Structural and Geochemical Constraints on the Emplacement of the Monakoff Oxide Cu-Au (-Co-U-REE-Ag-Zn-Pb) Deposit, Mt Isa Inlier, Australia

Within the Eastern Succession of the Australian Mount Isa Inlier, Monakoff is a 1 million tonne (mt) Mesoproterozoic, oxide Cu-Au deposit only 13 km from the large (167 mt) Ernest Henry mine. The two deposits share similar geochemical signatures (Ba-Cu-Au-U-Pb-Zn-As-Sb-Co-W-Mo-Mn-F-REE), suggesting commonality of origin. This signature is far more complex than those of most other Eastern Succession Cu-Au oxide systems, but it is extremely similar to the signatures of some recently discovered large Brazilian examples, such as Alemao. Monakoff ore has a barite-carbonate-fluorite-magnetite-chalcopyrite-dominated mineralogy, and contains economic quantities of Cu, Au, Co, U and Ag; the 1-2% levels of both Pb and Zn are unusually high for oxide Cu-Au deposits. However, it lacks the distinctive K-feldspar alteration halo of Ernest Henry. It occurs on the northern south-dipping limb of the Pumpkin Gully Syncline, considered to be a regional, EW-oriented, D2 fold, bounded to the north and west by Di thrust contacts. A splay of the northern thrust hosts the main Monakoff mineralisation. Naraku Batholith elements outcrop ~2 km north of Monakoff; and ore alteration records post-ore hornfels recrystallisation.

Two lenses of mineralisation occur: the main Monakoff Western Zone, which is a sub-vertical sheet within the shear, and the Monakoff Eastern Zone, which replaced meta-dolerite in the core of a tight D2 fold adjacent to a Di shear, forming a narrow west-plunging pipe of unknown depth extent. A 10-20 m wide symmetric alteration halo grades inward from carbonate spotting and garnet overgrowths in regional muscovite-quartz-plagioclase schist, through biotite-spessartine-magnetite-plagioclase, to chlorite-spessartine-magnetite. This zoned sequence overprints an earlier biotite-magnetite alteration system that was focussed upon the Dl shear. A large F anomaly extends into adjacent amphibolites and porphyroblastic garnet-biotite schists, at least 130 m across strike, expanding the geochemical halo by more than 1 Ox the dimensions of the observed alteration. The Monakoff Shear experienced movement during the regional Di and D2 to D2.5 events. Dextral shear during Dl was the major fabric-forming event. Ore formation was synchronous with D2.5 on the basis of: (1) pseudomorphing of earlier fabrics by the ore and alteration assemblage; (2) crenulation of the biotite alteration by D3; (3) preferential development of some ore phases (carbonate, sulphide) in D2.5 crenulations; (4) a lack of pressure-shadowing on alteration garnets; and (5) use of D2 fold structures as fluid conduits. Although evidence mainly favours an epigenetic timing for mineralisation, the footwall shows evidence of minor pre-deformational Mn-Al-K-Fe metasomatism, associated with BIF formation.

Our Monakoff ore formation model involves flexural slip and production of dilatancy in broad, near-horizontal, D2.5 folds, a mechanism which accounts for the sheet-like ore geometry in vertical beds. Flexural slip was concentrated on the lubricated Dl shear system, with packages of ore fluid being drawn in by extensional failure at various scales. The geometry of extension during D2.5 favoured the inflow and mixing of fluids from above and below. As at Ernest Henry, mixing deposited barite, U-phases, carbonate, Mn-minerals, and fluorite; the components of these phases are not effectively transported and deposited from one fluid. The metals and oxidised S were carried in a corrosive, F-rich, >500° C magmatic vapour and fluid, which condensed in, and reacted with, reduced K-Ca-Ba-Mn-Cl-rich waters, as well as Fe2+-rich assemblages in the wallrocks, such as pre-existing magnetite.