Interaction of magmatic fluids and silicate melt residues with saline groundwater in the footwall of the Sudbury Igneous Complex, Ontario, Canada: New evidence from bulk rock geochemistry, fluid inclusions and stable isotopes

Abstract

Along the northern margin of the 1.85Ga Sudbury Igneous Complex (SIC), Canada, a volumetrically minor rock type known as the footwall granophyre (FWGP) preserves evidence of the interaction of magmatic fluid and groundwater. The FWGP comprises veins, dikes and irregular bodies of a quartz–alkali feldspar–plagioclase intergrowth that are most abundant near footwall-style Cu–Ni–platinum-group element mineralization. Textures and cross-cutting relations show that the emplacement of the FWGP created favourable sites for the later deposition of sulfides. Geochemical and physical evidence show that the FWGP did not form by in-situ partial melting of the Archean country rocks in the footwall beneath the SIC. It represents a mobilized (injected) silicate residue from either the contact between partially melted country rocks and the SIC, or from the crystallizing SIC itself. Its bulk composition can be modeled as the product of fractional crystallization of a liquid with an initial composition equivalent to granophyric matrix (trapped liquid) in the lowermost units of the SIC (norite and sublayer).Primary fluid (saline, H2O-poor) and melt (silicate, H2O-rich) inclusions hosted in quartz from the FWGP have bulk and trace element compositions consistent with those hosted in quartz from the granophyric matrix of the lower units of the SIC (norite, sublayer). Fluid inclusions contain mixtures of a high salinity (44–70wt.% NaClequivalent; n=39), Na–Fe-rich aqueous magmatic fluid and a lower salinity, Ca- and Sr-rich groundwater. The magmatic end-member is thought to be an SIC-derived volatile phase. Estimates of the mixing proportions show that primary and secondary inclusions in the FWGP contain 60–100% and 30–80% of the magmatic end-member by mass, respectively. Fluids responsible for the remobilization of ore metals represent mixtures containing a much higher proportion of the groundwater end-member (not less than 70% by mass) compared to the fluids trapped in the FWGP. At considerable distance (~400m) from the SIC, the chlorine isotope composition of biotite in the FWGP is enriched in 37Cl (δ37Cl=0.98‰ to 1.61‰). Scapolite from the high temperature veins and the interstices of rock types in the main mass of the SIC shows similar enrichment in 37Cl (up to 1.34‰) whereas biotite from the Archean country rocks (δ37Cl=−0.88 to −0.53‰) and associated groundwater are 37Cl-depleted.The results of this study lead to two important conclusions concerning genetic and exploration models for the footwall ore zones. First, magmatic end-member fluid introduced into the footwall with the melt from which the FWGP crystallized contained very little Cu (rarely >~100ppm), the main ore metal in the footwall ores. This result questions hydrothermal models for footwall ore formation that hypothesize that Cu and other ore metals were remobilized from sulfide deposits along the SIC contact by magmatic-hydrothermal fluids and redeposited in the footwall. It is more likely that ore metals were only locally remobilized in the footwall ore zones when metal-poor hybrid fluids interacted with pre-existing sulfide veins of magmatic origin. Second, inclusions containing the hybrid fluid occur in the FWGP rock type throughout the footwall, in both mineralized and unmineralized areas. Therefore, although the FWGP preserves stable Cl isotope and fluid inclusion evidence that magmatic fluid was introduced into the footwall, detection of this magmatic component alone will not serve as a useful exploration method. In contrast, mapping the distribution and abundance of FWGP may be ore-predictive since FWGP emplacement and its higher abundance in mineralized areas appears to have been a structural and textural prerequisite for footwall ore emplacement.