Eloise Cu‐Au deposit, East Mt Isa Block: Structural environment and structural controls on ore

Abstract

The blind Eloise Cu‐Au deposit is hosted by the subsurface eastward extension of the Proterozoic East Mt Isa Block. The host rocks comprise highly deformed, amphibolite‐grade metasediments and amphibolite of the Soldiers Cap Group. The deposit formed during the waning phase of the Isan Orogeny (D3, 1540–1500 Ma) in a regional Cu‐Au metallogenic event synchronous with emplacement of the Williams Batholith (1540–1490 Ma). The deposit lies within and adjacent to a series of anastomosing shear 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 Cloncurry Fault Zones to the west. Alteration and mineralisation in the jog were synchronous with ductile‐brittle deformation, and were emplaced in ductile shear zones and cleavage, and brittle stockworks and faults. Brittle mineralisation styles were developed in competent enclaves in more competent primary host rock or silicic/feldspathic alteration. Three principal paragenetic stages are recognised. Stage I albitisation post‐dated peak (D2) metamorphism, and was overprinted by high temperature (>450°C) stage II hornblende + biotite alteration. Stage III sulfide mineralisation fractured and replaced the mafic silicates during a subsequent cooler event (200–450°C). Deposition of stage III ore, in refracture‐infill of stage II veins and selective replacement of stage II alteration, was controlled by sulfidisation of the mafic stage II assemblages. The changing distribution of alteration during the mineralising event (silicic/feldspathic versus hornblende/ phyllosilicate) controlled the competency architecture and in consequence the local mineralisation style. Each mineralisation stage has ductile and brittle styles, each locally overprinting the other. Ductile‐brittle deformation fluctuated in time and space at all scales, producing mineralisation characterised by locally ambiguous textures with respect to the timing of deformation. Massive sulfide veins show durchbewegung texture and breccias are ductilely deformed, yet identical mineralisation assemblages elsewhere occur in veins cross‐cutting cleavage. Ambiguity is compounded by a strong component of replacement in the mineralisation, which produced selvages along cleavage‐parallel veins which mimic cleavage, The paragenesis is determined from unambiguous overprinting relations in the brittle styles, and is then extrapolated to the more equivocal ductile styles.