Magmatic iron enrichment in high‐iron metatholeiites associated with ‘Broken Hill‐type’ Pb‐Zn‐Ag deposits, Mt Isa Eastern Succession*

Abstract

Ortho‐amphibolites associated with the Cannington and Maramungee Pb‐Zn‐Ag deposits and with mineralised magnetite‐rich ironstones at Fairmile help define rock associations in the Mt Isa Eastern Succession similar to those that host the giant orebody at Broken Hill, New South Wales. Thin amphibolites at Maramungee are strongly differentiated and iron‐rich (FeO(t) 11.60–17.73 wt%; FeO(t)/(FeO(t)+ MgO) 0.614–0.820). They resemble the most iron‐rich magmatic suites known from modern environments (Galapagos Rift) but their iron enrichment is not as extreme as occurs in metabasic rocks at Broken Hill. The ratio FeO(t)/(FeO(t) + MgO) at Maramungee is negatively correlated with Ni and Cr whereas Y, Zr, Nb, REE, Hf and Th exhibit systematic 3–4 x enrichment from the most primitive to most evolved compositions. TiO2, P2O5 and V display inflections from incompatible to compatible behaviour implying that the differentiation is magmatic and due to crystal fractionation including separation of magnetite and apatite from the most iron‐rich magmas. Metatholeiites in thick bodies at Fairmile and Cannington display little variation of incompatible trace‐element concentrations and their chemistry probably represents differentiation during primary crystallisation of the host bodies. Emplacement of exceptionally high Fe tholeiite liquids at Maramungee and Broken Hill required specialised extensional tectonic settings that allowed high density, low fo2 magmas to escape from magma chambers that had evolved without replenishment. Greater Fe‐enrichment and lower concentrations of Ti and Nb in the lode sequence amphibolites at Broken Hill may reflect suppression of magnetite saturation by very low fo2 and concomitant ilmenite fractionation. These data emphasise similarities between the Broken Hill district and the Mt Isa Eastern Succession but do not imply a causative relationship between exceptionally high Fe magmatism and Broken Hill‐type ore deposits.