Cryptic variation and the petrology of the Sudbury nickel irruptive

Naldrett, A. J.; Bray, Joshua G.P.; Gasparrini, E.; Podolsky, T.; Rucklidge, J. D.

doi:10.2113/gsecongeo.65.2.122

Cited by 95 publications

(38 citation statements)

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“…The SIC displays no unambiguous evidence for crystal fractionation [Naldrett et al, 1970], although it is modally and compositionally stratified on the scale of tens to hundreds of meters [e.g., Chai and Eckstrand, 1994]. The source of the ultramafic material has therefore been interpreted as crucial to the understanding of the origin and development of the sulphide ores.…”

Section: Contact Sublayermentioning

confidence: 99%

Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

Prevec

Cawthorn

2002

J. Geophys. Res.

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[1] The Sudbury Igneous Complex (SIC) and associated Ni-Cu-PGE mineralization has been interpreted in terms of a large meteorite impact event. In this study, the thermal relationship between the large cooling melt sheet and the surrounding country rock is examined in terms of its role in an evolving thermal gradient rather than as a passive receptacle for the melt sheet above. Thermal modeling of this environment is undertaken using physical and thermal constraints appropriate to the SIC and assuming heat dissipation from the 2.5-km-thick superheated melt sheet (!1800°C) by either diffusion with zero convection or by rapid convection within the melt sheet. With zero convection, basal cooling produces a solid base, which lowers conductivity such that the immediate footwall rocks reach 1000°C, producing partial melting that extends 200 m into the footwall. In a rapidly convecting melt sheet the initial footwall chill is remelted and high temperatures maintained within the sheet close to the contact. This results in higher temperatures being attained in the immediate footwall (1100-1200°C), inducing complete melting of proximal footwall and partial melting to depths of 500 m below the melt sheet. Proximal footwall consists of Paleoproterozoic Huronian basalts, granitoids and sediments, exposed in the south range, overlying Archaean gneisses and granitoids. Total and partial melting of this material early in the cooling history of the melt sheet and the subsequent gravitational accommodation of these melts according to density would produce a basalt-dominated basal liquid corresponding to the so-called contact sublayer. The thermal aureole predicted by our models is consistent with that preserved around the north range of the SIC assuming $800 m of thermally induced erosion at the contact.

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Section: Contact Sublayermentioning

confidence: 99%

Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

Prevec

Cawthorn

2002

J. Geophys. Res.

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“…The Main Mass comprises a norite-quartz gabbro-granophyre sequence, which was probably formed by the in situ differentiation of the impact melt pool (Ostermann and Deutsch, 1997). Certain geochemical and petrological differences exist between the North and South Ranges of the SIC (Naldrett, 1970). Emanating from the Main Mass are the so-called Offset Dykes (Coleman, 1903), which are disposed radially and concentrically about the SIC (Fig.…”

Section: Introductionmentioning

confidence: 99%

Origin and emplacement of Offset Dykes in the Sudbury impact structure: Constraints from Hess

Wood

Spray

1998

Meteorit & Planetary Scien

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Abstract-The Hess Offset is a steeply dipping dyke located 12-15 km north of the 1.85 Ga Sudbury igneous complex (SIC) within the 200-250 km diameter Sudbury impact structure. It is up to 60 m wide and strikes subconcentrically to the SIC for at least 23 km. The main phase of the dyke is granodioritic, but it conforms with what is locally referred to as Quartz Diorite: a term used for all the Offset Dykes of the Sudbury impact structure. Rare earth element data shows that the Hess Offset is genetically related to the SIC. Hess is most closely affiliated with an evolved Felsic Norite component of SIC and not bulk impact melt. This indicates that Hess was emplaced during fractionation of the impact melt sheet, rather than immediately following impact. The main Quartz Diorite phase of the dyke comprises a quartz + plagioclase + hornblende + biotite ? clinopyroxene 2 orthopyroxene assemblage. Critically, the Hess Offset occupies a concentric fault system that marks the northern limit of a pseudotachylyte-rich, shatter cone-bearing annulus about the SIC. This fault system was active during the modification stage of the impact process.

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“…In this, the quartz content increases very progressively towards the contact over the outer 300m (Naldrett et al 1970). This is unlikely to be due to contamination since the increase in quartz occurs as much where the footwall is composed of SiOrdeficient greenstone as it does where it is composed of granite.…”

Section: Main Massmentioning

confidence: 83%

“…This indicates that if contamination is involved, it is not in situ contamination. The increase in quartz is accompanied by an equally progressive decrease in the average Mg/(Mg + Fe) ratio of the pyroxenes (Naldrett et al 1970). This decrease is due to the pyroxenes becoming progressively more strongly zoned, with…”

Section: Main Massmentioning

confidence: 96%

Geochemistry of the Sudbury Igneous Complex: A Model for the Complex and Its Ores

Naldrett

1986

Special Publication No. 4 of the Society for Geology Applied to Mineral Deposits

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A recent interpretation of regional gravity and magnetic data (Gupta et al. 1984) has indicated that the Sudbury Igneous Complex is underlain at depths of 5 -8 km by a 60 x 40 km mass of mafic and ultramafic rock that is not part of the exposed Complex.The marginal rocks of the Complex are thought to have crystallized in situ. The high Si02 and K20 and low CaO contents, and low Na20/K20 ratio of these rocks in comparison with those of continental flood basalts are suggestive that the magma responsible for the Complex experienced extensive contamination by felsic country rocks. The REE profiles and major elements can be modelled if a 1 : 2 mixture of quartz monzonite and tonalite that form much of the basement at Sudbury, is combined on a 1: 1 basis with a fairly primitive flood basalt (MgNo = 0.61). The sub layer is more fractionated than the marginal unit of the main mass, but its major element and REE concentrations also suggest significant assimilation of a similar contaminant. The high Sr initial isotope ratios of both main mass and sublayer are consistent with the contamination hypothesis.Mafic and ultramafic inclusions are restricted to variants of the sub layer that are also mineralized. These inclusions have REE profiles with similar high La/Yb ratios to those of the main mass and sublayer. The Fo content of olivines in the inclusions indicates that they crystallized from liquids spanning the same range of Mg Nos as is spanned by samples of the sub layer . It is proposed that the inclusions have been derived from cumulate layers that formed as the sublayer magmas fractionated.The contamination that gave rise to the SiOrrich composition of the Complex is believed to be the cause of the segregation of large amounts of sulfide. It is suggested that the close association between sulfides, inclusions and sublayer magmas is the consequence of the sulfides and ultramafic and mafic cumulates settling together in hidden sills that are present peripheral to and beneath the Complex. These are responsible in part for the gravity and aeromagnetic anomalies. They were injected into the fractures in the rocks beneath the Complex as offshoots from the main magma conduit. As they cooled and fractionated, residual magma rose to the floor of the crater that now holds the

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Cryptic variation and the petrology of the Sudbury nickel irruptive

Cited by 95 publications

References 0 publications

Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

Origin and emplacement of Offset Dykes in the Sudbury impact structure: Constraints from Hess

Geochemistry of the Sudbury Igneous Complex: A Model for the Complex and Its Ores

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