It's a Demon of a Fault

Many people have requested that I do a post on the Demon Fault. I've struggled to put something together because structural geology is not one of my strong points and secondly because there was so little published information about it, except for some specific papers in the 1970’s. Thankfully, a few months ago Babaahmdi & Rosenbaum (2013) published a detailed paper summarising what was known in the 1970s, presenting how the fault appears, how it seems to have developed and how it may fit into the development of eastern Australia. It is worth noting that Gideon Rosenbaum from The University of Queensland has been the major researcher on New England structural geology for the last 5 years. If it was not for him and his student’s research we would be struggling to understand some of the basic features of the older rocks of the region including the Demon Fault.

Large faults are usually have quite distinctive landscape features. The Demon Fault is mainly a transverse type fault (movement on the fault horizontally rather than vertically) which displays very obvious topographical features.  Transverse faults often form valleys, where the rock of the fault has been broken down into what is called gouge or rock-flour. Gouge is very weak material. It is easily eroded and rivers often preferentially follow the route of the fault carving out the gouge into deep valleys.  The presence of deformational features in the surrounding rock can give an indication of how deep the fault was when it was active.

The Demon Fault is a prominent feature because it is evident from a series of valleys from near the Queensland Border to Dorrigo. At the Queensland end it is partly obscured by the Cenozoic Main Range Volcanics and in the Dorrigo area the end is obscured by the Cenozoic aged Ebor Volcanics. Geological maps of the area show a nice linear feature with obvious truncation of pre-existing geological units. Aerial photos also show the fault up nicely with streams preferentially flowing along the trace of the fault and contrasting with the rugged forested mountains surrounding it. I’ve never taken a photo of any part of the Demon Fault but a nice photo taken from an aeroplane can be found here: http://www.panoramio.com/photo/35890361

The Timbarra River has followed the Demon Fault creating linear valley
http://www.panoramio.com/photo/35890361 (used with permission)

Korsch et al (1978) observed that the Demon Fault had displaced several geological units including intrusions of the Bungulla Monzogranite (now known as the Rocky River Monzogranite), Dumbudgery Granodiorite and Newton Boyd Granodiorite as well as the Drake Volcanics. The fault was interpreted as a dextral strike-slip fault (a fault where the eastern side had moved south relative to the western side). Korsch et al (1978) calculated that the fault had displaced these units 17km which is substantial in Eastern Australia. Dating of the displaced granite intrusions provides a possible maximum date of within Triassic period (249-232 million years). The nature of deformation features adjacent to the faulting indicates that the fault was shallow and/or was created in a brittle environment. Badaahmadi & Rosenbaum (2013) speculate that the timing of the faulting may actually be similar to that of faulting and extension in the earth’s crust that formed the Ipswich and Clarence-Moreton Basins (more about this in future posts).

There are many factors in understanding the Demon Fault. It is interesting to note that other authors have come up with different lengths of displacement including 30km in the northern part of the fault and 23Km in the central part. Badaahmadi & Rosenbaum (2013) have calculated that the northern part of the fault displaced 35km, in the centre by 25km and south by 19km. Some components of reverse faulting (where one side of the fault slid down and away from the other side) were observed. Additionally, it was noted that the Demon Fault did not appear to follow one big long line but instead had numerous splays (deviations, splitting, etc) especially in the south. Badaahmadi & Rosenbaum (2013) suspect that there may be two causes to the different lengths:

1. Splays may have created movement of the fault which had a vertical component as well as horizontal.
2. There may have been some fault reactivation of the northern part of the fault as recently as the Cenozoic era.

Both of these possibilities really need a discussion in their own right, rather than cursory mention. So, I’ll get back to these in a future post.  I also want to cover the significance of the Demon Fault in formation of the Texas and Coffs Harbour Oroclines which are incredibly large features that I’ve briefly touched on in an earlier post about the South Solitary Island.

References/bibliography:
*Babaahmadi, A. & Rosenbaum, G. 2013. Kinematics of the Demon Fault: Implications for Mesozoic strike-slip faulting in eastern Australia. Australian Journal of Earth Sciences. V.60
*Korsch, R.J., Archer, R. & McConachy, G.W. 1978. The Demon Fault. Journal and Proceedings, Royal Society of New South Wales. V111.

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