| Summary: | The River Valley Intrusion (RVI) within the ~2.48 Ga East Bull Lake Intrusive Suite in Ontario, Canada, is an example of a mafic igneous intrusion with 'contact-type' Ni-Cu- PGE sulfide mineralization along its base. Whereas many 'contact-type' deposits are thought to form from in situ contamination of the magma by the addition of crustal S during emplacement, there are some intrusions, including the RVI, which appear to have a much more complex history where the timing of S saturation, and thus the critical ore genesis processes, may have occurred much earlier, prior to emplacement. The RVI is made up of a basal ~100 m of unlayered, inclusion-bearing units, overlain by layered cumulates. The basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites, all within a gabbroic matrix. Platinumgroup element-rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides. The matrix and inclusions can be separated into two distinct textural types: hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages. The platinum-group mineral (PGM) assemblages vary only between textural types, and not between inclusions and matrix, being dominated tellurides in all rock types. The hydrothermally altered rocks, however, have fewer tellurides, and an increased amount of Sb- and As-bearing PGM, indicative of minor fluid interaction, although the PGM have not been mobilised significantly away from the base metal sulfides. Precious and base metal geochemistry shows all rock types to have an excellent correlation between all the PGE, indicating the presence of a single, well homogenised, PGE-rich sulfide liquid. However, Au and Cu appear to be decoupled from the PGE at low concentrations, although correlate well with each other, which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins. The overlying Layered Units above the mineralized units are not PGE ...
|
|---|