Abstract:

The Ni-Cu-PGE deposits associated with the 1.8 Ga Sudbury Igneous Complex occur within 3 distinct environments: within quartz diorite offset dykes associated with the SIC, within embayments along footwall contacts of the SIC, and within brecciated and veined footwall rocks. The ores in contact and deposits are commonly fractionated: contact deposits are Fe-(Ni)-rich and footwall deposits are Cu-PGE-(Ni)-rich. Information on the transition between these two types of deposits is limited and in order to aid in understanding the genesis of the ores, we have studied the transition zone at the McCreedy East deposit, which is a contact-type deposit in close proximity to a footwall deposit. Detailed mapping and sampling in the lowest cut of the Lower Main ore body indicate that there are several megascopic differences between Contact- and Footwall-type ores, including variations in the proportions of clasts and matrix material, and in the compositions, angularity, and orientations of the clasts. Three zones can be distinguished based on these characteristics (from footwall toward the SIC): a Footwall Ore Zone (FOZ), a Transition Ore Zone (TOZ), and a Contact Ore Zone (COZ). Changes in lithology and texture from the FOZ into the TOZ are usually gradational and subtle, whereas changes in lithology and texture from the TOZ into the COZ are more abrupt. Sulfide textures and wallrock contacts change markedly across the zones: from stringer sulfides with sharp wallrock contacts in the FOZ to diffuse disseminations and blebs of sulfide in the COZ. Thin rinds of chalcopyrite are common on stringer margins in the FOZ, are absent in the TOZ, and are present as thin, discontinuous veinlets in massive sulfides of the COZ. On a microscopic level, the presence and shapes of pentlandite flames can be used to define the same zones, which are most common spanning the contact between the COZ and the TOZ. Based on these characteristics and geochemical data, the Transition Zone appears to represent an advancing zone of fracture propagation, thermomechanical erosion, and sulfide fractionation in which the contact ores in the COZ contain accumulated Mss and the footwall ores in the FOZ contain more residual sulfide liquid.