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Names Database the Hillgrove Monzogranite was until recently known as the Hillgrove Adamellite (Adamellite being the outdated synonym for Monzogranite).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

Don't you hate it when...

Don't you just hate it when you have information to share but you are not permitted by contracts and commercial in confidence so share it? Our society is more and more being constrained by bureaucratic regulations set up by people who are career managers but have very little understanding of the real world. Little understanding that science and engineering knowledge benefit all people and that some perceived public image issue is more important than the wider good. This means that innovation can be stifled... at least in my personal opinion. The perceived public perception of scientific discoveries hinders the development of knowledge from Climate Change to Panadol!


By way of one a specific local example, I was helping an environmental centre have a ground water bore installed. The Department of Education which runs the environmental centre put absurd restrictions on access to the groundwater. One such limitation was that school children had to wear gloves when touching the groundwater in case it was contaminated. After running some tests it is apparent that the groundwater is actually better quality than the filtered tank water that they are currently drinking... but still the safety controls need to be in place! I'm happy to drink the water but the children must still wear gloves... go figure!


Another example is a cutting edge research project in the Woodburn-Evans area. The information gained from this research is very important for most coastal sand groundwater systems in eastern Australia. Alas, the words from a senior manager in a NSW government department are that no scientific information gained from the research is to be released to the public in the short term. What a shame. I understand that people are risk adverse today especially with regards to perceived public opinion but I don't think scientific knowledge should be hidden away.


On a slightly different note I have received a copy of an in-press paper on Coal Seam Gas monitoring in the Northern Rivers area. I provided some minor assistance in the paper and so an author kindly showed me before it was published. It is expected to be released in a few weeks and is likely to be in the newspapers too. Keep an eye out for those three letters C, S & G.
Sorry for the rant... just had to get that off my chest... hopefully a less opinionated post coming up shortly!