A big pluton cut by the Clarence River

Yulgibar Bridge on the Clarence River

It has been some time since I spent a lot of time in the Clarence River area but occasionally I’ve got back there. Not too long ago I travelled along the Clarence Way. I took a quick detour down Lionsville Road when I came to the village of Baryulgil. Only a few kilometres down the road there is a quaint long low thin bridge over the Clarence. Just on the opposite side is a spot reguarly used as a swimming spot, there is also a tourist attraction for geologists (A road cutting).

The road cutting and the stream bank expose boulders of ‘granite’ rock which make up part of the New England Batholith. Right there at the bridge is a great spot to see one of rocks that make up what is called the Clarence Supersuite, a suite of ‘granites’ that have been derived from the melting of older igneous rocks. According to Bryant et al (1997) the Clarence River Supersuite for which this rock is a member is a type of ‘granite’ called an I-type. The ‘I’ stands for melted igneous in origin (as apposed to S-type for melted sedimentary). There is a lot to say about the New England batholith, its different granite types and its models of formation, such that I will do several blog posts in the future to cover this topic better.

The actual pluton in this area is called the Dumbudgery Granodiorite and it extends a few more kilometres to the north and for many kilometres to the south. Good outcrops can be seen in the hills on the southern side of Lionsville Road if you continue to the west a bit further. Indeed quartz veins in this area also contain small amounts of cinnabar (mercury ore), others contain some gold. If you want to take a sample or have a look at a fresh piece, a hammer (preferably a big one) with appropriate safety goggles is required. It is hard rock! But a fresh piece of granodiorite reveals a lovely white, pink and black speckled appearance. It can be so pretty that is is worth going on display.

Dumbudgery Granodiorite, fresh samples are very bright coloured

The colours of the rock reflect the mineral composition. Normal granite has a large proportion of alkali feldspar (sodium and potassium rich) relative to the amount of plagioclase (calcium and sodium rich) feldspar. A granodiorite like the Dumbudgery Granodiorite contains more plagioclase than alkali feldspar but still enough to be common in the rock. In the specimens at the Clarence River the plagioclase is a cloudy grey colour, sometimes difficult to distinguish from the quartz (tends to clearer) but the other alkali feldspar is a lovely bright pink colour. The lighter colours are contrasted by the two black minerals which are hornblende and biotite. The hornblende is identified by its hardness relative to the biotite which is a form of mica and therefore very easy to scratch. The Dumbudgery Granodiorite has been previously dated at 249 million years (very early Triassic period which is part of the mesozoic era).

Oddly the mass of granodiorite is actually bisected by the Clarence River. This is surprising given the prominent hills (and very hard rock) that the Dumbudgery Granodiorite is made from compared with the relative softness of the Clarence-Moreton Basin sedimentary rocks a short distance to the east. I’ve discussed why it is likely and surprising that the river has created this route in a previous post. Additonally, I’ve quickly discussed in another post the nearby unusual rock called the Gordonbrook Serpentinite which was mined at Baryulgil for asbestos. The Gordonbrook Serpentinite forms the eastern contact with the Dumbugery Granodiorite in Baryulgil area.

The Clarence river here is quite wide with large sand and gravel deposits moving every time it floods and altering its course. Historically some gold was found in this sand and gravel and is thought to be mainly sourced from gold in mineralised granitic rocks further up the river and in its tributaries. Some of the little deposits in the hills and much of the river itself was mined by the old timers, around the end of the 19th Century.

The area around Baryulgil is off the beaten track and Baryulgil itself is a bit of a delapidated little community but the area is worth a visit for its wonderful scenery and geological significance given its location at the edge of the mountainous New England region and the edge of the Clarence-Moreton Basin. Apparently the swimming and fishing are lovely too.

References/bibliography:

*Bryant, C.J., Arculus, R.J., & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology V38.

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.

References/bibliography:

*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.