Wednesday, 14 December 2011

From deep within the earth lies Baryulgil

Deep within the earth below the seas (so deep in fact we begin to enter the Earths upper mantle) we find material that is solid but so hot that it is viscous. This material is very low in quartz and when we see this rock on the surface it is unusual. The only way for such rock to come to the surface is through great wedges being thrust on to the edges of continents as the great oceanic plates move on the mantle. The upper units of rock from oceanic plates is greywacke from turbidites from collapsing continental shelves or the pelagic sediment accumulated over vast periods of time. But also you will find volcanic rocks erupted under the water at mid-ocean ridges and below these great thicknesses of basalt cooled into columns and even further below these great plutons of the mafic rock called gabbro which is the source of the basalt on the surface. Yet even deeper we start transitioning into the mantle and here we find rock that contains very little silica (ultramafic rocks) but is rich instead in iron and magnesium. These are called peridotites and dunites when found in rock form. From top to bottom the section is called an ophiolite sequence and these occur infrequently on the earths surface.

Given that the highlands of the New England region are derived from accretionary material scrapped off the sea floor during collision with the Australian Plate we have a good chance to find some. And we are in luck. I know of three significant areas in this region where ophiolite is preserved the two biggest are located north of Tamworth along the peel fault and at Port Macquarie. A smaller area can be found north-west of Grafton at the little village of Baryulgil, located midway between Tabulam and Copmanhurst.
Sepentinite from a location south of Baryulgil, the host rock for the asbestos
The ophiolite at Baryulgil is unusual because only a portion of the ophiolite is preserved, this being the peridotite and dunite altered to a rock called serpentinite and a small area of gabbro. It is also worthy of note because of the damage such a rock has caused the local people. The serpentinite at Baryulgil is known as the Gordonbrook Serpentinite and includes such serpentine minerals as chrysotile – better known as a mineral of the asbestos group. Mining of this industrial mineral by Australian Asbestos and later by James Hardie occurred at Baryulgil for quite some time and it is this that has caused many problems.

Stepping slightly into the area of politics and aboriginal relations (and then quickly away again) the Baryulgil asbestos mine was often held as a wonderful example of how an indigenous population could be assimilated into the good things of western culture. Alas, as we know too well today that model of assimilation was flawed, in part in the case of Baryulgil because of the harm to its workers from such a carcinogenic material. Reportedly the mine and its processing plant had an appalling reputation for dust which is the main mechanism that causes the entry into the body and the subsequent long term damage including a massive increase in the risk of cancer. As an aside, it is worth noting that even the Nazi party in Germany before the Second World War (and greater than 40 years before the closure of the Baryulgil mine) introduced regulations to ensure that dust was minimised when working with asbestos because of the probable heath effects.

The Gordonbrook Serpentinite is a body approximately 25km long elongated unit right on the edge of the New England Fold Belt accretionary terrain. Geophysical surveys including gravity and magnetics indicate that the unit probably much larger than the area exposed as it appears to underlie the Clarence Morton basin just to the east of Baryulgil. The unit shows a gravity anomaly given its composition from heavy minerals and the magnetic signature shows up because of the richness of iron when compared to the more recent Jurassic aged sediments (Laytons Range Conglomerate and Gatton Sandstone) of the Clarence Moreton Basin and the accretionary complex meta-sediments to the west.

The gabbro unit of the ophiolite sequence is present as a small remnant unit on the north western most part of the serpentinite body on the northern side of the Clarence River. Interestingly the Clarence River pretty much runs straight though the middle of the serpentinite as it meanders from the mesozoic clarence moreton basin sediments into and out of the older accretionary terrain. This meandering has implications for indicating the history of the river development of the Clarence. But more about the Clarence River in another future post.

The minerals present in the serpentinite are mainly comprised of serpentine (a type called antigorite) but there is asbestos (chrysotile) occurring naturally in vein systems. Altered serpentinite also locally forms magnesite which is a white chalk like mineral formed through the affects of carbon dioxide rich ground water. The nature of the serpentinite and ground water alteration and reposition of secondary minerals is such that metals such as arsenic, and particularly nickel and cobalt are also quite rich in small patches. But these minerals are hard to come by unless intersected by cuttings or mine workings.

If you pass through that way to explore the more remote corners of our region take note of the roads. The councils that managed the area have previously maintained and unpgraded the roads with locally sourced rock. This means that the road base is often made from serpentinite. This has caused made road management problematic because the current Clarence Valley Council to minimise the risk of exposure to asbestos when staff or contractors are maintaining the roads!

Another feature of the Baryulgil Serpentinite is that it helps to demonstrate a theory about a major period of deformation in Eastern Australia. This formed tectonic features called the Coffs Harbour Orocline and the Texas Orocline, but there is too much to discuss about this now so I will have to dedicate a post about this in the future.


*Cornwell, J 2004 Hitlers Scientists: Science, War and the Devil's Pact. Penguin Books
*Henley, H.F. , Brown, R.E. , Brownlow, J.W. , Barnes, R.G. , Stroud, W.J. 2001 Grafton-Maclean 1:250 000 Metallogenic Map SH/56-6 and SH/56-7: Metallogenic Study and Mineral Deposit Data Sheets Geological Survey of New South Wales.
*Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.


  1. A very interesting post Rod.

  2. Thanks again Mark. Note that I have just added a bit more info on the roads in the area that might be surprising too (if you were not already aware).

  3. Interesting post Rod, I seem to remember Paul Ashley saying the asbestos from Baryulgil was more toxic than the asbestos from Woodsreef. Do you know if this is true? and any ideas why?

  4. Hi Dylan,

    Both mines have chrysotile (blue asbestos) as the dominant serpentine mineral which I think is the nastiest variety. Maybe the way the asbestos occurs in the rock and the methods of separation make it more available to be inhaled etc? However, I wouldn't be surprised if Professor Paul Ashley knows a lot more about this than me!

  5. Morning Rod,

    What an excellent blog!

    Firstly I better introduce myself. I have just started the arduous task of a PhD at down at Wollongong, focusing on the evolution of the Great Serpentinite Belt along the Peel Fault (I have just recently stated reading on the area, so my knowledge on the New England Orogen is limited) Supervisors are Sol Buckman and Allen Nutman (You should check out “Middle Carboniferous - Early Triassic eclogite-blueschist blocks within a serpentinite mélange at Port Macquarie, eastern Australia: Implications for the evolution of Gondwana's eastern margin” by them, presents some new data from the dating of zircons).

    I am planning on collecting some samples from the Gordonbrook Serpentinite in a couple of weeks’ time, to see how they compare to the serps along the Peel and Manning system. You say there is a gabbro unit in the north western portion of the body. Is this accessible by Peckhams Rd? I’m assuming the outcrop is along a road cutting? If you could be of any help, that would be great, my time around the Baryulgil area is limited to a rushed day trip.

    To add to your comments above, I have also heard that the
    chrysotile around Baryulgil is particular nasty, in comparison to other deposits elsewhere on earth even.


    1. Hi Ryan,

      Great to hear from you. Thanks for commenting and recommending the paper. I don't have access to Gondwana Research, which I think is the Journal that they published in? But I'll try and see if I can find it somehow!

      As for access, I'm not sure, sorry. I think... that is THINK... that using Peckhams Road would be the best route since Lionsville Road is probably too far to the south to get very close. I've actually not seen the outcrop of Gabbro myself, just seen the maps and a hand specimen (altered to a nice green colour).

      However, I'd love to know how you go. If you like feel free to contact me on the email on the "about this blog" tab at the top of the page.

      Good luck! Enjoy the trip. It is a nice area (almost as good as the eastern side along the Peel fault side!)