Hematite-barite alteration in the Owen Conglomerate, North Lyell, Tasmania

The North Lyell mine area is located within the ML Read volcanics adjacent to the north-south trending Great Lyell Fault (GLF). It was one of the more copper rich deposits within the Mt. Lyell mining field having produced 5mt of ore with 5.3% Cu, 33 g/t Ag and 0.4 g/t Au. The main zone of mineralisation at North Lyell is located near the intersection of the GLF with the WNW trending North Lyell Fault. Although most of the bornite mineralisation at North Lyell occurred within the volcanics, there was bornite recognised within the Cambrian-Ordovician siliciclastic conglomerate which is also adjacent to the GLF. The timing of alteration/mineralisation at North Lyell is reflected in the extent and style of alteration observed within the overlying conglomerates, sandstones and limestones. Extensive hematisation with associated barite occurs along the GLF between the volcanics and sediments. The hydrothermal hematisation associated with the mineralisation at North Lyell and Lyell Tharsis extends for some distance into the sediments. As the fluids progressed through the North Lyell system their compositions were modified by their interaction with the various host lithologies, producing distinct alteration assemblages within the surrounding units. Three main geochemical associations related to the alteration can be recognised in the North Lyell area; phosphate-hematite (Fe2O3+P2O5+La+Sb), sericite (K20+Al203+Cr+Rb) and barite (Ba+Sr+Sb). The barite assemblage is the least dispersed, being confined to the main fluid conduits and wallrocks. The change in alteration styles from the weakly acidic, reduced sulphide rich volcanic environment, to the highly oxidised conditions within the conglomerate is reflected in the 'dumping' of hematite and pyrite on the GLF boundary. Computer modelling of the fluids show that this redox front alone cannot account for the observed mineralisation at North Lyell. The most successful mechanism for metal deposition was a thermal gradient coupled with the redox boundary. A fluid dominated system with input from the oxidised brines of the Owen Conglomerate produced an assemblage representative of the North Lyell mineralisation. The recognition of detrital hematite clasts adjacent to the Haulage Unconforrnity, coupled with the concept of metal deposition in a relatively shallow environment, suggests the hydrothermal enrichment at North Lyell took place during Late Cambrian-Early Ordovician sedimentation.