that of the ore-forming magma. However, the weighted median Cu contents of these rocks is only 90 ppm and hence too low to provide all of the Cu required by the ore-forming magma. What the mass balance calculation shows is that a contribution of Cu (and Ni) in the ore-forming magma was derived from pre-existing Ni-Cu-PGE sulphides associated with the mafic target rocks. This contribution was diluted by the lower concentrations of Ni and Cu from the Proterozoic metasedimentary rocks and the Archean granitoids and gneisses. For this reason, a contribution from magmatic sulfides in the mafic target rocks is required, and this may have been similar to the known disseminated concentrations of Cu-PGE-Ni sulfides present in mafic intrusions in the Sudbury area.
... Large Igneous Provinces (LIPs) that record a major mantle-plume driven, intracontinental rifting event (Keays and Lightfoot, 2020). These intrusions include the ca. ...
... 2.48 Ga Matachewan dike swarm, East Bull Lake, Agnew Lake and River Valley intrusions, which have correlatives in the Wyoming, Hearne, Kola and Karelian cratons; and the ca. 2.2 Ga Nipissing dike swarm (Heaman, 1997;Easton et al., 2004;Ames et al., 2008;Ciborowski et al., 2015;Keays and Lightfoot, 2020). The intrusions are important components of the Huron-Nipissing magmatic belt (Peck et al, 1993) and contribute to the 350 km-long Elliot Lake-Englehart (ELE) Gravity High located inboard of the southern margin of the Southern Province Hearst and Morris, 2001). ...
... It is hosted within one of Earth's largest preserved impact craters. The unusually high Ni-Cu-PGE endowment of the SIC is not well understood but possible remobilization of metals from a Ni-Cu-PGE endowed basement by the impact event has being studied by analyzing the geochemical composition of ultramafic and mafic inclusions in the SIC sublayer and offset dikes (Begg et al., 2010;Keays and Lightfoot, 2020;Wang et al., 2020) as well as Fe-oxide compositions from fertile and barren igneous complexes (Dare et al., 2014). ...
The Temagami Geophysical Anomaly (TGA) is situated in the Paleoproterozoic Huronian Supergroup near the southern margin of Archean Superior craton, along strike from the Temagami Greenstone Belt, 50 km northeast of world-class metal endowment in the Sudbury Igneous Complex. It has been attributed to coincidental geophysical responses from Neoarchean iron formation and a Paleoproterozoic mafic–ultramafic intrusion. The TGA is investigated using integrated magnetotelluric, magnetic, gravity and seismic reflection datasets to further define the geometry of its sources; to define structures in overlying Huronian Supergroup rocks; and to examine the geometrical relationship of Archean basement structure, the Proterozoic mafic–ultramafic intrusion, and Proterozoic rift structures.
Geophysical results reveal a strong conductor, interpreted to be iron formation, extending downwards from the Archean basement surface and enveloping the upper northern margin of a large magnetic, dense body at 5 km depth. The geometry of the features indicates mafic–ultramafic intrusion adjacent to greenstone rocks. Alignment of the long-axis of the intrusion with the greenstone belt suggests control on the intrusion process by the Neoarchean structures. The 60x10x10 km TGA intrusion lies within the mantle-plume, rift-related Huron-Nipissing magmatic belt and interpreted to be a member of the 2.491–2.475 Ga East Bull Lake intrusive suite. Based on this genetic relationship, the intrusion probably contributes significantly to Ni-Cu-PGE endowment of the Sudbury region. The spatial correlation of the intrusion with an overlying 20 km wide, 4.5 km deep, fault-bounded rift basin is attributed to crustal subsidence triggered by the intrusion. Seismic reflection and magnetotelluric results show younger Huronian Supergroup rocks record the transition to sedimentation on a laterally-extensive passive margin. An electrically-anisotropic, moderately-conductive layer in the Gowganda Formation is related to alteration during intrusion of the 2.200 Nipissing Diabase.
... The vast majority of Canadian Ni and Co production is sourced from: 1) Ni deposits from the Sudbury basin. The Ni-and Co-bearing pentlandite and pyrrhotite at these deposits segregated from a melt sheet that formed in response to a Paleoproterozoic meteorite impact (Keays and Lightfoot 2020). Although these impactrelated Ni deposits are economically significant to Canada and globally, the probability of finding additional examples of this mineralization style outside of the Sudbury basin is low. ...
... National gravity and magnetic survey compilations, re-processed using multiple methods to highlight the edges of density and magnetic anomalies, were also essential for mapping mineral system pathways under surficial cover and at multiple depths within the crust (discussed below). Interaction between mantle-derived melts and the S-rich rocks, either during transport along these pathways or at the depositional trap, is interpreted as one of the possible processes for triggering the rapid deposition of immiscible sulphide liquid droplets within the magma to form a mineral deposit (Barnes and Lightfoot 2005;Keays and Lightfoot 2020). The presence of black shales or other carbonaceous rock types contained within geological map databases were used to assess the potential for this depositional trapping Earth Science, Systems and Society | The Geological Society of London December 2022 | Volume 2 | Article 10064 mechanism across Canada. ...
Electrification of Canada’s energy and transport sectors is essential to achieve net-zero emissions by 2050 and will require a vast amount of raw materials. A large proportion of these critical raw materials are expected to be sourced from as yet undiscovered mineral deposits, which has the potential to accelerate environmental pressures on natural ecosystems. Herein we overlay new prospectivity model results for a major source of Canada’s battery minerals (i.e., magmatic Ni ± Cu ± Co ± PGE mineral systems) with five ecosystem services (i.e., freshwater resources, carbon, nature-based recreation, species at risk, climate-change refugia) and gaps in the current protected-area network to identify areas of high geological potential with lower ecological risk. New prospectivity models were trained on high-resolution geological and geophysical survey compilations using spatial cross-validation methods. The area under the curve for the receive operating characteristics (ROC) plot and the preferred gradient boosting machines model is 0.972, reducing the search space for more than 90% of deposits in the test set by 89%. Using the inflection point on the ROC plot as a threshold, we demonstrate that 16% of the most prospective model cells partially overlap with the current network of protected and other conserved areas, further reducing the search space for new critical mineral deposits. The vast majority of the remaining high prospectivity cells correspond to ecoregions with less than half of the protected areas required to meet national conservation targets. Poorly protected ecoregions with one or more of the five ecosystem services are interpreted as hotspots with the highest potential for conflicting land-use priorities in the future, including parts of southern Ontario and Québec, western Labrador, and northern Manitoba and Saskatchewan. Managing hotspots with multiple land-use priorities would necessarily involve partnerships with both Indigenous peoples whose traditional lands are affected, and other impacted communities. We suggest that prospectivity models and other machine learning methods can be used as part of natural resources management strategies to balance critical mineral development with conservation and biodiversity values.
... The SIC and to a lesser extent its brecciated footwall are the principal host of magmatic Ni-Cu-PGE (±Au ±Co) sulphide mineralization. Consensus exists that the metals that are currently being extracted from the SIC were not derived from the impactor itself, either a comet or chondritic asteroid (e.g., Petrus et al. 2015;Mougel et al. 2017), but are solely due to reworking of an unusual mafic, pre-enriched crust in the target area of the impact (Keays and Lightfoot 2020). Consequently, there is an incentive to gain a better understanding of the pre-impact crustal architecture and target rock stratigraphy of the uniquely endowed Sudbury Impact Structure. ...
The area northeast of Sudbury, Ontario, is well-known for hosting one of the largest unexplained geophysical anomalies in the Canadian Shield, the Temagami Anomaly. In search of a geological explanation for this anomaly, low-grade metamorphic ultrabasic dykes have been discovered in the overlying Huronian Supergroup sedimentary rocks, both in outcrop and in a deep drill core. Here we report on the first geochemical and geochronological data obtained on these dykes and compare these data with known magmatic units in and around the 1850 Ma impact-generated Sudbury Igneous Complex (SIC). The NW-striking dykes, which cut across sedimentary rocks of the ~2.3 Ga Cobalt Group, and which are, in turn, crosscut by pseudotachylitic breccia, are characterised by distinctively high concentrations of Ti, P, Nb and Zr, highly fractionated REE patterns (La/YbN 7.6–15.5), and a lack of crustal contamination (Nb/Th >10). Such features are typical of modern ocean island basalt (OIB) but very different from Palaeoproterozoic rocks previously documented in the wider region. Multigrain U-Pb LA-ICP-MS analyses performed on magmatic titanite and apatite with high Th/U ratios yielded 1876.0 ± 8.7 Ma and 1880.9 ± 8.3 Ma, respectively, which we interpret as the intrusion age of the dykes. This interpretation is supported by similar whole-rock Sm-Nd model ages of 1890–2000 Ma (initial εNd +2.5). This magmatic event in the footwall of the SIC shortly before the impact was coeval with, and likely genetically related to, the 1.88–1.87 Ga Circum-Superior Large Igneous Province.
The lowermost, discontinuous parts of the impact-generated Sudbury Igneous Complex (Canada), comprising the Sublayer and Offset Dikes, are distinguished from overlying Main Mass norite rocks by the presence of abundant inclusions and Ni-Cu-PGE (PGE-platinum group element) sulfide mineralization. The majority of the felsic to mafic inclusions appear to be derived from the exposed country rocks, but the volumetrically important olivine-bearing mafic and ultramafic inclusions have only very rare equivalents in the surrounding country rocks. We record the discovery of abundant shock metamorphic features (e.g., mosaicism in olivine; strong fracturing and partial isotropization of plagioclase) in the olivine-bearing mafic and ultramafic inclusions consistent with a shock pressure of 20-30 GPa. Olivine compositional data are inconsistent with a local country rock or mantle origin for these inclusions. Abundant plagioclase, the absence of garnet or Mg-spinel, and calculated low pressures ( < 500 MPa) provide evidence for derivation of the inclusions from unexposed mafic-ultramafic intrusions in the upper to middle crust that were disrupted during formation of the transient crater, incorporated into the impact melt sheet, and preserved because of their relatively refractory compositions. These observations support models involving intermediate, rather than very deep or very shallow, excavation for the Sudbury impact event.
A classic case study forging research from the fields of economic geology, petrology, geochemistry, geophysics, and the study of large terrestrial and extra-terrestrial impact structures KEY FEATURES • The only reference book focusing entirely on the Sudbury Igneous Complex • Brings together an understanding of ore deposit and impact melts as a basis for future exploration • Authored by a leading expert on the geology of the Sudbury Igneous Complex, with 35 years of experience working on nickel sulfide ore deposits DESCRIPTION Nickel Sulfide Ores and Impact Melts presents a current state of understanding of the geology and ore deposits of the Sudbury Igneous Complex in Ontario, Canada. The first complete reference on the subject, this book explores the linkages between the processes of meteorite impact, melt sheet formation, differentiation, sulfide immiscibility and metal collection, and localization of the ores by magmatic and post-magmatic processes. The discovery of new ore deposits requires industry and government scientists and academic scholars to have access to the latest understanding of ore formation process models that link to the mineralization of their host rocks. The ore deposits at Sudbury are one of the world's largest ore systems and are a classic case study that brings together very diverse datasets and ways of thinking. Nickel Sulfide Ores and Impact Melts is designed to emphasize concepts that can be applied across a broad range of ore deposit types beyond Sudbury and nickel deposit geology. It is an essential resource for exploration geologists, university researchers, and government scientists, and can be used in rock and mineral analysis, remote sensing, and geophysical applications.
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It is commonly possible to identify groups of orebodies within one cluster or camp of temporally and spatially constrained deposits that have common empirical traits, and to then develop successful exploration technologies based on these similarities. Models developed for exploration for contact, footwall, and offset styles of mineralization at Sudbury are examples at the camp scale, whereas the models for massive sulphide and disseminated sulphide deposit types associated with the ultramafic rocks of the Yilgarn represent examples where the models have been applied on the scale of greenstone belts. Exploration at the edges of economically mineralized systems is often rewarding as the detailed quantitative data can be used to inform robust predictive models, and the exploration risk is commensurately lower. Inherent in this approach is the bias towards exploring with known technologies and models that do not consider previously unrecognized geologic settings. The risk of missing a new style of mineralization in a mature camp or mine environment or in a new geological environment is thus heightened. In greenfields exploration, data sets are often unavailable or incomplete, and successful exploration requires holistic geological models that embrace the common characteristics of nickel systems and maximize value from empirical geoscience observations that form the basis for project selection and then targeting of exploration using advanced geochemical and geophysical technologies. The petrologic characteristics of the host rocks can effectively be used to rank exploration approaches in prospective environments; for example, the petrological associations and geochemical signatures of the associated mafic and ultramafic rocks provide an indication of whether the magma is from a productive source and has equilibrated with sulphide. At the deposit scale, linkages between the abundance of metal in the sulphides within the rock and the geological context offer ways to inform geological models, and they can focus efforts to define drill targets and to better delineate orebodies. This paper summarizes the state of present understanding of the key requirements for the formation of large nickel sulphide systems. It also reviews recent advances in our understanding of these systems and illustrates how geological models are increasingly informed by an array of geochemical and geophysical data.
The Sudbury Igneous Complex is widely believed to be the result of melt generation due to meteorite impact, and the Ni and Cu ores associated with the sublayer and offsets of the complex are considered to be a consequence of this event. The noritic and granophyric rocks of the main mass have similar yet elevated ratios of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy rare earth elements (HREE). This indicates that the magma,vas homogenized in terms of the incompatible trace element ratios and had a large chemical and isotopic contribution from an upper crustal source. Through the mafic and felsic norite there is an upward decline in MgO (14.6-4.4 wt %) and a small increase in incompatible element abundance (Ce = 35-50 ppm). The granophyre becomes slightly more evolved from the top toward the base (MgO = 0.6-2 wt %; Ce = 90-120 ppm). Some of these variations may be due to differentiation, but we believe that the crystallization of the original melt took place from the base upward and the top downwards this implies that the melt sheet was fundamentally density stratified into noritic and granophyric layers, Estimates of the bulk composition of the melt sheet are within the range of quartz diorite from the different Sudbury, offsets, and we believe that all of these rocks are the products of impact-triggered melting of upper crustal rocks of the Sudbury region. Mafic norite from the base of the Sudbury Igneous Complex has 1 to 5 modal percent pyrrhotite, plus chalcopyrite and elevated Ni (40-1,000 ppm) and Cu (40-1,140 ppm); this is overlain by felsic norite, which contains trace pyrite, and low Ni (15-40 ppm) and Cu (7-40 ppm). For a similar range of MgO, the upper portion of the felsic norite unit has 5 to 10 times lower Ni and Cu than within-plate basalts and crustal rocks. There is an increase in Ni (1-5 % wt %) and Cu (1-5 wt %) tenor of sulfide toward the base of the norite sequence, The depletion of the upper part of the felsic norite in Ni, Cu, and platinum group elements (PGE) is most likely due to equilibration of the magma with magmatic sulfide and accumulation of the dense sulfide liquid to the base of the melt sheet, The parental magmas giving rise to these disseminated sulfides contained similar to210 ppm Ni and 109 ppm Cu based on realistic partition coefficients and magma/sulfide ratio; the process of sublayer and offset ore formation did not entirely strip the Ni, Cu, and PGE from the main mass, About 40 percent of the total Ni-Cu-PGE inventory of the Sudbury Igneous Complex is related to quartz diorite in offset dikes and Sudbury breccia belts. The marginal sulfide-free quartz diorite of the Worthington offset has a Ni and Cu abundance that is similar to crustal rocks over the same range in MgO content (3-6 wt % MgO). We believe that the quartz diorite initially formed from an S-undersaturated magma that was injected downward away from the melt sheet, The Worthington offset quartz diorite averages 83 ppm Ni and 98 ppm Cu, which is similar to sulfide-poor quartz diorite from the other offsets (Lightfoot et al., 1997b). Saturation of the melt sheet in S was achieved shortly after the injection of the quartz diorite magmas but before the center of the offset had entirely crystallized, the presence of immense resources of magmatic sulfides in the Copper Cliff offsets with sulfide metal tenors of similar to13 m percent Cu, similar to6 wt percent Ni, similar to18 ppm Pt. and high Pd/Ni, indicates that these sulfides formed early and from a parental magma with a Ni/Cu ratio of similar to1. The parental magma is calculated to have contained 310 ppm Ni and 311 ppm Cu, and this is not characteristic of the bulk crustal melt. Presumably density segregation accompanied or predated sulfide saturation in the melt sheet, and this mechanism provides a way to produce a more metal-rich parental magma with a Ni/Cu similar to1. The formation of the geologically complex and heavily contaminated sublayer is believed to either accompany or postdate offset formation. The sublayer at the Whistle and Creighton mines occupies embayment structures at the base of the Sudbury Igneous Complex, which are directly connected to small offsets. Ores from the Creighton embayment have sulfide metal tenors of similar to7 wt percent Cu, similar to5 wt percent Ni, and a lower Pd/Ni than the offset ores, Calculated parental magmas would have 265 ppm Ni and 172 ppm Cu, which indicates that the metals were not all removed by the formation of offset ores. Mass-balance calculations show that there is more than enough Ni and Cu in the initial melt sheet to produce all of the known sulfide deposits at Sudbury. In this model there is no requirement for metal contributions from mantle sources or from protores. The mineral potential of offset and embayment structures appears to be empirically linked to the thickness of the overlying noritic rocks, and this is consistent with our model.
U–Pb ages have been determined for two diabase intrusions from Kerns and Hudson townships that are part of the Nipissing diabase suite from the Proterozoic Southern Province, Canada. The ages of the Kerns and Triangle Mountain intrusions are 2217.2 ± 4 and 2209.6 ± 3.5 Ma, respectively, based on analysis of primary baddeleyite from both intrusions. Together with a published U–Pb age for the Castle Mine intrusion, these data indicate that the Nipissing sills were emplaced separately over a time span of 2.8–1.6 Ma, given by the maximum and minimum overlap of errors on the U–Pb ages.
An investigation has been made of the trace-element and Nd-isotopic effects of assimilation at the roof of the Proterozoic Kerns sill in the 2.2 Ga Nipissing diabase province in Ontario. The ratios Th/Zr, La/Zr, and U/Zr and the concentrations of incompatible elements all tend to increase with decreasing Mg#, Ni, and Cr. These variations have been simulated by computer models in which assimilation and fractionation are coupled (AFC) and the most fractionated magmas (identified by low Mg#, Ni, and Cr and by high incompatible-element concentrations) are also the most contaminated (indicated by higher Th/Zr, La/Zr, and U/Zr and lower 143Nd/144Ndo). The results suggest that the ratio (r) of the change of magma mass due to assimilation relative to the change due to fractionation gradually increased. The latent heat of crystallization may have contributed sufficient heat to melt the roof of the intrusion where ponded crustal melts were separated from the underlying basic magma by a double-diffusive interface. Field relations suggest that this interface was progressively destroyed by convective erosion; thus the degree of contamination increased as the magma became more fractionated. These results are consistent with laboratory investigations designed to simulate assimilation at the roof of basaltic magma chambers.
Integrated paleomagnetic and U–Pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of Early Proterozoic tholeiitic dykes of northeast trend in the southern Superior Province, previously referred to collectively as Preissac dykes. It is demonstrated that they are readily separated on the basis of paleomagnetism into subsets, referred to as the Biscotasing and Senneterre swarms. In addition a pair of unnamed dykes may be associated with the north-and northwest-trending Matachewan swarm farther west.Biscotasing dykes have a down-west magnetization of single polarity with a corresponding paleopole at 27.8°N, 136.7°W (dm = 12.3° and dp = 9.4°). Senneterre dykes carry an up-north (or occasionally down-south) direction with corresponding paleopole at 15.3°S, 75.7°W (dm = 7.0°, dp = 4.4°). The Senneterre direction is indistinguishable from the primary N1 remanence direction that dominates the magnetization of Nipissing sills of the Southern Province. Paleomagnetic field tests described herein or in earlier studies indicate that Biscotasing and Senneterre directions are primary and, hence, that two ages of intrusion are involved, with the age of Senneterre dykes coinciding with the intrusion of most Nipissing sills. U–Pb dating of baddeleyite conducted at a paleomagnetic sampling site yields an age of 2214.3 ± 12.4 Ma for the Senneterre swarm, indistinguishable from the age of 2217.2 ± 4 Ma reported from an N1 Nipissing sill site in another study. A U–Pb age on baddeleyite and zircon of 2166.7 ± 1.4 Ma was obtained from a paleomagnetic site in the Biscotasing swarm. The primary paleopoles for the Senneterre, Nipissing, and Biscotasing rocks define a direction of polar wander opposite to that of the most widely used polar wander paths for North America for this period, suggesting that these paths should no longer be used.
Major and trace element data are presented for 2.2 Ga Proterozoic diabase sills from across the Nipissing magmatic province of Ontario. In situ differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions. Uniform trace element and isotope ratio signatures, such as La/Sm (2.8 – 3.7) and εNdCHUR (−2.7 to −5.9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and is displaced towards the composition of shales. Most chilled diabases and quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content (98 and 127 ppm), and it is argued that the magma differentiated at depth and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz diabase has La/Sm similar to the Nipissing magma type, which suggests that they came from the same source, the Mg# (0.68–0.71) and Ni content (130–141 ppm) are higher, which may suggest that they are either slightly more primitive examples of the normal Nipissing magma or that cumulus hypersthene has been resorbed. The light rare earth element enriched signature of the Nipissing magmas was perhaps introduced from the continental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (>20%) of crustal contamination would be required. An addition of 1 –3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce the compositional features of the Nipissing magma type. The source characteristics were perhaps imparted during subduction accompanying the terminal Kenoran orogeny.
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, parautochtho- nous 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 contribu- tion 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.
Large Igneous Provinces (LIPs) are intraplate magmatic events, involving volumes of mainly mafic magma upwards of 100,000 km3, and often above 1 million km3. They are linked to continental break-up, global environmental catastrophes, regional uplift and a variety of ore deposit types. In this up-to-date, fascinating book, leading expert Richard Ernst explores all aspects of LIPs, beginning by introducing their definition and essential characteristics. Topics covered include continental and oceanic LIPs; their origins, structures, and geochemistry; geological and environmental effects; association with silicic, carbonatite and kimberlite magmatism; and analogues of LIPs in the Archean, and on other planets. The book concludes with an assessment of LIPs' influence onnatural resources such as mineral deposits, petroleum and aquifers. This is a one-stop resource for researchersand graduate students in a wide range of disciplines, including tectonics, igneous petrology, geochemistry, geophysics, Earth history, and planetary geology, and for mining industry professionals.
The copper-uickel sulfide ores of the Sudbury basin are associated with sublayer intrusions along the outer margin of, and as outward radiating offset dykes from, the main Sudbury Irruptive. Two major variants of sublayer are recognized: igneous-textured gabbro-norites and metamorphic-textured leucocratic breccias. Both are characterized by the presence of sulfide and abundant xenoliths. The gabbro-norites show enrichment in quartz and alkali feldspar towards the footwall, while their pyroxenes exhibit iron enrichment. These rocks are thus upside down with respect to normal crystal-settling paragenesis, suggesting that assimilation phenomena played a large part in their formation. Despite controversial interpretations, I submit that the gabbro-norites and the leucocratic breccias are contemporaneous, both pre-main Irruptive. The sulfide minerals in the sublayer are dominantly monoclinic and hexagonal pyrrhotite, pentlandite and chalcopyrite with minor but locally important cubanite, millerite and pyrite. Zoning trends within individual orebodies are attributed to subsolidus migration down a thermal gradient imposed by the main Irruptive. Comparison of Cu-Ni ratios in sulfides with average MgO content for various host rocks suggests that the hosts are not the liquid from which the sulfides precipitated. Two processes may explain the origin of the sublayer: (1) segregation of a sulfide-rich magmatic differentiate in the lower levels of an impact-triggered magma and its early intrusion along the contact between brecciated footwall rocks and overlying Onaping Formation; (2) direct emplacement of sulfide-enriched impact melt along the walls of the crater, the sulfides being derived from previous concentrations in basic magmatic country rocks. In either case, the leucocratic breccias were formed by mechanical attrition of brecciated footwall rocks as the igneous sublayer was intruded.
U-Pb ages have been determined for two diabase intrusions from Kerns and Hudson townships that are part of the Nipissing diabase suite from the Proterozoic Southern Province, Canada. The ages of the Kerns and Triangle Mountain intrusions are 2217.2±4 and 2209.6±3.5Ma, respectively, based on analysis of primary baddeleyite from both intrusions. Together with a published U-Pb age for the Castle Mine intrusion, these data indicate that the Nipissing sills were emplaced separately over a time span of 2.8-1.6Ma, given by the maximum and minimum overlap of errors on the U-Pb ages. -Authors
The 2.49 to 2.475 Ga intrusions of the East Bull Lake intrusive suite occur in an east-nortbeast-trending discontinuous belt along the presently exposed boundary between the Archean Superior and the Proterozoic Southern provinces of the Canadian Shield near Sudbury, Ontario. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic magmatic event, extending from 2.49 to 2.44 Ga, which also includes bimodal volcanic rocks, felsic plutons, and the regionally extensive Hearst and Matachewan dike swarms. This regional magmatic event is thought to be the result of a mantle plume-driven, intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the Huronian Supergroup. The East Bull Lake intrusive suite appears to have been emplaced along a major axial-rift fault related to this rifting event. Compelling field and geochernical evidence summarized here indicates that the three largest East Bull, Lake suite intrusions, the East Bull Lake, Agnew Lake, and River Valley intrusions, crystallized from similar, low Ti, high Al tholeiitic parent magmas that originated in deeper, more basic chambers. It is proposed that the primary magmas for the East Bull Lake suite intrusions were second-stage melts derived from a sublithospheric depleted mantle source that had been modified by Neoarchean subduction and accretion events. Most of the petrological characteristics of the East Bull Lake suite intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe enrichment trend in the residual magmas. Orthopyroxene is the high-temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite. Local olivine-rich cumulates, present in all of the major East Bull Lake suite plutons, in some instances appear to be comagmatic with associated plagioclase-rich cumulates. A distinctive marginal facies, comprising brecciated and locally thermally recrystallized and/or partially melted footwall rocks, is a common feature of the East Bull Lake suite plutons and indicates that a high-energy flow regime existed during the initial stages of emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gab-bronorite that contains erratically distributed mafic autoliths and footwall-derived xenoliths. Chemical contamination of the mineralized matrix of these inclusion- bearing rocks can be extreme at a local scale. The inclusion-rich zone is overlain by a thick interval of undifferentiated, plagioclase-rich cumulates that locally display spectacular glomerocrystic textures. Rhythmically and irregularly, modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy. Massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the Agnew Lake intrusion. Varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy. The East Bull Lake suite is currently being explored for contact-type platinum-group element (PGE)-Cu-Ni mineralization that has a clear spatial association with orthopyroxene-rich cumulates which are otherwise poorly represented in these intrusions. Disseminated PGE- and Cu-rich sulfide mineralization is concentrated within the inclusion-bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense, mafic residual liquid. The resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma. Feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background PGE contents compared to those for other known magmatic PGE deposits; however, the economic potential of the contact-type mineralization hinges on the efficacy of the physical concentration process, which upgraded the metal content and size of the original sulfide particles inside the magma chambers. Based on the proposed basal accumulation model, the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions.
The River Valley intrusion within the ∼2.48 Ga East Bull Lake intrusive suite in Ontario, Canada, is an example of a mafic igneous intrusion with "contact-type" Ni-Cu-PGE sulfide mineralization along its base. Whereas many contact-type deposits are thought to form from in situ contamination of the magma by the addition of crustal S during emplacement, there are some intrusions, including the River Valley intrusion, which appear to have a much more complex history where the timing of S saturation, and thus the critical ore genesis processes, may have occurred much earlier, prior to emplacement. The River Valley intrusion is made up of a basal ∼100 m of unlayered, inclusion-bearing units, overlain by layered cumulates. The basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites, all within a gabbroic matrix. Platinum-group element-rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides. The matrix and inclusions can be separated into two distinct textural types: hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages. The platinum group mineral (PGM) assemblages vary only between textural types, and not between inclusions and matrix, being dominated tellurides in all rock types. The hydrothermally altered rocks, however, have fewer tellurides and an increased amount of Sb- and As-bearing PGM, indicative of minor fluid interaction, although the PGM have not been mobilized significantly away from the base metal sulfides. Precious and base metal geochemistry shows all rock types to have an excellent correlation between all the platinum group elements (PGE), indicating the presence of a single, well homogenized, PGE-rich sulfide liquid. However, Au and Cu appear to be decoupled from the PGE at low concentrations, although correlate well with each other, which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins. The overlying Layered units above the mineralized units are not PGE depleted. Trace element data, including (Th/ Yb)PM and (Nb/Th)PM ratios, demonstrate that all River Valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber; although some samples have S/ Se ratios indicative of crustal S, most have S/Se ratios lower than the mantle range, indicative of S loss. We propose a multistage model for the formation of the mineralization in the River Valley intrusion with a major contamination event at depth with the addition of S from local crustal rocks, inducing sulfide saturation. Sulfide droplets were then enriched in PGE within a conduit system with possible further upgrading of sulfide metal tenors (and reduction of S/Se ratios) via partial dissolution of sulfide. The PGE-enriched sulfide liquid then settled in a staging chamber and partially crystallized before a major pulse of magma entrained sulfide liquid, eroded blocks of precrystallized and mineralized gabbro and footwall rocks, and emplaced an inclusionbearing package as the lower 100 m or so of the River Valley intrusion. Later emplacement of main River Valley magma was from an S-undersaturated, PGE-fertile magma. The River Valley intrusion is thus an example whereby contact-type mineralization is purely a function of the earliest magma intruded containing preformed sulfide mineralization, rather than contamination triggering sulfide saturation in situ. In such cases, processes at depth determine the generation and subsequent tenor of the mineralization. In particular, dissolution of the sulfide can upgrade metal tenor, but subsequently will reduce S/ Se ratios, masking the signature of crustal contamination. In addition, a multistage emplacement such as this will not necessarily preserve the characteristic increase in Cu/Pd ratios in the overlying cumulates that is often used in exploration for PGE deposits in mafic intrusions. Thus, a full understanding of all the field, geochemical, and mineralogical factors is required to construct genetic models for such deposits and especially in the interpretation of S/Se and Cu/Pd ratios as an indicator of crustal contamination and the presence of PGE mineralization.
The 1.85 Ga Sudbury structure is the largest, well-preserved, exposed example of a terrestrial impact crater; it contains world-class Ni-Cu-platinum-group element (PGE) ores associated with the igneous component. A new understanding of the geology and geochemistry of the vritric crater-fill sequence, the Onaping Formation, provides constraints on the evolution of 1.85 Ga igneous rocks in the Sudbury structure. The Onaping Formation has a well-defined mappable stratigraphy composed of three members, seven stratigraphic units, and two types of intraformational dikes that are overprinted by an impact-generated hydrothermal system. Stratigraphic and alteration mapping provided a basis for well-constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit, including vitric bombs and blocks and intraformational vitric dikes. Significantly, the identification of a least altered composition of the vitric Onaping Formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater-fill sequence and the Sudbury igneous Complex and offset dikes. The least altered vitric Onaping Formation, represented by the massive, xenolith-poor cores of vitfic bombs, blocks, and intraformational dikes, is andesitic in composition, consistent with a crustally derived melt. The melt composition (61.6 wt % SiO2, 4.28 MgO, 60 ppm Ni) is remarkably uniform throughout the Onaping Formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative Nb and Ti anomalies. Incompatible trace element ratios and bulk concentrations of the Onaping Formation coincide with the main mass norite and offset quartz diorite compositions of the underlying Sudbury Igneous Complex but differ distinctly from the granophyre and quartz gabbro. Root mean squared differences in composition of the vitric Onaping Formation with the units of the Sudbury Igneous Complex and quartz diorite offset dikes show that the Atric Onaping Formation does not directly match any of these igneous units but more closely resembles the composition of the Ministic and Manchester offset dikes. The quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the Sudbury Igneous Complex. In detail, there are small compositional differences between North and South Range offset dikes and between these offset dikes and the concentric Manchester offset dike; however, such differences are lacking in the Onaping Formation. We propose that the least altered vitric composition of the Onaping Formation represents the best estimate of the bulk composition of an initial quenched melt, or shock melt, formed early in the evolution of the Sudbury structure and is a logical starting liquid to model the evolution of the Sudbury main mass and offset dikes. The offset dike compositions can be derived from Onaping Formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater. This mafic-contaminated melt, or impact melt, developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the Sudbury Complex. Fractionation of the mafic-contaminated melt by plagioclase and pyroxene accumulation produced the North Range felsic and mafic norite and the South Range norites. The chalcophile metal content of the Onaping Formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores. The vitric Onaping Formation and unmineralized offset dikes have low liquidus temperatures (1,115degrees-1,125degreesC) and, if these units represent the source of metals in the sulfide deposits, unusually high mass distribution coefficients for Cu and Ni and high R factors are necessary to account for the metal endowment.
Chalcophile element partitioning among base-metal sulfide, oxide and silicate phases during magmatic processes is poorly constrained in part because there are very few studies of reliable sulfide melt–silicate melt partition coefficients (Ds) and because the crystallization history of the sulfide liquid is ignored in most studies. Here we present laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of sulfide droplets and their host fresh mid-ocean-ridge basalt (MORB) glasses. The compositions of the sulfide droplets suggest that they formed in magma chambers beneath the mid-ocean ridges and equilibrated with their host silicate melt, enabling us to calculate new Ds for chalcophile elements. These are Co (45 ± 4.5), Ni (776 ± 98), Cu (1334 ± 210), Zn (3.5 ± 0.9), Se (345 ± 37), Ag (1138 ± 245), Cd (107 ± 47), Sn (11 ± 1.6), Te (4478 ± 1146), Pb (57 ± 10) and Bi (316 ± 38). The contents of the highly siderophile elements in the glasses were too low to be determined using LA-ICP-MS. The whole rock values were used as a proxy for the glass and these allow an estimate of minimum Ds in the 10 000 to 40 000 range for the platinum-group elements (PGE) and 870 for Re. Partition coefficient values are affected by oxygen fugacity; comparison of our values with those from experimental studies suggests that oxygen fugacities of the MORBs considered here were between FMQ and FMQ-1. From the determined D values we calculate the contribution of the sulfides to bulk partition coefficient during mantle melting. Considering these values in combination with what is known about the behavior of the chalcophile elements during mantle partial melting, we suggest that Co, Ni, Zn, Te, PGE and Au behave as compatible elements during mantle melting, with Ni, Co and Zn being controlled mainly by the silicate and oxide minerals and the PGE, Au and Te being controlled mainly by the sulfides (or other discrete metallic phases). Copper, Se, Ag, Cd, Sn, Re, Pb and Bi are slightly to strongly incompatible during mantle melting. MORB sulfide droplets consist mainly of monosulfide solid solution (MSS), which is the first mineral to crystallize from a sulfide liquid, and intermediate solid solution (ISS), which crystallizes from the remaining liquid. The distribution of the chalcophile elements within the sulfide droplets shows that Co and Re have a slight preference for MSS, whereas the Cu, Zn, Ag, Cd, Sn, Te, Au, Bi and Pb partition into ISS. Selenium is present in approximately equal amounts in both MSS and ISS, and Pt and Pd are also present in both phases. Previous experimental and empirical studies have shown that Os, Ir, Ru and Rh partition into MSS and a portion of these elements exsolve from the sulfides as platinum-group minerals (PGM) during cooling. The same studies show that most of the Pt and Pd partition into the liquid and eventually crystallize from the late fractionated liquid as PGM. We examined our MORB sulfide droplets closely for platinum group-minerals (PGM) but none were found. We suggest that because the rocks sustained rapid cooling PGM were unable to exsolve from the sulfide minerals and the liquid did not fractionate sufficiently to permit the crystallization of the Pt or Pd minerals. Thus the PGE are present in MSS and ISS.
Radiometric dating, including numerous high-precision U-Pb baddeleyite and zircon ages, of Paleoproterozoic flood basalts, dike swarms, and layered mafic intrusions worldwide, indicates that a substantial volume of mafic magma was produced on Earth ca. 2.45 Ga. New U-Pb data presented here for the 2473 Ma Matachewan and 2446 Ma Hearst diabase dike swarms in North America establish a critical temporal link with ancient flood-basalt volcanism in Karelia and constrain the timing of a very ancient magnetic field reversal on Earth. The potential volume, magnitude, and extent of this magmatism may approach many of the better-known Mesozoic flood-basalt provinces and represent the oldest recognized large igneous province. The initiation and subsequent proliferation of flood-basalt and associated magmatism near the Archean-Proterozoic boundary may reflect a fundamental change in heat flux at the core-mantle boundary that heralded the breakup of a Late Archean supercontinent.
The Sudbury Igneous Complex (SIC) is interpreted as the solidified impact melt body of the 1.850-g.y.-old Sudbury impact structure. First results of cratering and thermal modeling for this ∼250-km sized multi-ring structure are presented. The numerical calculations were done for the vertical impact of a stony (granite) body (cylindrical projectile, 12.5 km in diameter and height) impacting at a granite target with a velocity of 20 km s-1. These simulations yield estimates of the transient cavity dimensions and the temperature field below the impact structure just after the modification stage. One-dimensional heat transfer modeling sets constraints for the thermal history of the impact melt. Cooling of the melt sheet, the present SIC, from the initial temperature of 2,000°K to the liquidus at 1,450°K lasted several 100 k.y, and below the solidus at 1,270°K, about 300 k.y. to 2 m.y., depending on the initial melt thickness H(SIC). The cooling sequence was modeled for H(SIC) of 2.5, 4, and 6 km. Given this long duration of cooling, postimpact tectonic processes during the Penokean orogeny may well have deformed the melt sheet prior to its final solidification. Prolonged cooling as well as this large-scale deformation may explain the present structural position and the composition of the Offset Dikes, consisting of differentiated impact melt.
The Sudbury Igneous Complex of Ontario, Canada, is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1.85 Ga ago. The transient cavity and melting zone reached the Moho and instantly (∼2 min) relaxed to form a more familiar large, shallow crater holding a thick, superheated (∼1700 °C) melt sheet covered by ∼2 km of breccia. There is little about the resulting bimodal igneous complex that resembles crystallization of well-known sheet-like bodies of similar composition. Yet, the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions, suggesting an intimate common parentage from the surrounding crust. This petrogenetic enigma is explained here as a natural, unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion, which we take as the high-temperature equivalent of breccia. A wide spectrum of viscously discrete, interdispersed parcels of mafic and felsic liquids, reflecting the compositional heterogeneity of the target crustal materials, formed the emulsion. Within days to months, the emulsion components separated according to their relative densities into a bimodal norite-granophyre assemblage that formed the basic structure of the present Sudbury Igneous Complex. There is clear evidence of this emulsion in the earliest dikes (i.e., offsets), which likely give the earliest state of the nascent melt sheet. Immediately following emulsion separation, the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer. These convective motions homogenized and rapidly cooled the magma to the liquidus temperatures, whereupon convection ceased. The pattern of convection was pinned in place by the embayment topography of the crater floor, which in turn played a pivotal role in directing sulfide deposition into the embayments. All further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins. There is clear evidence of solidification from the floor upward and the roof downward. Minimal differentiation and compositional modification took place throughout cooling and solidification. Nevertheless, during the solidification stage, granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying Onaping Formation became unstable and entered the melt sheet, and the partially melted remnants collected at the interface between the norite and granophyre. Some interstitial melt from the norite also percolated upward, and, altogether, the blocks and melt produced the distinctive chemical and physical characteristics of the unusual Transition Zone. The Sudbury melt sheet is, in essence, a full-scale magmatic experiment. The conditions of formation, relative to any other large terrestrial magma, are "precisely" known. Thus the clear lack of any significant modal layering, the overall homogeneous nature of each unit, and the lack of any significant chemical differentiation through crystal fractionation establish Sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers.
This paper presents a detailed petrographic and mineralogic study of the rocks of the Nipissing diabase sill near Cobalt, Ontario, Canada. The sill has an undulatory form, and the petrology of the sections in the basins differs from that of the arches. Olivine-bearing hypersthene diabase is confined to the basins. Augite, pigeonite (in part inverted), and hypersthene occur in the sill; the systematic manner in which they vary and are related to each other is detailed herein. The diversity of major facies is reasonably explained by gravity control of differentiation accompanied by some lateral movements. Continuous exposures of rock at several mines have facilitated detailed studies.
The Worthington offset dike extends for approximately 15 km away from the southwestern margin of the 1.85 Ga Sudbury Igneous Complex. The dike is zoned with respect to inclusion and sulfide contents. Marginal chilled quartz diorite is transitional into medium-grained quartz diorite. These rocks are sulfide undersaturated, contain small inclusions from the wall rocks, and are preserved along much of the dike. Locally, the dike contains a core of inclusion-rich quartz diorite, which can be choked with inclusions surrounded by semimassive to massive sulfide. The more heavily mineralized inclusion-rich quartz diorite contains 10 to 75 percent amphibolite inclusions, which are petrologically and geochemically similar to the immediately adjacent country-rock amphibolites, locally termed "Sudbury gabbros." The semimassive to massive sulfide zones form subvertical pipes, much like the deposits of the Copper Cliff offset dike, and these are associated with locations where the Worthington dike widens from 20 to 30 in to 50 to 80 in. The average metal tenors of the sulfide with > = 5% sulfur are calculated to be 7 percent Ni and 13 percent Cu. Thus the dike ores have a much higher Cu/Ni ratio than orebodies within the contact sublayer (Cu/Ni similar to 1). The medium-grained inclusion-poor quartz diorite and inclusion-rich quartz diorite differ in Ni, Cu, Pt, and Pd abundances, but they have similar major and lithophile trace element abundance levels despite having different inclusion and sulfide contents. Assimilation of inclusions has therefore not significantly changed the composition of the silicate matrix of the inclusion-rich quartz diorite. The composition of the inclusion-poor quartz diorite is a close approximation to average crust and also to the bulk composition of the Sudbury Igneous Complex rocks. Regional differences in offset geochemistry exist between the North and South Range offset dikes, but we believe that the formation of the inclusion-poor quartz diorite and inclusion-rich quartz diorite silicate magmas predated significant silicate differentiation or silicate gravitational segregation of the melt sheet and that the differences may record primary silicate heterogeneity of the melt sheet. These differences may have developed in response to the different proportions of Archean granitoids relative to Proterozoic sediments and volcanics that contributed to the melt sheet on the North and South Ranges of the Sudbury Igneous Complex. The first quartz diorite melt was sulfide undersaturated and devoid of amphibolite inclusions. The introduction of this melt took place during or shortly after the generation of the melt sheet and represented a very rapid introduction of quartz diorite magma into radial and concentric dikes with local incorporation of metasedimentary inclusions. The second phase of activity involved the introduction of a sulfide-bearing melt into portions of the quartz diorite melt that were not completely crystallized at the center of the offset dike; this produced the inclusion-rich quartz diorite. The main control on the location of injection of the sulfide-bearing melt appears to be widened domains of the partially crystallized quartz diorite dike that often correspond to contacts between different country-rock types and sulfides, especially where there are Sudbury gabbro country rocks and/or local development of Sudbury breccia. The injection of sulfide-enriched melt is interpreted to have taken place along steeply oriented pipes through these heavily brecciated country rocks; this appears to explain why there is a contact relationship between inclusion-rich quartz diorite and inclusion-poor quartz diorite, and also explains the presence of fragments of inclusion-poor quartz diorite and country-rock amphibolites in the inclusion-rich quartz diorite. The introduction of sulfides into these pipes is interpreted to have taken place from the overlying melt sheet and it is suggested that the high Cu/Ni ratios and platinum-group element tenors of these sulfides, compared to contact-style sulfides, indicate that these were some of the earliest sulfides to segregate from the overlying melt sheet. The type and style of brecciation developed in the country rocks along different offset dikes controlled the continuity of the quartz diorite and the location of the orebodies. Intersections of the quartz diorite with domains of Sudbury breccia appear to have been important in the Copper Cliff offset dike and the development of mineralization may be a response to the presence of favorable channels through which magmatic sulfides were introduced into the dike from the overlying melt sheet. We propose that the quantity of sulfide in a dike is related to the thickness of the overlying melt sheet. The melt sheet thickness was presumably greatest at the Copper Cliff and Frood-Stobie offset dikes, the Worthington offset dike is located below a thinner melt sheet, and the Manchester and Foy offset dikes developed beneath very thin portions of the melt sheet.
The composition of any sample of a magmatic sulfide ore depends on the composition of the silicate magma from which the sulfide segregated, the appropriate sulfide liquid-silicate magma partition coefficients, the 'R' or 'N' factor operating during the interaction of the sulfide and silicate host, the subsequent fractionation of the resulting sulfide liquid, and the proportion of cumulus minerals to sulfide liquid represented within the sample. All these factors have been considered during the investigation of platinum-group elements (PGE) in 2,500 samples representative of 12 mineralized systems (ore deposits or areas of mineralization) at Sudbury. The data indicate that samples from most of these systems fit within envelopes defined by the compositions of cumulus mss and the associated sulfide liquid. Once the fractionating liquid reaches about 32 wt percent Cu, iss replaces mss as the liquidus phase, and Rh and Rh/Cu increase with continued fractionation. The initial composition of the sulfide liquids responsible for each of the mineralized zones has been determined by adjusting the position of the mss-sulfide liquid envelope to best include the data for the zone on an Rh/Cu versus Rh plot. The initial liquid compositions determined in this way vary widely, and, for most deposits, can be explained as the result of the equilibration of sulfide liquid with the same magma at R factors ranging from 1,200 (i.e., the liquids with the highest Ni, Cu, and PGE) to 140 (i.e., the liquids with the lowest Ni, Cu, and PGE). Deposits along the South Range are characterized by sulfide liquids with a distinctly higher R factor than those along the North Range. The data suggest that two deposits, Creighton and Victor, cannot be explained as the result of fractionation of a single sulfide liquid but that two or more liquids with different R factors were involved. The data also show that the mss-sulfide liquid partition coefficients for Rh, Ir, and Os are 4, 4.4 to 5.2, and 4.2, respectively, essentially confirming the experimentally determined coefficients reported in the recent literature. In the Trillabelle mineralized system, sulfide within sublayer norite started crystallizing mss before the sublayer norite itself was completely solidified; where the sulfide content exceeded 20 percent sulfide, Cu-rich, fractionated liquid was able to escape, leaving behind mss cumulates. In most areas with less than 15 percent sulfide, fractionated liquid has not escaped, and the sulfides have a composition close to that of the original sulfide liquid. Examination of a zone of subeconomic mineralization in this way can help assess the likelihood of there being high-grade zones of Cu- and PGE-rich fractionated sulfides in the vicinity. Comparison of the estimated composition of the sulfide liquid obtained from the sulfide-poor samples with that obtained from fitting a fractionation envelope to the sulfide-rich samples indicates that the rich mineralization crystallized from a liquid with a composition different from that found in much of the sublayer and mafic norites. At the Lindsley deposit, there is a close correlation between the presence in the footwall of a pod of massive sulfide with a fractionated bulk composition and sulfides within the adjacent sublayer norite that are 'wetter' (i.e., have a higher phi index) than is typical for the greater part of the sublayer norite mineralization at the deposit, suggesting that fractionating sulfide liquid has moved through the sublayer norite, leaving behind 'dry' cumulus mss and a 'wet' trail at the location where it leaked out into the footwall.
Cu-Ni sulfide deposits of the Sudbury Igneous Complex are hosted by the inclusion-rich sublayer, which occurs discontinuously between the footwall and main mass of the Sudbury Igneous Complex, by brecciated foorwall in the vicinity of the Sudbury Igneous Complex, and by offset dikes protruding for tens of kilometers into the country rocks. The genesis of mafic-ultramafic inclusions in the sublayer and the relationships between the sublayer, main mass, and offset dikes have long been at the center of debate on Sudbury geology. This paper reports U-Pb isotope data on zircon and baddeleyite from samples representing norite matrix, melanorite and olivine melanorite pods, a metapyroxenite inclusion, and diabase from the sublayer at the Whistle embayment. These samples yield ages ranging from 1848.1 ± 1.8 to 1849.1 ± 1.1 Ma, similar to a U-Pb age of 1849.8 ± 2.0 Ma obtained for zircons in quartz diorite from the Copper Cliff offset, and indistinguishable from the previously published average age of 1850 ± 1 Ma for the main mass of the Sudbury Igneous Complex. None of the samples yield evidence for inherited zircon components. The isotopic data prove that the exotic mafic-ultramafic inclusions and pods in the sublayer are not xenoliths scavenged from older layered complexes but require the existence of primitive, moderately Mg-rich magmas that crystallized in the early stages of development of the Sudbury Igneous Complex. Such Mg-rich cumulates are unlikely to have crystallized from melted average upper crust.
Formations of the "original Huronian" occur in a 200-km.-long belt along the north shore of Lake Huron. The Huronian formations lie unconformably upon an Archean basement complex of granite, gneiss, and metasedimentary and metavolcanic rocks. Previous age determinations on this basement complex indicate that it is about 2,500 m.y. old. Several igneous formations are intrusive into the Huronian formations. The oldest of these is the Nipissing diabase, which forms a complex of sills and dikes throughout the area studied; granophyric differentiates occur at several localities. In the southeastern part of the area two granitic intrusive bodies occur, the Cutler granite, previously dated at 1,750 m.y. by Wetherill, Davis, and Tilton, and the Eagle granite. An olivine diabase dike which cross-cuts the Cutler granite and adjacent Huronian formations is the youngest intrusive rock in the area. Age determinations were carried out by the Rb-Sr method on mineral and whole-rock samples from the crystalline rocks of t...
Petrological models relating the different rock types constituting the 1.85 Ga Sudbury Igneous Complex are constrained with extensive new geochemical data. We show that the main mass felsic norite, transition zone quartz gabbro, and granophyre have similar ratios of the highly incompatible trace elements (e.g., La/ Sm = 4.5-7, La/Nb = 2.8-4.2, Th/Zr = 0.04-0.05) and that these variations are consistent with the crystallization and differentiation of magma types largely (>80%) derived from the upper crust, with a smaller contribution from a mantle source. Although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact, we find no principal reason why this model should be rejected. However, we propose that a small contribution of mantle-derived picritic magma is required to explain the abundant Ni, Cu, and platinum-group elements (PGE) in the Sudbury deposits, as well as the compositions of the ultramafic inclusions (MgO = 12-36 wt %; Fo 68-87 olivines with 450-3,700 ppm Ni, and abundant chrome-rich spinel), and the magnesian composition of the mafic norite (8-14 wt % MgO) and the sublayer (6-12 wt % MgO). We believe that the main mass of the Sudbury Complex achieved its present composition through incorporatation of up to 20 percent mantle-derived picritic magma emplaced along crustal fractures produced by the impact event. These picritic magmas entered the melt sheet as a dense plume, vigorously mixing with it, and due to the marked compositional shift, the mixed magma formed magmatic sulfides which sank through the magma column, depleting the melt in Ni, Cu, and PGE. Since both the felsic norite and granophyre have indistinguishable ratios of the incompatible trace elements, we see no requirement to derive these units of rock by the crystallization of magmas derived from different sources. Rather, the compositional difference between the felsic norite and granophyre is attributed to the in situ differentiation of the magma. We show that the main mass has many compositional traits similar to those of most of the offset dike quartz diorites (e.g., the Parkin offset dike: La/Sm = 6.3, La/Nb = 4.5; Th/Zr = 0.05) and of embayment-related leucocratic norites from the Whistle mine (La/Sm = 6.2, La/Nb = 5.0, Th/Zr = 0.02). These rocks have compositions intermediate between the felsic norite and the granophyre, and therefore crystallized from the same magma type; arguably, the unmineralized quartz diorites provide the best possible estimate of the original magma from which the Sudbury Complex crystallized. In detail, there are subtle variations in composition within and between offset dikes, with the largest difference being between the North and South Range offsets; the North Range offset dikes cut Archean granitoids and gneisses and have elevated Sr, La/Yb, La/ Sm, and Gd/Yb and low TiO 2 whereas the South Range dikes cut Early Proterozoic sediments, mafic volcanics, and intrusions, and have low Sr, La/Yb, Gd/Yb, La/Sm, and high TiO 2. These differences may be caused by the assimilation of different country rocks during emplacement of the dike. A strongly mineralized offset dike at the Creighton mine has geochemical variations that are different when compared to the main mass, and in the case of Creighton, are more similar to the local mineralized sublayer. These data suggest that mineralized and barren quartz diorites have different geochemical compositions, and that these traits may be of value in mineral exploration.
Platinum and palladium minerals occur with Cu-Ni-Fe sulphides in sheared, propylitized gabbronorite near the margin of the Wanapitei intrusion. Most of the Pd minerals are disseminated in the altered silicates rather than in the sulphides - implying direct precipitation rather than exsolution from sulphides. Based solely on sulphide-mineralized dump samples, the occurrence is judged to be hydrothermal. -G.J.N. Dept. of Geology, Univ. of Western Ontario, London, Ontario, Canada N6A 5B7.
The East Bull Lake intrusion is the type example of a series of 2.48 to 2.49 Ga layered mafic intrusions that developed within the rifted southern margin of the Superior craton, central Ontario. The East Bull Lake intrusion contains contact-type PGE mineralization, e.g., strata-bound, PGE-rich, disseminated sulfide mineralization occurring within 200 to 300 m of tilt, contact with footwall and sidewall units. Although the mineralization locally occurs in massive plagioclase cumulates (anorthosite zone), most of the sulfides are concentrated within an underlying, hetcrolithic, inclusion-rich unit inclusion-bearing zone) that grades into art intrusive breccia. Disseminated sulfides also occur in Fine-grained gabbroic veins within brecciated footwall rocks (border zone). Geochemical characteristics of the contact-tt;pe sulfide mineralization and its host rocks include: (1) high-average Pd abundances throughout the East Bull Lake intrusion, including an average of ca. 200 ppb in the Lower Series: (2) low-average S contents (typically. <1%); (3) average Pd/Pt similar to3 and Cu/Ni similar to3: and (4) PGE concentrations and relative abundances similar to disseminated sulfide mineralization from the contact environment of other mafic-ultramafic intrusions. Contact-type PGE mineralization in the East Bull Lake intrusion reflects flow-enhanced gravitational settling of a Cu-rich sulfide liquid followed by local fractionation of Ni-rich monosulfide solid solution to produce minor amounts of Cu- and Pt-rich, residual sulfide liquid. Most of the observed compositional variation in the sulfides can be modeled using variable silicate liquid/sulfide liquid mass ratios. Field and geochemical observations suggest that the parental magmas to the mineralized parts of the East Bull Lake intrusion were S saturated and PGE rich at the time of their emplacement. These fertile parent magmas most likely originated as buoyant. feldspathic residual magmas that escaped from a deeper, more primitive subchamber. Although first described from the East Bull Lake intrusion, contact-type PGE mineralization is now recognized in all of the major East Bull Lake suite intrusions. Similar styles of magmatic sulfide mineralization art developed in the contact environments of the Bushveld Igneous Complex (Platreef deposits! anti the Duluth Complex, Minnesota (e.g., Dunka Road deposit). These contact type deposits are part of the family of "basal accumulation" magmatic sulfide deposits. The economics of low sulfide tenor, contact-type, PGE-Cu-Ni mineralization. such as that developed ill the East Bull Lake intrusion will be west favorable in systems in which (1) the parent magmas possessed high initial PGE tenors and reached S saturation prior to emplacement; (2) the parent magmas experienced vigorous convection in the early; stages of chamber development; and (3) segregated sulfide was not diluted by coprecipitating silicate minerals-a scenario that is most likely to occur in feldspathic magmas in which plagioclase is the principal high-temperature liquidus mineral.
AssrRAcr Three hundred and ninety inclusions from the igneous sublayer of the Sudbury Igneous Complex have been ex- amined, and many of them analyzed by microprobe. The 264 samples from the Strathcona mine of the North Range vary from dunite through harzburgite, wehrlite and clinop;roxenite to norite and gabbro, whereas the 126 samples from mines of the South Range include only rock types vdth onhopyroxene dominant, such as harzburgite and melanorite. The texture of the rocks ranges from that of a well-developed adcumulate, mesocumulate or or- thocumulate to an extensively granulated and recrystalliz- ed fabric. On both the North and South Ranges, olivine varies from 85 to 73 mole Vo Fo, hypersthene from 13.5 to 31 mole 9o Fs, augite from 5 to 15 mole go Fs, plagioclase (including both cumulus and intercumulus phases) from 90 to 30 mole 9o An, and chromite contains up to 50.8 wr. u/o Cr2O3. The textures, presence of plagioclase and evidence of olivine reacting to give orthopyroxene suggest that the inclusions crystallized at crustal pressures. The range of rock types and the observed correlation of increas- ing MgO/(MgO+FeO) ratio in mafic minerals wittr increas- ing content of mafic minerals in the host rocks suggest strongly that these rocks are derived from one (or more than one) hidden layered intrusive bodies. Our data do not allow us to state that this intrusive body is cogenetic with the Sudbury norite, but they are consistent with this interpretation.
The Talnakh, Kharaelakh, and Noril'sk I intrusions of the Noril'sk region are coeval with the Permo-Triassic Siberian trap flood basalt and contain one of the largest known resources of Ni-, Cu-, and platinum group element (PGE)-enriched sulfide mineralization. The similar to 3,5-km-thick stratigraphy of the basalts consists of a Lower sequence of high Ti alkalic, subalkalic, and picritic basalts, and an Upper sequence of low Ti basalts. The stratigraphy of the Upper sequence consists of a group of picritic and tholeiitic basalts assigned to the Tuklonsky Formation, overlain by the Nadezhdinsky Formation which is represented by a sequence of contaminated tholeiites that show an upward decline in the degree of contamination into the overlying Morongovsky and Mokulaevasky Formations. The Nadezhdinsky Formation is depleted in Ni, Cu, and PGE, with the most depleted basalt; having concentrations below the analytical determination limits of 0.06 ppb for Pd and 0.1 ppb for Pt and a factor of at least 100 less than the Pd and Pt concentrations of the rest of the Upper sequence. The greatest PGE depletion occurs a short distance above the base of the Nadezhdinsky,, and these flows are overlain by basalts that exhibit a gradual recovery in PGE concentration toward values typical of the Mokulaevsky Formation and the overlying basalts (similar to 6.8-12.1 ppb Pt and similar to 7.2-16.5 ppb Pd). Within the Nadezhdinsky Formation, there is a strong correlation between Pd/Zr and La/Sm ratios that indicates that the magmas with the largest contribution from crustal material are also the most PGE depleted, Moreover, the contribution of crust to the magma and the degree of metal depletion decreased through time, The PGE-undepleted Upper sequence tholeiites have Pd/Pt of 0.45 to 2.0, and the ore-forming sulfides (Pd/Pt similar to 3-4) were probably formed from magmas with Pd/Pt ratios at the upper limit of this range; the metal-depleted basalts have lower values (Pd/Pt similar to 0.3). The high Pd/Pt ratio of the ores and the low Pd/Pt ratio of the basalts indicate that the processes of ore formation and flood basalt magmatism were important because they indicate that the basalts have the Pd/Pt ratio of a magma that has segregated sulfide with the same Pd/Pt ratio as the Noril'sk ores. Moreover, the ore deposits are located in the region where the metal-depleted Nadezhdinsky Formation is similar to 500 m thick and forms a > 5,000-km(3) volcanic center. The continuous changes in basalt chemistry through the stratigraphy of the Upper sequence are consistent with processes that took place in a very large staging chamber rather than within multiple similar to 5- to 10-km(3) discrete high-level magma chambers on the scale of the mineralized intrusions. In the staging chamber, Tuklonksy-type magmas interacted with crust to produce the contaminated basalts of the Nadezhdinsky Formation. The staging chamber containing the crustally contaminated magma was then replenished with PGE-undepleted magma. Initially, the replenishing magma became sulfide saturated, thereby producing the ore-forming sulfides, and the magma in the staging chamber remained sulfide-saturated, As further injections of PGE-undepleted magma entered the staging chamber, the melt became sulfide undersaturated and magmas tapped from it were progressively less PGE depleted. The deposits of the Noril'sk, Talnakh, and Kharaelakh intrusions were formed by, injection of olivine-bearing melts containing immiscible sulfide that were produced in the staging chamber. The ores inherited a heavy sulfide isotope composition possibly by, reaction with evaporite-laden country rocks. The unradiogenic composition of the Os in the ores is consistent with interaction between the crustally contaminated magma (which had become depleted in the PGE and radiogenic Os) in the staging chamber with later pulses of uncontaminated mantle-derived magma, The high PGE contents of PGE-undepleted Siberian trap are due to interaction of plume-generated picritic magmas with lithosphere that had been depleted in sulfide by previous melt extraction.
The Palaeoproterozoic Southern Province comprises a thick, continental rift related volcanic-sedimentary sequence along the southern margin of the Archaean Superior Province. The Agnew Intrusion (50 km2), which is a member of the East Bull Lake suite of layered intrusions, occurs adjacent to the Superior Province - Southern Province boundary in central Ontario, Canada, and provides an opportunity to examine the early tectono-magmatic evolution of a Palaeoproterozoic rifting event. The Agnew Intrusion is a well-exposed, 2100 m thick, layered gabbronoritic to leucogabbronoritic pluton. It was the product of at least four recognizable, but chemically similar, high-Al2O3 and low-TiO2 magma pulses. Structural data, coupled with excellent stratigraphic correlations between the Agnew Intrusion and other East Bull Lake suite layered intrusions, suggest that these plutons are erosional remnants of one or more sill-like bodies that may originally have formed an extensive, subhorizontal mafic sheet. We argue on the basis of field evidence that the early evolution of the Southern Province was characterized by a large, mantle plume induced magmatic event that gave rise to a Palaeoproterozoic continental flood basalt province. However, the incompatible trace element characteristics of the Agnew Intrusion parental magma (i.e., large ion lithophile and light rare earth element enrichment and high field strength element depletion) are more typical of modern subduction-modified subcontinental lithospheric mantle. Given that this is a prevailing geochemical signature of mafic rocks in the Archaean-Palaeoproterozoic, we suggest that there was a fundamental difference in both the composition and structure between the ancient and more modern mantle. "Subduction-like" geochemical signatures may have been imparted to the entire developing mantle during early Earth differentiation.
Nipissing Diabase sills and baked host sediments of the Coleman Member of the Huronian Supergroup east of Englehart, Ontario, are shown to have a characteristic remanent magnetization direction (called N3) that is steeply up and to the west (D = 268.0°, I = −59.0°, k = 42, α95 = 6.0°). Petrographic study indicates that fresh pyroxene gabbro carries the N3 component at most sill sites. A baked contact test with the Coleman Member suggests that the magnetization is primary. The N3 magnetization direction is unlike either the N1 or N2 magnetization direction reported for Nipissing sills at other localities in the Southern Province. Three distinct ages of Nipissing sill emplacement are proposed. A single Nipissing sill site in the sampling area carries the N1 direction.A northeast-trending diabase dyke crosscuts both the Nipissing sills and Coleman sediments. It carries an N2 direction and has overprinted nearby intrusive and sedimentary rocks (D = 282.0°, I = 61.1°, k = 48, α95 = 5.8°). Several N3 sill sites far from the dyke may also carry a softer N2 overprint. However, the N3 and N2 directions and the direction of the present Earth's magnetic field fall near a great circle, making it difficult to separate the N2 and present-field components in such cases.
U–Pb analyses of primary baddeleyite and vein-related secondary rutile, separated from a Nipissing diabase sill at the Castle mine, Gowganda, yield ages of 2217.5 ± 1.6 and 2217.0 + 6 Ma, respectively. The data allow for the possibility that (i) the vein formed at essentially the same time as diabase intrusion, defined within the limits of the intercept age , or (ii) the vein could have formed up to 8.1 Ma later than intrusion. Paleomagnetic analyses of the diabase suggest that it is part of the older (N2) phase of this widespread intrusive event. Paleomagnetic analyses of a pre-vein granite dike, intrusive into the diabase, provide a minimum time interval of 20 000 – 50 000 years separating intrusion and vein formation.40Ar–39Ar analyses of silicates obtained from ore veins in the Cobalt camp yield highly disturbed age spectra. Paleomagnetic analyses of altered wall rocks to ore veins in both the Castle mine and the Cobalt camp provide a wide range of apparent ages, inconsistent with the U–Pb data. The 40Ar–39Ar and paleomagnetic data are interpreted as indicating a prolonged history of post-vein disturbances during regional tectonism.
Concordant baddeleyite and rutile analyses from mineralized Nipissing diabase at Gowganda's Castle mine yield a U–Pb age of . Baddeleyite is a primary phase of the diabase; therefore the age dates the magmatic emplacement. The genesis of rutile is less certain: rutile could have formed during an episode of alteration, veining, and Ag mineralization, in which case the U–Pb age would also date the secondary event and demonstrate that Ag mineralization was related to the Nipissing magmatism. However, a primary magmatic origin of the rutile cannot be completely excluded for now; thus the U–Pb age may not constrain the time of Ag mineralization. Further research is needed to clarify this point.
Field evidence shows that the Huronian section of metasediments in Ontario is younger than the Archean and older than the Nipissing diabase. Previous radiometric investigations show the age of the latest Archean granitic intrusions to be about 2500 m.y. and the Nipissing diabase to be 2155 ± 80 m.y. In this paper we present whole-rock Rb–Sr data which (1) confirm the age of the diabase in a single sill at Gowganda (2162 ± 27 m.y.), and (2) show the age of the Gowganda Formation (which the sill intruded) to be 2288 ± 87 m.y. In marked contrast metasediments and metavolcanics in the Huronian section southwest of Sudbury show younger ages ranging between 1800 and 2200 m.y. This is interpreted in terms of known orogenic disturbance of the Huronian rocks south and west of Sudbury and its absence in the Cobalt–Gowganda area to the northeast. Both minerals and whole rocks have responded by showing low and discordant ages in the disturbed area. The higher and accordant ages in the undisturbed environment agree with the geologic succession and are believed to be the true ages of these rocks.
The 2217 Ma Shakespeare intrusion is part of the extensive 2?2 Ga Nipissing gabbro suite and is hosted within 2?45–2?2 Ga Huronian Supergroup metasediments in the Southern Province of the Canadian Shield, close to the border with the Superior Province. The Shakespeare intrusion is a complex differentiated sill approximately 14 km in strike length and approximately 300 to 430 in thickness. It comprises two distinct magmatic packages: (a) a Lower Group composed of unmineralised pyroxenite and gabbro, and (b) an Upper Group composed of Ni–Cu–PGE mineralised melagabbro, quartz gabbro, and biotite quartz gabbro-diorite. The Shakespeare intrusion formed from a tholeiitic parental magma. All of the rocks in the intrusion are enriched in highly incompatible lithophile elements (HILE: Cs, Rb, U, Th, Nb, Ta, LREE) relative to moderately incompatible lithophile elements (MILE: Zr, Ti, HREE) and are strongly depleted in Nb and Ti relative to elements of similar incompatibility. These geochemical characteristics suggest that the Shakespeare magma underwent extensive degrees of upper crustal contamination before emplacement. Although other parts of the Nipissing gabbro suite exhibit similar geochemical characteristics, the Shakespeare intrusion is more enriched in HILE and more strongly depleted in Nb–Ti, and therefore appears to have undergone greater degrees of crustal contamination. Heavily disseminated to patchy net-textured (10–15%) Fe–Cu–Ni sulphides (pyrrhotite–chalcopyrite–pentlandite) occur in the upper portion of the melagabbro of the Upper Group, near and at the contact with the overlying quartz gabbro, and in melagabbro dykes. This is a new style of magmatic Ni–Cu–PGE mineralisation in the Nipissing gabbro suite. The mineralised zone contains abundant inclusions of quartzite, blue quartz eyes and rare diorite. The ores have compositions consistent with having been derived from the Shakespeare magma and to have equilibrated at moderate magma/sulphide ratios (R=500–1000). The Ni–Cu–PGE mineralisation in the Shakespeare deposit appears to have resulted from the following processes: (a) generation of Nipissing magmas via partial melting of mantle peridotite, (b) contamination of Nipissing magmas by continental crust during ascent, (c) introduction and crystallisation of contaminated but sulphide–undersaturated magmas into the Shakespeare intrusion, forming the Lower Group, (d) additional, apparently relatively local, crustal contamination and sulphide saturation resulting in the incorporation of abundant xenoliths of country rocks and the generation of moderate amounts of Ni–Cu–(PGE) sulphide melt, and, (e) introduction of the xenoliths and sulphide-bearing magma into the Shakespeare intrusion, forming the Upper Group, with heavier sulphides settling at the base of the new crystallisation floor, resulting in low-moderate R factor values.