Monday, 6 October 2014

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'.

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


  1. We crossed Rocky River several time on our way home from Lismore. We went the Tenterfield Way, and I couldn't help but see the Rocky R signs along the way.

    1. Hi Wangiwriter,

      You may have seen signs for Rocky River or maybe there are some Rocky Creeks in that area. The main Drake-Tenterfield Road (Bruxner Highway) does not cross the specific river covered in this post... It is a bit to the south of the Bruxner Highway.

      Either way you illustrate an interesting point about Australian names of natural features... they are descriptive but still not descriptive enough to really distinguish between one from another!