Structural‐metasomatic environment of the East Mt Isa Block base‐metal‐gold province

Abstract

The East Mt Isa Block experienced major metallogenesis (6 million oz Au, 3 Mt Cu, 7 Mt Pb, 8 Mt Zn) in the waning phase of the Isan orogeny. D1 thrusting of the Cloncurry terrane over the Mt Isa terrane interleaved crustal sheets of siliciclastic‐rich and carbonate‐rich rocks. Late‐D2 and D3 retrogressive metamorphism and intrusion of the Williams Batholith, between ca 1540 and 1490 Ma, produced an oxidised highly saline metal‐rich fluid. Major Au+Cu deposits, and an uncertain proportion of co‐genetic Pb+Zn mineralisation, were deposited in the siliciclastic Maronan Supergroup sheets within and adjacent to a regular array of steep D2/D3 shear zones and faults. The high‐tonnage deposits are dominated by replacement, characteristically of precursor ironstones, some of which developed by syntectonic Fe‐replacement of early shear zones. All deposits formed at sites of abnormal dilation of the host structure (fault intersections, dilational jogs, competency contacts including granite) or in contact with replaceable wall‐rock units. Mineable deposits are yet to be found in several interpreted favourable environments. Appropriate exploration models include polymetallic base‐metal+gold deposits, active search for ‘non‐identical twin’ deposits of Pb+Zn adjacent to Cu+Au and vice versa, and epigenetic Pb+Zn deposits. Exploration models to find a metal resource may differ from models for mineralisation. Coherent tonnes‐grade plots indicate potential for 50 Mt deposits grading 1 g/t Au and 2% Cu, and 300 Mt deposits grading 0.3 g/t Au and 0.8% Cu. A coherent positive‐slope Au‐Cu plot indicates a trend for high grades of Au and Cu to occur together. Faults are a prime factor in metal deposition. The combined effects of D2 and D3 were a kinematically remarkably coherent array, consisting of ‘orthogonal’ and ‘diagonal’ fault sets FN, FNE, FNW and FE. These are respectively reverse, right‐lateral, left‐lateral, and extensional. All are meta‐somatic channels. FN and FNW are regional structures which host major deposits, particularly the Mt Dore fault zone. FE are broad, long‐lived crustal structures, with the Yellow Waterhole corridor hosting major deposits. The intersection of the latter with the Williams magmatic arc is a principal locus of East Mt Isa Block metal deposition. This intersection is also transected by the newly identified Century‐Cannington corridor, which hosts most of Australia's Proterozoic combined base‐metal‐gold endowment. Phanerozoic tectonic reconstructions for northeastern Australia must be consistent with east‐west compressive kinematics clearly preserved in the East Mt Isa Block.