GEOCHEMICAL RELATIONSHIPS IN THE SUDBURY IGNEOUS COMPLEX - ORIGIN OF THE MAIN MASS AND OFFSET DIKES

Petrological models relating the different rock types constituting the 1.85 Ga Sudbury Igneous Complex are constrained with extensive new ge ochemical data. We show that the main mass felsic norite, transition z one quartz gabbro, and granophyre have similar ratios of the highly in compatible trace elements (e.g., La/ Sm = 4.5-7, La/Nb = 2.8-4.2, Th/Z r = 0.04-0.05) and that these variations are consistent with the cryst allization and differentiation of magma types largely (>80%) derived f rom the upper crust, with a smaller contribution from a mantle source. Although there is presently no conclusive proof that magma was genera ted in situ as a melt sheet produced by meteorite impact, we find no p rincipal reason why this model should be rejected. However, we propose that a small contribution of mantle-derived picritic magma is require d to explain the abundant Ni, Cu, and platinum-group elements (PGE) in the Sudbury deposits, as well as the compositions of the ultramafic i nclusions (MgO = 12-36 wt %; Fo(68-87) olivines with 450-3,700 ppm Ni, and abundant chrome-rich spinel), and the magnesian composition of ti le mafic norite (8-14 wt % MgO) and the sublayer (6-12 wt % MgO). We b elieve that the main mass of the Sudbury Complex achieved its present composition through incorporation of up to 20 percent mantle-derived p icritic magma emplaced along crustal fractures produced by the impact event. These picritic magmas entered the melt sheet as a dense plume, vigorously mixing with it, and due to the marked compositional shift: the mixed magma formed magmatic sulfides which sank through the magma column, depleting the melt in Ni, Cu, and PGE. Since both the felsic n orite and granophyre have indistinguishable ratios of the incompatible trace elements, we see no requirement to derive these units of rock b y the crystallization of magmas derived from different sources. Rather , the compositional difference between the felsic norite and granophyr e is attributed to the in situ differentiation of the magma. We show t hat the main mass has many compositional traits similar to those of mo st of the offset dike quartz diorites (e.g., the Parkin offset dike: L a/Sm = 6.3, La/Nb = 4.5; Th/Zr = 0.05) and of embayment-related leucoc ratic norites from the Whistle mine (La/Sm = 6.3, La/Nb = 5.0, Th/Zr = 0.02). These rocks have compositions intermediate between the felsic norite and the granophyre, and therefore crystallized from the same ma gma type; arguably, the unmineralized quartz diorites provide the best possible estimate of the original magma from which the Sudbury Comple x crystallized. In detail, there are subtle variations in composition within and between offset dikes, with the largest difference being bet ween the North and South Range offsets; the North Range offset dikes c ut Archean granitoids and gneisses and have elevated Sr, La/Yb, La/ Sm , and Gd/Yb and low TiO2 whereas the South Range dikes cut Early Prote rozoic sediments, mafic volcanics, and intrusions, and have low Sr, La /Yb, Gd/Yb, La/Sm, and high TiO2. These differences may be caused by t he assimilation of different country rocks during emplacement of the d ike. A strongly mineralized offset dike at the Creighton mine has geoc hemical variations that are different when compared to the main mass, and in the case of Creighton, are more similar to the local mineralize d sublayer. These data suggest that mineralized and barren quartz dior ites have different geochemical compositions, and that these traits ma y be of value in mineral exploration.