Geology of Mount Lyell

Mount Lyell is a world famous mining field that has produced more than 1.9 million tonnes of copper metal, 1.9 million ounces of gold and 1,300 tonnes of silver from more than 150 million tonnes of ore. First discovered in the 1880,s, Mount Lyell is the oldest, continually operating mining field in Australia. Six major and numerous smaller ore-bodies have been mined to date and large copper resources and exploration potential remain.

Regionally Mount Lyell is located within the Cambrian Mount Read Volcanics, interpreted to have formed in an island arc setting some 500 million years ago. Mineralization is hosted by a large hydrothermal alteration zone within the Central Volcanic Complex, a mix of rhyolitic, andesitic to basaltic composition intrusives, volcanics, volcaniclastics and volcanic sediments. Mount Lyell is located at the intersection of the north south trending Mount Read Volcanics and a regional cross cutting structure, the Linda Disturbance. The major structural break known as the Great Lyell Fault defines the eastern boundary of the mineralized zone. East of this regional structure is a thick sequence of unmineralised, younger siliclastic sediments (Owen Conglomerate). The field is bound to the west by a thick sequence of younger acid to intermediate composition volcanics, intrusives and sediments on the Tyndall Group.

All known primary mineralization is located within a zone of intense hydrothermal alteration that extends 5 km along strike north – south and is up to 1 km wide within the Central Volcanic Complex. Within this zone original textures and composition have been variably altered to propylitic and argillic alteration assemblages with a moderate to locally texturally destructive schistose foliation. Locally intense silicification, advanced argillic (pyrophyllite – alunite – topaz), haematite – barite and weak potassic alteration are important associations with mineralization.

Mineralization can be classified into five main types. The most productive style is disseminated chalcopyrite associated with disseminated pyrite and chloritic to sericitic and minor potassic altered rocks. Chalcopyrite occurs as fine disseminations and locally important vein stockworks and semi massive banded accumulations. Associated minerals include pyrite, magnetite, fluorite and apatite. Typical ore grade for this style is 1.2% Cu and 0.3 g/t Au. Ore-bodies of this style include Prince Lyell (current underground operation), West Lyell open cut, Western Tharsis and Cape Horn. The second main style is disseminated bornite, associated with chalcopyrite, pyrite and more intensely argillised and silicified alteration styles. Copper grades are typically higher at 2 to 3% Cu and have been typical of the North Lyell ore-bodies. This style is restricted to locations near to the eastern boundary of the field. Other, less significant styles of mineralization include massive pyrite bodies with disseminated chalcopyrite plus high sulfidation sulfosalt minerals (Iron Blow); Banded massive lead – zinc sulfides (Comstock & Tasman Crown) and secondary, supergene concentrations of native copper, copper oxides and low sulfur content copper sulfides situated in clay altered limestones down slope and adjacent to other sulfide mineralized zones.

Mineralization and the host rocks have been subjected to at least two periods of deformation, resulting in variable northwesterly – southeasterly striking cleavage and schistosity development and steep to shallow dipping cross and strike parallel faults. The main large ore-bodies are vertical to steeply dipping to the south-west. The Prince Lyell – West Lyell orebody is typically 150m wide, 450m long and is vertically continuous for more than 1,500 m. This orebody has a grade boundary within the altered volcanic sequence and is apparently truncated at depth by the steeply south-west dipping Great Lyell Fault.