Integrated stratigraphic–structural–hydrothermal alteration and mineralisation model for the Kangaroo Caves zinc–copper deposit, Western Australia

Abstract

The Kangaroo Caves volcanic-hosted massive sulfide deposit is located in the Archean Panorama district in the northern Pilbara Craton, Western Australia. The deposit contains an indicated and inferred mineral resource of 6.3 Mt at 3.3% Zn, 0.5% Cu and 12.1 g/t Ag. The Kangaroo Caves area is characterised by predominantly tholeiitic volcanic rocks of the ca 3240 Ma Kangaroo Caves Formation, which is overlain by turbiditic sedimentary and volcanic rocks of the Soanesville Group. Zinc–copper mineralisation is hosted mainly by the regionally extensive Marker Chert, the uppermost unit of the Kangaroo Caves Formation, and structurally controlled by D1 synvolcanic faults. The upper area of the deposit is characterised by quartz–sphalerite ± pyrite ± barite ± chalcopyrite, whereas the lower area contains mainly chlorite–pyrite–quartz–carbonate–sericite ± chalcopyrite ± sphalerite.Laser-ablation inductively coupled-plasma mass-spectrometry analyses show that pyrite in the Marker Chert is enriched in Zn, Ag, Pb and Sb when compared with dacite from the Kangaroo Caves Formation, Paddy Market Formation, and Dalton Suite. Pyrite from the Marker Chert also displays a negative correlation between Zn and As, i.e. with increasing Zn levels As is decreasing. The Co/Ni ratios in pyrite are significantly greater in the upper, Zn-rich area (median ratio = 0.4) of the deposit than the lower, Zn-poor area (median ratio = 5). Copper is low irrespective of the host rocks. Sphalerite displays low Fe (<10 000 ppm), Cu (<100 ppm), Ag (58 ppm) but high Cd (<500–1000 ppm). Some elevated Cu values suggest the presence of chalcopyrite inclusions, whereas elevated Cu and Sb values suggest tetrahedrite–tennantite inclusions.Structural analysis of the Kangaroo Caves area, combined with three-dimensional modelling of ore element distribution, shows that the deposit is predominantly an elongate pipe of Zn mineralisation, which plunges ~30° to the northeast and is approximately 1000 m in length. The morphology of the Kangaroo Caves deposit was largely retained from its original formation, despite rotation of the host rocks and deposit during the D2 event. Variations in hydrothermal alteration assemblages, including the Co and Ni contents of pyrite within the deposit and underlying dacite, are interpreted to be the result of variations in the influx and mixing of seawater with upwelling volcanogenic fluids during Zn–Cu mineralisation. In the Kangaroo Caves area, the Co/Ni of pyrite can be used as an exploration vector towards high-grade Zn–Cu mineralisation.