Geology, Geochemistry, and Mineralogy of the Worthington Offset Dike: A Genetic Model for Offset Dike Mineralization in the Sudbury Igneous Complex

Abstract

1993) and ~0.8 g/t Pt + Pd. The actual amount of mineralized rock is much larger, since large quantities of sulfides below cut-off grades are not mined. The sulfide mineralization is partly localized along the lower contact of the Sudbury Igneous Complex, where ores occur in depressions in its base in thick units of inclusion-rich norite and partially melted and AbstractThe Worthington offset dike extends for approximately 15 km away from the southwestern margin of the 1.85 Ga Sudbury Igneous Complex. The dike is zoned with respect to inclusion and sulfide contents. Marginal chilled quartz diorite is transitional into medium-grained quartz diorite. These rocks are sulfide undersaturated, contain small inclusions from the wall rocks, and are preserved along much of the dike. Locally, the dike contains a core of inclusion-rich quartz diorite, which can be choked with inclusions surrounded by semimassive to massive sulfide. The more heavily mineralized inclusion-rich quartz diorite contains 10 to 75 percent amphibolite inclusions, which are petrologically and geochemically similar to the immediately adjacent country-rock amphibolites, locally termed "Sudbury gabbros." The semimassive to massive sulfide zones form subvertical pipes, much like the deposits of the Copper Cliff offset dike, and these are associated with locations where the Worthington dike widens from 20 to 30 m to 50 to 80 m. The average metal tenors of the sulfide with > = 5% sulfur are calculated to be 7 percent Ni and 13 percent Cu. Thus the dike ores have a much higher Cu/Ni ratio than orebodies within the contact sublayer (Cu/Ni ~ 1).The medium-grained inclusion-poor quartz diorite and inclusion-rich quartz diorite differ in Ni, Cu, Pt, and Pd abundances, but they have similar major and lithophile trace element abundance levels despite having different inclusion and sulfide contents. Assimilation of inclusions has therefore not significantly changed the composition of the silicate matrix of the inclusion-rich quartz diorite. The composition of the inclusion-poor quartz diorite is a close approximation to average crust and also to the bulk composition of the Sudbury Igneous Complex rocks. Regional differences in offset geochemistry exist between the North and South Range offset dikes, but we believe that the formation of the inclusion-poor quartz diorite and inclusion-rich quartz diorite silicate magmas predated significant silicate differentiation or silicate gravitational segregation of the melt sheet and that the differences may record primary silicate heterogeneity of the melt sheet. These differences may have developed in response to the different proportions of Archean granitoids relative to Proterozoic sediments and volcanics that contributed to the melt sheet on the North and South Ranges of the Sudbury Igneous Complex.The first quartz diorite melt was sulfide undersaturated and devoid of amphibolite inclusions. The introduction of this melt took place during or shortly after the generation of the melt sheet and represented a very rapid int...

Differentiation and emplacement of the Worthington Offset Dike of the Sudbury impact structure, Ontario

“…The chemical compositions of QD and IQD samples vary in the drill core profile (Fig. 8), whereby the variation of most element concentrations (Table 3) overlap with the compositional ranges of the respective Worthington quartz diorite phases (Lightfoot et al 1997a,b;Lightfoot and Farrow 2002). On average, the QD and IQD samples are slightly more siliceous and less mafic (i.e., lower Mg and Fe contents).…”

“…Previous studies of the Worthington Offset Dike were focussed on a 6 km long dike segment between the SIC, at Victoria Mine, and the Totten Mine area (Grant and Bite 1984;Lightfoot and Farrow 2002). In this study, we have investigated two localities that are more distal to the SIC than the aforementioned dike segment.…”

“…At the dike margins, quartz-diorite (QD) is fine-to medium-grained and generally devoid of any inclusions, although some local host rock fragments may occur (Lightfoot and Farrow 2002). A chilled margin between the QD and the host rock is frequently developed.…”

“…Based on structural and geochemical evidence, various emplacement modes for the Offset Dikes have been suggested that are either related to a particular stage in cratering or to post-cratering deformation. For example, models include injection of quartz-dioritic melt into crater floor fractures during transient cavity formation (Grant and Bite 1984;Lightfoot and Farrow 2002;Murphy and Spray 2002) and melt injeion into rocks participating in the formation of a central uplift structure (Wood and Spray 1998;Tuchscherer and Spray 2002). Both processes would have occurred within minutes after impact (Melosh 1989).…”

“…Although most authors believe that the dikes represent a melt that was injected from the Main Mass of the SIC downward into the target rocks, i.e., the crater floor, it remains unclear at what evolutionary stage of the SIC the quartz-diorite dikes formed. Existing rival interpretations are for example: (1) the Offset Dikes represent a melt that was geochemically equivalent to the Main Mass of the SIC before it differentiated (Lightfoot et al 1997b;Lightfoot and Farrow 2002;Tuchscherer and Spray 2002). (2) The dike melts derived from intermediate stages of fractional crystallization (± contamination) of the Main Mass of the SIC (Wood and Spray 1998;Prevec et al 2000;Therriault et al 2002), or (3) radial dikes including the Sublayer sensu stricto (Fig.…”