Metasomatic interaction between disseminated nickel sulphides and reduced metamorphic fluids,Honeymoon Well komatiite complex,Western Australia

Abstract

Within the Honeymoon Well komatiite complex located within the Archaean Agnew-Wiluna greenstone belt, Western Australia, disseminated Ni sulphide deposits are hosted by completely serpentinised olivine-rich cumulates. These are unusual deposits as they contain large tonnages of mineralisation that are devoid of Cu despite containing up to >2%Ni and having magmatic Co and PGE values. About 55% of the 43 Mt Hannibals deposit consists of this style of mineralisation. The Cu distribution is spatially and geochemically related to bulk-rock changes in S and Fe with the Cu-absent parts of the deposit being variably depleted in these elements. Mimicking these changes in bulk composition is a zonation in sulphide–oxide assemblages of Ni-poor pentlandite-magnetite±pyrrhotite±chalcopyrite, pentlandite-magnetite±chalcopyrite, Ni-rich pentlandite-heazlewoodite±magnetite, and heazlewoodite-only with the latter occurring in zones showing the strongest S and Fe depletion. The sulphide mineral assemblages and bulk compositions correspond to variations in the non-sulphide mineralogy with much of the heazlewoodite mineralisation hosted by lizardite-iowaite±brucite-(magnetite absent) assemblages. Formation of highly reduced fluids during serpentinisation of olivine adcumulate and mesocumulate host sequences resulted in dissolution of sulphides in parts of the disseminated deposits. Copper, Co and Fe, soluble as chloride species, and S in the form of H₂S were variably leached and removed from parts of the deposits. Passage of these reduced fluids through barren ultramafic rocks caused partial leaching of Fe and resulted in the formation of lizardite-rich alteration assemblages devoid of magnetite. Metamorphic Fe-Cu±Zn±Ni±Co bearing sulphides are common in pyroaurite-brucite magnetite veins and in former pyroxene-rich lithologies indicating the presence and mobility of these elements in some of the metamorphic fluids at Honeymoon Well. The location of the sulphide deposits immediately adjacent to major shear zones that bound the ultramafic complex allowed the infiltration and passage of large volumes of metamorphic fluids. During the lowermost greenschist facies metamorphism this caused semi-regional scale metasomatism of the ultramafic complex.

Keywords

NICKEL SULPHIDE KOMATIITE ARCHAEAN MINERALOGY METAMORPHISM

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