Geological Environments for PGE-Cu-Ni Mineralization in the East Bull Lake Gabbro-Anorthosite Intrusion, Ontario

Exploration & Mining Geology, Vol. 2, No. 1, 1993

D.C. PECK, R.S. JAMES and P.T. CHUBB, Department of Geology, Laurentian University, Sudbury, Ontario, Canada

The East Bull Lake gabbro-anorthosite intrusion is one of several Early Proterozoic layered mafic intrusions occurring along the Southern Province - Superior Province boundary in the Elliot Lake - Sudbury region, Ontario. The base of the East Bull Lake intrusion comprises abundant plagioclase cumulates (Leucogabbronorite Zone) hosting two distinct styles of sulfide mineralization: (1) disseminated magmatic sulfides, and (2) disseminated to massive, structurally controlled sulfides. Both types of mineralization are enriched in Pd, Pt, Au, Cu and Ni. The host rocks to the sulfide mineralization display a range of interesting features including nodular textures, dendritic-pegmatitic mafic pods, inclusion-rich zones (pyroxenite, anorthosite and granitic inclusions) and magmatic breccias. Disseminated chalcopyrite, pyrrhotite and pentlandite occur in concentrations of up to 10% within erratically distributed gossan pods near the base of the intrusion. The sulfides were originally precipitated from intercumulus liquids, but were subsequently recrystallized and locally remobilized along fractures during the major period of metamorphism and deformation that affected the East Bull Lake intrusion. The base- and precious-metal tenor of the structurally controlled sulfides can be explained by extensive remobilization and reconcentration of the pre-existing magmatic sulfides in the vicinity of a major shear zone. Both styles of sulfide mineralization locally contain ppm abundances of PGE + Au and are generally Pd- and Cu-rich, although both display large variations in Pd:Pt and Cu:Ni ratios. The magmatic sulfides are more strongly enriched in PGE and contain up to 1470 ppm Pd in 100% sulfide. Several factors, including source characteristics, magmatic processes (crustal assimilation, fractional crystallization) and post-magmatic interaction with hydrous fluid, have influenced the present distribution of PGE, Cu and Ni in the East Bull Lake intrusion. The magmatic sulfide occurrences from the East Bull Lake intrusion compare favorably with marginal-type deposits from rift-related intrusions (Noril'sk-Talnakh region, Siberia; Duluth Complex, U.S.A.) in terms of their distribution, but are most similar to reef-type PGE deposits with respect to their base- and precious-metal geochemistry. The combination of high PGE tenors and large tonnages that is observed for the PGE-Cu-Ni mineralization from the East Bull Lake intrusion suggests that a good potential exists for the presence of economic concentrations of Pd, Pt, Au, Cu and Ni in the intrusion. On a global scale, the East Bull Lake intrusion is part of a suite of Early Proterozoic plutons, many of which contain significant volumes of stratiform and/or marginal-type PGE-Cu-Ni mineralization. The East Bull Lake intrusion is also one of the oldest mineralized intrusions derived from a sub-Superior Province mantle source region that has given rise to several significant magmatic PGE-Cu-Ni deposits during the period 2.5 Ga to 1.1 Ga.