OzGeographics

Just a quick post to draw attention to a great YouTuber... and to keep a post or two going to show that this blog is still active (just). The YouTube channel is OzGeographics and can be found here OzGeographics - YouTube. However, I need to draw attention to a really good video which outlines a lot of the geological history of the northeast of this state, this video I found really interesting because it also shows just how potentially devastating the volcanism in our region was. Super-volcano after super-volcano apparently formed much of the New England and Northern Tablelands areas.

The Chain of Super Volcanoes That Caused The Worst Mass Extinction on Earth - YouTube

Blog Update #11 - More rocks in our region

Not a lot to mention as far as the blog goes at this point, except that I've added photographs of three more stratigraphic units to the Rocks of the Region page. These are three of the many 'granites' in the Armidale district:

Gara Monzogranite --- Fickr Photos --- Stratigraphic Names Database
Glenburnie Leucomonzogranite --- Flickr Photos --- Stratigraphic Names Database; and
Rockvale Monzogranite --- Flickr Photos --- Stratigraphic Names Database

Typical landscape and outcrop characteristics of the Rockvale Monzogranite, Wollomombi area

On another note, while visiting a friends property near Armidale I observed a brecciated jasper in the Sandon Beds. I was aware of an abundance of jasper beds (red chert) in the region ever since my university days, however, I'd never seen a brecciated type and this was quite attractive. More to come in a week or so.

Cutting Through Mysterious Granite on a Country Highway

Australia is known for its remoteness. There are some quite remote areas in the Northern Rivers too. Along the escarpment there are rugged areas and visitors are rare. This means that sometimes rocks even though mapped broadly have geological units that have not been researched enough to relate them to surrounding units. It is a rare thing though, and rarely have rock units not been named, and categorised, even rarer is when a rock is found by the side of one of the national highways!

un-categorized granite on the New England Highway, Glencoe

The picture shows a granite that is currently mapped as "unassigned Permian intrusive - felsic". There may have been some investigations here in the past. I just can't believe some place so obvious like this one has not been investigated in detail.

A Rock of Gibraltar Range National Park - Part 2

Dandahra Creek Leucogranite

This post is a follow-on from an earlier post which can be read here.

The Dandahra Creek Leucogranite is mainly composed of granite which is depleted in dark (mafic) minerals. The crystals are of very similar size and medium to coarse grained. The crystals are mainly quartz with feldspars and occasional biotite mica. The term Leuco- simply refers to the light colour and lack of mafic minerals. There are also small amounts of other minerals that are disseminated through the rock these include the mineral zircon which is used for dating.

The dating of the Dandahra Creek Leucogranite was only conducted in the last couple of years. It is an example of using multiple techniques together to get an answer. The mineral Zircon is formed in magma chambers of granite and granite-like composition. This is a very stable mineral. Zircon locks up uranium in small amounts and this uranium undergoes radioactive decay to lead. By measuring the proportions of uranium to lead it is possible to determine how long ago the zircon had formed. In the past in some cases the whole zircon crystal have been used to determine the ratio. However, this method has some complications.

Not all of the zircon crystals in rocks show the same age. In the case of the Dandahra Creek Leucogranite some seemingly having much older ages. These crystals are actually inherited from the parent rock. The stability of the zircons means that they have not fully melted in the magma chamber. Often a good way to determine if a zircon is older than the magma chamber is to look at the shape and determine whether there has been any melting of the edges of the crystal. However, sometimes it is very hard to tell because the zircon often builds itself up again with an old core and a new crystal face.

To overcome the problem of age zoning in zircon crystals an alternative method was developed measure the ratio of lead and uranium. A high accuracy ion beam is aimed at the different portions of crystal. The ion beam vaporises the elements in that tiny area. The vapour is then measured for the abundance of each element and then the ratio of elements can be calculated. This is called the Sensitive High Resolution Ion Micro Probe or SHRIMP.

SHRIMP was a method developed right here in Australia. It is regarded as one of the most reliable ways to analyse microscopic crystals to determine when and how they formed. The need for the special machine came from dating the Rocks that make up the oldest parts of Western Australia which are the oldest in the world. It has no become a recognised tool around the world (Ireland et al 2008). There are 20 SHRIMP analysers around the world with four built in the last couple of years in Japan, China and Poland. Like Wi-Fi, the Hills-Hoist and Pavlova it is another example of Australian scientific ingenuity.

The age of the intrusion given for the Dandahra Creek Leucogranite using the SHRIMP method is 237.6 Ma (plus or minus 1.8Ma). This makes it the youngest example of the Stanthorpe Suite of Granites (Chisholm et al 2014) and nearly the youngest in the whole Standthorpe Supersuite (Thanks for the correction rockdoc!).

References.Bibliography:

*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia

*Clarke, Peter J. & Myerscough, Peter J. 2006. Introduction to the Biology and Ecology of Gibraltar Range National Park and Adjacent areas: Patterns, Processes and Prospects. Proceedings of the Linnean Society of New South Wales

*New South Wales National Parks and Wildlife Service 2005. Gibraltar Range Group of National Parks (Incorporating Barool, Capoompeta, Gibraltar Range, Nymboida and Washpool National Parks and Nymboida and Washpool State Conservation Areas) Plan of Management. February 2005. ISBN 0 7313 6861 4

*Ireland, T.R., Clement, S., Compston, W., Foster, J. J., Holden, P., Jenkins, B., Lanc, P., Schram, N. & Williams, I. S. (2008), "Development of SHRIMP", Australian Journal of Earth Sciences V55 p937–954

A rock of Gibraltar Range National Park - Part 1.

A lookout on the Gwydir Highway

I was going to write a very long post on the Dandahra Creek Leucogranite but I think it lends itself to two posts. This post will focus on the amazing Gibraltar Range National Park and the second will focus on Australian ingenuity and dating of the Dandahra Creek Leucogranite.

A few months ago I travelled from Glen Innes to Grafton via the Gwydir Highway. The landscape in this area is wonderfully diverse and surprisingly contradictory. For example usually Sandy soils on the plateau give rise to swamps with peat. It is a special area because the link between the geology, vegetation and even bush fire patterns is quite obvious. I'd like to focus on one rock unit that makes up the balance of the Gibraltar Range National Park area, the Dandahra Creek Leucogranite.

The Dandahra Creek Leucogranite was often referred to as the Danhahra Granite (and still regularly called this in botanical circles). It is part of the New England Batholith and has recently been dated at at 237.6 Ma (Chisholm et al 2014). It is the youngest member of the Stanthorpe supersuite of granites. Outcrops are very frequent in the Mulligans Hut area and the Gwydir highway transverses the unit.

The spectacular tors which are major features of the landscape of Gibraltar Range National Park arise from weathering from the Dandahra Creek Leucogranite. These tors form through onion peel weathering (technically called exfoliation or spheroidal weathering). This weathering process is where water enters cracks in the rocks and then freezes over night. As water turns to ice it expands and sheets off rock just like an onion skin. This is usually a fairly slow process except with the last sloughing off of the onion peel occurring quite rapidly.

Tall open forest is a major feature of the landscape of the Dandahra Creek Leucogranite. These eucalyptus dominated forests can have an open, grassy understorey featuring grass-trees and/or tree-ferns. These landscapes are quite fire prone. Indeed their structure is dependent on multi-decadal scale fires.

There are also some more unusual vegetation communities on rock outcrops because the tor outcrops lend themselves to protecting some vegetation from fires. They are also very thin soils with low nutrient content so even carnivorous plants can be found.

Heathlands and grasslands occur around the rock outcrops and are particularly important as they contain the greatest concentration of rare, threatened or geographically restricted species, or species found at the limits of their distribution (NPWS 2005). The grass and heath land burns very frequently often with bush fires only every several years.

The shallow wide valleys that are formed on the sandy granitic derived soils result in common large peat swamps. The shape of the valleys slows down water and the underlying massive granite means that the water does not infiltrate. The swamps contain sedges and other water loving plants.

If you are interested in the bush or interested in rock the Gibraltar Range National Park is for you. If you are in to camping, bush walking, amazing views of rugged valleys the Gibraltar Range National Park is for you. If you are in to spectacular flowers, rainforests, exploring a rocky creek the Gibraltar Range National Park is for you. If you are in to staying in a lodge, want to see some snow, or bathe in a rock pool on a summers day the Gibraltar Range National Park is for you.

References/Bibliography:

*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia

*Clarke, Peter J. & Myerscough, Peter J. 2006. Introduction to the Biology and Ecology of Gibraltar Range National Park and Adjacent areas: Patterns, Processes and Prospects. Proceedings of the Linnean Society of New South Wales

*New South Wales National Parks and Wildlife Service 2005. Gibraltar Range Group of National Parks (Incorporating Barool, Capoompeta, Gibraltar Range, Nymboida and Washpool National Parks and Nymboida and Washpool State Conservation Areas) Plan of Management. February 2005. ISBN 0 7313 6861 4

Hillgrove Monzogranite

Hillgrove is known for its mining history. The fortunes of the place have been directly related to gold and antimony mining for more than a hundred years. Armidale in comparison was tiny, a village in comparison with Hillgrove at its peak. Hillgrove still operates a mine for antimony and gold but is now quite a sleepy place with a handful of inhabitants. Most people working in the mine commute from Armidale. But the mine itself is not what I want to write about, it is about the attractive rock that is known as the Hillgrove Monzogranite. Despite its name the Hillgrove Monzogranite is not the extensive source of gold and antimony that is mined in the area. Most of the ore mineralisation is either directly or indirectly related to the nearby Bakers Creek Diorite or remobilisation of material from the adjacent marine sedimentary rocks.

Hillgrove Monzogranite on the Waterfall Way

According to the Australian Stratigraphic Names Database the Hillgrove Monzogranite was until recently known as the Hillgrove Adamellite (Adamellite being the outdated synonym for Monzogranite). It was previously classified as part of the Hillgrove suite which in turn is part of the Hillgrove Supersuite.  However, based on geochemical properties (and possibly just to confuse people) the Hillgrove Monzogranite is no longer considered part of the Hillgrove suite instead just being a member of the Hillgrove supersuite! However, it is clearly one of the S-type plutonic rocks collectively known as the New England Batholith (Bryant et al 2003).


Monzonite is unsurprisingly the dominant rock type of the Hillgrove Monzonite. It is an S-Type granite (derived from melted sedimentary rock). It is comprised mainly of quartz and feldspars (roughly equal potassium feldspar and sodium-calcium Feldspar), quartz, biotite mica and hornblende. The biotite often shows a foliation, which is a preferred alignment in the rock. The age of the Hillgrove monzogranite is estimated at between around 270 to 290 million years. To my knowledge, the age has not been directly measured but instead is based on its relationship to the surrounding rocks with their either calculate or approximate ages.


The landscape formed by the Hillgrove Monzogranite is one of my favourites. It forms a relatively large plateau which contains low rolling hills and lovely boulder outcrops. These outcrops often form lovely torrs (see pictures) formed by “onion-skin” weathering. Water enters cracks in the rock and during winter this freezes and expands gradually wedging the layers off the boulder. This is correctly termed frost wedging.


The Bakers Creek gorge has cut into some of the unit near the Hillgrove area but overall the appearance of the country is quite gentle. The rock unit extends a long distance from the location of Argyle in the west almost to Chandler Gorge in the east. The Waterfall Way (Armidale-Dorrigo Road) crosses in and out of the Hillgrove Monzogranite and Girrakool Beds into which it has intruded. Therefore it is an easy stop on the road when travelling this route.


The soils are sandy and not very fertile leading to an area used for cattle and sheep grazing on native and improved sown pastures. The forest is an open dry sclerophyll snow-gum type bush which is one of the typical environments of the New England high country. I love the appearance of this country. It is the quintessential high-lean New England landscape.
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.
*Bryant, C.J., Chappell, B.W. & Blevin, P.L. 2003. Granites of the Southern New England Orogen. Abstracts of the Ishihara Symposium: Granites and Associated Metallogenesis. GEMOC, Macquarie University

Rocks in the Rocky River

Rocky River Monzogranite (Bungulla Suite).
The Monzogranite here contains large crystals of twinned pink K-feldspar.

The Rocky River Road is a very quiet, scenic and out of the way route to travel. It is slow and windy, but a pretty alternative to the Bruxner Highway route between Drake and Tenterfield. I had the pleasure of a trip along Long Gully Road and Rocky River Road just last week. I enjoyed it very much for the scenery and the clear water of the Rocky River (also known as the Timbarra River). The area is also very interesting in a geological sense. The rock that is found along Rocky River Road (the Rocky River Monzogranite) is actually remnants of outer part of a very large batholith that makes up Timbarra Tableland.

Previously, understanding of the inner rocks of the Timbarra Tableland were incorrectly thought to be Moonbi Supersuite, while the outer rocks were correctly part of the Stanthorpe Supersuite. Having two parts of an intrusion being apparently related to different Suites was all quite confused. Mustard (2004) suggested an informal renaming of the Bungulla Monzogranite in the area of Rocky River to the Rocky River Monzogranite. The Rocky River Monzogranite would in turn be part of the Bungulla Suite. The Bungulla Suite being rocks that are I-type (derived from melted igneous rocks) of the Stanthorpe Supersuite.  Although the nomenclature by Mustard (2004) was suggested as informal it is quite reasonable to adopt the name of Rocky Creek Monzogranite as formal. The previous identification of some rocks in the Timbarra Tableland as Moonbi Supersuite has since been shown to be incorrect - they are all Stanthorpe Supersuite.

The Rocky River Monzogranite is in the extensive eastern edge of the Timbarra Tablelands. It is comprised mainly of the rock monzogranite. This rock is comprised of abundant quartz and roughly equal proportions of plagioclase feldspar (sodium and calcium feldspar) and potassium feldspar. There are also smaller amounts of dark biotite mica and amphibole in the rock. The Rocky River Monzogranite is quite a course grained and the crystals are very, very large. The monzonite is notable as it has many 'inclusions' called xenoliths. These are blobs of rock are of a less granitic composition. They are very, very common in some areas as the rock comprises of about 10% or more xenoliths. The xenoliths indicate that mixing of different composition magmas was occurring when the intrusion formed.

A monzogranite tor in the sandy bed of the Rocky River.
Note different sized irregular shaped xenoliths.

Along the very margin of the intrusion (I didn't get to see this) the crystals are smaller in size and the feldspars are even more potassium rich forming the rock syenite. The central area of the Timbarra tablelands is comprised of granitic rocks that were high in fluids when the rock was crystallizing. These fluids (formed by residual enrichment of the original magma chamber), has resulted in the concentration of metals, most notably gold (Mustard 2004). The Timbarra gold mine targeted this inner zone of the tablelands as the outer granite (Rocky Creek Monzogranite) do not contain nearly as much gold. The erosion of the gold has led to alluvial gold deposits in the Rocky River and Clarence Rivers but the gold is very fine grained so fossickers panning can be tricky.

The many components of the Timbarra tablelands intrusion were emplaced in the Triassic period. They intruded the Drake Volcanics. The size of the granite plutons has caused significant contact metamorphism, creating a large metamorphic aureole around the intrusion.

There is much more to say about the zones in the Timbarra tablelands intrusion described by Mustard (2004). This includes the neatness of the tablelands cross section, the way that the slightly different granites tapped different parts of a deeper magma chamber and the way that differentiation of granite types occurred are all worthy of a discussion. Though, this needs more than just a few paragraphs and so I will have to cover these matters in future posts. In the mean time I hope this post gives a taste for some of the 'granite'.

References/bibliography:
*Mustard, R. 2004. Textural, mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton, New South Wales. Australian Journal of Earth Sciences, 51.

The Great Dividing Ranges and Stonehenge

Granites occur throughout much of the north coast and New England region. I use the term granite here loosely, in reality the rocks I’m referring to have a range of compositions and ages. The things they have in common are their relatively high quartz content and they are igneous intrusive (plutonic) rocks. They have cooled slowly and therefore have allowed large crystals to form – giving them that typical granite appearance. I’ve covered a few granites in previous blog posts but in this post I’ll cover one New England “granite” called the Wards Mistake Monzogranite. I’ll continue to cover others in future posts.

Stratigraphically the Wards Mistake Monzogranite is part of the Wards Mistake Suite which in turn is part of the Uralla Supersuite. The Wards Mistake Monzogranite outcrops in a relatively extensive area between Glen Innes and Guyra. In places it straddles the Great Dividing Range but mainly occurs just on the eastern side within the upper reaches of many Clarence River tributaries. The unit was formed around 250million years ago, during the Lower Triassic to Lopingian (early Permian period).

The Wards Mistake Monzogranite consists of monzonite (a rock containing moderate quartz with equal parts potassium and sodium-calcium feldspar) with some granodiorite (abundant quartz and calcium-sodium feldspar). It has a typical equigranular black and white speckled appearance which is common of the Uralla Supersuite. It is like the other Uralla Supersuite granites as it is derived from the melting of other igneous rocks - I-Type Granites (Bryant et al 2003). However, it does contain some xenoliths (inclusions of other rock) which are sedimentary. It is possible that when the Wards Mistake Monzogranite was emplaced into the crust it incorporated bits of the surrounding sedimentary rock. This may have affected the chemistry of the magma and may be one of the reasons why there is both monzonite and granodiorite in the unit.

Typical tor outcrops of the Wards Mistake Monzogranite near Glen Innes

Many New England granites contain mineral deposits. Being an I-Type granite usually means a good chance of mineral deposit formation. However, the Wards Mistake Monzonite contains very sparse mineralisation with only a few small areas where there is some alteration zones that have more concentrated ore minerals. These include wolframite (tungsten), molybdenite (molybdenum) and cassiterite (tin) (Brown 1997). Other surrounding granites such as the Kingsgate Granite and Red Range Leucogranite have abundant mineralisation that was historically mined and is still under active mineral exploration permits.

A lovely feature of most New England granites is the interaction with the climate. This produces wonderful looking granite tors. This is a result of onion skin weathering (frost wedging) where water penetrates into the rock and freezes during the cold winters. This repeated action causes large flakes of rock to peel off. Some of these Tors are given their own names. In the Stonehenge area on the New England Highway you can stop and walk among these Tors and see the Balancing Rock which looks like it will topple over at any moment.

The landscape around Stonehenge between Guyra and Glen Innes is my favourite landscape in Australia. The high country agriculture, the cold weather and the geological conditions that form the rolling hills and special tors make it a special place. The picture above is of a portion of the Wards Mistake Monzogranite and partly shows the landscape I’m talking about. The accessibility of the granite is certainly worth a quick stop if you are travelling on the New England highway.

References/bibliography:

*Barnes, R.G , Willis, I.L. 1989. Preliminary geological plan of the 1:250 000 Grafton-Maclean sheet area - SH 56-6, SH 56-7. New South Wales Geological Survey Report

*Brown, R.E. 1997. Mineral deposits of the Glen Innes 1:100 000 map sheet area. Geological Survey of New South Wales. Quarterly Notes 103 p1-19

*Bryant, C.J. , Chappell, B.W. , Blevin, P.L. 2003. Granites of the southern New England orogeny. In Blevin, P. et al (eds) Magmas to Mineralisation: the Ishihara Symposium Geoscience Australia. Record 14 - extended abstracts.

The Road to The Gorge

Note that since this post was written the Towgon Grange Granodiorite has been renamed the Towgon Grange Tonalite.

Many people in the region know about “The Gorge”. It is a remote, yet popular area on the Clarence River. The road to The Gorge is interesting because of the change in geology that is experienced. The main route to The Gorge is via Grafton and Copmanhurst. By travelling west from Copmanhurst along the Clarence Way you move from the sedimentary rocks of the Clarence-Moreton Basin. First,  the rugged cliffs made from the Kangaroo Creek Sandstone give way to the rolling hills of the Walloon Coal Measures then Koukandowie Formation. Some road cuttings show weathered examples of these rocks. Turning off the Clarence Way and passing over the camping ground, swimming hole and bridge at Lilydale leads you to The Gorge turn-off. The Lilydale and Newbold areas have some of the oldest rocks of the Clarence-Moreton particularly the Laytons Range Conglomerate. But on the day I was there, I was not so interested in those rocks… because I was getting into the New England Orogen.

It is rare opportunity for me to explore the foot hills of the New England region. I love the feeling of the place, the wonderful landscape, climate, history and even culture. The place just seems to have a feeling of connection with the people who live there. Luckily, I managed to visit the edges of the New England escarpment for a little while on the weekend. While there I managed to experience more of the rocks that are the foundations of the landscape of New England.

Towgon Grange Tonalite - on The Grange Road, Middle Clarence River area

Driving along The Gorge road the rocks of the Silverwood Group are passed by. These are slightly enigmatic rocks of the New England Orogen, interpreted as subduction complex rocks (Van Noord 1999). Mainly outcropping in streams the rock of the Silverwood Group in this area are none the less quite hard and old metamorphosed marine sedimentary and volcanic rocks. The Silverwood Group is interesting because it also occurs near Texas in Southern Queensland and it is only partially understood in our region. But more about the Silverwood Group in a future post. 

Round tors appear by the road side near Table Creek about 15km south of The Gorge. These tors are a classical shape formed by the weathering and erosion of granite type rocks. Here are rocks that make up part of the New England Batholith. The batholith is numerous masses of intrusive igneous rocks plutons that were molten well before Australia was separate from Gondwana. The ‘granite’ here is called the Towgon Grange Granodiorite. Like the Dumbudgery Creek Granodiorite that occurs about 20-30km further north the pluton is bisected by the path of the Clarence River. This helps to illustrate the unusual behaviour of the Clarence river as it travels backward and forward over soft and hard rocks. In fact the other side of the pluton can be easily found on the other side of the river just off the Clarence Way.

The Towgon Grange Granodiorite intrudes into the Silverwood Group meta-sediments. The rock sample at Table Creek (pictured) is actually not a granodiorite. It is notionally similar in appearance but contains much less potassium-feldspar. The main minerals are light coloured plagioclase feldspar, quartz and darker clinopyroxene and amphibole. The rock sample shows that much of the clinopyroxene is mantled (surrounded) by amphibole. The lack of potassium-feldspar means that this particular sample is probably a Tonalite according to the most popular rock classification (QAPF). In fact Bryant et al (1997) actually notes that the Towgon Grange Granodiorite only contains small amounts of Granodiorite, with most being Tonalite or Quartz Diorite. This is a good example how stratigraphic names may be misleading to first time geologists!

Bryant et al (1997) classifies the Towgon Grange Granodiorite as an I-type granite of the Clarence River Supersuite. This means that the Towgon Grange Granodiorite is derived from the melting of other igneous rocks. The Towgon Grange Granodiorite is also comparatively low on silica (quartz) in comparison to other Clarence-River suite intrusions. It still contains enough quartz that it is generally visible in hand specimens. The age of the Towgon Grange Granodiorite is about 248-249Ma old. The younger sedimentary rocks of the Clarence-Moreton Basin overlie parts of the Towgon Grange Granodiorite and Silverwood Group.

The Towgon Grange Granodiorite is one of those rocks that just about no one in the general public has heard of. But, it is a good example of rocks that illustrate many points about the landscape evolution of the New England Orogen and the Clarence River. It occurs in a scenic area and is also a very attractive rock in its own right.

References/bibliography: 
*Bryan, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-Type Intrusions in Eastern Australia. Journal of Petrology V.38 No. 8.

*van Noord, K.A.A. 1999. Basin development, geological evolution and tectonic setting of the Silverwood Group IN Flood, P. G. (ed.) Regional Geology Tectonics and Metallogenesis: New England Orogen - NEO '99 Conference University of New England.

How wonderfully marbleous!

There are some rock types that are very common around the country and around the world that just don’t seem to rate much of a mention in the Northern Rivers. One very common rock is limestone formed from corals in a shallow sea, just like the Great Barrier Reef. Limestone is made almost entirely of the mineral calcite. Some parts of the world have vast terrains dominated by limestone called karst landscapes and it is quite distinctive. Limestone terrains sometimes form amazing subterranean cave systems as the stone is dissolved by rainwater infiltration into the formation. These karst terrains include north-west Mexico and other parts of North America, a giant band through northern England and a wide area of South Australia along the Great Australian Bight. However, it is a landscape absent from the Northern Rivers.

Outcrop of limestone north west of Tabulam

Having said that vast areas of limestone don’t exist in the region it is worth noting that they do exist in small areas here and there within the older rocks of the New England Orogen. The reason for this is interesting. The New England Orogeny was a period of mountain building during periods of plate collision which included a period of subduction of an oceanic plate under the Australian continental landmass during the Silurian period. The material on the surface of the oceanic plate was often accreted, that is scraped off and squashed onto the Australian continent. Seamounts are old islands in the middle of the sea. Such as, those around modern day Hawaii or Fiji. The seamounts were accreted onto the continental mass where they created little pockets of limestone in midst of the jumbled, squashed mass of deep seafloor sediments.

This means that if you find limestone in the New England area you are actually finding the preserved remnants of a little tropical island reef or lagoon. An especially nice thought, when you find some limestone on a cold frosty New England winter morning. One relatively accessible place to see some limestone is an old quarry on the Pretty Gully Road just north-west of the town of Tabulam which sits on the Bruxner Highway crossing of the Clarence River. The stratigraphic unit that the limestone of the area is part is the Emu Creek Formation which also includes areas of interesting fossils (more about that in yet another post). However, the quarry is interesting for more reasons than just as an occurrence of limestone.

Following the period of subduction and accretion a period occurred where intrusions of molten magma pushed their way into the accretionary sedimentary rocks. It occurred a couple of times including during the Late Permian to Early Triassic and created one part of what is referred to as the New England Batholith. The batholith is an array of granitic rocks that stretches through the whole New England Tablelands. The intrusions of the Late

fresh face of limestone - note the sparkles from the calcite crystals

Permian to Early Triassic included the emplacement of the Bruxner Monzogranite, a type of granite pluton (more about this specific rock in a future post). This pluton heated up and metamorphosed the rocks around it and one of which was that body of limestone near Tabulam. Contact metamorphism of limestone creates the rock called marble and this has happened at Tabulam. Although, the quality of marble is questionable because of the amount of impurities.

Other things happened to the limestone during metamorphism too. The transfer of fluids into and out of the cooling magma created chemical reactions which concentrated elements such as iron. This process develops what is called a skarn, a body of altered limestone with sometimes economic amounts of minerals. The minerals in a skarn can be diverse and very, very valuable but the minerals are based on the chemistry of the granite pluton. In the case of the chemistry of the Bruxner Monzogranite, there was not much of value except lots of iron which formed abundant amounts of the minerals magnetite and haematite. This has been considered for mining in the past but the small size and low grade means it is not a viable iron mine.

There are other small limestone deposits all around the New England and all of them are interesting for one reason or another. Some north of Inverell have lovely caves, others near Tamworth are mined for lime on a large scale. While others, just have interesting little features that illustrate what happened during the formation of our region.

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.

*Lishmund, S.R., Dawood, A.D. & Langley, W.V. 1986. The Limestone Deposits of New South Wales. 2nd Ed. Geological Survey of New South Wales

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.

The Dummy you'll find north of Armidale

One of the imposing landscape features on the north side of Armidale is the 1400m high Mount Duval. Some of my secondary education was in Armidale and I remember that the logo of my school actually had Mount Duval in it. Mount Duval is part of granite-like pluton called the Mount Duval Monzogranite. It was previously called the Mount Duval Adamellite; however the term Adamellite is no longer formally recognised. The intrusion actually extends in a crescent shape further to the west and includes Little Mount Duval which is roughly were the watershed for the Great Dividing Range sits, draining to the east all the way to the Macleay River. The monzogranite is considered to be middle Permian in age and intrudes several different complex rock units, one of these is a relatively small unit called the Dummy Creek Conglomerate.

Dummy Creek Conglomerate in the Sunnside area
metamorphosed by the Highlands Igneous Complex

The Dummy Creek Conglomerate is situated to the north of Mount Duval and extends to the east to the area of Puddledock, the northern side is intruded by the Highlands Igneous Complex. The Dummy Creek Conglomerate is comprised mainly of conglomerate but not exclusively. Lithic sandstone is a major component and it is actually what is in these sandstones that allow us to determine when the unit was formed, but more of that later. The abundance of conglomerate as well as sandstone and rarity of fine grained sediments like mudstones shows us that the sediments, gravels, etc that made up the Dummy Creek Conglomerate have not travelled far from their source. The clasts in the conglomerate show that the source rock was the underlying Carboniferous aged Sandon Beds (part of the Texas-Woolomin Block).

Korsch (1982) concludes that the original Sandon Beds was domed and uplifted by the intrusion of granite bodies of the New England Batholith such as the Mount Duval Monzogranite and the Highlands Igneous Complex. The hills formed from the deformation of the Sandon Beds began shedding rock, eroding and the sediments were deposited a short distance from these new hills. The intrusions continued to intrude shortly after the sediments were deposited which according to Holland (2001) created a complex system of overlapping zones of contact metamorphism. The intrusions were therefore emplaced in a very shallow crustal situation and volcanism was abundant and the Dummy Creek Conglomerate was quickly covered and preserved by a volcanic unit that is called the Annalee Pyroclastics which includes lavas, pyroclastic deposits and the like. It is worth noting that other models of formation by various other authors were summarized by Holland (2001) for instance some authors suggest that rock fabric studies may show a source only from the south.

A lot was happening in the Mount Duval-Tilbuster-Puddledock area during a relatively short period of geological time, indeed even during this time of change a substantial forest must have been growing in the area. The sandstone layers in the Dummy Creek Conglomerate preserve fairly common plant fossils. Most of the fossil remnants are fragments but there is enough to identify many plants with certainty. The most common fossil identified was the deciduous plant Gangopteris, a relative of the more commonly known Glossopteris, the main plant that formed the coal of the Sydney Basin. This plant existed abundantly in the middle of the Permian and so given that many of the rocks appeared to be forming at the same time these can be assumed to be close to this age too.

References/bibliography:

Holland, R. 2001. South western Margin and Contact Rocks of the Highlands Igneous Complex near Orana Falls, North of Armidale, NSW. Unpublished undergraduate research thesis, University of New England.
Korsch, R.J. 1982. The Dummy Creek Association: Rim Syncline Deposits. Journal and Proceedings of the Royal Society of New South Wales. V115.

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.

Lots of heat but low pressure between Ebor and Guyra

In the headwaters of the Aberfoyle and Guy Fawkes Rivers, tributaries of the Clarence River lies a rare zone (for Australia) of rocks that have experienced high temperatures but surprisingly low pressure. This is another one of those “we don’t have a clear answer” posts, in particular what has actually caused the metamorphism of the rock in this area, but research just published in January (Craven et al. 2012) has shed a lot of light on the matter.

Originally it was thought that the metamorphism at Wongwibinda (The Wongwibinda Metamorphic Complex) was directly associated with the emplacement of the Granites since the most intensely metamorphosed rocks are close to the Permian aged Abroi Granodiorite and other Permian granites with a decrease in intensity of metamorphism further away from these intrusions (Wilkinson 1969). The depth of metamorphism was never considered very deep because the minerals that are present in the metamorphic rocks are not formed where intense compression is found. However, it has since been observed that contacts with some of the granites shows no or little metamorphic effects, notably along the contact with the southern part of the Abroi Granodiorite. Additionally, the Abroi Granodiorite itself displays some metamorphic textures making the picture relatively unclear.

An old geological map of the area (NSW Geological Survey)-
Note that the Glen Bluff fault should not define the edge of the schist (gradational)
(Phag - Abroi, Plr - Ramspeck, Pl - Girrakool and Dyambarin, Tb - Basalt)

Like many parts of the New England, the geology can be quite complex with many aspects and relationships not fully understood and this holds for the Wongwibinda area which is a area of metamorphism, abundant faulting, granite intrusions, deep sea sedimentary and volcaniclasitic rocks and basalt lavas. Describing the generalgeology may be easiest from east to west. Before coming to the properties of Abroi and Springfield is the Palaeozoic aged Dyambarin Beds which is neatly faulted off to the east. The eastern side of the Wongwibinda Fault lies rock of the Abroi Grandiorite (a type of granite), sometimes referred to as the Abroi Gneiss (in this case metamorphosed granite which appears to have been affected by the Wongwibinda Fault). Then just to the west of this we enter what is significantly altered Girrakool Beds and this is were it gets even more interesting.

The eastern most part of the Girrakool beds has been significantly affected by heat maybe up to about 700 degrees Celsius, but has experienced very low pressures and this has created an unusual texture called migmatite. Migmatite is a type of rock were the parent (in this case sandstone and mudstone sedimentary rocks of the Girrakool Beds) has been heated so much that it just starts to become liquid like, here it also shows Ptygmatic folding. The liquid usually accumulates or is formed in some individual layers creating essentially layers of molten rock between sediments. Sometimes the hot liquid rock follows cracks in the rocks creating little dyke like structures too.

Moving further to the west we enter a zone of schist (called the Rampsbeck Schist), which is a medium grade metamorphic rock that has had some of the crystals in the rock reform into layers, this schist extends further west showing less and less metamorphic effects until it is indistinguishable from the rest of the Girrakool Beds. There are also some areas of quartzite and amphibolite (other metamorphic rocks) in the schist zone. I’ve also seen a pegmatite dyke a bit further to the west, which I have no idea where it fits into the picture.

Over the top of all this are remnants of comparatively recent Basalt referred to as Tertiary (or Mid-Cenozoic aged) Alkali Basalt. Given its location this basalt is probably Doughboy Basalt, part of the Cenozoic aged (~40Ma) Doughboy Volcanic Province.

But what caused the formation of the metamorphism? Many mechanisms have been proposed by different authors such as Wilkinson (1969), Danis et al (2010) and Craven et al (2012) and other authors. Craven (2012) has carried much work, including dating to try and gain an understanding:

• Was it the Wongwibinda fault? No – otherwise the rock would pressure related textures. 
• Was it the intrusion of the Abroi Granodiorite (or other granites in the area)? No – otherwise we’d see metamorphism around all the Abroi Granodiorite that we don’t see, and the age of the Abroi Granodiorite is older than the date of metamorphism. 
• Was it an intrusion that we can’t see because it fairly deep underground? No – gravity surveys have been conducted and these don’t show any deep granites other than the Abroi Granodiorite in the area of maximum metamorphism.
• Was it the eruption of the Cenozoic Basalts? No, the age of metamorphism vastly predates the Cenozoic period.

Wow, so what options are left? Craven et al (2012) have come up with a theory: following the tectonic events that formed the granites in the area there was a period of stretching of the earth, this thinned out the crust and allowed for heat to be more easily transferred from the mantle. All of the other options are more common elsewhere in Australia and around the world, but each option has been refuted by different evidence until the only reasonable explanation left at this stage is the extension of the crust to allow convective heat transfer from the mantle to very shallow levels during the Permian.

I seem to have lost my old photographs of the migmatite but I have found a good website on Finnish migmatites that has some great pictures. It can be linked to from here.

A follow up post on the Wongwibinda migmatites can be found here.

References/bibliography:

*Danis, C.R., Daczko, N.R., Lackie, M.A. and Craven, S.J. 2010. Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences V57.
*Craven, S.J. Daczko, N.R. and Halpin, J.A., 2012. Thermal gradient and timing of high-T-low-P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia. Journal of Metamorphic Geology V30.
*Wilkinson, J.F.G. 1969 The New England Batholith - introduction. IN Packham G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.