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Papers by Peter Lightfoot

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

Canadian Mineralogist, Sep 1, 2017

Constraining the Genesis of Sudbury Breccia, Ontario: Implications for Exploration of Footwall Cu-Ni-PGE Deposits

Geology and geochemistry of intrusions and flood basalts of the Noril'sk region, USSR, with implications for the origin of the Ni-Cu ores

Economic geology and the bulletin of the Society of Economic Geologists, Jul 1, 1992

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Geochimica et Cosmochimica Acta, Oct 1, 2010

The largest known terrestrial impact melt sheet occurs within the 1850 Ma Sudbury Structure, Onta... more The largest known terrestrial impact melt sheet occurs within the 1850 Ma Sudbury Structure, Ontario. In order to evaluate the relative contributions of different target lithologies to the melt sheet, we have investigated the Pb isotope compositions of feldspar separates from early-formed quartz diorite magmas within Offset Dykes from around the impact structure. The samples define a linear array on plots of age-corrected 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb. Samples from Offset Dykes hosted by the Huronian Supergroup (South Range) have a range of 206 Pb/ 204 Pb 1850 from 15.424 to 17.255 and 207 Pb/ 204 Pb 1850 from 15.390 to 15.801, whilst those hosted by Archean gneisses of the Superior Province (North Range) cluster around 206 Pb/ 204 Pb 1850 % 14.8 and 206 Pb/ 204 Pb 1850 % 15.1. These values can be approximated by binary mixing between the two major groups of target lithologies. A mix of 60-70% of Superior Province gneisses with 30-40% of Huronian metasedimentary material closely matches the Pb isotope compositions of North Range Offset Dyke samples, whereas in the South Range the required Huronian component is up to ca. 80%. These mixing proportions are consistent with Sr, Nd and Os isotope and trace element constraints. A third minor component, either locally-exposed Paleoproterozoic mafic rocks or the lower crust is also required. However, the isotopic, trace element and Ni-Cu-platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component. A major contribution of Huronian supracrustal material, which had a pre-impact thickness of up to 12 km, is required to explain the chemical characteristics of the impact melts, which also have a strong upper crustal affinity (e.g. Eu/Sm = 0.22, Rb/Sr = 0.2-0.35). As such, a shallower level of melting is apparent than that predicted by many previous impact models for the Sudbury event. This can be accommodated by considering approach trajectories for the impactor oblique to the Earth's surface. In addition, the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage, and may not have been completely homogenised during crater formation. Our findings have significant implications for the nature of the Sudbury impact event, the evolution of the melt sheet and the crustal sources of metals contained in Sudbury's world class Ni-Cu-PGE sulphide ores.

Significance of the precise determination of trace element abundances by ICP-MS in continental flood basalts with specific reference to the Keweenawan and Siberian traps

Canadian Mineralogist, Feb 1, 1984

The Insizwa Complex, located in Transkei, southem Africa, consists of four layered intrusive bodi... more The Insizwa Complex, located in Transkei, southem Africa, consists of four layered intrusive bodies. Wholerock geochemical and mineral compositional data for the marginal gabbro and picrite units are consistent with a low-Mg parentage. The modal mineralogy, petrography and geochemistry of the marginal gabbro unit point to olivine enrichment of a low-Mg magma by the settling of olivine crystals toward the contact and reaction of these crystals with the host liquid.

Transactions, Apr 2, 2016

Journal of Petrology, Jun 1, 2020

The c. 1Á85 Ga Sudbury Igneous Complex (SIC) is the igneous remnant of one of the oldest, largest... more The c. 1Á85 Ga Sudbury Igneous Complex (SIC) is the igneous remnant of one of the oldest, largest and best-preserved impact structures on Earth and contains some of the world's largest magmatic Ni-Cu-PGE sulfide deposits. Most of the mineralization occurs in Sublayer, Footwall Breccia and inclusionbearing quartz diorite (IQD), all of which contain significant (Sublayer and IQD) to minor (Footwall Breccia) amounts of olivine-bearing mafic-ultramafic inclusions. These inclusions have only rare equivalents in the country rocks and are closely associated with the Ni-Cu-PGE sulfide mineralization. They can be divided into three groups on the basis of petrography and geochemical characteristics. Group I (n ¼ 47) includes igneous-textured olivine melanorite and olivine melagabbronorite inclusions in the Whistle and Levack embayments on the North Range with Zr/Y, Zr/Nb, Nb/U and Zr/Hf similar to igneous-textured Sublayer matrix. Group I inclusions are interpreted to be anteliths that crystallized from a mixture of SIC impact melt and a more mafic melt, probably derived by melting of ultramafic footwall rocks. Group II includes Group IIA (n ¼ 17) shock metamorphosed wehrlite and olivine clinopyroxenite inclusions in the Levack embayment and Group IIB (n ¼ 2) shock metamorphosed olivine melanorite inclusions in the Foy Offset on the North Range. Group II inclusions have similar trace element patterns [e.g. negative Th-U, Nb-Ta-(Ti), Sr and Zr-Hf anomalies] and overlapping Nb/U to a layered mafic-ultramafic intrusion in the footwall of the Levack and Fraser deposits, which together with their limited distribution suggests that Group II inclusions are locally-derived xenoliths. Group III (n ¼ 21) includes phlogopite lherzolite and feldspar lherzolite inclusions with igneous, recrystallized and shockmetamorphic textures in the Trill, Levack and Bowell embayments, and the Foy Offset dike on the North Range. They have no equivalents in the exposed country rocks. The calculated parental magma is similar to continental arc basalt formed by approximately 5% partial melting of garnet peridotite. Ol-Cpx-Pl thermobarometry of several Group III inclusions indicate equilibration at 900-1120 C and 210 6 166 MPa to 300 6 178 MPa, suggesting crystallization in the upper-middle crust (7Á7 6 6Á6 to 10Á9 6 6Á5 km), prior to being incorporated into the lower parts of the impact melt sheet during impact excavation. The exotic xenoliths provide information about the depth of impact and composition of upper-middle crust in the Sudbury region at 1850 Ma, the local xenoliths provide information about the thermomechanical erosion process that followed generation of the impact melt, the anteliths provide information about the early crystallization history of the SIC, and all of the inclusions provide constraints on the genesis of Sublayer, IQD, footwall breccia, and associated Ni-Cu-PGE mineralization.

Assimilation and crystallization in basic magma chambers: trace-element and Nd-isotopic variations in the Kerns sill, Nipissing diabase province, Ontario

Canadian Journal of Earth Sciences, Apr 1, 1989

An investigation has been made of the trace-element and Nd-isotopic effects of assimilation at th... more 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.

Economic geology and the bulletin of the Society of Economic Geologists, Jun 1, 2000

New data on the Voisey's Bay and Mushuau intrusions indicate that the two bodies, which were form... more New data on the Voisey's Bay and Mushuau intrusions indicate that the two bodies, which were formerly grouped as the Reid Brook Complex, differ markedly in age, petrology, geochemistry, and the scale of known mineralization. U-Pb dating of baddeleyite and zircon has revealed that the Mushuau intrusion has an age of 1.313 Ga, which is ~20 m.y. younger than the 1.333 Ga Voisey's Bay intrusion. The Voisey's Bay intrusion is host to the 124 million tons Ni-Cu-Co sulfide deposit (reserves + resources). Mineralization occurs at the base of an upper subchamber (the Eastern Deeps subchamber) and in a conduit that connects the Eastern Deeps to a lower subchamber (the Reid Brook subchamber). The mineralization is associated with a basal breccia that contains abundant reacted inclusions of paragneiss. Within the Eastern Deeps, the basal mineralization is overlain by a varied-textured troctolite that contains variable amounts of disseminated sulfide and gneiss inclusions. By comparison, only minor amounts of sulfide mineralization have been observed within the Mushuau intrusion. The Mushuau intrusion comprises a layered unit that has been intruded by a stellate-textured troctolite along its contact with footwall orthogneiss. The layered unit consists dominantly of melatroctolite at the margin with leucotroctolite toward the center and olivine gabbro at the core. The cumulates of the Mushuau intrusion are richer in olivine and thus have higher MgO contents than those of the Voisey's Bay intrusion. Notwithstanding this, olivines in the melatroctolite inclusions of the Voisey's Bay intrusion (which are thought to represent disrupted cumulus layers) contain up to 81 mole percent forsterite, whereas those in the melatroctolite of the Mushuau intrusion contain no more than 70 mole percent forsterite. Nickel depletion in olivine is more pronounced in the Voisey's Bay intrusion than in the Mushuau intrusion. Our data indicate that the parental magmas of both intrusions were broadly basaltic (MgO < 8.1 wt %) and that the magma of the Mushuau intrusion was more evolved than that of the Voisey's Bay intrusion, yet still capable of forming olivine-rich cumulates. The troctolites of the Voisey's Bay intrusion have Ce/Yb = 22 to 27, whereas most troctolites of the Mushuau intrusion have Ce/Yb = 12 to 17. The rocks of the Mushuau intrusion are simple mixtures of mantle-derived magma and local orthogneiss, whereas the rocks of the Voisey's Bay intrusion are formed by multiple stages of crustal contamination including an early stage of contamination at lower midcrustal level and a late stage of contamination by Tasiuyak paragneiss at upper crustal level. The assimilation of the sulfide-bearing Tasiuyak paragneiss by magma at Voisey's Bay may have promoted initial sulfide saturation and/or increased sulfide oversaturation, thus contributing to the segregation of large amounts of sulfide liquid from magma. Continuing flow of magma through a dynamic conduit system at Voisey's Bay has caused the local accumulation of sulfide of sufficient size and grade to constitute mineable reserves.

Ore Geology Reviews, Oct 1, 2018

A large proportion of ores in magmatic sulfide deposits consist of mixtures of cumulus silicate m... more A large proportion of ores in magmatic sulfide deposits consist of mixtures of cumulus silicate minerals, sulfide liquid, and silicate melt, with characteristic textural relationships that provide essential clues to their origin. Within silicate-sulfide cumulates, there is a range of sulfide abundance in magmatic-textured silicate-sulfide ores between ores with up to about five modal percent sulfides, called "disseminated ores," and "net-textured" (or "matrix") ores containing about 30 to 70 modal percent sulfide forming continuous networks enclosing cumulus silicates. Disseminated ores in cumulates have various textural types relating to the presence or absence of trapped interstitial silicate melt and (rarely) vapor bubbles. Spherical or oblate spherical globules with smooth menisci, as in the Black Swan disseminated ores, are associated with silicate-filled cavities interpreted as amygdales or segregation vesicles. More irregular globules lacking internal differentiation and having partially facetted margins are interpreted as entrainment of previously segregated, partially solidified sulfide. There is a textural continuum between various types of disseminated and net-textured ores, intermediate types commonly taking the form of "patchy net-textured ores" containing sulfide-rich and sulfide-poor domains at centimeter to decimeter scale. These textures are ascribed primarily to the process of sulfide percolation, itself triggered by the process of competitive wetting whereby the silicate melt preferentially wets silicate crystal surfaces. The process is self-reinforcing as sulfide migration causes sulfide networks to grow by coalescence, with a larger rise height and hence a greater gravitational driving force for percolation and silicate melt displacement. Many of the textural variants catalogued here, including poikilitic or leopard-textured ores, can be explained in these terms. Additional complexity is added by factors such as the presence of oikocrysts and segregation of sulfide liquid during strain-rate dependent thixotropic behavior of partially consolidated cumulates. Integrated textural and geochemical studies are critical to full understanding of ore-forming systems.

Data in Brief, Apr 1, 2021

This contribution describes maps of the Copper Cliff Embayment (CCE) and Offset (CCO) dyke. The a... more This contribution describes maps of the Copper Cliff Embayment (CCE) and Offset (CCO) dyke. The associated study attempts to unravel the mode of melt emplacement and the role of pre-impact faults in the deformation of the southern part of the Sudbury Igneous Complex (SIC). This contribution summarizes field observations (maps and images) and structural measurements. In addition, perspective views of the 3D Move model of the CCE and CCO dyke are provided. This data can be used by researchers and exploration geologists working in the Sudbury mining camp as a basis for future mapping, research and exploration effort s in the Copper Cliff area. This article is a co-submission to the following article: L. Mathieu, U. Riller, L. Gibson, P. Lightfoot (2021) Structural controls on the localization of the mineralized Copper Cliff embayment and the Copper Cliff offset dyke,

List of Data repositories Data repository 1: Table of precision and analytical ranges for the lit... more List of Data repositories Data repository 1: Table of precision and analytical ranges for the lithogeochemical methods used to obtain data from the whole-rock samples of the South Manasan intrusion. Digital file in Excel 2010 format. Data repository 2: Table of lithogeochemical results from the whole-rock analysis of the South Manasan intrusion. Digital file in Excel 2010 format. Data repository 3: Table of mineral chemistry results from the SEM-EDS analysis of mineral grains in carbon-coated polished thin sections from the South Manasan intrusion. Digital file in Excel 2010 format. Data repository 4: Table of mineral chemistry results from the Microprobe analysis of mineral grains in carbon-coated polished thin sections from the South Manasan intrusion. Digital file in Excel 2010 format.

Ore Geology Reviews, Jun 1, 2021

The Main Mass of the Sudbury Igneous Complex (SIC) is a 1.5-5 km thick, layered sheet of impact m... more The Main Mass of the Sudbury Igneous Complex (SIC) is a 1.5-5 km thick, layered sheet of impact melt rocks, intensely studied because of the magmatic Ni-Cu-PGE sulfide deposits associated with the base of the Main Mass and connected dykes, known as the Sublayer and Offset dykes, respectively. The mode of emplacement of the mineralized Offset dykes that connect to the Main Mass through morphologic craterfloor irregularities (embayments) and the style of post-cratering deformation that affected the Offset dykes is not fully understood. This field-based study of the Copper Cliff Embayment (CCE) and Offset (CCO) dyke contributes to unraveling the mode of melt emplacement and the role of pre-impact faults in the deformation of the southern SIC. Field relationships indicate that the CCO dyke formed before the CCE and Sublayer were chemically fully evolved. Respective melts were injected into footwall rocks weakened by pre-impact deformation and cratering as a protracted event, with barren quartz diorite (QD) emplaced prior to mineralized, inclusion-bearing quartz diorite (IQD). Massive sulfide ore bodies appear to have formed late in the evolution of the dyke and physical separation (decoupling) of silicate magma and sulfide melt is required. NW-SE-shortening folded and faulted the strata hosting the CCO dyke and deformation was facilitated through reactivated E-W-striking, pre-impact faults. Restoring the initial geometry of the dyke and embayment, using 3D modelling and field constraints, helped to refine total slip estimates along major faults and confirmed that melts migrated gravitationally downward into the CCO dyke.

Ore Geology Reviews, Mar 1, 2016

Geology, Mar 14, 2018

The lowermost, discontinuous parts of the impact-generated Sudbury Igneous Complex (Canada), comp... more 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.

Ni-Cu-Pge Ores Of The Noril’sk Region Siberia

Terra Nova, Mar 24, 2018

The post-impact orogenic evolution of the world-class Ni-Cu-PGE Sudbury mining camp in Ontario re... more The post-impact orogenic evolution of the world-class Ni-Cu-PGE Sudbury mining camp in Ontario remains poorly understood. New temporal constraints from ore-controlling, epidote-amphibolite facies shear zones in the heavily mineralised Creighton Mine (Sudbury, South Range) illuminate the complex orogenic history of the Sudbury structure. In situ U-Pb dating of shear-hosted titanite grains by LA-ICP-MS reveals new evidence for shear zone reworking during the Yavapai (ca. 1.77-1.7 Ga), Mazatzalian-Labradorian (1.7-1.6 Ga), and Chieflakian-Pinwarian (1.5-1.4 Ga) accretionary events. The new age data show that the effects of the Penokean orogeny (1.9-1.8 Ga) on the structural architecture of the Sudbury structure have been overestimated. At a regional scale, the new titanite age populations corroborate that the Southern Province of the Canadian Shield documents the same tectonothermal episodes that are recorded along orogenic strike within the accretionary provinces of the southwestern United States.

Isotopic heterogeneity in the Sudbury impact melt sheet

Earth and Planetary Science Letters, 2010

A unique terrestrial large impact melt sheet is preserved in the 1850Ma Sudbury Structure, Ontari... more A unique terrestrial large impact melt sheet is preserved in the 1850Ma Sudbury Structure, Ontario. We have undertaken a Pb isotope investigation of the southern limb of the melt sheet, termed the South Range Main Mass. The model initial Pb isotope ratios (207Pb/204Pbm) vary stratigraphically through the predominantly quartz monzogabbroic Lower Unit, varying from 15.40 to 15.45 at the base

Petrogenesis of rhyolites and trachytes from the Deccan Trap: Sr, Nd and Pb isotope and trace element evidence

Contributions to Mineralogy and Petrology, 1987

Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and... more Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and isotope features which mark them out from typical crustal melts. Their highly incompatible trace element and Sr-, Nd and Pb isotope ratios are similar to those of the associated Deccan flood basalts. Thus the rhyolites and trachytes are closely related to the basalts, and a striking