Sedimentary and metamorphic factors in the development of the Pegmont stratiform Pb-Zn deposit, Queensland, Australia.

Abstract

The Pegmont deposit occurs in a thin, finely laminated, Fe-rich metasedimentary unit in the Middle Proterozoic eastern geosyncline succession of the Mt Isa-Cloncurry district; it appears to have developed as a result of hydrothermal contribution to a shallow basin or depression on the floor of a sea or lake. Detailed chemical and mineralogical investigations of drill-cores through the sulphide-bearing horizon show a large range of metamorphic Fe-rich silicates and aluminosilicates (probe analyses are reported for garnets, fayalites, grunerites, hornblendes, clinopyroxenes and greenalites) together with minor magnetite and traces of iron sulphides. Reconstruction of the original basin of chemical sedimentation indicates that the Fe-rich unit was deposited as a silicate-facies Pb-Zn-(Cu)-S-rich phosphatic, manganiferous iron formation; a fayalite microfacies is dominant at the basin centre, a hornblende-pyroxene microfacies on the basin slope and a garnet microfacies at the basin edge. Significant Pb-Zn sulphide occurrence is virtually confined to the sedimentary beds of the fayalite microfacies. All the evidence is that metamorphism has been isochemical. This Pegmont occurrence suggests that where stratiform ore exploration uses the occurrence of Fe-rich and related 'exhalites' along favourable horizons, careful attention should be paid to the possible incidence of significant sulphides closely related to specific microfacies within the broader facies patterns.-R.A.H. Show more