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Old gold mine reveals its secrets

A NEW cavity scanning system is being used to map old underground gold mine workings beneath a huge opencast pit in Western Australia.

UK based manufacturer MDL Rock Lasers (part of Measurement Devices) developed the C-ALS cavity scanning laser system. The device can be lowered into underground voids through boreholes as small as 110mm diameter and then rotated in three directions, enabling it to carry out vertical and horizontal plan section scans in real time at specified increments. This data can then be processed to create 3D computer maps.

The system was recently used to carry out detailed mapping trials of abandoned workings for Kalgoorlie Consolidated Gold Mines at its Fimiston Super Pit in Kalgoorlie, Western Australia. It will be used to improve the management of opencast mining through the affected ground.

The opencast mine, which will eventually be 3.7km long, 1.5km wide and be up to 540m deep, is in Kalgoorlie's 'Golden Mile' area, where an estimated 1300t of gold have been recovered since 1893. The earliest mining was carried out in narrow underground stopes and the area below the pit is known to be riddled with up to 2,000km of headings to depths of around 1km.

Although plans were available of these old workings, it was unclear whether sections had been completely backfilled or left open. The worry was that with time, collapsing pillars would cause voids to migrate towards the surface, posing a problem for the new operation.

Conventional borehole investigations based on the plans were used to build up models of the stopes, but it became clear that these were inaccurate, especially where there were many workings in close proximity. It was decided that a faster and more accurate method was needed.

A field trial with the C-ALS was conducted near the east wall of the Office Pit, where a number of ore bearing lodes intersect and old stopes centred on these lodes come together to form an underground 'ballroom' known as the Tetley Stope.

Existing records were used to create a 3D model of the cavern, allowing the sinking of a 165mm diameter borehole that intersected it at a depth of 71.5m. The borehole was lined with casing, leaving a 1.5m gap between its base and the roof crown.

The probe was then lowered to 76m and an initial survey of the stope carried out, taking measurements every 5degrees in the horizontal and vertical planes. A further survey of the section of the floor at 1degrees intervals was then carried out to create a more accurate model of the stope.

The mapping confirmed that the eastern hanging wall was formed by a steep, east dipping fault, where mineralisation stopped.

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