Sunday, 23 August 2015

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Names Database the Hillgrove Monzogranite was until recently known as the Hillgrove Adamellite (Adamellite being the outdated synonym for Monzogranite).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.
*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


  1. Hi Rodney

    Came across a booklet that you would like called "The tramways of Hillgrove" which is one in a series on the light railways of Australia. It has a great historical review of the mine and shows quite graphically the intestinal fortitude of the early miners and how they dealt with 500m escarpments surrounding the mine.

    1. Yes, doesn't Hillgrove have an interesting history. They didn't let anything stop them. I seem to remember hearing that Hillgrove had dozens of pubs... I suppose that helped with the courage of working on those tramways!

      I'm quite sure that Hillgrove also used to be known as Elanora. I don't know why it changed its name or when (I think it was in the 1800's). Sadly much of the engineering history (including a hydro-power scheme) are barely evident any more... however, the environmental legacy is. The present mine operators are still cleaning up the old dumps from the 1800's. In fact without the help of the recent mine operators the downstream rivers would be in a much worse state.

    2. Here is an account of the first gold discoveries at Hillgrove in 1857.

  2. Yes Rodney, I was reading in the booklet that coal power cost around £80 per day to run the mine and only £30 per day on hydro. Many of the houses including at least one pub were relocated into Armidale. Jim Belshaw would know all about it.

    Wasn't Elanora the station the gold was found on and also name of the first lease pegged? Another one for Jim B., I'm sure if I went looking in the local paper I would find he has already done an article on it.