Petrographic and mineral chemistry evidence for the origin of sulfide mineralization in the Main Sulfide Zone, Sebakwe Subchamber of the Great Dike, Zimbabwe

Abstract

The Great Dike, one of the largest repositories of platinum-group elements (PGEs) and largest layered mafic intrusions in the world, is currently being exploited for its PGEs. We undertook a petrographic and mineral compositional investigation in the Sebakwe Subchamber to place some constraints on the genesis of its PGEs. Sulfides, which host some of the PGEs, occur either as inclusions within medium-grained euhedral orthopyroxenes or in association with biotite and intercumulus plagioclase. Sulfides which occur as interstitial phases or as inclusions in silicates are consistent with a magmatic origin of PGE sulfide mineralization. However, some sulfides which occur in association with biotite, are consistent with concentration of PGEs by magmatic fluids. Average sample Mg# range from 83.5–80.1 for orthopyroxenes, 93.1–84.4 clinopyroxenes, and 79.2–76.7 for phlogopite, whereas average sample An# for feldspars range from 81.3 to 5.9. Pyroxene thermometry yields temperatures of 837–986 °C that are late magmatic, with biotite thermometry yielding temperatures of 478–645 °C that are interpreted to be consistent with recrystallization of earlier primary biotite during hydrothermal alteration by late- to post-magmatic fluids. Observations from petrographic and backscattered electron images are consistent with these interpretations. Primary, magmatic PGE mineralization is inferred to have been cogenetic with sulfides which occur either as inclusions within, or as intercumulus phases between, orthopyroxenes. Sulfide minerals coexisting with biotite likely represent primary, magmatic sulfides that were remobilized by late- to post-magmatic fluids. The inferred depth of crystallization and hydrothermal alteration of the Sebakwe Subchamber of ∼10 km, is broadly comparable to other Subchambers of the Great Dike, lending support to the view that similar processes operated in the uppermost part of Great Dike ultramafic rocks in all its Subchambers. The involvement of fluid migration in the possible remobilization and concentration of primary magmatic PGE-mineralization in most layered mafic intrusions where it has been documented, must be a more significant process in concentrating PGEs in layered intrusions than currently thought.