ELOISE CU-AU DEPOSIT, EAST MT ISA BLOCK - STRUCTURAL ENVIRONMENT AND STRUCTURAL CONTROLS ON ORE

The blind Eloise Cu-Au deposit is hosted by the subsurface eastward ex tension of the Proterozoic East Mt Isa Block. The host rocks comprise highly deformed, amphibolite-grade metasediments and amphibolite of th e Soldiers Cap Group. The deposit formed during the waning phase of th e Isan Orogeny (D-3 1540-1500 Ma) in a regional Cu-Au metallogenic eve nt synchronous with emplacement of the Williams Batholith (1540-1490 M a). The deposit lies within and adjacent to a series of anastomosing s hear zones at a local jog within the regional-scale Levuka Shear Zone. This was a major fluid channel similar to the parallel Mt Dore and Cl oncurry Fault Zones to the west. Alteration and mineralisation in the jog were synchronous with ductile-brittle deformation, and were emplac ed in ductile shear zones and cleavage, and brittle stockworks and fau lts. Brittle mineralisation styles were developed in competent enclave s in;more competent primary host rock or silicic/feldspathic alteratio n. Three principal paragenetic stages are recognised. Stage I albitisa tion post-dated peak (D-2) metamorphism, and was overprinted by high t emperature (>450 degrees C) stage II hornblende + biotite alteration. Stage III sulfide mineralisation fractured and replaced the mafic sili cates during a subsequent cooler event (200-450 degrees C). Deposition of stage III ore, in refracture-infill of stage II veins and selectiv e replacement of stage II alteration, was controlled by sulfidisation of the mafic stage II assemblages. The changing distribution of altera tion during the mineralising event (silicic/feldspathic versus hornble nde/phyllosilicate) controlled the competency architecture and in cons equence the local mineralisation style. Each mineralisation stage has ductile and brittle styles, each locally overprinting the other, Ducti le-brittle deformation fluctuated in time and space at all scales, pro ducing mineralisation characterised by locally ambiguous textures with respect to the timing of deformation, Massive sulfide veins show durc hbewegung texture and breccias are ductilely deformed, yet identical m ineralisation assemblages elsewhere occur in veins cross-cutting cleav age, Ambiguity is compounded by a strong component of replacement in t he mineralisation, which produced selvages along cleavage-parallel vei ns which mimic cleavage. The paragenesis is determined from unambiguou s overprinting relations in the brittle styles, and is then extrapolat ed to the more equivocal ductile styles.