Wednesday, 5 December 2012

Drake mining: managing a muddy mess

Sorry it has taken some time for me to post. I have had very little time of late because of some health problems my daughter has been having. But she is better than ever so time to get some time back into geology matters again.

Drake has a history of gold mining spanning back to 1886 when gold was dredged from Plumbago Creek. Since then the source of much of the alluvial gold was found just to the north of Drake. Many pits were created in the search for gold since the 1920s. These pits were relatively large mines in themselves and were given names such as White Rock, Carrington, Strauss, Lady Hampden and others. The mines were a source of wealth (during the good times) and a source of debt (during the bad times) with the mining operations completely ceasing in the 1990’s.

One of the old pits at Drake shortly after treatment with red mud
The formation of gold in the gold fields just north of Drake are a little difficult to put together as there seems to be more than one period of mineral formation in the rock. The parent rock is lavas and pyroclastic deposits including tuff which is of andesite to rhyolite in chemical composition. These rocks are called the Drake Volcanics which are part of the spatially significant Wandsworth Volcanic Suite. It appears that a caldera once developed in the area and fluids heated by magma transported gold and other metals and concentrated them. This is called an epithermal mineral deposit. However, Houston (1999) demonstrated that overprinting much of this epithermal stage is another chemically different period of mineralisation possibly related to different intrusive introducing mineralised fluids. And finally much of the area has been affected by supergene enrichment, which is enrichment caused by natural transport of minerals in groundwater and the percolation of rainwater.

Because financial stresses encourage people to take shortcuts to save money several things have happened at Drake that has caused elevated metal contamination to the environment of Plumbago Creek, a tributary of the Clarence River. Though sometimes people are just lazy or even ignorant of the possible impacts of incorrectly disposing of waste materials (Just like at home). Mineral deposits of the type at Drake contains minerals called sulphides, these include pyrite (iron sulphide), chalcopyrite (copper-iron sulphide) and sphalerite (zinc-iron sulphide). When exposed to air and water these minerals break down creating acids (called acid mine drainage) that cause the metals to be dissolved in any waters and therefore easily discharged into the environment. This is what has happened at the old pits near Drake and also at the waste dumps and even the access roads which were surfaced with waste rock.

But the story of the Drake mines also involve another waste material deliberately brought in from central Queensland. This material is referred to as Red Mud and is caustic (highly alkaline) waste material from aluminium refineries. But this is actually a good news story! Basic chemistry demonstrates that when you add acid and alkaline material together the material becomes neutral and metal contaminants precipitate out meaning any discharged water is decontaminated. Essentially an environmentally serious problem (disposal of aluminium refinery waste) has actually proven to be a resource. The trials and remediation of the pits was so successful that the technique was patented and a commercial product developed out of the Red Mud and given the name TerraB.

Application of the Red Mud was both as slurry pumped by ‘sprinkler’ directly into contaminated water left at the site or incorporated into waste rock or used as treatment liners. The picture shows one of the pits that I visited more than a decade ago when this technique was being trialled. It may look bad but really it is just suspended sediment that will settle out, while the acid and heavy metals have been neutralised. Some trials in waste rock have even found that Red Mud can actually reduce the uptake of heavy metals by plants, better than traditional rehabilitation techniques such as lime (Maddocks et al 2009).

The area around drake is interesting for many a geological reason, from its formation, the minerals found, the historical mining, contamination and rehabilitation. Who would have thought that adding two waste products together would fix both problems?! Two wrongs do make a right!

References/bibliography:

*Clark, M.W., Walsh, S.R. & Smith, J.V. 2001. The distribution of heavy metals in an abandoned mining area; a case study of Strauss Pit, the Drake mining area, Australia: implications for the environmental management of mine sites. Environmental Geology v40.

*Houston, M.J. 1999. The Geology and Mineralisation of the Drake Mine Area, Northern New South Wales. Papers, New England Orogen Conference, Armidale 1999.

*Maddocks, G., Lin, C. & McConchie, D. 2009. Field scale remediate of mine wastes at an abandoned gold mine, Australia II: Effects on plant growth and groundwater. Environmental Geology

3 comments:

  1. A really interesting post Rod. Haven't been to Drake for ages, knew it was a gold mining village but didn't know they were still mining in the 90's.

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    1. Actually, I think it was 1990 that was the last year of operation. exploration continued for a few more years but the mines were not reopened.

      I think a new exploration lease has been taken out in the last year or two.

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  2. In 1996 David McConachie took a class of undergrad Geology students from SCU (Lismore) on an excursion to the Drake area and among other things, showed us this tailings dam (?) prior to treatment. Some years later I was amazed to see on TV the extraordinary transformation.

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