Quarries excavating marble are a feature of the old mining town of Chillagoe. Marble quarrying is one of the few visibly active mineral exploitation industries in the area – compared with the way it was in the mining boom of the 19th century.
Earlier posts have described aspects of the limestone karst scenery in the Chillagoe District of Queensland, Australia. Limestone is a sedimentary rock formed from the shells of minute marine organisms which accumulated on the sea bed millions of years ago. The layers of sediment become compressed, compacted, hardened, and eventually folded by earth movements, and raised above sea level to their current location.
The Chillagoe limestone has in places been transformed to marble, baked by the extreme heat generated during ancient volcanic activity that has forced molten lava through, and between, the layers of limestone before reaching the surface. The calcium carbonate in the limestone has been melted and re-crystallised. Pure calcium carbonate is white but there are many other minerals in the original limestone and these give the newly formed marble its characteristic streaks and patterns.
Pink and cream colourations are caused by iron oxide inclusions. Impurities such as organic matter or globules of oil, are responsible for blue, grey, and black marble.
I was lucky enough to have a guided tour to see some of the marble, and evidence for the marble industry, while staying at Chillagoe Observatory and Eco Lodge. Rhondda, the manageress of the hotel, took us to see where massive outcrop rock faces, isolated natural boulders, and individual blocks cut out of the rock pavement, had been sliced open to examine the quality of the stone. I could compare the natural rough outer surfaces of the rock with the detailed colours and patterns revealed by the smooth cut surfaces. Many of these natural designs could be better seen when water was poured on them to bring out the detail – although we needed most of the water to drink as it was fiercely hot out in the open with no shade.
Where quarrying was still being carried out from the flatter, weathered, and smooth rock pavement, massive cutting equipment could be seen in position to excavate not far below the surface. The marble bedrock is cut into rows of huge regular blocks with geometric precision by vertical cuts at right angles to each other – and finally separated from the ground by a horizontal cut below.
A useful reference book to the geology of Queensland is:
Rocks, Landscapes & Resources of the Wet Tropics by Bernd G Lottermoser et al, edited by Warwick Willmott & bernd G Lottermoser, Geological Society of Australia, Queensland division, 2008, ISBN 9780975789483, pp 36-37.
COPYRIGHT JESSICA WINDER 2012