Controls on the formation of contact-type platinum-group element mineralization in the East Bull Lake intrusion

The East Bull Lake intrusion is the type example of a series of 2.48 to 2.4 9 Ga layered mafic intrusions that developed within tile rifted southern ma rgin of the Superior craton, central Ontario. The East Bull Lake intrusion contains contact-type PGE mineralization, e.g., strata-bound, PGE-rich, dis seminated sulfide mineralization occurring within 200 to 300 m of tilt, con tact with footwall and sidewall units. Although the mineralization locally occurs in massive plagioclase cumulates (anorthosite zone), most of the sul fides are concentrated within an underlying, hetcrolithic, inclusion-rich u nit inclusion-bearing zone) that grades into art intrusive breccia. Dissemi nated sulfides also occur in Fine-grained gabbroic veins within brecciated footwall rocks (border zone). Geochemical characteristics of tile contact-t t;pe sulfide mineralization and its host rocks include: (1) high-average Pd abundances throughout the East Bull Lake intrusion, including an average o f ca. 200 ppb in the Lower Series: (2) low-average S contents (typically. < 1%); (3) average Pd/Pt similar to3 and Cu/Ni similar to3: and (4) PGE conce ntrations and relative abundances similar to disseminated sulfide mineraliz ation from the contact environment of other mafic-ultramafic intrusions. Contact-type PGE mineralization in tile East Bull Lake intrusion reflects f low-enhanced gravitational settling of a Cu-rich sulfide liquid followed by local fractionation of Ni-rich monosulfide solid solution to produce minor amounts of Cu- and Pt-rich, residual sulfide liquid. Most of the observed compositional variation in the sulfides can be modeled using variable silic ate liquid/sulfide liquid mass ratios. Field and geochemical observations s uggest that the parental magmas to tile mineralized parts of the East Bull Lake intrusion were S saturated and PGE rich at tile time of their emplacem ent. These fertile parent magmas most likely originated as buoyant. feldspa thic residual magmas that escaped from a deeper, more primitive subchamber. Although first described from the East Bull Lake intrusion, contact-type PG E mineralization is now recognized in all of tile major East Bull Lake suit e intrusions. Similar styles of magmatic sulfide mineralization art develop ed in tile contact environments of the Bushveld Igneous Complex (Platreef d eposits! anti the Duluth Complex, Minnesota (e.g., Dunka Road deposit). The se contact type deposits are part of the family of "basal accumulation" mag matic sulfide deposits. The economics of low sulfide tenor, contact-type, P GE-Cu-Ni mineralization. such as that developed ill the East Bull Lake intr usion will be west favorable in systems in which (1) the parent magmas poss essed high initial PGE tenors and reached S saturation prior to emplacement ; (2) the parent magmas experienced vigorous convection in tile early; stag es of chamber development; and (3) segregated sulfide was not diluted by co precipitating silicate minerals-a scenario that is most likely to occur in feldspathic magmas in which plagioclase is the principal high-temperature l iquidus mineral.