Overview

Deposit type

• Vein / narrow vein
• Hydrothermal

Summary:

At the Tasmania Mine, the mine sequence of metasedimentary rocks dip 50° to 80° towards the northeast. The gold mineralised Tasmania Reef is situated along a dextral strike-slip transfer fault system which obliquely traverses the Cabbage Tree Thrust slice stratigraphy. The reef is composed of a quartz-ankerite-sulphide vein system occupying a dilational regime within a northeast trending fault. The strongly developed portion of the Tasmania Reef in the Denison Group sediments has a strike length of approximately 400m with an overall dip towards the southeast of approximately 50° to 70°. The Tasmania Reef and associated fault structure is confined laterally along strike by the Cabbage Tree Thrust fault to the southwest and by the Cobblestone Creek Thrust fault to the northeast. The Tasmania Reef outcropped on Cabbage Tree Hill (120m above sea level) and has been intersected in drillholes to a maximum depth of over 1,500m below sea level, remaining open at depth.

The reef is divided into the West, East and Central zones, primarily on the basis of reef thickness and sulphide content, which is controlled to some extent by the host stratigraphy through which the reef system cross-cuts at a high angle. The Salisbury Hill Formation contains three discrete quartz pebble conglomerate units (Cabbage Tree, 2CG, and Wet Beds conglomerates) which are significant in the mine due to strong reef development in proximity to these units, thus generally defining the Central zone. The conglomerate units are relatively competent compared to adjacent sandstone and limestone units of the Denison Group. The average width of the Tasmania reef is approximately 2.7 metres, although this increases to approximately 5 metres in the Central zone, which also has the highest grades (approximately 20g/t to 25g/t Au). The Central zone also contains the highest proportion of gravity-extractable gold. The East zone is hosted in the Eaglehawk Gully Formation and, to a lesser extent, the adjacent Flowery Gully Limestone. Compared to the high grade Central zone, the East zone has a lower gold grade, and relatively high sulphide and arsenopyrite content. Therefore, the East zone also has a high sulphur-gold ratio as well as relatively high zinc and silver grades. The West area has lower sulphide content compared to the Central zone, but gold grades are also lower and water ingress is higher.

The Tasmania Reef has been formed as a result of multiple phases of hydrothermal fluid ingress into the dilational zone. Jones (2001), has distinguished five distinct vein stages. This work suggests that gold mineralization is limited to the first and third stages, the first event being responsible for the greatest volume of gold. This early phase of pyrite + arsenopyrite + chalcopyrite + gold mineralisation is associated with laminated quartz vein material. Gold concentrations within this first stage reach 3000 grams per tonne. Far less gold is associated with the third phase, which consists of pyrite + arsenopyrite + chalcopyrite + sphalerite + galena + gold. Mineralisation occurs with ankerite veining and overprints the earlier phases of mineralisation.

Alteration of the country rock by a quartz-ankerite-sulphide assemblage is limited to the immediate vicinity of the main structure.

A stockwork style of mineralisation has been identified within the Cabbage Tree Conglomerate (CTC) in the immediate footwall to the F4 splay structure at the western extremity of Tasmania Mine defined resource. This zone has been named the West Stockwork Zone and extends from just above the 805 level to just below the 915 level.

The stockwork zone is characterised by swarms of narrow, discontinuous tensional veins that as a group dip around 75° to the southwest (mine grid), with the orientation of individual veins varying considerably. The veining consists of quartz +/- ankerite and sulphides with disseminated sulphides also noted within the conglomerate.

The mineralised zone displays a trapezoidal shape in plan section with the length of the east west axis up to 50m and a width of up to 20m. The southern and western edges are well defined by the F4 structure and the contact with the Blyths Creek Formation respectively. However the eastern and northern edges are less well defined and were based upon grade constraints in the diamond drill holes and the general strike direction of the veining.

Mining Methods

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Comminution

Crushers and Mills

Processing

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Production

Operational metrics

Metrics	2020
Annual processing capacity	....