Antimony and the Macleay River

Antimony is a metal that is very well represented in our region. Many people have not heard of antimony as it is one of those elements that is ‘hidden away’ in many metal alloys and plastics and therefore often outshone by the more well known ones such as Iron, Nickel, Cobalt etc. It is a very important element for use in electronics and to modify the properties of rubber and plastics. It is even used in the cosmetics industry and HIV treatment medication (Wilson et al 2010). The main antimony mineral is called stibnite, an antimony sulphide mineral with the chemical formula Sb2S3, though there are many other less common antimony minerals.

The geographical distribution of antimony mineralisation in the Northern Rivers and New England closely follows certain geological units intruded by granite type plutons during the Permian (Ashley & Craw 2004). Essentially these deposits fall into the category of mesothermal mineral deposits meaning that they were formed through the action of hot fluids under pressure within the earth. The heat source is from regional heat increase due to the intrusion of many granites and sometimes from the actual contact zone of individual intrusions. The source of the fluids can be existing water in sedimentary rock pore space and/or derived from the breakdown of hydrous minerals such as clays. This hot water (often accompanied by elevated salts) can dissolve elements such as antimony as well as others such as gold and silver and then as they cool these elements are redeposited. In practice this tends to mean that the elements are located within veins of quartz or carbonate.

Probably the best known deposit of antimony is the Hillgrove Mine east of Armidale. The mine is in the headwaters of the Macleay River and was first mined for gold at the end of the nineteenth century. Indeed Hillgrove had a gold rush of such size that it was much bigger than Armidale (now its population is less than a hundred, I think). But many other areas have extensive mineralisation of antimony such as the area to the west of Bowraville in the headwaters of the Nambucca River catchment, areas north of Dorrigo in the headwaters of the Nymboida River catchment and even areas as far north as Tooloom which is to the north of Drake in the upper portions of the Clarence River catchment. Some of these deposits have been mined historically, though in the main gold has been the target and antimony just a by-product.

Antimony is an interesting element because it is chemically closely related to arsenic and therefore behaves in a similar way. This means it can also be dangerous in high concentrations and its environmental impact can be significant at even moderate to low levels, however, the nature of antimony has not been as extensively researched as arsenic and therefore the drinking water and environmental limits in Australia have been set lower than arsenic to increase the safety margin in assessing whether there is likely to be an adverse impact (Ashley et at 2004).

Interestingly, unlike many other elements that can be mobilised by the creation of sulphuric acid during the oxidation of the parent sulphide mineral, antimony tends not to remain in solution for long because the nature of the mineralisation model is such that carbonates are often present which neutralises the acids and leads to settling out of the antimony from the water column with iron and other metals. However, if the sediment is transported then this can be deposited a huge distance from its source and in some situations can be re-mobilized because of local stagnant water during dry periods combined with the presence of natural humic acids. This behaviour has been observed in the Macleay River catchment as suspended sediment from the areas around Hillgrove has been deposited on the flood plains as far away as Kempsey, very low concentrations of antimony are usually found in clear, clean water in the region. However, Wilson et al (2010) has shown that sometimes high antimony contents of alluvial soils can lead to uptake by flora and therefore this contaminant can then be accumulated in animals that graze on these plants.

References/bibliography:

*Ashley, P.M. & Craw, D. 2004. Structural controls on hydrothermal alteration and gold-antimony mineralisation in the Hillgrove area, NSW, Australia. Mineralium Deposita v39.
*Ashley, P.M., Craw, D., Graham, B.P. & Chappell, D.A. 2003. Environmental mobility of antimony around mesothermal stibnite deposits, New South Wales, Australia and southern New Zealand. Journal of Geochemical Exploration v77
*Craw, D, Wilson, N. & Ashley, P.M. 2004. Geochemical controls on the environmental mobility of Sb and As at mesothermal antimony and gold deposits. Applied Earth Science (Transactions of the International Mineralogy and Metallurgy Bulletin). v 113.
*Wilson, S.C., Lockwood, P.V., Ashley, P.M., & Tighe, M. 2010. The chemistry and behaviour of antimony in the soil environment with comparisons to arsenic: a critical review. Environmetnal Pollution v158.

Shaping the Australian Nation

A free ebook was published by the Australian National University Press in August this year. It is the geological history of the Australian Continent by numerous authors and edited by Richard Blewett. The book is titled called Shaping a Nation: A Geology of Australia. If you are keen (or old fasioned like me) you can buy a hard cover copy of the book for $70. Have a quick look at the PDF first and you'll see how good it is and worth the expense.

The electronic copy of the book can be obtained at the following site:

http://epress.anu.edu.au/titles/shaping-a-nation

A southern solitary island

How do you find out about something you can’t visit? From time to time I’ve wanted to visit sites that were on private land but I was unable to contact the landholder. More recently I find that the landholders do not want me on their land because of fears that I’m something to do with a gas company exploring for coal seam gas reserves (which I’m not). However, there is one place that is nearly impossible to get to because of its remoteness and the level of control that a government department have (for good reasons). I’d dearly like to visit this place because of the history, biology and of course the geology. The place is South Solitary Island off the coast of Coffs Harbour and Woolgoolga.

North Solitary Island is also considered part of the Coramba Beds

South Solitary Island has a lighthouse and an old lighthouse keepers residence which is disused and slowly deteriorating. It is perched on a rock that just sticks straight out of the sea. A few small islands are part of the island group but they are all really just rocks sticking out of the ocean. I understand that the National Parks and Wildlife Service licence visits by tourists to the island lighthouse once a year by helicopter. I’d love to go but unfortunately I don’t think I could afford such a trip.
The Solitary Islands (and the South Solitary Island in particular) is known to be rock comprised of turbidites (marine mass wasting derived sediments) derived from volcanic parent rock and ash-fall tuff (Korsch 1993). This same assemblage is present on the mainland throughout the area called the Coffs Harbour Block or Coffs Harbour Association and is considered Carboniferous in age. The stratigraphic unit is probably the Coramba beds which mean there is also the possibility that chert, jasper and metabasalt are present as they are elsewhere on the mainland. 

I had no idea about the geology of South Solitary Island until I read Korsch (1993) in which he was permitted to visit all of the solitary islands to determine whether the concept of a giant fold (called an orocline) was present off the coast. If the orientation of the rock strata was right it would demonstrate that the area between Brooms Head and Coffs Harbour and then inland up through the Orara region and eventually looping back up into Queensland was a giant fold in the earth. Korsch (1993) did observe just such features and this has resulted in much further interest and research (including papers published in the last 12 months) about the tectonic history of the New England and Northern Rivers. I will go into more details about the extraordinary folding and tectonic history in future posts as there is an incredible amount of detail and unknowns when it comes to our area.

Oddly, Weber et al (1978) mentioned that a report from 1945 that there is an area of molybdenum mineral deposit on the South Solitary Island. The size of the island (and being a national park) is such that it could never be mined but it is such an unknown curiosity. Webber et al (1978) describes the deposits:

Worthy of passing mention is an occurrence of molybdenite at the eastern extremity of the Demon Block. Narrow, molybdenite-bearing quartz veins have been reported from South Solitary Island, 16.5km northeast of Coffs Harbour, by Fisher (1945, p10). The host rock is unknown.

The reason this is a little odd in my mind is because molybdenite is not very common in the Coffs Harbour Block. Some molybdenum formed in areas related to specific types of intrusions to the south in the nearby Nambucca Block (e.g. see my earlier post on the Valla Monzogranite) but to my knowledge this has not occurred to any significant extent in the Coffs Harbour Block. Just another slightly out of place geological feature in our region.

References/bibliography:

Korsch, R.J. (1993) Reconnaissance geology of the Solitary Islands: constraints on the geometry of the Coffs Harbour Orocline. New England Orogen Conference 1993, University of New England.

Weber, C.R., Paterson, I.B.L & Townsend, D.J. (1978) Molybdenum in New South Wales. Geological Survey of New South Wales 43.

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