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Papers by Peter Lightfoot

Canadian Journal of Earth Sciences, 1997

Three distinct tonalite-trondhjemite-granodiorite (TI'G) magma series occur syngenetically with b... more Three distinct tonalite-trondhjemite-granodiorite (TI'G) magma series occur syngenetically with bimodal mafic-felsic cyclical volcanics in the middle to late Archean Michipicoten Greenstone Belt (MGB), Canada. Between 2.72 and 2.60 Ga, two additional series are recognised as pre-tectonic, late tectonic, and post-tectonic syenite-quartz monzonite granitoid stocks (SMG series) and an alkali feldspar syenite-alkali feldspar quartz syenite stock (SS series). The synvolcanic q'TG series, such as the Hawk Lake Granitic Complex and the Jubilee Lake Stock, are characterised by moderate to low K20 ( < 2.5 wt.%), Rb ( < 60 ppm), and Sr concentrations ( <400 ppm), moderate TiO2 levels (0.1-1.0 wt.%), La/Sm (2-12), and Gd/Yb (0.8-2.9), moderate to large negative Eu-anomalies, and small negative Ta + Nb troughs on ocean ridge granite (ORG)-normalised plots. The "ITG series are compositionally similar to synvolcanic MGB felsic volcanics of cycles 1 to 3. The high La/Sm, La/Ta and low Zr/Th are features of sialic crust; a contribution of ancient sialic crust in the genesis of the felsic volcanics and TTG-series granitoids of all three felsic volcanic cycles and synvolcanic "ITG stocks is required in the MGB. The parental magma, like that of the TTG series of the Abitibi Greenstone belt (AGB) may originate in oceanic mantle. The SMG series have elevated K20 (2.0-6.3 wt.%), Rb (40-280 ppm), Ba ( 10-2800 ppm), and Sr (250-1350 ppm), steep REE patterns reflected in elevated La/ Sm (3-12) and Gd/Yb (1.8-5.0) without Eu-anomalies, a moderate to large negative Ta+Nb trough on ocean ridge granite-normalised plots, and low TiO2 ( < 0.5 wt.%) and CaO/(Na20 + K20). The SMG series is compositionally different to the MGB felsic volcanic rocks and to the "ITG-series granitoids. The source is tentatively placed in the mantle wedge overlying a southward-dipping subducted plate of oceanic crust, and the elevated LILE signature is attributed to metasomatism of this source by fluids derived from the subducted slab of oceanic crust. The SMG-series granitoids, like those of the Abitibi, are different when compared to the sanukitoids in that they are compositionally more evolved; however, they may be related to these rocks by low-pressure fractionation, and may be derivatives of sanukitoid-like magmas. The alkali feldspar syenite-alkali feldspar quartz syenite, represented by the Herman Lake Stock of the SS series, has a primitive composition ( < 60 wt.% SiO2) with low REE abundances and relatively flat LREE and HREE, but steep MREE profiles. These rocks are strongly LILEenriched, but do not develop pronounced Ta + Nb troughs on ORG-normalised profiles. These rocks may be derived by low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab. The elevated Ti/Y and Zr/Y of these rocks appear to suggest that small amounts of partial melting were responsible for the genesis of these rocks. The granitoid rocks record an overall progression from contributions made from the continental crust 0301-9268/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSD!0301-9268 (95)00021-6

Process models for ore formation in magmatic Ni-Cu-platinum group element (PGE) sulfide systems r... more Process models for ore formation in magmatic Ni-Cu-platinum group element (PGE) sulfide systems require that S saturation is achieved in a mafic-ultramafic magma. Traditional models explain the achievement of S saturation or sulfide saturation either by the addition of crustal S, by the felsification of the magma by crustal contamination, or by mixing between primitive and evolved magmas. Which process matters most is important to industry-oriented exploration models where crustal S sources are believed to be encouraging features of a metallotect. Studies of the Siberian Trap flood basalts at Noril'sk have demonstrated that chalcophile element depletion is linked to assimilation of silica-rich crust, but it is less clear whether this contaminant contained an appreciable amount of S. At Noril'sk, the Ni-Cu-PGE sulfide deposits are associated with subvolcanic intrusions that were emplaced into Permian and Carboniferous sedimentary sequences rich in shales, marlstones, and evaporites. Similar to the Siberian Trap basalts, the Deccan Trap contains a volumetrically important suite of crustally contaminated tholeiitic basalts. We present new PGE data for samples from a stratigraphic sequence of basalts from the southern Deccan province. Two of the formations in this sequence (the Bushe and Poladpur Formations) have geochemical signatures indicative of a wide degree of crustal contamination of a magma type that gave rise to the stratigraphically higher Ambenali Formation (a product of transitional midocean ridge basalt magmatism). There are no known deposits or occurrences of Ni-Cu-PGE sulfides associated with subvolcanic intrusions in the Deccan province. Despite the fact that the Bushe Formation exhibits a stronger crustal contamination signature than the most contaminated Siberian Trap basalt formations, and the Poladpur lavas are also strongly crustally contaminated, the Bushe and Poladpur basalts are undepleted in Ni, Cu, or PGE. This indicates that the contaminated Deccan Trap lavas did not achieve S saturation. This, in turn, places constraints on the potential of the Deccan Trap in southern India to host significant magmatic sulfide deposits. Conversely, this observation also indicates that an S-rich crustal contaminant is required for the genesis of magmatic Ni-Cu-PGE sulfide deposits.

Earth and Planetary Science Letters, 1988

Geochemical and isotope results are presented from a new study of the most southern basalts in th... more Geochemical and isotope results are presented from a new study of the most southern basalts in the Deccan Trap, India. Three chemical formations are recognised, two of which can be correlated with the established stratigraphy in Mahabaleshwar and imply a regional southerly dip of 0.06° over a distance of 250 km. In detail Sr-isotope variations within the Ambenali and Mahabaleshwar

Mineralium Deposita, 2012

The Voisey's Bay deposit is hosted in a 1.34 Ga intrusion composed of troctolite, olivine gabbro,... more The Voisey's Bay deposit is hosted in a 1.34 Ga intrusion composed of troctolite, olivine gabbro, and ferrogabbro. The massive sulphides are compositionally different when compared to the disseminated sulphides, and it is believed that the sulphide melts underwent different S saturation histories. Compositional variations in the sulphides are consistent with controls by grain size and texture, fractionation, magma/sulphide ratio, and silicate magma composition. Variable-textured troctolites and normal-textured troctolites of the Eastern Deeps that contain trace sulphide have 0.1-3 ppb Pt and 0.1-3 ppb Pd, whereas weakly mineralised to heavily mineralised variable-textured troctolites from the same stratigraphy have 1-2 orders of magnitude higher Pt and Pd abundance levels. Troctolites and olivine gabbros from other Nain Plutonic Suite intrusions have low Pt and Pd abundances. It appears likely that Pt and Pd abundance levels are consistent with low degrees of melting with retention of PGE in the mantle source. This helps to explain the low PGE abundance levels in the Voisey's Bay ores, and it also indicates that PGEdepletion should be used with caution as an exploration tool in the Nain Plutonic Suite.

Mineralium Deposita, 2007

Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nu... more Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ∼1,000 m of picritic lavas and discrete 10-to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaîgat and Maligât Formations of which the former includes the Naujánguit member, which consists of picrites with 7-29 wt% MgO, 80-1,400 ppm Ni, 5.7-9.4 ppb Pt and 4.2-12.9 ppb Pd. The Naujánguit member contains two horizons of contaminated basalts, the Asûk and Kûgánguaq, which have elevated SiO2 (52-58 wt%) and low to moderate MgO (7.5-12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16-0.63 ppb) and Pd (0.13-0.68 ppb) abundances, and in the case of the Asûk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril'sk region. These basalts belong to the 0-to 500-m thick, ∼5,000to 10,000-km 3 Nadezhdinsky Formation, which is centered in the Noril'sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metaldepleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10-20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N-S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril'sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be Miner Deposita : [319][320][321][322][323][324][325][326][327][328][329][330][331][332][333][334][335][336] unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril'sk system.

Geostandards and Geoanalytical Research, 1996

Mineralium Deposita, 2012

Canadian Journal of Earth Sciences, 1997

Three distinct tonalite-trondhjemite-granodiorite (TI'G) magma series occur syngenetically with b... more Three distinct tonalite-trondhjemite-granodiorite (TI'G) magma series occur syngenetically with bimodal mafic-felsic cyclical volcanics in the middle to late Archean Michipicoten Greenstone Belt (MGB), Canada. Between 2.72 and 2.60 Ga, two additional series are recognised as pre-tectonic, late tectonic, and post-tectonic syenite-quartz monzonite granitoid stocks (SMG series) and an alkali feldspar syenite-alkali feldspar quartz syenite stock (SS series). The synvolcanic q'TG series, such as the Hawk Lake Granitic Complex and the Jubilee Lake Stock, are characterised by moderate to low K20 ( < 2.5 wt.%), Rb ( < 60 ppm), and Sr concentrations ( <400 ppm), moderate TiO2 levels (0.1-1.0 wt.%), La/Sm (2-12), and Gd/Yb (0.8-2.9), moderate to large negative Eu-anomalies, and small negative Ta + Nb troughs on ocean ridge granite (ORG)-normalised plots. The "ITG series are compositionally similar to synvolcanic MGB felsic volcanics of cycles 1 to 3. The high La/Sm, La/Ta and low Zr/Th are features of sialic crust; a contribution of ancient sialic crust in the genesis of the felsic volcanics and TTG-series granitoids of all three felsic volcanic cycles and synvolcanic "ITG stocks is required in the MGB. The parental magma, like that of the TTG series of the Abitibi Greenstone belt (AGB) may originate in oceanic mantle. The SMG series have elevated K20 (2.0-6.3 wt.%), Rb (40-280 ppm), Ba ( 10-2800 ppm), and Sr (250-1350 ppm), steep REE patterns reflected in elevated La/ Sm (3-12) and Gd/Yb (1.8-5.0) without Eu-anomalies, a moderate to large negative Ta+Nb trough on ocean ridge granite-normalised plots, and low TiO2 ( < 0.5 wt.%) and CaO/(Na20 + K20). The SMG series is compositionally different to the MGB felsic volcanic rocks and to the "ITG-series granitoids. The source is tentatively placed in the mantle wedge overlying a southward-dipping subducted plate of oceanic crust, and the elevated LILE signature is attributed to metasomatism of this source by fluids derived from the subducted slab of oceanic crust. The SMG-series granitoids, like those of the Abitibi, are different when compared to the sanukitoids in that they are compositionally more evolved; however, they may be related to these rocks by low-pressure fractionation, and may be derivatives of sanukitoid-like magmas. The alkali feldspar syenite-alkali feldspar quartz syenite, represented by the Herman Lake Stock of the SS series, has a primitive composition ( < 60 wt.% SiO2) with low REE abundances and relatively flat LREE and HREE, but steep MREE profiles. These rocks are strongly LILEenriched, but do not develop pronounced Ta + Nb troughs on ORG-normalised profiles. These rocks may be derived by low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab. The elevated Ti/Y and Zr/Y of these rocks appear to suggest that small amounts of partial melting were responsible for the genesis of these rocks. The granitoid rocks record an overall progression from contributions made from the continental crust 0301-9268/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSD!0301-9268 (95)00021-6

Process models for ore formation in magmatic Ni-Cu-platinum group element (PGE) sulfide systems r... more Process models for ore formation in magmatic Ni-Cu-platinum group element (PGE) sulfide systems require that S saturation is achieved in a mafic-ultramafic magma. Traditional models explain the achievement of S saturation or sulfide saturation either by the addition of crustal S, by the felsification of the magma by crustal contamination, or by mixing between primitive and evolved magmas. Which process matters most is important to industry-oriented exploration models where crustal S sources are believed to be encouraging features of a metallotect. Studies of the Siberian Trap flood basalts at Noril'sk have demonstrated that chalcophile element depletion is linked to assimilation of silica-rich crust, but it is less clear whether this contaminant contained an appreciable amount of S. At Noril'sk, the Ni-Cu-PGE sulfide deposits are associated with subvolcanic intrusions that were emplaced into Permian and Carboniferous sedimentary sequences rich in shales, marlstones, and evaporites. Similar to the Siberian Trap basalts, the Deccan Trap contains a volumetrically important suite of crustally contaminated tholeiitic basalts. We present new PGE data for samples from a stratigraphic sequence of basalts from the southern Deccan province. Two of the formations in this sequence (the Bushe and Poladpur Formations) have geochemical signatures indicative of a wide degree of crustal contamination of a magma type that gave rise to the stratigraphically higher Ambenali Formation (a product of transitional midocean ridge basalt magmatism). There are no known deposits or occurrences of Ni-Cu-PGE sulfides associated with subvolcanic intrusions in the Deccan province. Despite the fact that the Bushe Formation exhibits a stronger crustal contamination signature than the most contaminated Siberian Trap basalt formations, and the Poladpur lavas are also strongly crustally contaminated, the Bushe and Poladpur basalts are undepleted in Ni, Cu, or PGE. This indicates that the contaminated Deccan Trap lavas did not achieve S saturation. This, in turn, places constraints on the potential of the Deccan Trap in southern India to host significant magmatic sulfide deposits. Conversely, this observation also indicates that an S-rich crustal contaminant is required for the genesis of magmatic Ni-Cu-PGE sulfide deposits.

Earth and Planetary Science Letters, 1988

Geochemical and isotope results are presented from a new study of the most southern basalts in th... more Geochemical and isotope results are presented from a new study of the most southern basalts in the Deccan Trap, India. Three chemical formations are recognised, two of which can be correlated with the established stratigraphy in Mahabaleshwar and imply a regional southerly dip of 0.06° over a distance of 250 km. In detail Sr-isotope variations within the Ambenali and Mahabaleshwar

Mineralium Deposita, 2012

The Voisey's Bay deposit is hosted in a 1.34 Ga intrusion composed of troctolite, olivine gabbro,... more The Voisey's Bay deposit is hosted in a 1.34 Ga intrusion composed of troctolite, olivine gabbro, and ferrogabbro. The massive sulphides are compositionally different when compared to the disseminated sulphides, and it is believed that the sulphide melts underwent different S saturation histories. Compositional variations in the sulphides are consistent with controls by grain size and texture, fractionation, magma/sulphide ratio, and silicate magma composition. Variable-textured troctolites and normal-textured troctolites of the Eastern Deeps that contain trace sulphide have 0.1-3 ppb Pt and 0.1-3 ppb Pd, whereas weakly mineralised to heavily mineralised variable-textured troctolites from the same stratigraphy have 1-2 orders of magnitude higher Pt and Pd abundance levels. Troctolites and olivine gabbros from other Nain Plutonic Suite intrusions have low Pt and Pd abundances. It appears likely that Pt and Pd abundance levels are consistent with low degrees of melting with retention of PGE in the mantle source. This helps to explain the low PGE abundance levels in the Voisey's Bay ores, and it also indicates that PGEdepletion should be used with caution as an exploration tool in the Nain Plutonic Suite.

Mineralium Deposita, 2007

Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nu... more Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ∼1,000 m of picritic lavas and discrete 10-to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaîgat and Maligât Formations of which the former includes the Naujánguit member, which consists of picrites with 7-29 wt% MgO, 80-1,400 ppm Ni, 5.7-9.4 ppb Pt and 4.2-12.9 ppb Pd. The Naujánguit member contains two horizons of contaminated basalts, the Asûk and Kûgánguaq, which have elevated SiO2 (52-58 wt%) and low to moderate MgO (7.5-12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16-0.63 ppb) and Pd (0.13-0.68 ppb) abundances, and in the case of the Asûk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril'sk region. These basalts belong to the 0-to 500-m thick, ∼5,000to 10,000-km 3 Nadezhdinsky Formation, which is centered in the Noril'sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metaldepleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10-20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N-S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril'sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be Miner Deposita : [319][320][321][322][323][324][325][326][327][328][329][330][331][332][333][334][335][336] unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril'sk system.

Geostandards and Geoanalytical Research, 1996

Mineralium Deposita, 2012