Geochemistry of Deformed and Hydrothermally Mobilized Magmatic Ni-Cu-PGE Ores at the Garson Mine, Sudbury

摘要

The Garson Ni-Cu-platinum group element (PGE) deposit is a deformed, overturned, contact-type deposit along the contact between the Sudbuiy Igneous Complex and underlying rocks of the Huronian Supergroup in the South Range of the 1850 Ma Sudbury structure. The main Garson orebodies occur along steeply S dipping ductile shear zones, which formed as D_1 north-over-south amphibolite facies thrust zones that were rotated and overturned during buckling of the Sudbury Igneous Complex. The shear zones were reactivated as south-over-north greenschist facies reverse shear zones during D_2. The mineralization is primarily composed of pyrrhotite-pentlandite-chalcopyrite-magnetite, occurring as four main types: (1) disseminated to matrix-textured sulfides in Main Mass norite and slivers of norite within shear zones, (2) semimassive breccia ore within the shear zones, (3) quartz-calcite-sulfide veins spatially associated with breccia ores, and (4) Cu-Pd-Pt-Au-rich "footwall-type" mineralization in the nearby Garson Ramp deposit. Garson disseminated sulfides and breccia ores have relatively low Pd/Ir ratios and restricted ranges of Ni tenors and Fe/Ni ratios, very similar to those of undeformed disseminated sulfides and contact-type massive ores at the less deformed Creighton deposit. This suggests that the mobilization of the breccia ores by ductile plastic flow during D_1 and D_2 did not significantly alter their original magmatic geochemical signatures. Garson breccia ores are slightly depleted in Cu-Pt-Pd-Au and slightly enriched in Rh-Ru-Ir relative to disseminated sulfides, similar to other contact-type deposits at Sudbury. In contrast, quartz-calcite-sulfide veins at Garson are characterized by very low Rh-Ru-Ir abundances and very high Pd/Ir ratios, consistent with their mobilization and deposition from metamorphic-hydrothermal fluids. Despite being strongly enriched in Cu-Pd-Pt-Au, there are too few of these veins to account for the broad depletion in Cu-Pd-Pt-Au from the main mass of breccia ores, suggesting that the breccia ores are mono-sulfide solid solution cumulates that left a residual sulfide melt represented by the Cu-Pd-Pt-Au-rich ores in the Garson Ramp deposit.