The petrology, geochemistry, and fluid history of calc-silicate rocks and associated primary Cu-Co mineralization in the Ethiudna Mines area, Olary Province, South Australia

Abstract

Textures in calc-silicate rocks of the Lower Proterozoic Bimba Suite in the Ethiudna mines area, Olary Province, South Australia, record multiple fluid infiltrational episodes associated with both prograde, amphibolite-facies, and retrograde, greenschist-facies metamorphism. Pervasive infiltration of H2O-rich fluids (XCO2<0.18) during prograde metamorphism, was associated with the large-scale development of wollastonite-rich lithologies within the Bimba Suite. Stable oxygen isotope evidence suggests these fluids were equilibrated with siliciclastic sequence(s) prior to infiltrating the Bimba Suite.

A second, retrograde metamorphic (temperatures -300oC) infiltrational episode was associated with the development of a grossular-quartz assemblage. Fluid flow was channelized during this episode, the H2O-rich fluids being focussed along structurally-induced zones of high permeability. Large variations in fluid/rock ratios are recorded in the calc-silicates as large variations in the progress of the grossular-quartz producing reaction.

The formation of cross-cutting quartz, calcite, and rare laumontite veins postdates both these infiltrational episodes. Fluid inclusion evidence suggests these veins were deposited by fluids at temperatures of less than 250°C.

Fluid inclusion data and scapolite compositional data indicate fluids associated with all three episodes were highly saline, consistent with conclusions made by earlier workers that the Bimba Suite represents a meta-evaporite sequence.

The microcline-rich rocks of the. Quartzofeldspathic Suite, which underlies the Bimba Suite, host stratiform and disseminated Cu-Co sulphide mineralization. Geochemical and petrological evidence suggests this mineralization is of the genetic type known as a red-bed-associated Cu deposit. The distribution, geochemistry, and textural characteristics of sulphides in the Bimba Suite, suggest this mineralization represents a metasomatic remobilization of pre-metamorphic Quartzofeldspathic Suite mineralization. The sulphide-silicate-carbonate textural relationship in the Bimba Suite suggests this remobilization took place during the high temperature, wollastonite-producing infiltrational episode, and that precipitation of ore metals was effected by a pH rise.