Geology of the Golden Mile
The Golden Mile at the centre of the Kalgoorlie Goldfield is one of the richest gold deposits in the world. Composed of a series of mineralised faults known as lodes, the deposit mainly occurs in a host rock known as the Golden Mile Dolerite.
The Kalgoorlie Goldfield is part of the Norseman-Wiluna Belt, a greenstone belt consisting of volcanic and sedimentary rocks and surrounded by extensive granite. These rocks all belong to the Archaean period of the Earth’s history and the goldfield is between 2.6 and 2.9 billion years old.
The main host rock for mineralisation in the Kalgoorlie district is the Golden Mile Dolerite, the largest of the dolerites in the area. There are two main styles of mineralisation. The Fimiston Style consists of classic Golden Mile lodes with abundant sulphides and tellurides. The Charlotte Style describes younger white quartz veins which are typically mined at Mt Charlotte, but have been found in other smaller deposits in the Kalgoorlie District.
The Golden Mile lodes have several characteristic orientations, each with slight variations in the style of the mineralisation. More than 1,000 individual ore lodes occur within the Golden Mile, some extending up to 1,800 metres long, 1,200 metres deep and 10 metres wide. They are all within an area over five kilometres long, two kilometres wide and more than one kilometre deep. The highest gold grades are typically associated with gold tellurides and alteration minerals with high vanadium contents.
The First Step: Exploration
Although the Golden Mile has been explored for well over a century, potential remains for new discoveries or re-interpretation of the results of previous exploration. KCGM’s Exploration Team is responsible for finding new deposits or extending those currently being mined. However, even in “brownfields” areas where the geology is already known, exploration is only the first step in the process toward a sustainable mining operation.
An ore body can only be classified as a resource based on its geologic certainty and economic value in accordance with set standards. There are many factors which must be considered, including the gold price, capital and operating costs, equipment and infrastructure relocation. Land access, government regulations and community expectations can all determine whether an area can be mined – regardless of what minerals exist under the ground.
Looking at the Rocks
The technology available to today’s exploration geologist may be light years away from most of the techniques employed by prospectors like Paddy Hannan and the early geologists on the Golden Mile. However two important tools still in use today would be immediately recognisable to those pioneers: a hammer and a good eye. A geologist still has to get out and look at the rocks.
Today a large array of sophisticated geological, geochemical and geophysical technology is available to help geologists add to their understanding of what lies below the surface. Satellite mapping, showing the distribution of minerals in the Earth’s surface, and hand-held x-ray fluorescence instruments which rapidly analyse the composition of rocks, are just two of the latest developments used by exploration geologists to identify areas of possible gold mineralisation.
Routine exploration work now involves measuring the Earth’s magnetic and gravitational fields and analysing a wide array of elements. Exploration geologists also use aerial photographs, published geological maps and historical data to build a picture of the ground and any riches it may contain.
The next step is drilling for samples, using a reverse air blast (RAB) drill rig which drills to depths of up to 100 metres. If the results are favourable a reverse circulation (RC) drill rig is used to collect deeper samples at depths of up to 800 metres, or a diamond drill may be used to drill depths of 1,000 metres or more.
The drill samples are analysed for gold and, depending on the drilling method, can be geologically assessed to determine rock type, structure and rock strength. Further analysis of this data provides the basis for an economic assessment of the resource on which the mining plan is based. This will dictate the style of mining, that is, whether an underground or open pit mine is more feasible, and the shape of the mining area.
Drill core samples from the Golden Mile are stored in coreyards at several locations around KCGM’s operations. They form open-air libraries of rocks containing the geological information of the Golden Mile.