Mineralization at the Golden Grove Cu – Zn deposit, Western Australia. I: Premetamorphic textures of the opaque minerals

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Abstract
The competent opaque minerals in the Archaean Golden Grove deposit, pyrite and magnetite, retain pre-regional metamorphic textures despite the lower greenschist-facies grade of metamorphism. The pre-regional metamorphic textures and structures recognized include the development of pyrite and magnetite overgrowths, the replacement of pyrrhotite by pyrite, the conversion of a primary hematite–goethite mineralogy to magnetite and, as a result of thermal metamorphism, further local replacement of pyrrhotite (and sphalerite) by magnetite. Comparisons between pyrite from the Cu-rich mineralization at the base of the deposit and that from the Zn-rich mineralization in the hanging wall indicate that postdepositional modification and recrystallization were more extreme at the base of the deposit. The pre-regional metamorphic textures and structures indicate that pyrite and magnetite overgrowths developed almost immediately after primary precipitation ceased and that overgrowths continued to develop into the late hydrothermal–diagenetic stage of mineralization. A large proportion of the sulphide–Fe-oxide mineralization was formed at shallow depth within the volaniclastic host rocks, but at two horizons (the base and hanging wall) the mineralization formed at or very near the sea floor. These two periods of near sea-floor sulphide precipitation are separated by an oxide-dominated opaque-mineral assemblage, originally hematite–goethite and secondary marcasite but now converted to magnetite and secondary pyrite. The microtextural evidence supports a three-stage evolution of the ore deposit, two sulphide exhalative phases of mineralization separated by a stage of more oxidized hydrothermal activity, or, alternatively, sea-floor weathering during which hematite–goethite formed and marcasite partly replaced earlier formed sulphides.

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