Bruxner Monzogranite on the Bruxner Highway

In a previous post, I discussed the metamorphism of limestone at an area north-west of Tabulam. I thought I’d take the opportunity to discuss the intrusion itself  that caused the metamorphism (a rock unit called the Bruxner Monzogranite). Also briefly, put it in the context of the formation of the broader New England Batholith.

Typical Bruxner Monzogranite monzogranite

The Bruxner Monzogranite is a geological unit that is composed of a series of ‘granite’ plutons (intrusions of molten magma). These occur in a hourglass shape between Drake and Tabulam. The biggest areas occurring north and south of the Bruxner Highway and the central thin part of the ‘hourglass’ occurring where the Bruxner Highway crosses it.

The Bruxner Monzogranite is part of the Clarence River Super Suite of granites which is an I-type granite (Bryant et al 1997). I-type granites are derived from melted igneous rock. It contains two different varieties of ‘granite’ (Thomson 1976). One variety is the rock type monzogranite which contains roughly equal amounts of the two main feldspar groups (plagioclase feldspar and alkali feldspar). It also includes quartz, amphibole and biotite mica.

The slightly less common variety is the granodiorite which contains more alkali feldspar than plagioclase. Therefore, it is richer in the elements sodium and potassium . It is worth noting that the granodiorite is often more altered and is more quartz rich. The easiest way to distinguish between the two Bruxner Monzogranite varieties in the field is their colour: The granodiorite usually has a pink colour and the monzogranite grey. The relationship between the two varieties of granite is not very clear to me. The following questions immediately spring into my mind:

• Does one granite intrude the other? 
• Were they both molten when they were emplaced? Or was one crystallised first? 
• Was it fluids from the crystallising monzogranite that caused the alteration of the granodiorite?

Maybe, they are questions that someone knows about but has not published their work on, or maybe they are just one of the many geological questions unanswered.

Bryant et al (1997) gives the potassium-argon age of the Bruxner Monzogranite as 250Ma. This places it in the Triassic Era, the same age as the other nearby Clarence River Supersuite. Such as, the Jenny Lind Granite which occurs a few kilometres north of the Buxner Monzogranite. The Clarence River Supersuite ‘granites’ are a similar age to many other granites which occur throughout the New England. This was certainly a busy time for intrusions. Indeed, these granites probably represent the magma source for an eroded volcanic arc system. It was caused by a large west dipping subduction zone that was active during this time (Scheibner & Basden 1998).

The Bruxner Monzogranite was intruded into Emu Creek Formation which is Carboniferous to Permian aged (Bottomer 1986). It is comprised of mudstones, greywacke, siltstones, shale, sandstones, conglomerate and limestone. As mentioned in my earlier post on limestone in the area, metamorphism of these rocks has in places been quite pervasive with a distinct metamorphic aureole. This has created some interesting rocks and altered zones such as marble and iron rich skarn.

The Bruxner Monzogranite is overlain in some areas by sediments of the Clarence-Moreton Basin. In particular, the Woogaroo Subgroup of the Bundamba Group, mainly the Laytons Range Conglomerate. Weathered exposures of the Laytons Range Conglomerate can be seen in road cuttings on the Paddys Flat Road.

The Bruxner Monzogranite was once called the Bruxner Adamellite (the term adamellite is no longer recognised). It is named after the Bruxner Highway which passes right through the unit.  Adjacent to the Bruxner Highway, approximately 2-3km west of Plumbago Creek, is one of the best places to see the outcrops of both the monzogranite and granodiorite. A good place to see the monzogranite is along the ridges along Sugarbag Road which is in the northern part of the unit, off Paddys Flat Road.


References/bibliography:

*Bottomer, L.R. (1986), Epithermal silver‐gold mineralization in the Drake area, northeastern New South Wales, Australian Journal of Earth Sciences. V33.
*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cirdilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. V38.
Scheibner, E. & Basden, H. 1998 Geology of New South Wales – Synthesis. Volume 2 – Geological Evolution. Geological Survey of New South Wales, Memoir Geology 13.
*Thomson, J. 1976 Geology of the Drake 1:100 000 sheet, 9340. Geological Survey of New South Wales 1v.

Tharz gold in them hills!

Jim Belshaw, blogger and New England self government advocate has several interesting blogs. I thought I'd take the opportunity to share his latest New England History blog post on gold in the Timbarra/Rocky River area.

Jim has a very interesting writing style and I enjoy his blogs. He also seems to capture many parts of the New England landscape and history that go poorly documented. I understand this fascinating article was also published in the Armidale Express Extra which is not available online.

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