Summary:
Capricorn Copper consists of several structurally controlled, sediment-hosted copper deposits located within the Western Fold Belt of the Mt Isa Inlier.
There are two dominant styles of mineralisation at Capricorn Copper:
- breccia-hosted within quartzites – Mammoth and Greenstone deposits; and
- shear-hosted within shales/siltstones – Esperanza, Esperanza South and Pluto deposits.
Esperanza South
Esperanza South is a steeply plunging breccia located between the NNE-SSW-striking hangingwall and footwall margins of the Esperanza Fault zone. The footwall is defined by the easternmost shear within the Esperanza Formation. The fault zone envelope is approximately 50 – 70m wide.
Mineralisation dips sub-parallel to the hangingwall at around -75° to the west, with a SSW plunge which steepens at depth from around -50° to -75°. The hypogene mineralisation at depth consists of chalcopyrite and pyrite exhibited as fracture fill, breccia matrix and massive forms. Supergene enrichment processes play a significant part of localising mineralisation at ESS, particularly in the upper 500m of the orebody. This weathering profile is represented by a broad weathering cap to the base of oxidation under which structural pathways have promoted downward percolation of meteoric fluids. These pathways have created supergene enrichment pathways which broadly run sub-parallel to the main structural envelope and in the most well developed zones consist of a barren, massive earthy haematite core (the centre of the structural zone), peripheral haematite and chalcocite (“chalcocite group” minerals), grading outwards to chalcocite-pyrite and eventually chalcopyrite-pyrite. Development of these enrichment zones varies on a local scale dependent on the structural permeability, availability of hypogene ore, and intensity of weathering. The effects of these zones lessen with depth but remains present in variable amounts to the deeper portions of the orebody, where the primary chalcopyrite-pyrite assemblage becomes more dominant.
Greenstone
The orebody is located within a wedge of Whitworth Quartzite constrained by the Mammoth Extended Fault. Here, the fault strikes roughly ENE and dilates sinusoidally in the vicinity of the GST orebody, with apparent dextral movement. This has brought a fault bounded block of Whitworth Quartzite into contact with Surprise Creek Formation sediments in the north (referred to as the hangingwall side), and Bortala Formation and Alsace Quartzite sediments to the south (footwall side). At the eastern and western extremities, the zone is highly fractured likely due to the convergence of the dilatant zone. The orebody sits within the core of this zone yet does not extend to surface due to significant weathering and vertical convergence of this zone. With depth, the hangingwall and footwall diverge and bound the Whitworth Quartzite wedge. Whilst structurally hosted, highly fractured zones do not tend to contain mineralisation. Mineralisation consists as chalcocite, bornite or chalcopyrite mineralisation hosted within fracture to breccia fill and is controlled as irregular, anastomosing fracture packages within the quartzite.
Pluto
Hosted within strongly oxidised siltstones and breccia of the Paradise Creek Formation. The formation is a sequence of light to dark grey rhythmically bedded dolomitic and carbonaceous siltstones and lesser stromatolites. The Pluto deposit is centred around the Mammoth Extended Fault and bounded by the localised Foschi’s Fault. Intense leaching and oxidation occur within the structural core, with mineralisation occurring peripheral interpreted at a reaction front with the surrounding Paradise Creek Formation sediments. Bedding dip and strike of favourable stratigraphic units coupled with bedding parallel faulting plays an additional role in localising mineralisation. Copper is typically presented as supergene chalcocite and as cuprite and native copper in the more highly leached and oxidized zones. Gangue minerals included pyrite, hematite and kaolinite. Ore contacts are typically sharp along with the oxidation fronts. Minor cobalt is also noted as a significant mineralisation type at Pluto and is typically seen within cobaltite and/or cobaltiferous pyrite as a halo around the more locally confined Cu mineralisation. The oxidation zone is approximately 200 m long by 20 – 30 m wide
Mammoth
The Mammoth orebodies occur within the Whitworth Quartzite of the Myally Sub-Group. The sequence strikes North-northeast dipping 65-85°W and is dominated by massive pink to grey felspathic, medium to coarse grained, poorly bedded and homogenous quartzite. Localised siltstones are present within the unit. Three major faults are important in localizing mineralisation at the Mammoth Mine – the Mammoth Fault, the Portal Fault and the Mammoth Extended Fault. The Mammoth Extended Fault bounds the overall zone to the north and west, the Mammoth Fault localises the main strike of mineralisation which can occur either side of the fault, and the Portal Fault acts as a hard boundary on the east and controls the plunge of the mineralisation. The overall Mammoth domain plunges roughly at 65° to the SW. Mineralisation at Mammoth is found in three styles: massive, brecciated and veined; Massive mineralisation occurs adjacent to the Mammoth and Portal Faults and contains minor host rock fragments. Brecciated mineralisation occurs further away from the major faults and consists of angular and sometime fragmented clasts; Veined mineralisation is the most distal mineralising style from the faults. Individual ore lodes (“lenses”) are locally controlled by the interplay between these major faults, minor local faults and shears, structural permeability and bedding.
Esperanza
Hosted by the Esperanza Formation at the confluence of the Mammoth, Mammoth Extended and Foschi’s faults. Primary mineralisation is recorded as chalcopyrite and pyrite veining with locally massive zones. Supergene mineralisation is typically located in the upper and northern parts of the orebody (largely mined) under the silica cap and is characterised as massive, vein and disseminated chalcocite, native copper and reported digenite-djurleite-covellite.
Dimensions
Esperanza South: strikes approximately 25 degrees NNE, 50 m below surface extending to 1,100 m below surface, 850 m long and up to 70 m wide. Copper mineralisation width within the corridor varies greatly from several metres to full corridor width and is continuous down dip.
Greenstone: strikes approximately 65 degrees NE; The top of the orebody is 150 m below surface extending to date to 400 m below surface, 300 m long and 150 m wide. Copper mineralisation currently presents as an upper, sub-vertical core, and a deeper southerly dipping lode which is offset to the north.
Mammoth: A very extensive complex multi fault-controlled mineralisation complex with multiple lodes and orientations extending from surface to approximately 1,200 m below surface and open at depth. Mineralisation widths vary from several metres to several hundred metres with mineralisation continuous down dip. Overall strike is approximately 1,400 m.
Pluto: strikes approximately 45 degrees NE, 100 m below surface extending to 700 m below surface, 500 m long and 100 m wide, as discrete, thin (5 – 25 m) mineralised lodes.
Esperanza: strikes approximately between 45 degrees (NE) on the eastern side to 70 degrees (ENE) on the western side. The orebody commences between 20 m to 150 m below natural surface (now mined out) and extending to 400 m below natural surface, 700m along strike and 20 m to 80 m wide tapering at depth.