Greeny Stuff at Port Macquarie

Serpentinite at Port Macquarie

A fascinating area of the northern rivers is right at Port Macquarie. This is a post that I’ve wanted to start for a long time but because the area is quite complex I’ve baulked at the prospect. I’ve just wanted to discuss too much, to dive into the deep end. I now realise that I should just start with an introduction to the formation of the fascinating rocks and come back for many more posts about more specific details in the near future.

The picture to the right shows the nature of one of the rock types at Port Macquarie. If I recall correctly this photo was taken at the southern end of Flynns Beach. It is a characteristic rock and given the odd shapes preserved in it implies quite an interesting history. The rock is serpentinite in two forms. The first being the banded appearing one which is called serpentinite schist. The second is a block of serpentinite which has not had the schistose fabric developed in it. I’ve discussed serpentinite occurring elsewhere such as at Baryulgil in previous posts but as far as Serpentinite goes the Port Macquarie area has heaps of it.

Serpentinite is a rock mainly comprised of the mineral group Serpentine. This is a very silica poor rock formed by the regional metamorphism of Olivine rich rocks such as Dunite or Peridotite. These parent rocks are from deep below the oceanic crust in the deepest parts of a layered sequence called Ophiolite and because of this it is rarely preserved on land. The metamorphism of the serpentinite is actually at the same time as large blocks of the Dunite and Peridotite rich oceanic crust are thrust and rotated during tectonic plate collision. Because serpentinite tends to be ‘slippery’ it is mostly present around major regional fault systems where it is ‘squeezed’ into place. However, its relationship to other nearby tectonic blocks is detailed and requires a separate blog post on its own.

At Port Macquarie the parent rock appears to have been a calcium rich variety of Peridotite called Harzburgite. There are also other rocks mixed in with the Serpentite, so much so that the area is often referred to as a melange. These other rocks are sometimes (but not always) part of the Ophiolite. For example slightly shallower ones such as gabbro which has been metamorphosed to rocks called Blue Schists. Also occurring are non Ophiolite rocks such as marble and other types of schist. Because of the complexity some 'inclusions' in the melange are from a different source than the Ophiolite, that is a story for another post.

As for the age of the Serpentinite unit, direct dating is impossible due to metamorphism re-setting the dating clock of the rock. The best that can be achieved is the last date of metamorphism. Even then the ultramafic (silica poor) nature of the rock means that minerals that can be used for dating (such as zircons) are uncommon or simply absent. Therefore the age of the Port Macquarie Serpentinite is only estimated from the surrounding rocks. However recent work by Nutman et al (2013) has narrowed the age of metamorphism and probable emplacement of the serpentinite to 251-220Ma which is the late Permian to early Triassic. How they found the date is quite interesting with adopting multiple techniques physical, nuclear and chemical.

Bibliography/references:

*Aitchison, J.C. & Ireland, T.R. (1995). Age Profile of Ophiolitic Rocks across the Late Palaeozoic New England Orogen, New South Wales: Implications for tectonic models. Australian Journal of Earth Sciences. Vol.42.
*Nutman, A.P., Buckman, S., Hidaka, H., Kamiichi, T., Belousova, E., Aitchinson, J.C. 2013. Middle Carboniferous-Triassic eclogite-blueschist blocks within a serpentinite melange at Port Macquarie, eastern Australia: Implications for the evolution of Gondwana’s eastern margin. Gondwana Research.
*Och, D.J., Leitch, E.C. & Caprarelli, G. 2007. Geological Units of the Port Macquarie-Tacking Point tract, north-eastern Port Macquarie Block, Mid North Coast Region of New South Wales. Quarterly Notes of the Geological Survey of New South Wales. Vol.126.

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.

A rock forming mineral: Olivine

Everyone has heard of the very common mineral called quartz, most people have heard of the very common mineral called feldspar, but surprisingly few people have heard of the very common mineral called olivine. I speculate that this is for two reasons. one being that quartz is resistant to weathering and is very easy to find, feldspar often occurs in big crystals and is also somewhat resistant to weathering, whereas olivine quickly breaks down into clay and occurs in mafic (quartz poor) rocks. the second being that it is often only obvious as large crystals in some basaltic rock.

But firstly olivine is made from similar components as most of the other common minerals. In particular it is comprised of silica with either/or some magnesium (Mg2SiO4), known as forsterite or iron (Fe2SiO4), known as fayalite. Its chemical formula is often given as ((Mg,Fe)2SiO4) because the magnesium or iron can substitute for each other and are usually present together. Because of the nature of the chemical bonds between the magnesium, iron and the silica group the mineral weathers quite rapidly (geologically speaking). Forsterite (mg rich) tends to be an olive green colour and because of the iron content fayalite is more browny-green.

Bowens Reaction Series from Encyclopedia of Earth

Olivine is crystalised in volcanic rocks at high temperatures. This means that as a mafic (basalt like) magma chamber cools the first mineral to form into crystals is olivine (see figure opposite). This indirectly means that if you see olivine crystals in the field it is usually because the rock was a lava that was erupted relatively rapidly to the surface from deep in the earths crust or upper mantle. But, sometimes you can come across rocks that are almost entirely made from olivine. These rocks are called dunite. It is formed at the boundary between the crust and the mantle and has crystalised there. It is thought that it has been bought to the surface through the action of plate tectonics where sometimes large chunks of oceanic crust can be scraped onto a continental plate as the process of subduction takes place. This is called an ophiolite sequence.

A metamorphic source of olivine is through the contact metamorphism of dolomite limestones.

Particulars:

Chemical Formula: (Mg,Fe)2SiO4
Fracture:Conchoidal
Hardness (Moh): 6.5-7
Specific Gravity:
Colour: Olive Green (Forsterite) to Browny-Green (Fayalite)
Luster: Vitreous (glassy)
Crystallography: Orthorhombic
Gem: Peridot
Common accompanying Minerals: Not found with free quartz crystals. reguarly found with feldspar, pyroxene, augite

More information on olivine can be found one the Mineralogy Database.

Just a quick note on dunite and ophiolite sequences, this rock type is named after Dun Mountain in the northern part of the South Island of New Zealand. Dun Mountain is almost exclusively made from dunite and is part of a geological feature known as an ophiolite sequence which stretches along and off the Alpine Fault in New Zealand. Another ophiolite sequence is present in New Caledonia. Closer to home, the Peel Fault which runs along the western side of the New England Tablelands past Tamworth eventually to somewhere near Port Macquarie, also resembles an ophiolite sequence. I will discuss the Port Macquarie part of the Peel Fault at some time in the near future.

References/Bibliography:

*Klein, K. Hurlbut, K. Manual of Mineralogy (After Dana, J.D.). Wiley 21st Ed.
*Encyclopedia of Earth: www.eoearth.org