Sunday, 16 July 2017

Rockvale Arsenic mine


Rockvale Monzogranite and my toes for scale
It has been a long time since I wrote a blog post. Life has got in the way with a unexpected sorrow in the death of my little lady. It has been a year of change and that includes work and new ventures. Now, I begin the journey of working for myself. In saying that, if you have any work that might interest me I’d love to chat to you about it. I don’t have a web page at this stage, but I have my contact details on this page. Already, I have had the opportunity to work in the North Coast again, renewing old and making new contacts. I am still based in the New England area and as always it is an adventure to explore the land and environment.

The first post I want to do is one about an arsenic mine I visited near Armidale. When I visited the Rockvale Arsenic Mine it was quite evident that the site has contaminated the soil. The most widespread contaminant in the region is antimony metal which when mined (along with gold) in the Hillgrove district and was discharged in large volumes into the Macleay River catchment. This antimony continues to be dissolved in water flowing from the old Hillgrove and other abandoned areas. However, further upstream from Hillgrove, in the Wollomombi River catchment from is Rockvale which has a similar geological history.

Rockvale Arsenic mine
Possible evidence of contaminated soil transport (gully erosion)
Rockvale is named for its rocks! Granite types that cover a very large area. The rock unit is called the Rockvale Monzogranite of the Hillgrove Supersuite. It was formerly known as the Rockvale Adamellite, in the old nomenclature. According to Kent (1994) the Rockvale Monzogranite consists of 20 individual plutons all intruded during the Carboniferous (at approximately 303Ma). The surrounding rock into which it intruded and metamorphosed the Girrakool Beds. The metamorphic effects are quite significant and extend for quite some distance from the Rockvale Monzogranite. Interestingly, there are many hydrothermal mineral deposits in the Rockvale Monzogranite and adjacent metamorphic rocks, yet the mineral deposits were formed 50Ma later (approximately 250Ma). This situation is similar to the nearby Hillgrove mines which although spatially appear to be directly related to the Hillgrove Monzogranite but are actually later concentrations of hydrothermal minerals.

The number of old mines is the Rockvale area is quite significant. Some have been rehabilitated, some badly rehabilitated and others still discharge metal contaminants into the receiving environment. The Rockvale Arsenic mine is a good example, it appears to be the northern most of several mineral deposits that occur along a line a couple of kilometres long, possibly terminating at the Ruby Silver Mine. The main mineral mined from the hillgrove arsenic mine was arsenopyrite, though other less common arsenic minerals were also part of the ore. The arsenic was 'roasted' to drive of volatile elements and concentrate the ore.

The legacy of historical operations can be seen from the photograph which shows extensive bare areas and gully erosion due to the metal toxicity and acid mine waste from oxidised pyrite, arsenopyrite and other sulfide minerals. So, although the metal contamination from downstream Hillgrove is well known, a baseline study will obviously provide some indication that contamination is coming from other upstream sources too. It will be interesting to see how much.

References/bibliography

*Ashley, P. & Graham, B. 2001. Heavy metal loadings of streams in the Macley River catchment. Report to the Mid North Coast Catchment Management Board, NSW Department of Mineral Resouces & Armidale Dumaresq Council.
*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 Sciences (Transactions of the Institute of Mining and Metallurgy) vol 133 B3.
*Kent, A.J.R. (1994) Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region, southern New England Orogen, New South Wales. Australian Journal of Earth Sciences 14:4
*McClatchie & Sylvester. 1970. The Tulloch Silver mine. Records of the Geological Survey of New South Wales. Vol 12, part 1.


Thursday, 9 March 2017

Where you are: GDA2020 on the way

Australia's main datum for mapping and locating points on the australian continent is changing. You can occasionally find old maps using AGD66 in some shops, these are out of date. In 2000 GDA94 was adopted and now it too is changing. GDA2020 is here! Have no idea what I'm talking about?

Here is a good video that explains the change:

https://vimeo.com/191566518

While I'm on spatial data... why not increase the accuracy of your GPS? Many new GPS units have a WAAS option. Enable this and you will have greater accuracy in all three spatial dimensions. Of course it only works when certain satellites are visible to your GPS, in northern Australia it will detect the Indian and Japanese satellite systems but in Victoria and Tasmania you may find it harder to find a satellite that has WAAS type capabilities.

Want to know more about WAAS:

https://www.youtube.com/watch?v=CeIEs9l9aws

Saturday, 28 January 2017

Stunning in Red and White

A friend and colleague showed me his new lifestyle property on the edge of Armidale a couple of weeks ago. He is an observant soil scientist and noted that his land consisted of poor quality soils which grew only resilient grasses and some typical New England gum, stringy-bark and box woodland. He was curious about the rocks that were common on the surface of the dusty grey-brown soil. I was not surprised of the poor soils because the property is located on a geological unit called the Sandon Beds.

The Sandon Beds are common in the Armidale district, especially just to the north of the town. They were laid down sometime during the Devonian to Carboniferous periods. The rocks of the Sandon Beds are varied and include mudstones, conglomerates, volcanics and bio-chemical sedimentary rocks. The deposition of the unit was in the ocean debris flows from the continental shelf would form turbidites (coarse to fine grained repeating sequences), layers of fine mud would accumulate and occasional basalt volcanic rocks would occur. Sometimes, while a long distance from landmasses or spreading ridges very little would happen - only the gentle settling of dead primitive ocean organisms with silica skeletons.
Brecciated Jasper (chert) of the Sandon Beds

The settling of silica on the sea bed produces a rock called chert. It is common in the Sandon Beds with a red colour. The chert occurs in beds interspersed with dull mudstones, siltstones and the like. Possibly because of regional scale metamorphism or the effect of fluids in the rock the chert has been affected and displays its red colour. Because of the red colour it is often referred to as Jasper which is seen by some as a semi-precious stone.

Throughout my friend's property could be traced lines of chert running essentially north-south. This is because the beds have been tilted to a nearly vertical direction. There was nothing out of the ordinary with these beds but in one area some of the red chert caught my eye. I could not see the actual outcrop but scattered around one little area was red chert with bright white quartz veins. The chert had been broken apart and re-cemented together with the quartz rich fluid. The result was quite striking, a stunning red and white. In this one little area, at some time after the chert had formed and turned into hard rock it had been blasted apart apparently by hot fluids. A 'brecciated jasper' occurring in a little area that just happened to be on a friends new property just ready to be discovered.

Monday, 16 January 2017

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.