Geology, geochemistry and chromium-nickel-copper-platinum group elements mineralization of the Bird River sill: Evidence for a multiple intrusion model.

摘要

The Bird River sill (BRS) is composed of several layered mafic-ultramafic intrusive bodies located in the northern portion of the Bird River greenstone belt in southeastern Manitoba. Layered intrusions, such as those that collectively make-up the BRS, are important hosts to base and precious metal deposits. This study was initiated to examine and develop a petrogenetic model for the emplacement of the western half of the BRS and to establish the controls on Cr-Ni-Cu-PGE mineralization.;The mafic-ultramafic bodies of the BRS complex were emplaced as separate intrusions through multiple-magmatic injections into different stratigraphic levels of the Lamprey Falls Formation. The intrusions share a common magma source which is geochemically akin to modern back-arc basin basalts but is also similar to mid-ocean ridge basalts. It is interpreted that the central BRS intrusions are connected and represent a part of a single conduit system. The BRS complex and the Lamprey Falls Formation are overlain by the metasedimentary rocks of the Peterson Creek Formation and were tilted southward from their original horizontal positions. Metagabbroic intrusions present in the Lamprey Falls Formation to the north of the BRS complex represent coarsegrained flows genetically related to the mafic volcanics of the Lamprey Falls Formation and do not appear to be related to the BRS.;The stratigraphy of the BRS complex is divided into four basic magmatic series which are from the base upwards: (1) marginal mafic series, (2) ultramafic series, (3) transition series, and (4) mafic series. All significant concentrations of Ni-Cu-PGE and Cr in the study area are contained in the ultramafic series of the BRS. Mineralization is magmatic in origin with significant Ni-Cu and PGE remobilization associated with late felsic magmatism. Ni-Cu remobilization is also associated with mineralized shear zones that cross-cut the BRS and extend into the Lamprey Falls Formation. The sulphur source could not be determined unambiguously based on sulphur isotopes alone but the delta34S values of the separate BRS bodies suggests that the sulphur in the BRS is magmatic in origin and that two of the BRS bodies may have assimilated external sulphur from iron formations present within the Lamprey Falls Formation.;The findings of this investigation have considerable economic implications. The model that each BRS body is an individual intrusion implies each body may contain its own style of mineralization. The Page body is the most northern BRS intrusion and is interpreted to represent a turbulent magmatic environment and to be the first intrusion to form at the lowest stratigraphic level. The magmas that formed the stratigraphically higher BRS intrusions are believed to have passed through the Page intrusion. Therefore, the Page body is an excellent PGE exploration target as it represents a turbulent environment in which significant amounts of primitive magma have passed through which are two key factors in the formation of Ni-Cu-PGE deposits.