Geochemistry of the Sudbury Igneous Complex: A Model for the Complex and Its Ores
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 × 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 SiO2 and K2O and low CaO contents, and low Na2O/K2O 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 sublayer 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 sublayer 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 sublayer. 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 SiO2-rich 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 Complex to form the presently exposed sublayer. Where magma from a deeper sill cut and disrupted an overlying sill, it picked up sulfides and inclusions, and carried them upwards to form the ore deposits.
The magma of the main mass of the Complex, cooling in part within the central conduit, did so more slowly than the sublayer. It thus became contaminated more rapidly, but fractionated less rapidly. It was injected into its present position as a series of pulses at essentially the same time as the sublayer, possibly in response to structural adjustments taking place in the overlying crater.
KeywordsCountry Rock Flood Basalt Main Mass Layered Intrusion Quartz Monzonite
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