PETROGENETIC SIGNIFICANCE OF CHROMIAN SPINELS FROM THE SUDBURY IGNEOUS COMPLEX, ONTARIO, CANADA

Chromian spinels occur in mafic-ultramafic inclusions in the Sublayer of the Sudbury Igneous Complex (SIC) as well as in mafic-ultramafic ro cks in the immediate footwall of the Sublayer. The host rocks are pyro xenite and melanorite with minor dunite, harzburgite, and melatroctoli te. As common accessory phases in these rocks, the chromian spinels di splay euhedral or subhedral forms and are included in olivine and orth opyroxene. Chromian spinel grains generally have ilmenite lamellae and contain abundant inclusions (zircon, olivine, diopside, plagioclase, biotite, and sulfide). All the chromian spinels have similar trace ele ment abundances and are rich in TiO2 (0.5-15 wt.%). They have constant Cr# (100Cr/(Cr + Al)) (55-70) and exhibit a continuum in composition that traverses the normal fields of spinels in a Al-(Fe3+ + 2Ti)-Cr tr iangular diagram. This continuum extends to that of the composition of chromian magnetite in the host norite matrix to the mafic-ultramafic inclusions. This continuum in composition of the spinels suggests that the noritic matrix to the Sublayer formed from the same magma as the inclusions. A positive correlation between the Cr and Al contents of t he spinels was probably produced by dilution of these elements by Fe3 contributed, perhaps, by a plagioclase-saturated melt. Zircon inclusi ons in a chromian spinel grain reflect incorporation of crustal, felsi c materials into the magma before crystallization of chromian spinel. The chemical characteristics and mineral inclusions of the spinels sug gest that the Sublayer formed in response to magma mixing. It is sugge sted that subsequent to the formation of the crustal melt, mantle-deri ved high-Mg magmas mixed vigourously with this and generated the magma tic sulfides that eventually formed the Ni - Cu - platinum-group eleme nts sulfide ore deposits. Some of the early crystallization products o f the high-Mg magma settled to the chamber floor, where they partially mixed with the crustal melt and formed the mafic-ultramafic inclusion s and footwall complexes.